Testing of Metal-enclosed

14/419

:.:~,'.~i;'.

Testing of Metalenclosed Switchgear Assemblies

..:::~-./.,,.::.:..

::: ..,,....j~;.7~ :;.~:.:;

:i;!i~i:ii!!~:i

...........

:2:,.::-'.-i;"

.!'.!,.:..;:.

~...:::.:.:;~..::,.~ ..~.~:.,..;..~ .~:..,..,.... :.. .... .....,.....

! . .",..~.:..:..~:: :. ": :,,:::~ .:::,....i ~ ,.... .:,",,, "I~".L[ !i: ::,,:,Qn~ty'as'sur~.r1691,41421 ; .....

, '

:

....

~ i ":.,:~! ~:. ~,:.

: .".".:.. " ": .:".i" ,, , ",,'" : , i"i, : i,.'

,:

l l

9"::.....:".i::..::;:,i.:I,.:',!.i,:;::.:. :.!(. ': . .:: ":"17.1"."~.".:i : ,',:, ' " ,: ".!i.'..':.:..':.:~ i.' .~.:)!.'ii.:.,...,.i~

):.. : ~::;...,.~., :':~:.' ..:, .'.!.!?, "...:.!:.

: :, .' :

,, '..

:: : ~

, :7

, ,::," ,!',::,: , :::,"",:~ "

....., .

~.,: !4,~.2:.

I

'

:> :' :! ::"~.

' ": . ........ ,' .. ........:... ,.i !":! i .:: ".. : ,:.: :: . :,!:::: : ,": ,,,, : ,, ,....... , :,,,, ,~..?,,,,,.,~.;"::~,~:, !4/421:" : ,, "~ :: ," :", i:', ,:!,,,,!::~:::,,,: :: : i:':: ' ::: ,'.?.~'.i!i:~,~"ii.:,.

~!:41~:~: !

:~ i :: ,,,'~,'i',~,14.3:4,:,~,:

: ~:.14:3.5

,,::~: :

II I l l l I '~ II: I l' l' : IIII ~ Illl~ I'l' l~~ I:l II l:....

.': ~~ ~,,i!::~,.~.. :..i.~.:

i~trpose of te~n_g i~/42~

":. !".!'.i ii""".:i ':" .i i: ii..4i2'" !'il~~0"~de~l.::t~sts. !4/42.1 '. . : .. .....: ' ...... .. ~::~,:::: : ": ~,, 1~,2.!t-: .: . :.Type: te:m 14/~2t: .. ,, ..... :,,: i,, ,, ,: ::, ,:"" ,, " : ,",,,14.2:,2 , ,, Ro~tir~t~sts i,4/421. "":",,", :: "" : ,", , " , ' :," ', /1,,4,2.3 , : :: ~.'S~ distnrbaiie6s ::: :." ::i'. .... : :, .:: i:.: .:1.~1i2~4. :l~idd:t.asts !4/422 :

'

"".

":.

l '~l

l .... i

' Ill I II II : I ~l Il~4"3"6

,, ,::" ' : :, ," ....:" " " ~ ' ...... ,i,::" ' : .

:' .

'

' "1.4.3.7'

.

..

: ,: ,: :.:i::,.!,. :!: ,, :, :: ':!i; ,:," :?.:,i," !: ,,, ,' ;~:: :, : :,::., ::: ..... ~4',~:9" ,,:

.:!;.i.iT::.~,:~::,.:.,:,:..i!:.,::!.i'....'..,: , :..:::,! :~.:::..:. IIII

,i~ II~ I,I I

I:~ "'.l I:~' Ii,~::~l I

....

II' :I ,'I~'i'i, I

.....9l::,~:l......,:iii..i i I i:l~,:ii !liiIiil,i i,iI :li :l~:~:~l:IIi.i iIll.:;l::ii:~: iI ii.~ i ,,,,,,/,:,,,,/, ?:: ,,: ':!(,,,, ,:, : ....., :~: ';

.

.,

.. ', ...

,,

.... ,;

:

I I

I~ i i

,

.....

.

,, ,,

,

,,,

, ,,

......:, ""

,,:

,,

,.

.

,

.

.

.

,

,

,

14A.

.

:...

; .. ,.

.

..

i

..

Procedure

for routine tests i4/435.

,,

'

....

,

,,

":

....: ::,,

.,,:,

'

,:

.. ....

14,5

Pn)cedu~ 14.5.1

.1:4.6

An..Introdaction. toe~ake e,~ineeriag and ,,, .... ,~,'- ;..!; testing methods 14/436 .". : " " :i . :,':. :7. :: 1.4;6,1 ' Seismic.Disturbances.14/436 . .... ,. ~:...... ,.: 1.4,.61.2 , R~cording.~a e,ar~aa, ke 11,4/439 :.., '

. . .

..

.

.

.,..

.

.

'.

' .....:,

....

,;' " "

..

' ::

:. i

,

..

..:

...

,

, ..'

.

'..

:' ..

....

for field tests .14/435 . " ,.,, .... ": Verification Of insulation: resis .tahoe or :: ":: ;: :: ': . measare~ment of,tl~le~e current t41435 .;, ~"., ."i i" .

.

.

.

'

..

....

, i ,~ (

,

. . . . .

.

'

.

..... !,;;, ,,, ",i::: ~":.!,,i. ,: ,", ..

~

,

:.

. .;

, ,:,,,,,,

.

:,.....

.

....

31:~'0

,...,

.

. .:....

I~ 41

:+,.:

'....

. . .

,

,

,:

:,

. ..

II

i i i i .:~i.ii.ii:4::i:;ii i.i.!iiil!..li :~ ~ii:ii.iIi,; i, .......i,i1t4:3.,.~i .. i.

,

,

..,.. . "

:.":""

'

iii.i

. ~ . .

,

..!...

.. '

,

:~'..

9 . "~i!.::i ' ~ '" ~ '

II

..:

".'i

'

::"

.

""

'

'.!

..

:.'..

" .. ::. '" ~.~i"

. , i,~.i...i/,".: ',,,~:." ...: :..'L., '~..' ~,.::.+. )..,

: ..::, ....' '/... :.;

..

I ]]i~] I ,, '

::

~, "~',

.,... ~. , . :'...' ,,.

'., :' ':, ," .:'"'~,~., ' '".:.... ,i, i,'..

~,.~, ' , .~ ,~'~

,",,,

,..

" ' .".

:~:i:~,.,~".~.:,.:.~',,,:".!.....:.ii: .....

::. :::."

!

.

'

,. ' "

:...: .: . .. :.

.: .

~ "

., 14~6,7.

.....

14,6.8

...Test

:equi,~

:141449

: Test proc~lur~l.4/450

: .,

:

,:. ' ","

".

i"..: : : .i.:.. :: : i.:!?:...~:".

, . :' .'.: :~ :~:i,,

..

,,,

.:'

!,:

.

..'

.

,:,

..

,,'

,,

,

,,

,

.,

' .,,

;~ ,r

~, ~ .... ~.~,,,,, ,,,

,~:~. r

:, ~ , ,, .~,.... ,~

...':'.;!.'::":i.~;~. ,i.V.i';' '. ',:'~::

,,,

:.. "

.. .....

,

'~ ':"..

, ',,: ."," '.i

,,

,,..

'....: ,

.

,

,

,, ,,::;,.i,,,:i:,': .....:,,"::i:: :::!, .... :,,",, ' " ...

...

, ....:.+

Testing of metal-enclosed switchgear assemblies 14/421

14.1 Philosophy of quality systems This has been covered inSection 11.1.

14.1.1 Quality assurance To fulfil quality requirements, the material inputs going into the making of assemblies such as MS sheets (for their surface finish, thickness and bending properties), hardware (for their correct sizes, quality of threads and tensile strength etc.), busbars and cables (for their area of cross-section, conductivity and quality of insulation in the case of cables), insulators and supports (for their sizes and quality of SMC, DMC or any other material used), and all other switchgear components such as breakers, contactors, switches, fuses, CTs, PTs and meters etc. (for their ratings, duty class, class of accuracy (wherever applicable) and specifications etc.) must be properly checked and recorded, according to the manufacturers; internal quality checks and formats. This should be carried out before they are used in assemblies to eliminate any inconsistency in a material or component. Similar stage inspections are necessary during the course of manufacturing to assure quality at every stage and eliminate any mistakes in construction, assembly or workmanship during the course of manufacturing. Thus, a product of desired specification and quality is ensured.

14.1.2 Purpose of testing The purpose of testing a switchgear or a controlgear assembly is to assure its compliance with the design parameters, material inputs and manufacturing consistency. Here we discuss this for a switchgear assembly only. For tests on a controlgear assembly, the relevant tests may be chosen from those prescribed for a switchgear assembly.

14.2 Recommended tests The following are the recommended tests according to IEC 60439-1 for LT, IEC 60694, IEC 60298 for HT and ANSI C-37/20C for LT and HT switchgear assemblies that may be carried out on a completed switchgear assembly.

14.2.1 Type tests Type tests are conducted on the first enclosure of each voltage, current rating and fault level to demonstrate compliance with electrical and constructional design parameters. The tests provide a standard reference for any subsequent enclosure with similar ratings and constructional details. The following tests may be conducted to demonstrate verification of the following: 1 Insulation resistance or measurement of leakage current, both before and after the dielectric test 2 Dielectric properties:

3 4 5 6 7 8

9

9 Power frequency voltage withstand or HV test on power as well as control and protective circuits 9 Impulse voltage withstand test for system voltages 2.4 kV and above Temperature rise limits (or rated continuous current capacity) Short-circuit strength Momentary peak or dynamic current Protective circuits Clearance and creepage distances Degree of protection: 9 Enclosure test 9 Weatherproof test Mechanical operation.

14.2.2 Routine tests Routine tests are conducted on each completed assembly, irrespective of voltage, current, fault level and constructional details and whether it has already undergone the type tests. The following steps will form routine tests: 1 Checking for any inadvertent human error during assembly. This check may be carried out both at the works, after final assembly, and at the site after the installation before conducting routine tests or energizing the assembly. The checks can be carried out at a lower voltage to detect any shorting links or spanners left inadvertently on live terminals, weak insulation or insufficient clearance or creepage distances between the phases or phase to ground conductors. In the case of a defect the same must be rectified immediately. 2 Inspection of the switchgear assembly to check the following: 9 General arrangement and appearance 9 Mechanical operation of all movable parts and interlocks 9 Random checking of bolted connections for proper jointing and tightening 9 Generally all important requirements as discussed in Section 13.4 that may have been required in the specifications of the switchgear or controlgear assembly. 3 Inspection and verification of electrical wiring: 9 For power connections 9 For controls, metering and sequential operations if any 9 For protective circuits 9 For grounding of instrument transformers 9 For checking the insulation of the control wiring 9 For a polarity test of CTs. 4 Verification of insulation resistance or measurement of the leakage current, both before and after the dielectric test. 5 Verification of dielectric properties, limited to power frequency voltage withstand or HV test on power as well as control and protective circuits.

14.2.3 Seismic disturbances In Section 14.6 we provide a brief account of these

14/422 Industrial Power Engineering and Applications Handbook

disturbances as well as the recommended tests and their procedures to verify the suitability of critical structures, equipment and devices for locations that are earthquakeprone.

14.2.4 Field tests Generally the following tests may be carried out at site after installation and before energizing an assembly: 1 Checking for any human error as described in Section 14.2.2. 2 Visual inspection of the switchgear assembly 3 Inspection of electrical wiring (see Section 14.2.2) 4 Verification of insulation resistance or measurement of the leakage current, both before and after the dielectric test, when an HV test is being conducted at site. 5 Verification of dielectric properties, limited to power frequency voltage withstand test. This test is neither mandatory nor recommended, but may be required if a modification is carried out in the switchgear assembly at site.

14.3 Procedure for type tests Below we outline the procedure for conducting the above tests at the manufacturer's works.

14.3.1 Electrical measurements

Selection of testing instruments The instruments used for electrical measurements must conform to IEC 60051-1. Instruments with the following accuracies must be used:

9 For routine tests: class 1.0 accuracy or better, and 9 for type tests: not below class 0.5 accuracy. The current transformers and potential transformers must conform to IEC 60044-1 and IEC 60044-2 respectively. Instrument transformers with the following accuracies must be used. 9 For routine tests" Class 1 accuracy 9 For type tests" Class 0.5 accuracy

14.3.2 Verification of insulation resistance This test is covered under field tests, Section 14.5(4).

14.3.3 Verification of dielectric properties

Power frequency voltage withstand or HV test The test voltage must be as close to a sine wave as practicable and frequency as noted in column 3 of Tables 14.1 and 14.2, for series II and Table 13.2 for series I voltage systems, and applied for one minute as follows: 1 If relays, instruments and other auxiliary apparatus, have a voltage other than the main voltage, they should be disconnected from the main circuit, while conducting this test and tested separately, according to their voltage rating as shown in Table 14.3. 2 All the CT (current transformer) secondaries must be shorted and grounded while conducting the test on the main circuit. This condition is redundant while conducting test on the auxiliary circuit. 3 PT (potential transformer) windings must be disconnected by removing the control fuses from its both sides. 4 The test voltage at the moment of application should not exceed 50% of the rated test value and should be

Table 14.1 For series II voltage systems Insulation levels, power frequency and impulse withstand voltages for metal-enclosed switchgear assemblies

Nominal system voltage

Rated maximum system voltage

One-minute power frequency voltage withstand at a frequency not less than the rated 60 Hz (phase to ground)

kV (r.m.s)

kV (r.m.s.)

kV (r.m.s.)

kV (r.m.s.)

Dry (1 min.)

Weta (10 s)

List I

List II

List I

kV (peak)

List II

List I

List II

30 45 60

60 75 95 125

60 95 110 150

150 250 350

200 250 350

4.16 7.20 13.80 23.0

4.76 8.25 15.0 25.8

19 26 35 50

19 35 50 70

24 30 45

34.5

38.0 48.3 72.5

70 120

95 120

60

80

100

100

160

160

140

140

69.0

Standard lightning impulse (1.2/50 Its) voltage withstand (phase to ground)

m

For still higher voltages refer to IEC 60694. For impulse withstand voltages across the contact gaps of an interrupting device, refer to IEC 60694 and IEC 60298. aWet or dew test is a weatherproof test (Section 14.3.10) and is meant for outdoor assemblies. Based on IEC 60694

Notes

Testing of metal-enclosed switchgear assemblies 14/423 Table 14.2 For series II voltage systems Insulation levels, power frequency and impulse withstand voltages for metal-enclosed switchgear assemblies i

Nominal system voltage

Rated maximum system voltage

kV (r.m.s.)

kV (r.m.s)

One-minute power frequency voltage withstand at a frequency not less than the rated 60 Hz (phase to ground) 3 kV (r.m.s.) b

0.48 0.6 4.16

0.508 0.635 4.76

Standard lightning impulse (1.2/50 Its) voltage withstand (phase to ground)

kV (peak) c

b

kV c

b

m

2.2

One-minute d.c. voltage withstand (phase to ground)

c

3.1

2.2 19.0

19

60

60

3.1 27

75 95

50 50

7.2 a 13.8 14.4

8.25 15.0 15.5

36.0 36.0 -

26.0 36.0 50.0

95 95 -

110

23.0 34.5 69.0

25.8 38.0 72.5

80.0 -

60.0 80.0 160.0

-

125 150 350

150 -

-

27 37 50 70

aANSI C-37/20C specifies this rating only for metal-clad switchgear assemblies and not for metal-enclosed bus systems. bWith breaker assemblies cWith isolator assemblies Based on ANSI C-37/20C

Notes 1 (a) The procedure for a d.c. test is same as for a.c. Due to variable voltage distributions when conducting d.c. tests, the ANSI Standard recommends that the matter may be referred to the manufacturer for system voltages 25.8 kV and above. (b) For a power frequency voltage withstand test, the d.c. test is generally not recommended on a.c. equipment, unless only d.c. test voltage is available at the place of testing. The d.c. test values, as provided above, are therefore for such cases only and are equivalent to power frequency a.c. voltage withstand test values. 2 Power frequency tests after erection at site, if required, may be conducted at 75 % of the values indicated above. Also refer to field tests (Section 14.5, Table 14.8).

Table 14.3 For series I voltage systems. Dielectric test voltages for control and auxiliary circuits and also LT power circuits i

Nominal auxiliary~power voltage

Control

and auxiliary

One-minute power frequency voltage withstand at any frequency between 45 and 65 Hz (between phases and ground)

circuits

1 Where the rated insulation voltage does not exceed 60 V

1000 V (r.m.s.)

2 Where the insulation voltage exceeds 60 V

(2 Vr + 1000) volts with a minimum of 1500 V (r.m.s.).

Power

(i) F o r f i x e d as well as d r a w - o u t a s s e m b l i e s The trolley is in the service position in draw-out assemblies (Section 13.3.2). 1 B e t w e e n phase to phase and each phase to ground with the switching devices in the closed position 2 B e t w e e n phase to phase and each phase to ground with the switching devices in the open position. This test is to be conducted on both line and load sides of the switching device. 3 B e t w e e n the line and the load terminals of each phase, the main switching devices being in the open position.

circuits

3 For voltages between 300 and 660 V

2500 V (rms)

As in IEC 60439-1

(ii) F o r d r a w - o u t a s s e m b l i e s The trolley is in test position and the main switching device in the closed position,

raised gradually but rapidly and then maintained at that level for one minute, reduced rapidly up to 50% of its value and then disconnected. 5 During the test, one pole of the testing transformer should be connected to ground and the frame of the assembly.

1 B e t w e e n phase to phase and each phase to ground on both line and load fixed terminals. 2 B e t w e e n line and load fixed terminals of each phase. A N S I - C 3 7 / 2 0 C r e c o m m e n d s this test to be conducted at a value 10% higher than specified in Table 14.2 for both p o w e r frequency and impulse voltage withstand tests.

14/424 Industrial Power Engineering and Applications Handbook

(iii) Additional test requirements for outdoor HT assemblies The following are a few additional test requirements for an outdoor HT assembly, recommended by IEC 60694 for rated voltages up to 100 kV. 9 Wet test While an indoor type switchgear assembly requires only a dry power frequency voltage withstand test, the outdoor type switchgear assembly also needs a wet test under wet conditions to check the external insulation. For the test procedure refer to IEC 60060-1. 9 Artificial pollution test The purpose of this test is to provide information on the behaviour of the external insulation while operating in polluted conditions. The test may be performed only if thought necessary, depending upon the degree of contamination at the place of installation. For the test procedure refer to IEC 60060-1. 9 Test results Any disruptive discharge or electrical breakdown during the application of high voltage should be considered as a dielectric failure.

14.3.4 Impulse voltage withstand test (for system voltages 2.4 kV and above) The impulse voltage is applied as shown in Tables 14.1 and 14.2 for series II or Table 13.2 for series I voltage systems, with a.full wave standard lightning impulse 1.2/50 ~ (Section 17.6.1) a front time equal to or less than 1.2 ~ and the crest value equal to or more than the rated full wave impulse withstand voltage. According to ANSI C-37/20C, three positive and three negative impulses must be applied without causing damage or flashover. Should a flashover occur in only one or any group of three consecutive tests, three more tests are allowed to be conducted. If the equipment passes the second group of three consecutive tests, it should be considered as acceptable. The flashover which occurred earlier may be considered as random and irrelevant.

14.3.5 Verification of temperature rise limits (or rated continuous current capacity) The test must be carried out indoors, reasonably free from draughts. Thermocouples should be used to measure the temperature, and the ambient temperature can be measured by thermocouples or thermometers.

Measurement of ambient temperature The ambient temperature should be measured during the last quarter of the testby at least three thermometers or thermocouples placed equally around the switchgear assembly, at almost the centre level and at about 1 metre from the body of the enclosure. The ambient temperature to be considered must be the average of these readings and should be within 10-40~ To ensure that the ambient temperature is unaffected by magnetic field, alcohol thermometers must be used and not mercury thermometers.

Measurement of temperature of enclosure, insulation and current-carrying parts The thermocouples or RTDs should be located such as to measure the hottest spot, even if this means drilling a hole in the current-carrying parts.

Procedure 1 The test may be conducted on a completed assembly of a switchgear or a controlgear or a part of it, whichever may be regarded as a complete section. The purpose is to achieve a near-service condition. To do this in a multi-section switchgear or controlgear assembly it is advisable to test at least three vertical sections joined together and measuring the temperature rise on the middle section. This is to restrict the extra heat dissipation, through the sides, except natural heat transfer, and also to simulate the influence of heat transfer to this section through other sections. 2 The test must be conducted at the rated* current, at a frequency with a tolerance of +2% a n d - 5 % of the rated frequency and the voltage in a sinusoidal waveform, as much as practicable. See also Section 11.3.2. The test is carried out until the temperature reaches almost a stable state, i.e. when the variation does not exceed I~ per hour. To shorten the test duration, the current may be enhanced during the initial period to reach a fast, stable state. 3 Since it may not be practical to create the actual operating conditions at the place of testing, normal practice is to simulate these conditions on the following basis. The heat generated by a current-carrying component or conductor is its watt loss and is expressed by I2R, where I is the current and R the resistance of the circuit under consideration. The watt loss of each current-carrying component installed in the test assembly is estimated and added to arrive at the approximate watt loss during the actual operation. Based on this loss is calculated of the total heaters required. These heaters are then suitably located in the test assembly to represent all the incoming and outgoing feeders, their power cables and any other current-carrying component.

Sources generating heat 1 Power circuits 9 Interrupting devices - switches, breakers, MCCBs, power contactors and fuses 9 Thermal elements of the overload relays 9 Incoming and outgoing power contacts. In these circuits the watt loss is ascertained, by measurement of the resistance of their conducting paths. 2 Control and auxiliary circuits Coils of the power contactors Coils of the auxiliary contactors (relays)

*All the circuits may carry current based on the diversity factor. The loads may be substituted by space heaters.

Testing of metal-enclosed switchgear assemblies 14/425

Coils of the timers Control fuses Coils of the measuring instruments (A, kWh and kW meters etc.) Wattage of the indicating lights VA burden of the instrument and control transformers and Control terminals etc. The VA burden and the corresponding p.f. of all such components are provided by their manufacturers. VA cos~ is the content of watt loss. For more details see Section 15.6. l(iii). 3 Power connections and control wiring The loss within such components is measured by their resistance, which, in the case of cables, is a function of their size and length. The loss in the external power cables is calculated similarly, parts of which run inside the assembly to connect the various feeders, by measuring their average length inside the assembly. Calculating the resistance of each current-carrying component separately is a very cumbersome and lengthy procedure, in addition to being not very accurate due to the large number of approximations. Some of the joints and components may still have been omitted from these calculations. The easier and more often recommended procedure is to measure the resistance between the extreme ends of each feeder in its ON condition by an Ohm-meter. This resistance will also include the contact resistance of each terminal and joint. With the rating of each feeder and the resistance so obtained, the I 2R loss of each feeder can be calculated Table 14.4

and totalled. This is the loss at the ambient temperature at the test place. It may be corrected to the operating temperature of the assembly as shown in Table 14.5. For sample calculations, we have considered it to be 90~ Table 14.4 provides a step-by-step procedure for estimation. The total watt loss of the assembly so determined is an estimate for the required heaters that may be installed inside the assembly, to achieve almost a true replica of the watt loss, as during actual operation. These heaters are located within the assembly under test to circulate heat uniformly to all parts to reach a rapid thermal equilibrium. To provide heaters for individual feeders is very cumbersome and serves no purpose. The operating conditions are simulated similarly in the adjacent panel sections and heaters are provided there also. If the bus rating of these sections is different from the rating of the section under test, then the heating effect of these busbars should also be estimated and the rating of the heaters altered to account for this.

Main busbars (horizontal and vertical) During the test the main busbars are fed at the rated current, for which the switchgear assembly is designed. They are heated naturally and therefore no resistance of the main bus need be measured. The busbars are shorted at one end and the current is fed from the other through a variable-current injection set at a reduced voltage of 3-10 V, or enough to achieve the rated current. The arrangement saves on power requirement and consum-

Computation of heat losses for temperature rise test (single-line diagram, Figure 14.1)

Sr. no.

Component

(A) 1

P o w e r circuits 55 k W D O L feeder, Comprising; 1 No. S w i t c h - 250A 3 Nos. F u s e s - 200A 1 No. R e l a y - 9 0 - 5 0 A

Watt loss at operating temperature of 90~ W o watts

No. of feeders

Feeder rating, (I) A

Component resistance per pole R ml2/pole

Watt loss (I2R) at room temperature (32 ~ Wr watts

1

110

1 x 3 x 1 1 0 2 x 0 . 3 3 x 10 -3 = 11.98

Since W ~: R and R90 = R32 [1 + oc20 ( 9 0 - 32)] .'. Wo at 90~ -- W r 9 [1 + ~ 2 0 ( 9 0 - 3 2 ) ] - 11.98 [1 + 3.93 x 10-3(58)] - 14.71 (~20 for copper from Table 30.1 - 3.93 x 10 -3 per ~

5.5 k W D O L feeders, each comprising 1 No. Switch - 63A 3 Nos. Fuses - 25A 1 No. Contactor 16A 1 No. R e l a y - 9 - 14A

2

12

Circuit resistance between I/C and O/G p o w e r terminals covering all components and p o w e r wiring or metallic links and their contact resistances, including end terminations. By measurement = 0.33 As above = 0.52

2 x 3 x 122x 0.52 x 10 -3 - 0.45

0.45 [1 + 3.93 x 10 -3 (58)] - 0.55

SFU feeder comprising; 1 No. Switch - 63A 3 Nos. F u s e s - 32A

1

32

As above = 0.46

Total loss in power circuits

1 x 3 x 3 2 2 x 0.46 x 10 -3 1.41

-

= 16.99 W

-

1.4111 + 3.93 x 10 -3 (58)] 1.73

(A)

(Contd)

14/426 Industrial Power Engineering and Applications Handbook Table 14.4

(Contd.)

(B) C o n t r o l circuits

1 Component

Contactor coil CT Ammeter ON light Auxiliary contactor Loss at 40~ (I)

Watt loss 63/32A SFU feeders

Qty

VAa

Watt loss

1 1 1 1 1

85 at 0.3 p.f. 7.5 b 5b 7b 15 at 0.35 p.f.

25.5 7.5 5.0 7.0 5.25

15 at 0.35 p.f. 5b 5b 7b

Total

50.25

Total

W4o

Qty

VAa

5.25 5 5 7

W90 = 22.25[1 + 3.93 x 10-3(50)] W90 = W40[ 1 + oc20 (90 - 40)] = 50.25 [1 + 3.93 x 10 -3 (50)] = 26.62 for 2 feeders = 53.24 = 60.12 (o~20 for copper)

Control cables, 2.5 mm 2 (copper flexible). 9 Cable resistance at 20~ (Table 13.15) s 9 Control circuit current

@ 10 m/feeder

@ 5 m/feeder

7.6 less than 1A

7.6 less than 1A

(For 1A)

Watt loss at operating temperature (90~ W9o-- W2o [1 + ~ 2 o ( 9 0 - 20)]

NIL

22.25

loss at 90~

9 Watt-loss at 20~ (II)

5.5 kW feeders

55 kW feeders

NIL

10

• 7.6 = 0.076

- x 7.6 = 0.038 per feeder 1000

1 x 0.076 [1 + 3.93 x 10-3(70)]

2 x 0.038 [1 + 3.93 x 10-3(70)]

= 0.097

= 2 x 0.048 = 0.096 53.34

Loss in control circuits (I + II) 60.22 Total loss in control circuits

(B)

= 113.56 W

If the loss in the control cables is small, this can be ignored for a quicker estimation of losses. (C)

P o w e r cables (aluminium) (mm 2) 9 Average length from cable gland to the power terminals (m) 9 Cable resistance at 20~ (Table 13.15) D./km 9 Watt loss at 20~ Watt loss at, operating temperature (90~ W9o -- W2o [1 + ~20 ( 9 0 - 20)]

3•

3x4

31/2 X 25

1.5

2.0

2.5

1 • 3 • 1102 X ~

1.2

7.54

0.32 1.5

• 0.32

2x3x122

2 • 1--6-6-~ x 7.54

1•215

2.5 2 x 1--6-0~ x 1.2

= 17.42

= 13.03

= 9.22

17.42 (1 + 4.03 • 10 -3 • 70) = 22.33

13.03(1 + 4.03 • 10 -3 • 70) = 16.7

9.22(1 + 4.03 • 10 -3 • 70) = 11.82

oe20 for aluminium from Table 30.1 --- 4.03 • 10 -3 per ~ Total loss in power cables

= 50.85

Total watt losses A + B + C

= 181.40 W

(C)

9 Heaters required for 180 W, which may be arranged in the sizes of 3 of 50 W each and 1 of 30 W (or as convenient) and located as shown in Figure 14.2.

a From manufacturers' catalogues at 40~ b We may consider these at unity p.f.

Testing of metal-enclosed switchgear assemblies 14/427

Table 14.5

Temperature rise limits: for buses, bus connections and other parts of a switchgear assembly

Limit of hottest spot total temperature ~

Type of bus connection

Limit of hottest spot temperature rise above an ambient of 40~ ~

(A) 1 For busbars and busbar connections of aluminium or copper 2 For busbars and busbar connections of aluminium or copper silver plated or equivalent 3 Terminals for external insulated cables

50

90

65

105

70

110

15 a 25

55 65

30 b 40

70 80

(B) For parts exposed to contact by a human body 1 Parts handled by operator (i) of metal (ii) of insulation 2 External surfaces, covers (i) of metal (ii) of insulation

Note: For details of temperature rise of various parts and materials of an HT switching device refer to IEC 60694. Based on IEC 60439-1 and 2 aparts that are not frequently handled, may be allowed a higher temperature rise. bparts that are exposed but need not be touched during a normal operation, may have higher temperature rise by 25~ for metal surfaces and 15~ for insulating surfaces.

ption. If the ratings of the main bus and the sectional bus (vertical bus feeding a group of feeders) are different, as in large switchgear assemblies, then two separate current sources may be used, one to feed the main bus and the other the sectional bus. The sectional bus can now be detached from the main bus as shown in Figure 14.2, and applied with the appropriate diversity factor, as shown in Table 13.4, to simulate the test condition to obtain almost the operating condition. For the purpose of illustration, we consider the single-line diagram of a power

io

distribution circuit shown in Figure 14.1. The general arrangement of its switchgear assembly is illustrated in Figure 14.2. Locate the RTDs at the likely hot spots, as at the joints of the busbars. Figure 14.2 illustrates the likely locations of the RTDs. The test may be carried out as noted earlier and temperature readings tabulated at 30-minute or 1hour intervals, whichever is more appropriate. The temperature rise, estimated with the highest temperature recorded by any of the RTDs, would refer to the ambient

ii

................

,((r

;l I

i ;|174 i ;|174 i. ;|174

"

i

i

~D

|

55 kW

DD

|

D~

| 2 x v55 kW"

|

1 x 63/32A SFU

Figure 14.1 Single-line diagram for an assembly under a heat run test

14/428 Industrial Power Engineering and Applications Handbook Remove the links and short the vertical bus separately

Main busbars

View-A

Shorting -+- -+- ,r'+--, +

+

]$] ]+]

9

',+I

",kiE ~

* :1: '*j !

it +

5 o $ .t $ (~) 63/ 32A 6i

!

7 9

(~

I

Main busbars

+

]l

I

'

View-A

s5

Vertical sectional bus 200A [(sum of all the vertical feeders of this row) x (diversity factor). But considered as min 200A]

80

J

55 kW

Q

Shorting links

,~,

J

)

links

I

kW

)

S

'

5.5 kW

|

o', olt ol

_ <

{El

{c

Oq

-T

TTI 1"

111111 Variable current'injection set for the main bus 0-2000 A, 3-10 V

IZI

~

171

v Dummy panel

I-ml "l \

I-m-I

I-ml

v Dummy panel

V

Panel under test

J

Variable current injection set for the vertical bus 0-1000A, 3-10 V

Note: Dummy panels are also heated simultaneously, the same way, as the panel under test

9

Location of RTD'S

r--I Location of heaters Figure 14.2

Temperature rise test on a switchgear assembly

temperature of the test location and may be corrected to the desired ambient temperature at which the assembly is likely to operate. The temperature thus obtained is the temperature rise that the assembly would attain during continuous operation. The corrected temperature rise must fall within the limits prescribed in Table 14.5. If

Or = temperature rise estimated at the test location 0o = ambient temperature at the test location Oh = highest temperature recorded by any of the RTDs 0a = ambient temperature at the place of installation Then 0r= 0h-- 0o and corrected temperature rise = Or + ( 0 a - 0o)

A successful test will ensure: 1 For insulating materials: the highest temperature rise

will not exceed the hot spot temperature rise as recommended in Table 14.6. 2 For busbars and busbar connections: the highest temperature rise will not exceed the hot spot temperature rise as recommended in Table 14.5. Table 14.6 Temperature limits for insulating materials as used in a switchgear assembly

Class of insulating material

Limit of hottest spot temperature rise above an ambient of 40~

Limit of hottest spot total temperature ~

Y A E B F H

50 60 80 90 115 140

90 100 120 130 155 180

Testing of metal-enclosed switchgear assemblies 14/429

3 Other parts of the switchgear assembly and the auxiliary components, for which limits have been specified, will not exceed the hot spot temperature rise, as recommended in Table 14.5.

The test conditions as noted above may over-estimate the rise in temperature during actual operation. Some latitude may therefore be considered while analysing the final results if the temperature rise thus estimated exceeds the prescribed limits only marginally.

General notes on testing procedure 9 The main bus through its entire length is fed with its rated current, while in operation it would carry a diminishing value after every feeder or a sectional bus. 9 The sectional bus is fed similarly. 9 If a control bus is also used add for its heat loss. A third current source may be required if a temperature rise in this bus is also desired. 9 Keep the control circuits energized if possible, to further save on calculations and to obtain more accurate results. In the sample calculations as shown in Table 14.4 we consider this in a de-energized condition for the sake of more clarity. 9 Each feeder is considered at its optimum rating, based on the current rating of the motor or the rating of the power fuses in a SFU or FSU feeder while the current may be much less in actual operation. If the temperature rise, as determined above, exceeds permissible limits it will be desirable to provide extra louvres, a forced cooling arrangement, larger busbars or a change in their configuration whichever is more convenient and easy to implement.

@ Short circuit generator

14.3.6 Verification of short-circuit strength This test is conducted to verify the suitability of the equipment to withstand a prospective short-circuit current that may develop on a fault. It may also be termed the steady state symmetrical fault current Isc or the shorttime (withstand current) rating of the equipment. When the equipment is an interrupting device, it is referred to as its symmetrical breaking current. It is permissible to test just one panel of a multi panelassembly so long as the construction of other panels is similar and busbar arrangement and supports are the same. The value of the prospective short-circuit current may be determined from a calibrated oscillogram. The test current in any phase should not vary by more than 10% of the average in the three phases and must be applied for a predetermined time of 1 or 3 seconds. Unless specified otherwise, this should be considered as to be 1 second. The oscillogram must reveal continuity of the current during the test period. The frequency of the test circuit

L

R

@1-,-~ I

@ Circuit breaker

]

@ H.T. transformer

I r f-IJ

@~(,a.sterbriaker" "-

.

.

" .

9

I

]

L_~ r-L J

L I~t-- --

-

[

.

@ _

_

_

,

6(~ Source side reactance 'L' (to control the magnitude of the test current)

I;.;Y;B

|

V" VyVB 1

Source side resistance 'R' (to control the p.f. as per the test requirements) High current step down transformer Test object Current shunt R-C voltage dividers Recording instruments: EMO : Electro-magnetic oscillograph, to record power frequency quantities such as short circuit average and peak currents in each phase, voltage across each phase during and after the test, generator voltage and time duration of test, as recorded in Figure 14.4 Figure 14.3

CRO : Cathode ray oscillograph, to record voltages of a transient nature. For instance, re-striking voltages (TRVs), whose frequency of oscillations is beyond the response range of EMO.

General arrangement of a power circuit to conduct a short-circuit test

14/430 Industrial Power Engineering and Applications Handbook

can have a tolerance of up to 25 % of the rated frequency for LT and 10% for HT assemblies. Figure 14.3 illustrates a general arrangement for such a test.

short-circuit current, then by using the Simpson formula, Iav can be calculated by using

Iav =~ 3~ [I~ +4(12 + I~ +I52+12 +192)+2(12 + I42+ I~ +I~)+ I120]

Inference from the oscillogram From the oscillogram, shown in Figure 14.4 one can easily determine the average r.m.s, value of the shortcircuit current, lav, its duration and the momentary peak current. For easy evaluation, this oscillogram has been divided into ten equal parts (1 to 10) and is redrawn in Figure 14.5 for more clarity. The short-circuit commences at point D1 and concludes at point A2, A1A2 being the original zero axis. At the instant of short-circuit, the zero axis shifts to B1A2. D1B1 is the initial d.c. component that decays to zero at A2 at the conclusion of the test. I0, I1 . . . . . Ilo etc. are the r.m.s, values of the a.c. components of the asymmetrical fault current at instants 1, 2 . . . . . 10 as indicated. They diminish gradually and reach their steady-state condition about the original axis, A1A2, in about three or four cycles of the short-circuit condition (Section 13.4.1(8)). The values of Io, I1 ..... 110 can be calculated from the d.c. components and the r.m.s, values of the symmetrical a.c. components, Iac0, lacl ..... lacl0 at the instants of 1, 2, .... 10 at which are referred the values I0, 11. . . . . Ilo. Say, for I0, if Iac0, is the r.m.s, value of the symmetrical c o m p o n e n t of the a.c. fault current and ldc0 the corresponding d.c. component on a B1A2 curve then I0 = 4 la2c0 + I2c0 (since Iac0 and Idc0 are almost 90 ~ apart). The values of 11, 12. . . . . I10 can be determined along similar lines. The curve C1C2defines the asymmetrical average fault current lay. If Iav is the average r.m.s, value of the asymmetrical

This is also known as the asymmetrical breaking current and tends to become the symmetrical r.m.s, value of the fault current Isc after almost four cycles from the instant of fault initiation, as discussed in Section 13.4.1(8). For more clarity and a better understanding of the oscillogram and also to determine lac0 and ldc0 more accurately, a few cycles of the first section of the oscillogram are shown in Figure 14.6. The d.c. component is assumed to decay quickly and approach zero by the instant B2, i.e. within the first section of the test oscillogram. The asymmetrical fault current envelope C3C4will also approach an almost steady state about its original axis A1A 2 by B2. O1 and 02 are considered arbitrary instants of current zeros on the asymmetrical current wave. ?? If la'~o and laco are the peak symmetrical a.c. components of the fault current at these instants as noted in Figure 14.6 and l~co and Id'co are the corresponding d.c. components then ##

laco and Iac----2-~

45 will represent the symmetrical r.m.s, short-circuit current (or symmetrical breaking current of an interrupting device) at the instants O1 and O2 respectively, and

I(

,;co) '

J +/go

(58 completed cycles) 1.16 sec. 39.6 kA(rms)

uR 47.2 kA(rms)

Uy

First major peak 110.6 kA (peak)

UB

Figure 14.40scillograms of an actual short-circuit test carried out on a power distribution panel (Courtesy: ECS)

Testing of metal-enclosedswitchgear assemblies 14/431

Ca h

~ / I ~z,,///

Momentarypeak or makingcurrent(/M),Table 13.11

[. / l / ~

011/~ 1

\

Asymmetricpeak al currentenvelopes Asymmetricalaveragefault current, /av-

, ~ 7

/

1

~

c~176

#

A1D1 1

,

i

5 ;'i/i

7 i 8ill 7 / 10'A~~ F~~~_..~

D ~

i

I D3 /

-

E

Sub-transientstate

--

Transient state

Duration of fault

-J

J"

Steady state

Note The curves C1 C2 and C3 C4 are considered symmetrical after the first few cycles say 4 to 5. For better clarity, these 4 cycles are redrawn in an enlarged form in Figure 14.6 Figure

14.5 Determiningthe average r.m.s, value of the short-timecurrent/scfrom the oscillogramobtained during a short-circuittest

will represent the asymmetrical r.m.s, short-circuit current (or the asymmetrical breaking current of an interrupting device) at the instants O1.

sustain the asymmetry for the period up to its opening. It may also be referred to as the interrupting duty of the device.

Notes

Making current

1 The d.c. component,/de, at any instant should be a minimum of 50% that of the corresponding peak value of the a.c. component of the symmetrical fault current Iaco, Iacl, ..., Iacl0, i.e. at any instant, during the period of the short-circuit condition, Idc should be > 0.5 9 x/2 9Iac. Otherwise the asymmetry may be ignored, being insignificant. 2 The peak value of the asymmetrical fault current determined for the first maximum peak may be considered as the momentary peak value of the fault current IM. 3 The oscillogram also reveals the following vital information for an interrupting device, if used in the circuit, to make or break on fault;

Symmetrical breaking current This is the steady-state symmetrical fault current, which the faulty circuit may almost achieve in about three or four cycles from c o m m e n c e m e n t of the short-circuit c o n d i t i o n at point D1 (Figure 14.5) and w h i c h the interrupting device should be able to break successfully.

This is the same as the m o m e n t a r y peak value of the fault c u r r e n t I M and defines the c a p a b i l i t y of the interrupting device to make on fault.

Short-circuit generator (Figure 14.3) Since this equipment is short-circuited repeatedly it is specially braced to withstand repeated voltage transients and is m o u n t e d on a resilient base to m i n i m i z e the mechanical shocks transmitted to the base. If the generator is motor driven it is disconnected just before creating the short-circuit condition, otherwise the generator may have to feed the motor and be subject to stress. The stator has low reactance to give m a x i m u m short-circuit output and has two windings per phase as illustrated in Figure 14.7. These are arranged so that they can be connected in series or parallel, in star or delta etc., to provide four basic three-phase voltage systems as follows:

R.M.S. asymmetrical breaking current This is the r.m.s, value of the asymmetrical fault current I0, 11. . . . . 110 that the faulty circuit may generate, i.e.

lav -" ~/Ia2c +1% It will determine the ability of the interrupting device to

1 2 3 4

Stator connections

Nominal voltage

Parallel delta Parallel star Series delta Series star

6.35 kV x 6.35 = 11.0 kV 12.7 kV x 12.7 = 22 kV

14/432 Industrial Power Engineering and Applications Handbook

Testing of the main circuits (with short-circuit protective devices)

c~

~

I/ll

~ component aco'"

B1

-r

A1

.....

A

B2

2

1 . . . . . . . . .

3

~ Time Sub-transient state (period of asymmetry)

I-

Figure 14.6

R

-i

Illustration of asymmetry during a short-circuit

Y

R I3.veY

(ii) Parallel star

(i) Parallel delta

R

Y

B

B

Y

B

<

I (iii) Series delta Figure 14.7

(iv) Series star

Arrangements of windings in a test generator

9 For a switching device (which has not been previously tested for a short-circuit test). This should be closed and held in the normal service position. The test voltage (that would generate the required level of fault current) may be applied on one set of terminals, the other terminals being shorted. The test may be continued until the short-circuit device operates to clear the fault, but in no case for less than 10 cycles. In LT assemblies the point where the short-circuit is created should be 2 + 0.4 m from the nearest point of supply. 9 For a switching device having no protection (e.g. an Isolator). The required test current may be applied for the necessary duration (1 or 3 seconds) and the dynamic and thermal strengths should be verified. 9 For the main busbars - In LT assemblies, when the test is conducted on busbars, the length of busbars should be minimum 2 m. If it is less than this, short-circuit may be created at the ends of the busbars. - If the busbars consist of more than one section in cross-section, or different distances between the supports or the busbars, the test may be conducted separately on each section. 9 Test results - A successful test should reveal no undue deformation. Slight deformation of busbars is acceptable provided that the clearance and the creepage distances, as given in Tables 28.4 and 28.5, are maintained. The insulation of the conductors and the mounting supports should show no sign of deterioration. The degree of protection will not be impaired. - For withdrawable parts, such as a draw-out breaker or a draw-out chassis, proper movement of the movable parts and making of the contacts should be ensured. To verify this requirement, the chassis may be moved in and out for at least 50 times. - Clearance and creepage distances must be maintained in the service, test and isolated positions. 14.3.7 Verification of m o m e n t a r y p e a k or dynamic current

This test is carried out to verify the mechanical fitness of the buses, their interconnections, other current-carrying parts and the mounting structure to withstand the electrodynamic forces developed during a fault. It is measured by the first major peak (IM) of the oscillogram as discussed above and is obtained during the course of the short-time rating test (Section 14.3.6). The value obtained will not be less than those specified in Table 13.11. For more details see Section. 13.4.1(8). The test procedure and the test current are generally the same as for the short-time rating test, except that when the test is being conducted exclusively to determine the momentary peak current, the duration must not be less than 0.3 second, i.e. 15 cycles for a 50 Hz system as in IEC 60694. Referring to the oscillogram of Figure 14.5, the momentary peak

Testing of metal-enclosed switchgear assemblies 14/433

value of the fault current of the first major loop, after commencement of the fault condition at instant D~, is indicated as IM. Referring to the original oscillogram, (Figure 14.4), it occurs in phase Y.

2 Test current /R = 39.6 kA /y = 47.2 kA /a = 41.2 kA Highest of the above exists in phase Y at 47.2 kA 99 equivalent current rating of the equipment under test for

Example 14.1 For more clarity we have reproduced in Figure 14.3 an actual test circuit and in Figure 14.4, the oscillograms of the test results of a short-circuit test successfully carried out on an LT power distribution panel (Figure 14.8) for a system fault level of 50 kA for 1 second, at CPRI (Central Power Research Institute). From a study of these oscillograms (Figure 14.4), we can infer the following test results: 1 No. of completed test cycles = 58 58 9 duration of test for a 50 Hz system = 5-O = 1.16 second

1 second = 47.2 9 ~j-l.16

[sincel2.tl=12.t2,

"/2=/1.

t~

)

= 47.2 x 1.077 = 50.836 kA 3 The maximum peak current also appears in phase Yand measures at 110.6 kA at the first loop of the current wave. This loop is 110.6/50, i.e. 2.21 times the test current and satisfies the requirement of Table 13.11.

Analysis of the test results As discussed above we establish two basic parameters from a short-time withstand test, i.e. ,

,,

"

.,++t

+

,~,,,

2'+,?,i,

+! I

~ . . .......... H . : ,:, ~ ,

Allit

. ~kt+ t ~ y : : +,~ ; + ,

Ab'TERTE,ST

4-:

+

+

t

1 Thermal capability of the equipment under test, i.e. Isc and its duration 2 Mechanical compatibility through the peak making current, IM, as in Table 13.11. Before creating a fault condition, to obtain the required Isc the impedance of the test circuit is adjusted so that the required fault current is obtained in all the phases on creating a short-circuit. To provide the required thermal effect (Is2c 9t), the duration of test, t, is then adjusted accor-dingly. The relevant standards therefore stipulate that the test current may be higher or lower than required and can be compensated by adjusting its duration, t. However, due to the minor variations in the phase impedances, all the phases may not be subjected to identical severity of faults. For instance, in the above test each phase has recorded a different fault current. To evaluate the fault level from these test data, the general practice has been to consider the phase that has recorded the highest fault current as the base, which may occur in any of the phases. In the above test, it has occurred in phase Y. For this fault current, the test duration is adjusted to achieve the required severity of fault in terms of thermal effect (502 x 1 in the above case). Some users/consultants, however, are of the opinion that by this method the other phases are not subjected to the same severity. Accordingly, they prefer to consider the phase that is subjected to the least fault current as the base. Accordingly, the test durationshould be adjusted. for this phase. In the above case, the minimum severity has occurred in phase R, with only 39.6 kA. According to this philosophy the test duration should be enhanced to 502

xl

(39.6) 2

or 1.59 s

I

Figure 14.8 test at CPRI

A motor control centre (MCC) after short-circuit

as against 1.16 s in the above test9 Even then it is essential, that the peak making current, lu, of the required magnitude is achieved during the test. This is one parameter that cannot be established by

14/434 Industrial Power Engineering and Applications Handbook

hypothesis. It is therefore imperative that the minimum peak current according to the multiplying factor, shown in Table 13.11, is obtained during the course of the test itself, for example a minimum of 2.1 x 50, i.e. 105 kA in the above test. The multiplying factor will correspond to the specified fault current, (50 kA) and not the test current, e.g. 47 kA in the above test. If during the course of the test, the peak making current is less than this, the test will be considered invalid.

14.3.8 Verification of the protective circuits All protective circuits must be checked for continuity and the operational and sequential requirements, if any, in addition to the following: 9 Checking for the grounding of instrument transformers by means of a low-voltage source (10 V or so) using a bell, buzzer or a light. 9 Control wiring insulation test, as in Table 14.3. 9 Polarity test: to check the connections through the potential transformer to ensure that the connections between the transformer and the meters or relays have a correct relative polarity. Otherwise the meters would show erratic readings, while the relays would transmit wrong signals. This test may also be conducted with a low-voltage source of 10 V by observing the deflection of the instruments.

14.3.9 Verification of clearance and creepage distances The clearances may be verified as in Table 28.4, whereas for creepages Table 28.5 may be followed.

than 3 m from all sides through a square shaped nozzle of a capacity of 30 e/min +10% at a pressure of 46 N/cm 2 +10% and a spray angle of 60-80 ~ (See Figure 14.9). The rate at which the water is impinged on the surface under test should be almost 5 ram/minute. Standard nozzle designs are also available which can ensure the desired quantity of water at a pressure of 46 N/cm 2 + 10%. Refer to IEC 60298. For a uniform spray on the entire test surface more nozzles may be employed. Normally, surfaces of up to a width of 3 m may be tested at a time. For larger widths the test may be conducted in two steps. Normally, only one vertical surface is tested at a time. Besides the vertical sections, the test will also be conducted on" 9 The roof surface, from nozzles located at a suitable height 9 The floor outside the enclosure for a distance up to 1 m in front of the switchgear assembly, the assembly being in its normal position. Figure 14.9 illustrates the above requirements.

Test results The test may be considered successful if 9 No water droplets can be observed on the insulation of the main and auxiliary circuits 9 There are no water droplets on the electrical components or mechanism of the equipment 9 There is no significant accumulation of water on any part of the structure or other non-insulating parts. This requirement is to minimize corrosion.

14.3.10 Verification of degree of protection Enclosure test

60~

I ...- ---- "" ~//~,r

The types and degrees of enclosure protection are generally the same as defined for motors in Section 1.15, Tables 1.10 and 1.11. The testing requirements and methods of carrying out such tests are also almost the same as for motors, and as discussed in Section 11.5.3.

i I

Weatherproof test This test is applicable to all outdoor metal-enclosed switchgear and controlgear assemblies, as in IEC 60298, IEC 60694 and ANSI C-37/20C. The enclosure to be tested should be complete in all respects including its mounts, bushings (for HT switchgear assemblies, 1 kV and above) and wiring. One or more vertical units can be tested simultaneously as may be convenient, but not more than 3 m panel width can be tested at a time. For a multiple unit switchboard, however, at least two vertical units should be tested together to check the joints between the units.

Procedure All surfaces of the enclosure must be tested uniformly for 5 minutes each. The water will be impinged on the surface of the enclosure from a distance of not more

~

//

I

i / //

I 11 i// ,"

i" C~z/,/i i / /,~1 i I / /i II .Z/ ---7("SL~ ~ ~ / _ - "/I...r l//IZ"

/

A

/ I//,/// 4 P//

~

'

/

I

Enclosure %/i/'3//ii

Nozzles

,// /r" I/II " / / / ,4///

Platform \]

! I/ -

GL

~

"

'

~

i I

[[

D

c

A

-2.0m

B

-- 1.0m

C

= 2.5 to 3.0 m

D

Minimum height above floor level

Figure 14.9

Arrangement for weatherproofing test

Testing of metal-enclosed switchgear assemblies 14/435

14.3.11 Verification of mechanical operation (LT and HT) This test is conducted to establish the satisfactory functioning of mechanical parts, such as switching devices and their interlocks, shutter assembly, draw-out mechanisms and interchangeability between identical drawout modules. A brief procedure to test these features is as follows.

Switching devices These should be operated 50 times and removable parts inserted and withdrawn 25 times each.

Mechanical interlocks The interlocks should be set in the intended position to prevent operation of the switching device and insertion or withdrawal of the removable parts. Fifty attempts must be made to operate the switching device and removable parts inserted and withdrawn 25 times each. The test may be considered successful if the operations of the interrupting mechanism, the interlocks and other mechanical features after the test are found satisfactory.

Insulation resistance method The insulation resistance can be checked with the help of an appropriate megger (Mf2-meter), as recommended by IS 10118-3, and shown in Table 14.7. According to this, for an LT system, an insulation resistance of 1 Mr2 with a 1 kV megger for a completed switchboard, irrespective of the number of outgoing circuits, is considered to be safe. With this insulation resistance, the switchboard can be put to an HV test or actual use. The values of insulation resistances for higher system voltages and the recommended rating of megger are indicated in Table 14.7. IEC 60439-1 recommends the minimum insulation resistance for an LT system to be 1 kf~/V per circuit referred to the rated voltage to the ground9

Leakage current method The leakage current is measured as in IEC 60298. This method is generally applicable to an HT system by applying the full rated voltage between the insulating surface, say, between a phase and the ground. The leakage current thus measured should not exceed 0.5 mA. Illustration 1 Referring to Table 14.7, the recommended insulation value will ensure the following leakage current for an LT system:

14.4 Procedure for routine tests

System voltage - say, 415 V Insulation resistance = 1 Mf~ = 1 0 6 ~

The tests against step numbers 1, 2 and 3 in Section 14.2.2 are of a general nature and no test procedure is prescribed. The rest are similar to the tests covered under type tests. The procedures of tests and requirements of test results will remain the same as discussed earlier.

14.5 Procedure for field tests

9 Maximum leakage current = 415 A = 0.415 mA

If the system had been 660 V, this current would exceed 0.5 mA, which is permissible for an LT system9 2 For an 11 kV system, the recommended insulation resistance according to the same table = 100 Mf~ = 100 x 106f~ 9 Maximum leakage current =

The tests against step numbers 1, 2 and 3 are of general nature (see Section 14.2.4) and no test procedure is laid down for them.

4. Verification of insulation resistance or measurement of the leakage current The purpose of this test is to check for proper insulation of all the insulated live parts and components and to ensure protection of a human body against electrical shocks while the equipment is energized and is in operation. The test detects weak insulation, if any, and this must be rectified before putting the equipment into service. This test should be conducted both before and after the HV test if this test is to be carried out. The test before the HV test ensure the quality of insulation and the test after the HV test checks that this has not deteriorated after the HV test. If the insulation resistance is found to be lower each circuit and component must be checked separately to identify the weak area and corrective steps taken to improve the resistance to the required level. The test can be conducted in two ways.

11 x 10 3 A 100 x 108

= 0.11 mA and for 33 kV = 0.33 mA

Table 14.7 Insulation resistance for different voltage systems

System voltage

Minimum insulation Typeof megger resistance MI2 kV(d.c.)

1 Auxiliary and control 1 for one or more circuits (all secondary circuits wiring circuits) 2 For a completed 1 switchboard of up to 1000 V with a number of outgoing circuits

Manual, 0.5

3 Above 1000 V and up to and including 33 kV 4 Above 33 kV

100

Motorized, min. 2.5

1000

Motorized, min. 2.5

As in IS 10118-3

Manual, 1.0

14/436 Industrial Power Engineering and Applications Handbook The values thus shown in Table 14.7 take cognisance of the recommendations of IEC 60298 to maintain the leakage current at less than 0.5 mA for all HT systems.

The dielectric test may be limited to a power frequency voltage withstand test. This test is considered neither necessary nor advisable at site if already conducted at the manufacturer's works unless there is a major modification or repairs at site in the existing switchgear assembly. Repeated application of a high voltage may degrade the properties of the insulation system used. If such a test becomes necessary at site, it may be carried out at a reduced test voltage of 85% for LT systems as in IEC 60439-1 and at 80% of the test values for HT systems as prescribed in note 2 of Table 32.1 (a) as in IEC 60298, for series I voltage systems or at 75% as prescribed in note 2 of Table 14.2 for series II voltage systems, as in ANSI C 37.20C. When the required test voltage is not available at site, the reduced voltage power frequency withstand test may be carried out at still lower test voltages, depending upon the voltage availability at site. Then the duration of the test must be increased as shown in Table 14.8, and IS 10118-3 and BS 159. In this case also verification of insulation resistance or the measurement of leakage current will be carried out before and after the HV test. Table 14.8 Duration for dielectric test at reduced test voltages Rated test voltage % 100 83.5 75 70 66.6 60 57.7

Test duration in minutes 1 2 3 4 5 10 15

14.6 An introduction to earthquake engineering and testing methods The consequences of an earthquake on life and property have caused increasing concern among scientists, engineers and educational institutions. All are becoming more conscious about preventive measures for buildings and critical installations against possible earthquakes to mitigate, if not eliminate, their devastating effects. It is becoming common practice to conduct seismic studies on a region where a large and/or critical project is to be located before commencing work and locating the more important buildings in seismically safer areas. Buildings, other structures and important machines are designed to withstand earthquakes. Analytical methods and laboratory test facilities have also been developed to demonstrate the suitability of structures to withstand seismic events. Additional information on newer aspects on the behaviour of the earth during an earthquake has been

obtained on a regular basis. This has been possible through seismographs and accelerographs installed at the various strategic locations throughout the world (see Section 14.6.2). Availability of new information on aspects on the behaviour of the earth's body is making it possible to update and improve the above test methods and facilities. With such information, it is also hoped that our scientists may soon be able to predict an earthquake, its intensity and location well in advance. The likely 'responses' of buildings, structures and installations to such events are now available. These help us to study earthquakes more closely and their effects and enable us to take more authentic and appropriate preventive measures at the design stage. We offer an introduction to this subject with a view to make the students and the engineers more conversant with and aware of these geological phenomena and to be more concerned about safety for life and property. A few examples may be houses, buildings, hospitals, industrial plants, power generating and distributing systems, dams, bridges and handling of hazardous materials. These and similar systems should be given constructional and design considerations to make them reasonably safe against such effects. In further discussions we consider only the secondary systems that are supported on the primary system and consist mainly of the electrical and mechanical machines, devices and components. The primary systems, which include houses, buildings and main structures, (columns, beams, trusses, floors, walls), dams and bridges etc., fall within the purview of civil and structural engineering and are not discussed here. Our present discussions relate only to the laboratory testing of safety-related secondary systems, as are employed in critical areas such as areas of emergency power supply and reactor power control supply etc. of a nuclear power plant (NPP) according to IEEE 344 and IEC 60980. There are other codes also but IEEE 344 is referred to more commonly. Basically, all such codes are meant for an NPP but they can be applied to other critical applications or installations that are prone to earthquakes. It should be noted in subsequent discussion that the design of structures and foundations for machines and equipment play a vital role in absorbing or magnifying seismic effects. A proper design consideration in such areas at the initial stage can save the primary (and, in all probability the secondary) systems from such effects to a large extent. Testing may be essential only for the critical equipment, such as being used in the critical areas of an NPP or similar, more critical installations.

14.6.1 Seismic disturbances Random vibrations, such as those caused by an earthquake, cause shocks and ground movements and are termed seismic disturbances. Shocks and turbulence caused by a heavy sea, landslides and volcanic eruptions are also examples of shocks that may cause vibrations and result in tremors, not necessarily earthquakes. Nevertheless, they may require design considerations similar to those for an earthquake, depending upon the application (e.g. naval applications, hydro projects, dams and bridges),

Testing of metal-enclosed switchgear assemblies 14/437 Temperature ofthe

and their vulnerability to such effects. Our present discussions relate to shocks and vibrations caused by an earthquakes and laboratory testing of equipment mounted on primary systems against such effects, particularly those required for critical areas of an NPP.

Rigid plates (= 20 Nos.) floating on the mantle in the lithosphere

deepest parts ofthe crust can go up to 870~ Molten mantle (C) (sphere of hot rock and / metal)

Causes of seismic disturbances Scientists suggest different theories for the causes of an earthquake. One of many such theories is the Elastic Rebound Theory. This suggests that with the evolution of the earth, several tectonic processes have been taking place within it. These processes have caused severe deformations in the crust and have resulted in the formation of ocean basins and mountains. These impose elastic strains on the earth's crust. These strains build up with the passage of time, and eventually they overcome the resilience of the earth's crust and result in its rupture. The rebound of the ruptured crust causes an earthquake. Geologists and seismologists have explained this theory more comprehen-sively through the Plate Tectonic Theory which can be briefly explained as follows. The outer shell of the earth, consisting of the upper mantle and the crust (Figure 14.10), is formed of a number of rigid plates. These plates are 20 in number and are shown in Figure 14.11. Of these, six or seven are major plates, as can be seen in the map. The edges of these plates define their boundaries and the arrows indicate the direction of their movement. These plates contain the continents, oceans and mountains. They almost float on the partially molten rock and metal of the mantle. The outer shell, known as the lithosphere, is about 70 to 150 km thick. It has already moved great distances below the earth's surface, ever since the earth was formed and is believed to be in slow and continuous motion all the time. The plates slide on the molten mantle and move about 10 to 100 mm a year in the direction shown by the arrows. The movement of plates is believed to be the cause of continental drifts, the formation of ocean basins and mountains and also the consequent earthquakes and volcanic eruptions. The movement of these plates carries with it continents, ocean basins and mountains. Scientists believe that convection currents are generated as a result of great heat within the earth, as illustrated in Figure 14.10. Below the crust, the hot rocks and metal in liquid form rise to the crust, cool and sink into the mantle causing a turbulence through heat convection. The hot rocks become hardened at the surface of the mantle and push the crust which is part of the hug plates that are afloat the mantle. This movement of plates can cause the following: 1 When the plates move away from each other, the molten rock from the mantle fills the gaps between them to form ocean basins. 2 While moving away from one plate, they will be moving closer to another and may collide. One plate may pile up over the other and form mountains. 3 If the plates pull down, they would sink into the mantle and melt to form ocean basins. Some of the molten rock of these plates may travel to the earth's surface through the crevice so formed due to heat convection and cause a volcano.

Crust (E)

E

B _A

- - - l , , t i,,

yl~

A Vl~

B yl

~

i

C

E

~1 ''~~'~

- 6490 km = 12980 km

Section of the earth body

Name of the

section

Approx. thickness

Temperature Outer part Inner part

A

Inner core

1300 km

----upto

B

Outer core

2250km

up to 2200~

up to 5000~

C

Molten core

2900 km

up to 870~

up to 2200~

D

Lithosphere

70 km to 150 km

E

Crust

Figure 14.10

,-----

5000~

----

870oc

8 km (under . , ~ Below the crust --,. up to 870~ the oceans) to 40 km (under the continents) Construction of the earth

4 When the plates slide past each other, they may cause stresses at the edges of the crust. The stresses may build up and at some stage exceed the resilience of the earth's crust and cause a fault, i.e., cause the crust to rupture and shift. When this occurs, it causes an earthquake in the form of violent motion of the earth's surface and/or large sea waves. Major earthquakes occur because of this phenomenon.

Magnitude and quantum of energy released The magnitude of shocks and vibrations caused by an earthquake is the measure of energy released (E) at the focal point in the form of seismic waves. It is measured on a Richter scale. An American seismologist called

14/438 Industrial Power Engineering and Applications Handbook Eurasian plate .... io, . . - ~ " ' " ~J/7"-~ ~7, (..NpP.:....~", ~ . ~ Ae.geanTurkish~ P.hi!ipp~ne/// p ? a ~ ~ a t e ~)e.NtoorthnA~e~r -7 '-,,~ ~latelr~platelraai~,~ \ p,ate/~..~.~ "~ ........ '-..... ~ 7 c~?Caribb, ~.._.___: ~ ~ ~ ~ ~ ~ / ~

~, '~~-~m rica l--")

~

c

t

i

c

plate

14.11 Map showing tectonic plates and their boundaries. The arrows (Courtesy: World Book Encyclopaedia)

M = log A where M = magnitude of the earthquake A = maximum amplitude, as recorded by the Wood Anderson seismograph in microns at a distance of 100 km from the epicentre. Since the distance of the instrument from the epicentre will usually not be exactly 100 km, a distance correction must be applied to obtain the magnitude of the earthquake, defined as, M = log A - log Ao. Distance correction curves between epicentral distance of the seismograph versus log Ao (which are also sometimes referred to as attenuation curves) are used for this purpose. One standard attenuation curve is shown in Figure 14.12(a). This definition of the magnitude of earthquake is used for the records of Wood A n d e r s o n type torsion seismograph. This has a dampening equal to 80% of the critical natural, period of 0.8 second and a magnification of 2800. The value of M is determined from seismograph records at different locations and a mean value is obtained to define the magnitude of the earthquake. The minimum value of M which may cause appreciable damage is considered to be about 5. The extent of damage caused by a higher M will depend upon the depth of focus, the distance and the soil stratification. Generally, an earthquake can have a focus varying from 5 to 150 km from the earth's surface. It is generally seen that an earthquake of M = 5 may be felt up to a radius of 150 km and can cause substantial damage within a radius of up to 8 km while an earthquake of M = 7 may be felt up to 400 km and can cause damage up to a radius of 80 km. An M = 8 may be felt up to 800 km and can cause damage up to a radius of 250 km. At Koyna (India), for instance, an

( '~ate}

""

Figure

Charles Richter suggested that the magnitude of an earthquake can be expressed by

) plate)r../j

indicate the direction of their movements

earthquake having M = 6.5 was felt up to a radius of 400 km and caused destruction up to 60 km or so*. The energy thus released is considerable and can be gauged by its magnitude as shown in Table 14.9. To obtain an idea of the energy that may be released and the destruction that it can cause, one may compare it with the energy of 8 x 1020 ergs released during the atomic explosion at Hiroshima, Japan, in 1945. This is equivalent to an earthquake of M = 6.33. The extent of destruction may be equivalent to an explosion of 10 such bombs if M is 7.0 and many times more at yet higher magnitudes.

5

?=

E4 ,< ~3 .0

82

]

10

100

1000

Distance in km

Amplituderecorded by Wood Anderson Seismograph 'A' is in mm. Figure 14.12(a) Distance correction curve for determining the magnitude of an earthquake Note

*The recent earthquake (2001) with its epicentre at Bhuj (India) was felt upto > 1000 km and caused destruction upto > 400 km. It was measured as M = 8.1 and lasted for about 45-50 seconds.

Testing of metal-enclosed switchgear assemblies 14/439 Table 14.9 Likely energy released at different magnitudes of an earthquake

M E(1020 ergs a)

5.0 0.08

6.0 2.5

6.5 14.1

7.0 80

7.5 446

8.0 2500

aAn erg is the unit of work in the cgs system and is equal to the energ~r required by a force of 1 dyne to move an object 1 cm (1 erg = 10-"joule).

The intensity of an earthquake is a subjective assessment of its effects on the primary systems and inhabitants in surrounding areas and is measured on the Mercalli scale. As noted above, this decreases with distance from the epicentre while the magnitude remains the same. For details refer to DD ENV 1998. Generally, the magnitude and intensity of an earthquake at a location are interrelated. The energy so released propagates in the form of waves and travels through the stratification of the earth's crust in all directions, longitudinal (X axis), transverse (Y axis) and vertical (Z axis) at the same instant, with varying magnitudes subjecting to vibrations whatever stands in its way on the earth's surface, such as buildings and trees etc. These waves are recorded in the form of irregular broad bands, i.e., multi-frequency waveforms, composed of many sine waves of different frequencies, similar to a harmonic waveform, discussed in Section 23.5.2 and as shown in Figure 23.7. It is known as the time history of an earthquake. The normal practice to describe the motion of an earthquake for a particular location, based on seismic studies and data obtained for and from nearby areas, is in the form of a response spectrum (RS), as discussed later.

Duration of an earthquake An earthquake may last for 4-6 seconds only for M = 5.5 or less and for over 40 seconds for M > 7.5. The greater the magnitude, longer will be the duration. An earthquake of M > 6, for instance, may last for 15-30 seconds and produce a maximum horizontal ground acceleration of the order of 0.1 g to 0.6 g (98 cm/s 2 to 590 cm/s 2) and higher, inflicting maximum damage in the first 5-10 seconds only, and a frequency band between 1 and 33 Hz (IEEE 344). Figure 14.12(b) represents an actual time history (accelerogram) of the earthquake that occurred in Chamoli, India, on 29 March 1999. It had apeak ground acceleration of nearly 0.15 g and a predominant frequency of about 2 Hz. It is also accepted that after such an event, the ruptured earth surfaces may try to settle down again. It is possible that during the course of such a realignment there may still remain pockets of energy between the two plates until they finally settle. These may develop into releases of stresses once again, leading to occasional tremors or earthquakes even for several days after a major earthquake or volcanic eruption. The earthquakes in Turkey are examples where two equally devastating earthquakes occurred between September and November 1999.

14.6.2 Recording an earthquake This is carried out with the help of an instrument known

as a seismograph which amplifies and records small movements of the earth's surface and helps to identify the epicentre and focal depth and determines the magnitude of an earthquake. More than a thousand earthquakes with a magnitude of at least 2 (corresponding ground acceleration, < 0.01 g to 0.02 g) occur daily. But earthquakes less than M = 5 are considered minor, as they are generally harmless. Earthquakes of the same magnitude may cause varying amounts of damage at different locations, depending upon the soil stratification and design considerations of the primary systems. Conventional seismograph are not suitable for recording major ground movements and go off-scale (and sometimes are even damaged) when severe earthquakes occur in their vicinity. For recording significant ground movements, strong motion accelerographs are used. Records of accelerographs are called accelerograms. They are basic requirements for seismic analysis, to design earthquake-resistant structures and buildings and for other engineering applications. Large numbers of accelerograph stations have been established in vulnerable locations throughout the world to record seismic waves for further research and to take preventive measures by improving design practices. Geologists and scientists make use of these data to determine the magnitude of an earthquake and analyse the source mechanism to further their quest to predict an earthquake more accurately, while design engineers use them for developing earthquake-resistant systems, structures and equipment etc. So far these records have proved insufficient to provide required forecasts about an earthquake's location, time of occurrence and magnitude. Nevertheless, with continued efforts in this direction, it is hoped that one day it will be possible to predict an earthquake more accurately.

Response spectrum (RS) A response spectrum (RS) is analytically determined by calculating the peak response (also called the spectral response) of a linear single degree of freedom system with different natural periods and dampening for a given acceleration time history of ground movements recorded during an earthquake. It forms a part of seismic studies carried out for a particular area, and provides information about far responses of different types of structures during an earthquake. It takes cognisance of the fact that an earthquake can be expressed indirectly in the form of a response spectrum. This spectrum is the peak response of a linear single degree of freedom system on the occurrence of an earthquake, as a function of its natural frequency (periods) for different dampening. They are in the shape of a curve, natural frequency or natural period versus peak amplitude of vibrations of the system, as illustrated in Figure 14.13. They have a broad band as noted later and can be expressed in any of the following forms for different dampening: 1 Ground displacement response spectrum in terms of spectral displacement versus natural period of oscillations (Figure 14.14). 2 Ground velocity response spectrum in terms of spectral velocity versus natural period of oscillations (Figure 14.15).

14/440 Industrial Power Engineering and Applications Handbook 200 r

O ~0

0.15g ( 153.73 cm/sec 2) -200 0

,',

5 Time in seconds

Peak ground acceleration -~ 0.15 g Predominant frequency* --- 2 Hz

.~

*This can be determined by drawing a Fourier spectrum, which would identify the fundamental frequencies that build up the multi-frequency spectrum. (a) Acceleration

10

r G)

0

0

W"

_o

-10

o

40 Time in seconds

(b) Velocity 10

,..-..,

E G)

E

0

0

a

-10

I

I

I

I

5

10

15

20

I

25

Time in seconds

(c) Displacement Figure

14.12(b)

Time history of earthquake at Chamoli, India, which occurred in March 1999

3 Ground acceleration response spectrum in terms of spectral acceleration, g, versus natural period of oscillations (Figure 14.16). These spectra represent the nature of peak displacement/ velocity/forces and their magnitudes that may generate in a vibrating system of different damping levels and periods on the occurrence of an earthquake. They form

the basis of equipment design and their seismic testing and are provided by the user to the equipment manufacturer. In the above instance, the response spectra shown in Figures 14.14-14.16 are for 5% damping. The damping levels described here are generally in the range 1-10% and frequency in the range of 1-33 Hz for systems at ground level and 0.5-10 Hz for floors above the ground

Testing bf metal-enclosed switchgear assemblies 14/441

1.80

1.50

1.20 Damping %

0.90

3

0.60

0.30

0.00

,

0.00

Figure 14.13

i

'

0.20

,

0.40

'

I

0.60

'

I

~

I

'

35 EL Centro - Sd (USA)

25 20

I

'

I

1.60

'

I

1.80

'

I

2.00

'

I

2.20

'

I

'

2.40

I

2.60

a mathematical model for analytical assessment or deciding the level of ground movement and frequency of excitation etc. To assist those in the field, the International Association of Earthquake Engineering (IAEE) has prepared a world list, coding the various countries on the following bases:

Damping - 5%

-Sd a)

Chamoli 9

.,..,

'

Typical broad band floor response spectra showing the floor acceleration at different damping levels

40

30

I

0.80 1 . 0 0 1 . 2 0 1 . 4 0 Period in seconds (7)

E ~15

to

9 9 9 9 9

Seismic zones Seismic coefficient Acceleration of ground motion (~) Velocity of ground motion (j), and Displacement of ground (x).

The above factors must be taken into consideration while constructing an RS.

lO

Floor response spectrum (FRS)

0.4

0.8

1.2

Period in seconds

1.6

2.0

2.4

2.8

(T)

Figure 14.14 Displacement response spectra of the Koyna (India), Chamoli (India) and EL Centro (USA) earthquakes

level, due to filtration, discussed later. It is, however, observed that most of the vibrating bodies fall in a frequency range of 2-15 Hz. Any of the three RS is adequate to derive a time history of an earthquake to simulate test conditions in a laboratory. This, however, being a complex subject, assistance must be obtained from experts in the field for constructing an RS for laboratory testing, preparing

Some equipment will be on the ground, and some on floors above the ground in which case floor movement must be considered rather than ground movement. Floor movement is different from ground movement because of structural behaviour, characteristics and filtration of ground frequencies. Filtration of ground frequencies may lead to resonance and quasi-resonance conditions and magnify the floor movements, compared to ground movements.

Required response spectrum (RRS) This is the response spectrum, constructed for a particular location, for a future earthquake. It is based on seismic studies conducted for that region and past seismic records of and around that region, if available. It forms the basic parameters for the design and testing of an object.

14/442 Industrial Power Engineering and Applications Handbook lO0 Damping - 5% EL Centro - Sv

80

Chamoli

r

60

~o f

Koyna - Sv

40

20

0.4

0.8

1.2

1.6

2.0

2.4

3.0

Period in seconds (7")

Figure 14.15

9

9

Velocity response spectra of the Koyna (India), Chamoli (india) and EL Centro (USA) earthquakes

peaks which, in fact, represent the resonance or quasiresonance conditions. The RS may drop sharply immediately before or after such peaks. It is possible that during an earthquake, the corresponding peaks may occur shortly before or after the peaks considered

The RRS is defined for different levels of damping, as shown in Figure 14.13. The RRS that is more appropriate for the test object is chosen. The damping level will be assessed as noted earlier, otherwise 5% d a m p i n g m a y be considered. Artificial broadening of the spectral accelerations at the peaks of RRS: If we refer to an RS such as that s h o w n in Figure 14.16 we will notice a few

in the RS as it m a y not be possible to estimate the frequency of the structure so accurately, at the time of constructing the RS. Since the test conditions

1.4 1.80

Damping - 5%

r-

" ~1

1,2-

ZPA-"

1.50

1.0O

1.20

0.90 .r

0.60

0.30

0.2 .......

' 014 ' 0'.8 ' 1'.2 ' 1'.6 ' 2'.0 ' 2'.4 ' 2'.8 Period in seconds (7")

Figure 14.16 Acceleration response spectra of the Chamoli (India), Koyna (India), and EL Centro (USA) earthquakes

0.00

'

0.00

I

'

I

'

I

'

I

'

I

'

I

0 . 2 0 0 . 4 0 0 . 6 0 0 . 8 0 1 . 0 0 1 . 2 0 1.40 Period in seconds (T)

Figure 14.17

Broadening of the spectral peaks of RRS

Testing of metal-enclosed switchgear assemblies 14/443

on the ground. However, for equipment mounted on the floors of buildings, the floor spectra are determined. To do this from ground movements the building/structure is analysed and the response time history at various floors is determined. From these time histories the FRS for different floors is established. The floor response spectrum (FRS) so obtained is used as the required response spectrum (RRS) for floor-mounted equipment (secondary systems). The test conditions are developed to simulate floor spectra. They must also be regarded as the basis of the design response spectra for all critical equipment and devices. The following are the main parameters that must be considered to arrive at the most appropriate response spectra:

Towards rigidity --~-~---~ Towards flexibility (___15 Hz) (< 15 Hz)

ZPA =

0

*

0.03 sec

1 sec

ZPA Zone I Periodic (sec) t 33 Hz = ZPA = (Zero period acceleration)

1 f

1 Hz f, frequency (Hz)

Peak ground or floor acceleration as recorded by the time history. The rest is a response record.

Figure 14.18

A ground or floor response spectrum

will only trace back the RRS, it is possible that the object is not sufficiently loaded for such periods and may fail during an earthquake while the test may not be able to detect it and the object may successfully withstand the test. To overcome such an uncertainty, it is normal practice to artificially broaden the spectral acceleration in the peak regions of the RRS by + 15% T or so (T being periods of peaks). The broadened spectral peaks are illustrated in Figure 14.17.

Zero period acceleration (ZPA) The maximum ground or floor acceleration, as a result of an earthquake, can be obtained from a given RRS. It corresponds to acceleration at high frequency, i.e. more than 33 Hz. This is illustrated in Figure 14.18, and represents the peak ground or floor acceleration of a time history of an earthquake, from which the RRS is developed. During a test, the peak acceleration of the shake table motion (ZPA) should be at least 10% greater than the ZPA of RRS, according to IEEE 344, to account for any likely severity in the event of an earthquake.

14.6.3 Constructing the RRS Seismic analysis is carried out for all important engineering structures such as dams, bridges and nuclear power plants. For regions where these are to be located the likely expectations of an earthquake as well as the extent of its magnitude must be assessed on the basis of the seismic history and the earthquake records of the region (Figures 14.12 to Figure 14.16). Based on these and other factors such as soil stratification, site dependent response spectra are determined. These are the RRS for equipment mounted

1 Magnitude of the earthquake, hypocentral distance and soil stratification. 2 Based on above peak value of ground acceleration, duration and frequency range. An RRS is normally constructed for several levels of critical dampings as illustrated in Figure 14.13. The most appropriate of these is then chosen for the purpose of testing. Any of the above response spectra can be developed into a time history of the earthquake, similar to that in Figure 14.12(b).

Hypocentre This defines the focus, i.e. the point of source within the earth's body, from where the stored energy is released. It causes an earthquake and travels outwards in the form of seismic waves to the earth's surface.

Epicentre This identifies the part of the earth's surface directly above the hypocentre and where it produces the most severe ground movements. Away from the epicentre, the acceleration and the intensity of ground movements diminish.

Soil stratification (rocky, alluvial or sedimented etc.) From their focal point to the earth's surface seismic waves travel through the earth's crust and the soil. The stratification of soil, i.e. the earth's layers above the crust, plays an important role, as the intensity and frequencies of an earthquake, as felt on the earth's surface, will depend upon the type of soil strata. It is observed that as a result of damping of soil, the soil may absorb some or most of the energy produced during an earthquake, depending upon the thickness and type of strata. Hence this may help to diminish, to a great extent, ground vibrations, i.e. ground acceleration, velocity and displacement. Further studies on the subject have revealed the following: 9 Bedrock Ground displacement in bedrock is less and hence there is no or only a small settlement of a structure

14/444 Industrial Power Engineering and Applications Handbook

built on this rock. But since the seismic forces act directly on the structure, there is no damping of these forces or filtration of frequencies. The structure resting on such rock therefore should be adequate to absorb and sustain all the energy of an earthquake. Rock, however, forms a solid part of the earth's crust and provides a stable foundation for a building or a structure. It is least affected during a seismic event, as there is very little settlement. But in many places, the rock may be deep below the earth's surface and it may not be practical or economical to build the foundations on such rock. The universal practice, generally, is to rest the foundations on shallow soil layers only (Figure 14.19). 9 Small or moderate thickness of soil Where there is some soil, ground displacement will be greater and seismic waves will pass through the soil. There may be some settlement of the structure due to soil compaction. While the structure will now be less subject to seismic forces, this may prove to be a worse case, as in addition to the structure being subject to almost the full intensity of the earthquake, there may also be settlement of the soil, which may result in settlement of the structure and cause it to collapse or develop cracks.

Building/ structure ,,~,

/

'

[:3

GL .

D

[3

IS3

E3

[3

O

C3

C3

C3

C)

.

D

N .

Alluvial 7 il

9 Reasonable depth of soil When the soil is deeper there may be considerable settlement before seismic waves reach a structure. This soil consolidation may cause a substantial differential settlement of the structure and damage it. Although the intensity of the shock and ground movements will now be less damage may be severe as a result of settlement rather than the intensity of the earthquake, as most of the energy will be absorbed by the soil. At an increasing distance of the structure or object from the focal point of the earthquake, ground movements will diminish. 9 Greater depth of soil when there is a deep layer of soil, the intensity of the earthquake will reduce. The greater the distance from the focal point, the smaller will be ground movements. In such cases it is seen that the settlement of the soil below the structure may be negligible as it would have already settled by the time the shock reached the surface, and hence damage to the structure would be reduced. Soil does not provide as solid a base as rock. The strength of a foundation built on soil and its ability to withstand an earthquake will therefore depend upon the quality and depth of soils which may be formed of a number of soil layers of different stratifications and depths. Sandy soil or soil with sedimentary deposits, for instance, will have less strength and will provide a weaker base, as such soils may settle more during a ground movement.

14.6.4 Theory of testing a system for seismic effects

Foundation Depthof foundation

.

i i:i

*Alluvial soil formedof a numberof layers of non-uniform non-homogeneous soil of different stratifications

Figure 14.19 A typical stratification of soil

A study of seismic effects on a structure, equipment or device will reveal its worthiness to withstand an earthquake without appreciable damage and perform satisfactorily during and after sudden shocks and vibrations. It is possible to study their performance through prescribed seismic withstand tests. Where a test is not possible, due to the size and/or weight of the object, performance can be assessed through mathematical analysis. Seismic testing is a complex subject. To provide the full details here is neither possible nor the purpose of this text. We have covered this subject only broadly to provide an introduction to the applicability of earthquake engineering to more constructive use structures, particularly to take safety measures in the initial stages when commencing a new project. For those in this field and who are seeking more details/clarifications on the subject, references have been provided at the end of this chapter. Whatever minimum information is considered necessary to familiarize an engineer with this subject are provided below. National and international specifications on rotating machines, switchgears and switchgear and controlgear assemblies and bus systems as discussed in Chapters 11, 14 and 32, respectively, do not normally require such tests. They become vital when such equipment is installed in a nuclear power plant and where, by virtue of its failure Or malfunctioning during or after such a disturbance, they may cause a process destabilization. Such a destabilization may jeopardize the safety and integrity of the main plant, and result in an accident or radioactive radiation beyond critical limits. The radiation may cause a catastrophe to

Testing of metal-enclosed switchgear assemblies 14/445

the inhabitants in the vicinity. Such tests are advisable even for machines, devices and components that are to be installed in other critical areas, such as a refinery, a petrochemical project, handling and filling areas of inflammable liquid, gas or vapour where also as a result of failure of such machines system process or control may be jeopardized and cause serious accidents, and resulting in heavy loss of life and property. The seismic worthiness relate more to the primary system than to the secondary. For a secondary system, it applies only to safety-related equipment or devices installed in critical areas as noted above. The suitability of primary systems is verified through analytical means only, as laboratory test for such systems are not practicable. Hydro projects, dams, bridges, naval equipment and any installations that are prone to continuous shocks and vibrations also require their primary and secondary systems to have a better design and operational ability to withstand seismic effects or other ground/surface vibrations. No specific tests are presently prescribed for such applications. But response spectra can be established even for such locations and the primary and secondary systems analysed mathematically or laboratory tested. We define below some common terms in earthquake engineering to clarify test requirements and methods: 9 Ground acceleration This is the time history of ground acceleration as a result of an earthquake, where multiple frequency excitation predominates (Figure 14.12(b). A ground response spectrum (GRS) can be derived from this history. 9 Floor acceleration This is the time history of acceleration of a particular floor or structure caused by a given ground acceleration (Figure 14.16). It may have an amplified narrow band spectrum due to structural filtration, where single frequency excitation and resonance may predominate, depending upon the dynamic characteristics of the structure. A floor response spectrum (FRS), as shown in Figure 14.18, can be derived from this history. Consideration of GRS or FRS will depend upon the location of the object under test. 9 Broad band This means multiple frequencies of ground movements. During an earthquake these assume multi-frequency characteristics, which are represented by broad band response spectra (Figure 14.13). When, however, such a response is transmitted to secondary systems and objects mounted on floors, it becomes a narrow band, due to floor and structural filtration, and the amplitude of vibration is magnified. The magnification will depend upon the natural frequency and damping of the secondary systems and the objects. As normal practice, all systems and objects, mounted on the ground or a floor, must undergo multi-frequency tests. The shake table is excited to achieve a movement that represents a broad band waveform which will include all frequencies in the range of 1-33 Hz. 9 Natural frequency When an object is mounted in situ (as in normal operation) and given an initial external displacement or velocity in any direction and then released, the body will oscillate about its initial position in a sinusoidal waveform as illustrated in Figure 14.20.

The amplitudes of oscillations will depend upon weight, stiffness and configuration. The record of these oscillations is known as free vibration record. The rate of oscillations will determine the natural frequency of the object. Figure 14.20 shows one such free vibration record. Damping is the characteristic of a vibrating system which defines how fast the amplitudes of a freely vibrating system will decay. The greater the damping of a system, the faster the amplitude will decay and vice-versa. The magnification of vibrations of a system, as a result of ground movements, will depend upon its natural frequency and level of damping. Generally, all systems are flexible to some extent, except a few that may be completely rigid. A flexible system can be represented as shown in Figure 14.21, where 'resistance' represents the restoring force developed within the system, when applied with a force to displace it from its original axis X - X ' . It will try to regain its original shape and position and vibrate about its axis until it attenuates due to damping. Vibrations are caused as the system (which may be any object) returns to its original position and overshoots the original axis X - X ' to the other side. Thus vibrations of the system about its axis, commence until they attenuate. The 'dashpot' represents the resilient characteristic of the body that would try to damp the oscillations thus developed and attenuate it. The following mathematical expressions derive the properties of a system when excited by an external force F(t)" F(t ) =

rex" + cx' + kx

where F(t ) = force applied to the object as a function of time

x - displacement of the object x ' = velocity of the mass attained when affected by vibrations x'" - acceleration of the mass attained during the course of restoring force. m - mass of the object c - coefficient of viscous damping K - coefficient of restoring force (or stiffness of the foundation) This equation can also be rewritten as F~t~ m

-'X

,,

c

+~X m

,

k

+~'X m

If (on is the natural angular frequency of vibration of an object in rad/s, i.e. 2nf, fbeing the natural frequency of vibration in Hz, then (On = ~ k = 2 f f f 1 or f = - ~ -

~k m

Since the natural time for one full vibration (one cycle),

T= 1 f

14/446 Industrial Power Engineering and Applications Handbook

x0 = 1.000 0.800 xl = 0.600 Damping 0.400 l

0.200 0.000

/ I

xj

~

I -0.200

o

0 c~

~-

O O

v

O O

O O

~o

00

o

c~

....

~

c~

c~

O

o

,

o

O ,~"

d

v-

v-

-0.400

I/~d 0

~t

-0.600-

Time (seconds)

-0.800 -1.000" -~1-~ 0.208 Figure

14.20 A typical free vibration record (sine wave) illustrating natural frequency of vibration and level of damping of an object F(t)

Force

- - System

--X---t--- Object ( m ) - - - - F -x'

~~l~(

/ Restoring force ~ / (resistance) k ,~

~ Dampingforce (dashpot)

response

F(t)

sPec~kalesp;nSnee)

~---

Ground motion

~

Peak acceleration (ZPA)

/

Object | ams~ tedtfb/e

mx"

~

kx

cx"

(Inertia (Restoring (Damping force) force) force) Free body diagram of mass

Figure

14.21 A single degree of freedom system

9 T= 2zr ~ m "" k c

and q = 2~/km O is the fraction of the critical damping constant, then c = 2r/~ m

m

= 2r/~~--

=2/7.(.0

n

and F = x " + 2.77. COn "X' "]" 0) 2 "X m

This is an important equation that defines the behaviour of a vibrating body under different conditions of applied force or motion F(t ). From this it can be inferred that the response or movement of object 'x' will depend upon 77 and COn9q is termed the fraction of critical damping and COn, the angular natural frequency of the system. With the help of these equations, the response characteristics of an object to a force F(t ) can be determined.

D a m p i n g characteristics

An object can acquire the following four types of damping characteristics:

Testing of metal-enclosed switchgear assemblies 14/447

1 Undamped systems (Figure 14.22(a)) Where there is no damping force, such as friction or air resistance, the object will continue to oscillate about its initial position for ever (but this does not happen as it is natural). Now c=0

x0 = Xl = x2 etc. 2 Underdamped systems (Figure 14.22(b)) Most systems fall in this category. As a result of the restoring force, the object returns to its original position, overshoots its original axis and goes to the other side and hence the oscillations commence. For mathematical convenience, it is generally assumed that damping is viscous in nature, which means that the amplitude of a vibrating body will decay exponentially, i.e.

X1 X2

3 Critically damped systems (Figure 14.22(c)) The object may just reach its original position. By the time it does so, it loses all its restoring force due to damping and does not overshoot. Such systems do not oscillate. For critically damped systems 7/=1

.'. 7/= 0 and

X0 X1 and

For underdamped systems, 0 < U < 1

Resonance leading to negative damping When the natural frequency of a system, or object 09n,!COn = ~mk~)coincides with that of the ground or

X2 X3

log e x 1 : 2zcr/ - 2zoo (for q to be too small) x2 ~/1 - U2 1 x1 or q = ~-~ log e - X2

Original_i~'ol \ i /,~1\ T /i2/ p~176 .~ [ /x~$/ \ x ~ / \Xo= x6 = xl = q/ q/ \ etc. T/= 0 '

4 0 v e r d a m p e d systems (Figure 14.22(c)) The damping strength of the object is such that it may absorb most of its restoring force before it reaches its original position. There are no oscillations. For overdamped systems, U > 1.

x;

[.Qnecycl~

(T) t -~ (a) Undamped free vibrations

t

Or, ,na, pos,,,on -IT,/

floo co, where resonance may occur due to a ground movement, this will cause magnification of vibration amplitudes, which will rise in successive cycles and destabilize the system or object and render it highly vulnerable to failure. The maximum magnification occurs at a ground or floor frequency, 09, slightly less than the natural frequency, (On.The magnification of vibrations is a function of its dampening characteristics, as noted earlier. Figure 14.23 illustrates the influence of the natural frequency of the system or object on the magnification of its vibration amplitudes, as a result of a ground movement (F(t)), having a frequency, co, for different levels of system or equipment damping. Similarly, a build-up of amplitudes and consequent higher swings (oscillations) are sometimes noticed in transmission line conductors, tall poles or suspension bridges due to strong winds at critical speeds. This is a condition of negative damping, when the amplitudes magnify and must be avoided at the design stage. This can be done by selecting suitable lengths of transmission lines between two poles, the height of poles or the lengths of suspension bridges or by carrying out constructional changes in the system to achieve a natural frequency safe to avoid a possible resonance.

14.6.5 Test response spectrum (TRS) X~amping t

=

x0> x6>xl> x; etc. 1>0>0

(b) Underdampedfree vibrations ~ ~,,,~ Original position

Overdampedr/>l Critical~/damped7/= 1 ',

-[ t = (c) Criticallyand overdampedsystem(no oscillations)

Figure 14.22 Different damping levels of a free vibrating system

This is a response spectrum obtained during a test in a laboratory while exciting the shake table with ground movements as in the RRS. The test object is mounted on the shake table. The test object should respond normally during such movements. The test conditions (i.e. TRS) should closely overlap the required seismic conditions (i.e. RRS) of Figure 14.25.

14.6.6 Test requirements To test large to very large objects such as primary systems, where a laboratory test is not practical, seismic checks are established through analytical means. Similarly, for very large and heavy secondary systems such as turbines, alternators or transformers and reactors also their

14/448 Industrial Power Engineering and Applications Handbook

analysed during and after the tests. Where tests in all the three axes are not possible, two axis tests (two orthogonal horizontal and one vertical) are also permissible, which can also simulate the three axes conditions. For more details refer to IEEE 344 or IEC 60980.

i

770

01

Free vibration test

This is conducted to check the dynamic behaviour of the test object. It determines its natural frequency of oscillations, fn, and level of damping, 77, under in-situ condition before conducting the seismic tests. The test object is mounted rigidly on a platform as in its actual service position. It is then shaken gently by a collapsible string, tied at the centre of the top of the object and is pulled horizontally parallel to the X - X ' axis. The string will snap at a certain force and will make the object swing sinusoidally. A graph may be plotted of amplitude versus time as shown in Figure 14.20, from which can be determined its fundamental or natural frequency, fn, and the level of damping, 77. From Figure 14.20 it can be found that fn --~ 4.8 U z

1

r / = 0.035

.-.~--~< 1

fn = -f, r = O.2O8

(l~ - - . loge xx = 2zc, 1Oge o+6/

(.On

Amplification of vibrations vs frequency ratio Note 1. The peak of all the curves represent the condition of resonance that occurs when the system or equipment frequency ~ is a little CO

.

higher than of the ground motions 09or ,,-;- is slightly less than 1. 2. Curve 1 represents a system that is undanrnped, fig(a) of 14.22 3. Higher the level of damping, lower will be the amplifications, 171 < 172 < 1/3"

Figure 14.23

Amplification of vibrations versus frequency ratio

worthiness is established through mainly on mathematical analysis. Below we briefly discuss guidelines for testing such objects that can be tested in a laboratory. Now that it is possible to establish test facilities in a laboratory to simulate the time history of an earthquake seismic tests are conducted by creating the ground movements in the test object. Other methods, such as by analysis or by combined analysis and testing, are also available. Refer to IEEE 344 and IEC 60980 for more details. For this purpose a shake table, able to simulate the required seismic conditions (RRS) is developed on which the test object is mounted and its performance observed under the required shock conditions. Since it is n0teasy to create such conditions in a laboratory, there are only a few of these facilities available. The better equipped laboratories are in Japan, the USA, the UK, Greece, Germany, India and China. In India the Earthquake Engineering Department (EQD) of the University of Roorkee (UoR) is equipped with these facilities. The test object is mounted on the shake table and subjected to movements at the desired level and frequency in the horizontal and vertical directions simultaneously and its performance is critically observed, recorded and

Similar tests are conducted for other directions.

Duration of testing This is the duration sufficient to simulate seismic conditions. It depends upon the algorithm used to find time history from the required response spectrum (RRS). The minimum duration of a strong movement, as recommended by IEEE 344, is 15 seconds as illustrated in Figure 14.24(b). This will require a total duration of the order of 20 seconds, including the movement's times of rise and time of decay. A duration of 20.48 seconds, as noted in the figure, is typical of a test conducted at University of Rorkee. The following tests may be conducted: 9 Fragility testing This is a proprietory test to provide future reference data about an object, on its worthiness to operate under certain seismic conditions. When no seismic requirements are defined by the manufacturer the object is tested for its optimum capability. 9 Proof testing This is conducted when the seismic requirements in the form of floor response spectra (FRS) or required response spectra (RRS) have been pre-determined, and consequently test response spectra (TRS) have been established. This test will verify whether the test object can withstand an earthquake, of this magnitude and characteristics. The above tests are performed on the following basis:

1 Operating basis earthquake (OBE) test

This is defined by an earthquake that may be expected to occur during the operating life of the object for which

Testing of metal-enclosed switchgear assemblies 14/449

shutdown. For the purpose of design, the peak ground movements should not be considered to be less than 0.5 g (IEEE 344). In other words, equipment conforming to this test is capable of safely shutting down the whole plant and maintaining it in the event of an earthquake of this intensity. It will also be able to perform its duties when normal conditions are restored. Such equipment is subjected to at least five OBE tests before applying the SSE test. The logic behind such a stipulation is a statistical study of earthquakes which suggested a higher probability of moderate-intensity earthquakes and a lower probability for the most severe earthquakes. A small earthquake may occur on more than one occasion for which the test object must be suitable, whereas the most severe earthquake may occur only once in lifetime.

it is designed. For the purpose of design, the peak ground movements should not be considered to be less than 0.1 g (lEE 344). The object should function normally during and after the earthquake without malfunctioning or affecting the safety and the integrity of the nuclear plantthroughout its operating life. All the equipment installed in a nuclear power plant must undergo this level of testing. 2 Safe shutdown earthquake (SSE) test This is defined by' the most severe earthquake that could occur, producing the maximum vibratory ground movements at the place of installation. Safety-related machines, devices and components should remain functional during an earthquake of this magnitude and maintain the safety and integrity of the plant until a safe

14.6.7 Test equipment

Iv

. . . .

f.

9

~

!~ ........~,~ ?,~,~~i~i!ilili!:~!~(il ~!:i.!~.~./84184184184 i :.....

ii~84 i ., ..................

Figure 14.24(a) tests at UoR

....

:~i:::i:.:~.:.:i.::.:iii:i.lili:iiiiiii:!~il!.i~.i::i=,,., ~

Panels mounted on a shake table during seismic

This is a shake table, dynamically balanced, consisting of a platform on which the test object is mounted (Figure 14.24(a)). Special arrangements may be required to accommodate very large objects, such as rotating machines, motors, generators turbines and transformers, except extremely large machines, where it may be impractical to establish such laboratory test facilities. UoR, India, possesses a shake table of 3.5 m • 3.5 m with a payload capacity of up to 20 tonnes. It is provided with electrohydraulic actuators with feedback controls suitable to develop the required one horizontal and one vertical movement of the test table up to 3 g (ZPA). The test 'is usually conducted for a time history compatible with the RRS between a frequency range of 0.2-50 Hz. The TRS is calculated conventionally at 74 frequency points, as recommended by the United States Nuclear Regulatory Commission (USNRC). The time duration, considered at about 20 seconds, will be adequate for the attack and decay periods of the shake table. Of this, the duration of a strong movement is considered for at least 15 seconds, as illustrated in Figure 14.24(b). Time of

100 samples/sec

AI,IIIi1 'TIlVIvvIVv

r'T IIv

IVYVV 'IV'/'

0.00 -~

2.00

3.00

4.00

.oo

8.00 Time ( S e c o n d s )

Note

,

o.o

4.0

,

Strong motion time (>15 sec.)

Rise time I

.~

The TRS will envelop the RRS by minimum 10%

Figure 14.24(b)

Typical time history (RRS) of shake table movements during a laboratory test

18.0

20.0

=1~ Decay time-~

14/450 Industrial Power Engineering and Applications Handbook

rise, t 1, is adjusted according to the shape of the RRS. The data is acquired at 100 samples per second, enough to meet the requirements of accurate recording of frequency in earthquake movements.

14.6.8 Test procedure Simulating an R R S in a test laboratory Normally the user provides the nature of a probable earthquake in the form of RRS, i.e. acceleration characteristic curves, period versus spectral acceleration, such as those in Figure 14.18. The first objective is to generate a signal which should be able to produce a time history, on a shake table, whose response spectra match those of the RRS. The RRS is simulated on the shake table, generally without the test object to protect it from repeated shocks. This is done in all three directions, X, Y and Z, by shaking the table in the respective direction and adjusting its movement according to the relevant RRS. The maximum acceleration of the table is kept higher than the spectral acceleration corresponding to the ZPA of the relevant RRS, subject to a minimum of 0.1 g for an OBE and 0.5 g for an SSE test, as discussed above. The movement of the table is recorded as 'time versus g' (Figure 14.24(b)). If the three directional facility is not available, the two orthogonal RRS of the horizontal directions X-X" and Y-Y' can be superimposed on each other to obtain an equivalent horizontal RRS analytically as shown in Figure 14.26. This is a permissible procedure equivalent horizontal RRS is then translated into the shake system.

With the recording of the three RRS or two, as the case may be, the table is now actuated in the respective direction, according to the simulated RRS and the response spectra of the history of table acceleration obtained, which can be termed TRS and compared with the original RRS (Figure 14.25). This is plotted for 74 frequency points, 0.2-50 Hz, as prescribed by the USNRC, most of which are concentrated in the range 1-15 Hz, as noted earlier. The test response spectra should overlap the RRS by at least 10% at most of the 74 frequency points, to be on the safer side, as prescribed by IEEE 344 and illustrated in Figure 14.25. The table is actuated to obtain an acceleration equal to at least the ZPA of the respective RRS as noted above. Shortcomings in the TRS, if any, are compensated by readjusting the control signals to the table actuators and the actual tests are carried out on the test object as follows.

1 0 B E test

Passive test

The test object is mounted on the shake table and subjected to both horizontal (X and Y) movements or one cumulative orthogonal horizontal and one vertical ground movement simultaneously. Accelerometers are mounted on the shake table to measure its movements and also on the test object at its most vulnerable points. These points may be identified by the manufacturer or the user to monitor the behaviour of the object in such locations. The test is conducted for nearly 20 seconds, of which at least 15 seconds are covered for the strong movement (Figure 14.24(b)) the remaining 5 seconds

_~--_10%at most of the periods 1.2g Damping 5.00% 1.0g

0.8g

.(b

Required response spectrum (RRS)

a) 0.6g

I, "

tO

Test response __~ spectrum (TRS) ~ |/

I

//_!.

..-.i-il-i/lllilIll [l.=..

0.4g

0.2g

....... 10.0 ms

100.0 ms

II 1.0 s

]

,I

i

,I 10.0 s

Natural period (seconds) Figure

14.25 Comparison of the test conditions (TRS) with the required conditions (RRS). This is established in all three directions

Testing of metal-enclosed switchgear assemblies 14/451

/

//

• • - Cumulative - - -RRS of two / ~

~ - -

~176176 h~176

Test object turned by 90~

Y

RRS'S

I

I I i

RRS in X-X' direction

i X'

_x,I,

XX'---

.

.

.

.

.

.

.

I I i

I

i

I y, Position-1

/

y,

Position-2

Shake table

z~

Test object

.03 sec 33 Hz

i

Period t (sec) ~--.~ f, Frequency (Hz)

Test object turned by 90~ X

Fy

1 sec 1 Hz

Figure 14.26 Analytical construction of an equivalent horizontal RRS

Y Accelero-meter

being for the table pickup and decay periods. Figure 14.25 illustrates one test result, indicating the TRS exceeding the RRS by at least 10% at most of the frequency points. The behaviour of the object is assessed for its structural and mechanical worthiness and the test is repeated in the following four positions: 1 Equipment axis X-X' parallel to the X-X' axis of the shake table (Figure 14.27 position 1). 2 E q u i p m e n t turned by 90 ~ from the first (position 2). 3 Equipment axis X-X' at 45~ to the line ofhorizontal actuator of the shake table (position 3). 4 Equipment turned by 90 ~ from position 3 (position 4). Objects symmetrical through their length and width and by the distribution of their weight about the vertical axis can be tested in position 1 only. The rest can be tested in all four positions. 9 Testing under energized conditions Testing equipment under energized conditions should be performed only after tests under passive conditions have been conducted successfully. The same test in four different positions, under energized conditions, should be conducted and the behaviour and performance of the equipment assessed. 2 SSE test In this test equipment in a passive state is tested first in any of the four positions noted above for an OBE test. When successful, it is tested for one SSE test. If these tests are successful, then the following tests may be conducted to complete seismic testing. Five n u m b e r s - OBE tests one after the other.

Position-3

Figure 14.27

X'

Accelero-meter Position-4

Mounting of test object and applying force

One n u m b e r - SSE test under passive state or energized conditions as desired and the behaviour and performance of the equipment assessed. 9 Test results After each test, the test object is subjected to visual and routine checks. For electrical equipment, component and devices the test requirements will generally remain those discussed in Chapters 11, 14 and 32 for rotating machines, switchgear and controlgear assemblies and bus systems respectively, unless there is a specific requirement for an additional test. If so, this must be specified by the user. In a switchgear or a control assembly in particular, the contacts of all breakers, contactors, and moving and fixed power and control contacts of a draw-out assembly should be monitored during the course of seismic tests to ensure that there is no chattering (bouncing), or breaking and making of contacts that may destabilize the plant functions or throw it out of control. The indicating and measuring instrumentation and devices must give correct indications and readings. Equipment having to perform more complex duties, such as a process control, flow control, or sequence control, or when the control panel is also controlling and monitoring many other drives or devices, must be monitored more closely during seismic tests to detect malfunctioning of any component that may jeopardize the safety and integrity of the plant. The actual operating conditions are simulated as far as is practical, before performing such

14/452 Industrial Power Engineering and Applications Handbook

tests, to represent the behaviour of all components and devices during the test and their repercussions on the process they are controlling. 14.6.9 Preventive m e a s u r e s

In general, this discussion relates more to primary systems, which include civil foundation and structures, on which the whole building rests. Correct civil foundations, structures, columns, beams and trusses are major components in mitigating the effects of an earthquake. All these must be capable of withstanding the shocks and vibrations of an earthquake, according to the response spectra constructed for that location. It may be noted that at higher floor levels, the building tends to act like a vibrating filter and may transmit to the object frequencies close to the natural frequency of the secondary structure. In other words, the multi-frequency band of the ground movements may reduce to a narrow frequency band, almost dominating the natural frequency of the secondary system and may become a potential cause of a likely resonance with the structure. Objects on upper floors may thus be subject to higher accelerations, sometimes many times more than ground accelerations. Hence the necessity to avoid a resonance at the design stage itself. It is possible to do this, by keeping the fundamental frequency of the floor or structure, where the secondary systems are to be mounted, away from the predominant frequency that may filter out from the ground level. More precautions in mounting the electrical or mechanical machine, component and device on each floor or structure would contain the magnification of seismic effects to a large extent. Critical machines and components then need be designed with such a natural frequency that they do not resonate with the filtered-out predominant frequency of the structure or foundation. This is possible by suitably modifying the method of mounting or making small alterations in the construction of the object. It may be noted that a resonance may take time to build up and an earthquake may not last that long. The occurrence of a resonance, therefore, may not be as significant as the quasi-resonance (i.e. initial stages while the resonance is still building up) and this is reflected by the jagged peaks of the RS. The latest practice in the field of civil and structural engineering is to build a resilient ground floor rather than a rigid one, which can absorb the most vibrations of a seismic event and filter out to the upper floors only the frail motions. For more details, refer to the work available on the subject, as mentioned in the references.

By exercising these checks, most problems can be averted at the installation stage. The remainder can be avoided in the design of mechanical systems/construction of a machine rather than its electrical design, even in the case of an electrical machine, to ensure their required behaviour during an earthquake. The following may be a few such design areas that may be considered, in improving the performance of a machine during an earthquake: 9 Bearings and bearing housings in a rotating machine, in view of the very small gap between the stator and the rotor. 9 Rigidity of doors, bus and wiring systems etc. in a switchgear or controlgear assembly. 9 Limiting the use of brittle metals such as ceramic or porcelain. Although, avoiding the use of such materials in some cases may not be practical, as in the case of lightning arresters, bushings and insulators used in an HT switchgear, instrument and power transformers and reactors. 9 It is mandatory to employ only static or microprocessorbased relays, components and devices wherever possible. 9 Extra care needs be taken on the quality of fasteners and the method of jointing and bolting. 9 Resilient but rigid foundations such as by providing spring mounts or rubber pads for machines on the floor or for components and devices mounted on the machine so that they are able to absorb the vibrations, caused by resonance and quasi-resonance effects, due to filtered out narrow band ground movements. The stiffness of the foundation (coefficient of the restoring force, k) may be chosen such that it would make the natural frequency of the equipment OJn=~ k m much less than the ground (exciting) frequency, co. It is recommended to keep ~ > 2, to achieve an effective P vibration isolation. 9 The foundation and machine body/structure must prevent internal resonance, which is possible by slightly altering their designs. The critical items may then be checked for the required response spectra (RRS) as discussed above.

Relevant Standards IEC

Title

IS

BS

60044-1/ 1996

Specifications for current transformers General requirements Measuring current transformers Application Guide for voltage transformers Specification for voltage transformers General requirements Method for measuring partial discharges in instrument transformers

2705 Part 1/1992 Part 2/1992 4201/1991 4146/1991

BS 7626/1993

11322/1990

BS 6184/1992

60044-2/1997

60044-4/1980

BS 7625/1993 BS 7629/1995

Testing of metal-enclosed switchgear assemblies 14/453 60044-6/1992

Protective current transformers Requirements for transient performance

2705-3/1992 2705-4/1992

BS 7626/1993

60186/1995

Capacitive voltage transformers General requirements for measurement and protection (all voltages)

3156-1 to 4/1992

BS 7625/1993 BS 7729/1995

60051-1/1997

Direct acting indicating analogue electrical measuring instruments and their accessories Definitions and general requirements

1248-1 to 9

BS 89-1/1990

60060-1/1989

High voltage test techniques - General definitions and test requirement

2071-1/1993

BS 923-1/1990

60060-2/1994

High voltage test techniques. Measuring systems

2071-2/1991 2071-3/1991

BSEN 60060-2/1997

60298/1994

A.C. metal enclosed switchgear and controlgear for rated voltages above lkV and up to and including 52kV

3427/1991

BSEN 60298/1996

60439-1/1992

Low voltage switchgear and controlgear assemblies Type-tested and partially type-tested assemblies

8623-1/1993

BSEN 60439-1/1994

60439-2/1987

Particular requirements for busbar trunking systems (busways)

8623-2/1993

BSEN 60439-2/1993

60694/1996

Common specifications for high voltage switchgear and controlgear standards

3427/1991

BSEN 60694/1997

60898/1995

Electrical accessories: circuit breakers for over current protection for household and similar installations

8828/1996

BSEN 60898/1991

60980/1989

Recommended practices for seismic qualification of electrical equipment of the safety system for nuclear generating stations Code of practice for selection, installation and maintenance of switchgear and controlgear up to 1 kV. Installation

10118-3/1991

Criteria for earthquake resistant design of structures

1893/1991

DDENV 1998 (1-5) /1996

Specification for high voltage busbars and busbar connections

8084/1992

BS 159/1992

Relevant US standards ANSI/NEMA and IEEE

IEEE 4/1995

Standard techniques for HV testing

ANSI C.37.51/1989

Switchgear metal enclosed low voltage a.c. power circuit breaker switchgear assembliesmconformance test procedures

ANSI C.37.55/1989

Metal clad switchgear assemblies. Conformance test procedures

ANSI C37.57/1990

Metal enclosed interrupter switchgear assemblies. Conformance test procedures.

ANSI/IEEE 37.81/1989 Guide for seismic qualification of class IE metal enclosed power switchgear assemblies ANSI/IEEE-308/1992

Criteria for class IE power system for nuclear power generating station

ANSI/IEEE-323/1991

Qualifying class 1E equipment for nuclear power generating station

ANSI/IEEE-336/1991

Standard Installation, Inspection and testing requirements for power instrumentation and control equipment at nuclear facilites

ANSI/IEEE-344/1993

Recommended practices for seismic qualification of class 1E equipment for nuclear power generating stations (NPGS)

ANSI/IEEE-649/1992

For qualifying class IE motor control centres for nuclear power generating stations (NPGS)

ANSI/IEEE-693/1991

Seismic design of substation equipment

NEMA 250/1991

Enclosure for electrical equipment up to 1000V. Test criteria and design tests

NEMA/ICS 6/1996

Industrial controls and systems. Enclosures

Notes

1 In the tables of relevant Standards in this book while the latest editions of the standards are provided, it is possible that revised editions have become available. With the advances of technology and/or its application, the updating of standards is a continuous process by different standards organizations. It is therefore advisable that for more authentic references, readers should consult the relevant organizations for the latest version of a standard. 2

Some of the BS or IS standards mentioned against IEC may not be identical.

3

The year noted against each standard may also refer to the year of its last amendment and not necessarily the year of publication.

14/454 Industrial Power Engineering and Applications Handbook

Further reading 7 1 Agrawal, P.N., Engineering Seismology, Oxford and IBH Publishing Co. Pvt. Ltd, New Delhi, India. 2 Basu, S., Kumar, A. and Lawama, K.K., 'System identification of a circuit breaker pole'. Proceedings of Tenth Symposium on Earthquake Engineering, Roorkee, pp. 979-491 (1994). 3 Basu, S., Prajapati, G.I., Bandopadhyay, S., Kumar A. and Arya, A.S., 'Design and development of a shake table facility', Proceedings of the Eighth Symposium on Earthquake Engineering Vol. I. pp. 1 to 8 (1986). 4 Bressani, M., Bobig, P. and Secco, M., 'A support experimental program for the qualification of safety-related medium-voltage induction motors for nuclear power generating stations. Presented at the International Conference on the Evolution and Modem Aspects of Induction Machines Torino, July (1986). 5 Gass, Smith, and Wilson, Plate Tectonics in Understanding the Earth, Oxburgh, E.R. (ed.), pp. 263-285 (1973). 6 Krishna, J., Chandrasekaran, A.R. and Chandra, B., Elements

8 9

10 11

of Earthquake Engineering, South Asian Publishers (P) Ltd, New Delhi, India. Kumar, A. and Basu, S. 'Isolation in a shake table system', Workshop Proceedings on Base Isolation, New Delhi, June, (1989). Majumdar, S., 'Seismic qualification testing of switchboards', Siemens Circuit, 2/91 Vol. XXVI April (1991) and 1/92 Vol. XXVI January (1992). Pankaj, Basu, S. and Kumar, A., 'Seismic qualification of equipment- a need for greater understanding,' Proceedings of National Conference on Role of Continuing Engineering Education in Industrial Restructuring, University of Roorkee. February 1995, pp 157-162. Shun Zo Okamoto. Introduction to Earthquake Engineering, University of Tokyo Press, Japan (1973). Symposium on Earthquake Effects on Plant and Equipment (Vols I and II), Organized by BHEL energy system group Hyderabad, India, and Indian Society of Earthquake Technology, Roorkee, India (1984).