Power System Control Centre Display of Voltage Phase Angle

IFAC Symposium 1977 Melbourne, 21-25 February 1977

Power System Control Centre Display of Voltage Phase Angle M.J. BRITT and G.H. COUCH Engineers, Electricity Commission of New South Wales, Sydney

SUMMARY The significance of voJ tage ~Jhase angle across important transmission lines in the Nelll South Wales pOlller system is discussed. A microprocessor-based unit located in a pOlller system control centre IIIhich computes the phase angle difference betllleen tlllO remote locations from the data normally telemetered is described. The unit complements the role of a conventional pOllJer system static state estimator. INTROOUCT ION

1

As shollln in Figure 1, the N.S.W. pOlller system (6 GW) is characterised by relatively large distances betllleen generation sources and loads. The largest generating station (Liddell, Z GW) is situated about ZOO kilometres north of the main load centre (Sydney). The SnolllY Mountains Scheme (3.7 GW) is approximately 400 kilometres to the south-Illest of Sydney. The netlllork interconnects at 330 kV to the pOllJer system of the State of Victoria (4 GW). The electrical and geographic configuration of the pOlller system leads to its operation being at times constrained by steady state and transient stability considerations (Callolll, 1974).

Reactive pOllJer flOllJ from remote to local bus, measured at local bus

R

Resistance of transmission line betllleen remote and local bus Series reactance of transmission line betllleen remote and local bus Shunt susceptance of transmission line betllleen remote and local bus

X B 3

DERIVATION OF VOLTAGE PHASE ANGLE

Flolllers (1973) has described a method of voltage phase angle telemetry used by the Pacific Gas and Electric pOlller utility. The method requIres the communication of instantaneous values of voltage to a central location IIIhere a direct measurement of relative phase angles is then made. Practical difficulties arise in assessing the propagation delays on each communication channel. These difficulties are multiplied IIIhen alternative channels are available for backup purposes. The cost of large bandlllidth channels required by this approach must also be considered.

The essential function of pOlller system control centre equipment and its related telemetry is the provision to operating staff of information to allolll proper co-ordination of system operation. The Nelll South Wales pOlller system is co-ordinated from a State System Control Centre. Active and reactive pOllJer flollls on all 330 kV transmission lines are telemetered, and displayed to operating staff. These quantities are adequate IIIhen operation is constrained by transmission line thermal ratings. HOlllever IIIhere stability limitations related to transfer of pOllJer over relatively long distances apply, display of additional quantities can be of valuable assistance. Such useful information may, for example, take the form of summations of selected line f 101lls.

The computation of voltage phase angle difference betllleen the tlllO ends of a transmission line from knollln line parameters, and telemetered voltage magnitude, active pOlller flolll and reactive pOlller flolll avoids the problems discussed above. Furthermore, line flOllJS and voltage magnitude are quantities IIIhich are commonly telemetered to pOlller system control centres.

Practical telemetering difficulties have prevented quantities more fundamental to the assessment of stability being provided for operating purposes. In particular, the display of voltage magnitudes and phase angles could permit stability limits to be more easily monitored and help to improve operating staff understanding of the physical nature of system capabilities. Z

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This method is being implemented at the N.S.W. State System Control Centre. It involves the follOllJing on-line computations:

NOTATION

I(VZ)

= VIBR _ QR

and

b

Imaginary voltage at remote bus (VI as reference) Voltage phase angle difference betllleen remote and local buses

P

Active pOlller flolll from remote to local bus, measured at local bus

Vz

4

VIXB

+ VI + VI - -Z-

Z

Real voltage at remote bus (VI as reference)

QX

= VI

Voltage magnitude of local bus

5

PR

R(VZ)

+ PX

(Z)

VI

VI

= J[R(VZ)] Z

(1)

+

[I(VZU Z

= tan- l I(VZ)

(3) (4)

ffi2J

APPLICATION TO THE N.S.W. POWER SYSTEM

Relative phase angles IIIhich are useful for monitoring the pOlller system are across transmission lines linking Liddell POlller Station, Sydney West

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NEW SOUTH WALES

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VICTORIA

j Major Power Statians A Other Pawer Stations • Substa t ion. - 330kV Transmission Lines - - 132 & 66kV Transmission Lines

Figure 1

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200km

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Scale

Geographic outline of the N.S.W.

Substation and the Snowy Mountains Scheme (Fig. 1).

po~er

system

(b)

Line · fl~s are telemetered using a pulse frequency format ranging from 5 to 20 pulses per second;

(c)

Simultaneous sampling of line flo~s and voltage is necessary to compute instantaneous voltage angle during changing conditions;

(d)

The computation should not contribute significantly to the error in the derived angle;

(e)

Filtering of input and output quantities should be provided for, ~ith a simple means of modification to facilitate commissioning of the unit and to accommodate telemetry alterations;

(f)

Extension of the equipment to include further transmission lines should be simple;

The following factors were influential in the choice of equipment to perform the on-line computation.

(g)

Alterations to the line parameters necessitated by power system modifications or by better approximations should be simple;

(a)

(h)

The means to interface to digital equipment (such as a control centre computer) is required.

All the 330 kV line flo~s into Sydney West Substation and line flo~s north into Yass Substation are telemetered to the State System Control Centre. The Sydney West Substation bus voltage is also transmitted to the State System Control Centre as an accurate digital quantity and the transmission of voltage from Yass Substation is to be added in the near future. Thus the mInImum information required to calculate the voltage phase angles across this major portion of the system is centrally available. In addition, the voltage magnitudes can be calculated for a number of stations from which voltage is not telemetered. 5

EQUIPMENT

Insufficient circuit breaker status indications from the ends of the two transmission links are available at the State System Control Centre to include the on-line determination of line status; 411

access memory, programmable read-only memory and input/output logic.

Based on the above factors a design employing digital microprocessor components has been adopted. The arithmetic speed and accuracy of commercially available microprocessors is consistent with the requirements of the on-line computation. Furthermore expansion and interfacing to other equipment is simplified by microprocessor bus structures, and the use of programmable read-only memory enables stored constants and programs to be modified by reprogramming while providing permanent storage. 6

Random access memory (RAM) is used to store the sampled line flows and voltages, and computed volt_ ages and phase angles. The most recent 256 voltages and phase angles computed by the microprocessor are stored for each remote location in a RAM cyclic buffer for post-mortem purposes. Power fail detection circuitry and battery backup ensure that the contents of this store are saved in the event of loss of supply. The writing of data into this post-mortem store can be inhibited by operating staff to preserve voltage phase angle information for later analysis.

IMPLEMENTATION

A simplified block diagram of the Voltage Angle Display Unit appears in Figure 2. All components are contained on 110 mm x 110 mm printed circuit cards, which are plugged into sockets in a rack mountable sub-frame. Communication between cards is via control lines and a 16-bit multiplexed address / data bus which are wire-wrapped to the edge connector pins.

Programmable read-only memory is used for storing transmission line parameters, programs and static data. A special input / output card has been developed t o contain all the buffers, storage conversion and multiplexing logic required for one remote locat i ~ This rationalises the incorporation of additional remote locations. To minimi s e conversion errors

Design and construction have been facilitated by the commercial availability of standard printed circuit cards containing microprocessor, random

DATA ,CONTROL & POWER BUS

• Limits • Post-mort t:m Iiofa rdrinal

• Post-mort t:m data rt:trit:val

• ON/OFF • Slngl~ / doubl( lint: • Post- mor tt:m stort: inhibit

rI Control -------, cmtrt: ~ ____ .. L _ ~~pu~r __ J

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Pulst

inputs

T~It:mt:t~r intt:rfac~

L

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• Puls~ frt:q u~ncy-digitol conversion • Input bUf f~rs • Multiplex ing logic

rt:ad- only mt:mory

• Powt:r systt:m poramt:tt:rs • Softwart: • ARCTAN tablt:

r -Incr(mt:ntal - - ----,1- ______

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L ~~~~o~d~ _ .J Manual inputs

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Random accns mt:mory

~ Micraproct:ssor

Input / output timing

t~l~m~t~r

1 Programmablt:

J L

!

Op~rotor

outputs

J Digital 1

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-,

•

Digital procnsing

•

Arithmnic ~rror Plausibility rrror

•

J

T(I~m~t~r

input logic

• Powt:r systt:m variabln

• Post- mottt:m start:

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1

display logic

• OUtput

•

buff~r

Multipl~xing

Pow~r-'ail d~t~ction. batt~ry back-up

BCD inpu ts

1

Digital input logic

• Input bu ff~rs • Multiplu ing logic

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Powrr Supply

Figure 2

Simplified block diagram of voltage phase angle unit

412

logic

Digital displays

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and to simplify interface design, conditioned telemetry pulses are fed directly to the input/output cards. The card accommodates two pulse-frequency telemeters (line flows), one BCD input (voltage) and BCD outputs (voltage and angle). Timing signals for the input/output cards are generated on a separate printed circuit card and are bussed to the input/output cards.

accumulation of errors and to provide some plausibility checking of the computation it is desirable to use additional telemetered voltages. It is planned to use the voltage phase angle display unit to complement a conventional power system static state estimator which will form part of a proposed security assessment monitor for the State System Control Centre. The dedicated unit will provide backup for state estimated voltage phase angles in the event of computer failure as well as permitting checks on the estimation process.

As received telemetry pulses can be up to 200 millisecond s apart, computations are carried out at approximately 250 millisecond intervals. Each computed voltage and phase angle is written into the post-mortem store to allow subsequent off-line analysis of dynamic behaviour. Computed angles are displayed to the nearest degree and voltage magnitudes to the nearest kV. The displayed values are formed from four adjacent computed quantities and are updated approximately once per second on 7segment displa ys in the Control Room. 7

In addition, the speed of computation of the phase angle display unit will permit system changes to be followed more rapidly than is practical with a full static state estimator. 8

REFERENCES

CALLOW, J., DEMBECKI, J.A., DOUGLAS, D.W., MAGUIRE, A.F., PETERSON, H. and SPROULE, J. (1974). Design and operation of the interconnected power system in South-East Australia. Elect. Engr. Trans. I.E. Au s t., Vol. EEIO, No. 2, pp 63-68.

CONCLUSIONS

The voltage phase angle calculation equipment will provide operating staff with information to enable the N.S.W. power system voltage and phase angle profiles to be monitored. Only one 330 kV voltage magnitude is required. However, to reduce

FLOWERS, W.A. (1973). Phase angles displayed for dispatcher. Electrical World, May, p 48.

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413