Controllable Static Reactive-Power Compensators in Electrical Supply System

CONTROLLABLE STATIC REACTIVE-POWER COMPENSATORS IN ELECTRICAL SUPPLY SYSTEM H. Achenbach, W. Hanke and W. Hochstetter Siemens Aktiengesellschaft, 8520 Erlangen, Germany

ABSTRACT

As in power-station construction, and improvement of the efficiency is an important reqUirement for the operation of transmission systems.

Long-distance transmission of electrical power through extra-high voltage overhead lines and power distribution through cable system call for controllable reactive-power compensating circuits on an increasing scale. The transmission losses are thereby reduced, voltage deviations kept within permissible limits and the system stability is improve~ The achievements in the field of thyristor techniques made it possible to build steplessly controllable compensating reactors which are combined with a three-phase thyristor power controller. Selection of optimum voltages, current and reactor reactance of the static compensator is dealt with and the current and voltage ratings of the thyristors are explained. The problem of harmonics, the influence of unbalanced conditions, and the protective and monitoring arrangements are also referred to.

Since the losses Pv of a high-voltage line are proportional to the square of the transmitted power P and inversely proportional to the square of the transmission voltage U, the losses can be expressed as P v

=k

p2 • 1 u2cos2cp

where k 1

cos cp

Proportionality factor Line length Power factor of the transmitted power P

With increasing transmission power P, a correspondingly higher value of transmission voltage U is selected to keep the losses Pv in an economically justifiable relationship to the power P. The proportional increase of the losses with increasing line length 1 is also allowed for in the selection of a suitable transmission voltage. Furthermore, equation 7 indicates that the line losses P; are inversely proportional to the square of the power factor of the transmitted power. Reduction of the reactive power is therefore the main prerequisite for obtaining efficient operation of the high-voltage transmission system with minimum losses and voltage fluctuation.

INTRODUCTION Many problems associated with environmental protection preclude the possibility of installing the generating plant required to cover the increasing energy demand in conurbated areas, and the electrical energy must therefore be transmitted over large distances in many cases. One possibility is to utilize hydropower, which is abundantly available in some countries, but reqUires transmission of the power generated through long extra-high-voltage overhead lines. In densely populated areas, a cable system have to be installed on an increasing scale for transporting and distributing the power. In both cases, transmission losses and a strongly varying reactive-power demand result.

L-1

Owing to the operating capacitance and the inductance of the line, the reactive-power balance of the transmission system depends on the loading conditions, i.e. the amount and power factor of the transmitted power. When the high-voltage transmission system is operated at the

917

918

H. Achenbach, W. Hanke and H. Hochstetter

surge-impedance loading, the capacitive charging power is just compensated by the magnetizing reactive power. When the line is at no load, the full charging power occurs and would result in considerable voltage increases if no compensating means were employed. Therefore, the charging power during no-load operation of the transmission line has to be fully compensated and the compensating equipment should be controlled automatically. It would then be possible to operate the line at any partial load between no load and rated load with the voltage not exceeding the permissible limits. Compensating reactors installed at the line ends and in intermediate substations can compensate the charging power to up to about 70 %. The remaining 30 % compensating power required at no load would have to be provided by underexcited operation of the generators supplying the line. When the load increases, the generator excitation is increased to overexcited operation while the reactor units are gradually cut out. Another possibility is to switch in capacitor banks step by step, as a function of the voltage increase, the capacitors could be installed in the load centres and in the intermediate substations. Continuous control of the compensating power to improve the voltage and system stability has so far been effected by means of synchronous compensators installed in the intermediate substations and/or at the line ends.An advantage of the synchronous compensators is that they can be controlled steplessly within a reactivepower range corresponding to the range between overexcited operation (synchronous machine acts as a capacitor) and underexcited operation (synchronous machine acts as a reactor) • Like the generators supplying the system, the compensators are thus capable of carrying out dynamic reactive-power compensation, i.e. to make available the required reactive power within a short time in the case of sudden load variations or in the event of faults, thus keeping the voltage within permissilbe Hmi ts. Since the charging power of a highvoltage line can be compensated by reactors, it appears to be a logical step to assign the automatic control function to the reactor circuits. Today, a three-phase thyristor power

controller is available which permits this idea to be put into effect. One of the circuit arrangements presently known (1 to 7) is described in the following by reference to Fig. 1.

4

I 1

2 3

4 5

5

1

Transformer Linear reactor Thyristor controller Capacitor banks Filters

Fig. 1

Controllable static reactive-power compensator, basic circuit SELECTION OF OPTIMUM VOLTAGES AND CURRENTS

The capacity can be increased by series and/or parallel connection of several units. With parallel connection, each individual thyristor should have a series-connected fuse. In the case of the threephase power controller shown in Fig. 1, failure of one thyristor causes a current which corresponds to that at zero-delay firing angle and is not fully transferred to the faulted circuit before the control setting of the thyristors in the parallel circuit of the same and opposite polarity is automatically reduced. Selective operation of the fuses is not possible before the number of parallel circuits or the ratio between the overload current and the continuous operating current is adequately high. Uniform distribution of the current during normal operating conditions over all parallel-connected thyristors is obtained through careful selection of the components and a suitable circuit arrangement.

Controllable static reactive-power compensators

Fuses are not employed if the thyristors are connected in series. Here, the required redundancy, i.e. the means required to avoid service interruptions on the failure of one thyristor, is obtained by connecting additional thyristors in series. If one thyristor fails, the blocking function is assumed by the seriesconnected thyristors. Interruptions have not been experienced, but it may occur that the firing circuits of individual thyristors in the series circuit will be blocked. In this case, an electronic auxiliary circuit comes into operation through which firing of the thyristors is initiated without endangering these thyristors. RC circuits ensure uniform distribution of the voltage over the series-connected thyristors. For a given thyristor type, a capacity increase results in a higher voltage in the case of the series connection and in a current increase with parallel connection. This has, of course, effects on the equipment connected in the external circuits of the thyristor controller, such as the reactor, switchgear and transformer secondary winding. A threephase arrangement without parallel circuit using the highest-power thyristors presently available would involve currents of the order of 1000 A, and very high currents would result if the power were increased by parallel connection. This is undesirable, also in view of the connecting leads. With the series arrangement, isolation of the firing circuits from the power circuit presents problems. Today, this problem is solved by the use of signal transmission through light-optical conductors. The energy for the firing pulses has to be drawn from the voltage across the thyristor through a capacitor store. Depending on the capacity, unusually high voltage values, e.g. between 30 kV and 110 kV, result with the series connection. This is of no importance in the case of the system transformers, since selection of a suitable secondary voltage has only a slight effect on the costs. In the case of the capacitor banks, which are made up of small units arranged in series and parallel circuits, only the insulators between the capacitors involve a slight increase of the costs. SWitchgear is available for certain insulation ratings

919

and the slight increase in costs for the equipment for a higher insulation rating is compensated by the advantage of lower currents and smaller conductor cross sections, particularly if isolating switches are employed. A reduction of the overall expense

may, under circumstances, be obtained through a combination of series and parallel circuits. If, for instance, two thyristors are connected direct in parallel, a saving may be obtained from the less complicated auxiliary and firing circuits. SELECTION OF REACTOR REACTANCE

1 2 3

Voltage characteristic Current characteristic at 0(. 90 0 Current characteristic at 0(= 120 0

Fig. 2

Thyristor-controlled reactor; control method

Selection of the reactor reactance is closely associated with the overload capacity when reactive power is required, and with the firing angle selected for the rated current. The reactor itself with its copper Winding can be designed for a high ove~load capacity without appreciable additional expense. The type rating is in this case reduced, since the reactor is designed for 1 2 • X in continuous duty. For economical reasons, it is therefore a logical step to operate the equipment at firing angles D( >90 0 • A disadvantage of this method is that higher harmonic currents result. The control method for the thyrist~ controlled reactor is illustrated in Fig. 2.

920

H. Achenbach, W. Hanke and W. Hochstetter

So

--l~ .x

u= cons!. IN=consl

0.5

0.1

------

o

0.5

-xO

calculating the junction temperature. If high powers are involved, an accurate method, e.g. that described in (8) is employed for the calculation. In addition to the exact thyristor data, transient conditions, e.g. those occurring when the controllers are connected to the system, IIUSt be known. The basis is the continuous operating current, which determines the initial teaperature for the following cases of overload, but certain overload conditions have 'to be allowed for as well. It .ay be assused that the XD/XN ratio in operation under rated conditions will be decisive factor determining the losses.

XN

t!.L KW

Sn

Xc

Reactor kVA Reactor reactance

X UN N T: fictitious nominal reactance

11eW 1000A

N

UN IN

Rated voltage Rated current of fundamental oscillation I max Overload current of thyristor controller at firing angle ()(:: 90 0

Fig. 3

A

0.5

750 A SOOA

Design criteria for the reactor

Fig. 3 shows the reactor type rating SD versus the ratio between the reactor reactance and the rated reactance. The highest type rating is obtained if the reactor reactance XD is equal to the rated reactance IN, i.e. with the thyristor controller at the full control setting in operation under rated conditions. The short-time overload capacity can be taken frQm the Ima:lC7IN curvel also shown are the currents due to the 5th and 7th hanaonic. The harmonic currents transferred to the system can be reduced by filter circuits or other means. CURRENT RATING OF THE

THYRISTORS

The main factor determining the current rating of the thyristors is the temperature at the thyristor junction. Owing to the lillited thermal capacity of the thyristors, shorttime current overloads have to be allowed for in addition. There are various approxillation lIethods for

Ol_

0+----,----,----.....-90"

180·

lSO'

120'

ol

-

CJt

I

:

1(

I

bl

()(=150·

Average losses during one cycle R.M.S. value of fundamental coaponent current Instantaneous value of thyristor current Fig. 4 Losses in thyristor a) as a function of firias angle ~ b) Comparative current curves for points A and B at the same fundamental-component current

Controllable static reactive-power compensators

Fig. 4a shows these losses Pv in a thyristor at an operating current with a constant fundamental coaponent and for different firing angles«. In Fig. 4b, the strongly differing current curves at ~ = 90 0 and ~::a 1500 are shown for points A

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and B, the fundamental current component being the same in both cases. A surprising fact is that the losses hardly vary within a wide range of firing angles (up to approx. 140 0 ). Selection for a high overload without appreciable increase of the

J" U.I

A

I

2 1.5 1\

U

0~'50~

I --------r----j- - - - - - - ' - 1

0.1

0.,,-1

cl

I! ex. J

90°

et.

60°

I,

300

0

dl

acl System configuration b Oscillogram for U and I Crest values taken from osclllograll d) Quantities calculated from oscillograa

0.1

0.25

_I

Voltages applied to threephase thyristor controller IT Currents across the thyristor controller Crest values Fundaaental-component current Firing angle

Fig. 5 Model investigation; load shedding in system; characteristics of Toltage and current at thyristor controller

922

H. Achenbach, W. Hanke and W. Hochstetter

losses in continuous operation is thus possible. For deteraining the overload conditions, the following design criteria have to be foundl a) Overcurrent required to keep the system voltage within permissible limits on system disturbances and period for which this overcurrent has to be maintained. Consideration has to be given to the fact that the other voltage regulators connected to the system react as well aad will become effective within about 0.5 to 1 second, so that the supplying generators will contribute to the voltage reduction. b) Magnitude of the voltage at the connecting point of the thyristor controller and the controllable static compensator before the compensator comes into action. This delay may result in correspondingly increased currents which aay last for 1 to 3 cycles. c) Is there the possibility of aperiodic components occurring during these first cycles until the new state has been reached. Aperiodic components may have a strong influence on the temperature rise on a part of the thyristors. Fig. 5 exemplifies the result of a model analysis during which the current and voltage at the thyristor controller was oscillographed for the case of a load-shedding operation in the system (Fig. 5a). The peak values of system voltage appear on the thyristor (Fig. 5b), and the rise of the system voltage at the instant of load shedding is thus olearly shown (Fig. 5c). The amount of the current rise in the 1st half-wave corresponds to that of the voltage rise. The effect of the controller, i.e. the reduction of the firing angle and the resulting further increase of the current, can already be noticed in the 2nd cycle. This causes a strong reduction of the voltage rise rate. As a result of the system optimization of the controller, a further reduction of the firing angle now follows i.e. to the smallest possible value (Fig. 5d) in this case, and the maximum current across the thyristor controller is obtained after about 5 cycles, corresponding to 100 ms. If the system voltage is thereby reduced to a value near the setpoint, the current must be maintained

(Figs. 5c and d) until all the reaaining regulating gear in the systea can become effective. This will be the case after 1 second at the latest. If the thyristor controller carried the maximum continuous current before the load-shedding operation, this oscillogram can demonstrate the maximum loading of the controller. Whereas the voltage rating is primarily determined by the peak values during the first cycles, the current rating is determined by the temperature characteristic of the thyristor junction. If - like in the oscillogram - pronounced unbalance or overcurrents do not occur during the first cycles, the current characteristic may be represented by a rectangular function, and a temperature characteristic with respect to time as shown in Fig. 6 is obtained. 6Vt K

I

I-

t

I I

100

I

._1

....... "-: -:...-: -:.. _ - - - 2

I I I

,,",..-

3

"" /:-/

I //

1//

I',

50

;=-~~

__ I

o+--t------.-------,-o 0,5 15 Fig. 6 Temperature/time characteristic for thyristor junction 1 max. temperature within a cycle 2 temperature on current interruption and at voltage in reverse direction 3 temperature on current interruption of the opposite thyristor and built-up of voltage in the forward direction Since an alternating current is applied to the thyristor, unequal temperatures result at discrete instants within a cycle (Fig. 6). The maximum temperature and the temperature resulting when the thyristor has to carry the inverse voltage are of particular importance. There

Controllable static reactive-power compensators

is a certain difference between the blocking function in the positive and negative directions. If critical overloads are to be expected during the first cycle, the maximua permissible temperature may occur even during the first cycles. In addition to the design of the heat sink, these conditions also de~end on the overload conditions (9). It will be expedient to choose a heat sink with a low thermal resistance through which the initial temperature is kept low even before the overload occurs. If, however, the limit temperature is reached at the end of the overload period, a heat sink with a higher thermal capacity is employed. The initial coolant temperature has, of course, a decisive influence. Depending on the climatic conditions, it will be expedient to provide for artiticial cooling, e.g. by evaporation coolers or retrigeration machinery, instead ot using additional thyristors. Finally, unbalance conditions and aperiodic components should be allowed tor in the current rating of the thyristors. VOLTAGE RATING OF THYRISTORS For high powers, thyristors having the highest inverse-voltage ratings are employed. Since- with a series circuit - a protective tiring ot the thyristors is provided, an etfective protection will be available against high overvoltages. However, the protective firing arrangement must not come into action too early, otherwise the control capability of the thyristor controller would be attected. This condition is eased by the tact that simultaneous tiring will also be required at high voltages and that the delay angle of the controller is anyhow changed into that particular direction. The rating for a high overload capacity takes into account that reduction ot the current, e.g. by a current limiting control, must still be possible. Limitation ot the rate ot voltage rise is normally ettected by the series-connected reactor. An undesirably high rate ot voltage rise may only occur under certain tault conditions, e.g. on incoming lightning surges or on earth taults. Such stresses can be eliminated by a suitable circuit arrangement ot the

923

equipment and its protection by buildings or other screening means. For operation at the maximua continuous service voltage, the voltage design tactor tor the thyristors is chosen so as to allow tor non periodic overvoltages to be expected. The number of redundant series-connected thyristors can be strongly reduced if suitable means are provided tor indicating the failure ot individual series thyristors. ELIMINATION OF HARMONIC CURRENTS FROM THE SYSTEM In the case of balanced operation, which is the normal case if the controllers are operated in conjunction with a supply system, the current due to the triplen harmonics can be kept away trom the system by using a delta connection ot thyristor controllers. The harmonics ot the other modal numbers are ettectively suppressed by a permanently connected capacitor and coupling through a reactance provided by the system transformer. A tilter circuit will, however, be necessary in most cases tor eliminating the 5th harmonic (and the 7th in some cases). In any case, possible points ot resonance must be investigated to prevent amplitication of certain harmonic oscillations. Here, the properties and contiguration ot the system to which the compensator is connected plays an important role. The reactance of the transtormer, i.e. its reactance drop, may be adapted to the particular requirements, but this is not normally done on account ot the economical advantage obtained through the use ot standard-type transtormers. It very high powers are involved,

suppression ot the 5th and 7th harmonics may be ettected by two sepa rate compensators whoae 8u8Ply phases are displayed by 30 • In this case, the system transtormer is prOVided with two secondary windings, one connected in delta and the other one in star. Balancing to the two thyristor controllers connected is by means of the closed-loop and open-loop control.

924

H. Achenbach, W. Hanke and W. Hochstetter

To prevent radio interterence by the thyristor tiring circuits, a screen i8 installed in the building housing the thyristor controllers. An important requirement is that the seriesconnected reactor should be installed close to the building; the connecting cable should be properly screened.

pensator trom the system. Standardtype protective arrangements are used for the system transformer, the reactor, the capacitor bank and the filter circuits. The entire equipment arrangement may also be protected by a differential protection.

INFLUENCE OF UNBALANCE DURING STEADY-STATE OPERATION AND TRANSIENT CONDITIONS The individual phases ot the static reactive-power compensator can be controlled separately, which is not possible with a synchronous machine. This possibility is maily used on reactors in industrial plants where unbalance resulting trom the operation of arc turnaces and other loads have to be compensated (10). In electrical supply systems, the compensator is normally operated in symmetry, the applied system voltage being normally symmetrical as well. Care should be taken to see that the compensator symmetry is inherently maintained; this is ensured by suitable balancing controls. In addition to the possibility of unbalanced control of the three phases, manufacturing tolerances of the components ot the thyristor tiring circuits may cause different control settings for the positive and negative halt-waves, which may result in the generation of an aperiodic component. It can theretore not be excluded that - with high powers and a comparatively low magnetizing current - the transformer is driven into saturation (11). To avoid this, suitable balancing controls are employed. The characteristics of the magnetizing current ot a model transtormer with and without balancing control are shown in Fig. 7. PROTECTION AND MONITORING Where the rating does not provide tor overload capacity the seriesconnected reactor protects the thyristor controller against current overloads. If the overload capacity is included, the thyristor controller may be subjected to high currents, e.g. if the limiting control or the firing circuits fail with the thyristors being continually conductive. In such a case, an overcurrent protector must isolate the com-

0)

Fig. 7

Transformer magnetizing current; characteristic with respect to time a) with b) without balancing control

Monitoring of all the components of the compensator is of particular importance. The thyristors of the series-connected groups, for instance, are moni toried for conductivity and readiness for gating. If unbalance results in the capacitor banks, appropriate indications are given as a first step. Partial disconnection under load of plant sections, e.g. capacitor banks or filter circuits, is not provided for, since such an operation may entail undue stressing of other parts. CONCLUSIONS The conventional reactive-power compensating arrangements for highvoltage systems, such as fixedvalue reactors, capacitors and synchronous compensators, have now been supplemented by the controllable static reactive-power compensator. This consists essentially of a thyristor-controlled three-phase reactor which is distinguished by the possibility of continuous controllability within a kVA range covering both the supply of reactive power to the system and the drawing of such power from the system. The control range depends on

Controllable static reactive-power compensators

the system conditions to which the static compensator can easily be adapted by suitable selection and rating of its components. It is superior to any other type of compensating equipment as it is capable of making available the reactivepower demand of the high-voltage system practically without delay.

kompensation fUr Industrienetze, Techn. Mitt. AEGTELEFUNKEN 66, 7, 286-290

( 1976)

(6)

It is expected that the controllable static reactive-power compensator Will, in future, be used on an increasing scale in addition to the conventional compensating eqUipment, and will partly replace the latter.

(2)

W. Hochstetter, Eigenschaften der regelbaren statischen Blindleistungskompensatoren in der Elektrizitatsversorgung, Siemens-Zeitschrift 51,

3, 141-145 (1977)

(8)

w.

Herbst, J. K!uferle, F. Peneder, K. Reichert, Statische regelbare Blindleistungskompen8ation fUr Hochspannungsnetze, Brown Boveri Mitt. 9/10, 433-439

(9)

I.D. Ainsworth, E. Friedlander, K.J. RaIls, Recent developments towards long distance A.C. transmission using saturated reactors, lEE Conf.,London, 242 (1973) A. Chit, W. Horn, Neuzeitliche ruhende Blindleistungs-

T. Salanki, Berechnungsmethode fUr LuftkUhlk6rper von Thyristoren hoher Leistung, Siemens-Zeitschrift

50, 3, 172-177 (1976)

(10)

Z. 19, 11-16 (1974)

(4)

R. MUller, Untersuch\1lU!;en zur thermischen Beanspruchung der !hyristoren in einer Hochspannungs-GleichstroaUbertragung, Elektrotechnische Zeitschrift A 97, 7,

422-426 (1976)

(1974)

H. Frank. S. Ivner, Blindleistungskompensation von Lichtbogen6fen mit Hilfe von thyristorgeschalteten Kondensatoren System TYCAP, ASEA.-

M. Erche, H.P. Hettler, D. Povh, Performance of Static Compensators, Cana4ian Communications & Power Conference, Montreal, 350-353

(1976)

REFERENCES L.O. Barthold et al., Static Shunt Devices for Reactive Power Control, CIGRE Paper 31-08 (1974)

925

W. Meusel, H. Waldmann, Coordinate Transformations of Multi-Term Regulation Systeas for the Compensation and Symmetrization of Three-Phase Supplies, Siemens Forsch.und Entwickl.-Ber. 6, 1

(1977)

(11)

H. Bold, Stromrichtertransformatoren fUr die Hochspannungs-GleichstrolllUbertragung Cabora Bassa-Apollo, Brown Boveri Mitt. 7, 432-441

(1976)