Series capacitor equipment

Abstract: A series capacitor equipment for connection into an electric power line (A) has a capacitor bank (C) and a controllable inductor connection (L, T1, T2) connected in parallel therewith and comprising a series connection of an inductor (L) and a controllable semiconductor connection (T1, T2). Further, the equipment comprises control members (CU) adapted, on the occurrence of a subsynchronous resonance oscillation, to control the semiconductor connection with separate control angle values during positive and negative half-cycles of the capacitor voltage for generation of a subharmonic voltage over the capacitor which counteracts the resonance oscillation.

Description: The invention relates to a series capacitor equipment according to the precharacterising part of claim 1. A series capacitor equipment as described in the preamble of claim 1 is known from the WO-A-870 45 38. With the means described in this document it is intended to be able to rapidly change the line impedance of the power line in order to control the transfer capacity of the power line. For this reason, the impedance of a circuit consisting of a line capacitor bank and a controllable inductor connected in parallel thereto is made variable within a large range. This is achieved by controlling thyristors connected in series with said inductor in a conventional symmetrical mode over a large range of the control angle between zero and 180°. The WO-A-870 45 38 does not deal with the problem of eliminating subharmonic voltages present in the line voltage. In an electric power network, so-called subsynchronous resonance (SSR) oscillations may arise by the cooperation between mechanical oscillations in, for example, turbo-generators connected to the network and the electrical system. The oscillations may, for example, have a frequency of 20 Hz. They may have positive feedback (negative damping) and reach such amplitudes as to damage the shafts of the generator units. The SSR oscillation has proved to be particularly serious in such networks which include power lines with series capacitors for compensation of the line inductance in order to increase the transmission capacity of the line. It has been proposed to connect, in series with each phase in the block transformer of a turbo-group and/or the star point of the generator, an ohmic damping resistor in parallel with an inductor and with a series resonance circuit tuned to the power system frequency. This solution is known from the SE-A-446 289. These damping circuits may be permanently connected or alternatively be put into operation under the control of a disturbance detector. This device comprises a large number of components, which must be dimensioned for the maximum current of the line. Therefore, the device will be expensive and space-demanding. The desired damping of SSR is further obtained by the consumption of power in the resistors of the device. These must therefore be either so largely dimensioned or so efficiently cooled that they are able to take care of the large energies which are consumed during a damping process without being overheated. From, for example, US-A-4,292,545 and US-A-4,434,376 damping circuits for SSR oscillations are known, which are used in series capacitors in power lines. With the aid of thyristor valves, damping resistors are connected into the network circuit, for example in parallel with the capacitors, when an SSR oscillation occurs. The connection is initiated by a detector which connects a damping resistor each time that the length of a half-cycle of the line voltage exceeds a predetermined value, preferably approximately equal to the length of a half-cycle at a power system frequency equal to the system frequency of the network. Also in this device a heavily dimensioned resistor is required, which is a clear disadvantage from economical and practical points of view. The invention aims at providing a series capacitor equipment of the above-mentioned kind which, with a minimum of additional components and practically without losses, provides an efficient damping of SSR oscillations. To achieve this aim the invention suggests a series capacitor equipment according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of the invention are characterized by the features of the additional claims. The invention is based on the surprising realization that efficient damping may be obtained without the usual consumption of energy in damping circuits. Instead, the energy in the subharmonic oscillation is transformed into fundamental frequency power and returned to the power network. This is done with the aid of an inductor and a controllable semiconductor valve which, in the vicinity of the zero crossings of the capacitor voltage, reverses the charge of the series capacitor and hence its voltage. How great the voltage change of the series capacitor because of the voltage reversal will be depends on how close to the voltage zero crossing the voltage reversal is started. By using suitable control members for varying the time of initiation of the voltage reversal from fundamental frequency half-cycle to fundamental frequency half-cycle concurrently with an SSR oscillation, a subharmonic voltage can be generated across the series capacitor, which subharmonic voltage counteracts the SSR cillation that is to be dampended. By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1 series capacitor equipment according to the invention, in which, in parallel with the series capacitor bank, an inductor is connected in series with two thyristor valves connected in antiparallel with each other, Figure 2 an alternative embodiment of the equipment according to Figure 1, in which the single inductor is replaced by two inductors, each one series-connected to a thyristor valve, Figure 3a the inductor-thyristor connection being connected to the series capacitor via a transformer, Figure 3b the thyristor valves only being connected to the series capacitor via a transformer, Figure 4 an example of a control device in the equipment according to the invention, Figure 5 the mode of operation of a thyristor-controlled inductor, Figure 6 how, in equipment according to the invention, the control of the inductor may be used for generation of a subsynchronous voltage component across the series capacitor, Figure 7 the mode of operation of equipment according to the invention, which is controlled with the aid of a simplified control device. Figure 1 shows an example of series capacitor equipment according to the invention. A series capacitor C is connected into a power line A. For the sake of simplicity, the equipment and the power line are shown to be of single-phase design, but in practice power networks of the kind in question are normally of three-phase design. The power line may be of any arbitrary kind, but in practice series capacitor banks occur primarily in power lines with relatively high voltages and extending over relatively large distances. An overvoltage protection device for the series capacitor may consist, in its simplest form, of the zinc oxide varistor V shown in Figure 1 but is in practice often considerably more complicated, for example comprising controllable or not controllable spark gaps, damping circuits, etc. In parallel with the series capacitor C, an inductor L is connected in series with two thyristor valves T1 and T2 which are connected in antiparallel with each other. The inductor L suitably consists of an air inductor and its inductance is chosen such as will be described in greater detail below, suitably such that the reactance of the inductor at the system frequency of the power network is lower than, preferably considerably lower than, the reactance of the series capacitor C at the same frequency. The system frequency of the power network is the nominal operating frequency, that is, normally 50 Hz to 60 Hz. Each one of the thyristor valves may consist of one single thyristor or of a plurality of series-connected thyristors, the number of thyristors being sufficiently large for withstanding the maximum capacitor voltage arising. For control of the equipment, a control device CU is provided. The control device is supplied with a signal which corresponds to the capacitor voltage U c . This signal may be obtained with the aid of a voltage measuring device (not shown), for example a measuring voltage divider arranged across the capacitor bank. The control device delivers firing pulses to the thyristors in the two thyristor valves. The operation of the control device will be described in greater detail below with reference to Figures 4-7. In the schematic circuit diagram of Figure 1, the control device is shown in galvanic connection to the power components of the equipment. These components, that is, the capacitor bank, the inductor and the thyristor valves, are, however, located at the potential of the power line A, that is, at a voltage relative to ground which is often very high. For practical reasons it is often suitable to arrange the control device CU at ground potential, the connections between the control device and the other parts of the equipment then taking place via galvanically separated transmission channels, suitably via light guides. Figure 2 shows an alternative embodiment of the inductor-thyristor circuit shown in Figure 1. In Figure 2, two separate inductors L1 and L2 are arranged in parallel with the capacitor bank C, each one of the inductors being arranged in series with one of the two thyristor valves T1 and T2. The function is the same as in the equipment shown in Figure 1. Figure 3a shows how the inductor-thyristor circuit L-T1-T2 may be connected to the series capacitor bank C via a transformer TR with the windings W1 and W2. With the aid of this connection, such an adaptation of the operating conditions of the inductor-Thyristor circuit can be obtained that the current handling capacity of the thyristor valves is fully utilized, which may entail such a reduction of the operating voltage of the valves and hence of the number of required series-connected thyristors in each valve that this saving is larger than the additional costs of the transformer. Figure 3b shows a further alternative embodiment, in which only the two antiparallel-connected thyristor valves are connected to the main circuit via the transformer TR. In this case, the inductor L operates at the current and voltage level of the main circuit. In both of the examples shown in Figure 3, the necessary inductance may consist of the leakage inductance of the transformer, thus eliminating the need for a separate inductor L. Figure 4 shows an example of a possible configuration of the control device CU in Figure 1. The capacitor voltage U c is supplied to a bandpass filter 1, which lets through the system frequency component of the network but blocks higher and lower frequency components. Thus, the output signal U co from the filter corresponds to the fundamental frequency component of the capacitor voltage U C . This output signal is supplied to a phase-locked oscillator PLO. In a phase comparison circuit 2, the output signal U OSC of the oscillator is compared with the input signal U CO . A signal corresponding to the phase difference is supplied, via an integrator 3, to a voltage-controlled oscillator 4. The output signal from the oscillator will be controlled so that it will have the same frequency as the input signal U CO to the oscillator. Further, the output signal will be kept at a constant phase position in relation to the input signal. This constant phase position is dependent on the design of the comparison circuit 2, and in the following the signal U OSC is assumed to have a constant phase lag of 90 degrees related to the input voltage U CO . The signal U OSC consists of a sinusoidal signal. Further, the capacitor voltage U C is supplied to a band reject filter 5, which blocks the power system frequency component. The output signal from this filter is supplied to a low-pass filter 6, in which signal components with frequencies exceeding the expected subsynchronous oscillations are filtered out. The output signal from the filter 6 thus consists of a signal U CS which corresponds to a subsynchronous resonance oscillation in the capacitor voltage. The latter signal is supplied to a linearizing circuit 7. This circuit has such a characteristic, for example approximately of the type shown in Figure 4, that the output signal U X of the circuit, independently of the amplitude of the fundamental frequency component of the SSR oscillation, controls the thyristors such that a subharmonic voltage is obtained across the capacitor, which with the desired degree of compensation counteracts the SSR oscillation arising. The signals U X and U OSC are supplied to a summation circuit 8, the output signal of which is supplied to a level detector 10. The output signal of the level detector is supplied to the thyristor T1 as firing signal via a signal convertor 12 and a signal channel 14. The signal convertor 12 may, for example, consist of a power amplifier and a light-emitting diode, and the signal channel 14 then suitably consists of a light guide. Further, the signals U X and U OSC are supplied to inverting inputs of a second summation circuit 9, the output signal of which is supplied to a level detector 11. The output signal of this detector is supplied as firing signal to the thyristor T2 via a signal convertor 13 and a signal channel 15. The firing signals from the level detectors 10 and 11 are designated U P1 and U P2 , respectively. A control signal U S for symmetrical control of the inductor current and hence of the effective capacitance of the equipment is supplied to inverting inputs of the two summation circuits 8 and 9. The mode of operation of the equipment described above will now be described in greater detail, first with reference to Figure 5 and with the simplified assumption that the SSR component U X is zero. Figure 5 shows a number of diagrams of quantities occurring in the equipment as functions of the time t. The uppermost diagram shows the fundamental frequency component U CO of the capacitor voltage and below this diagram the output voltage U OSC from the phase-locked oscillator PLO is shown. Then the control signal U S is shown which, since it is assumed to vary slowly, may be regarded as constant during the time interval shown. Below this diagram the output signal U OSC - U S from the summation circuit 8 is shown. When this output signal is positive, the level detector 10 delivers an output signal in the form of a firing pulse U P1 to the thyristor T1. As shown in Figure 5, the thyristor T1 will therefore be fired at the time t = t1, t = t3 and so on. Each firing takes place at an electrical angel β prior to the zero crossing of the fundamental frequency component U CO of the capacitor voltage, that is, the thyristor T1 is controlled with the control angel β relative to these zero crossings. The output signal from the summation circuit 9 is shown in the lowermost but one diagram in Figure 5, that is, the signal -U OSC -U S . The level detector 11 supplies firing pulses U P2 to the thyristor T2 when this signal is positive. Thus, the thyristor T2 will be fired at the times t = t2, t = t4 and so on, that is, this thyristor is controlled with the control angle β relative to the zero crossings of the fundamental frequency component of the capacitor voltage. The two thyristors are controlled with the same control angel β, that is, the control is performed with the same control angle during the positive and negative half-cycles of the capacitor voltage. Therefore, the control may be said to be symmetrical. The control angle β is 90° when the control signal U S is zero and decreases with increasing control signal U S . By varying the control signal U S , the equivalent inductance of the inductor L for the fundamental frequency component may be varied from a lowest value, which is the inductance of the inductor itself, up to an inductance value which, in principle, is infinite. The susceptance of the series capacitor may then be varied from a minimum value B C + B L to a maximum value B C , where B C is the susceptance of the capacitor bank at the system fundamental frequency and B L is the susceptance of the inductor L at the system fundamental frequency (B L is negative). The above-described symmetrical control of the thyristors with the aid of the control signal U S may advantageously be used together with the control of the invention which will be described in the following. As is shown in Figure 4, the subharmonic component U X is supplied to the summation circuits 8 and 9 with different signs (positive for the summation circuit 8 and negative for the summation circuit 9). Thus, a positive value of the signal U X , for example, will, as far as the summation circuit 8 is concerned, act in the same way as a reduction of the control signal U S and hence cause an increase of the control angle for the thyristor T1. A positive signal U X , on the other hand, will, as far as the summation circuit 9 is concerned, act as an increase of a control signal U S and hence cause a reduction of the control angle for the thyristor T2. In a corresponding way, a negative value of the signal U X causes a reduction of the control angle of the thyristor T1 and an increase of the control angle of the thyristor T2. Thus, the influence on the control, caused by the signal U X , gives rise to an unsymmetry between positive and negative half cycles of the capacitor voltage. This unsymmetry results in a mean value of the capacitor voltage different from zero, and this mean value depends on and may be controlled by the signal U X . Figure 6 shows how a detected SSR component U X , via the firing of the thyristors, controls the mean value of the capacitor voltage. The control signal U S is assumed to be somewhat lower than the peak value of the voltage U OSC , which corresponds to the control angle β O . The curve shape of the capacitor voltage and the control pulses for the case where U X = 0 (no SSR oscillation) are shown in shaded lines in Figure 6. The corresponding signals and the voltage U X and the current pulses I L through the inductor L are shown in unbroken curves in Figure 6 for the case where an SSR oscillation is detected, that is, the signal U X ≠ 0. The reactance at power system frequency of the inductor L is assumed to be considerably smaller than the reactance of the capacitor bank at the power system frequency, which means that the natural frequency of the oscillation circuit L-C is considerably higher than the system frequency of the network. Upon firing of the thyristor T1, the oscillation circuit just mentioned performs a half-cycle of a natural oscillation, which entails a reversal of the voltage of the capacitor C from the positive value it had when the thyristor was fired to a negative value of, in principle, the same magnitude. The current pulse occurring during the reversal of the voltage is shown at the bottom of Figure 6. During the first part of the time interval shown in Figure 6, the signal U X is positive. This means that the control pulses to the thyristor T1 are somewhat advanced in time and the control pulses to the thyristor T2 are somewhat retarded in time in relation to the cases where U X = 0. This causes the capacitor voltage to swing over more in the negative direction upon firing of T1 and then not to swing back to the same extent upon firing of T2. Thus, the capacitor voltage swings over more in the negative direction than in the positive direction during the positive intervals for U X . During the negative intervals for U X , the control pulses to T2 are advanced in time and the control pulses to T1 are retarded in time. This causes the capacitor voltage during these intervals to swing over more in the positive direction than in the negative direction. Thus, in the manner now described, the mean value of the capacitor voltage U C will be controlled by the detected SSR component U X . As will be clear from Figure 6, a positive value of U X gives a displacement of the mean value of the capacitor voltage in the negative direction and a negative value of U X gives a displacement in the positive direction of the mean value of the capacitor voltage. Thus, the capacitor voltage will contain a subsynchronous component with the same frequency as the signal U X . In practice, this component will be somewhat offset in phase in relation to the signal U X . By using suitable filters for the formation of the signal U X , this signal may be given such a phase position that the generated subharmonic component of the capacitor voltage is in opposition to the SSR oscillation. In this way, a subsynchronous resonance oscillation may be effectively counteracted and dampended. The equipment described has an inherent damping effect on the SSR oscillation even if the detection of the signal U X (units 5, 6, 7 in Fig. 4) is omitted. In Figure 7 it is assumed that U S is equal to the peak value of U OSC , that is, that the control angle β = 0. The broken-lined curve shows the capacitor voltage in the absence of any SSR oscillation. At t = t0 it is assumed that an SSR oscillation suddenly occurs and displaces the capacitor voltage U C in the positive direction (unbroken line). At t = t1, U C would have been zero but now has a positive value because of the SSR oscillation. Because the thyristor T1 is fired at t = t1, U C is reversed to the corresponding negative value. During the subsequent negative half-cycle, because of the SSR oscillation a continued displacement of U C in a positive direction takes place so that, at t = t2, U C is again zero. By the firing of T1 at t = t1, a displacement of U C in a negative direction is obtained, which cancels the positive displacement caused by the SSR oscillation. The result is a damping of the SSR oscillation. In the embodiment described with reference to Figure 7, the capacitor voltage may alternatively be sensed at the zero crossings of the fundamental frequency component and the firing be delayed until the voltage has dropped to a certain fraction, for example 50%, of the sensed value. Common to the described embodiments of the invention is that the control angle of the semiconductor valves during the two half-cycles of at least certain pairs of consecutive half-cycles of the capacitor voltage assumes separate values, the control angle being referred to the total capacitor voltage (the sum of the fundamental component and the harmonic components). As will be clear from the description above, with the aid of the invention series capacitor equipment can be obtained which provides a possibility of efficient damping of subsynchronous resonance oscillations. According to the invention, this can be accomplished, in principle, without any consumption of active energy, which results in small dimensions of the equipment, a low price, and a minimum of cooling requirement. The pieces of equipment described above are only examples of embodiments according to the invention and a large number of other variants are feasible within the scope of the invention. Thus, in Figure 6, the reversal of the capacitor voltage is schematically shown in vertical lines, indicating a natural oscillation frequency of the reversal circuit which is much higher than the system frequency of the network. In practice, of course, the natural frequency of the reversal circuit may be chosen lower than what has been indicated in Figure 6. For the invention to provide the desired function, however, it has proved that the natural frequency of the reversal circuit should be at least five times as great as the system frequency of the power network. Expressed in other terms, this means that the reactance of the inductor L at the system frequency of the network should be at least twenty-five times lower than the reactance of the capacitor C at the system frequency.

Claims: Series capacitor equipment with a capacitor bank (C) adapted for connection in series into an electric power line (A) and with a controllable circuit (L, T1, T2) connected in parallel with said capacitor bank, said controllable circuit including inductor means, controllable in both directions and comprising at least one inductive component (L) and a controllable semiconductor valve (T1, T2) connected to said component (L), characterized in that the equipment comprises control members (CU) adapted, on the occurrence of a subsynchronous resonance oscillation in the line voltage, to control said semiconductor valve (T1, T2) with control angle values, which are asymmetrical with respect to the two half-cycles of the line voltage, such that a subharmonic voltage is generated across the capacitor bank, which subharmonic voltage counteracts said subsynchronous resonance oscillation in the line voltage. Series capacitor equipment according to claim 1, characterized in that the inductive component (L) has an inductance value which is so chosen in relation to the capacitance of the series capacitor bank (C) that the oscillation circuit formed by the inductive component and the capacitor bank has a natural frequency which is at least five times greater than the system frequency of the power line. Series capacitor equipment according to claim 1 or 2, characterized in that the controllable circuit has an inductor (L) connected in series with a bidirectional controllable semiconductor valve (T1, T2). Series capacitor equipment according to claim 3, characterized in that the semiconductor valve comprises two antiparallel-connected thyristor valves (T1, T2). Series capacitor equipment according to claim 1 or 2, characterized in that the controllable circuit (L, T1, T2) comprises two inductors (L1, L2), each connected in series with a semiconductor valve (T1, T2), said semiconductor valve being connected oppositely in relation to each other, and that these two series connections of an inductor and a semiconductor valve are connected in parallel with each other. Series capacitor equipment according to any of claims 1 to 5, characterized in that the inductor means and/or and the semiconductor valves are connected to the capacitor bank (C) via a transformer (TR), whereby the inductor means may consist merely of the stray reactance of the transformer (TR). Series capacitor equipment according to any of the preceding claims, characterized in that the control members are adapted, on the occurrence of a subsynchronous resonance oscillation, to control the semiconductor valves with different control angle values relative to the total capacitor voltage (U c ) during the two half-cycles of at least certain pairs of consecutive half-cycles of the capacitor voltage (U c ) for generation of the subharmonic voltage counteracting the resonance voltage. Series capacitor equipment according to any of the preceding claims, characterized in that it comprises control members (CU) adapted to make the controllable semiconductor valves conductive at the zero crossings of the fundamental frequency component of the capacitor voltage. Series capacitor equipment according to any of claims 1 to 7, characterized in that it comprises control members adapted to control the controllable semiconductor valves with a variable control angle (β) relative to the fundamental frequency component (U CO ) of the capacitor voltage. Series capacitor equipment according to claim 9, characterized in that the control members are adapted to form a signal (U X ) corresponding to a subharmonic oscillation in the capacitor voltage and, at each firing of a controllable semiconductor valve, to determine the control angle relative to the fundamental frequency component (U CO ) of the capacitor voltage in dependence on the current value of said signal.

Inventors: KLERFORS BERTIL, KLERFORS, BERTIL
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1991
Main classification: H02J3
IPC codes: H02J3