GB2105885A - Static var generation for network stabilisation - Google Patents

Abstract

Subsynchronous resonance in a network is controlled by monitoring the frequency of an electrical generating system 10 and controlling the resonance by phase-firing thyristors to insert compensating inductance L into the network. The frequency is monitored by measuring half-periods and comparing with a standard frequency, using pulse counting to establish an error signal. The network voltage may also be stabilised by controlling the firing of the thyristors. <IMAGE>

Description

SPECIFICATION Static var generation for transmission line compensation This invention relates to means utilized for reducing subsynchronous resonance in synchronous machines and more particularly is related to means for reducing subsychronous resonance in synchronous machines for AC power system stability, and automatic voltage regulation.

In the analysis of power systems stability, it is well known that a synchronous machine connected to an infinite bus will oscillate or "hunt" under certain circuit conditions. In turbine driven generators, periodic variations in the torque applied to the generators caused periodic variations in speed or rotor oscillations. This results in periodic variations in voltage and frequency which are transmitted to the electrical power generating system.

These mechanical periodic variations in torque, ZM, (mechanical resonance frequency), cause modulation of the generator voltage which results in small side band components of the electrical system nominal electrical frequency, o,. It has been found that when a synchronous machine supplies power to a long transmission line to which series capacitors are connected for voltage regulation, the resultant line reactance may have a resonant frequency that may match the mechanical resonance frequency, SM that greatly amplify the rotor oscillations (mechanical resonance frequency SM) referred to as negative damping. In this case, the lower, or subsynchronous side band component may cause an extremely high current to flow in the electrical system.This high current may feed back magnetically through the air gap of the generator so as to excite further the oscillation of the rotating mechanical apparatus and may cause considerable physical damage such as shaft breakage in the generator.

Although various methods have been proposed for stabilizing an electrical generating system, in a known instance a delta-connected, thyristor-controlled, three-phase reactor bank is employed. The current in the reactor bank is modulated according to the torsional oscillation of the rotating mechanical system using the usual technique of thyristor conduction angle control. The modulating signal used to control the thyristors is derived by measuring the velocity variation of the generator shaft, using some mechanically coupled device, such as a tooth-wheel pick-up.

According to the present invention, a static VAR generator and network stabilizer comprises a reactance means disposed for connection into an AC network, a frequency monitoring means for detecting subsynchronous resonance in said AC network, a control means connected to said reactance means and said frequency monitoring means for connection of said reactance means into said AC network to damp subsychronuous resonance in said AC network.

Conveniently, this invention provides a static VAR generator means that provides the capability to damp subsychronous resonance should it occur. A means for controlling subsynchronous resonance is provided by monitoring the frequency of the electrical generation system and controlling subsynchronous resonance in response to disturbances caused by changes in load, switching, pulsating driving torque, self-excitation, or other power to a long transmission line.

Advantageously a static VAR generator means provides voltage regulation in combination with the capability to damp subsynchronous resonance should it occur. This double utilization of the static VAR generator is clearly economically both from the standpoint of initial capital investment and reducing operating losses by providing complete power disturbances that may result in voltage and frequency pulsations. A means for controlling subsynchrous resonance is provided by monitoring the frequency of the electrical generation system and controlling in response to disturbances caused by changes in load, switching, pulsating driving torque, self excitation, or other disturbances that may result in voltage and frequency pulsations.

The invention will now be described, by way of example, with reference to the following drawings: Figure 1 is a schematic system arrangement for a static VAR generator used for damping of subsychronous resonance; Figure 2 is a block diagram of the control arrangement for subsynchronous resonance damping; Figure 3a-3d are waveforms illustrating the operation of the proposed controls shown in Fig. 2 at frequencies lower than the nominal frequency; Figure 4a-4d are waveforms illustrating the operation of the proposed controls shown in Fig. 2 at frequencies higher than the nominal frequency; Figures 5a and 5b are schematic diagrams and operating waveforms of the zero crossing pulse generator; Figures 6a and 6b are the schematic diagrams and operating waveforms typical of the half period reference generators;; Figures 7a-7c are the schematic block diagrams and operating waveforms for the error pulse generator and integrator.

Figure 8 is a schematic system arrangement for a static VAR generator used for both terminal voltage regulation and damping of subsynchronous resonance; Figure 9 is the schematic diagram for the error amplifier with analog switch; and Figure 10 is the schematic diagram for the switch activating circuit.

Fig. 1 shows a VAR generator scheme. The electrical system comprises a turbine driven AC generator 10 feeding a transmission line via a step-up transformer 11. The receiving end of the transmission line is terminated by an infinite bus that represents the remaining part of the power system. The inductance of the transmission line is partially compensated by a seried connected capacitor CT. At an intermediate terminal of the transmission line, a static VAR generator consisting of a three phase thyristor-controlled reactor, L, is con nected via a step-down transformer 1 2. The current in the reactor, L, is controlled by delaying the firing pulse with respect to the time reference at which the applied AC voltage is maximum.The appropriate delay of the firing pulses, in response to an analog control signal is provided by the firing pulse generator 1 3. Possible realization of the firing pulse generator is described in U.S. Patent No. 3,999,117, entitled "Method and Appa ratus for Static VAR Generator and Compensator". The control signals that initiate the firing of the pulse generator 1 3 are derived by positive and negative half-period measuring circuits 14 and generators 1 5. An error pulse generator 1 6 develops an error signal from the two half period measuring circuits.An error integrator 1 7 develops a voltage signal proportional to the frequency of the AC network and initiates firing of the pulse generator 1 3 from a quiscent value determined by the bias signal supply 18.

The basic idea of the control arrangement to damp subsynchronous resonance is to mea sure the half cycle period times of each of the three terminal voltages and compare these to a reference half cycle period corresponding to unmodulated 60 Hz. terminal voltage. Referring to Figs. 2, 3 and 4, there is shown at the zero crossings of the terminal voltage, Vab, Vbc, Vca, pulses PTp(ab) (positive going zero crossing) and, PTN(ab) (negative-zero crossings) and, PTP(bc) and PTN(bc) and PTp(Ca) and, PTN(ca) respectively, produced by zero crossing pulse generator 1 9. Each zero crossing pulse initiates the generation of the reference half periods, TP(ab) and TN(ab), TP(bc) and TN(bC) TP and TN(caX, each of which is derived from a precision high-frequency clock pulse generator 20 shown in Fig. 2.The half period reference generator produce pulses RTp(ab) and PTN(ab), TTP(bc) and RTN(bC) RTp(ca) and RTN(ca) The time difference between the reference half periods and the actual one marked by the pulses RTP(ab) PTN(ab) and RTN (bc) PTP(bc)t RTP(ca) PTN(ca) and RTN(Ca) PTP(Ca) is the time error representing the variation of the steady state period time.This time difference or time error for a preterminal voltage is represented by pulses Eab, EbC, and Eca of constant magnitude and a polarity that indicates whether the actual half period time is longer, that is the generator frequency is lower (positive polarity) or shorter, that is, the generator frequency is higher (negative polarity), than the reference half period as illustrated in Figs. 3 and 4. By integrating these error pulses, a voltage proportional to the frequency (and rotor velocity) change is obtained. As illustrated in Fig. 1, this voltage can be used to modulate the firing delay of thyristor from a quiescent value (determined by the output voltage level of the bias signal supply) and thereby insert effective inductance to damp subsynchronous resonance.

The operation of the proposed control scheme is further explained by the more detailed functional diagrams in operating waveforms shown in Figs. 5, 6, and 7. Fig. 5 illustrates for phase AB the derivation of the pulses PTp(ab) and PTN(ab) at the positive-going and, respectively the negative-going zero crossings of the terminal voltage Vab utilizing comparator 30 as shown.

Fig. 6 illustrates the derivation of the reference half period, TP(ab), and the corresponding pulse RTp(ab, and the corresponding pulse, RTP)ab(, using the high frequency clock pulse generator 20 and a gated counter 21. The gate of the counter 21 is opened by the zero crossing pulse PTp(ab) and pulses of the high frequency clock pulse generator 20, appearing at precisely defined time intervals (for example, at every 1/12,000 interval of the half-period corresponding to 60 Hz.), are counted. When the count reaches the number corresponding to the reference half period (in the example used, 12,000), the last pulse closes the gate, and the counter stops until the next PTp(ab) pulse restarts the process again.

Fig. 7 illustrates a possible logic arrangement to derive the polarized error pulses representing the time difference between the reference half period and the measured one.

The derivation of the error integral is also shown. The error integral is provided as an analog voltage signal, whose magnitude and polarity follows precisely the frequency variation of the terminal voltage. The high frquency clock pulse generator 20 can be a precision crystal oscillator resonating at some multiple of the 60 Hz. generator frequency (for example, at 2 x 12,000 X 60 = 1.44 MHz). Because of the different steady-state accuracy between a frequency regulator of the generator and the crystal oscillator, a constant steady-state error may develop. To prevent the integrator to respond to slow or steady-state (dc) error, its dc gain has to be reduced. This can be accomplished, for example, by the simple T-network consisting of a capacitor and two resistors, connected across the integrator's capacitors, as illustrated in Fig. 7a. The high frequency clock pulse generator 20 can also be realized by a phase-locked loop incorporating a high frequency voltage control oscillator. Making the response of the phase locked loop rather slow, the frequency of the voltage control oscillator will correspond to the main frequency of the generator voltage and will not follow the subsynchronism modulation caused by the torsional oscillation.

Since the frequency of the voltage control oscillator in the phase-lock loop represent the exact multiple of the steady-state generator frequency, the previously described dc gain reduction for the error integrator is not necessary.

Fig. 8 shows the arrangement of the double purpose VAR generator scheme. The electrical system comprises a turbine driven AC generator 110 feeding a transmission line via a stepup transformer 111. The receiving end of the transmission line is terminated by an infinite bus that represents the remaining part of the power system. The inductance of the transmission line is partially compensated by a series connected capacitor Ct. At an intermediate terminal of the transmission line, a static VAR generator consisting of a three-phase thyristor-controlled reactor, L, and a threephase power factor correcting capacitor, C, is connected via a stepdown transformer 11 2.

The current in the reactor, L, is controlled by delaying the firing pulse with respect to the time reference at which the applied AC voltage is maximum. The appropriate delay of the firing pulses, in response to an analog control signal is provided by the firing pulse generator 11 3. Possible realization of the firing pulse generator is described in the specification of U. S. Patent No. 3,999,117. The control signals that initiate the firing of the pulse generator 11 3 for subsynchronous resonance are derived by positive and negative half-period measuring circuits 114 and generators 11 5. An error pulse generator 11 6 develops an error signal from the two halfperiod measuring circuits.An error integrator 11 7 develops a voltage signal proportional to the frequency of the AC network and initiates firing of the pulse generator 11 3 from a quiescent value determined by the bias signal supply 118.

At the right-hand side of Fig. 8, is shown schematically the control arrangement used to regulate the terminal voltage. This is described in the specification of U.S. Patent No.

4,1 56,1 76, of generating an error-signal from the difference of the measured terminal voltage as determined by the voltage measuring circuits 1 22 and a reference voltage, then amplifying appropriately this error by the error amplifier 1 23 feeding the firing pulse generator 11 3. In the proposed arrangement, the error amplifier is compensated with an electronic analog switch shunting the output to input feedback resistor, as illustrated in Fig.

9. When the switch is open the error amplifier has the normal gain needed to regulate the terminal voltage. When the switch is closed the gain of the amplifier is greatly reduced and thus the output current of the VAR generator will not respond significantly to the level, or variation of the terminal voltage. The electronic switch is driven by a switch activator circuit 1 25 which compares to rectified and filtered value of an input signal to a reference level by comparator 131, as illustrated in Fig.

10. Whatever the reference level is exceeded the switch activator circuit closes the electronic switch, thereby reducing the gain of the voltage error amplifier.

At the left-hand side of Fig. 8 is shown schematically the control arrangement to damp subsynchronous resonance. The basic idea is to measure the half cycle period times of each of the three terminal voltages and compare these to a reference half cycle period time corresponding to the unmodulated 60 Hz. terminal voltage.

From the foregoing, it is readily seen that there is described a means to damp subsynchronous resonance without directly measuring the shaft velocity change of the generator.

A novel method is devised to measure, with a negligible time delay, the frequency variation of the terminal voltage, and to generate a corresponding signal to modulate the conduction angles of the thyristor in the VAR generator.

Claims (6)

1. A static VAR generator and network stabilizer comprising a reactance means disposed for connection into an AC network, a frequency monitoring means for detecting subsynchronous resonance in said AC network, a control means connected to said reactance means and said frequency monitoring means for connection of said reactance means into said AC network to damp subsynchronous resonance in said AC network.

2. A static VAR generator and network stabilizer as claimed in claim 1 wherein said control means includes a network voltage frequency pulse generator, a high frequency reference clock pulse generator, an error pulse generator responsive to deviations in the network voltage frequency pulse generator and the high frequency reference clock pulse generator, an error pulse integrator disposed to produce a voltage proportion to the frequency of said subsynchronous resonance, and a firing pulse generator responsive to said error pulse integrator to modulate firing delays of thyristors disposed to insert said reactance means to dampen said subsynchronous resonance.

3. A static VAR generator and network stabilizer as claimed in claim 2 wherein said high frequency reference pulse generator is a crystal oscillator resonating at some multiple of the network nominal frequency.

4. A static VAR generator and network stabilizer as claimed in claim 2 wherein said high frequency reference pulse generator is a phase-locked loop incorporating a high frequency voltage controlled oscillator.

5. A static VAR generator and network stabilizer, as claimed in any one of claims 1 to 4 including a VAR monitoring means for monitoring the reactive requirement of said AC network, the control means connected to said reactance means and said VAR monitoring means for connection of said reactance means into said AC network in response to the reactive requirements of said AC network, and said control means connected to said reactance means and said frequency monitoring means for connection of said reactance means into said AC network to dampen subsynchronous resonance in said AC network.

6. A static VAR generator, constructed and adapted for use, substantially as hereinbefore described and illustrated with reference to the accompanying drawings.