This example shows the operation of a +300 Mvar/-100 Mvar Static Var Compensator (SVC).
Pierre Giroux and Gibert Sybille (Hydro-Quebec)
A 300-Mvar Static Var Compensator (SVC) regulates voltage on a 6000-MVA 735-kV system. The SVC consists of a 735kV/16-kV 333-MVA coupling transformer, one 109-Mvar thyristor-controlled reactor bank (TCR) and three 94-Mvar thyristor-switched capacitor banks (TSC1 TSC2 TSC3) connected on the secondary side of the transformer. Switching the TSCs in and out allows a discrete variation of the secondary reactive power from zero to 282 Mvar capacitive (at 16 kV) by steps of 94 Mvar, whereas phase control of the TCR allows a continuous variation from zero to 109 Mvar inductive. Taking into account the leakage reactance of the transformer (15%), the SVC equivalent susceptance seen from the primary side can be varied continuously from -1.04 pu/100 MVA (fully inductive) to +3.23 pu/100 Mvar (fully capacitive). The SVC controller monitors the primary voltage and sends appropriate pulses to the 24 thyristors (6 thyristors per three-phase bank) in order to obtain the susceptance required by the voltage regulator.
Use Look under Mask to see how the TCR and TSC subsystems are built. Each three-phase bank is connected in delta so that, during normal balanced operation, the zero-sequence tripplen harmonics (3rd, 9th... ) remain trapped inside the delta, thus reducing harmonic injection into the power system. The power system is represented by an inductive equivalent (6000 MVA short circuit level) and a 200-MW load. The internal voltage of the equivalent can be varied by means of programmable source in order to observe the SVC dynamic response to changes in system voltage. Open the voltage source menu and look at the sequence of voltage steps which are programmed.
Dynamic response of the SVC
Run the simulation and observe waveforms on the SVC scope block. The SVC is in voltage control mode and its reference voltage is set to Vref=1.0 pu. The voltage droop of the regulator is 0.01 pu/100 VA (0.03 pu/300MVA). Therefore when the SVC operating point changes from fully capacitive (+300 Mvar) to fully inductive (-100 Mvar) the SVC voltage varies between 1-0.03=0.97 pu and 1+0.01=1.01 pu.
Initially the source voltage is set at 1.004 pu, resulting in a 1.0 pu voltage at SVC terminals when the SVC is out of service . As the reference voltage Vref is set to 1.0 pu, the SVC is initially floating (zero current). This operating point is obtained with TSC1 in service and TCR almost at full conduction (alpha=96 degrees). At t=0.1s voltage is suddenly increased to 1.025 pu. The SVC reacts by absorbing reactive power (Q=-95 Mvar) in order to bring the voltage back to 1.01 pu. The 95% settling time is approximately 135 ms. At this point all TSCs are out of service and the TCR is almost at full conduction (alpha = 94 degrees) . At t=0.4 s the source voltage is suddenly lowered to 0.93 pu. The SVC reacts by generating 256 Mvar of reactive power, thus increasing the voltage to 0.974 pu. At this point the three TSCs are in service and the TCR absorbs approximately 40% of its nominal reactive power (alpha =120 degrees). Observe on the last trace of the scope how the TSCs are sequentially switched on and off. Each time a TSC is switched on the TCR alpha angle changes suddenly from 180 degrees (no conduction) to 90 degrees (full conduction). Finally, at t=0.7 s the voltage is increased to 1.0 pu and the SVC reactive power is reduced to zero.
Misfiring of TSC1
Each time a TSC is switched off a voltage remains trapped across the TSC capacitors. If you look at the 'TSC1 Misfiring' scope inside the "Signals and Scope" subsystem you can observe the TSC1 voltage (first trace) and the TSC1 current (second trace) for branch AB. The voltage across the positive thyristor (thyristor conducting the positive current) is shown on the 3rd trace and the pulses sent to this thyristor are shown on the 4th trace. Notice that the positive thyristor is fired at maximum negative TSC voltage, when the valve voltage is minimum. If by mistake the firing pulse is not sent at the right time, very large overcurrents can be observed in the TSC valves.
Look inside the SVC Controller block how a misfiring can be simulated on TSC1. A Timer block and a OR block are used to add pulses to the normal pulses coming from the Firing Unit. Open the Timer block menu and remove the 100 multiplication factor. The timer is now programmed to send a misfiring pulse lasting one sample time at time t= 0.121 s. Restart simulation. Observe that the misfiring pulse is sent when the valve voltage is maximum positive immediately after the TSC has blocked. This thyristor misfiring produces a large thyristor overcurrent (18 kA or 6.5 times the nominal peak current). Also, immediately after the thyristor has blocked, the thyristor voltage reaches 85 kV (3.8 times the nominal peak voltage). In order to prevent such overcurrents and overvoltages, thyristor valves are normally protected by metal oxide arresters (not simulated here).