This example shows a Static Synchronous Series Compensator (SSSC) used for power oscillation damping.
Pierre Giroux and Gibert Sybille (Hydro-Quebec)
The Static Synchronous Series Compensator (SSSC), one of the key FACTS devices, consists of a voltage-sourced converter and a transformer connected in series with a transmission line. The SSSC injects a voltage of variable magnitude in quadrature with the line current, thereby emulating an inductive or capacitive reactance. This emulated variable reactance in series with the line can then influence the transmitted electric power. The SSSC is used to damp power oscillation on a power grid following a three-phase fault.
The power grid consists of two power generation substations and one major load center at bus B3. The first power generation substation (M1) has a rating of 2100 MVA, representing 6 machines of 350 MVA and the other one (M2) has a rating of 1400 MVA, representing 4 machines of 350 MVA. The load center of approximately 2200 MW is modeled using a dynamic load model where the active & reactive power absorbed by the load is a function of the system voltage.The generation substation M1 is connected to this load by two transmission lines L1 and L2. L1 is 280-km long and L2 is split in two segments of 150 km in order to simulate a three-phase fault (using a fault breaker) at the midpoint of the line. The generation substation M2 is also connected to the load by a 50-km line (L3). When the SSSC is bypassed, the power flow towards this major load is as follows: 664 MW flow on L1 (measured at bus B2), 563 MW flow on L2 (measured at B4) and 990 MW flow on L3 (measured at B3).
The SSSC, located at bus B1, is in series with line L1. It has a rating of 100MVA and is capable of injecting up to 10% of the nominal system voltage. This SSSC is a phasor model of a typical three-level PWM SSSC. If you open the SSSC dialog box and select "Display Power data", you will see that our model represents a SSSC having a DC link nominal voltage of 40 kV with an equivalent capacitance of 375 uF. On the AC side, its total equivalent impedance is 0.16 pu on 100 MVA. This impedance represents the transformer leakage reactance and the phase reactor of the IGBT bridge of an actual PWM SSSC.The SSSC injected voltage reference is normally set by a POD (Power Oscillation Damping) controller whose output is connected to the Vqref input of the SSSC. The POD controller consists of an active power measurement system, a general gain, a low-pass filter, a washout high-pass filter, a lead compensator, and an output limiter. The inputs to the POD controller are the bus voltage at B2 and the current flowing in L1. Use the Edit/Look under mask" menu to see how the controller is built.
1. SSSC Dynamic Response
We will first verify the dynamic response of our model. Open the "Step Vqref" block (the red timer block connected to the "Vqref" input of the POD Controller).This block should be programmed to modify the reference voltage Vqref as follows: Initially Vqref is set to 0 pu; at t=2 s, Vqref is set to -0.08 pu (SSSC inductive); then at t=6 s, Vqref is set to 0.08 pu (SSSC capacitive). Double-click on the POD Controller block and set the POD status parameter to "off". This will disable the POD controller. Also, make sure that the fault breaker will not operate during the simulation (the parameters "Switching of phase A, B and C" should not be selected).
Run the simulation and look at Scope1. The first graph displays the Vqref signal (magenta trace) along with the measured injected voltage by the SSSC. The second graph displays the active power flow (P_B2) on line L1, measured at bus B2. We can see that the SSSC regulator follows very well the reference signal Vqref. Depending on the injected voltage, the power flow on line varies from 575 to 750 MW. In a real system the reference signal Vqref would typically be changed much more gradually in order to avoid the oscillation we see on the transmitted power (P_B2 signal). Double-click on the SSSC block and select "Display Control parameters". Modify the "Maximum rate of change for Vqref (pu/s)" parameter from 3 to 0.05. Rerun the simulation. The power oscillation on the active power should now be very small.
2. SSSC damping power oscillation
We will now compare the operation of our SSSC with and without POD control. Open the "Step Vqref" block and multiply by 1000 the time vector in order to disable the Vqref variations. Double-click on the fault breaker and select the parameters "Switching of phase A, B and C" to simulate a three-phase fault. The transition times should be set as follows: [ 20/60 30/60]+1; this means that the fault will be applied at 1.33 s and will last for 10 cycles. Run a simulation and observe the power oscillation on the L1 line (second graph on Scope1) following the three-phase fault.
Now, you will run a second simulation with the POD controller in operation. Double-click on the POD Controller block and set the POD status parameter to "on". Start the simulation. Looking again at the second graph on Scope1(P_B2 signal), we can see that the SSSC with a POD controller is a very effective tool to damp power oscillation. To see a figure showing a comparison of the SSSC operation with and without POD control, double-click on the blue block on the bottom right of the model.