This example shows a 12 MVA, 34.5 kV Static Synchronous Compensator using 22 power modules per phase
Pierre Giroux (Hydro-Quebec)
In electrical grids, shunt compensation is often used for reactive power and voltage control. This example models a shunt compensation device that is increasingly used in modern grids: the Modular-Multi-level(MMC) STATCOM. The MMC-STATCOM is built using the Full-Bridge MMC block to represent a power electronics converter of 22 modules per phase. In order to speed-up simulation while keeping simulation fidelity, the Switching Function model is selected.
The STATCOM can absorb or generate reactive power. The reactive power transfer is done through the phase reactance. The converter generates a voltage in phase with the grid voltage. When the amplitude of the converter voltage is lower than that of the grid voltage, the STATCOM acts like an inductance absorbing reactive power. When the amplitude of the converter voltage is higher than that of the bus voltage, the STATCOM acts like a capacitor generating reactive power.
Run the simulation and observe waveforms on Scope1. You can see that the simulation starts in steady state and that the STATCOM operates in inductive mode following its set-point Qref (-5 Mvar ). At 0.1 second, the set-point changes from -5 to +10 Mvar. The STATCOM control system reacts very rapidly to modify the inverter output voltage in order to generate 10 Mvar of reactive power (capacitive mode).
In order to analyze the operation of the low-level control (DC voltage balancing) run the simulation with the following changes:
1. Set the simulation time to 2 seconds.
2. Set the initial value of Qref (red Step block) to 10e6. With this modification, the setpoint will not change during the simulation.
3. Set the Switching time of the red Breaker block to 1 second. This will produce a power unbalance at 1s by switching-in a small resistive load on phase C only.
4. Run the simulation and observe some of the capacitors voltage on the Vc's & Vc_mean Scopes, located inside the green Additional scopes subsystem. You can see that the low-level control system performs well to maintain the capacitor voltages balanced.
5. Double-click on the blue Low-Level Control block and turn-off the Phase Balancing regulator of the DC voltage balancing system. Run the simulation and notice that the STATCOM DC voltages balance between phases is lost when the load is switched-in on phase C.