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MMC-STATCOM with 22 Power Modules per Phase

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This example shows a 12 MVA, 34.5 kV Static Synchronous Compensator using 22 power modules per phase

Pierre Giroux (Hydro-Quebec)

Description

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.

Principle of Operation

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.

Simulation 1: Dynamic Response

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).

Simulation 2: Low-Level Control - Capacitor Voltage Balancing

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 capacitor voltages 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. Note: The individual capacitor voltages balancing control generates a small voltage in phase/180 degrees phase-shift with the current flowing into the arm in order to produce/absorb active power from each capacitor. This control system will then not work properly if the current (capacitive or inductive) flowing into the capacitors is too low to produce sufficient active power to charge/discharge them. For this reason, the Qref_Limits subsystem does not permit Qref values between -0.75 and +0.75 Mvar. See reference [2] for more details on this control. Other voltage balancing approaches, such as a sorting algorithm, do not have this limitation. This method is used in the example power_Grid_STATCOM.

References

[1] M. Pereira, Member, IEEE, D. Retzmann, Member, IEEE, J. Lottes, M. Wiesinger, G. Wong, SVC PLUS: An MMC STATCOM for Network and Grid Access Applications 2011 IEEE Trondheim PowerTech

[2] Hirofumi Akagi ; Shigenori Inoue ; Tsurugi Yoshii, Control and Performance of a Transformerless Cascade PWM STATCOM with Star Configuration IEEE Transactions on Industry Applications (Volume: 43, Issue: 4 , July-august 2007)