MMC-STATCOM Connected to a 735-kV Transmission System

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 power modules (PM) per arm. The three MMC arms are delta-connected to an 18kV/735kV power transformer.

Principles 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 and absorbs reactive power. When the amplitude of the converter voltage is higher than that of the bus voltage, the STATCOM acts like a capacitor and generates reactive power.

Transmission System

The 60 Hz power system is a simplified model of a series-compensated 735 kV transmission system. The grid is both series- and shunt-compensated using fixed capacitors and inductors. It comprises the following main components: - Thirty-two three-phase buses - Six power plants. Each power plant is represented by a synchronous machine, an hydraulic turbine and governor, an excitation system, and a power system stabilizer (PSS) - Six three-phase power transformers - Twenty three-phase shunts and loads - Seventeen 735 kV distributed parameters lines. Eleven lines are series-compensated by capacitors in order to increase the transmission capability (compensation factor = 15% to 42% of the line reactance). - 50 MVA STATCOM with twenty-two cascaded full-bridge converters per arm.

Control System

The STATCOM control system consists of the following main subsystems: - Measurements computes the d-axis and q-axis components of the primary voltages and currents by performing an abc-dq transformation in the synchronous reference provided by a three-phase PLL synchronized on the primary voltages. - Reactive Power Regulator controls the reactive power absorbed or generated by the STATCOM by determining the required reactive current reference value (Iq_ref). - DC Voltage Regulator maintains the average value for all capacitors PM to the specified reference value (Vnom_PM = 1600V). To perform this task, the regulator controls the active power absorbed or generated by the STATCOM by determining the required active current reference value (Id_ref). - Current Regulator generates the converter output voltage signals (Vd,Vq) in order to have measured current values (Id/Iq) equal to the reference values (Id_ref/Iq_ref). - Insertion Indices Computation determines the insertion indices (n) using the nearest-level modulation technique. The indices are calculated based on the current regulator outputs (Vd,Vq), and the number and nominal voltage of the capacitors power modules. - Power Modules Selection selects which power modules will be inserted or removed based on the requested insertion indices and on a voltage balancing algorithm. The voltage balancing algorithm is based on sorting the capacitor voltages and on measuring the polarity of the current flowing into the arm. Double-click on the subsystem to see how the algorithm is implemented.

Simulation

The simulation starts in steady state with the STATCOM operating at 0 Mvar. At 0.1 seconds, the set-point Qref value is changed to -30 Mvar (inductive var). At 0.3 seconds, the set-point changes from -30 Mvar to 40 Mvar (capacitive var). The STATCOM control system reacts to modify the inverter output voltage in order to follow the new Qref set-point. At 0.5 seconds a three-phase fault at the SAG7 substation force the STATCOM operation.

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] Mattia Ricco, Laszlo Mathe, Manel Hammami, Francesco Lo Franco, Claudio Rossi and Remus Teodorescu, A Capacitor Voltage Balancing Approach Based on Mapping Strategy for MMC Applications Electronics Open Access Journal, April 2019