Research on an Asymmetric Fault Control Strategy for an AC/AC System Based on a Modular Multilevel Matrix Converter

Abstract

This paper studies control strategies for an AC/AC system based on a modular multilevel matrix converter (M3C) when an asymmetric fault occurs in the secondary side ac system. Firstly, the operating principle of M3C is briefly introduced and verified by simulation. Then, based on its mathematical model by double αβ0 transformation, the decoupled control strategies for the primary side and secondary side systems are designed. In view of the asymmetric fault condition of the secondary side system, the positive sequence and negative sequence components of voltages and currents are separated and extracted, and then a proportional resonant controller (PR) is used to regulate the positive and negative sequence currents at the same time to realize decoupled current control in the αβ reference frames. The capacitor voltage balancing control, which consists of an inter-subconverter balancing control and an inner-subconverter balancing control, is realized by adjusting four circulating currents. Finally, the proposed control strategy is validated by simulation in the PSCAD/EMTDC software (Manitoba HVDC Research Center, Canada). The result shows that during the period of the BC-phase short-circuit fault occurring in the secondary side system, the whole system can still operate stably and transmit a certain amount of active power, according to their set values. Furthermore, the capacitor voltages are balanced, with a slight increase during the fault period. The simulation results verify the effectiveness of the proposed control strategy.

Comprehensive optimization framework for transmission expansion planning with fractional frequency transmission system

“…Based on the mathematical model of M3C [15] and the design of main circuit parameters [11], in engineering practice, M3C is often equivalently represented as a back‐to‐back Voltage Source Conver inverter in steady state. According to the operational data of the M3C fractional frequency transmission demonstration project already built in China, the loss rate of M3C is less than 1%, which can be ignored for planning purposes.…”

“…A mathematical model of the M3C converter was derived based on the extended MMC topology and the multiple αβ0 and dq transformations. On this basis, a control strategy based on nearest level control (NLC) was proposed [15]. This model focuses on control equations and transient models.…”

Recurrence of Sub-Synchronous Oscillation Accident of Hornsea Wind Farm in UK and Its Suppression Strategy

“…Since the sub-synchronous oscillation event of the Hornsea wind farm is caused by the asymmetric drop fault of power grid voltage, there are many asymmetric components and sub-synchronous components in the system. Hence, it reduces the system control accuracy, making it more difficult to suppress the sub-synchronous oscillation and increasing the dynamic coupling between the equipment and the power grid [22][23][24][25][26]. If DFIG control systems do not consider the voltage unbalance, the stator currents can become highly unbalanced even with the small voltage unbalance.…”