Control Strategy Optimization of Voltage Source Converter Connected to Various Types of AC Systems

Connecting the voltage source converters (VSCs) to various types of AC systems results in different operation characteristics and core problems associated with traditional control strategies. Therefore, it is necessary to optimize the control strategies of the VSCs according to the types of AC systems. For the VSCs connected to islanded renewable power plants, a voltage/frequency (V/f) droop control strategy is proposed to damp fluctuations of AC voltage and frequency in the island, which is vital for bipolar VSC control. In addition, a multi-branch impedance equivalent method for renewable power plants is proposed, with which large-scale renewable power plants can be modeled accurately in the frequency domain to prevent wide-band oscillation. For the VSCs connected to strong AC systems, smart AC voltage and coordinated frequency transient control strategies are proposed, which can improve AC system transient stability. For the VSCs connected to weak AC systems, the relationship between the system stability and strength is analyzed, and then the control strategy of inner-loop control parameter optimization and outer-loop power limiting (if necessary) is proposed to improve the stability of the allied system. The proposed strategies are verified by both software simulation and field commissioning.