Secondary Frequency Regulation Control Strategy of MMC-MTDC Converter Based on Improved VSG

Abstract

In modular multi-level converter based multi-terminal HVDC system (MMC-MTDC), PI double-loop direct current (DLDC) control cannot provide inertia for the system. Although conventional virtual synchronous generator (CVSG) control can provide inertia for the system and realize primary frequency regulation of AC system, it still belongs to differential frequency regulation and fails to give full play to the advantage of flexible regulation of converter. To address this issue, combined with the improved droop control (IDC) method of MMC-MTDC system, this paper proposes an improved virtual synchronous generator (IVSG) converter control strategy. Based on retaining the inertia and damping parameters in CVSG control, frequency deviation is introduced into the integral controller to make the converter participate in the secondary frequency regulation of AC system, and realize deviation-free frequency control of AC system through coordinating with other converters. To verify the proposed control strategy, a model of 5-terminal 31-level MMC-MTDC system interconnected with wind farm is built on Opal-RT simulator, and different operating conditions of the MMC-MTDC system are designed. Simulation results show that the proposed control strategy can reasonably distribute the received power, increase the inertia of converter and participate in secondary frequency regulation of AC system when the power of DC system fluctuates.