Tolerant Control of Voltage Signal Fault for Converter Station Based Multi-Terminal HVDC Systems

Voltage source converter (VSC)-based multi-terminal high-voltage direct current (MTDC) technology is one of the most promising ways to integrate several non-synchronous weak ac grids into one hybrid ac/dc system. To enhance reliability and avoid unscheduled maintenance, it is desired that the VSC can remain operational without having to shut down despite some of its insulated gate bipolar transistors (IGBTs) or other devices are failed. Based on the dc voltage and ac frequency droop ( $V_{\text {dc}}-f$ ) control, this paper proposed a control strategy for coping with the condition of lacking ac voltage signal. This fault condition may be caused by measurement fault or part of communication malfunction. The controller is similar to a virtual synchronous generator (VSG), where the dc voltage (or power) $V_{\text {dc}}$ is the input and the ac frequency is the output. With the new control strategy, the VSC could still run stably and provide frequency support for an ac grid, even though the ac voltage measurement device breaks down. Besides, it could provide a reference frequency for passive ac grid in which there is no synchronous generator providing the reference frequency. An ac/dc hybrid system model is built in MATLAB/Simulink. The model analysis and simulations are performed to demonstrate the performance of the proposed strategy.