Impact of variable DC reactors in voltage-source converter based multi-terminal high-voltage DC transmission systems

A novel variable DC reactor is proposed to cooperate with DC circuit breaker (DCCB) for breaking DC fault currents, which can satisfy the requirement of fast protection in high-voltage DC grids. Meanwhile, the variable reactor possesses other advantages such as smaller size, less loss and a small impact on the design of DCCBs. According to an electromagnetic characteristic of the reactor, a mathematical model of variable DC reactor is built on the base of experiment analysis. A three-terminal VSC-MTDC system with variable DC reactors is established in power systems computer-aided design/electromagnetic transient design and control. Simulation results are obtained as follows. First, the variable DC reactor possesses a low inductance during normal operations. When the received power increases, the DC reactors can reduce the adverse impact on the system stability to a great extent. Second, during a DC fault, the inductances of the variable DC reactors can increase automatically to limit the rising rate of short-circuit current and to extend the arriving time of the peak current. The application of variable DC reactors can provide the condition for DCCBs breaking fault currents successfully.