Modular Hybrid DC Breaker-based Adaptive Auto-Reclosing Method for MMC-HVDC Systems

In this paper, the modular hybrid DC breaker is employed to inject active pulses after fault-current interruption and arc extinction due to a pole-to-pole fault in MMC-HVDC systems with half-bridge submodules. The power electronic branch of hybrid DC breaker is made of n series modules, which allows generating active pulses with a controllable amplitude and pulse width. Each module is a combination of metal oxide varistor (MOV) and parallel IGBTs, which can be bypassed by the IGBTs turn-on. In the proposed method, when the fault current is interrupted and the arc is extinguished, a specific number of modules are bypassed and by this action the operating point of remaining MOVs is pushed from high resistance to temporary overvoltage (or switching surge) mode. At the new operating point, the behavior of MOVs is equivalent to a DC voltage source in series with a low resistance and the voltage difference of MMC DC-link and the equivalent DC source is applied to the line. After a short delay (equal to the desired pulse width), the breaker modules are switched off and the pulse generation is ended. This action is repeated for several times and based on the monitored waveforms on the DC line, the fault type and its characteristics are recognized. In case of temporary fault, the voltage of DC line is gradually increased by sequentially turning the breaker modules on. Finally, the validity of proposed method is verified by simulations and experiments on a scaled down prototype.