Bypassing transients of pre-insertion resistor during energization of MMC-HVDC stations

In a modular multilevel converter (MMC) station, a circuit breaker (CB) with a pre-insertion resistor (PIR) is required to be installed to suppress the inrush current due to the saturation of the converter transformer during the energization of the MMC station. The insertion time of the PIR depends on the specific design of the MMC-HVDC projects, usually in the order of seconds. When the PIR is bypassed from the energization circuit, the resulting switching transients can lead the converter transformer to be saturated. The caused high inrush currents are able to endanger the MMC station and reduce the transformer life-cycle. In order to understand the corresponding transients and perform the simulation study, this paper presents a test model, including a MMC station, a HVDC feeder and an external network. Two cases with different bypassing strategies are simulated in this paper to identify an appropriate bypassing time.