Fault Limiting Circuit based protection for DC and AC Faults in HB-MMC HVDC Systems

The Half bridge Modular Multilevel Converter (HB-MMC) based High Voltage Direct Current (HVDC) Transmission systems faces various challenges due to DC faults and AC faults. In case of DC faults (pole-to-pole and pole-to-ground fault), the high fault current flowing through MMC arms may damage the devices present in the MMC. Among the AC faults, the valve side single phase to ground (SPG) fault in HB-MMC based asymmetrical monopolar and bipolar HVDC system is one of the most serious faults. It produces some special characteristics like non-zero crossing of the grid side currents which prevent the AC Circuit breakers (ACCBs) present in the grid side from operating; further, capacitor overvoltage happens in upper arm submodules. In this paper, a Fault Limiting Circuit (FLC) is proposed to solve the above problems. The proposed circuit acts as a parallel LC resonant circuit formed by inserting a capacitor in parallel to arm inductor during fault condition which provides high impedance to the fault current path and reduces the fault current. This circuit is activated during fault condition only, hence the efficiency of the converter is not compromised. The operation of the FLC with HB-MMC is analyzed and verified by simulation in EMTP platform, showing a good agreement between simulation results and theoretical analysis.