Solution to reduce voltage stress of sub-module in LCC–MMC transmission system at the condition of communication fault

Abstract

Line commutate converter–modular multilevel converter (LCC–MMC) hybrid high voltage direct current (HVDC) system combines the advantages of both LCC-HVDC and MMC-HVDC, and will be wildly used in the future. The sub-module will withstand a very high voltage if a fault occurs in MMC converter station. In order to reduce this voltage, the LCC converter must operate in inversion state through gate-shift (GS) control strategy as quickly as possible. Normally, the fault signal was sent to LCC station through communication and GS control strategy was triggered due to this signal. At the condition of communication fault, LCC converter station cannot receive a fault signal, thus the control strategy will not change. In this way, the SM voltage will be as high as 4100 V which is beyond the maximum withstanding voltage of insulated gate bipolar transistor (IGBT). It will endanger the safety of devices. To solve the over voltage problem of IGBT, a new solution that designs a leakage thyristor valve between DC line and ground is proposed as well as the coordination control strategy. A simulation based on power systems computer aided design (PSCAD) is conducted to verify this method. The simulation results show that the proposed solution is valid.

Keywords

• HVDC power transmission
• HVDC power convertors
• power transmission faults
• voltage-source convertors
• commutation
• voltage stress
• LCC–MMC transmission system
• communication fault
• line commutate converter–modular multilevel converter
• LCC-HVDC
• MMC-HVDC
• high voltage
• MMC converter station
• inversion state
• gate-shift control strategy
• GS
• fault signal
• LCC station
• LCC converter station
• SM voltage
• maximum withstanding voltage
• coordination control strategy
• voltage 4100.0 V