Enhanced line commutated converter with embedded fully controlled sub-modules to mitigate commutation failures in high voltage direct current systems

An enhanced line commutated converter (E-LCC) with embedded fully controlled sub-modules (SMs) to mitigate commutation failures (CFs) in high voltage direct current (HVDC) system is proposed in this study. The main feature of E-LCC is that each arm is composed of thyristor valves and series connected fully controlled SMs. The operation modes and voltage-current stresses of embedded SMs in E-LCC are studied. Then the control strategy, including the pre-charging scheme of SMs and the coordination control method between the thyristor valves and the SMs, is presented. On the basis of the derived equivalent commutation circuit of E-LCC-based HVDC (E-LCC-HVDC), the dynamic characteristics of the commutation process are investigated. Then, the impact of E-LCC-HVDC on CFs mitigation is theoretically analysed. To validate the proposed E-LCC-HVDC, the simulation study is conducted in the electromagnetic transient simulation program PSCAD/EMTDC. The results show that with the embedded SMs, the proposed E-LCC-HVDC has the ability to mitigate the CFs and alleviate the power interruption during the AC faults effectively. Furthermore, the theoretical analysis and the presented control method for E-LCC-HVDC are also validated.