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.