Fault characteristics analysis of DC transmission system embedded in 220kV AC power grid

Short-distance HVDC transmission system within the same AC grid, known as embedded DC transmission technology, can significantly enhance the transmission capacity, controllability, and flexibility of the power grid within limited transmission corridors. This technology will be applied in the 220kV power grid in Jiangsu, China in 2024. This paper focuses on the 220kV embedded DC transmission system with symmetric unipolar wring and DC single-side grounding. Fault characteristics of the system are analyzed based on electromagnetic transient simulation modelling, and the fault characteristics differing from conventional long-distance HVDC transmission systems are investigated. The research results indicate that, due to the DC wiring mode, when a unipolar grounded short-circuit fault occurs at the embedded HVDC transmission line, there is a significant short-circuit current in the non-faulted pole which may affect the performance of travelling wave protection and differential under-voltage protection. Additionally, due to the electrical coupling between the sending and receiving ends, a short-circuit fault on the AC side at the sending end can cause a voltage drop at the receiving end, possibly leading to commutation failure.