DC fault current approximation and fault clearing methods for hybrid LCC-VSC HVDC networks

The DC-side fault analysis of Line commutated converter based high voltage direct current (LCC-HVDC) and Voltage source converter based high voltage direct current (VSC-HVDC) networks has been studied by most researchers and achieved fruitful results. However, the traditional fault analysis method does not consider in detail the change process of the trigger angle of the LCC converter station and the AC current feeding of the VSC converter station caused by the control strategy. Therefore, this paper proposes a new DC side fault transient analysis method suitable for hybrid multi-terminal HVDC transmission networks. In addition, considering the deficiency that the active fault current suppression method only relying on the control strategy cannot funda-mentally clear the DC fault, this paper proposes a new Z-source current-limiting circuit (ZCLC) with reference to the topology of the Z-source dc circuit breaker (DCCB). Then, ZCLC will cooperate with DCCB to improve the fault clearing capacity of the whole system. Firstly, this paper establishes the equivalent Laplace transform model of the three-terminal LCC-VSC-VSC, and analyzes the short-circuit current expression on both sides. Then, the change process of trigger angle is approximately linearized by the least square method, which obtains a new expression of fault transient current on the LCC side. Similarly, the composition of AC current fed to DC side is deduced by using the commonly used inner-loop PI controller model, which also obtains a new expression of fault transient current on the VSC side. Secondly, referring to the traditional crossed-type Z-source DCCB, series -type Z-source DCCB, and parallel-type Z-source DCCB, this paper also proposes a new ZCLC and analyzes its working principle and parameter design. Then, the newly proposed ZCLC is combined with DCCB to be applied to the hybrid multi-terminal HVDC network. Finally, the simulation results based on Matlab/Simulink verify the effectiveness of the fault analysis and passive fault clearing methods, and the semi-physical experimental plat-form based on PLECS/RTbox verifies the superiority of the new ZCLC.