Fault analysis for simultaneous AC-DC power transmission in distribution system integrated with distributed PV generation

The use of simultaneous AC-DC power transmission can not only increase line capacity, but also alleviate the shortage of power supply in existing distribution system. Moreover, renewable energy is used efficiently by cancelling the energy conversion equipment. Simultaneous AC-DC power transmission in distribution system integrated with distributed photovoltaic generation has a great prospect of application. However, due to the special structure of simultaneous AC-DC power systems, their fault characteristics are very complex, which may render traditional protection strategies ineffective, and existing research on fault analysis for such systems is limited. Unlike existing studies that mainly focus on fault analysis of individual AC and DC systems or VSC-based hybrid AC/DC systems, a comprehensive fault analysis for simultaneous AC-DC power system is presented in this paper, which is essential to relay protection design. Firstly, the structure of system is introduced. Secondly, two types of common faults, i.e., single line-to-ground (SLG) and three-phase-ground (TPG) faults, are analyzed. The fault process is divided into several stages, each stage describing the fault circuit and main equations. The simulation model is built to prove the correctness of the analysis. Then, the factors affecting fault characteristics, including grounding resistance, DC-link capacitance, and initial phase angle of the fault are discussed. Finally, the fault characteristic is studied with various DC injection modes. Furthermore, a method for suppressing overvoltage in DC system under SLG fault is proposed.