An overview of series-connected power electronic converter with function extension strategies in the context of high-penetration of power electronics and renewables

Series-connected voltage sources converters (SVSCs), (i.e., dynamic voltage restorer, fault current limiter, electrical spring, etc.), have been widely used in the electricity grid to improve the electricity grid performances in terms of voltage fluctuation compensation, flexible power flow regulation, fault overcurrent limitation, flexible access of renewable energy, etc. However, SVSCs have a relatively short effective operating time and a low utilization rate, because they are generally operated during abnormal grid conditions. The new trend is to increase the utilization by extending the functions of SVSC, e.g., integrating the voltage compensation and overcurrent limiting in a single SVSC. On the other hand, the series connection architecture results in the SVSCs experiencing overvoltage and overcurrent from the main grid, and consequently the reliability of SVSC is challenged. Based on these details, this paper systematically studies the state of the art multifunctional techniques and the corresponding strategies of SVSCs, which can be mainly divided into three categories as follows: function integration between different SVSC; function coordination between SVSC and parallel-connected VSC(PVSC); function integration between SVSC and PVSC. The most representative topology and control optimization method in each category is elaborated, and a brief comparison of them is conducted as well. On this basis, the key problems and challenges in the research and engineering application of multifunctional SVSC system are summarized.