Circuit breakers in HVDC systems: state-of-the-art review and future trends

High voltage direct current(HVDC)systems are efficient solutions for the integration of large-scale renewable energy sources with the main power grids.The rapid development of the HVDC grid has resulted in a growing interest in DC circuit breakers(DCCBs).A fast and reliable circuit breaker is a necessary requirement in the development of large scale HVDC grids.This paper provides a comprehensive review and survey of the HVDC CBs and discusses potential research directions.Operational principles and the main features of various DCCBs are described and their merits and shortcomings are also highlighted.

A novel control strategy for load converter of DC isolated distribution system under unbalanced loading conditions

A novel control strategy for the load converter supplying the unbalanced AC load in a DC isolated distribution system is presented. The control algorithm results in balanced and sinusoidal load voltages under unbalanced AC loading. The unbalanced load is characterized in the d-q-0 rotating coordinate based on symmetrical sequence components. Also, the mathematical model of the load converter in both a-b-c and d-q-0 coordinates is derived by using the average large signal model. Then, two control strategies for the load converter are presented. The first one uses the conventional d-q-0 controller to ensure the voltage and current regulation. The second one is a newly proposed control strategy based on the decomposition of the voltage and current into in-stantaneous positive, negative, and zero sequences. These three sequences are controlled independently in their own reference frames as DC signals. The performance of the load converter using these two control strategies is compared. Simulation results show the validity and capability of the newly proposed control strategy.

基于级联变压器的二极管桥式固态故障电流限制器（英文）

目的:为控制故障电流,提出一种新型基于级联变压器的二极管桥式固态故障电流限制器(SSFCL)。创新点:所描述的SSFCL具有简单且易用的结构,能够将故障电流幅度限制于安全范围内。并能减少谐波畸变与开关电压过冲,调控公共耦合点电压。方法:将一串联RLC支路连接于隔离串联变压器的次级端,针对此RLC支路,提出两种限流模式。(1)放电模式:电阻R与电容C被并联的功率电子开关(IGBT)旁路,而电感L仍与变压器次级端相联,从而构成直流电抗器。(2)充电模式:若串联电抗器阻抗不足以限制故障电流,可以通过选择恰当的并联IGBT开关持续时间(on-off duration),将高阻抗的串联电阻R和电容C插入电路,从而限制故障电流。结论:通过控制直流电抗器电流幅度,可以降低故障电流并将公共耦合点(PCC)的电位限制于合理水平。此外,在本文所述的新型SSFCL中,与IGBT并联的RC支路可作为缓冲电路用于降低开关状态下IGBT的瞬态恢复电压(TRV)。因此,SSFCL可以提升电能质量。本文还给出了在已有的原型系统上的实验测量结果以支持理论和仿真研究。所述的SSFCL可以无延迟地限制故障电流并有效平滑故障电流波形。