Analysis of the Voltage-Grading Circuit for Power Electronics in HVDC CBs

High voltage DC circuit breaker (HVDC CB) is the key equipment to form the meshed voltage source converter (VSC) DC transmission system. In HVDC CBs, many series-connected devices are used to satisfy the voltage withstand requirement. During the transient switching operation, an uneven voltage distribution between these devices is unavoidable due to many factors like as manufacturing variations, switching dispersibility, and stray characteristics. An effective technical solution to deal with the imbalanced voltage distribution is the voltage-grading circuit. In this paper, we discuss the performance of a voltage-grading circuit for power electronics in HVDC CB under the conditions of capacitance differences and switching delays in a Modular Multilevel Converters (MMC) system. The study finds that for the cascaded power electronics with voltage-grading circuits, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm 2.5\%$</tex> capacitance difference of the voltage-grading circuit can cause an imbalanced voltage distribution ratio of about 10%. For power electronic modules divided by the parallel-connected metal-oxide surge arresters (MOSAs), the voltage distributions between modules are well regulated while the difference in energy dissipation between corresponding MOSAs is also acceptable. In addition, the study also finds that the unsynchronized switching will pose a greater challenge to voltage grading compared to the capacitance difference caused by manufacture.