Study on flexible DC grid based on H7-CSC

Abstract

In flexible DC transmission, if the DC fault occurs, not only the 2-level voltage source converter(VSC) and the modular multilevel converter(MMC) based on the half-bridge submodule(HBSM) cannot suppress the DC fault current, but also the DC capacitor will increase the DC fault current. Furthermore, DC circuit breakers are too expensive. In order to overcome the issue that the converter can not block the DC fault current, this paper took advantage of the unidirectional conductivity of current source PWM converter(CSC) which is a dual with VSC, studied H7-CSC topology which can greatly reduce the CSC loss, and provided the modulation and control strategy of H7-CSC. In addition, the start-up control strategy and the DC fault treatment strategy of DC network consist of H7-CSC were proposed; the three terminal DC network simulation model was set up which composes a line-commutated converter(LCC) as a rectifier and two H7-CSCs as the inverters, then simulation verification was carried out in PSCAD/EMTDC. The results show that the DC network model proposed by this paper can achieve the smooth start-up, stable operation, DC fault removal and system restart. This model realizes DC fault removal through the control of each converter, without DC circuit breaker, which provides new ideas for the development of DC power grid in the future.