| Nowadays,more and more flexible DC projects and large-scale AC power grid are interconnected in China while the research on the failure characteristics analysis and protection strategy of AC/DC hybrid power transmission system containing MMC-HVDC is not comprehensive.In order to develop fault protection,it is necessary to study the operation characteristics and protection strategy of AC/DC hybrid system.Based on a study on the topology and principle of Modular Multilevel Converter(MMC),this paper derives a mathematical model of MMC,proposes the Modular Multilevel approximation modulation(NLM),and discusses the control strategy of MMC in detail.An AC/DC hybrid power transmission system including MMC-HVDC is constructed on PSCAD.This model refers to a DC power transmission project in Xiamen and a simulation experiment is carried out to check its stability.Furthermore,three-phase short-circuit fault of AC system,single-pole short-circuit fault,and double-pole short-circuit fault in DC system are studied.In this paper,a DC side clamp resistance grounding protection strategy is adopted for the single-pole short-circuit fault of DC system.To deal with the bipolar short circuit fault,a comprehensive study is incorporated on current limiting circuit protection and blocking converter station tripping protection strategy,and then an optimized method that combines advantages of both strategies is proposed.The current-limiting circuit is installed at the outlet of the converter station’s positive and negative terminals.When a fault happens,the converter station will be immediately closed,the circuit breaker will trip and the maintenance will be carried out.When the maintenance finishes,the converter station will be unlocked,and the current limiting resistor will be put in before the converter station will be unlocked,and then the system will restart and the current limiting resistor will be removed.Simulation results of DC system protection strategy show that the single-pole short-circuit protection strategy adopted in this paper can shorten the voltage recovery time after the failure and improve the stability of the system.Among the three bipolar short-circuit protection strategies adopted in this paper,the current limiting circuit protection,blocking converter station,tripping protection and the optimal protection strategies of current limiting circuit with blocking converter station and tripping have different effects on limiting the fault current and raising the voltage level of sub-modules during the fault.However,the current limiting effect of the improved optimal protection strategy is better than the previous two protection strategies,and the voltage level of the sub-module during the lifting of the fault is also optimized,and the stability of the system is improved. |
