Bipolar Short-circuit Fault Analysis And Protection Technology Of VSC In DC Grid

Due to the increase of the power quality requirements and the power load,the traditional AC network is facing more and more challenges.Because of flexible control,plug and play,low power loss and flow conversion convenient,the MTDC technology based on full controlled rectifier devices and pulse width modulation is becoming the trend of future grid.However,due to the lack of practical engineering experience and the limitation of DC circuit breaker technology,the research of DC grid protection becomes the bottleneck of DC grid system development.This paper takes the DC grid system as the research object,focuses on the mathematical model and control method of VSC in the system,the fault transient analysis and fault detection and isolation of VSC in DC grid system under different topologies.1?The mathematical models of VSC under abc three-phase static coordinate system and dq two-phase rotating coordinate system are established,the double loop control method of VSC converter is studied,the current inner loop controller based on the current state feed-forward and decoupling theory is designed,and the active power external loop control methods such as constant DC voltage,constant active pwer are discussed to realize the decoupling control of active power and reactive power.2?The bipolar short-circuit fault characteristics of VSC under different DC topologies are discussed.For the two-terminal DC system,the fault process is divided into capacitor discharge phase,diode freewheeling phase and uncontrolled rectifier steady-state stage.For tree topology and ring topology MTDC system,the fault circuits are equivalent simplified,deduces the transient expression of the current after fault.3?The methods of fault detection and isolation for DC grid system protection are studied.According to the DC faults transient process and mixed with current limiting inductor,DC breaker and DC direction switch,a new protection scheme based on fault feather di/dt without using communication system is investigated.Aiming at the problems of research difficulties in high capacity DC circuit breaker and the poor reliability of AC circuit breaker,a new fault isolation method using small capacity DC circuit breaker and AC circuit breaker is put forward.