Research On Fault Ride-Through Strategy And Key Technologies Of Pole-to-Pole Short Circuit In Flexible DC Grid

With the development of high voltage direct current(HVDC)transmission technology,multi-terminal HVDC and HVDC grid are the focus of academic research and the important direction of HVDC project construction.The DC grid based on modular multilevel converter(MMC)can effectively solve the imperative requirement of connection and absorption of the large-scale new energy in China.However,there are new technical difficulties in DC grid that need to be solved:the lack of mature fault detection and protection strategies is one of the key difficulties.Due to the low damping of the DC power grid and the high rising speed of DC pole-to-pole short-circuit fault,fault isolation time is usually less than 5ms,which is much lower than that of AC system.At the same time,there is no natural zero crossing point in DC fault current,which makes the interruption more difficult than that of AC system.Therefore,how to design a DC short-circuit fault treatment device for DC power grid is a big challenge.This thesis focuses on the ride-through challenges of DC grid pole-to-pole short circuit,and it is organized as follows:(1)For non-selective fault ride-through strategy,from the point of view of converter side fault ride-through strategy,a novel MMC sub-module(SM)topology with low power loss and DC fault ride-through capability is proposed.This strategy is achieved by blocking the SMs in MMC.the DC fault ride-through mechanism of proposed topology is investigated and the corresponding fault ride-through strategy is designed.The cost and power loss among different SM topologies are compared.The device usage of the proposed SM is the lowest level of the existing SMs,and the number of IGBTs and diodes in the current path under normal operation is lower than that of the existing SMs.The simulation results show that the DC fault current can be interrupted rapidly by blocking the proposed SMs.(2)For the selective fault ride-through strategy,from the point of view of grid side fault ride-through strategy,the composition and operation principle of hybrid direct current circuit breaker(DCCB)are studied.A reciprocating DCCB is proposed which can increase redundancy for DC fault detection delay.It presents low reactance value under normal operation,and the reactance value can be rapidly increased by current limiting operation under DC fault state.Current limiting operation can be started when suspected fault occurs,and circuit breaking operation can be adopted after the DC fault is clear.This strategy has the following benefits:on the one hand,it can prevent the rapid increase of the fault current;on the other hand,long fault detection time can be allowed to prevent the misoperation.Finally,for occasions where only fault current limitation is required,a low-cost reciprocating fault current limiter is designed.(3)For the selective fault ride-through strategy,the research is mainly earied out from the perspective of converter side and grid side coordination fault ride-through strategy.Considering the fault current commutation branch is a necessary condition for the interruption of ultra-fast disconnector(UFD),and each line should be configured with this high cost proportion branch independently,the cost of existing DCCB schemes is relatively high.To solve this difficulty,a dual capacitor fault current commutation branch based on thyristors and capacitors is designed.The coordinated control of MMC converter,dual fault current commutation branch and residual current discharging switch(RCDS)branch is studied and the corresponding control strategy is designed to create the electrical condition of isolating fault line by UFD.The scheme allows multiple lines connected with the same DC bus share the fault current commutation branch,so it can reduce the investment costs and improve the resource utilization.(4)For the selective fault ride-through strategy,from the perspective of converter side and grid side coordination fault ride-through strategy,an auxiliary fault ride-through strategy based on MMC is designed.To further reduce the investment cost,this strategy uses converter as the fault current commutation branch,thus the additional fault current commutation branches are omitted.Aiming at the power loss of IGBTs in load commutation switch(LCS)in DC line,the optimization scheme of low loss fault ride-through strategy is studied,and IGBTs in LCS is omitted.Aiming at the current and power fluctuation problem in healthy lines and MMCs under DC faults,a coordinated fault ride-through scheme with reciprocating fault current limiters is designed.