Research On Converter-Based DC Fault Current Clearing Methods For VSC-HVDC Transmission Systems

In order to achieve the carbon peaking and carbon neutrality goals,voltage-source converter-based high-voltage direct-current transmission(VSC-HVDC)should be developed preferentially to improve the utilization of renewable energy.At present,modular multilevel converters(MMCs)are widely used in VSC-HVDC.DC fault current clearing is the key point of VSC-HVDC transmission systems since the dc fault current rises rapidly and has no natural zero crossing,as well as the semiconductor devices in MMCs cannot withstand large over-current.DC fault clearing methods can be divided into two categories according to the main device they rely on: those relying on the converters and those relying on the dc circuit breakers.Currently,dc circuit breakers cannot be applied widely due to their high cost.Using MMCs to clear the dc fault current is more cost-effective than using dc circuit breakers.Among the converter with dc fault current clearing capability,the hybrid MMC is highly recognized and has been applied in China.However,the hybrid MMC still has some problems.Firstly,severe full-bridge submodule(FBSM)overvoltage may occur during the dc fault clearing process since FBSMs need absorb the energy stored in inductors and transmission lines.Besides,the cost and loss of the hybrid MMC are still high and the dc fault clearing capability of the MMC can be further improved.These problems will be fully researched in this thesis.The main work of this thesis is as follows:(1)The unbalanced energy absorption between FBSMs and half-bridge submodules(HBSMs)will increase the maximum submodule voltage.Thus,a submodule choosing algorithm is proposed.By inserting FBSMs to discharge preferential when a dc fault may have occurred,the FBSM voltage will decrease and the HBSM voltage will increase before submodules are blocked.This algorithm can ease the FBSM overvoltage.Thanks to the reduced voltage difference between FBSMs and HBSMs,the restarting of the converter can be accelerated.(2)The unbalanced energy absorption of arms will increase the maximum submodule voltage.Thus,the arm current changing process after submodule blocking is analyzed.A submodule control method is proposed accordingly.By bypassing some FBSMs according to transient ac voltages,ac currents can be limited,thereby reducing the current stress in arms and easing FBSM overvoltage.(3)The converter absorbing excessive energy will lead to submodule overvoltage.Thus,an energy absorption branch based on metal oxide varistors and thyristors is proposed.By choosing proper protection voltage of metal oxide varistors,the dc fault current of the converter will decrease faster after submodule blocking,thereby reducing the energy absorbed by the converter and suppressing submodule overvoltage.(4)To reduce the loss and cost of the converter,an improved HBSM-based MMC is proposed.The dc fault current is cleared only by FBSMs in the conventional hybrid MMC.However,the proposed topology only relies on HBSMs,which has low cost and loss.By changing the direction of one arm current with auxiliary circuit,HBSMs in the arm can provide reverse-biased voltage to clear the dc fault current.The dc fault clearing capability of the proposed topology is equivalent to that of the hybrid MMC,while the cost and loss are much lower.Besides,the proposed topology can provide reactive power to the ac grid during dc faults.(5)To improve the dc fault clearing capability of the converter,an improved hybrid MMC is proposed,in which an auxiliary circuit is installed at the dc port of the hybrid MMC.After a dc fault occurs,the proposed topology can change the direction of the dc fault current at the converter’s dc port rapidly,enabling all submodules of the hybrid MMC to provide reverse-biased voltage to clear the dc fault current.The dc fault clearing ability of the proposed topology is significantly stronger than that of the conventional hybrid MMC,while the semiconductor costs and power losses are lower.