Coordinated Method Of Fault Current Suppression In Flexible DC Grid

The high voltage direct current(HVDC)grid based on the modular multilevel converter(MMC)is considered to be an effective solution for transferring renewable energy and for AC grid interconnection.DC fault protection is the major challenge that limits the wide application of DC grids.Therefore,significant attention has been paid to DC fault handling methods from the industry and academia.Due to the small circuit impedance of DC grid,DC fault currents grow rapidly,and the DC fault propagates very fast.A cost-effective,fast,and reliable DC fault protection scheme is necessary for DC grids.The DC fault characteristics of DC grid are obviously different from that of AC system.DC grid is a low inertia system with large energy storage capacitors,and DC fault current rising rate can reach 3-5 kA/ms.DC fault isolation time is required to be less than 5 ms to prevent the converter from blocking.However,there is no natural current zero-crossing point during DC fault,so an artificial zero-crossing point is necessary for DC fault isolation.The challenge of DC fault protection requires high capability for DC Circuit Breakers(DCCBs).Therefore,the suppression of rapid rising of fault current,reasonable artificial current zero-crossing method,and low-cost DC protection equipment have attracted many researchers' attentions.Aiming at the transient process after DC fault occurred,this paper mainly contains the following aspects:(1)A classification study of different DC fault protection methods in DC grid is given.Due to the flexibility of power electronic system,there are many DC fault protection schemes in DC power grid.The existing researches mainly focus on the analysis of one certain scheme,but lack of the overall evaluation of different kinds of schemes.More importantly,they lack the overall induction of the existing fault clearing schemes,also the insight of different schemes.This paper classifies existing fault clearance methods from two aspects:"decrease DC voltage or increase fault line resistance",which will have effect according to the Ohm's law.In DC grid,there is another way to protect fault line from“source-side”and“line-side ".The existing fault protection methods are either independent or combined use of the above types.The classification study can better distinguish the advantages of different methods.Moreover,the classification scheme proposed in this paper can guide further study of fault protection.(2)By reducing the DC voltage on the source side,this paper proposes an adaptive fault current suppression control strategy for MMC.As the converter is the dominating fault current source in the grid,and thus preventing the discharging of converter can significantly reduce the fault current.The existing converters lack the ability to suppress and control the fault current after fault.The converters either do not operate or directly switch to fault ride-through process.By reducing the number of inserted sub-modules,the DC side voltage can be reduced.The proposed scheme does not depend on the fault detection signal,and can adaptively suppress the overcurrent.Moreover,it has the ability of multi-converter coordinated fault current suppression.Furthermore,an adaptive sub-module capacitor voltage control method is proposed to limit sub-module voltage fluctuation during fault.The proposed scheme can not only suppress the fault current,but also suppress the power fluctuation of the DC grid and speed up the system recovery process.(3)By increasing the line side impedance,the topology and control method of the self-bypassing fault current limiter(FCL)are investigated.As it is hard to interrupt large current,using FCL to suppress overcurrent becomes an effective way.The application of FCL in existing projects is still insufficient.This paper presents a current limiter topology based on the single clamping sub-module(SCSM).The reliable insertion of FCL can be realized by flexible insertion of reactors.Further considering the subsequent circuit breaker breaking process,the coordination method of FCL and DCCB is studied.The self-dissipation of the FCL energy can reduce the requirement of DCCB.The principle,control method,and electrical stress of the proposed FCL are studied,a new index of evaluating the performance of FCL is proposed.The performance of proposed FCL is verified in a 4 terminal DC grid.Further analysis shows that FCL topology has potential application value in DC grids.(4)By reducing the fault line voltage,a clamping type DC circuit breaker is proposed.The conventional DC circuit breaker(DCCB)employs metal-oxide varistor(MOV),which is actually a resistance type device,is used to isolate the faulted line.In the conventional DCCB,fault isolation process is coupled with the energy dissipation.In this study,a clamping type DCCB(CTCB)uses internal capacitors to clamp the converter voltage is proposed.In the proposed configuration,fault isolation and energy dissipation are decoupled,resulting in a fast fault isolation and low energy dissipation compared to the conventional DCCB.The working principle of the proposed CTCB is presented and verified in a DC grid simulation model.A comparison is made with the traditional DCCB.(5)The coordination principles and methods of different protection equipment are investigated.By analyzing the requirement of fault current limiting,the directional installation principle of FCL and DCCB is proposed.The coordination control of multiple DCCBs is discussed.The conventional methods only use the faulty line DCCB to withstand the fault stress,while this paper presents a coordination method of multiple DCCBs to protect the system.As many adjacent DCCBs are tripped to interrupt the fault current,the fault energy is shared,and the requirement for the faulty line DCCB is reduced.The primary protection,backup protection,and reclosing logic of multiple DCCBs are studied.