Researches On Short Circuit Fault Detection And Clearance For DC Grids Considering Resistive Type Superconducting Fault Current Limiters

Modular multilevel converter-based multi-terminal HVDC(MMC-MTDC)grids become a promising option for large-scale renewable energy transmission.Under the goal of carbon neutrality and improving the energy structure,the DC power grids will play an increasingly important role in the future.However,one of the concerns resitricting the development of DC power grid is the fast dc-side short circuit fault detection and isolation.On the one hand,being composed of power electronic devices,the DC power grid is a low damping system.This will lead to a fast development of short circuit faults,so that the fault detection needs to be completed within several miliseconds.On the other hand,MMCs based upon half-bridge submodules,the dominant topology in DC power grids,has no fault isolation capability.Besides,the DC fault current has no zero-crossing point,which makes it more difficult to timely isolate the fault area.With such advantages as no effect on the system during normal operation and automatically quench after a short circuit fault,the resistive type superconducting fault current limiters(RSFCLs)are a prominent solution to tackle the issue of dc short circuit fault in DC power grid.Taking RSFCLs into consideration,this paper studies the following four aspects in terms of the fast fault detection and isolation in DC power grid.(1)The impact of the installation of RSFCL on the time domain characteristics based fault detection scheme of DC power grid is investigated.First of all,the DC voltage and current after fault in the DC grid are compared,in terms of the amplitude and waveform,under the conditions of with and without the RSFCLs.It is concluded that the addition of RSFCLs increases the impedance of the fault discharging loop,which boosting and suppressing the amplitudes of the voltage and current on the fault line,respectively.The waveforms of the signal are also affected by the change of the refraction and reflection process caused by RSFCLs.Subsequently,the time for different fault characteristics reaching their threshold value are compared,according to the PSCAD/EMTDC simulation results.It was found that the addition of RSFCLs could lead to a more than 40%addition of the fault detection delay.The relationship between the extent of the change to the measured signals and four key parameters is further revealed.(2)In order to tackle the problem that RSFCLs affects the fault detection of DC power grids,a RSFCL based fault detection scheme is proposed.By analyzing the characteristics of both the fault currents and the quench process of RSFCLs,the theoretical foundations of the scheme are established,which is the voltage across RSFCLs on the faulted line will be greater than those on non-faulted lines,Based on the theoretical analysis.the fault detection scheme for metallic short circuit faults is proposed.Considering the circumstances of the short circuit fault via high resistance,fault detection is achieved by the current information from both side of the fault line.Extensive simulations confirm that the proposed detection scheme can identify the metallic short circuit faults and faults with high resistance within the time requirement of the DC grid.The proposed RSFCL based scheme will not cause mal-operation of DCCB as a result of faults on adjacent lines and power order changes.It is also confirmed that the scheme has immunity to noise of 20dB.Besides,with the installation of RSFCLs,the proposed scheme shows 20%fault current suppression rate in the fault detection process.(3)To over come the shortcomings that the communication is needed for the RSFCLs based fault detection scheme in detecting fault with large resistance,a single-ended fault detection scheme for long transmission lines in DC girds is proposed based upon a support vector machine(SVM).Subsequent to the theoretical analysis of the measured voltage after fault using fast Fourier transform(FFT)and symmetrical components analysis(SCA),the characteristic of frequency spectrum and the zero-mode component of the positive voltage are compared under different short circuit fault conditions.An integrated SVM-based fault detection scheme with start-up criterion is presented.The two inputs to a trained SVM are the summation of the second-level detail coefficient of line-mode set of the positive voltage DIE obtained from multiresolution analysis,and the summation of zero-mode set of the positive voltage DFT.The output of the SVM determines the fault types.Cases with various fault conditions are generated in PSCAD/EMTDC platform,and an appropriate SVM structure is obtained.Test results show the proposed scheme can achieve 100%accuracy in detection short-circuit faults with high resistance for the whole length of the protected line.The proposed scheme is compared with other methods,and its robustness against measurement errors,time delay of the detection process,and the fault detection range are evaluated.It is concluded that the proposed single-ended fault detection scheme with only voltage measurement can provide fast and accurate fault detection in DC grid with RSFCLs or on a larger scale.(4)To deal with short circuit faults clearance problem in DC grids,a novel solution based on the combination of RSFCLs and hybrid DC circuit breakers(DCCBs)is proposed.Based on the analysis of the fault current in a symmetrical monopole DC grid and the operating principles of a DCCBs,the effectiveness and necessity of the addition of RSFCL into the system and the combination of RSFCLs and DCCBs after fault is established.Subsequently,the sequence coordination scheme of the combination of RSFCLs and DCCBs for fault clearance in DC grid is proposed.Simulation results on PSCAD/EMTDC platform verifies the effectiveness of the proposed scheme.During the most severe dc side pole-to-pole short circuit fault,the proposed fault clearing scheme makes it possible to maintain all converters unblocked.Thus,the DC grid can quickly recover after a fault through DCCB reclosing.The requirement of the breaking capacity of DCCBs is reduced for the suppressed fault current.The influence of the typical parameters of RSFCLs and DCCBs are further discussed.The results provide a theoretical reference for parameter design of RSFCLs and DCCBs in a combined operation scheme in DC grids.