Analysis Of Fault Characteristics Of DC Distribution Network And Development Of Sic Solid State Circuit Breaker

With the development of power electronics and distributed generation technologies and the significant increase of DC loads,DC power distribution technology is now gaining more interest as an effective energy technology.However,when the distributed power generation is connected to the DC distribution network,the fault current characteristics will be diversified and complicated during the DC fault period,so that original fault detection methods of DC systems without distributed power generations cannot be fully applicable.This may cause the protection system to malfunction or refuse to operate.Moreover,a DC short-circuit fault will pose high transient fault currents since the existence of large DC-link capacitors and characteristics of low DC systems impedance.Thus,a reliable high-speed DC circuit breaker is needed to quickly and effectively interrupt these fault currents to greatly improve the reliability of DC distribution.This thesis aims to solve the problem mentioned above and carries out researches on the key theory and method of short-circuit fault transient analysis of DC distribution network with distributed generators.The mechanism and response characteristics of fault current generation are also analyzed.At the same time,the DC solid-state circuit breakers(SSCBs)based on normally-on silicon carbide(SiC)junction gate field-effect transistor(JFET)devices are studied in terms of its topology,driving circuit and corresponding protection control methods.The main achievements and contents of this paper include the following aspects:(1)This study investigated the short-circuit fault characteristics of VSC-based DC distribution networks containing distributed generation systems.In consideration of three kinds of distributed generators(supercapacitor energy storage,photovoltaic power generation and wind power generation)being connected to DC distribution system,respectively,the mathematical models of transient DC-link voltage and fault current of the distributed power converter in the insulated gate bipolar transistor(IGBT)blocking mode are studied while short-circuit fault occurring on its DC side.The fault response characteristics of different stages are analyzed and compared.The effects of these distributed generators on the magnitude and peak time of fault current are analyzed.It revealed that the main factors affect the fault behaviors of dc distribution networks are the dc fault distances and these distributed power generators' dc-link capacitors.Based on PSCAD/EMTDC simulation results and RT-LAB hardware in the loop simulation experiment results,the correctness of the mathematical models of fault current in this paper is verified.(2)The influences of the control strategy(the gate signal of IGBT of the distributed power converter is not locked)of the supercapacitor energy storage,the photovoltaic power generation and the wind power generation on the fault characteristics are studied when a short-circuit fault occurs in DC distribution network.The transient fault equivalent models of each distributed power converter are analyzed,and the influences of the control modes of the converters on the fault current characteristics are compared.The results of PSCAD/EMTDC simulation and RT-LAB-based control hardware in the loop semi-experimental simulation show that the peak value of fault current will not be affected by either control mode or IGBT direct blocking when DC short circuit fault occurs.However,the steady-state fault current of each converter under different control modes is smaller than that of IGBT when it is locked directly.Therefore,in order to ensure the safe and reliable operation of DC distribution network with distributed generators,the high transient fault current generated by distributed generators requires the system to be equipped with the fast DC circuit breakers with microsecond response capability.(3)Aiming at the problems that the fault current rises fastly and its amplitude is high caused by the distributed generators connected to the DC distribution network,this paper proposes a smart SSCB based on the normally-on SiC JFETs.It can quickly cut off the short-circuit current in as little as ten microseconds.In order to provide a flexible overcurrent protection for the DC distribution network,a pulse width modulation(PWM)current limiting delay protection method is proposed.This method can identify the permanent overcurrent and the inrush current caused by the normal starting of the power electronic load,thus effectively avoiding the protection false trip caused by the inrush current.Furthermore,the transient thermal characteristics of normally-on SiC JFET devices under overcurrent fault are analyzed.The failure mechanism of JFET under overcurrent fault is revealed,and the thermal failure time range of JFET under different overcurrent conditions is identified.In order to verify the dynamic response of the SSCB,a 400V/38 A SiC JFETs-based circuit breaker prototype is designed and fabricated for result confirmation.The SSCB realizes reliable and rapid isolation faults of the DC distribution network under different fault conditions.(4)The factors affecting the voltage imbalance during the series operation of the normally-on SiC JFET devices are analyzed in this study.A single gate driving circuit of SiC JFETs in series is proposed,which realizes dynamic and static voltage balancing of series devices.Moreover,based on the analysis of the factors affecting the current imbalance during the parallel operation of the SiC JFETs,a parallel driving circuit of SiC JFETs is designed to realize the current balance of operation parallel devices.A 2kV/38 A SSCB designed in series and a 400V/200 A SSCB designed in parallel based on SiC JFETs are used to verify the validity of the design method.These design methods provide technical support for the large capacity applications of SSCB.In this paper,based on the analysis of fault characteristics of DC distribution network with distributed generators and the study of SiC-based SSCB,the theoretical system of short-circuit fault analysis for DC distribution network is improved.These studies provide a support for the development of DC distribution network protection technology.Through the research of SiC-based SSCB with very low loss and fast protection capability,the requirements of fast protection response and non-arcing,noise-free interruption fault current in DC distribution network are realized,which provides a useful reference for the practical application of SSCB in DC distribution network.