Study Of Protection For Cable And Bus Short-circuit Faults In DC Microgrids

Replacing fossil energy generation with new energy generation is an important technology for achieving the energy saving and emission reduction.The microgrid is undoubtedly a main way for photovoltaic power,wind power and other new energy power sources to be connected into grid.DC microgrids have no need to take the frequency and phase angle matching into consideration,as a result,compared with AC grids,control system of DC grid is simpler.In addition,the dc lines have higher energy transfer efficiency and there are less energy transformation levels in DC grids,therefore,DC microgrids have lower network cost.Then,DC microgrids are usually connected with AC grids through the transformers and converters,which can effectively shield interference from AC grids,so the operation stability of DC microgrids is improved.In a word,DC mcrogrids have great development potential.However,as a low-voltage DC grid,DC microgrid has very low voltage level and limited power transmission range,as a result,there is very short line length in DC microgrids.Then,loads are connected into DC microgrids directly,which means DC microgrids should possess high power supply reliability.In addition,DC microgrid contains a large number of voltage stabilizing capacitors.Violent capacitors discharging caused by short-circuit faults would lead to rapid increasing of fault current and threaten the safery of fragile power electronic components,therefore,it is required that the protection should remove the fault very quickly.The characteristics that contain short line length,high power supply reliability requirement and high protection speed requirement lead to the lack of mature protection scheme in DC microgrids.In order to establish a protection scheme that can meet the requirements of DC microgrids protection,the following analysis and methods are proposed in this paper:According to the characteristics of the DC microgrids and the shortages of the existing bus structures,a novel method of deciding bus protection zones is proposed.Based on proposed method,a bus protection zone can hold several equipment and the DC microgrids can utilized less dc circuit breakers.Because the protection areas division in this paper is very flexible,backup lines could be added for important loads to improve their power supply reliability.Based on theoretical analysis and simulation research,the paper points out the flaws of the existing cable dc circuit breakers configurations and proposes an idea of improving the DC circuit breakers configuration,as a result,the overcurrent in converter after the protection action can be completely avoided.Meanwhile,a capacitor current limiter is proposed,which can ensure the safety of the voltage stabilizing capacitors and assist cable short circuit fault location methods.In DC microgrids,the fault analysis for grid-connected equipment with DC-DC converters is significantly insufficient,therefore,the fault transient characteristics of DC-DC converter are analyzed in detail in this paper.Based on the changing of fault circuits,the process of faults is divided into several stages,and for each stage,the fault current circuits,the related formulas of fault currents and changing trends of main electric parameters are given.The analysis points out the main phenomenons during the short-circuit faults and main reasons of the fault circuit changing.The simulation verifies the correctness of the fault analysis and influences factors of fault characteristics,such as fault distance and transition resistance,are discussed with the help of simulation.Then,the safery of fragile uncontrollable power electronic components during short-circuit faults are analyzed and the main threats to components safety are discovered.Based on analysis,the general fault characteristics of all kinds of grid-connected equipment in DC microgrids are proposed,which are essential foundations for establishing DC microgrid protection criterias.Aiming at the cable short-circuit faults in DC microgrids,a universal protection scheme is established for equipment with different transient characteristics.(1)Based on the conclusions of fault analysis,the intense capacitors discharging at the beginning of faults are inevitable for different power sources and loads in DC microgrids.Therefore,employing the capacitors discharging phenomenons,the protection criterions based on the capacitor current ratio and grounding current are proposed respectively for the pole-to-pole cable faults and pole-to-ground cable faults.In addition,in the end of cables,there are directional components.Based on the fault direction information given by the directional components,protection can find out if there is a shor-circuit fault inside the cable protection zone.The proposed protection criterions do not depend on specific types of power supplies,loads or converters and have great adaptability.The protection algorithms are convenient for engineering application can deal with large transition resistance well.(2)Based on the proposed configuration of DC circuit breakers and capacitor current limiter,an over-damped capacitor discharging circuit can be formed after the protection action.Then,the simplified expressions of capacitor discharging current could be obtained,according to the resistance is the dominant parameter in the capacitor discharging circuits.The fault location method of cable short-circuit faults could be deduced through the simplified expressions.The fault location method can provide accurate fault location results and has good ability to deal with transition resistance.Besides,because only the stable measurement datas after protection action are used for fault location,the measurement errors caused by current and voltage mutation could be effectively avoided.(3)By integrating the proposed protection criterions,the directional element criterions and the fault location method,a complete protection and fault location process for DC microgrid cable short-circuit faults could be eatablished,as a result,a fault protection scheme for DC microgrid cable faults is finally obtained.(4)Using DC microgrid simulation system,a grid-connected voltage source converter,a grid-connected PV power supply with DC-DC converter and a DC load directly connected into DC microgrids are chosen as three representational cases for cable short-circuit faults protection simulation.The simulation results verify the protection effectiveness,the protection generality,the ability of dealing with transition resistance and the precision of fault location.For the bus short-circuit faults in DC microgrids,this paper proposes a bus short circuit fault protection scheme.(1)The network topology information of DC mcirogrids is digitized.The protection zones,sectionalizing switches and cables are numbered respectively and a current direction coefficient matrix is proposed.(2)The fault direction informations from directional elements located in bus are necessary for the protection schemen.The directional elements utlize positive current faulty component and transverse differential current as action quantities.The positive current faulty component is used in the start-up criterion for the poleto-pole faults and transverse differential current is used in the start-up criterion for the pole-toground faults.The start-up criterions also employ dynamic thresholds,which are based on recorded measurement datas.Therefore,the directional elements are able to possess the sufficient sensitivity and deal with transition resistances effectively.Based on the direction of current faulty component and transverse current,directional elements could provide accurate fault direction information through a unified workflow.(3)Based on the digital topology and fault direction informations provided by directional elements,the protection scheme can discover the faulty area accurately with the longitudinal algorithm.The algorithm is convenient for the engineering application and has fault-tolerance ability and strong adaptability.The protection scheme is able to adapt to the complex operating environments in microgrids and it is advantageous for network expansion.After the faulty area is cut off,the operating states of the system can be adjusted through the sectional switches and standby lines,as a result,power would be supplied normally in the non-fault zones.(4)Simulation verifies the sensitivity,the accuracy and transition resistance resisting ability of the proposed direction elements.Then,based on the fault direction informations provided by directional elements in simulation system,the effectiveness and fault-tolerance ability of the longitudinal algorithm is verified,as even when some directional elements might not start up,the algorithm would be still able to discover the faulty zone correctly.In conclusion,based on the general conclusions of fault analysis,the paper proposes a complete protection scheme for both cable short-circuit faults and the bus short-circuit faults.The line short-circuit faults can be completely removed in time.The protection is able to deal with transition resistance well and has enough adaptability to operate in complex and changeable working environments of DC microgrids.