Fault Characteristics Study And Protection Design For DC Microgrids

Along with the development of power electronics technology and distributed generation,microgrids have attracted much attention as an effective way to connect DGs to the grid.In recent years,the advantages of DC microgrids become more prominent with the increasing of direct-current DGs,DC load and frequency conversion load,such as:easy accessing to DC equipment,high transmission efficiency,reduction of converter devices and simple control.However,its development is limited by the lack of effective protection.Different from the traditional single-terminal distribution networks,the DC microgrid is a multi-terminal network,which leads to the result that the existing protection cannot meet the reliability and selectivity.The relay setting is also difficult because of the short cable line.In addition,a large number of power electronic devices are contained in DC microgrid through the converters.The vulnerability of them requires high protection speed,especially in DC microgrids with small capacity and low voltage level.Therefore,the design scheme of DC microgrid protection is rich in challenges.An effective protection is generally designed based on fault characteristics which can be obtained through fault analysis.So it is necessary to study the fault performance of DC microgrids at first.In DC microgrids,various DGs connect to grid through different types of converters to match the voltage.Duo to the diversity in circuit structures and working principle of them,the fault characteristics of each DG model are significantly different for cable faults and also for their protection requirements.Therefore,a comprehensive study for the cable fault responses of DC microgrids with various types of DGs is needed to gain the fault features as a whole,which will be conducive to clarifying the protection requirements and formulating protection schemes accordingly.In this paper,the grid-connected interface,photovoltaic power,wind power and battery module in DC microgrids are taken into account.The fault processes of VSC and PV for cable faults are analyzed in depth.The protection requirements and components seriously affected by the fault are pointed out.Moreover,the factors leading to those phenomena are also researched.Then a protection scheme that combines current differential protection with directional protection is proposed,advantages of which are high speed and selectivity.Based on the PSCAD/EMTDC,DC microgrids model is built to simulate cable faults of each module.At the same time,the simulation results are compared with the theoretical faults waveforms,which verifies the correctness of theoretical faults analysis.What's more,the performance of the proposed protection scheme is tested by specific simulation examples.The main contents of this paper are as follows:1.The working principle of grid-connected interface devices,photovoltaic power module,wind power generation system and storage battery in DC microgrid is analyzed.Then the circuit structures of converters for each module are determined and the control strategies are designed.Based on this,a typical DC microgrid simulation model is built with PSCAD/EMTDC.This part of the work lays the foundation for the rest of the work,such as theoretical analysis and verification for cable faults,extraction of transient fault characteristics and design protection schemes.2.The transient responses of VSC,PV and storage battery module in DC microgrid to different cable faults are studied.The transient process analysis is carried out in stages with fault circuits and relative equations.Moreover,the fault current characteristics of cable and converters are pointed out.Considering the vulnerability of uncontrollable power electronic devices in converters,the components seriously affected by faults are studied,and the fault stages threatening the safety of converters are given.All of this provides theoretical basis for defining DC microgrid protection requirements and designing protection schemes.3.A protection method combining current differential protection with directional protection is proposed.In DC microgrid,the power supply current of each power module is random as a result of the changing of external environment,which makes the setting of overcurrent protection difficult.For line current differential,it is almost zero in all states except to the moment of fault occuring with larger peak value.Furthermore,there is the difference only in amplitude with the increase of fault distance and transition resistance.So it has full-line high speed.This paper presents a protection scheme which combines current differential protection with directional protection in consideration of their quickness and selectivity.For the former,a new setting principle is proposed.It selects the fault point where the diodes in the converter can be turned on simultaneously during the pole-to-pole cable fault as the set point.The current differential value is mainly a function about voltage-stabilized capacitors and fault cable line and is less affected by the type or the operation state of DGs.Therefore,it has the advantage of great generalization.For the latter,a starting condition using the reverse zero-crossing point of power at one end of the fault cable is given,which is helpful to improve the speed and reliability of directional protection and avoid the influence of load current.In addition,according to the difference of current characteristics between fault pole and another,the methods of fault type and fault pole selection are discussed.Finally,the protection performance is verified by the DC microgrid simulation.