Research On Microgrid Current-limiting Strategy And Protection Method

Recently,technological advancements and government incentives facilitated the emergence of microgrids.The microgrid can effectively manage the energy between the distributed generation unit(DG)and the main grid,which can not only improve the stability of the main grid but also enhance the productivity of DG.But,the characters of microgrid,including small inertia,the frequent power flow in feeders,limited fault current and so on,pose challenges for microgrid current-limiting strategy and protection.Hence,it is necessary to research these challenges to enhance the reliability of power supply and the construction of strong power system.In order to construct a stable microgrid,this paper firstly studies a droop controlled microgrid,because the droop control has the character of conventional synchronous generator,needs no communication and so on.Studying this microgrid consists of two parts:(1)based on the basic principle of droop control,its shortages and improved method are analyzed,then the Washout filter is utilized to improve the traditional method,which can ensure the stable operation of microgrid,that is,the voltage and frequency of microgrid would not be various when the loads change.(2)because the traditional current-limiting strategy is unable to adapt to different short-circuit types,a current-limiting strategy based on improved filtering inductance reference current(FIRC)is presented.The DG with droop control is selected for investigation.Relationship among DG output current and voltage and FIRC is discussed with proportional-integral controller and proportional-resonance controller respectively.In the condition of these two controllers,theoretical derivation demonstrates that DG output current can follow FIRC by adjusting parameters of control system and filtering capacitance and not be affected by fault types,grounding resistance and so on.The weak fault of microgrid includes high grounding resistance fault and nonlinear high resistance fault.There are also three types of normal disturbance including the conversion between islanded mode and grid connected mode,load switching,and DG switching.If the weak fault and normal disturbance are not distinguished,the weak fault would develop into the strong fault and the protection device would have error action.Hence,this paper proposes a microgrid fault detection method based on dq transformation and wavelet package.The analysis current,set up by d-axis and q-axis current,is decomposed by wavelet package.And then,the fault is detected by the wavelet energy from the wavelet package.Lots of simulation experiments prove the effectiveness of proposed detection method.The fault detection method does not have the ability of selection.In order to ensure the selection of protection device and simplify the threshold setting,a protection method based on geometric expression and differential scheme is presented for microgrid.Analyses are conducted on the electrical quantity after the microgrid occurs fault.Then,it is introduced how to obtain the feature cosine of bus using ellipse curve and Levenberg-Marquardt algorithm.And the differential direction is obtained through feature cosine,which is used to detect faulty zone.Lastly,the effectiveness of this proposed method is proved by lots of simulation experiments.