Research Of Microgrid Protection Strategy Based On Traveling Wave

As a new type of grid structure,Micro-grid has advantages in the control and management of distributed generation and reducing the fluctuations of distributed generation.However,due to the existence of distributed generation,micro-grid has featured with two-way flow,small short circuit capacity and short protective action time.These features make the Micro-grid system difficult to protect.Therefor,developing appropriate protection strategies for microgrid can promote the use of micro-grid technology,which is significantly important.Grid-connected inverter is the core of distributed generation system connecting to power grid.This paper chooses the three-phase voltage source inverter as representative for research,and focuses on the three-phase inverter s mathematical model and its double-loop control strategy.A simulation model of a three-phase grid-connected inverter based on constant power control was built using PSCAD/EMTDC.Then,the propagation law of transient traveling wave in microgrid is analyzed:the folding and reflection rule of voltage traveling wave and current traveling wave at impedance discontinuity point.The reflection characteristic of the traveling wave at the end of the line is related to the equivalent impedance at the end of the line,and the voltage wave and the current wave have the opposite reflection rule at the end.The existence of inverter interfaced distributed generation at the end of the line does not affect the detection of the fault traveling wave signal,we can select the appropriate signal to detect according to the actual situation.Then the wavelet theory is elaborated,and the multi-scale and time-frequency characteristics of the fault signal under wavelet transform are analyzed.Wavelet transform of the fault signal can be decomposed into a combination of components on different scales.There is a definite correspondence between the scale parameters of the wavelet transform and the frequency of the fault signal.Fault signal in a scale of performance,corresponding to the signal in a certain center frequency within the determined range of information.And an improved method based on wavelet modulus maxima method is proposed to reduce the ranging error.And verified in the simulation.Finally,a fault location method based on wide-area initial traveling wave polarity and time information is proposed.This method firstly configures the traveling wave detection unit in the network according to the principle of configuration based on graph theory,and divides the micro-network group into several monitoring domains.The traveling wave arriving at the traveling wave detecting unit is used to determine which monitoring region the fault point is in,and the fault branch is determined according to the time when the initial traveling wave reaches the traveling wave detection unit in the monitoring region.This method reduces the traveling wave detection unit configuration without influencing the accuracy of the original fault line selection,thus saving investment and improving the economy effectively.The problems of sampling frequency setting,clock synchronization,communication problems and the main factors affecting the fault location of micro-network lines are analyzed in detail.And the feasibility of the method is verified by simulation on PSCAD/EMTDC.At the same time,it verifies the feasibility of locating the faults of the microgrid group using the principle of the double-ended traveling wave fault location.It is proved that the change of fault transition resistance,fault location,different fault phases and different fault initial phase angles.