Islanding Detection Methods For Distribution Generation System

Distributed generation system improves the flexibility and economy of intelligent distribution network,it is the key to the sustainable development of electric power.Islanding detection is an indispensable function of DG,which is related to the stable and safe operation of the power system.However,the existing methods are not strong enough to adapt to the load and parameters of main grid,otherwise,debasing power quality.In this context,this thesis studies more effective and reliable active islanding detection methods(IDMs).A non-destructive IDM based on the consistency of quality factor under harmonic of different orders is proposed.Firstly,the calculation formula of quality factor of the load after DG islanding is derived;Secondly,the quality factor before and after the DG islanding is analyzed: Since the inherent quality factor of the load has nothing to do with the harmonic orders,the quality factor under harmonic of different orders after the islanding is consistent;The “quality factor” calculated during grid-connected has no physical meaning,and the values under harmonic of different orders are greatly different;Using this characteristic to determine whether an islanding occurs;Finally,an islanding detection criterion based on the consistency of the quality factor is given.Based on this,combined with the harmonic current perturbation method of inverter DG,an islanding detection scheme is given.This method can realize the rapid detection of small non-detection zone(NDZ)in islanding,and the harmonic disturbance required to meet the calculation requirements is small;Compared with the harmonic voltage or impedance detection method,this method is more adaptable to main network parameters,and the threshold setting is simpler.The IDM based on reactive power-frequency characteristic curve is studied.Firstly,an improved reactive power injection mode is proposed,through the analysis of the NDZ in this mode,the condition that the positive feedback gain of the disturbance curve should meet is obtained,so that less reactive injection is required under the same detection target;Secondly,a reactive power disturbance method with positive feedback of phase difference is proposed,which introduces the phase difference between DG output voltage and current into the perturbation equation,makes the frequency shifts reliably,and the detection is quick and simple.Finally,the theoretical analysis obtains the value range of the phase difference positive feedback gain to ensure that the method will not cause positive feedback of reactive power disturbance to the system during grid connection.The Simulink simulation models of DG grid-connected system are established.The above IDMs are simulated under a variety of scenarios of different load and line parameters.The results show that proposed methods have stronger applicability,less influence on power quality,and higher reliability.