| A microgrid is an important way to use renewable energy,which owns the characteristics of flexibility,efficiency and power supply reliability.Therefore,the rapid development of microgrid technology is promoted.The microgrid in isl anding mode is mainly deployed in remote islands or rural areas.To meet its power supply needs,wind turbines,photovoltaics,diesel generators,energy storage and other distributed generators(DGs)are connected to the islanded microgrid.Diesel engines and gas turbines are connected to the AC microgrid through synchronous generators(SGs)with large inertia and strong overcurrent capability,which belongs to SG-interfaced DGs and can guarantee the power supply reliability of islanded microgrid.New energy is connected to the AC microgrid through flexible and controllable inverters,which belongs to inverter-interfaced distributed generators(IIDGs)and can guarantee the power supply quality of islanded microgrid.However,SG-interfaced DGs and IIDGs have differences in physical structures and control modes(hereinafter referred to as heterogeneous),which would lead to transient instability of the microgrids under large disturbances and then lead to system collapse and emergency outage.At present,the research on transient stability of microgrid mainly focuses on a single type of DG connected to the system.There is a lack of systematic research on transient angle instability caused by the interaction between heterogeneous DGs and transient voltage instability caused by the interaction between DGs and loads.For this purpose,this paper focuses on the transient stability of islanded microgrid formed by heterogeneous DGs and different loads and conducts research on transient angle stability,transient voltage stability and the coupled problem of transient voltage/angle stability.The main contributions of this paper are given as follows:(1)For the transient angle instability of islanded microgrid under interaction between heterogeneous DGs,the mechanism of transient angle instability caused by“source-source” interaction is explored.It can be found that the microgrid with heterogeneous DGs is more prone to instability than the microgrid with IIDGs.A Lyapunov function based on T-S fuzzy model is used to further quantitatively analyze the influences of key parameters of microgrid on transient angle stability.Based on theoretical analysis,a transient angle stability improvement method based on torque adaptive adjustment is proposed.This method can regulate the equivalent input torque of the system through the angular frequency difference between heterogeneous DGs,and then the transient angle instability of islanded microgrid is improved.(2)For the transient voltage instability of islanded microgrid under interaction between DGs and loads,the mechanism of transient angle instability caused by“source-load” interaction is investigated.It can be found that the dynamic interaction between DGs would affect the input power and dynamic characteristics of the l oad,and then would deteriorate transient voltage stability of the microgrid.A calculation method of critical clearance time(CCT)is proposed to quantify the influences of microgrid parameters on voltage stability.Based on theoretical analysis,a voltag e stability enhancement method based on frequency fast tracking is designed.This method can enhance the transient voltage stability of islanded microgrid by reducing the angle difference between DGs during the transient.(3)For the transient voltage instability of islanded microgrid with different loads,the effects of different loads and DGs’ voltage regulation methods on transient voltage stability of the microgrid are comparatively studied.The results show that the microgrid may lose its voltage stability when induction motors(IMs)or rectifiers are connected.Moreover,the dynamic characteristics of loads are affected by the DGs’ voltage regulation methods,which would further influence the microgrid voltage stability.Further research shows that Q-V droop and constant reactive power control modes of IIDGs’ reactive power loop would deteriorate transient voltage stability of the microgrid,while SGs’ excitation systems would help restore the bus voltage after the fault is cleared.Based on this,a control method is proposed with considering the voltage stability enhancement and the limitation of inverter fault current.The method can effectively enhance the transient voltage stability of islanded microgrid without overcurrent produced by inverters.(4)For the coupled problem of transient voltage stability and transient angle stability,a coupling model of voltage stability and angle stability is derived,and the interaction mechanism of voltage instability and angle instability is explored.It can be seen that the microgrid with low inertia is more prone to the coupling phenomenon of voltage instability and angle instability.Furthermore,a transient instability identification method considering the instability coupling problem is proposed and the influence of different parameters on the coupling of voltage instability and angle instability of the microgrid is discussed in this paper.Based on theoretical research,a comprehensive strategy is proposed with considering voltage and power angle stability enhancement,which can resolve the problems of transient voltage instability and transient angle instability in islanded microgrids.(5)Combined with the research results of this paper on transient instability mechanism analysis and stability improvement methods,an islanded microgrid project is taken as a case,the transient stability of the microgrid project is analyzed,and the overall stability improvement scheme of microgrid is proposed.The hardware-in-the-loop system of the microgrid is established on the RT-LAB platform.The applicability of the theoretical results and the stability improvement scheme in the microgrid project is verified.This paper takes the islanded microgrid formed by heterogeneous DGs and different loads as the research object.The mechanism of different transient instability problems caused by “source-source” interaction and “source-load” interaction is studied and the transient stability margin of islanded microgrid is quantitatively estimated.On this basis,an improvement method for transient stability of islanded microgrid is proposed.The transient stability mechanism studied in this paper is the theoretical basis for the design of stability improvement scheme for the islanded microgrid,and provides a theoretical basis and technical guarantee for the safe and stable operation of islanded microgrids. |
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