Research On Voltage And Frequency Control Of Islanded Microgrid Based On Electric Springs

Without the support from the bulk grid,islanded microgrid is prone to power supply and demand imbalance when renewable energy sources(RESs)power generations are connected,resulting in voltage fluctuations and frequency instability.Electric Spring(ES)can change the power of the non-critical load(NCL)connected to it by controlling its own output voltage,which provides new ideas for alleviating power quality problems such as voltage and frequency fluctuations.This dissertation focuses on the voltage and frequency control of islanded microgrids based on ES.In this dissertation,the method of expanding the voltage stabilization range of ES-1,the control strategy for simultaneously compensating critical load(CL)voltage and line current in three-phase four wire power systems,the strategy for voltage and frequency controlling of islanded microgrids based on ES adaptive sliding mode control(ASMC),the strategy for voltage and frequency controlling of islanded microgrids based on multiple ESs are studied.The main research contents and contributions of this dissertation are as follows:(1)For the narrow voltage stabilizing range of ES-1,a method of adaptively changing CL voltage target value to expand the voltage stabilization range is proposed.By sacrificing small control accuracy,ES-1 ensures that the CL voltage remains stable even when the power supply voltage exceeds the original voltage regulation range of ES-1.Secondly,a generalized ES-1 structure(GES-1)composed of several ES-1 and its control method is proposed.The voltage regulation range of ES-1 is effectively expanded by the hierarchical control of NCLs,and the time of high priority NCL participating in CL voltage regulation is reduced.(2)For the issues of CL voltage control and neutral line current caused by load imbalance,a strategy for compensating CL voltage and line current simultaneous is proposed.Firstly,the fruit fly optimization algorithm is used to search for the optimal reference output voltage of ES that minimizes the amplitude of the neutral line current.Then,PI controllers force the actual output voltage of ES to track this reference output voltage to compensate the CL voltage and line current simultaneously,ensuring the CL voltage is stable at the target value and the neutral line current is reduced.(3)For the issues of narrow stability range and weak anti-interference ability of traditional linear controllers used ES,this dissertation proposes to introduce the ASMC strategy into ES system,and the corresponding controllers are designed for two scenarios: only voltage is controlled,voltage and frequency are controlled.It has been proven through Lyapunov stability theory that both ES control systems are asymptotically stable.The simulation results show that the ASMC strategy can not only stabilize voltage and improve frequency regulation performances,but also has better static and dynamic performances and stronger anti-interference ability compared to PR/PI control.(4)For the problem that a single ES cannot support voltage control of multiple nodes and the limited capacity is not enough to support system frequency regulation,a consensus control strategy is proposed to coordinate the participation of multiple ESs in voltage control and frequency regulation.Firstly,with node voltage and system angular frequency as control objects,node voltage stability,frequency regulation performance improvement,and consistent voltage deviation ratios of each NCL are achieved based on consens control.However,there is still a problem that the reactive power output ratio of each ES is only of the same sign but not the same numerical value.Therefore,the voltage observers are further introduced to obtain the corresponding global average voltage estimated value for each ES.By comparing the node reference voltage with the estimated value instead of the node voltage,the proposed consensus control strategy is improved,so that the reactive power output ratio of each ES is consistent.