Research On Key Technologies Of Inverters With Voltage-mode Control In AC Microgrids

Nowadays,the technology for microgrid is becoming a research focus due to the vigorous development of the renewable energy generation.Microgrid is proposed to integrate all kinds of distributed generation(DG)units and energy storage elements,supplying stable,reliable and high-quality power to customers.Generally,the DG units connect to the microgrid through the interfacing inverters,which can be classified as current-controlled mode inverters(CCMIs)and voltage-controlled mode inverters(VMCIs)according to the controlled mode.The VMCIs can directly supply the loads within the microgrid in the case of power failure of the main grid thus becoming the indispensible elements in a microgrid.In this paper,researches on several key technologies for the VMCIs using in microgrids are done.They are:the power decoupling strategy of virtual negative resistor(VNR)used for the VMCIs in a low-voltage,the small-signal power stability of the microgrid,the operation and control stragegy of the VMCIs in a distributed energy storage system within a low-voltage microgrid,and the control of the VMCIs in the aspect of the microgrid power quality.Firstly,the performance of the VNR power decoupling strategy used for the VMCIs in a low-microgrid is studied.The power coupling model for a droop-controlled VMCI is deduced,based on which the power coupling issue in a low-voltage microgrid is present.The effect of the basic VNR strategy on the power decoupling is discussed by ways of impedance analysis of the VMCI.However,it is found that the grid-connected VMCIs may present non-fundamental instability if the line resistance drifts to be smaller than the value of VNR with the basic VNR method.Furthermore,an interesting conclusion is obtained that,if the PI voltage regulator and the basic VNR are used,the dc cormponents of the grid-connected currents for VMCIs certainly reach to the critical stable state when the virtual resistance is exactly equal to the line resistance.In order to produce a more stable system,an improved VNR form based on a proportional-resonant structure is proposed in the ?? stationary frame.Using the improved VNR method,the goal of power decoupling can be achieved and the system damping can be increased as well.In addition,the effects of the transformer for isolating purpose and the local load on the performance of the VNR method are discussed.Secondly,a stability analysis method for the parallel VMCIs is proposed which is based on the using of P/?(Q/E)'admittance'.Power control of each VMCI is deduced to be a two-terminal network where one 'open-circuit voltage' connects in series with one 'admittance'reciprocal,thus,the power model of the whole parallel system can be obtained.Through theoretical analysis,a conclusion can be drawn that the stability for the grid-connected mode is determined by each 'admittance' in the network,while the stability for the stand-alone mode is determined by the reciprocal of the sum of all the 'admittances'.Furthermore,two practical conclusions used for the design of the stand-alone system are obtained by this method.Thirdly,the operation and control stragegy of the VMCI in a distributed energy storage system(DESS)within a low-voltage microgrid is disgusted.The system impedance of the DESS is designed to be mainly resistive,accordingly,the Q/? and P/E droop functions are used.In order to design the power controller and dc link voltage controller,the small-signal system models are deduced using the methods base on average power definition and instantaneous power definition respectively.Design results indicate that the latter small-signal model is more accurate to describe the dynamic performance of the system.An experimental microgrid platform consist of two 40-kVA DESSs is built to validate the accuracy of the latter-built model and actually realize the seamless switch between the grid-connecting and islanding mode for the microgrid.Finally,the application of the transient virtual resistor is studied using in the hybrid voltage and current(HVC)control approach for the VMCIs in a microgrid.The basic theory and implementation of the HVC control approach is introduced,and accordingly the small-signal model of the VMCIs controlled by the HVC method is deduced.Results of the stability analysis indicate that the low-order resonant controller in the HVC method may cause the low-frequency instability of the grid-connected currents.To improve the system stability,a transient virtual resistor is proposed to insert between the VMCIs and the microgrid.The inserted virtual resistor is handled by a high-pass filter in order to avoid the power coupling issue.