Research On Circulation Suppression Technology Of Photovoltaic Microgrid Parallel Inverter

With the increasing demand of AC load in photovoltaic microgrid,multiple small capacity photovoltaic inverters are often connected by the form of common AC bus in parallel for load power supply,so as to solve the high cost,low reliability and difficult maintenance problems of single large capacity inverter.However,the output voltage vector and the line impedance of each inverter are inconsistent,which will lead to larger circulation between photovoltaic microgrid parallel inverters.Circulation will accelerate the device loss,reduce the system efficiency,and seriously affect the stable operation of system.Therefore,the circulation suppression technology of photovoltaic microgrid parallel inverters is studied as follows in this paper.Firstly,the topology of three-phase three-wire voltage parallel inverter is studied,and the advantages and disadvantages of mathematical models are compared and analyzed under different coordinate systems.In two-phase static coordinate system,the QPR-P control strategy of voltage and current double closed-loop is designed to realize the output voltage non-static tracking of photovoltaic microgrid parallel inverters.Secondly,an improved robust droop control method based on virtual complex impedance is proposed aiming at the fundamental wave circulation problem of photovoltaic microgrid parallel inverters.The equivalent line impedance in fundamental wave domain is sensitive by introducing the virtual complex impedance,so as to eliminate the coupling of active and reactive power.The first order linear non-homogeneous differential equation about fundamental wave voltage is solved by adding the initial value of inverter output voltage and improving the integral coefficient.It reduces the fundamental wave voltage time of arrival in steady state to improve the circulation suppression speed.The output power of each inverter is calculated by final value theorem by setting the droop coefficient inverse with the rated capacity and the droop power standard value distribute with the inverter rated capacity ratio.It makes the steady-state output power reach power standard value to eliminate the bus voltage deviation of inverters with the same or different rated capacity after load switching and suppress the fundamental wave circulation effectively which is caused by inconsistent output voltage vector.The simulation results show that the maximum suppression effect is achieved in 0.13 s of fundamental wave circulation and the fundamental wave circulation is suppressed below 0.65 A.At the same time,the bus voltage is stable at 220V/50 Hz in steady-state.Therefore,this method can effectively improve the circulation suppression speed and system stability.Finally,a control method based on adaptive virtual harmonic impedance and multi-quasi proportional complex integral is proposed aiming at the harmonic circulation problem of photovoltaic microgrid parallel inverters.The equivalent line impedance in harmonic domain is assumed to be resistive by introducing the virtual impedance composed of virtual resistance and the main harmonic current.The difference of equivalent line impedance can be compensated adaptively next by using the inverse relation between harmonic current and impedance.It eliminates the harmonic circulation caused by the inconsistent of equivalent line impedance under nonlinear load.The harmonic voltage can track the zero instruction voltage well by designing MQPCI controller to adjust the gain and bandwidth of controller at the 5th and 7th harmonic frequencies.It eliminates the harmonic circulation caused by the inconsistent output harmonic voltage components from harmonic voltage suppression aspect.The simulation results show that the output voltage THD of each inverter is reduced to 0.52% and 1.56% after adding MQPCI controller,and the suppression effect of harmonic circulation caused by power reaches 90%.The harmonic circulation of parallel inverter is reduced from 0.7A to 0.063 A after the introduction of adaptive virtual harmonic impedance control,and the suppression effect of harmonic circulation caused by load reaches 91%.