The Research On Three-phase Inverter Parallel Control Strategy In Microgrid

In the microgrid composed by distributed power sources, inverters are the interfacebetween the all kinds of micro-sources and the grid, so the control technology of invertersare very important. As the increase of the capacity of distributed power system, the capacityof the inverters should also be enhanced. The parallel of inverters is a good way to satisfythe requirement.This paper researched some key technologies for the parallel of inverters inmicro-grid.First, the paper researched the droop control of three-phase inverters, analyzed theprinciple of the parallel strategy. The conventional droop control strategy will result in themagnitude and frequency deviation of the inverter output voltage. To reduce deviation,the parallel of low pass filter and band pass filter is used after the active power and reactivepower are calculated. The method can also improve the dynamic performance in the powersharing between inverters. The small-signal model of the droop control is estimated and theinfluence of the control parameters to the dynamic and stable performance is also analyzed.Second, the zero-sequence circulating current between parallel inverters with commondc bus is researched. The paper built the average model of the parallel inverters withcommon dc bus. Based on the model, the causation of zero-sequence circulation, thedifference of zero-sequence voltage introduced into the inverters’ output50Hz voltage, isgiven. The small-signal model of the parallel inverters with common dc bus is given andthe coupling between the different variables is analyzed. The quantitative analysis of therelationship between the active power circulation, reactive power circulation, theparameters of the LC filter and the amount of zero-sequence circulation is provided. Thestrategy to suppress the zero-sequence circulation is give, as well as the range of the actionof the strategy.Third, the model of three-phase inverter connected to the grid is built and thecontroller is designed. In order to eliminate the low frequency harmonics current in theoutput current of inverters, quasi-resonance controller is applied and the estimation of thecontroller is given. The effectiveness of the method is verified by simulation.Finally, an improved decoupled double synchronous reference frame PLL is provided.The transilience of phase, the asymmetric of three-phase voltage, the pollution of lowfrequency harmonics voltage is existed, which will make against the operation of inverters. The PLL can estimate the phase of the three-phase voltage, even if the voltage isasymmetric, which is verified by experiment and simulation. In order to suppress thenegative influence of the harmonics, band trap for6thharmonics voltage is added in thePLL control loop, which is verified by experiments.