Study On The Control Strategy Of Off-grid Parallel Inverters

Micro-grid is a novel topology of electricity system network, it can work both in grid-connected mode and off-grid mode. In off-grid mode, inverter serves as the interface unit for micro-grid system. The control performance of inverter and the issues existing in multiple parallel inverter applications will have an influence on the characteristics of micro-grid system. Taking two off-grid inverters as the research object, this thesis investigates its effectiveness and suppression method under the non-linear load and circulation current between the inverters.Based on the analysis of three phase off-grid inverter system model, the mathematical model is built. Control strategy is developed and simulation research verifies its correctness. In this thesis, dual-closed loop control strategy and droop control strategy of off-grid inverter are studied. Simulation and experimental results have verified the effectiveness and correctness.In this thesis, the influence of load harmonics effect on the output voltage of off-grid inverter is studied based on the angle of output impedance. After researching load current full feed-forward control strategy, a feed-forward compensation strategy of positive and negative sequence harmonics regulator(PNSHR) method is proposed. And by means of reshaping the output impedance at the harmonics frequencies, the parameters of PNSHR design method is derived. The modified load current feed-forward based on PNSHR compensation strategy can significantly reduce the output impedance of the 5&7 times harmonic loads, the effect of non-linear load on off-grid inverter output voltage is restrained.Parallel-connected inverter system can not only enhance the power level of the system, but also reduce the redundancy. However, circulation current can seriously influence the operation stability. Based on the proposed PNSHR feed-forward compensation strategy, the virtual impedance loop is adapted to depress the circulation current. Through the verification of simulations and experiments, this method can highly reduce circulation current, the accuracy of power sharing is improved.