Research On Transformer-isolated Three-phase Three-wire Inverter Parallel System Without Interconnection

In order to solve the city’s traffic congestion, the Metro system were or will be introduced in many cities. Auxiliary inverter power provide three-phase AC power for ventilation unit, lighting load and electric heaters of the Metro, which ensure the normal and orderly operation of the Metro. Because auxiliary inverter power must be reliable and redundant.three-phase inverters in parallel are necessary. To improve system reliability, three-phase inverters parallel without interconnection is proposed.The control strategies of transformer-isolated three-phase three-wire inverter parallel system without interconnection is given based on dqO coordinate transform and PQ droop control in this paper. So the PQ droop control method and parameter selection are firstly introduced.Three-phase ac component can be converted into dc component by dqO coordinate transformer,which can be controlled without error in steady-state.Power calculation has an important impact of transformer-isolated three-phase three-wire inverter parallel system performance. Through analysis and comparison of existing digital power calculation methods, instantaneous power calculation besed on dqO coordinate transformation is introduced according to the instantaneous p-q power theory.It can calculate active and reactive power quickly and accurately, improving the performance of parallel system dynamic response.Reliable operation of transformer-isolated three-phase three-wire inverter parallel system is effected by the performance of stand-alone transformer-isolated three-phase three-wire inverter, and module and PI controller of transformer-isolated three-phase three-wire inverter are given based on dqO coordinate transformation.The state space module of transformer-isolated three-phase three-wire inverter is built based on dq0coordinate transformation,and the parameters of traditional dual-loop controller is designed based on dqO coordinate transformation. Experimental results demonstrate the feasibility and effectiveness of the control method.Three-phase load is often asymmetric, so this paper design dual-loop controller based on dqO coordinate transformation and feedback linearization control method to restrain adverse effects of asymmetic load.To verify the validity of the feedback linearization method based on dqO coordinate transformation.this paper builds transformer-isolated three-phase three-wire inverter module with matlab’s simulink and simulate it.The simulation results and experimental results verify the effectivity of the control strategy.Experimental waveforms and current data of two and three transformer-isolated three-phase three-wire inverters parallel system without interconnection verify the feasibility and effectivity of parallel control strategy based on dqO coordinate transformer and PQ droop method.Steady-state and dynamic performance are greatly improved.