| Distributed energy has become the main form of power supply for microgrids through inverter interfaces,and inverters are of great significance in microgrids.At present,three-phase three-leg inverters are widely used.However,in microgrids,three-phase four-wire inverters are often required to provide a neutral connection for three-phase asymmetric loads.Among many four-wire inverter topologies,the four-leg inverter has been extensively investigated for its small size and high efficiency.In order to take into account,the existing three-leg inverter system,this paper adopts a three-leg/four-leg inverters hybrid parallel topology to meet the four-wire system requirements under asymmetric load conditions and improve the voltage balance.In this paper,the output voltage balance control of three-/four-leg inverters and the hybrid parallel control technology of three-/four-leg inverters are studied under three-phase asymmetric load conditions:To achieve stable operation of hybrid parallel systems,inverter voltage control should be achieved,with the first control goal being to balance the output voltage of inverters.In order to overcome the unbalanced output voltage caused by load disturbance and follow the controlling idea of adjusting the three-phase inductance currents to a certain unbalanced state to maintain the balanced state of three-phase capacitor voltages.Firstly,a phase-locked loop is used to detect the output voltages amplitude and phase imbalance index,Then the unbalance indexes of inductor currents are continuously adjusted by the error between voltage unbalance indexes and expected balance state to restore the balance state of capacitor voltages.According to whether the inductance currents have zero sequence component,Tan-Sun coordinate transformation or three-dimensional unbalanced coordinate transformation based on the inductance current unbalance indexes orientation is selected to realize the unbalanced control of the inductance currents,and the output voltage balance control of the three/four-leg inverter is realized by PI control in dq coordinate system or dq0 coordinate system respectively.Then,the hybrid parallel operation of three-/four-leg inverters and the control strategy of unbalanced load current sharing are studied.By calculating the output power of the inverter under asymmetric load conditions,the defects of traditional droop control and virtual impedance control under unbalanced conditions are found.To improve the sharing accuracy of power and unbalanced currents.The virtual impedance control is realized by combining the concept of virtual impedance and the voltage balance control strategy based on unbalanced coordinate transformation.Finally,the experimental platform of the three-/four-leg inverters hybrid parallel system is built,and the feasibility and effectiveness of the proposed three-/four-leg inverter parallel system,inverter output voltage balance control,and power and unbalanced load currents sharing control strategies are verified by experiments. |
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