Research On Three-phase Four-wire Gird-connected Inverter With LCL Filter Under Unbalanced Grid Voltage

In wind and photovoltaic power generation network operation, the three-phase DC-AC inverter with the corresponding L or LCL filter is a critical component of the power system.Due to unfavorable factors such as disturbances or interferences, the unbalanced ac voltages are proved to be one of the greatest obstacle for grid inverters to operate normally. Under the unbalanced grid, zero sequence voltage and current components are not considered in the typical three-phase three-wire gird-connected inverter. When the system is connected with grid, it will appear problems of distorted load currents, power oscillations,overcurrent in the fault phase and so on. This paper studies the three-phase four-wire grid-connected inverter which provides a flow path for zero sequence components. Its control performances can be optimized through utilizing the power generated by zero sequence components. The system can completely inhibit power oscillations or reduce current magnitude in the fault phase. So focusing on effective control strategies of the three-phase four-wire gird-connected inverter to improve network operation’s ability running during a grid failure is very significant. The three-phase three-wire and the three-phase four-wire grid-connected inverter separately with the LCL filter are researched analyzed comparatively.Firstly, mathematical models of the three-phase three-wire grid-connected inverter system without considering zero-sequence components are established respectively in the three-phase stationary coordinate system, two-phase stationary coordinate system and synchronously rotating coordinate system. Comparison analyses of the current control strategies have been conducted in the two-phase stationary coordinate system and synchronously rotating coordinate system. After that, power oscillations and overcurrent in the fault phase of the three-phase three-wire gird-connected inverter can be analyzed from the power fed into the grid side.Secondly, to solve the problems existed in the three-phase three-wire system, a split-capacitor three-phase four-wire system is adopted in this paper with the dc side midpoint connected to the neutral line which can provide access to zero sequence components. Thus the new total power constraint equations are formed from the power generated by zero sequence components. The six unknowns and six equations show that the three-phase four-wire grid-connected inverter can achieve variable controllability completely. Simultaneously suppressing active and reactive power fluctuations can be carried out. In the case of suppressing active power fluctuation and negative sequence current, reactive power fluctuation and current magnitude in the fault phase can be reduced.Again, this paper briefly describes the LCL filter parameter selection principle and constraint relations between parameter and performance. At the same time in order to suppress adverse effects caused by resonance, this paper explores passive and active damping methods. The active damping method is widely used because of its no additional damping resistance and low power consumption. This article uses an active damping method of the filter capacitor in parallel with a virtual resistor, which has no differential part and no noise. Under active damping controls of the filter capacitor in series and parallel the virtual resistance, current control block diagrams of grid-connected inverter in the two-phase stationary coordinate system are analyzed comparatively.Finally, simulation models of the three-phase three-wire system and the three-wire four-wire system are built through MATLAB/Simulink to carry on simulation verification. An experimental platform are set up for experimental verification. The feasibility and superiority of the three-phase four-wire gird-connected inverter system under network operation have been proved by comparative analyses of simulation and experiment.