Control Strategy For Single-Phase Grid-Connected Inverter With LCL Filter

Nowadays, the fossil fuels such as goal and oil are going to be exhausted, and meanwhile, the utilization of the fossil fuels has been seriously polluting the environment. Renewable energy including solar and wind energy has the advantages of cleanness, environment-friendliness, and it has been attracting more and more attentions. In the distributed generation (DG) system based on renewable energy, the grid connected inverter is the interface between DG and grid. This thesis is dedicated to the control strategies for single-phase grid-connected inverter with LCL filter.Compared with the single-inductor filter, the LCL filter provides better harmonic attenuation, provided that the sum of two inductors is equal to the one of the single-inductor filter. As a consequence, the size and cost of the LCL filter is smaller than the single-inductor filter. This thesis analyzes the output voltage harmonics of the single-polarity double-frequency SPWM controlled inverter, and the design guidelines for the LCL filter are presented. A 6 kW grid-connected inverter with LCL filter is taken as the example to illustrate the design of the LCL filter.The resonance of LCL filter may cause instability of system. Series- or parallel-connection of one resistor with the two filter inductors and filter capacitor, respectively, could damp the resonant peak. This thesis compares the characteristics of the six passive damp methods, and it is pointed out that the parallel-connection of one resistor with the filter capacitor can not only effectively damp the resonance peak, but also has no effect on the low-frequency and high-frequency harmonics attenuation, and the other five methods affect harmonics attenuation at low frequency or high-frequency. However, the paralleled resistor with the filter capacitor results in relatively large loss. The block diagram of the LCL grid-connected inverter with the resistor being paralleled with the filter capacitor is derived, and it is equivalently transformed to another block diagram, in which, the feedback of the filter capacitor current to the input of the SPWM modulator. It is revealed that the well-known active damping method with feedback of the filter capacitor current is equivalent to the passive damping method with one resistor in parallel with the filter capacitor. Furthermore, this active damping method has no loss.For grid-connected inverter, the grid voltage has great effect on the current injected into grid, including the steady state error and current harmonics. A full feed-forward of grid voltage is proposed for the grid-connected inverter with LCL filter to fully eliminate the effect of the grid voltage. This control strategy is simplified according to the grid voltage harmonics.Grid-connected inverter is capable of working in both grid-connected mode and stand- alone mode. This thesis proposed the method for seamless switching between the two modes and the current and voltage overshoot is avoided during the switching process.A 6 kW prototype of the grid-connected inverter with LCL filter has been built and tested in the lab. The experimental results are presented to verify the effectiveness of proposed feed-forward control strategy, seamless switching method, and the design of the LCL filters.