Current Control for a Single-Phase Grid-Connected Inverter Considering Grid Impedance
Type of DegreePhD Dissertation
Electrical and Computer Engineering
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The voltage source inverters are typically used to connect between distributed generation and the utility grid. In the grid-connected inverter, an output filter is often utilized in the inverter terminals to reduce the pulse width modulation (PWM) switching harmonics. To achieve a sinusoidal grid current with unity power factor, the single-loop proportional integral (PI) or proportional resonant (PR) controller is often utilized for current control in the grid-connected inverter. However, in the distribution system, the grid can have a large impedance, which will affect the inverter control performance, even the stability of the inverter system. Presented in this work are four control methods aimed at reducing the effect of the grid impedance. The impedance-based control is designed based on the relationship between the inverter output impedance and the grid impedance in the frequency domain. Through the analysis of the inverter output impedance in the frequency domain, the current controller parameters can be adjusted to regulate the output impedance to improve the control performance when the inverter is connected to a large grid impedance. State feedback control combined with a PI/PR controller is a robust control method. Based on the pole placement method, the system stability and dynamic performance can be specified directly by determining the closed-loop pole locations. The system stability, robustness to grid impedance uncertainties, as well as damping to reduce the LC filter resonance can be improved. Gain scheduling control is an adaptive control, which adjusts the controller parameters to make the system robust to grid impedance variations based on the grid impedance estimation. An optimal gain is determined by the desired controller bandwidth and the phase margin of the system. Therefore, the inverter control performance can be maintained even with the grid impedance variation. Grid-current observer-based compensation method aims to compensate for disturbances from the grid side; two different compensation control structures are proposed. The feed forward compensation uses the estimated grid current as a feed forward signal. The modified disturbance observer is utilized to compensate for the disturbance introduced into the inverter system. Both of the compensation methods are based on a grid current observer. Experiments were implemented on a 1 kW Texas Instruments single-phase grid-connected inverter with an LC filter to verify the effectiveness of the control methods introduced.