Study On The Control Strategy Of Three-phase Grid-connected Photovoltaic Inverter Under Unba-Lanced Grid Voltage Condition

Unbalanced grid voltage will worsen grid-connected photovoltaic inverter’sworking conditions, such as exacerbating grid current distortion, disturbing MPPTprocess and so on. If unbalance is serious, the inverter equipment may even be burnedwhen failure to take appropriate measures. This thesis studied the control strategy ofgrid-connected photovoltaic inverter under unbalanced grid voltage, aiming at inhi-biting DC-link voltage’s2nd harmonic or AC negative sequence current respectively.In this thesis, the mathematical model of LCL filter based three-phasegrid-connected photovoltaic inverter under unbalanced grid voltage condition wasdeveloped. Modeling results show that the model in two phase synchronous rotatingframe is comparatively complicated, while the model in two-phase stationary frame isrelatively simple.Illustrated by the case of the perturbation and observation method, a qualitativeanalysis about the impact of DC-link voltage’s2nd harmonic on the maximum powerpoint tracking has been given. The analysis results show that too large DC-link vol-tage’s2nd harmonic may interfere with maximum power point tracking process, andeven lead to tracking failure.This thesis compares three kinds of positive/negative sequence component ex-traction (PNSCE) method whith simulation analysis:1. PNSCE based on notch filter;2. PNSCE based on decoupled double synchronous reference frame phase-locked loop(DDSRF-PLL);3. PNSCE based on dual second order generalized integrator restingon a frequency-locked loop (SOGI-FLL). Simulation results show that:1. PNSCEbased on notch filter has a faster dynamic response, but weak harmonic suppression;2.PNSCE based on DDSRF-PLL or SOGI-FLL has a slower dynamic response, but ithas a certain degree of harmonic suppression ability.Based on the extension pq theory, this thesis studies the instantaneous powercontrol at the point of common coupling (IPC-PCC), the instantaneous power controlat the converter pole (IPC-CP) and the control strategy of inhibiting AC negative se-quence current. Among them, the IPC-PCC and the IPC-CP are aimed at inhibiting2nd harmonic in the DC-link Voltage, while the control strategy of inhibiting AC neg-ative sequence current is aimed at keeping the grid current balance. Considering thedecoupling is not easy and more controllers are needed when using PI control in thetwo-phase synchronous rotating coordinate frame, the PR control in the two-phase stationary coordinate frame is selected. The simulation and experimental results showthat, these3kinds of control strategies achieve their goals respectively and all canmake the inverter work properly under unbalanced grid voltage, proving the validityof these control strategies.