Controller Design for an Off-Grid Photovoltaic Solar Inverter

摘要

One of the key components in photovoltaic (PV) electrical systems is the inverter. It is the unit that converters the DC power generated from the solar panels or the batteries to an AC power that can supply the electrical home appliances. The issues associated with the inverter are power quality and harmonics. This paper introduces a controller design for a single phase full bridge inverter for an off-grid PV electrical system which supplies a typical home or an office. For a pure sinewave inverter, a sinewave pulse-width modulation (SPWM) scheme is used. This puts the switching harmonics far away from the fundamental 50Hz component, which eases the filtering process by a simple low pass filter (LPF). As a result, a sinusoidal AC voltage waveform is obtained. Then, a step up voltage transformer is used to step up the voltage to 220V. As the AC load increases, the drop voltage on the LPF and the transformer primary increases significantly. This causes the secondary voltage of the transformer to drop drastically bellow 220V. To solve this issue, a cascaded voltage and current control loops are designed to control the transformer primary voltage at exactly 12V so that the secondary voltage is held always at 220V. The inverter circuit with the closed-loop control is simulated fully using Power Simulation (PSIM) software. Then, the inverter circuit is built and tested experimentally in the laboratory using only the open-loop control, and this is due to the lake of LEM voltage and current sensors in the laboratory.