Three-Phase Boost-Type Grid-Connected Inverter

Abstract

Alternative energy sources, such as solar energy and fuel cells, are desirable due to their pollution-free property. In order to utilize the present infrastructure of the utility grid for power transmission and distribution, grid-connected DC-to-AC inverters are required for alternative energy source power generation. For many of these applications, the input dc voltage is usually below peak voltage of the output and may vary in a wide range. Thus single-stage buck-type inverters may not be adequate, since they have very limited input voltage range and require the input DC voltage to be higher than the peak of the output voltage. For this reason, two-power-stage topologies, cascaded topologies and multilevel topologies are reported for applications where the input voltage is lower than the peak of the output voltage. Typically, one DC-DC power stage is required to boost the DC voltage in addition to an inverter for DC-AC conversion, which yields increased circuitry complexity. One-stage inverters for low DC voltage to high AC voltage conversion have been reported for non-grid-connected inverters based on the topology of a current source inverter. In this paper, the one-cycle control (OCC) method and the pulse width modulation (PWM) method have been proposed for a three-phase boost-type grid-connected inverter. The inverter features a single power stage that converts dc power to grid-connected ac power by injecting three in phase sinusoidal currents into grids, which may reduce power losses and circuit complexity. The input dc voltage is lower than the peak grid voltage and can vary in a wide range, which greatly suits the power conversion from photovoltaic or fuel cells to grid lines. The DC inductance may be kept low because the average DC current is maintained constant in a switching cycle. With the OCC method, the inverter preserves the advantages of simple circuitry, good stability and fast dynamic response and maximum power point tracking (MPPT) function can be conveniently integrated into the control core. Experiments have been performed with a 1.5-kW laboratory prototype that demonstrated the good performance of the inverter and MPPT function.