Low-cost current-source 1-ph photovoltaic grid-connected inverter.

Abstract

This research investigates a complete analysis and investigation of a single-phase currentsource (CSI) grid-connected inverter topology that is based on a photovoltaic array as a supply and a DC link inductor acting as a constant-current source. The proposed low-cost system is implemented using an open-loop control to prove the concept. Then a well-known feedforward compensation control is implemented to achieve acceptable total harmonic distortion of the inverter output current. A single boost switch (based on a switched-mode rectifier) has a duty-cycle that is modulated sinusoidally at the mains (grid) frequency such that it produces an output current that appears a full-wave rectified sinewave that is synchronised to the grid voltage. Additionally, a H-bridge inverter circuit and a capacitive-inductive low-pass grid filter is used to unfold, filter and feed the sinusoidal output current into the grid. A number of detailed PV array models are studied and used in the simulations. The relationship between the PV array output ripple and the DC link energy storage (element) for single-phase grid-connected inverters is analysed. The “balanced" ripple definition is introduced to estimate the PV array output power reduction due to ripple. The proposed grid-connected CSI topology is idealised which ignores : component losses, voltage drops, PWM switching and low-pass output filter resonance effects. Normalised simulations are carried out to investigate DC link energy storage, modulation index and irradiance variation effects with an emphasis on meeting the power factor (PF) and total harmonic distortion (THD) grid requirements. The low-pass grid filter optimisation for the proposed topology is studied showing a trade-off between the output current THD, power loss, and quality factor. A 160 W inverter is implemented and a set of comprehensive test results obtained to verify the simulations using open-loop and feedforward compensation control. To conclude, a 1.2 kW gridconnected inverter based on the proposed low-cost topology was designed and simulated. Its simulated efficiency of 95% was higher than that of the 160 W inverter.