I. Introduction
Recently, the application of photovoltaic assisted uninterruptible power supply (UPS) systems for poor quality utility power grids has been reported [1], [2], [3]–, where a conventional bi-directional inverter is used in an “in-line” configuration as shown in Fig. 1. In this scheme, a dc-dc converter/charge controller is used to operate the PV generator at a DC voltage corresponding to the maximum power at varying solar insolation and ambient temperatures [4], [5]. The charge controller has to be rated for the full PV capacity regardless of whether the energy is intended to flow entirely into the battery or out to the AC grid system. The charge controller is not utilised or under utilised during the nighttime or when the solar power is unavailable. Since the maximum capacity factor for a PV system is less than 20%, most manufacturers prefer to omit the maximum power point tracking feature and resort to some simple on/off battery charge controller. A novel system that can extract maximum power from the PV panels, and provide reactive power support during the night, while retaining the UPS features of the system in Fig. 1, is proposed in this paper. The system has also the following additional features:
The PV panels operate at a higher DC voltage, resulting in greater overall efficiency and a reduction in wiring costs
Load voltage stabilisation
Unity power factor operation
Harmonic minimisation
Reduction in the overall converter power rating and hence cost
Operation in stand-alone and grid-connected modes.
Conventional grid-connected PV assisted UPS