Abstract
Nowadays, renewable energies are considered a suitable option to generate clean energy. As a result, a new generation scheme has gained interest in the recent years: the distributed generation. Due to the lack of a public electric power grid in remote areas, as well as the need of fostering energy cogeneration using renewable energy, it is necessary to consider small distributed generation (DG) systems with energy storage capacity. These DGs have to feed both critical and non-critical loads starting from various kinds of energy sources. This has led to the concept of a new DG system called microgrid.
An essential subsystem in every microgrid is the power electronics conversion equipment, which must be able to work in two different operation modes: the grid connected and the island modes.
In this Ph.D. Thesis a photovoltaic (PV) microinverter capable of operating in both island mode and grid connected mode by means of a reconfigurable control scheme is proposed. The main advantage of control reconfiguration is that in grid connected mode the microinverter works as a current source in phase with the grid voltage, injecting power to the grid. This is the operation mode of most commercial grid connected PV inverters. The idea is to provide those inverters with the additional functionality of working in island mode without changing their control algorithms, which have been developed and refined over the time for grid connected mode operation. It is proposed that in island mode the microinverter control is reconfigured to work as a voltage source using droop schemes. These schemes consist in implementing P/Q strategies in the microinverters, in order to properly share the power delivered to the loads.
Moreover, it is necessary to identify when the microinverter is disconnected from the main grid (islanding situation). Different islanding detection schemes have been analyzed in order to identify this situation. One solution reported in previous literature is based on the addition of a small-amplitude second order harmonic to the inverter output current. When the grid is disconnected (islanding situation), the PCC voltage follows the waveform of the current injected by the inverter, so that a small second harmonic can be detected. The proposed islanding detection method is based on measuring this second harmonic of the PCC voltage waveform by means of the Goertzel algorithm.
The general aim of the thesis is to show that the proposed control reconfiguration is possible without dangerous transients for the inverter or the loads. Simulation and experimental results on a 180 W, PV microinverter are provided to show the feasibility of the proposed control strategy.