Design and Control of an Inverter for Photovoltaic Applications

The energy demand in the world is steadily increasing and new types of energy sources must be found in order to cover the future demands, since the conventional sources are about to be emptied.

One type of renewable energy source is the photovoltaic (PV) cell, which converts sunlight to electrical current, without any form for mechanical or thermal interlink. PV cells are usually connected together to make PV modules, consisting of 72 PV cells, which generates a DC voltage between 23 Volt to 45 Volt and a typical maximum power of 160 Watt, depending on temperature and solar irradiation.

The electrical infrastructure around the world is based on AC voltage, with a few exceptions, with a voltage of 120 Volt or 230 Volt in the distribution grid. PV modules can therefore not be connected directly to the grid, but must be connected through an inverter. The two main tasks for the inverter are to load the PV module optimal, in order to harvest the most energy, and to inject a sinusoidal current into the grid.

The price for a PV module is in the very moment high compared with other sources. The lowest price for a PV module, inclusive inverter, cables and installation, is approximately 30 DKK! per Watt (app. 4.0 € per Watt), or about 5000 DKK (app. 670 € per system) for a standard PV module and inverter with a nominal power of 160 Watt. This corresponds to a production-price of 0.24 € per kWh over a time period of 25 years, which cannot yet compete with other energy sources. However, it might be profitable for domestic use, since in does not have to take duty, tax, and wage for regular cleaning of the PV module, etc, into consideration.

One method, among many, to PV power more competitive is by developing inexpensive and reliable inverters. The aim of this thesis is therefore to develop new and cheap concepts for converting electrical energy, from the PV module to the grid. Research has therefore been done in the field of inverter technologies, which is used to interface a single PV module to the grid. The inverter is developed with focus on low cost, high reliability and mass-production.

The project contains an analysis of the PV module, a specification based on the analysis and national & international standards, and a state-of-the-art analysis of different inverter topologies. Two new topologies are discovered, and a topology is selected for further design. The inverter, with belonging auxiliary circuits, is designed and a prototype is build. The prototype is tested at the test facilities of Teknologisk Institut. The project has resulted in an inverter, which can be massproduced within a short time.