Power-generation, Power-electronics and Power-systems issues of Power Converters for Photovoltaic Applications

Doctoral thesis, 2002

The thesis deals with issues related to solar energy systems, or more specifically, the electric systems of photovoltaic (PV) installations. Initially aspects of planning and estimating the performance of a PV-installation are considered. As the test site showed to have poor energy yield due to shading, a decentralised converter system was suggested for that and similar systems which result in a higher shading tolerance. The operation of single-phase inverters connected to the grid is investigated, especially the nature of hysteresis (bang-bang) controlled inverters and their interaction with the grid and other inverters. A novel switching method is suggested with similar features as of the hysteresis technique. The impact on harmonics on the grid from many inverters is investigated and showed to be decreasing with the increasing number of inverters. The design and construction of a 110 W, single-phase PV-inverter is presented and discussed. The advantages and disadvantages of a common ac-bus compared with a common dc-bus are treated. Voltage variations are discussed, for a low-voltage network with an extensive amount of photovoltaic generators or other types of embedded generation like fuel cells or microturbines. A number of analytic expressions are derived for the voltage profile of a feeder in these networks. Examples are given where the overvoltage on the grid due to active power injection is severe. However, allowing the inverters to inject or consume an amount of reactive power dependent on the local rms voltage is shown to eliminate this potential problem. A simple algorithm for this reactive power control is proposed and tested.