Autonomous Power Systems based on Renewables - On generation reliability and system control

Doctoral thesis, 2007

Today one billion people are living in non-electrified areas. A majority of the people are living in rural areas in developing countries where there are no possibilities, neither economical nor technical, for grid extension. Instead energy sources like wood and fossil fuels are used. However, these sources are not suitable due to environmental reasons and therefore electricity is favourable. Autonomous power systems are for that reason needed to secure a decent living standard and still preserving the environment. To be able to study the availability of solar power for electricity production in rural areas a method for solar insulation simulation is proposed. The method is based on cloud coverage according to the Oktas-scale. Cloud coverage is often available due to its low demand on technical equipment during measurement. Since continuous data series are hard to find the model is based on transitions between different levels of cloud coverage. A stochastic model for wind speeds for electricity production based on quantified wind speed measurements is also proposed. The model is only dependent on the site-specific yearly mean value of the wind speed if a distribution of wind speeds from a location with a similar distribution is available. Other models used are for a weak constant flow-of-river and for storage. The load considered was industrial with its maximum during daytime. The generation reliability of using only one production source was found to be low. Combinations of production sources or a storage capacity significantly improve the situation. Even if several different sources are used some over-capacity of power production is required. Normally, the generation is cut down to control the frequency of a system. The generation is cut down to save fuel but in systems with “free” fuel energy will only be wasted. To limit the waste a method based on load frequency control of a frequency converter is proposed. Some new frequency converters are equipped with an active rectifier also voltage control can be implemented. A control design for voltage control is also proposed. Both the frequency and voltage control are tested through simulations and are verified in a laboratory environment. The behaviour of the frequency control as well as voltage control is even better as compared to normal control methods. It has been shown that both the frequency and voltage controllers can easily be implemented in the same off-the-shelf frequency converter.