Integrated energy storage-wind energy conversion for the mitigation of power quality problems

Abstract

The study presented in this thesis was conducted to investigate the power quality of wind turbines installed at Dassiesklip Wind Farm and the impacts they could have on distribution networks. The key emphasis is on flicker and voltage fluctuations as grid connected wind turbines could have enormous effect on them, particularly when they are connected to weak distribution networks. Thereby, proposed an introduction of energy storage system as one of the solutions to mitigate the identified power quality problems. The main objectives and the outline of the thesis are: ▪ To review existing information on power quality of wind turbines and the effects they could have on distribution networks. ▪ To study different types of wind turbines and their behaviour. ▪ To conduct and analyse power quality studies of wind turbines ▪ To study energy storage and their contribution to power quality problems ▪ To draw conclusions based on studies conducted Voltage flicker was identified as the main issue that affects the power quality of wind turbines due to fluctuations in wind turbines power output. Therefore, the study has been carried out specifically on flicker emission by wind turbines to show how it affects the power quality of the distribution networks where wind turbines are connected. The findings of flicker emission studies conducted showed that flicker occurs during both switching and continuous operations. Flicker caused by switching operations is due to the turbine generator connection and capacitor switching. During turbine generator connection, high currents are drawn, which could results to voltage dips. Capacitor switching is followed by high frequency inrush currents, which could result to transient. The wind turbine distributed generator was modelled in DigSilent power factory with and without the inclusion of battery energy storage to mitigate the effect of power quality in the distribution network. Thereafter, load flow studies were carried on the developed models to determine the effect of RMS fluctuations and transient events. Based on the results obtained, it was discovered that at low X/R ratios, the voltage variations are high and the voltage at PCC increases with an increase in active power generated by the wind turbine induction generator when no energy storage system was connected. However, this voltage increase is low at high X/R ratios, because the active power generated by the generator result to a voltage surge caused by the grid resistance. However, the inclusion of energy storage (battery storage) provides a rapid response for either charging or discharging the battery and also acts as a constant voltage source for the critical load in the distributed network, thus provided a better power quality. It can be concluded that the Dassiesklip Wind Farm should easily conform to the harmonic requirements at the 66 kV busbar at Dassiesklip substation with the future fault levels, without any intervention such as shunt capacitors, filters or transformer impedance reduction. The Wind Farm will conform to the flicker requirements. The wind farm will also meet the rapid voltage change limit if it is assumed to be operating at unity power factor before it is switched off. The main barriers for widespread commercial implementations of ES with WPPs are high cost of ES technologies, immaturity of some technologies and uncertainty over the quantified benefits. Government subsidies for ES would also speed up widespread use of the new ES installations with WPPs.