On series-connected renewable generator capable of providing power quality enhancement
Date of Issue2015
School of Electrical and Electronic Engineering
Incorporation of renewable generation to low voltage distribution network will have significant impacts on the power quality and the reliability of electricity supply. This is because the renewable power is often based on the harnessing of energy from highly unsteady sources. In this connection, this research work begins with a review of power quality issues in grid systems and the impacts of distributed renewable generator (DRG) on power quality. In this part of the study, it is shown how incorporating the DRG with ancillary functionality can contribute toward reinforcing the distribution grid and improves the power quality of the supply system. As photovoltaic (PV) is one of the most promising renewable energy resources in distribution systems, the study focuses on the design of PV DRG, while taking into consideration the possibility of extending the approaches to other renewable energy resources as well. Furthermore, as series compensation technique is much more effective in maintaining voltage quality of sensitive loads in tightly coupled networks such as the Singapore network, this thesis examines the design of a series-connected photovoltaic generator (SPVG) capable of providing load low-voltage ride-through (LVRT) while maximizing the energy harness from the sun. Analysis of the SPVG operations under disturbance conditions shows explicitly how network voltage quality is affected by the SPVG injected power and its apparent power rating, and that voltage quality can be significantly improved even with a modest level of energy storage capacity incorporated into the SPVG. A control system for the SPVG is also proposed. Both simulation and laboratory tests confirm the efficacy of the proposed distributed generator system. The study then extends to include a probabilistic analysis of LVRT capability enabled by the SPVG, as a consequence of the stochastic behavior of solar irradiance, load changes and voltage disturbances. A statistical approach is used as a means to assess the effectiveness of the voltage quality enhancement, by treating the input solar power to the SPVG, the load level and the occurrences of voltage disturbances as random variables. The developed statistical model is then utilized to assess the load LVRT capability as it is impacted by the capacity and the aging effect of the energy-storage capacitor incorporated in the SPVG. The theoretical results are again validated through simulation and laboratory tests. A generalized approach is then proposed in the thesis to examine the capability of a DRG, incorporated with energy storage system (ESS), in providing load LVRT. With the renewable power, load demand and the occurrences of low-voltage incidents treated as random variables, the probability of successful load LVRT is assessed through the use of copula function to quantify the stochastic dependency between the load and the renewable power. The analysis is subsequently applied to the case of the proposed SPVG incorporated with capacitor energy storage, wherein the focus is to establish the analytical relationship between the probability of successful load LVRT and the rated power/energy capacities of the DRG-ESS. Determination of the optimal capacities of the DRG-ESS is achieved through the maximization of the expected economic benefits obtained from the renewable energy harness and load LVRT minus the cost of the DRG-ESS.
DRNTU::Engineering::Electrical and electronic engineering::Power electronics