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Abstract
The rapid and continuous growth of grid-connected solar photovoltaic (PV) power plants reflects the keen demand for sustainable energy. However, the output power of a PV power plant is intermittent and fluctuating, because of the variability of solar radiation. Such fluctuating power could challenge the stability and reliability of the electricity network. Currently, additional rules and standards are expected to be imposed on large-scale grid-connected PV power plants.
To satisfy future rules, one possible solution is the integration of an energy storage system (ESS) with the PV power plant to smooth the varying output power and improve its dispatchability. Consequently, this thesis features a novel rule-based power management (PM) algorithm for a large-scale grid-connected PV power plant including an ESS. This novel PM algorithm can effectively regulate the output power of a PV power plant, thus enabling such plant to operate effectively within the Australian National Electricity Market (NEM).
This thesis starts with an overview of various energy storage technologies considered suitable for large-scale power applications. The advantages and drawbacks of each technology are discussed in terms of power density, efficiency, stage of commercialisation to mention just few. However, only the vanadium redox battery (VRB) technology is considered in this thesis owing to its high scalability and design flexibility in terms of power and energy capacity.
A VRB-based ESS has efficiency issues when operating below 20% of its rated power. Therefore, a hybrid energy storage system (HESS) is considered for improving the efficiency and reliability of the VRB ESS. Several typical combinations of HESS are outlined. Owing to its high efficiency and long life span, a supercapacitor bank (SCB) is chosen for this thesis, resulting in an HESS comprising a VRB and a SCB. Additionally, the technical literature regarding the main challenge of using an HESS (i.e. power sharing between the different storage technologies of the HESS) is presented.
Power sharing between the VRB and the SCB is handled by the first part of the novel proposed PM algorithm, which considers the physical constraints of the HESS (i.e. power ratings and state of charge). The second part of the proposed PM algorithm mainly manages the operation points of the entire PV power plant in accordance with the dispatch rules of the Australian NEM. Furthermore, many technical issues caused by solar forecasting accuracies are specifically addressed by this PM algorithm.
Long-term continuous operation of the PV power plant is necessary to con-firm the benefit of the novel PM algorithm in terms of reliability, energy yield etc. Therefore, a trade-off between the accuracy and computational burden of the simulation model is made. Specifically, the large-scale PV array is modelled by a low-pass filter. The power converters are implemented using averaged models. The detailed equivalent circuit models of the VRB and the SCB are used to assess the performance of the HESS. All models and the PM algorithm are developed in a MATLAB/Simulink & PLECS simulation environment.
Comprehensive simulation results based on actual summer solar radiation profile are presented. First, a 30 MW PV power plant using a 7.5 MW VRB is demonstrated for short-term operation. Secondly, a 1.25 MW SCB is added to the above PV system to improve the operating performance of the VRB. Finally, a 30 MW PV power plant including an HESS based on the VRB and the SCB is discussed under many different scenarios.
Overall, the results reported in this thesis indicate that the proposed HESS with the novel PM algorithm provides a effective solution for large-scale grid-connected PV power plants, which significantly improves the dispatchability of the PV power plant in accordance with the Australian NEM. Results also indicate that the proposed HESS is able to improve the efficiency of the VRB and potentially prolong its lifetime. Additionally, this thesis, for the first time, documents a detailed and quantitative analysis of the impacts of forecast accuracies on the real-time operation of a PV power plant.