Reactive Power Control and Optimization of Large Scale Grid Connected Photovoltaic Systems in the Smart Grid

In future smart grid, DC-AC inverter based renewable energy sources will greatly participate in not only the real power generation but also reactive power compensation. Among all the renewable energy sources, grid-connected photovoltaic (PV) systems have received much attention by engineers and researchers. Grid-connected PV systems with DC-AC inverters are able to supply real power to the utility grid as well as reactive power. The real power extracted by the DC-AC inverters is usually at the maximum power point (MPP) of the attached PV arrays and the reactive power is used to compensate the grid demand. However, the considerable number of DC-AC inverters and the complicated operating environment result in challenges in the control and optimization of both real and reactive power.;For large scale grid-connected PV systems with multiple DC-AC inverters, due to the limited apparent power transfer capability of each inverter, the reactive power needs to be allocated among the DC-AC inverters in a proper way. In this thesis, an optimal strategy is proposed for the reactive power allocation in large scale grid-connected PV systems. The proposed method achieves the maximum reactive power transfer capability of the entire system by applying classic Lagrange multiplier method. The sufficient conditions of the optimal reactive power allocation strategy are provided and mathematically proved. Then, the optimal solutions of the reactive power allocation in the large scale PV systems are developed into a distributed optimization algorithm by using dual theory. The cost function of the optimization problem is proved to be convex and the original optimization problem is decomposed into multiple sub-problems. Each inverter only needs to solve the sub-problem subjected to it and then the optimal solutions of each sub-problem lead to the optimal solutions of the entire system. Later, control schemes are proposed by applying the balancing strategies for the reactive power. Reactive power balancing strategies have been discussed for uniform distribution and optimal distribution based on derived optimal solutions. Invariant sets are presented to denote the desired distribution of reactive power among inverters and stability analysis is conducted for the invariant sets for different conditions by using Lyapunov stability theory. Finally, the proposed optimal reactive power allocation strategies, the distributed optimization algorithm, and the reactive power balancing strategies are validated in case studies against a sample large scale grid-connected PV system.