Voltage Stability Enhancement of Distribution Systems with Renewable Energy

Due to the increasing use of distributed generation (DG) in existing distribution networks, it is essential to have prior knowledge regarding the relevant power system stability in different generation scenarios. To mitigate the constraints related to voltage and power, the development of a proper reactive power planning methodology and the design and implementation of high-performance controllers are crucial.The first contribution of this thesis is an investigation into the dynamic behavior of active distribution systems with different types of load models to identify critical issues. The analyses show that a high penetration of DG affects the voltage stability of distribution systems and dynamic characteristics of induction motors influence the transient voltage recovery phenomenon. The scope of such analyses demonstrates that it is important to consider dynamic characteristics of loads in order to specify any planning task. To ensure fast voltage recovery after a sudden disturbance, a reactive power planning algorithm is developed based on a new index which reduces the costs of compensation by reducing the required sizes of compensating devices. The effectiveness of the proposed algorithm is verified on widely used distribution test systems with different types of DG units.The second contribution of this research is to analyze the impact of various load compositions on the dynamic voltage stability of distribution networks with renewable energy and present a new control methodology for distribution static compensators (D-STATCOMs) to ensure grid code-compatible performances of DG-integrated systems. The results show that the stability of a system can be drastically affected by a higher proportion of induction motors in load compositions. As loads in a practical system constantly change due to variations in consumer demands, the controller design process takes into account different load compositions in the composite load model. The performance of the proposed control scheme is demonstrated through detailed simulations using nonlinear models of the devices. The other significant contribution of this dissertation is the design of a novel controller for damping oscillatory voltage modes of distribution systems. These modes have been identified as a result of the work in this thesis and they arise due to the proximity of a DG unit to an induction motor load. The critical parameters which affect these modes are carefully identified and, during the controller design process, a bound is provided for their variations. Thus, uncertainties are targeted very carefully which keeps the bound on the modeling uncertainty to a minimum level and it is found that the proposed voltage controller is robust against all types of domestic motor loads considered in this thesis. The final contribution of this research is an examination of controller interactions in distribution networks with multiple DG units and the design of decentralized output-feedback robust controllers which consider intermittent natures of renewable energy as modeling uncertainties. As the results show that nonlinear interactions among different DG controllers cause oscillatory behavior in the system, the interconnection effects of other subsystems are considered in the design process to avoid negative interactions among DG-subsystems. The performances of the designed controllers are compared with those of conventional controllers. Simulation results indicate that the designed controllers are robust and ensure stability under varying operating conditions