Transient stability and performance based on nonlinear power flow control design of renewable energy systems

In this paper, the swing equations for renewable generators are formulated as a natural Hamiltonian system with externally applied non-conservative forces. A two-step process referred to as Hamiltonian Surface Shaping and Power Flow Control (HSSPFC) is used to analyze and design feedback controllers for the renewable generator system. The results of this research include the determination of the required performance of a proposed Flexible AC Transmission System (FACTS)/storage device, such as a Unified Power Flow Controller (UPFC), to enable the maximum power output of a wind turbine while meeting the power system constraints on frequency and phase. The UPFC is required to operate as both a generator and load (energy storage) on the power system in this design. Necessary and sufficient conditions for stability of renewable generator systems are determined based on the concepts of Hamiltonian systems, power flow, exergy (the maximum work that can be extracted from an energy flow) rate, and entropy rate. An illustrative example demonstrates this HSSPFC methodology. It includes a 600 kW wind turbine, variable speed variable pitch configuration. The wind turbine is operated with a turbulent wind profile for below-rated wind power conditions. The wind turbine is connected in series through a UPFC to the infinite bus. Numerical simulation cases are reviewed that best demonstrate the stability and performance of HSSPFC as applied to a renewable energy system.