The Coordination of Back-to-Back Starting and Protections for Pumped Storage Units

Using the state equations of synchronous motor, this paper establishes a mathematical model of back-to-back starting. Simulations of back-to-back starting process for pumped storage units under different conditions are made, including the normal starting and starting processes with motor de-excitation and braking and so on. The variations of electrical quantities in the starting process are simulated. By analyzing the physical processes and the simulation results, two new issues are recognized that coordination of excitation systems between two units and the differential protections of transformer and units faced in back-to-back starting. Finally, appropriate solutions for these two problems are proposed. The two excitation systems can be removed simultaneously by adding a hard-wire circuit between LCU and the tripping circuit and each of these circuits is added with an additional relay. The corresponding differential protection should be locked according to different operating conditions in the process of back-to-back starting.