Investigation of the influence of series compensation in AC transmission systems on bus connected parallel generating units with respect to subsynchronous resonance (SSR)

Series capacitor compensation of AC transmission systems provides an economical solution for the power transmission companies to increase maximum power carrying capacity of existing transmission lines, control of load sharing of parallel lines and enhance transient stability. However, looking at it from generating utility point of view, the series compensation of lines may lead to possible subsynchronous resonance (SSR), in case of transient faults, of turbine generator units resulting in fatigue life reduction of turbine generator rotors and even damages which may prove to be very costly. In power stations having more than one bus connected parallel generating units, the SSR consequences depend on the number of units in parallel operation at the time of transient fault occurrence. In this paper, these consequences have been simulated for different combinations of parallel operating units for a power station having five generating units to determine the worst case combination. Simulations have been done for varying compensation ratio. Acceptable value of series compensation ratio has been determined considering worst case operating units combination and the fatigue limit of turbine geneerator rotor. Frequency domai analysis of electrical torque to determine the turbine generator rotor natural frequencies, frequency scanning of passive electrical network reactance viewed from a point behind the generator stator winding to determine the network natural frequencies, and finally the time domain simulation to capture the system dynamics following a transient fault have been demonstrated for different operating unit combinations. Simulation methodology demonstrated in this paper can prove to be very useful for system planning decisions related to determining SSR risks of series compensation for bus connected parallel generating units.