A new approach for determining series compensation ratio for transmission systems with bus connected parallel generating units considering subsynchronous resonance as a constraint

Transients following switching in the network can excite oscillatory torques on the turbine-generator-rotor-shaft system leading to reduction of fatigue-life and even severe damages. In the liberalised electricity markets there is a conflict between the transmission company and the generation companies with respect to the acceptable series compensation ratio. The former wanting to have more and the latter worrying about the consequences related to possible subsynchronous resonance (SSR) problems. Currently, there is a need to establish an approach to determining the acceptable series compensation ratio, particularly for the bus connected parallel generating units environment. This paper is a contribution in this direction. The influence of series compensation in the transmission lines leading to oscillatory torques and SSR phenomena is investigated in this work. Power system software NETOMAC and matlab/simulink have been used for the simulations in a complementary manner to get a deeper insight into generating unit's oscillatory behaviour. The SSR 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 and different combinations of transmission lines. Time domain simulation to capture the system dynamics following a transient fault using NETOMAC has been demonstrated for different operating unit and transmission line combinations. Further to get a deeper insight into the oscillatory behavior, the rotor shaft system has been modeled in matlab/simulink to simulate the turbine generator startup from turning gear speed to rated speed, block loading the generator and finally ramp loading the turbine generator to full load. Considering the generator synchronization and block loading condition, oscillations of (i) transmitted torques between adjacent rotor sections (ii) torsional angles between adjacent masses and.(iii) angular velocity of different masses have been investigated. This has very clearly revealed the complex dynamic interrelationship among variables responsible for causing fatigue life reduction. Using the understanding of this relationship, the NETOMAC simulation results are further processed using matlab for getting dynamic angular deviations for the SSR cases. Thus using the results obtained by the proposed approach and limits given by the turbine generator manufacturer, the acceptable value of series compensation ratio can be determined.