Effect of Load and Damper Modelling on Evaluation of Negative Sequence Capability of Large Generators

The paper examines negative sequence losses in solid generator rotors following sustained stator-terminal and HV system L-L short-circuits at full-load and no load with or without a connection to the grid supply. Components of stator current which give rise to rotor heating over the period of the transient are identified and effective intrinsic values ot I22 t for a range of system and machine parameters are discussed. The analysis uses a phase-variable model of a synchronous-generator with detailed and reduced damper representations to compute stator current following rated-voltage line-to-line disturbances on the supply. Simulations from full-load for a variety of system parameters with and without connection of a field discharge resistor on tripping of the generator breaker are performed. From the responses, components of stator current which give rise to rotor heating are isolated by Fourier analysis combined with symmetrical component analysis. These components are integrated over the period of the transient to give effective values of I22 t. Maximum supply-frequency components of stator current taking into account d.c. components have been assumed. Effective values of I22 t using detailed and reduced damper models in simulations with and without voltage regulator action are compared with values predicted by simpler calculations where no allowance is made for generator load or for the connection to the grid supply. A 500 MW, 3600 rpm and a 660 MW, 3000 rpm Machine are analysed.