Dimensioning of EHV series braking resistor for large thermal generators

This paper describes the possibility to improve transient stability for large generators using a series braking resistor. The method of dimensioning the resistor is presented together with results from dynamic simulations done in power system analysis tool PSS/E. It is found that adding a series braking resistor to a large thermal generator-turbine set results in a greatly increased transient stability when experiencing a close-up three-phase fault. The critical fault clearance time for the studied unit is increased from 150 ms to well above 250 ms. The simulations show that both the braking resistance and braking time influence the stability margin as well as the energy absorbed by the braking resistor. Results also show that the selection of braking resistance and braking time is not very sensitive to changes in the fault clearance time in terms of preserving transient stability.