Improvement of power system transient stability by coordinated operation of fast valving and braking resistor

Fast valving and braking resistors, individually, are an effective means of improving the stability of a power system under large and sudden disturbances. The fast valving schemes, like other methods, are not always suitable for all power systems for enhancing the transient stability. Minor variations in the switching parameters of the fast valving scheme drastically affect the stability of the system. Moreover, there are some other associated problems such as risk of safety valve operation owing to increased boiler pressure, the problem of second swing instability etc. Similarly the dynamic braking resistor has its own limitations such as excessive heat loss and the resultant temperature rise of the resistor. Therefore, coordinated fast valving and braking resistor control is proposed. The coordinated control scheme is very effective in reducing the mismatch between the mechanical input power and electrical power output of the generator, thereby reducing the generator accelerating power during the fault period. This dual control from load side and generation side substantially improves the transient stability performance of the system. Various schemes of fast valving control and coordinated control operation were tested on a single machine infinite bus system and the results are compared.