Effect of field failure of a d.w.r. generator

Performance of a divided-winding-rotor (d.w.r) generator equipped with its controls and connected to an infinite busbar, has been studied under abnormal operating conditions such as (a) external 3-phase short circuit, (b) sustained loss of excitation in either of the field windings, and (c) double faults such as an external 3-phase short circuit and simultaneous opening of the reactive-field winding. The digital simulation shows that quick reversal of the reactive-winding current during an external fault is responsible in bringing the rotor m.m.f. back towards the pole axis and thus helps to maintain stability especially for faults of long duration. It is also found that the reversal of the reactive-winding current is not absolutely essential for external faults of shorter duration as the angle regulator, located on the torque winding, alone is quite capable of maintaining stability. It is also found that permanent loss of excitation of the reactive-field winding does not affect stability, whereas similar loss of excitation in the torque-field winding causes instability.