The Science of Electricity

 1. Loss of Prime Mover: In case of loss of prime mover the machine continues to remain synchronized with the grid. As there is no prime mover the machine draws a small amount of Active Power (compared to its rating) from the grid in order to drive the turbine and meet the losses taking place in the machine. At the same time, the machine supplies reactive power to the grid since its excitation is intact. In this state, the generator acts as a synchronous motor. Running in this mode does not affect the generator but it is definitely harmful to the prime mover like a steam turbine. When the prime mover is lost, the generator starts drawing real power from the grid. This active/ real power is quite small compared to the generator rating.  The generator draws real power to meet the losses and the load put on it by the turbine. The stator current goes 180 degrees out of phase. Thus we use a directional relay with an MTA of 180 degrees.  2. Loss of Excitation: Causes: a) Loss of field to the