Abnormal operating conditions of Alternators

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 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 main exciter.

b) Accidental tripping of field breaker

c) Short circuit in the field winding

d) Poor brush contact in the exciter

e) Field circuit breaker latch failure

f) Loss of AC supply to excitation system

The generator delivers both real as well as reactive power to the grid. The real power comes from the turbine while the reactive power is due to the field excitation. Now consider that the field excitation is lost while the mechanical input remains intact. since the generator is already synchronized with the grid it would attempt to remain synchronized by running as an induction generator. As an induction generator, the machine speeds up slightly above synchronous speed and draws its excitation from the grid.



There are two possibilities either the grid is able to meet this reactive power demand fully or partially. If the grid is fully able to satisfy this demand for reactive power the machine continues to deliver active power and draws reactive power. But the generator is not designed as an induction machine, so abnormal heating of the rotor and overloading of stator winding will take place. If the grid meets reactive demand only partially then this would be reflected by the fall of the generator terminal voltage. The generator would be under excited. 

When the excitation fails, the speed of the rotor increases as the magnetic drag is decreased, then it starts acting as an induction generator. It takes the required reactive power from the grid and supplies active power. As an induction generator necessitates the flow of slip frequency current in the rotor, the current flows in damper winding, slot wedges, and the surface of the solid rotor winding. Thus temperature increases which cause abnormal heating of the rotor and overloading of stator winding to take place. On the loss of excitation, the current increases and impedance reduce this can be detected by offset mho relay.

3. Unbalanced Loading:

 In the unbalanced loading of the generator, the stator currents have a negative sequence component. Due to this, the stator field rotates at synchronous speed but in the opposite direction of the rotor field, Thus the field is at (- Ns) and the rotor field at (Ns) therefore the relative velocity is 2Ns.

This causes double frequency currents of large amplitude in the rotor. Eddy currents are directly proportional to f^2 (Bmax)^2 and Hysteresis losses are directly proportional to f (Bmax)^n. As eddy current and hysteresis losses are dependent upon frequency these losses will increase by increasing in double frequency component. Due to this, the rotor gets overheated as the stator carries unbalanced currents. The capacity of a particular machine to safely dissipate the energy is limited to a certain value k which generally varies between 3 to 20. 

The thermal characteristics of the machine are given as, I^2*t = k

The time for which the offending current I can be allowed to flow should be less than equal to k/I^2 Thus t <= k/I^2. An inverse type of overcurrent relay which is fed with the negative sequence component of stator current gives protection against unbalanced loading of the generator.

4. Over Speeding:

In normal operating conditions alternator supplies rated electrical power Pe to the grid. Its mechanical input Pm is nearly equal to Pe (except losses) and the machine runs at constant synchronous speed Ns. Noe due to fault generator is tripped and disconnected from the grid this Pe becomes zero, but mechanical power input Pm cannot be suddenly reduced to zero. In this situation where the generator has full input mechanical power but no output electrical power. This causes the machine to accelerate to dangerously high speeds if mechanical input is not quickly reduced. 



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