Suppose the Breaker is closed when the Generator is at standstill condition, the Generator will behave as an Induction Motor with surface of rotor core and rotor winding slot wedges acting as rotor current carrying conductors. This abnormal current in the rotor can cause arcing between the components like slot wedge to core leading to rapid overheating and damage. Generally, the time to damage the generator stator from the high in-rush currents received during energizing at standstill is in the order of a few seconds. The bearing, however, may be damaged more quickly due to the lack of oil pressure. Thus it is very important to provide a fast and reliable protection to protect Generator from accidental energization at standstill condition.
To better understand the protection philosophy used behind the Dead Machine Protection, let us first have a look at the values of electrical parameters when there is an accidental closer of circuit breaker when Generator is at standstill condition.
Voltage across the Terminal of Generator = Low as the Generator behaves as an Induction motor.
Current in the Armature winding = High
Thus we should use the above two conditions to design Dead machine protection scheme. In Dead Machine Protection scheme, an instantaneous over-current element in combination with under-voltage element is used to detect Dead Machine condition.
Logic of Dead Machine Protection: If an over-current is sensed in the armature winding of Generator when there exists under-voltage across the terminals of Generator, Relay shall give tripping command to breaker to isolate Generator from the circuit.
If you see the above logic carefully, you will notice that an element VT Supervision is also added in the OR Gate. Basically VT supervision logic checks for the healthiness of Voltage Transformer or Potential Transformer Fuse. It may happen so that Fuse of Voltage Transformer or Potential Transformer is blown due to which under-voltage will be sensed and at the time of normal operation also, the Dead Machine Protection will operate where it is not expected to operate. Thus in the logic of Dead Machine Protection, it is must that Voltage Transformer or Potential Transformer is healthy.
Another thing to notice is, a timer at the output of an OR Gate. In the timer, there are two things; one is pick-up time delay and drop-off time delay. A pick-up time delay is provided to prevent the initialization of the Dead Machine Protection during system fault. This pick-up time delay is normally set to 5 s. The drop-off time delay ensures that Dead Machine Protection remains initialized following an accidental closure of breaker when the under-voltage element may reset. This drop-off time delay is normally set to 500 ms.
The over-current element can be set to less than full load current as under normal operation of Generator, under-voltage element will be low (here low means 0 and high means 1. If there is under-voltage then it will be 1 else 0) and hence Dead Machine Protection will not operate. Normally over-current element is set to 10% of full load current.
Under-voltage element is normally set to 85% of the rated voltage.