Types of Faults in an Alternator

An Alternator is vulnerable to many types of faults.  Each of these faults can cause major damage which can be expensive to rectify and result in loss of generation.

The common faults are
  1. Stator Faults
  2. Rotor Faults
  3. Operational Faults

Stator Faults
Stator faults are those which occur on the stator of the Alternator.  These faults can be categorized into

1.  Phase-to-Phase Faults which occur between two phases

2.   Phase-to-Earth faults which occur between a phase and the ground and

3.  Inter-turn Faults which occur between the turns of a winding of the same phase.
Stator faults occur due to failure of the winding insulation.  The heat generated by these faults can cause serious damage to the laminated core of the Stator.  This may require expensive  re-insulation and rebuilding.

Rotor faults
Rotor Faults  on the Alternator when the rotor winding gets grounded or short circuited.  The rotor winding is usually ungrounded; hence the first earth fault is not always obvious.  However, if  a second earth fault occurs on the rotor, the fault becomes a virtual short-circuit through the rotor body.

Operational Faults are

Overloading 
Overloading causes the flow of high currents which causes the stator winding to heat up.

Reverse Power
This occurs due to failure of the prime mover and insufficient torque supplied to the generator.

Underexcitation
Underexcitation occurs when the excitation to the generator is cut off and the Power factor goes to the leading side.  This can lead to the failure of the diodes on the rotor and pole slipping.

Negative Phase Sequence
Negative Phase sequence occurs when the Alternator is loaded in an unbalanced manner.  That is,  the current on the three phases are not balanced.  This results in heating of the Alternator rotor.

Overvoltage 
Overvoltage occurs due to failure of the excitation control system.  If the excitation input to the alternator does not match the voltage.  It can result in the voltage rising above normal levels and the risk of the winding insulation getting damaged.

Overspeeding
Overspeeding is an extremely serious and dangerous condition.  This occurs when the speed controller regulating the speed of the prime mover fails.  When the speed of the alternator rises above the nominal speed, the centrifugal forces developed within the Alternator are so enormous that the poles of a salient pole rotor can get damaged and can come out of the rotor.  This can then hit the stator and the alternator will be severely damaged.

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