In: Electrical Engineering
1) List various types of faults occurring in generator and their consequences.
The common faults are
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. Rotor
winding short-circuit leads to an increase of excitation current
and a decrease of excitation voltage.
Operational
Faults:
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.