14-05-2012, 04:40 PM
Generator and Generator-Transformer Protection
[attachment=22022]
INTRODUCTION
The core of an electric power system is the generation.
With the exception of emerging fuel cell and solar-cell
technology for power systems, the conversion of the
fundamental energy into its electrical equivalent
normally requires a 'prime mover' to develop mechanical
power as an intermediate stage.
The nature of this machine depends upon the source of
energy and in turn this has some bearing on the design
of the generator. Generators based on steam, gas, water
or wind turbines, and reciprocating combustion engines
are all in use. Electrical ratings extend from a few
hundred kVA (or even less) for reciprocating engine and
renewable energy sets, up to steam turbine sets
exceeding 1200MVA.
GENERATOR EARTHING
The neutral point of a generator is usually earthed to
facilitate protection of the stator winding and associated
system. Earthing also prevents damaging transient
overvoltages in the event of an arcing earth fault or
ferroresonance.
For HV generators, impedance is usually inserted in the
stator earthing connection to limit the magnitude of
earth fault current. There is a wide variation in the earth
fault current chosen, common values being:
1. rated current
2. 200A-400A (low impedance earthing)
3. 10A-20A (high impedance earthing)
STATOR WINDING FAULTS
Failure of the stator windings or connection insulation
can result in severe damage to the windings and stator
core. The extent of the damage will depend on the
magnitude and duration of the fault current.
Earth Faults
The most probable mode of insulation failure is phase to
earth. Use of an earthing impedance limits the earth
fault current and hence stator damage.
An earth fault involving the stator core results in burning
of the iron at the point of fault and welds laminations
together. Replacement of the faulty conductor may not
be a very serious matter (dependent on set
rating/voltage/construction) but the damage to the core
cannot be ignored, since the welding of laminations may
result in local overheating. The damaged area can
sometimes be repaired, but if severe damage has
occurred, a partial core rebuild will be necessary.
[attachment=22022]
INTRODUCTION
The core of an electric power system is the generation.
With the exception of emerging fuel cell and solar-cell
technology for power systems, the conversion of the
fundamental energy into its electrical equivalent
normally requires a 'prime mover' to develop mechanical
power as an intermediate stage.
The nature of this machine depends upon the source of
energy and in turn this has some bearing on the design
of the generator. Generators based on steam, gas, water
or wind turbines, and reciprocating combustion engines
are all in use. Electrical ratings extend from a few
hundred kVA (or even less) for reciprocating engine and
renewable energy sets, up to steam turbine sets
exceeding 1200MVA.
GENERATOR EARTHING
The neutral point of a generator is usually earthed to
facilitate protection of the stator winding and associated
system. Earthing also prevents damaging transient
overvoltages in the event of an arcing earth fault or
ferroresonance.
For HV generators, impedance is usually inserted in the
stator earthing connection to limit the magnitude of
earth fault current. There is a wide variation in the earth
fault current chosen, common values being:
1. rated current
2. 200A-400A (low impedance earthing)
3. 10A-20A (high impedance earthing)
STATOR WINDING FAULTS
Failure of the stator windings or connection insulation
can result in severe damage to the windings and stator
core. The extent of the damage will depend on the
magnitude and duration of the fault current.
Earth Faults
The most probable mode of insulation failure is phase to
earth. Use of an earthing impedance limits the earth
fault current and hence stator damage.
An earth fault involving the stator core results in burning
of the iron at the point of fault and welds laminations
together. Replacement of the faulty conductor may not
be a very serious matter (dependent on set
rating/voltage/construction) but the damage to the core
cannot be ignored, since the welding of laminations may
result in local overheating. The damaged area can
sometimes be repaired, but if severe damage has
occurred, a partial core rebuild will be necessary.