This document discusses generator protection methods. It describes how large generators are connected directly to bus bars or via transformers. It also explains different fault conditions generators may experience like overloading, unbalanced loading, and stator winding faults. Various protection systems are discussed like using thermo-couples for temperature monitoring, negative sequence relays for unbalanced loads, and tripping exciters and dissipating stored energy for stator faults.

2.  Protection of turbo-generators is the most
complex and elaborate.
 A Generator is a large machine and is
connected to bus-bars. It is accompanied by
unit transformers, auxiliary transformer and
bus system.
 There are two methods by which generators are
connected to Bus:
 Generators connected directly to bus
 Generators connected via delta-star unit transformer
to HV bus i.e., to transmission line.
A turbo generator is the combination of a turbine directly connected to
an electric generator for the generation of electric power

3. Typical small generators
connection to system
Large Generator connection to
system

4.  Continued over loading may increase the
winding temperature to such an extent that
the insulation will be damaged and its useful
life reduced.
 Temperature rise can also be caused by
failure of cooling system. In large machines
thermal elements (thermo-couples or
resistance thermometers) are embedded in
the stator slots and cooling system.
 Electrical overcurrent protection can not
sense the failure of cooling system

5.  Un-balanced loading on generator can be due to:
 1. Unsymmetrical faults in the system near the
generating station.
 2. Mal-operation of a circuit-breaker near generating
station, the three phases not being cleared.
Negative sequence protection senses unbalanced
loading of generators.
Continued unbalanced loads, equal to or more than 10%
of the rated current cause dangerous heating of the
cylindrical rotor in turbo-generators. Salient pole
rotors in hydro-generators often include damper
windings and are, therefore, much less affected by
unbalance loading (negative phase sequence
currents).

6.  Stator winding faults involve armature
winding and must therefore be cleared
quickly by complete shutdown of the
generator. Only opening the stator circuit
does not help since the e. m.f. is induced in
the stator winding itself. The field is,
therefore, opened and de-energized by "Field
Suppression".

7.  The stator faults include:
1. Phase-to-earth faults
2. Phase-to-phase faults.
3. Stator inter-turn faults.
Phase to phase faults and phase inter-turn
faults are less common.
Inter turn faults are more difficult to detect.

8.  These faults normally occur in the armature
slots. The damage at the point of fault is
directly related to the selected neutral
earthing resistor. With fault currents less
than 20 A, negligible burning of the iron core
will result if the machine is tripped within
few seconds. The repair work then amounts
to changing the damaged coil without re-
stacking the core laminations.

9.  If, however, the earthing resistor is selected to
pass a much larger earth-fault current (> 200 A)
severe burning of the stator core, will take
place, necessitating re-stacking of laminations.
Even when a high speed earth-fault differential
protection is used, severe damage may be
caused owing to the large time constant of the
field-circuit and the relatively long time
required to completely suppress the field flux. In
the case of high earth-fault currents it is
therefore normal practice to install a circuit
breaker in the neutral of the generator in order
to break the path of earth fault current.

10.  As the source of fault current is itself the
stator winding in which voltage is induced
due to field flux, the exciter must be
disconnected and stored field energy of both
alternator and exciter be dissipated in the
resistance . For this purpose,
 one trip coil opens the connection of generator
field winding from exciter and simultaneously
shunts it to a resistance,
 second trip coil opens the neutral circuit
breaker to break the earth fault current path
 And third trip coil opens the exciter armature
circuit and shunts the exciter field winding
across a resistance for flux collapse

11. Star winding Delta winding

14.  Resistor earthing is used when generator is
directly connected to station bus. In this
case the IDMT relay used in neutral is
coordinated with other over current relays in
the system
 When generator is connected via delta-Y
transformer, no coordination is required with
other relays since earth loop is restricted to
generator and transformer primary(delta)
 For combined generator-transformer
protection system, neutral is grounded
through a voltage transformer.

15.  With Resistance earthing it is impossible to
provide protection to the whole winding, the
percentage of stator winding protected being
dependent on the value of earthing
resistance.
 Another problem with resistance earthing-
resonance may occur between generator
stator winding distribution capacitance and
Earth resistance. The limiting value is given
by
fC
Rn


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