ALTERNATOR PROTECTION
Large MW Generator Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Problems that demand Protection
Stator electrical faults
Overload
Overvoltage
Unbalanced loading
Over fluxing
Inadvertent energisation
Rotor electrical faults
Loss of excitation
Loss of synchronism
Failure of prime mover
Lubrication oil failure
Over speeding
Rotor distortion
Difference in expansion between rotating and stationary parts
Excessive vibration
Core lamination faults
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Electrical Fault Protection
Stator Earth Fault
Using Overcurrent relay
Using Overvoltage relay
Using Third Harmonic voltage
By low frequency voltage injection
Phase-Phase Fault
Differential Protection
Inter Turn Fault
Overcurrent protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Earth Fault Protection using Overcurrent Relay
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Earth Fault Protection using Overvoltage
Relay
Vph
95% 0%
Protection is achieved against
the faults occurred from 0% to 95% of the winding
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Restricted Earth Fault Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Phase to Phase fault protection using
Differential protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Generator Differential Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Inter-turn fault protection
Effect: Unnecessary heating & Unbalanced Flux
distribution
If Generator is with Parallel wdg, - Opposed connected
Diff. Relay can be used
If Generator is without access to parallel winding.
Third Harmonic Measurement can be used
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Stator Inter-turn Fault Protection-Using Inst
over current relays
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ALTERNATOR PROTECTION
Stator Inter-turn Fault Protection-Using 3rd
Harmonic measurement
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Loss of excitation
Cause :
Short or Open-circuit of the exciter.
A failure of automatic voltage regulator
An operator error under manual control
Opening the excitation switch by mistake
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Loss of excitation
Effects
Falling out of synchronism
Runs as Induction Generator drawing Reactive
power from grid causing deficiency in grid
reactive power.
Local hot spots in stator and rotor
Due to super synchronous speed >105% of
Synchronous speed, rotor gets damaged.
Damage to rotor insulation
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Behavior of Generator impedance during Loss
of excitation
Swing Curves and Loss of Synchronism locus
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ALTERNATOR PROTECTION
Protection against Loss of excitation
Loss of Excitation Protection using impedance relays
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Rotor Earth Fault Protection
To avoid unbalanced flux distribution and excessive
vibration.
Current Balance relay can be used to detect.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Rotor Earth Fault Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Rotor Earth Fault Protection- Operating principle
An ac voltage stepped down to 48 Volts from 100 V or 230 V is set up
via coupling capacitors to the rotor circuit towards earth
This ac voltage forms a small charging current to flow via the
coupling capacitor, resistances of the brushes and earth capacitance of
field circuit, depending upon the value of earth capacitanceCE the value
of this current varies.
The value of this current depends upon the value of earth capacitance,CE
which is a few mA during normal no fault operating condition
If an E/F arises in the field circuit this current increases and this current
is amplified up to 10 times by the CT T2
A sensitive definite time O/C relay is used to measure this current , and
an alarm/trip is generated from the relay
The relay selected should be insensitive to harmonics since there will
be considerable harmonics especially with thyristor excitation and
rotating rectifier system
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Over Voltage Protection
Cause:
Power frequency switching Over voltages & High
frequency Lightning Strokes.
Effect:
Stator Insulation failure.
Time delayed over voltage protection relay can be
used
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Current
Cause:
Unbalance loading of the three phases of a
generator results in NPS current
generation.
(Negative Sequence Current is given by:
I2 = Ia + a*Ib + a2 *Ic
a = 1 120o
a2 =1 240o
It is clear that for any Unbalance loading I2 will be present.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Current
Effects:
Associated negative-sequence component in the
stator current induces currents at double the
power system frequency in the rotor
Temperature rise mainly in the rotor retaining
rings
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Protection
Typical negative phase
sequence current
withstand of cylindrical
rotor generators:
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Negative Phase Sequence Protection
Trip time t for a typical relay element
characteristic:
Modern numerical
relays derive the
negative sequence
current level by
calculation.
For protection a true
thermal replica
approach is followed.
With trip time t
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Reverse Power Protection
This is also known as Protection against motoring of
Generator
Cause:
Supply of energy (steam, water or gas) to the
primover fails.
Effects:
In case of steam turbine: overheating of turbine
blades.
In case of Diesel generator turbine: Explosion
may occur.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Reverse Power Protection
Generator Reverse Power problems:
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ALTERNATOR PROTECTION
Reverse Power Protection
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ALTERNATOR PROTECTION
Pole Slipping Protection
Requirements To avoid cascade tripping.
Concept Tracking of system impedance.
Problems Tracking the transient nature of System
Impedance.
Solution use Blinder Relay.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Differential Protection for Generator transformer
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ALTERNATOR PROTECTION
Overall Differential Protection for TG,GT & Unit
transformer
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Overfluxing in Generator transformer
Cause:
Increase in the voltage
Decrease in the frequency
Basic governing equation for both the cases:
V *f
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Overfluxing in Generator transformer
Effects:
Saturation of Core
Heating of iron core
Flux leaks into other parts of Transformer,
producing eddy currents, which in-turn causes
more heat in windings and structural parts
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Protection against Overfluxing in Generator
transformer
Measure of degree of overfluxing of the transformer is the
Ratio of Saturation flux BS to rated flux BN
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Types of Protection
Class A Protection Operation Sequence
Gen Breaker Trips
Gen Field Breaker Trips
Incomers of UATs are tripped
Tie Breaker between Aux. Station Bus & Aux. Unit
Bus are closed.
Boiler Trips
Prime Mover Trips
Class A Annunciation will appear.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Types of Protection
Class B Protection - Operating Sequence
Boiler Trips
Turbine Trips
Class A protection will operate through low
forward power relay.
Class B Trip Annunciation will appear.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Types of Protection
Class C Protection
Trips Generator Breaker only.
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ALTERNATOR PROTECTION
Class A Protection
Differential Protection
Stator Earth Fault Protection
Interturn Fault Protection
Rotor Second Earth Fault Protection
Over Voltage Protection
Reverse Power Protection
Pole Slipping Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Class B Protection
Negative Phase Sequence Protection
Field Failure Protection
Back Up Impedance Protection
Under Frequency Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Class C Protection
Generator Transformer Back up Over Current
Generator Transformer Back up E/F
Protection
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Effect of Neutral Grounding through transformer
With a step down transformer of turns ratio n:1
The resistance size will be reduced by a factor of
(1/n2)
Current Transformer size will be reduced by a factor
of (1/n)
Electrical isolation is provided for the protection
equipment.
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
NGR Calculations
Earth fault current is limited to three different values:
Rated current
200A-400A (low impedance earthing)
10A-20A (high impedance earthing)
For limiting Earth fault current to Ifr the NGR required
is given by:
Vph
NGR =
Ifr
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
NGR Calculations
When it is earthed through Transformer of turns ratio
= n:1
Then, Resistance required is
Vph
NGR = n2 x Ifr
WLSA-IND/GKT/2006
ALTERNATOR PROTECTION
Thank You
WLSA-IND/GKT/2006
