Generator Protection
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Generator Protection
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Generator Protection
Outlines
Overview
Theory
Wiring
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Overview
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Typical Parts of a Power Plant
Substation
Busbar in Substation
HV - Breaker
Power plant
Main Transformer Auxiliary Transformer
Generator Breaker
Excitation Transformer
Excitation System
Turbine valve
Turbine - Generator G Field Circuit Breaker
Earthing System
HYBRID SYSTEM
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Possible Faults
Stator Earth Faults
Rotor Earth Faults
Stator Short Circuits
Stator/Rotor Interturn faults
External faults
Generator Protection
System
Ground
Stator
Ground
Exciter
The
"Wild"
G Power
System
Stator
Phase
System
Phase
Internal and External Short Circuits
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Abnormal operating conditions
Some of the abnormal conditions that must be dealt
with are the following:-
1. Winding faults: stator – phase and ground fault
2. Overload
3. Over-speed
4. Abnormal voltages and frequencies.
For generators we must consider the following:-
1. Under-excitation
2. Motoring and startup
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Generator Protection Review
Abnormal Operating Conditions
Loss of Synchronism
Abnormal Frequency
Over excitation
Field Loss
Inadvertent (Accidental) Energizing
Breaker Failure
Loss of Prime Mover Power
Blown VT Fuses
Open Circuits
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Abnormal Operating Condition
Abnormal conditions that may occur with generator include the
following .
1- Faults in the windings (Stator, rotor )
2- Overload
3- Overheating of windings and bearings
4- Over-speed
5- Loss of excitation
6- Motoring of generators
7- Single - phase or unbalanced current operation
8- Out of step
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Abnormal Operating Condition
overcurrent/overload
unbalanced load
Over-temperature
over- and under-voltage
over- and under-excitation
over- and under-frequency
over-fluxing
asynchronous running
out of step
generator motoring
failures in the machine control system
(i.e. AVR or governor failure)
failures in the machine cooling system
failures in the primary equipment (i.e.
breaker head flashover)
open phase
Protection Functions
Protection Functions Include:
• Generator Stator Differential
• Backup Distance Protection
• 100% Stator Ground
• Generator Unbalance
• Loss of Excitation
• Accidental Energization
• Breaker Fail
• Phase Overcurrent – Voltage Restraint
• Neutral Inst. / Timed Overcurrent
• Neg. Sequence Overcurrent
• Under / Over Voltage
• Under / Over Frequency
• Generator Thermal Model
• RTD Temperature
• Overexcitation – Volts/Hertz
Following are the various protections recommended for the generator and
generator transformer protection:
Type of fault ANSI Device Protection Functions
No.
GENERATOR STATOR
Short Circuits 87G Generator differential
87GT Overall differential
21G Minimum impedance (or alternatively
51 / 27 G Over current / under voltage)
Asymmetry 46G Negative sequence
Stator overload 51G Overload
Earth fault stator 64G1 95% stator earth fault
64G2 100% stator earth fault
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Loss of excitation 40G Loss of excitation
Out of step 98G Pole slip
Monitoring 32G / 37G Low forward power / reverse power
(double protection for large generators)
Blade fatigue 81G Minimum frequency
Inter turn fault 95G Over voltage or over current
Mag. Circuits 99G Overfluxing volt / Hz
Higher voltage 59G Over voltage
Accidental 27 / 50 G Dead machine
energisation
Monitoring 60 G PT fuse failure
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GENERATOR ROTOR
Rotor ground 64F Rotor earth fault
GENERATOR TRANSFORMER
Short Circuits 87GT Overall differential
51GT Overcurrent
87T Transformer differential
Ground fault 51NGT Earth fault over-current
87NT Restricted earth fault
Overhang 87HV HV winding cum overhang differential
UNIT AUXILIARY TRANSFORMER
Short circuit 87 UAT Transformer differential
51 UAT Over-current
Ground fault 51 UAT Restricted over-current
64 UAT Restricted earth fault
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50/51
Unit aux.
transformer
64F
Field winding
ground-fault
RAGRA
(RXNB4)
1) Instruments
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Generator Protection
Abnormal Operating Conditions
Overexcitation
Over Open
Power Loss of Field
Loss of Field Circuits
Overexcitation
Overexcitation
Exciter
The
"Wild"
G Power
System
Abnormal
Inadvertent Frequency
Abnormal Energizing,
Frequency Pole Flashover
Loss of
Reverse Breaker Synchronism
Power Failure
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Small Machine Protection
IEEE Buff Book
32 Reverse Power
40 Loss of Excitation
51V voltage restraint
51G Ground overcurrent
87 Differential
Small – up to 1 MW to 600V, 500 kVA if >600V
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Small Machine Protection
IEEE Buff Book
32 Reverse Power
40 Loss of Excitation
46 Negative Sequence
51V voltage restraint
51G Ground overcurrent
87 Differential
Medium – up to 12.5 MW
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Small Machine Protection
IEEE Buff Book
32 Reverse Power
40 Loss of Excitation
46 Negative Sequence
49 Thermal Overload
51V voltage restraint
51G Ground overcurrent
64 Ground Relay
87 Differential
Large – up to 50 MW
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Large Machine Protection
IEEE C37.102
Unit Connected,
High Z Grounded
32 Reverse Power
40 Loss of Excitation
46 Negative Sequence
49 Thermal Overload
51V voltage restraint
51G Ground overcurrent
64 Ground Relay
87 Differential
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Stator Protection
1-Phase and Ground Fault .
Generator differential connection with over current relay
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Three phase connection diagram for longitudinal
biased differential protection of a generator
(a) three phase connection (b) Typical characteristic of
diagram for longitudinal biased differential relay
protection of a generator.
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Connection diagram for transverse
differential protection of a generator
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Generator differential using percentage differential relay
The relay operating equation
|I -I | ≥ P |I +I |0.5
1 2 1 2
P: percentage of restrain coil winding to operating coil winding
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Sensitive earth fault current relay
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Restricted earth fault protection of the stator
windings.
It is used when the generator connected to delta/star transformer
The high impedance relay detects only earth faults in stator winding
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2- Stator Inter - Tur n Faults
Generator inter- turn fault protection
using harmonic restrained relay.
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(3) Stator over heating
Overheating of the stator may result from:
Failure of the machine's cooling system
Overloading
Short circuit in some of the stator laminations
Failure of core bolt insulation
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Stator overheat protection using resistance
temperature detector
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(4) Over voltage
(a) Connection diagram ,
(b) Relay characteristic for steam sets ,
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(c) Relay characteristic for hydro sets. 34
FIELD GROUND-FAULT PROTECTION
Because field circuits are operated ungrounded, a
single ground fault will not cause any damage or affect
the operation of a generator in any way
Thus, the probability of a second ground occurring is increased.
Should a second ground occur, part of the field winding will be
by-passed, and the current through the remaining portion may be
increased.
By-passing part of the field winding will unbalance the air-gap
fluxes, and this will unbalance the magnetic forces on opposite
sides of the rotor.
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Rotor Protection
(1) Ground Faults
Rotor earth fault detection using d.c. injection method.
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(2) Loss of Excitation ( Field Failure ) Protection
When a synchronous generator loses excitation, it
operates as an induction generator, running above
synchronous speed. Round-rotor generators are not
suited to such operation because they do not have
windings that can carry the induced rotor currents
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(3) unbalanced stator cur rents
The system conditions that would cause
these harmful unbalanced conditions are :
The open - circuit of one phase a line or the failure
of one contact of circuit breaker;
An unbalanced fault near the station which is not
cleared promptly by the normal relays;
A fault in the stator windings.
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Typical negative current withstand levels of
generators with different forms of cooling.
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Generator unbalance stator current protection
using negative phase sequence current relay
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Over-speed Protection
Over-speed protection is recommended for all prime-mover-
driven generators. The over speed element should be responsive
to machine speed by mechanical, or equivalent electrical,
connection
However, a direct-connected centrifugal switch is preferred. The
over-speed element should usually be adjusted to operate at
about 3% to 5% above the full-load rejection speed.
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(4) Over-speed Protection
Generator over-speed protection using wattmetric
relay
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(5) Protection Against Generator Motoring
Motoring protection is for the benefit of the prime mover
or the system, and not for the generator.
Diagram showing power and current locus of A. C. machine under various
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Reverse power relay setting for different
types of drive systems
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Schematic connections for Anti - Motoring protection of a
generator using reverse power relay
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Desirable Attributes - Generator
Reverse Power (32)
Should be sensitive to 0.05 pu to properly detect motoring in large steam turbines
Existing E/M and Static relays not sensitive enough
May cause shutdown difficulties
Negative Sequence (46)
Should be sensitive to low values to detect open poles/conductors on system after
GSU
Existing E/M and Static relays not sensitive enough
Long time low level events may cause excessive rotor heating, thermal tripping and
damage rotor
100% Stator Ground (59N / 59D)
Protects all of the stator winding
Existing schemes typically only see 90%
Stop low level fault near neutral from degrading into a high level fault and
causing large amounts of damage
Very secure
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Desirable Attributes - Generator
Inadvertent Energizing (50/27)
High speed tripping of unit after breaker accidentally closed
on dead machine, also pole flashover prior to syncing
Most existing schemes do not incorporate
Many schemes fail due to complexity (reliance on breaker
auxiliary switches, yard distance relays, etc.
If breaker is not tripped rapidly, mechanical and electric
damage can occur to machine in fractions of a second!
Pole flashovers must be cleared by high speed breaker
failure, as the breaker is open!
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Protection Functions Summary
Generator Stator Differential
• Stator Phase Differential
• Dual Slope Percent Restraint Operating
Characteristic
• Directional Supervision under saturation
conditions
Distance Backup Protection
• 2 or 3 Zone Mho Distance Protection Dual Slope Phase Differential
• Backup for primary line protection
• Configurable Reach and Angle Distance
Characteristic
100% Stator Ground Protection
• 95% Fundamental OverVoltage
• 15% Third Harmonic UnderVoltage Two Zone Distance Backup
Protection Functions Summary
Voltage Protection
• Phase Overvoltage
• Phase Under voltage
• Neutral Overvoltage (fundamental)
• Neutral Under voltage (3rd Harmonic)
• Voltage Phase Reversal
Voltage Restraint Overcurrent
Current Protection
• Phase, Ground and Negative Sequence
Overcurrent Tripping
• IEC, ANSI , IAC and Customizable
Overcurrent Curves
• Voltage Restraint increasing sensitivity under
low voltage conditions
Frequency Protection
• Overfrequency
• Underfrequency Undervoltage Trip Curves
Protection Functions Summary
Thermal Protection
• Generator Thermal Model
• RTD Alarming and Tripping
• RTD Biased Thermal Model
Inadvertent Energization
• Prevents Accidentally Closing a Stopped
Generator onto a Live Line
• Armed when Generator is Offline and Voltage is
Thermal Model – Voltage
below a pre-set level Dependant Overload Curves
Loss of Excitation (show Training CD)
• Uses Impedance Circle for loss of excitation
detection
• 2 Zones for fault detection and control
Volt/Hertz (Overexcitation)
• Detects changes in the Volts/Hertz ratio of the
generator or associated transformer Loss of Excitation
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Wiring
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Generator Protection
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Generator Protection with GSU Wrap
PTs on Low Side
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Generator Protection with GSU Wrap
PTs on High Side
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Generator Protection
PTs on Low Side
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