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UNIT PROTECTION
Boiler Protections• Loss of FSSS DC for > 2 Sec• Both ID Fan Trip• Both FD fan Trip• Both Air Heater Trip• Loss of all fuel• Flame Failure• Drum Level High / Low• Furnace vaccum High / Low• Boiler manual Trip• ALL BFP’s Trip• Turbine to Boiler Trip
Boiler Trip
Boiler Lockout
Turbine Protections
• Main Steam Temperature Low• Axial Shift High• Lub Oil Pressure Low• ESV/IV Closed• Turbine Manual Trip• HPH’s Level High High• Loss of 6.6KV Voltage
Turbine Lockout
Integrated Protections
Boiler Lockout Master Trip Relay
Generator Lockout
Turbine Lock out
86Y86T
86GT 86B
86G
FCB Contact 2/37 2/32
POWER SYSTEM PROTECTION KEY ASPECTS
• Reliability• Security• Sensitivity• Selectivity• Zone over lapping• co-ordination• Primary & Back up Relays• Speed
KEY ASPECTS• Reliability
• Security/Stability
• Sensitivity
• Protection zones
• Coordination
• Primary Relays
• Back up Relays
• Probability that the system will function correctly when required to act(for a fault in it’s zone)
• Refrain from unwanted operation in the absence of fault or fault out side it’s zone
• Ability of the system to detect the threshold value of an abnormal condition to initiate protective action.
• Regions of primary sensitivity• Determination of graded settings
to achieve selectivity
• Relays with in a particular zone that should operate for prescribed abnormalities with in that zone
• Relays outside a given primary protection zone, independently of the primary Relays.
Generator Protection Requirement
Generator faults are considered to be serious since they may cause severe and costly damage to insulation, windings, and the core may also produce severe mechanical torsional shock to shafts and couplings.
Fault current may continue to flow for many seconds even after the generator is tripped, because of trapped flux within the machine, thereby increasing the amount of fault damage.
As a consequence, for faults in or near the generator that produce high magnitudes of short-circuit currents, some form of high-speed protection is normally used to trip and shut down the machine as quickly as possible in order to minimize damage.
TRIP LOGIC OF GENERATOR PROTECTION
• TWO INDEPENDENT CHANNELS WITH INDEPENDENT CT/VT INPUTS/DC SUPPLY/TRIP RELAY
CLASS A TRIPS• ALL ELECTRICAL TRIP• TRIP TURBINE , FIELD, GENERATOR,GT,UT
CLASS-B TRIP• MECHANICAL TRIPS• AVOID OVER SPEEDING OF TURBINE DUE TO STEAM ENTRAPPED IN TURBINE.
TURBINE TRIP SIGNAL IS GIVEN FIRST AND THE ACTIVE POWER, SENSED BY THE LOW FORWARD RELAY (32G) GIVES THE TRIP SIGNAL TO THE UNIT BREAKER & FIELD BREAKER AFTER A TIME DELAY.
• IN GCB SCHEME, ONLY GCB AND FIELD IS TRIPPED,KEEPING UAT CHARGED THROUGH GT
• IN NON GCB SCHEME, HV CB,FIELD,UT LCV CB ARE TRIPPED.
• Class C• Trips only HV CB
Gen Stator Thermal Protection Field Thermal Protection Gen stator fault Protection Gen rotor field Protection Gen abnormal operating conditions System backup Protection Power transformer Protection
Generator Protection
Generator Protection
Stator Thermal protection
Thermal protection for the generator stator core and windings Generator overload
Winding Temperature Over current
Failure of cooling systems RTDs Thermocouple Flow and pressure sensor
Localized hot spots caused by core lamination insulation failures or by localized or rapidly developing winding failures Generator Core monitor
Generator Protection
Turbine-generator short time thermal capability for balanced three-phase loading
Generator Protection
Generator Field Thermal protection
Thermal Protection Direct rotor Body temperature measurement not possible Core Monitor may detect overheating
Protection for field over excitation IDMT/ Definite Time Excitation limiters
Generator Protection
Generator field short time thermal capability
Generator Grounding Practices
It is common practice to ground all types of generators through some form of external impedance
limit the mechanical stresses and fault damage in the machine, to limit transient voltages during faults, and to provide a means for detecting ground faults within the
machine.
Typical Grounding practicesUngroundedSolid GroundingHigh-impedance groundingLow-resistance groundingReactance groundingGrounding-transformer grounding
Generator Grounding Practices
High Impedance Grounding– High resistance grounding
• The high-resistance grounding method utilizes a resistor connected across the secondary of the distribution transformer to limit the maximum ground fault current.
• For a single-phase-to-ground fault at the machine terminals, the primary fault current will be limited to a value in the range of about 3 A to 25 A.
– Ground fault neutralizer grounding The ground fault neutralizer grounding method utilizes a secondary tunable
reactor to limit the maximum ground fault current. Low –resistance grounding
In this method, a resistor is connected directly between the generator neutral and ground.
For a single-phase-to-ground fault at its terminals the primary fault current will be limited to a value in the range of about 200 A up to 150% of rated full-load current.
Resistor cost and size usually preclude the use of resistors.
Current based system– For generators with split neutrals with all six terminals brought out on
neutral side.– Delayed low-set o/c relay which senses the current in the connection
between the neutrals of the stator windings
• Voltage based system – Relay compares the neutral NGT sec voltage and Genertaor terminal
open delta voltage. – Balance during external E/F or normal condition– During inter turn fault open delta voltage will be developed and NGT
sec voltage will be zero, resulting in a differential voltage which makes the relay operate.
Typical setting
Definite time type relays: minimum setting with 1 sec delay.
INTERTURN PROTECTION
Inter turn protection
Split Phase Protection
Voltga
Voltage Based
NEGATIVE SEQUENCE PROTECTION
Negative sequence protection• NEGATIVE SEQUENCE PROTECTION FOR GENERATOR PROTECTS THE GENERATOR
FROM EXCESSIVE HEATING IN THE ROTOR RESULTING FROM UNBALANCED STATOR CURRENTS
• CAUSED DUE TO– ONE POLE OPEN IN LINE– ONE POLE OPEN OF A CIRCUIT BREAKER– CLOSE IN UNCLEARED UNBALANCED FAULTS
• THE NEGETIVE SEQUENCE PROTECTION RELAYS SHALL BE SET TO THE NPS CAPABILITY OF THE MACHINE WHICH IS
• K = I22X T
• TYP FOR 500 MW PERMISSIVE NEG SEQ CURRENT = 5 – 8 % OF STATOR CURRENT
PERMISSIVE I22X T = 5 – 10
SETTINGS ADOPTED FOR NTPCI2 = = 7.5 % I2
2XT = 8.3
NEGATIVE SEQUENCE PROTECTION
TYPICAL NPS CAPABILITY
Negative sequence protection
Loss of field protection
Loss of field protection
• ACTS AS AN INDUCTION GENERATOR• INDUCED EDDY CURRENTS IN THE FIELD WINDING, ROTOR BODY,
WEDGES AND RETAINING RINGS • MW FLOW IN TO THE SYSTEM/ MVAR FLOWS IN TO THE MACHINE.• THE APPARENT IMP TRAVELS TO THE FORTH QUADRANT OF X-Y
PLANE
• METHOD OF DETECTION:
MINIMUM IMPEDANCE WITH U/V
SOME RELAYS ARE SET IN THE ADMITTANCE PLANE MATCHING WITH THE CAPABILITY CURVE OF THE MACHINE
TRIP CHARACTERISTICS OF LOSS OF FIELD PROTECTION
REVERSE /LOW FORWARD POWER INTERLOCK
LOW FORWARD AND REVERSE POWER INTER LOCK
• To allow entrapped steam in the turbine to be utilized to avoid damage of the turbine blade.
• To protect the machine from motoring action• Trip under class B after a short time delay in
case the turbine is already tripped ( typ set at 2 sec)
• Trip under class A, after a long time delay if turbine is not tripped (typically set at 10 sec)
• Power setting typ 0.5 % of rated power
Rev power/ LFPR INTERLOCK
• STATOR EARTH FAULT PROTECTION
• E/F CURRENT IS LIMITTED TO 10A• THIS MINIMIZES THE DAMAGE• FIRST FAULT LESS CRITICAL• NEEDS CLEARANCE AS
IT MAY DEVELOP INTO A PH TO PH FAULT SECOND FAULT WILL RESULT IN VERY HIGH
CURRENT
• TWO TYPES:• 100 % E/F• 95 % E/F
EFFECT OF STATOR E/F
Results In Voltage Shift Of Gen Neutral W.R.T Ground.
Detected By Voltage Relay Connected Across Grounding Resistor Or From The Generator Terminal Through Open Delta VT
Protect Approx 95% Of Stator Wdg
Typical Setting:– For Definite Time Delay Type: 5%of 110 V Ie, 5.5 V
At 1 Sec
95 % Stator Earth Fault
April 9, 2023 32PMI Revision 00
Three ways of providing E/F protection:• Voltage relay connected across grounding
resistor• A current relay connected to CT provided in
grounding transformer• Voltage relay connected to open delta in
generator VT
EARTH FAULT PROTECTION
April 9, 2023 33PMI Revision 00
100 % Stator E/F Protection
• Third Harmonic Principle• Relay responds to the reduction of the 3rd Harmonic
Component • For a Stator Phase-to-ground fault at or near the
Generator Neutral, there will be an increase in third Harmonic Voltage at The Generator Terminals, which Will Cause Relay Operation.
100% SEF based on third harmonics measurements
Disadvantages
Due to design variations, certain generating units may not produce sufficient third harmonic voltages.
This method does not protect the machine during stand still conditions.
100% stator earth fault protection (Low freq. injection principle)
20 Hz
RE
max.200 V
I
20 Hz
RE
max.200 V
I
Detects the ground faults by injecting a low frequency signal (say 20 hz) at the neutral earthing transformer and monitor the earth current in the winding.
COMPARISION BETWEEN E/F PROTECTIONS
ROTOR EARTH FAULT PROTECTION
• FIRST ROTOR E/F DOES NOT CAUSE IMMEDIATE DAMAGE• SECOND E/F RESULTS IN A WDG SC OF ROTOR • CAUSE MAGNETIC UNBALANCE/MECH FORCES /DAMAGE• METHODS OF DETECTION
– POTENTIOMETER METHOD• A CENTRE TAPED RESISTOR IS CONNECTED ACROSS THE MAIN
FIELD WINDING • THE CENTRE TAP IS CONNECTED TO EARTH THROUGH A VOLTAGE
RELAY• AN EARTH FAULT ON THE FIELD WINDING WILL PRODUCE
VOLTAGE IN THE RELAY, MAXIMUM VOLTAGE OCCURRING FOR END FAULTS
• A BLIND SPOT EXISTS AT THE TAPPING POINT, TO AVOID THIS , THE TAPPING POINT IS VARIED WITH A PUSH BUTTON OR SWITCH , AND IS TESTED PERIODICALLY TO DETECT BLIND ZONE
• SETTING IS 5% OF FIELD VOLTAGE
LOW FREQUENCY INJECTION METHODMODERN ROTOR EARTH FAULT PROT ECTION RELAY OPERATES ON THE PRINCIPLE OF LOW FREQUENCY INJECTION INTO THE FIELD WINDING VIA CAPACITORS.
CORRESPONDING CURRENT OR RESISTANCE DURING E/F IS SENSED
TYP SETTING (500 MW)
ALARM 40 K OHM TIME = 10 SEC
TRIP 5 K OHM TIME = 1 SEC
ACTUAL VALUES OF SETTING SHALL BE DECIDED AT SITE DURING COMMISSIONING TO ACCERTAIN THE HEALTHY VALUE OF THE PARTICULAR M/C.
ROTOR E/F PROT (contd)
Under and Over Voltage Protection:27/59
l Backup to generator’s AVRl V<: Protects against stalling
of auxiliary cooling fans or pumps at low voltage
l V>: Protects insulation from damage for sustained overvoltages
l V<1 & V>1: IDMT or DT operation
l V<2 & V>2: Definite time delayed
Time
VoltageV>1
Idmt curve (V>1)
t = k / (M-1)
M = Multiple of settingk = Time multiplier
MiCOM-P340-40
V/Hz Overfluxing Protection (24)
V f K
Primary function to detect overfluxing during machine run-up
Alarm : Definite time characteristic to initiate corrective action
Trip : IDMT or DT characteristic to clear overfluxing condition
Settings
Pick-up 1.5 to 3.0 i.e. 110V x 1.05 = 2.3150Hz
DT setting range 0 to 100 secondsMiCOM-P340-41
Typical Over Fluxing Withstand Capability
O/V PROTECTION
. TYP SETTINGS OF A 3 STAGE O/V RELAY IS AS FOLLOWS ALARM 110 % 2 SEC TRIP 120 % 1 SEC
140 % INSTANTANEOUS
U/F O/F PROTECTION
TYPICAL SETTING: U/F O/FALARM - 47.8HZ 1 SEC 51 Hz 1 SECTRIP - 47.4 HZ 2 SEC 51.5Hz 2.5 SECSETTING NEED TO BE CO-ORDINATED WITH THE RESPECTIVE GRID
AGENCY AND THE ISLANDING SCHEME SETTINGS AND THE M/C CAPABILITY.
NTPC Vindhyachal
Local Breaker Back up Protection (LBBGT-50Z):
All the protections of the Generator transformer and unit auxiliary transformers finally operate the Generator master trip relay. This master trip relay issues tripping command to Generator bay breaker. In the event, the Generator breaker does not open within preset time say 200 ms, the LBB scheme is energized.
For uncleared system fault
The backup protection is time delayed to coordinate with the zone 3 setting of lines
Detected by– over current – impedance– Impedance type preferred as the line is provided with
distance relays Setting should be made to cover the GT imp and the longest
line impedance. Setting should take care of the infeed from other generators
connected to the same bus also. Time setting 1.5 –2 sec
Backup impedance protection
Stator fault Protection
High Speed Differential protection– Will detect Phase to Phase Faults, Double phase faults involving
earth– Single phase to Earth will not be detected due to limited earth
fault current available.
Two types of high-speed differential relays are commonly used for stator phase fault detection:– High-impedance differential– Biased differential
High Impedance Differential Relay
Use two sets of identical dedicated CTs. PS class CT with stringent parameters to be used This scheme has higher sensitivity than the percentage
differential relay. Through fault stability achieved by using stabilising resistors
in the relay circuit.
High Impedance Differential
I1 I1
i1 i1’
i1 – i1’
Equipment to be protectedI2
i2i1 + i2Stabilising
Resistance
Operating relay
Biased Type Diff Relay
Less stringent CT parameters. CTs can be shared with other protections. Through fault stability achieved through biasing. CT mismatch (typ of the order of 1:5 ) can be accommodated. More suitable for numerical integrated protection systems as the CTs can be shared
for many functions. Modern numerical relays have flexible settings for
Low Impedance Differential
I1 I1
i1 i1’
i1 – i1’
Equipment to be protectedI2
i2
i1 + i2
Biasing/Restraining element
Differentialelement
Overall Differential Protection:
.This protection is provided to detect the faults of Generator, Generator transformer, unit auxiliary transformer.
• This is three winding type biased percentage differentialrelay.
• In case of any fault within the Generator neutral toswitchyard the unbalance will be created in the relay and relay
will operate.•This relay is fast operating (20ms) and is considered as primary
protection of Generator unit protection system. •Operation of this relay isolates the set from the system.
•The relay should have immunity to 2nd and 5th harmonic to avoid operation due to magnetizing in rush current and over
excitation of transformer.
April 9, 2023 52PMI Revision 00
87 GT
50Z
GT
GENERATOR
GT OVERALL DIFFERENTIAL PROTECTIION
Igt
Iuat1
Ig
Iuat2
NUMERICAL RELAYS
• These relays combine several type of Protections into one relay.
• These are micro processor based and can perform various type of calculation by programming.
S.K. Singhal 53
Numerical Relays
• They also display values of Current, Voltage, Power factor at the time of fault as well as during normal running.
• They can record and store number of faults
and events.
S.K. Singhal 54
DISADVANTAGE OF NUMERICAL RELAYS
• Failure of one relay can make a number of protections inoperative.
• Except changing of cards, no maintenance is possible
• Become obsolete very fast and manufacturers are not able to provide services for a long time.
• Requires cleaner and humid free / dry environment as compared to conventional relay.
S.K. Singhal 55
59
78
81
64G
Core-1
Core-3Core-2
Core-4
P1
P2
P1
P2
Resi
stor
Gen. Diff. Prot. (87G)AVR (Auto)
Gen.
LK
To Excit. Trf.To PT Ckt.
MeteringField Failure Prot. (40)
Neg. Phase Seq. (46)
Back Up Imp. Prot. (21G)
Reverse Power Prot.(37G)
Gen. Diff. Protection (87G)
Gen. O/A Diff. Protection (87G)
Over Voltage Relay
Out OF Step Relay
Under/ Over Freq. Relay
100% Stator E/F Protection
Gen. Trf. O/A (87GT) Diff. Protection
P2P1P1P2
Gen. Trf. O/A (87GT) Diff. Protection
UAT O/C (50/51) Protection
UAT Ref. Protection (64 Ref.)
UAT Ref. Protection (64 Ref.)
UAT
52U
GENERATOR PROTECTION SYSTEM
S.K. Singhal 56
• Differential ProtectionCurrent differential relaying can be used to protect network transformers. The relays are connected to current transformers on the high side and low side of the network transformer. The net operating current to the relays is the difference between input and output currents to the network transformer zone of protection. Differential relaying provides a clearly defined zone of protection.
• Biased type with adjustable bias setting of 10-50%• Triple pole• Harmonic restraint feature• No of bias wdgs as applicable• Operating current setting of 15% or less
• Over current protection– Given on LV side as primary protection– On HV side as back up protection– IDMT or def time as applicable– Need to be co-coordinated ( for time and current)with down stream
protection Directional if the power flow is bi-directional
• Back up earth fault protection– Single pole– Def time /IDMT As applicable– Time delay 0.3 – 3 sec for def time relay– Standard curves for IDMT– Setting ranges to suit the application
• Over fluxing– Operate on V/F principle– Inverse characteristics to suit the transformer o/f withstand capability– Shall have alarm and trip stages
Restricted earth fault Protection Single pole Generally high impedance type preferred Setting rage of 5-20% 0r 10 – 40 % Stable for through faults Include stabilizing resistor for through fault stability
NTPC Vindhyachal
Generator Out of Step (Pole slip) Protection:
This relay is connected between Generator neutral C.Ts. and phase side P.T.
When there is sudden increase or decrease of load in power system may be due to fault, switching ON/OFF of large loaded lines which disturbs the balance of power of the system
causing oscillations is called power swing. If the system recovers immediately, stable operation is resumed and generating set can sustain such small swings. However, if the
system is not recovered within say few seconds power swing become so large that synchronism between gen. and system is lost (which is called pole slip).
The pole slip is detected by the impedance relay. It detects the locus of the machine and system impedance and its residence time in the reactance diagram. Generally if the
impedance remains more than 25 ms in the impedance diagram it is considered as pole slip. The relay should not operate on normal recoverable power swings. This protection is very important for the mechanical protection of the machine as the power swings causes
severe pulsating stress on the T.G. set shaft. It is necessary to isolate the machine on unrecoverable power swings.
LA
Core-3
Core-2
Core-1
G.T. O/A Diff. Protection (87 GT)
G.T. Ref. Protection (64 R)
Link Line Diff. Protection (87 LL)
G.T. Ref. Protection (64 Ref.)
P2 P1
GENERATOR TRANSFORMER PROTECTION SYSTEM
S.K. Singhal 62
STANDARD PROTECTIONS USED FOR TRANSFORMERS
• Differential• Over current• Earth fault• Restricted earth fault• Over fluxing• Mechanical protection
buchholtzpressure relief device
• Temperature protectionWTI/OTI
GENERATOR Xer PROTECTION
• REF Protection• Back-up Earth Fault Protection• Overfluxing Protection• Differential Protection• Oil & Wdg Temperature Protection• Buckholtz Protection• PRV Protection