generator protection
DESCRIPTION
aTRANSCRIPT
GRIDTechnical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Generator Protection
Generator Protection - P 2
Generator Protection
The extent and types of protection specified will depend on the following factors :-
• Type of prime mover and generator construction
• MW and voltage ratings
• Mode of operation
• Method of connection to the power system
• Method of earthing
Generator Protection - P 3
Generator Protection
• Types of Prime Mover − Steam Turbines− Gas Turbines− Hydro− Diesel
• Construction− Cylindrial Rotor− Salient Pole (Hydro and small generators)
• Mode of operation− Base load− Peak lopping− Standby
• Ratings− Power from 200kVA to 1000MVA− Voltage from 440V to 24kV
Generator Protection - P 4
Connection to the Power System
1. Direct :
2. Via Transformer :
Generator Protection - P 5
Generator Protection Requirements
• To detect faults on the generator
• To protection generator from the effects of abnormal power system operating conditions
• To isolate generator from system faults not cleared remotely
• Action required depends upon the nature of the fault.
• Usual to segregate protection functions into :
− Urgent− Non-urgent− Alarm
Generator Protection - P 6
Generator Faults
Mixture of mechanical and electrical problems.
Faults include :-
• Insulation Failure− Stator− Rotor
• Excitation system failure• Prime mover / governor failure• Bearing Failure• Excessive vibration• Low steam pressure• etc.
Generator Protection - P 7
System Conditions
• Short circuits• Overloads• Loss of load• Unbalanced load• Loss of synchronism
Generator Protection - P 8
Generator Failure
Generator Protection - P 9
Generator Failure
Generator Protection - P 10
Generator Failure
Generator Protection - P 11
Generator Failure
Generator Protection - P 12
Stator Earth Fault Protection
Fault caused by failure of stator winding insulation
Leads to burning of machine corewelding of laminations
Rebuilding of machine core can be a very expensive process
Earth fault protection is therefore a principal feature of any generator protection package
TYPE OF METHOD METHODPROTECTION OF OF
EARTHING CONNECTION
Generator Protection - P 13
Method of Earthing
Machine stator windings are surrounded by a mass of earthed metal
Most probable result of stator winding insulation failure is a phase-earth fault
Desirable to earth neutral point of generator to prevent dangerous transient overvoltages during arcing earth faults
Several methods of earthing are in use Damage resulting from a stator earth fault will
depend upon the earthing arrangement
Generator Protection - P 14
Method of Earthing
Solidly Earthed Machines :
• Fault current is high
• Rapid damage occurs− burning of core iron− welding of
laminations
• Used on LV machines only
Generator Protection - P 15
Generator - Transformer Units
IF ~ 200 300 A
IF ~ 10 15 A
Method of Earthing
Generator Protection - P 16
Method of Earthing
Desirable to limit earth fault current :
− limits damage− reduces possibility of developing into phase -
phase fault
Degree to which fault current is limited must take into account :
− detection of earth faults as near as possible to the neutral point
− ease of discrimination with system earth fault protection (directly connected machines)
Generator Protection - P 17
Method of Earthing : Limitation of Earth Fault Current
Discrimination not required can limit current to very low value. Sometimes down to 5A
F
Earth faults on the power system are not seen by the generator earth fault protection.
Generator Protection - P 18
Method of Earthing : Limitation of Earth Fault Current
Limit To Generator Full Load Current
• Most popular.
• Used for ease of fault detection and discrimination.
• Residual connection of CTs can be used
• Can result in serious core damage.
Generator Protection - P 19
Stator Earth Fault Protection
Directly Connected Generators :
Earthed Generator : Earth fault relay must be time delayed forco-ordination with other earth fault protection on the power system.
Unearthed Generators : Other generators connected in parallelwill generally be unearthed.
Protection is restricted to faults on the generator, grading with power system earth fault protection is not required. A high impedance instantaneous relay can be used (Balanced Earth Fault protection).
51N
51N50N
Generator Protection - P 20
Percentage Winding Protected
xV
250/1A IS
33R
11.5kV; 75,000KVA
RxV
F Ι
0.8x 250
1 x x.200 Ι
x.200 33
x.6600
R
xV
Ι Ι
operationFor
Y)S(SECONDAR
FS(PRIMARY)
For protection of 90% of winding; x = 1-0.9 = 0.1 Relay setting = 0.8 x 0.1 = 0.08A = 8% of 1A
Generator Protection - P 21
Stator Earth Fault Protection
Generators connected via step-up transformer (resistance earthed) :
Instantaneous protection (50N) :
System earth faults ARE not seen by generator earth fault protection instantaneous relay may be used.
Set to 10% of resistor rating (avoids operation due to transient surges passed through generator transformer interwinding capacitance).
Advantage : Fast
51N 50N
Generator Protection - P 22
Stator Earth Fault Protection
Time delayed protection (51N) :
Time delay prevents operation on transient surges.
A more sensitive current setting may be used.
Set to 5% of resistor rating.
Advantage : Sensitive
On large machines considered worthwhile to use both
instantaneous and time delayed.
Generator Protection - P 23
Restricted Earth Fault Protection
64
RSTAB
Protects approx. 90 - 95% of generator winding.
Generator Protection - P 24
z
Terminal CT Inputs
E/F CT Input
P342/3 Relay
2000/1 ?
500/1 ?
Connections for Biased REF
• Smaller rating machines may have only one (neutral) tail CT brought out for connection
Generator Protection - P 25
0 1 2 3 4
1
2
3
Restrain
Operate
Biased REF Protection OperatingCharacteristic
K1
Slo
pe K
2
• High sensitivity (5%)
• Unit Protection
• FASTEffective bias (x In) = Max. phase current + k . I
N2
Differential current (x In)
= I + I + I + k . IA B C N
Generator Protection - P 26
Neutral Displacement / ResidualOvervoltage - Earth Fault Protection
P340Relay
3
1
2
(1) Derived measurement from 5-limb or 3 x 1 phase VT
(2) Directly measured from a broken delta VT input
(3) Directly measured across an earthing resistor
Generator Protection - P 27
Stator Earth Fault Protection
• 100% Stator Earth Fault Protection :
• Standard relays only cover 95% of winding.
• Probability of fault occurring in end 5% is low.
• On large machines 100% stator earth fault protection may be required.
• Two methods :
− Low Frequency Injection− Third Harmonic Voltage Measurement
Generator Protection - P 28
100% Stator Earth Fault Protection (27TN)
(1) Derived measurement from 5-limb or 3 x 1 phase VT(2) 3rd harmonic overvoltage(3) 3rd harmonic undervoltage
• 3rd harmonic undervoltage supervised by 3 phase undervoltage and W/VA/Var at generator terminals
P340Relay
3
1
2
Generator Protection - P 29
100% Stator Earth Fault Protection
Distribution of 3rd harmonic voltage along the stator winding
• (a) normal operation
• (b) stator earth fault at star point
• (c) stator earth fault at the terminals
Generator Protection - P 30
100% Stator Earth Fault - Low Frequency Injection
For Large Machines Only
InjectionTransformer
51 Alternative InjectionPoints
• Injection Frequency 12.5 - 20Hz
• Provides protection during run up & Standstill
• High cost due to injection equipment.
Generator Protection - P 31
Overcurrent Protection
• For small generators this may be the only protection applied.
• With solid earthing it will provide some protection against earth faults.
• For a single generator, CTs must be connected to neutral end of stator winding.
51
Generator Protection - P 32
Overcurrent Protection
• For parallel generators, CTs can be located on line side.
51
Generator Protection - P 34
Differential Protection
• Provides high speed protection for all fault types
• May be : High impedance type : Biased (low impedance)
type
CT’s required in neutral end of winding
Relay
Generator Protection - P 35
Differential Protection - Biased
OPERATE
BIASBIAS
Biased Differential Scheme
Generator Protection - P 36
Differential Protection
Overall Differential Scheme
INTERPOSINGC.T.
Generator Protection - P 37
Independent current settings per phase Single stage definite time delay
IA2
IB2
IC2
Interturn Protection (50DT)
Generator Protection - P 38
Neutral Displacement / ResidualOvervoltage - Interturn Protection (59N)
GenRelay
3
1
2
(1) Interturn, derived measurement from 5-limb or 3 x 1 phase VT (2) Interturn, directly measured from a broken delta VT input (3) 95% stator earth fault protection across an earthing resistor
Generator Protection - P 39
Prime Mover Failure
Isolated Generators :
Machine slows down and stops. Other protection initiates shut down.
Parallel Sets :
System supplies power - generator operates as a motor.Seriousness depends on type of drive.
Steam Turbine Sets :
Steam acts as a coolant.Loss of steam causes overheating.Turbulence in trapped steam causes distortion of turbine blades.Motoring power 0.5% to 6% rated.Condensing turbines, rate of heating slow. Loss of steam instantly recognised.
Generator Protection - P 40
Prime Mover Failure
Diesel Driven Sets :
Prime mover failure due to mechanical fault.Serious mechanical damage if allowed to persist.Motoring power from 35% rated for stiff machine, to 5% rated for run in machine.
Gas Turbines :
Motoring power 100% rated for single shaft machine, 10% to 15% rated for double shaft.
Hydro Sets :
Mechanical precautions taken if water level drops.Low head types - erosion and cavitation of runner can occur.Additional protection may be required.
Generator Protection - P 41
Prime Mover Failure
Reverse Power Protection :
Reverse power measuring relays used where protection required.
Single phase relay is sufficient as prime mover failure results in balanced conditions.
Sensitive settings required - metering class CTs required for accuracy.
Generator Protection - P 42
Reverse Power
• Blinders at 0.5 degrees reduces operation area for low power settings where the power factor is low to improve reliability of reverse power element
Operational limits
Trip a rea
Q
U nsta b le a reaU nsta b le a rea
P
as ta b le
n a tu ra la = 0 .1 6 o
-P= P0= 0 .5 o
Generator Protection - P 43
Low Forward Power
• To reduce the risk of overspeed damage to steam turbine generators a low forward power element is used for interlocking the generator CB and excitation for non urgent trips (eg thermal protection, stator earth fault for high impedance earthing).
• Turbine steam valves are tripped immediatelay and when power output has reduced the generator CB and excitation are tripped.
Operational limits
Trip a rea
Q
U nsta b le a reaExtend ed Trip a reaP
0
P= P0
Trip a rea
a s ta b le = 0 .5 o
Generator Protection - P 44
Loss of Excitation
Effects
Single Generator :
− Loses output volts and therefore load.
Parallel Generators :
−Operate as induction generator (> synch speed)−Flux provided by reactive stator current drawn from system-leading pf−Slip frequency current induced in rotor - abnormal
heating
Situation does not require immediate tripping,
however,
large machines have short thermal time constants - should be unloaded in a few seconds.
Generator Protection - P 45
Loss of Excitation
X
Load Impedance
RImpedanceLocus
Offset – Prevents operation on pole
slips
Diameter
Typically :Offset 50-75%X’dDiameter 50-100% XS
Time Delayed
Relay Characteristic
Impedance seen by relay follows locus shown below :
Generator Protection - P 47
Pole Slipping
Sudden changes or shocks in an electrical power system may lead to power system oscillations - regular variations of I and V and angular system separation
In a recoverable situation these oscillations will die away - a power swing
In an unrecoverable situation the oscillations become so severe that synchronisation between the generator and the power system is lost - out of step/pole slipping
Causes− Transient system faults− Failure of the generator governor− Failure of the generators excitation
control− Reconnection of an islanded system
without synchronisation− Switching transients on a weak system
Generator Protection - P 48
Pole slipping
Power Swing
Recoverable
Unrecoverable
Loss of Synchronism
Out-of-Step
(Power System)
Pole-Slipping
(Generator)
Generator Protection - P 49
Theory of pole slipping
Where:
EG represents the generator terminal voltage;
ZG represents the generator reactance;
ZT is the reactance of step-up transform;
Zs represents the impedance of the power system connected to the generation unit
Es represents the system voltage.
Simplified Two Machine System:
Generator Protection - P 51
Loss of synchronisation Characteristics
EG/ES<1
EG/ES>1EG/ES=1
R
X S
G
L
Generator Protection - P 53
Conventional Pole Slipping Protection
Reactance Line
R
Lens
Blinder
ZA
ZB
X
a
ZC
Zone 1
Zone 2
Generator Protection - P 54
Pole Slipping Protection - 78
• Conventional lenticular (lens) characteristic− 2 Zones defined by reactance line− Zone 1 - pole slip in the generator− Zone 2 - pole slip in the power system− Separate counters per zone (1-20)
• Setting to detect pole slipping when :− Generating− Motoring− Both (Pumped storage generator)
Generator Protection - P 55
Pole Slipping Protection - 78
• Pole slip when generating− Impedance position on RHS of lens characteristic− Impedance crosses lens on RHS− Impedance spends >T1 (15ms) in RHS of lens− Impedance spends >T2 (15ms) in LHS of lens− Impedance leaves lens on LHS − Zone 1 and 2 counter is incremented if in Z1− Zone 2 counter is incremented if in Z2− Trip when zone counter value exceeded
• Pole slipping when motoring is the opposite
Generator Protection - P 56
State Transition Diagram
ID LE
D E T E C T E D S T A R T
C O N F IR M
Zm = R 1 .R eset S tart_S igna ls;R eset F lag_Zone1;IF (Any T rip_S igna l) R eset C ounters; R eset T rip_S igna ls;
(Zm = R 4) & T im er2 > T2)If (C 2==0) S tart R eset_T im er;C 2++;Set Zone2_S tart;if(C 2>=C ount2) Set Zone2_Trip ;If (F lag_Zone1) C 1++; Set Zone1_S tart; if(C 1>=C ount1) Set Zone1_Trip ;R eset T im er2;
(Zm = R 3) & T im er1 > T1) F lag_Zone1=Zone1Pu();
R eset T im er1;S tart T im er2 ;
Zm = R 2Start T im er1
Zm = R 1 or R 2R eset F lag_Zone1;
R eset T im er2;
Zm = R 3 but T im er1<T1R eset T im er1
Zm = R 1 or R 3
Zm = R 2
Zm = R 3
Zm = R 4 or R 2 or R 3
(R eset_T im er T im e O ut)Actions are the sam e as
S ta te M ach ine EntryS tate M ach ine E ntry
R eset T rip_ S igna ls; R eset S tart_ S igna ls;
R eset F lag_Zone1;R eset A ll C ounters;
R eset A ll T im ers;
Zm = R 1 or R 4 R eset T im er1
Zm = R 4 bu t T im er2 < T2R eset F lag_Zone1;
R eset T im er2;
Zm = R 4IF (M ode_Both)F lag_M ode= !F lag_M ode;
*N o S igna l C ondition(V A<1V or I <0.02A )
N o S igna l C ondition*Actions are the sam e as
S ta te M ach ine Entry
V TS -FA S T-BLO C KActions are the sam e as
S ta te M ach ine Entry
Generator Protection - P 57
RTDS Pole Slip Simulation
Local Load
T/line 140 km 11 kV BUS132/13.5 kV
Yd1Grid System Generator with
AVR and Governor control
132 kV BUS
Generator Protection - P 58
Pole Slipping - 80% Load, Local 3 ph fault
Generator Protection - P 59
Loss of excitation at 100% machine loading
Generator Protection - P 60
Rotor ThermalProtection
• Unbalanced loading leads to negative sequence current
• Double frequency slip
• Rapid overheating of rotor
Generator Protection - P 61
Unbalanced Loading
• Gives rise to negative phase sequence (NPS) currents - results in contra-rotating magnetic field
• Stator flux cuts rotor at twice synchronous speed
• Induces double frequency current in field system and rotor body
• Resulting eddy currents cause severe over heating− Use negative sequence overcurrent relay− Relay should have inverse time characteristic to
match generator I22t withstand
Generator Protection - P 62
Unbalanced Loading
• Machines are assigned NPS current withstand values :−Continuous NPS rating, I2R (PU CMR)−Short time NPS rating, I22t (K)
• If possible level of system unbalance approaches machine continuous withstand, protection is required.
Generator Protection - P 63
Overload Protection
high load current
heating of stator and rotor
insulation failure
Governor Setting
Should prevent serious overload automatically.Generator may lose speed if required load can not be met by other sources.
Generator Protection - P 64
Stator Thermal Protection
Current operated−Over power protection−Overcurrent element −Thermal replica
RTD Thermal Probes−PT100 Platinum probes−Embedded in machine−Alarm and trip thresholds for each RTD
Generator Protection - P 65
Current
Time
Overload Protection (1)
• Thermal replica for stator overload protection− Current based on I1 and I2− Heating and cooling time constants− Non-volatile memory thermal state− Alarm output
Generator Protection - P 66
Rotor Earth Fault Protection
Field circuit is an isolated DC system.
• Insulation failure at a single point :− No fault current, therefore no danger− Increase chance of second fault occurring
• Insulation failure at a second point :− Shorts out part of field winding− Heating (burning of conductor)− Flux distortion causing violent vibration of rotor
• Desirable to detect presence of first earth fault and give an alarm.
Generator Protection - P 67
Rotor Earth Fault Protection
R
Exciter
Potentiometer Method
• Required sensitivity approximately 5% exciter voltage.
• No auxiliary supply required.
• “Blind spot” - require manually operated push button to vary tapping point.
Generator Protection - P 68
Rotor Earth Fault Protection
AC Injection Method
• Brushless Machines
• No access to rotor circuit
• Require special slip rings for measurement
• If slip rings not present, must use telemetering techniques (expensive)
R
AC AuxiliarySupply
Generator Protection - P 69
Rotor Earth Fault Protection
Brushless MachineA brushless generator has an excitation system consisting of:
− A main excitor with rotating armature and stationary field windings− A rotating rectifier assembly, carried on the main shaft line out− A controlled rectifier producing the d.c. field voltage for the main exciter field
from the a.c. source (often a small `pilot` exciter)
Hence:− No brushes are required in the field circuit− All control is carried out in the field circuit of the main exciter− Detection of rotor circuit earth fault is still necessary− Based on dedicated rotor-mounted system that has a telemetry link to provide
an alarm/data
Generator Protection - P 70
Generator Back-Up Protection
10 x FL
with AVR
no AVR
Cycles
FullLoad
Overcurrent Protection
Typical use :− Very or extremely inverse for LV machines− Normal inverse for HV machines
Must consider generator voltage decrement characteristic for close-in faults.With reliable AVR system, “conventional” overcurrent relays may be used.Otherwise, voltage controlled / restrained relays are required.
Generator Protection - P 71
Generator Back-Up Protection
Overcurrent Protection
Voltage Restrained
• Operating characteristic is continuously varied depending on measured volts.
• Alternatively, use impedance relay.
Voltage Controlled
• Relay switches between fault characteristic and load characteristic depending on measured volts.
F
Generator Protection - P 72
10
O/L CHARAC
FAULT CHARAC1.0
tsec
GENERATOR DECREMENTCURVE
0.1
0.01 10
0AMPS10,0003000100
0600
240
LARGESTOUTGOING FEEDER
6.6kV
5MVA115% XS
500/5200/5
Generator Back-Up Protection (2)
Generator Protection - P 73
I>
Terminal Volts
LoadFault
k.I>
Voltage control
I>
Terminal Volts
LoadFault
k.I>
Voltage restraint
Voltage Dependent Overcurrent Protection (51V)
Generator Protection - P 74
Impedance Relay
• 2 Zones of protection− Zone 1 - Set to operate at 70% rated load impedance.
Back-up protection for generator-transformer, busbar and outgoing feeders. Time delayed for co-ordination with external feeder phase fault protection.
− Zone 2 – Set to 50% transformer impedance. Back-up protection for generator phase faults. Faster time delay to co-ordinate with generator phase fault protection
R
X
LoadFault
Underimpedance
Generator Protection - P 75
Under & Over Frequency Conditions
Over Frequency
• Results from generator over speed caused by sudden loss of load.
• In isolated generators may be due to failure of speed governing system.
• Over speed protection may be provided by mechanical means.
• Desirable to have over frequency relay with more sensitive settings.
Generator Protection - P 76
Under & Over Frequency Conditions
Under Frequency
• Results from loss of synchronous speed due to excessive overload.
• In isolated generators may be due to failure of speed governing system.
• Under frequency condition gives rise to:−Overfluxing of stator core at nominal volts−Plant drives operating at lower speeds - can affect
generator output−Mechanical resonant condition in turbines
• Desirable to supply an under frequency relay.• Protection may be arranged to initiate load
shedding as a first step.
Generator Protection - P 77
df/dt+t: Time Delayed ROCOF
• Df/dt can operate quicker than underfrequency for large changes in frequency
• Rolling window is better than fixed window as gives faster operation
• Averaging cycles is typically 5 to provide some stability for power system oscillations
• Stages can be used for load shedding or alarm/tripping of the generator
df/dt (81R)Loadshedding
Generator Protection - P 78
Under & Over Voltage Conditions
Protection
• Under & over voltage protection usually provided as part of excitation system.
• For most applications an additional high set over voltage relay is sufficient.
• Time delayed under and over voltage protection may be provided.
Generator Protection - P 79
Under & Over Voltage Conditions
Over Voltage
• Results from generator over speed caused by sudden loss of load.
• May be due to failure of the voltage regulator.
• An over voltage condition :
− Causes overfluxing at nominal frequency− Endangers integrity of insulation
Under Voltage
• No danger to generator. May cause stalling of motors.
• Prolonged under voltage indicates abnormal conditions.
Generator Protection - P 80
Generator Abnormal Frequency Protection (81AB)
• 6 independent bands of abnormal frequency protection
• Accumulation of time up to 1000 hours in each band
• Band data provided by generator manufacturer
• Bands match resonance, blade stress frequencies …
• Dead band timer before accumulation starts allows time for resonance to established
• When generator is off-line bands can be blocked
Generator Protection - P 81
Generator Abnormal Frequency Protection (81AB)
Band 1f nom
Band 4
Band 3
Band 2
Timer 1
Timer 2
Timer 3
Timer 4
Generator Protection - P 82
Application Negative Sequence Overvoltage (47)
Generator/MotorCB
Negative Sequence Overvoltage
Swapping of 2 phases to motor (pump water)
47
Generator/Motor
47
b ac
Block CB Close
Busbar
Hydro machines can operate as motors/pumps by swapping 2 phases (phase rotation is reversed)
a
bc
Generator Protection - P 83
Generator differentialUnder & over voltageUnder & over frequencyReverse powerStator earth faultLoss of excitationVoltage dependent overcurrentNegative phase sequence
87G27 & 5981U & 81O32R51N4051V46
When the units are being used to generate power the protection could be as below:
When the units pump water the protection applied will change
2 31 4
Four groupsavailable
32R Reverse power
Use of Alternative Setting GroupsExample : Pumped Storage Unit
Generator Protection - P 84
Phase Rotation
• Phase rotation for hydro generator/motor applications where 2 phases are swapped to make the machine operate as a pump (motor)
G x
P340
PhaseReversalSwitches
CT1 CT2
Case 1 : Phase Reversal Switches affecting all CTs and VTs
G x
P343/4/5
PhaseReversalSwitches
CT1 CT2
Case 2 : Phase Reversal Switches affecting CT1 only
Generator Protection - P 85
Phase Rotation
• Phase rotation settings can be changed for generator/motor operation using 2 setting groups
Setting Range Default SYSTEM CONFIG Phase Sequence Standard ABC /
Reverse ACB Standard ABC
VT Reversal No Swap / A-B Swapped / B-C Swapped / C-A Swapped
No Swap
CT1 Reversal No Swap / A-B Swapped / B-C Swapped / C-A Swapped
No Swap
CT2 Reversal (P343/4/5 only)
No Swap / A-B Swapped / B-C Swapped / C-A Swapped
No Swap
Generator Protection - P 86
50
27
VTS
&tPU
tDO
& Trip
Unintentional Energisation at Standstill
• Overcurrent element detects breaker flashover or starting current (as motor)
• Three phase undervoltage detection
• VTS function checks no VT anomalies
Generator Protection - P 87
Check Synch (25)
• Check is used when closing generator CB to ensure synchronism with system voltage.
• Check synch relay usually checks 3 things:−Phase angle difference−Voltage−Frequency difference
Generator Protection - P 88
Check Synchronising (25)
• Phase angle difference−Single phase comparison
• Can select either A-N, B-N, C-N, A-B, B-C, C-A is settings −Typical setting is 20º to reduce mechanical stresses on
generators.
• Voltage −Check synch relay inoperative if :-
• Generator/busbar voltage is below or above preset limit (independent settings for generator and busbar under/overvoltages)
• voltage difference exceeds preset limit−Typical settings for undervoltage: 80 - 85% Vn−Typical settings for difference voltage: 6 - 10% Vn
• Frequency difference−Usually measured by time to traverse phase angle limits or direct
slip frequency measurement (Fgen – Fbus)• Eg Timer setting of 2 secs over 20º :
• Slip frequency = 2 x (20 x ½) / 360 = 0.055Hz = 0.11% (50Hz)
• Timer usually set to 2 secs or 10 x C.B. closing time whichever is greater).
Generator Protection - P 89
Check Synchronising (25)
• Check synch has 2 stages – Check Sync 1/2−Usually only 1 stage is required for generator
applications−Check Sync 2 has CB closing time compensation−Check Sync2 only permits closure for decreasing angles
of slip
• Check synch has vector compensation to account for phase shift across transformer with Main VT Vect Grp setting 0-11
• Check synch has ratio correction to correct ratio errors of VTs
• Voltage monitors for dead/live generator/busbar
• System Split output operates for phase angle > setting adjustable from 90 to 175 degrees
Generator Protection - P 90
Check Synch (25)
Check synch stages 1 and 2
GRIDTechnical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Typical Schemes
Generator Protection - P 93
Protection Package for Diesel Generator
G87
R64
R64
V5132
32 Reverse Power64R Rotor Earth Fault 64S Stator Earth Fault 51V Voltage Dependent
Overcurrent87G Generator Differential
Protection P343
Generator Protection - P 94
Overall Protection of Generator Installation
Generator Feeder Protn.
51 V
64R
32
40
87
46
64S
Overcurrent Voltage
Restraint
Restricted
E/F
Buchholz Winding Temp.
Reverse Power
Field Failure
Generator Differential
Rotor E/F Prime Mover Protection
Negative Phase Sequence
Stator E/F
Overall Gen/Trans Diffl Protn.
Generator Protection - P 95
Overall Protection of Generator Installation
Generator Feeder Protection
Low Steam Pressure, Loss of Vacuum
Loss of Lubricating OilLoss of Boiler Water
Governor FailureVibration, Rotor Distortion
O/C Circuit Breaker Fail
Busbar Protection
Restricted E/F
Buchholz Winding Temperature
V.T.sO/C
Transformer
Overfluxing
Restricted E/F
Standby E/F
BuchholzO/C + E/F
Unit Transformer Differential Protn.
Overall Generator
Transformer Differential
Protn.
Rotor E/F
Permissive (Low
Power) Interlock Pole
SlippingField Failure
Generator Differential
Negative Phase Sequence
Stator E/F Protection
GRIDTechnical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Embedded Generation
Generator Protection - P 97
PES system
PES system
81U/O
27/59
59N
df/dt
dV
Frequency
Voltage
Residual Voltage
81U/O
27/59
59NIslanded load fed unearthed
AR?
O/C & E/F50/51N
df/dt
dV
ROCOF
Voltage Vector Shift
Co-generation/Embedded Machines
NPS Voltage
NPS O/C
47/46
Check Synch25
Generator Protection - P 98
Embedded Generation
USED TO PROVIDE:
• Emergency Power Upon Loss Of Main Supply
• Operate In Parallel To Reduce Site Demand
• Excess Generation May Be Exported Or Sold
Generator Protection - P 99
Engineering Recommendation G59
• ER G59 relates to the connection of generating plant to the distribution systems of licensed distribution network operators (DNOs)
• ER G83/1 covers connection of generating units rated < 16A / phase in parallel with LV distribution system
• ER G59 COVERS:−Safety Aspects−Legal Requirements−Operation−Protection
Generator Protection - P 100
Engineering Recommendation G59
• The main function of the protection systems and settings is to prevent Generating Plant supporting an islanded section of the Distribution System when it would or could pose a hazard to the Distribution System or customers connected to it.
General Requirements Protective Equipment
Generator Protection - P 101
Engineering Recommendation G59
• To disconnect the Generating Plant from the Distribution System in the event of loss of one or more phases of the DNOs supply.
• LoM is required to ensure requirements for earthing and out of synch closure are complied with and customers are not supplied with voltage and frequency outside statutory limits
LoM (Loss of Mains = Islanding) Protection Requirements
Generator Protection - P 102
Loss of Mains Problem
• Loss of mains is where a generator is inadvertently isolated from the grid and continues to supply local load
• Loss of mains can be caused by:
−Protection tripping
−Accidentally due to network reconfiguration
33kV DISTRIBUTION
33/11kV
400V DG
SECTIONALISINGSWITCH
CIRCUITBREAKER BUS-BAR
FAULT
Generator Protection - P 103
Loss of Mains Problem
Islanding is unacceptable for a number of reasons:
−Safety risk - for example, through personnel working on the network under the assumption that no parts of the network are energised
−Stresses from out of synchronism re-closure
−Loss of system earth where the earth is on the star winding of a network transformer. This can cause problems for existing earth fault protection to detect earth faults if the system is unearthed.
−Utility is legally bound to maintaining quality of supply (frequency and voltage ) to local demand.
Generator Protection - P 104
Existing LoM Methods – Performance Assessment
• Loss of mains performance can be assessed in terms of sensitivity and stability
• Sensitivity
− Smallest possible mismatch between local generation and the demand at the instant of islanding.
− Also referred to as non-detection zone
• Stability
− Stability for different fault types with varying duration and retained voltage at the point of measurement
• When designing LoM method objective is to have a small non detection zone and be stable for as many fault characteristics as possible
STABILITY
SENSITIVITY
Generator/demand Imbalance
Network faults
Generator Protection - P 105
Existing Loss of Mains Methods
• Passive Methods
− Under/over frequency and voltage • Requires large change in load, time delayed
− Df/dt – rate of change of frequency• Sensitive, fast operating
− Voltage vector shift• Not as sensitive as df/dt, fast operating
− Direct inter-tripping
• Not load dependent, fast, expensive, signalling can be complex
• Active Methods
− Active frequency drift
− Reactive Error export
• There is an abundance of active methods proposed in the technical literature, however, their application in practice has been limited to date. The traditional protection philosophy of independence from other systems makes the introduction of these methods difficult.
Generator Protection - P 107
Single phase line diagram showing generator parameters
E
R
T
jX
VIL
Loss of Mains Methods – Voltage Vector Shift
Generator Protection - P 108
Vector Diagram Representing Steady State Condition
E
I XL
II R
LVT
L
Loss of Mains Methods – Voltage Vector Shift
Generator Protection - P 109
Loss of Mains Methods – Voltage Vector Shift
Transient voltage vector change due to change in load current IL
L
L
E
I
I XI RV
L
L
I X”
VT
L
T
I
Generator Protection - P 110
Loss of Mains Methods - ROCOF
The rate of change of speed, or frequency, following a power disturbance can be approximated by:
df/dt =
where P = Change in power output between synchronised and islanded operation
f = Rated frequency
G = Machine rate MVA
H = Inertia constant
df/dt
P.f
2GH
Generator Protection - P 111
Loss of Mains Methods - ROCOF
df/dt =
Two consecutive calculations must give a result above the setting threshold before a trip decision can be initiated
P341 df/dt calculation
F - f
3 cycle
n n - 3 cycle
Generator Protection - P 112
df/dt+t: Time Delayed ROCOF
t
f
df/dt Setting
Start
Trip Pick up cycles
Time delay
Loss of Mains Methods - ROCOF
Generator Protection - P 113
G59 Other Protection
• Neutral voltage
• Overcurrent
• Earth fault
• Phase unbalance
• Reverse power − Used when generator does not export power during normal operation
Generator Protection - P 114
G59 Protection Settings
Protection Settings for Long-Term Parallel Operation
Notes: K1 = 1.0 (low impedance networks or 1.66-2 (high impedance networks)K2 = 1.0 (low impedance networks or 1.6 (high impedance networks)A fault level of < 10% system design max fault level is high impedance* Might need to be reduced if auto-reclose time <3s
Prot Function Small Power Station Medium Power Station
LV Connected HV Connected
Setting Time Setting Time Setting Time
U/V st 1 Vph-n -13% 2.5s* Vph-ph -13% 2.5s Vph-ph -20% 2.5s*
U/V st 2 Vph-n -20% 0.5s Vph-ph -20% 0.5s
O/V st 1 Vph-n +10% 1.0s Vph-ph +10% 1.0s Vph-ph +10% 1.0s
O/V st 2 Vph-n +15% 0.5s Vph-ph +13% 0.5s
U/F st 1 47.5Hz 20s 47.5Hz 20s 47.5Hz 20s
U/F st 2 47Hz 0.5s 47Hz 0.5s 47Hz 0.5s
O/F st 1 51.5Hz 90s 51.5Hz 90s 51.5Hz 90s
O/F st 2 52Hz 0.5s 52Hz 0.5s
LoM (Vector Shift) K1 x 6 degrees K1 x 6 degrees Intertripping expected
LoM (RoCoF) K1 x 0.125 Hz/s K2 x 0.125Hz/s Intertripping expected
Generator Protection - P 115
G59 Protection for HV Generator connected to DNO HV System for Parallel Operation Only
Generator Protection - P 116
G59 Protection for HV Generator connected to DNO HV System for Standby and Parallel Operation
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