class on Protection.pptx

Discussion on:
Generator Protection ,
Generator & Unit transformer Protection.
Protection of Bus Bar,
Feeder Protection,
Transformer Protection and
Motor Protection

ELECTRICAL PROTECTION of
GENERATOR
• Generator Backup Impedance Protection 21G1
• • GT Over fluxing Protection 24 G1
• • Generator under voltage Protection 27G1
• • Generator Reverse Power Protection 32 G1
• • Generator Field failure Protection 40 G1
• • Generator Negative Phase Sequence Protection 46 G1
• • Generator Dead Machine Protection 50GDM1
• • Generator Over voltage Protection 59 G1
• • 95% Stator Earth Fault Protection 59 NG1
• • 100% Stator Earth Fault Protection 64G1
• • Rotor Earth Fault Protection 64F1
• • Generator Over/Under Frequency Protection 81G1
• • Generator Differential Protection 87 G1
• • Generator Low forward Power Protection 37 G1
• • Generator Over flux Protection 99GT
• • Generator thermal Overload Protection 51GTH 1
• • Generator Standby Earth Fault Protection
• Generator Stator Inter-turn Protection

GENERATOR PROTECTION
CLASSIFICATION
1. Protection against Electrical Faults.
A. Unit Protection (against Internal faults)
 Differential Protection
 Stator Earth Fault Protection
 Rotor Earth Fault protection
 Inter-turn Fault Protection.
B. Non-unit Protection (against Uncleared External Faults)
 Voltage Controlled / Restraint O/C Relays.
 Impedance Backup Protection.

CLASSIFICATION
2. Protections against Abnormal Operating Conditions
 Unbalanced Load Protection (NPS Current Relay)
 Field Failure Protection
 Pole Slipping (Out of step) Protection
 Anti-motoring Protection
 Over Excitation Protection (for G.Ts.)
 Under / Over Frequency Protection
 Over Voltage Protection
 Over Load Protection
 Dead Machine Energisation Protection.

INFLUENCING FACTORS
 Type of Connection / Earthing.
 Type of Prime Mover.
 Type of Excitation System
 Type of Stator Winding (Single / Split Phase)
 Mode of Operation (Isolated / Parallel)
 Size and Criticality of Machine.

A. DIRECT CONNECTED GENERATOR
CONNECTIONS AND EARTHING
 In case of NGR grounding, earth fault current is usually limited to
rated current of the machine.
 NGR is short time rated, usually 10 – 15 seconds.

B. UNIT CONNECTED GENERATOR.
CONNECTIONS & EARTHING

 IDMT E/F Relay with sensitive setting (about 10% In) provides about 90%
winding coverage.
 CT ratio nearer to the max. earth fault current can be selected.
DIRECT CONNECTED GENERATOR
STATOR EARTH FAULT PROTECTION.

UNIT CONNECTED GENERATOR
STATOR E/F PROTECTION USING NDR (64N)
 NDR (64N) with a sensitive voltage setting (about 5V) can provide
about 95% coverage of Stator Winding.
 The relay should have harmonic immunity and small time delay
(built-in or external) for transient free operation.

UNIT CONNECTED GENERATOR
STATOR E/F PROTECTION USING CURRENT RELAY
(64N)
 Current Relay with a sensitive current setting (say 5%) can provide about
95% coverage for stator winding.
 Relay should have harmonic immunity and small time delay (built-in or
external) for transient free operation.

STATOR E/F PROTECTION
STATOR WINDING COVERAGE
 NDR setting = 5 V
 Volts across E.T. Secondary for terminal earth fault
= 240 /1.732 = 138V.
 Winding coverage = (138 – 5) / 138 = 0.95 (95%)
 Identical coverage by a sensitive current relay with 5% setting.

INTER-TURN FAULT PROTECTION (87- I)

DIFFERENTIAL PROTECTION (87G)
A. HIGH IMPEDANCE
TYPE
 Either a Voltage Relay or Current relay (87G) with a Series
Stabilising Resistor is used.

DIFFERENTIAL PROTECTION- HIGH IPEDANCE TYPE
SETTING CRIETERIA
 Assuming one CT fully saturates while other fully active, on external fault, max.
Voltage across Relay Branch-
Vs = If/n (Rct + 2Rl)
 If Relay current setting is “Is” and Relay Ohmic burden is “Zr”, then Stabilising
resistor setting (Rs) = Vs/Is – Zr.
 In case of Generator, max. through fault current should be worked out based on
Sub-transient Reactance (Xd”).

DIFFERENTIAL PROTECTION (87G)
B. BIASED TYPE.
• Bias ensures stability on external fault in the presence of mismatch
produced by CT errors.
• A sensitive pick-up and low bias slope can be set, with Class PS CTs,
since CT error is the only contributing factor for mismatch on load or
through fault.

OVER-ALL DIFFERENTIAL PROTECTION FOR GEN. / GT AND
DIFFERENTIAL FOR UAT.
 Over-all Differential (87 O/A) covers GEN. / GT and requires CTs
identical to GEN. Neutral side CTs on UAT tap off.
 Separate Differential Relay (87UAT) should be provided for UAT, where
applicable, for better winding coverage.

GENERATOR BEHAVIOUR ON SHORT CIRCUIT.
• Initial high S.C. Current produces severe armature reaction, thereby,
reducing Machine induced voltage.
• This, in turn, progressively reduces sustained S.C. Current, as the
armature reaction sets in.
• AVR may provide some compensation to maintain fault infeeds by
field forcing but this may not be adequate for close-up faults, resulting
sustained S.C. Current less than full load current.
• Conventional IDMT O/C, relays set above F.L. Current, will not,
therefore, be suitable as backup for Generator application.
• Generally, Voltage Controlled / Restrained O/C relays or Impedance
type backup protection is used for Generators.

VOLTAGE CONTROLLED O/C RELAY (EM VERSION) - 51V
• On fault, U/V element drops off & shunts the shading coil resistor through its
contact. This increases the Shading coil current, thereby, increasing torque on
disc to 2.5 times. Consequently, effective pick-up setting reduces to 40% of the
nominal setting.
• The relay requires time co-ordination with down-stream backup. protection.

VOLTAGE CONTROLLED / RESTRAINT CHARACTERISTIC – COMPARISON.
• V/Vn = Fault Voltage (V) as pu of Nominal System Voltage (Vn)
• Ism / Is Nom = Modified O/C setting as pu of Nominal O/C Setting.

GNERATOR PROTECTION
IMPEDANCE BACKUP PROTECTION (21G)
 Impedance Relay (21G) used as Phase fault Backup.
 Delta / Star IVTs used to compensate phase shift produced by GT
at Line voltage level.
 Earth Fault Backup is provided by Stand by E/F relay(51N)
operated off CT at GT HV Neutral.

IMPEDANCE BACKUP PROTECTION (21G)
SETTING CRIETERIA
 Relay Impedance setting ZR = XT + (N+1).ZL OR
= 0.70 Z Load, which ever is greater.
 Time Delay = 1.5 – 2 seconds (to co-ordinate with downstream Protns)

ROTOR EARTH FAULT PROTECTION (64R)
• 1 st Rotor E/F enhances possibility of a second E/F causing partial S.C.
of Field winding. Consequent field current diversion may cause
burning of conductor.
• It may also cause flux distortion resulting unbalanced forces, causing
vibrations / mechanical damage.
• Methods of 1st Rotor E/F Detection
a) Potentiometer Method
b) A.C. Injection
c) D.C. Injection

1 ST ROTOR E/F PROTECTION
(64R1)
D.C. INJECTION METHOD.

COMBINED 1ST / 2ND ROTOR E/F PROTECTION (64R1/ R2)

UNBALANCE PROTECTION - NPS O/C RELAY (46G)
 NPS component of unbalanced stator current produces reverse
reaction field which cuts rotor at double the speed.
 Resulting double frequency eddie currents cause intense heating
of the rotor iron / field winding.
 Machine Capability specified in terms of:
1. Continuous NPS Rating = I2
2 Short Time NPS Rating = I2 SQ.t
 Typical NPS Withstand Values are:
a) Salient Pole M/C = 40% (I2) / 60 (I2SQ.t)
b) Cylindrical Rotor M/C = 10–15% (I2) / 7-20 (I2SQ.t)

UNBALANCE PROTECTION (46G)
N.P.S. FILTER
Typical Settings of NPS O/C Relay:
I2 = 7.5 – 30%, I2SQ.t = 1 – 160, Pre-trip Alarm = 70-100% Trip.

FIELD FAILURE PROTECTION
EFFECT ON PARALLEL GENERATORS.
 Defaulting M/C runs at super-synchronous speed as Induction
Generator.
 Continues Generating action by absorbing VARs from System to
support excitation.
 Slip Freq. Currents induced in damper winding, rotor iron and field
system, cause rotor heating.
 Defaulting M/C becomes a burden on the system since the latter has
to supply VARs.
 Methods of Detection:
a) Field Under Current relay (50)
b) Off-set MHO Relay (40G)

FIELD FALURE PROTECTION
FIELD UNDER CURRENT RELAY (50)
• Under Current relay to have setting below min. excitation current (usually
5%).
• Slip freq. Currents induced in field system cause cyclic operation /
resetting of U/C relay. Off-delay timer (T1) stabilises the scheme against
slip freq. effect.

FIELD FAILURE PROTECTION (40G)
RELAY CHARACTERISTIC & M/C TERMINAL IMPEDANCE LOCUS.
 VAR reversal by defaulting M/C shifts terminal impedance into relay
characteristic towards (-) X axis. Off-set stabilises protection during transient
swings.

FIELD FAILURE PROTECTION (40G)
SETTING CRIETERIA FOR OFF-SET MHO RELAY
A. Rotor Angle up to 90º & No Leading P.F operation contemplated.
 Off-set = 0.5Xd'
 Diameter = Xd
B. Rotor angle up to 120º & Leading P.F operation contemplated.
 Off-set = 0.75Xd'
 Diameter = 0.5Xd
C. Time delay = 2 Seconds (typical)

FIELD FAILURE PROTECTION – TRIP LOGIC
 For larger M/Cs, U/V relay (70% D.O. Setting) usually provided to over-
ride time delay, if system voltage collapses at M/C terminal due to
excessive VAR loading, endangering system stability.

POLE SLIPPING PROTECTION (78)
CAUSES
 System Faults.
 Weak Field conditions.
EFFECT
 Causes rotor oscillations with cyclic variations in voltage, current and active /
reactive power.
 Torque reversal cause torsional stresses on the shaft & stator end windings.
 Prolonged voltage depression may cause Aux. Motors to stall.
 Cyclic voltage variations with possible rise in frequency may affect consumer's
rotating plants (loss of synchronism for Synchronous Motors and stalling of
Induction Motors).

POLE SLIPPING PROTECTION
POLE SLIPPING PHENOMENON

POLE SLIPPING RELAY
(78)
 With fast acting AVR, & MVAR /
Rotor Angle limiters, risk of Pole
Slipping greatly reduced.
 If rotor angle exceeds stable
limits, rotor slips one pole. If
Synchronism not regained, Gen.
must be isolated.
 Sequential operation of Ohm1/2
relays occur when M/C slips one
pole (i.e swings upto +/-90º),
warranting isolation.
 For some relays, number of slip
cycles to be elapsed before
attempting isolation, particularly
for System Power Swings.

ANTI-MOTORING PROTECTION (32G)
1. Steam Turbine Generators.
 Cooling effect lost leading to heating/softening/distortion of blades.
 Motoring power : 1-3% for condensing type turbines but higher for Back
pressure turbines.
 Reverse power relay with high sensitivity (0.5% rated power) used.
2. Hydro Generators.
 Heating of blade not serious due to low speed but cavitation may occur
on loss of water.
 Motoring Power small (about 2%).Requires sensitive setting on the
Reverse power relay (0.5%).

ANTI-MOTORING PROTECTION (32G)
3. Gas Turbine Generator.
 Motoring power high (10-15% or higher).
 Reverse power relay with coarser settings (say 3%) can be used.
4. Diesel Generators.
 Sizable loss of energy due to high motoring power (10-25%)
 Unburnt fuel in the midst of combustion may cause fire or explosion.
 Relay with coarser setting (say 3%) can be used.

GENERATOR TRPPING LOGIC
A. CLASS-A TRIP (UNIT SHUT-DOWN)
 Generator completely shut down (simultaneous tripping of turbine and
electrical isolation).
 Protections operating on severe electrical faults in the Generator or
associated equipments are grouped for Class A trip (e.g. Gen.
Differential, Overall Differential, stator E/F, Inter turn fault protection,
GT / UAT Differential, 2 nd Rotor Earth Fault etc)

GENERATOR TRIPPING LOGIC
B. CLASS – B TRIP (INTERLOCKED TRIPPING)
 Turbine tripped immediately but electrical isolation interlocked with
Low Forward relay (37G – 0.5% setting)and hence delayed.
 Class B trip is applied to those protections for which electrical isolation
can be delayed without significant damage to Generator. This avoids
over speeding. (e.g. Field failure, GT winding temp. high, UAT LV
Standby E/F relay etc).

GENERATOR TRIPPING LOGIC
C. CLASS – C TRIP (ELECTRICAL ISOLATION)
 Only HV Control Breaker of GT is tripped and Generator allowed to run
on No Load.
 Applied to those protections which operate for faults external to
Generator / GT unit (e.g. Bus bar protection, Impedance Backup stage II
etc)

OVER FLUXING PROTECTION FOR GT (99)
CAUSES
 Over Voltage - AVR Malfunction
- Human error
- Loss of Load
 Under frequency- System Conditions
- M/C start up / Shut down
 Uncomfortable combination of Over voltage and Under Frequency
causes Over Fluxing because,
EMF V α Φ.f, Hence, Φ α V/f
EFFECT
 Large Magnetising Current.
 Core bolt heating
 Heating of frame & Support structure
 Increase in noise and vibration.

OVER FLUXING PROTECTION (99)
Typical Relay used (GTT21)
 V/f Setting = 1- 1.3 of 110V / 50Hz.
 Trip time delay = 12 – 120 Seconds.
 Pre-trip Alarm = 1 Second (fixed)

UNDER / OVER FREQUENCY PROTECTION (81)
A. UNDER FREQUENCY PROTECTION
 Results due to over loading of Generator beyond corrective limits of
governor. Sustained U/F operation causes turbine blade fatigue.
 Load shedding initiated as corrective measure. Also multi-stage U/F
relay provided to trip M/C on sustained under frequency.
 Some times, cumulative timers are used with different U/F stages to
record cumulative time of U/F operation in particular band. These are
used to sound alarm for maintenance shut down on elapse of set time
delay.
B. OVER FREQUENCY RELAY
 Primarily used as backup to Mechanical over speed protection.

OVER-ALL PROTECTION ARRANGEMENT OF A TYPICAL UNIT
CONNECTED GENERATOR.

11 January 2023 PMI Revision 00 45
Transformers

11 January 2023 46
Buchholz Relay

Buchholz's Relay
• This has two Floats, one of them with surge catching baffle and gas
collecting space at top. This is mounted in the connecting pipe line between
conservator and main tank.This is the most dependable protection for a
given transformer.
• Gas evolution at a slow rate, that is associated with minor faults inside the
transformers gives rise to the operation or top float whose contacts are
wired for alarm. There is a glass window with marking to read the volume
of gas collected in the relay. Any major fault in transformer creates a surge
and the surge element in the relay trips the transformer.size of the relay
varies with oil volume in the transformer and the mounting angle also is
specified for proper operation of the relay.

• Alarm element Operates When a specified volume of gas
gets collected in Chamber during
Broken down core bolt insulation
 Shorted Laminations
 Bad Contacts
 Overheating of winding parts
• Trip element Operates by Oil surge in the event of serious
fault
Short Circuit between Winding Phases or
within Windings
Puncture of Bushing
BUCHHOLZ PROTECTION

Off Load Tap Changer

Conservator
• Conservator With the variation of temperature there is
corresponding variation in the oil volume. To account for this,
an expansion vessel called conservator is added to the
transformer with a connecting pipe to the main tank. In
smaller transformers this vessel is open to atmosphere
through dehydrating breathers (to keep the air dry). In larger
transformers, an air bag is mounted inside the conservator with
the inside of bag open to atmosphere through the breathers
and the outside surface of the bag in contact with the oil
surface.

Buchholtz relay

Winding / Oil Temperature Indicator

Pressure Relief Device/Expansion vent
• Transformers tank is a pressure vessel as the inside
pressure can group steeply whenever there is a fault in
the windings and the surrounding oil is suddenly
vaporized. Tanks as such are tested for a pressure with
stand capacity of 0.35 Kg/ cm". To prevent bursting of
the tank, these tanks are in addition provided with
expansion vents with a thin diaphragm made of
bakelite/copper/glass at the end. In present day
transformers, pressure relief devices are replacing the
expansion vents. These are similar to safety valves on
boilers (spring loaded).

11 January 2023 57
Pressure Relief Device( PRD)

Temperature Indicators
• Most of the transformer (small transformers have only
OTI) are provided with indicators that displace oil
temperature and winding temperature. There are
thermometers pockets provided in the tank top cover
which hold the sensing bulls in them. Oil temperature
measured is that of the top oil, where as the winding
temperature measurement is indirect. This is done by
adding the temperature rise
• due to the heat produced in a heater coil (known as image coil)
when a current proportional to that flowing in windings is
passed in it to that or top oil. For proper functioning or OTI &
WTI it is essential to keep the thermometers pocket clean and
filled with oil.

Generator Transformer Protection

List of Relays for Generator Transformer Protection
NO NAME OF RELAY PURPOSE ACTION
1 OVERALL DIFFERENTIAL PROT. (87GT) PROTECTION TRIPPING , CLASS-A
2 HV RESTRICTED EARTH FAULT (64GT) PROTECTION TRIPPING, CLASS-A
3 HV STANDBY EARTH FAULT (95GT) PROTECTION TRIPPING, CLASS-C  CLASS-A
4 OVERLOAD (51GT) PROTECTION TRIPPING, CLASS-A
5 INCIPIENT FAULT PROTECTION TRIPPING, CLASS-C / CLASS-A
CLASS-A  MFR,TLR & GEN CKT. BKR.OPEN, CLASS-B TLR,GEN CKT BKR OPEN, CLASS-CGEN CKT BKR

Gen & Transf overall Diff.
Relay(87GT)
GEN
UAT
GT
Yd1
UAT
GCB
Class-A Trip
87GT
CT-2
CT-12
CT-4
CT-5
ICT
P
1/ 3. 849A
Yd1

GT HV Restricted Earth Fault
Relay(64GT)
GEN
64GT
Class-A Trip
3 Phase
GCB
CT10
(BCT)
CT-11

GT HV Standby Earth Fault(51NGT)
GEN
51NGT
Class-A Trip
with td
CT-11

GT Over Flux Protection(95GT)
GEN GT
95GT
Alarm action to AVR Control Inhibition
With td Class –A Trip

GT OverLoad Protection (51GT)
GEN
51GT
Class-A Trip
CT-12

Transformer Through Fault
• Time Graded O/C & Earth fault protection relay used
• Over-fluxing protection
• Back-up Earth Fault Protection
• LBB

Transformer Internal Fault
• Differential Protection (Biased & Harmonic Restraint)
• Restricted Earth Fault Protection.
• Incipient Fault Protection BUCHOLZ, PRV,WTI,OTI, OIL SURGE

GT Incipient Fault Protection
GT Wndg. Temp.very High(90 Deg C) Class-C Trip
GT Oil Temp. very High (100 Deg C) Class-C Trip
GT Buchholz Relay Trip optd. Class-A Trip
GT Pressure Relief Device optd. Class-A Trip

Unit Aux.Transformer
(UAT)Protection

List of Relays for Unit Aux.Transformer (UAT)Protection
NO NAME OF RELAY PURPOSE ACTION
1 DIFFERENTIAL PROT. (87UT) PROTECTION TRIPPING , CLASS-A
2 HV OVERLOAD (51HUT) PROTECTION TRIPPING, CLASS-A
3 HV SHORT CIRCUIT (50HUT) PROTECTION TRIPPING, CLASS-A
4 RESTRICTED EARTH FAULT (64LUT) PROTECTION TRIPPING, CLASS-A
4 STANDBY EARTH FAULT (50N/2 LUT) PROTECTION TRIPPING, CLASS-A
5 INCIPIENT FAULT PROTECTION TRIPPING, LV BKR / CLASS-A
CLASS-A  MFR,TLR & GEN CKT. BKR.OPEN, CLASS-B TLR,GEN CKT BKR OPEN, CLASS-CGEN CKT BKR

UAT Differential Relay (87UT)
GEN GT
UAT
UAT
87UT-B
87UT-A
Class-A Trip
Class-A Trip
Unit Bus
Unit Bus
CT-4
CT-5
Dyn11 ICT
P
1/ 3. 3727A
Yd1
ICT
P
CT-6
CT-7

UAT Over Load Relay IDMT
(HV)51HUT
UAT 51HUT-A
GEN GT
Class-A Trip
Unit Bus
CT-4

UAT Over Current Relay
Inst.(HV)50HUT
GEN GT
50HUT-A
Class-A Trip
UAT
Unit Bus
CT-4

UAT Restricted E/F(LV) Protection 64LUT
GEN GT
64LUT-A
UAT
Unit Bus
Class-A Trip
NGR-
9.95/400A.10s
CT-8
CT-7
CT-7

UAT Standby E/F Protection
UAT
Unit Bus
50N/2
LUT-A Class-A Trip
NGR-
9.95/400A.10s
CT-8

UAT Incipient Fault Protection
• UAT Buchholz Relay Operated Class-A Trip
• UAT Wndg. Temp Very High –LV side Bkr. Trip
• UAT Oil Temp Very High-LV side Bkr. Trip
• UAT Pressure Relief Device Operated Class-A Trip
• UAT OLTC Oil Surge Relay Operated Class-A Trip

List of Relays Transformer Protection (Grid connected)
NO NAME OF PROTECTION RELAY PURPOSE ACTION
1 BIASED DIFFERENTIAL PROT. (87) WITH HERMONIC RESTRAINT PROTECTION TRIPPING both end
2 DIR O/C & E/F PROT. (67 / 67N) HV SIDE TRIPPING both end
3 DIR O/C & E/F PROT. (67 / 67N) LV SIDE PROTECTION TRIPPING both end
4 LBB PROT. (50Z) HV SIDE Tripping all other CB
5 LBB PROT. (50Z) LV SIDE Tripping all other CB
6 CIRCULATING CURRENT DIFF. PROT-87T (HV & LV) BCT PROTECTION TRIPPING both end
7 NEUTRAL STANDBY EARTH FAULT (51N) PROTECTION TRIPPING both end
8 OVER FLUXING (61) HV SIDE TRIPPING both end
9 OVER FLUXING (61)LV SIDE TRIPPING both end
10 INCIPIENT FAULT TRIPPING both end

Bias Diff. Relay(87)
87GT
HV
LV
TV
0 2 4 6 8 10 12 14 16 18 20
THROUGH CURRENT IN MULTIPLE OF TAP
SENSITIVITY : 30 % of TAP
INSTANTANEOUS : 8 X TAP
15 %
20%
25%
30%
35%
40%
45%

Restricted Circulating Current Diff. Relay(87T)
87T
HV
LV
TV
# All CTs are BUSHING type
Ratio: 1000/1A Rct=5
SURGE
PROTECTOR
STAB RES.

Restricted Earth Fault Relay(64R)
64R
3 Phase
CB
CT10
(BCT)

Standby Earth fault Relay (51N)
51N
HV
LV
TV
A B C
N N
N

Over Flux Protection(61T)
61
HV
HV
LV
TV
61
LV

O/C & E/F Dir. & Non Dir.
67 /
67N
HV
LV
TV
67 /
67N
51 &
50N/2
DIRECTION
400KV
220KV
33KV
315MVA
TERCIARY 105MVA
455A
827A
1837A
500/1A
1000/1A
2000/1A
P/F :PSM 75% ;TMS 0. 3 (IDMT 3sec)
E/F P/U 10% ;T/D 0. 4sec
P/F : PSM 100% TDM 1.0; IDMT 3 sec
E/F : PSM 40% TDM 1.0 ; IDMT 3sec
Definite Time
STAGE-1 P/F : P/U 105% TD 10sec
E/F : P/U 25% TD 0. 5sec
E/F : P/U 30 % TD 0. 4sec
ISTN.
Stage-1 P/F : P/U 725% TD 0. 2sec
Stage-2 P/F : P/U 7.75% TD 0. 1sec
Stage-2 E/F : P/U 725% TD 0. 05sec
P/F : PSM 90% TDM 1.0; IDMT 3 sec
E/F : PSM 20% TDM 1.0 ; IDMT 3sec
Definite Time
E/F : P/U 25% TD 0. 5sec
E/F : P/U 30 % TD 0. 4sec
ISTN.
Stage-2 E/F : P/U 660% TD 0. 05sec

LBB Protection
50Z
HV
LV
TV
50Z
400KV
220KV
33KV
500/1A
1000/1A
P/U : 20% TD: 200 msec
P/U : 20% TD: 200 msec
+ ve
86
-ve
+ve
-ve
86

Transformer Through Fault
Time Graded O/C & E/F protection
Over fluxing protection
Back up E/F protection
LBB protection.

Transformer Internal Fault
DIFFERENTIAL PROTECTION (BIASED & HARMINIC RESTRAINT)
RESTRICTED / CIRCULATING CURRENT DIFFERNTIAL PROTECTION
INCIPIENT FAULT PROTECTION

GT Incipient Fault Protection
Transformer Wndg. Temp. very High (90°C)
Transformer Oil Temp very High (100°C)
Transformer Buchholz relay optd. trip
Transformer Pressure Relief Device optd Trip
OLTC oil surge relay optd trip (0.32kg/cm2 Alarm ,0.6kg/cm2 Trip

Protection Selection at a Glance-1
Sl
No
PROTECTION PRESENT PRACTICE RECOMENDATION
1 Fuse on primary side and
MCCB/ACB on secondary side
Transformer upto 1.0MVA rating
(upto 11kv voltage) and upto
2.5MVA rating (upto 33kV voltage)
ACB shall be
recommended for
630kVA rating and
above on secondary side
2 Lightning Arrestors HV winding of all outdoor located
transformers connected with
overhead lines.
Distance from
transformer bushing to
live part of LA shall not
be more than 1.0 feet per
kV
3 Buchholtz relay with alarm &
trip contacts
For transformer of capacity 750kVA
onwards
For indoor transformers
this can be provided in
630kVA transformer and
onwards.
4 Oil temperature indicator with
alarm and trip contact
-do- -do-

Sl
No
5 Winding
temperature
indicator with
alarm & trip
contacts
-upto 33kV class transformer.
-Shall be provided for 66kV class above
voltage grade transformers and for generator
transformer of 16MVA and above capacity.
Indicator shall have additional contacts for
Auto start of cooling fans (whenever required)
It is recommended to provide
for transformer of 1MVA and
above rating. As an optional
item temp. monitoring device
(4-20mA) can be provided,
which can be connected to
SCADA system
6 Magnetic oil
level gauge with
alarm contact
For transformer of 66kV class and above &
generator transformer of 16MVA and above.
on all HT transformers.
7 Oil surge
protection for
OLTC diverter
tank with trip
contact
To be provided wherever applicable For indoor transformers this
can be provided in 630kVA
transformer and onwards.
8 Explosion vent ?
Pressure relief
device with
alarm and trip
contact.
Pressure relief device shall be provided in
above 40MVA and above rating of 132kV
class.all transformer of 220kV class and
above voltage class 100MVA and above
capacity.
-do-

Sl
No
9 Transformer fire
trip (from
Emulsifier
system)
To be provided wherever applicable It is recommended to provide
for transformer of 1MVA and
above rating. As an optional
item temp. monitoring device
(4-20mA) can be provided,
which can be connected to
SCADA system
10 Over current and
earth fault relay
-IDMT on HV
side
-IDMT relay
with high set
element on
primary side
To be provided wherever CB are used.
Provided with transformer3.15MVA and
above rating
Two no of Instantaneous Over
current with one timmercan be
provided as an option toIDMT
relays.
in all transformers having CB
on primary side.
11 Differential relay Percentage biased differential relays for 33kV
class and above voltage level, 3.15MVA to 10
MVA rating transformers.
High speed diff relay with harmonic restrain
feature for transformer above 10MVA rating
and all transformer of 132kV class and above
voltage level.
In addition it is recommended
to provide differential relay for
11kV class transformers of
above 3.15MVA rating.

Sl
No
12 Restricted earth
fault on star
winding
For transformer of 132k class and above and
for generator transformer (16MVA and above)
In addition it is recommended
to provide for all HT
transformer having resistance
earthling system
13 Overflux relay -do-
14 Neutral over
current relay
against sustained
external earth
fault
For all transformer having C.B. on primary
and secondary side
Not applicable for unearth
system.

Protection Scheme Dia
M
Motor
Prot.
Relay

CBCT
CB

Thermal Overload (49/51)
t= τ.Ln
t
=
2
I²–Kн/с.Iр²
I² –(k.Iв)²
t -operating time
τ–Time Constant
I–Relay CurrentIeq
Iв–Full Load current
k–Mult. Factor for Th count
start(105%).
Ip–Steady state current prior to O/L
Ieq–Relay current considering NPS
KH/C– Constant to provide Hot Cold Ratio
At cold Ip=0
for<2xIeq
t= τ·.Ln
I²–Ip²
I²–(k.Iв)²
Hot
Ieq=I1²+K.I2²

Short Circuit (50)
This is for protection against the short circuit fault
For DOL starter motor Motor Start Over Logic may be included for better sensitivity during
running.At the starting period the short ckt setting may become DOUBLE the actual Relay
setting value.

Stalling (Locked Rotor) (48)
• This is to protect the motor from Stalling and locked rotor condition
• Case 1: whenever the start time is less than Stall withstand time then Relay will operate
when all the three phase current exceed the set value and Time.
• Case 2: whenever the start time is greater than Stall withstand time then additional input for
speed(10%) switch used to discriminate between Starting and Stalling.

Prolong Start
• Protection against the prolong starting is intended primarily
for supervising the starting sequence of motors for detecting
unduly long starting period

Negative Phase Seq. (46)
• Protection against unbalanceI/P or single phase condition.
Two type characteristics used –
• 1. Definite type
• 2. Inverse type-
1
Op time for ph.
Diff. t=
(I/Iθ)²
xTm
Op time for NPS t=
1
(I2/Iθ)²
X Tm
I=IMIN-IMAX
I =Thermal O/L setting
Tm =Time Mult setting

Earth Fault (50N/51N)
• Protection against Earth Fault
– Core Balance type
– Residual type

Differential Protection(87)
• Large capacity motor this protection is used
generally.

class on Protection.pptx

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