CENTRAL TESTING
CIRCLE,DVC, MAITHON
TRANSFORMER PROTECTION
BY
JAYANTA DUTTA,
SUPERINTENDING ENGINEER
CRITM , DVC,
MAITHON
PHYSICAL ARRANGEMENT OF A THREE PHASE
TRANSFORMER
CORE MATERIALS USED FOR CONSTRUCTION OF
TRANSFORMER
 Grain oriented Electrical Steel CRGO is
undoubtedly the most important soft magnetic
material in use today. Whether in small transformer,
distribution transformer or in large transformer &
generator, grain oriented electrical steel CRGO is a
must for the production of energy saving electrical
machines.
 Grain oriented Electrical Steels are iron-silicon
alloys that provide low core loss and high
permeability needed for more efficient and
economical electrical transformers. CRGO Grain
oriented grades of electrical steel are typically used
for transformer cores and large generators
CORE MATERIALS USED FOR CONSTRUCTION OF
TRANSFORMER
 Important physical properties of Electrical
steels (CRGO) include Resistivity, saturation
induction, magneto-crystalline anisotropy,
magnetostriction, and Curie temperature.
Resistivity, which is quite low in iron, increases
markedly with the addition of silicon. Higher
Resistivity lessens the core loss by reducing the
eddy current component. Raising the silicon content
will lower magnetostriction, but processing
becomes more difficult. The high Curie temperature
of iron will be lowered by alloying elements, but the
decrease is of little importance to the user of CRGO
Electrical steels.
DERIVATION EQUATION OF TRANSFORMER
STANDARD CONNECTION GROUP 1 (ZERO DEGREE)
STANDARD CONNECTION GROUP 2 (180 DEGREE)
STANDARD CONNECTION GROUP 3 (-30DEGREE)
STANDARD CONNECTION GROUP 4 (+30DEGREE)
PARALLELLING OF TRANSFORMERS






The theoretically ideal conditions for
paralleling transformers are:
Identical turn ratios and voltage ratings.
Equal percent impedances required for equal
Load sharing.
Equal ratios of resistance to reactance.
Same polarity.
Same phase angle shift. (Vector Group)
Same phase rotation.
TYPES OF TRANSFORMER FAULTS
TRANSFORMER FAULTS ARE GENERALLY
CLASSIFIED INTO FIVE CATEGORIES:WINDING AND TERMINAL FAULTS. (ABOUT 60%)
CORE FAULTS. (ABOUT 12%)
TANK AND TRANSFORMER ACCESSORIES FAULT.
( ABOUT10%)
ONLOAD TAPCHANGER FAULT.(15%)
ABNORMAL OPERATION CONDITIONS.(2%)
SUSTAINED AND UNCLEARED EXTERNAL
FAULTS.(1%)
FAULTS IN AUXILLIARY EQUIPMENT
 Transformer oil
 Oil level low.
 Moisture absorption
 Transformer cooling system.
 Failure of insulation between lamination
and core bolt.
 Badly made joints and connections.
WINDING AND TERMINAL FAULTS
 Insulation failure between winding and core
 Between (phase to earth)
 Between phases
(phase to phase)
 Between HV and LV winding
 Inter turn faults.
 Cause may be
 Excessive over load,
 Loose connection
 Improper Installation and Commissioning
 Constant Over voltage,
 Aging of winding insulation
 Consequence of minor faults.
CONSTANT OVER VOLTAGE
Over voltage conditions are of two kinds;
 TRANSIENT OVER VOLTAGE arise from
switching and lightning disturbances and are
liable to cause interturn faults.
 POWER FREQUENCY OVER VOLTAGE
causes both an increase in stress on the
insulation and proportionate increase in
working flux. Increase in working Flux will
increase the working Flux and thereby
increase in the Magnetizing Current.

EXTERNAL FAULTS
Sources of abnormal stress in a transformer
are: External overloads
 Short circuits
 Over voltage
 Reduced system frequency (v/f ratio)
FAULTS IN OLTC
FAULTS IN BUSHINGS
TYPES OF TRANSFORMER FAULTS
CRITERION FOR SELECTION OF PROTECTION
SCHEME
 Kva or Mva rating
 Voltage ratio
 Winding connections.
 Per unit / per cent reactance
 Neutral point Earthing resistance
 Value of system Earthing resistance.
 Whether indoor or outdoor
 Dry or oil filled.
 With or without conservator
 Fault level at the power transformer terminals
 Network diagram showing the position of transformer
in the system network.
BASIC PROTECTIONS OF TRANSFORMERS
BIASED DIFFERENTIAL PROTECTION
RESTRICTED EARTH FAULT PROTECTION.
HV BACK-UP OVERCURRENT AND EARTH
FAULT PROTECTION. EARTH FAULT
PROTECTION MAY HAVE AN ADDITIONAL
DIRECTIONAL ELEMENT.
LV BACK-UP OVERCURRENT AND EARTH FAULT
PROTECTION.
OVERFLUXING PROTECTION
DIFFERENTIAL PROTECTION
 Protects the transformer against winding faults
such as phase-to-phase fault and phase to earth
fault.
 It works on circulating current principle.
 Balanced three phase through current suffers a
phase change (30 degree for Group 3,4) which
must be corrected in CT secondary leads by
appropriate connection of the CT secondary
windings.
 Elimination of Zero sequence through CT
secondary Connections for Star Connected
Winding.
DIFFERENTIAL RELAY CURRENT BALANCE CONDITION
Problems / Difficulties, While Incorporating
Differential Protection
:
 Different voltage levels at primary and secondary of








the transformer.
Possible ratio mismatch of the current transformers.
30º phase shift in case of delta / star transformer.
Operation of the transformer at different tap position.
Ct ratio errors
Different CT performance at high fault currents.
Inherent difference in the CT characteristic
Saturation of CTs cause large ratio errors – ip = k*is +
im.
Presence of d. C. Component in the fault current.
MAGNETISING INRUSH
6 to 10 times the rated current.
 Present only on the primary side of the
transformer.
 Depends on residual flux of the transformer and
voltage wave position at the instant of switching
(i.e. Zero or peak).

MAGNETISING INRUSH CURRENT WAVEFORM
BIAS OF DIFFERENTIAL RELAY
DUAL SLOPE OF BIAS OF DIFFERENTIAL
RELAY
BIAS OF DIFFERENTIAL RELAY
There may be some ratio errors in CTs (upto 5%) and some
difference in primary and secondary currents due to tap
changing (upto 10%).
Biasing is used to avoid false trippings from these
effects.
The differential protection uses some percentage of through
current for biasing (or restraining). say we use tripping
slope as 20% and differential & restraining currents in a
typical relay are :
Differential current = |I1+I2|
Restraining current = (|I1|+|I2|)/2
The relay will trip when:
Differential current / Restraining current> 0.2
RESTRICTED EARTH FAULT PROTECTION
 Provides sensitive, instantaneous earth fault
protection within the protected zone of the
transformer.
 Operates on circulating current principle.
 CAG 14 – high impedance relay is used with
external stabilizing resistor for the stability
against through faults.
 Operating coil of the relay is connected with L. C.
Circuit in series to immune harmonic produced
by ct saturation.
 The residual current of 3 CTs is balanced against
the output of a CT in neutral conductor.
HIGH IMPEDENCE RESTRICTED EARTH FAULT PROTECTION
 RATIO AND ACCURRACY CLASS ALONG WITH KNEE
POINT VOLTAGE OF ALL THE 4 CT’S SHOULD BE
IDENTICAL.
CT’s TRANSFORMER
A
LINE
CT’s
TRANSFORMER
A
R
B
B
R
C
C
R
RELAY & SR
NEUTRAL
CT’s
RESTRICTED EARTH FAULT PROTECTION CONNECTIONS
CALCULATION OF STABILISING RESISTANCE FOR
R/E/F PROTECTION
 Example:- For a 50MVA 230/36KV transformer, through




fault Stability req. up to say 20 Times Full Load Current
with Neutral CT assumed completely saturated. Line CT
ratio & Neutral CT Ratio considered as 400/1. CT resistance
= 5ohms. Relay resistance 4.5 ohms Cable and Lead
Resistance - 1 ohm. Relay Pick Up - 0.1 Amps. Calculate
the value of Stabilizing Resistance.
Full Load current = 50,000/  3 x 230 = 125.51A
Maximum Current Stability = 125.51 x 20 = 2510.218A =
2.5kA
Considering the Saturation of the Neutral CT, Voltage
developed across the relay = 2510.218 ( 1+5+1 ) = 43.93
Volts
Now Volts to operate the relay: - IR = 0.1 x 4.5 =
0.45Volts.
CALCULATION OF STABILISING RESISTANCE FOR R/E/F
PROTECTION
 To ensure Stability of Protection Circuit Voltage drop
across the S.R = 2510.218 ( 1+5+1 ) = 43.93 Volts
 Hence resistance required - 43.47/0.1 = 434.7ohms.
 Now the relay must pick - up at internal faults.
 For 0.1A pickup requires 0.1( 5+1+434.7+4.5+1) =
44.62 Volts.
 Now Im at Vk = 800mA
 Total Mag. Current for 4 CT = 4 x 0.8 = 3.2A
 Effective Relay setting = Irelay + Im = 0.1 + 3.2 =
3.3A
 Or in terms of primary the Line Current = 3.3 x 400
= 1320A.
 Hence for a internal E/F current of 1320A the relay is
found to operate.
COMBINED DIFFERENTIAL AND RESTRICTED EARTH FAULT
PROTECTION
NUMERICAL DIFFERENTIAL RELAY PROTECTION WITH
HIGH IMPEDENCE R/E/F PROTECTION
Connection of transformer differential protection with high
impedance REF (I7) and neutral current measurement at I8
OVERFLUXING PROTECTION
 E = 4.44 *  * f * T
 E/f
 WHEN THE FLUX DENSITY INCREASES BEYOND SATURATION POINT, A
SUBSTANTIAL AMOUNT OF FLUX IS DIVERTED TO STEEL STRUCTURAL
PARTS AND INTO THE AIR. AT SATURATION FLUX DENSITY THE CORE
STEEL WILL OVER HEAT.
 STRUCTURAL STEEL PARTS WHICH ARE NU-LAMINATED AND ARE NOT
DESIGNED TO CARRY MAGNETIC FLUX WILL HEAT RAPIDLY. FLUX
FLOWING IN UNPLANNED AIR PATHS MAY LINK CONDUCING LOOPS IN
THE WINDINGS, LOADS, TANK BASE AT THE BOTTOM OF THE CORE
AND STRUCTURAL PARTS AND THE RESULTING CIRCULATING
CURRENTS IN THESE LOOPS CAN CAUSE DANGEROUS TEMPERATURE
INCREASE.
 IT MAY BE SEEN THAT METALLIC SUPPORT STRUCTURES FOR CORE
AND COIL, WINDINGS, LEAD CONDUCTORS, CORE LAMINATION, TANK
ETC. MAY ATTAIN SUFFICIENT TEMPERATURE WITH THE EVOLUTION OF
COMBUSTIBLE GAS
F=(V/F)/(Vn/Fn)
1.1
Duration of withstand time(min.) Continuous
1.2
1.25
1.3
1.4
2
1
0.5
0
BACK UP O/C AND E/F PROTECTION
 BACKUP PROTECTION OF TRANSFORMER IS O/C AND E/F

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
PROTECTION APPLIED AGAINST EXTERNAL SHORT
CIRCUIT AND EXCESSIVE OVER LOADS.
NORMALLY IDMT RELAYS WITH STANDARD INVERSE OR
VERY INVERSE CURVES.
GENERALLY INSTALLED IN THE HV AND LV SIDE OF THE
TRANSFORMER.
FOR HV SIDE, THE TRIPPING SHOULD BE OF BOTH
SIDES.
FOR LV SIDE, THE TRIPPING WILL ONLY BE FOR THE LV
SIDE.
BACKUP PROTECTION OF TRANSFORMER HAS FOUR
ELEMENTS, THREE OVER CURRENT RELAYS CONNECTED
EACH IN EACH PHASE AND ONE EARTH FAULT RELAY
CONNECTED TO THE COMMON POINT OF THREE OVER
CURRENT RELAYS.
INHERENT PROTECTIONS OF TRANSFORMERS
BUCHOLZ PROTECTION FOR MAIN TANK.
OIL SURGE RELAY PROTECTION FOR
DIVERTOR TANK.
PRESSURE RELEASE VALVE (PRV).
OIL TEMEPRATURE HIGH ALARM AND TRIP.
WINDING TEMPERATURE HIGH ALARM AND
TRIP.
OIL LEVEL LOW ALARM.
BUCHOLZ RELAY
SAMPLING
COCK
GAS BUILD
UP
MERCURY TILT SWITCH
STOP
HINGE
STOP
OIL SURGE TOWARDS
CONSERVATOR
OIL SURGE FROM
TRANSFORMER TANK
HINGE
BUCHHOLZ
GAS
BUCHHOLZ
OIL
CONSERVATOR
DRAIN PLUG
BUCHHOLZ RELAY
TRANSFORMER
TANK
TRANSFORMER PROTECTIONS
PROTECTION
TRANSFORMER
RATED BELOW
500KVA
TRANSFORMER
RATED BETWEEN
500KVA AND 5MVA
TRANSFORMER
RATED ABOVE 5MVA
DIFFERENTIAL
X
X

R.E.F.
X
X

BACK UP O/C
X


BACK UP E/F
X


TRANSFORMER
OVERFLUXING
X
X

BUCHOLZ
X


P.R.V.
X


O.T.I AND W.T.I.
X


OIL LEVEL
MONITORING
X


FUSES

X
X
EARTHING SYSTEM FOR UNGROUNDED DELTA
POWER TRANSFORMER PROTECTION IN DVC
PRESENT DAY POWER TRANSFORMER PROTECTION IN
132KV FOR DVC
PRESENT DAY POWER TRANSFORMER PROTECTION IN
220KV FOR DVC
PRESENT DAY ICT PROTECTION IN 400KV INSTALLATION
FOR DVC (6 CT SCHEME)
STANDARD A.C. SCHEMATIC OF TRANSFORMER
PROTECTION.
MAINATAINANCE PRACTICES REQUIRED FOR
TRANSFORMER

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The general cleaning of bushings and
transformer top on main tank.
The small oil leakage attending.
The tightness checking of control wire and
terminal blocks.
The cleaning of contactor coils at marshalling
box.
The test of Buch. relay by loss of oil method.
The OTI & WTI checking by both hot oil bath
method and dial rotating method. The
cooling system also are checked.
MAINTAINANCE PRACTICES REQUIRED FOR
TRANSFORMER

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The OTI & WTI pocket oil are checked.
The alarm for Main tank & diverter tank low
oil level.
The trip test by PRV and diver tank OSR.
The Phase marking of bushings.
The IR Value of Transformer are measured by
5.0KV Megger.
The power factor test.
The main tank oil BDV and moisture content
test.
The DGA test of both main tank and diverter
tank oil at CRITL
MAINATAINANCE PRACTICES REQUIRED FOR
TRANSFORMER

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


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Without shut down activities.
Checking of bushing oil level :- M/D2
Checking of oil level in conservator :- M/D3.
Checking of oil level in OLTC conservator :-M/D4.
Manual actuation of cooler oil pumps and fans
:- M/D5.
Checking of oil leaks
:-M/D6.
Checking condition of silica gel in breather :- M/D7.
Checking of oil level in oil seal of breather :-M/D8.
Testing of oil for DGA and other oil parameters
:-HY
CHANGE OF DIFERENTIAL CIRCUIT WITH CHANGE OF
VECTOR GROUP THROUGH PRIMARY CONNECTIONS.
Monitoring test on a Transformer
 There are certain tests, which after rectifying the




value nearer to permissible limit, minimizes the
failure of transformers due to internal fault.
DGA test: DGA will indicate heating, burning,
sparking/arcing, loose joint, etc. inside the xmer.
Capacitance Tan Delta / P.I. & A.I. tests:
Capacitance Tan Delta and P.I. & A.I. values will
indicate insulation condition of transformer.
Oil sample test: As per IS - 1866. Also, the trend
of change in the oil gives useful information (can be
known from its colour and odor)
Oil sample test will indicate oil quality, which is
directly affecting transformer health and life both.
MONITORING OF TRANSFORMER BY INSULATING OIL
EVALUATION
MINERAL OILS USED IN TRANSFORMERS AS
COOLANTS AND INSULANT SHOULD HAVE THE
FOLLOWING PROPERTIES:HIGH DIELECTRIC STRENGTH TO WITHSTAND
ELECTRICAL STRESS IMPOSED IN SERVICE.
SUFFICIENTLY LOW VISCOCITY SO THAT ITS
ABILITY TO CIRCULATE AND TRANSFER HEAT IS
NOT IMPAIRED.
ADEQUATE LOW TEMPERATURE PROPERTIES
DOWN TO THE LOWEST TEMPERATURE EXPECTED
AT THE INSTALLATION SITE.
RESISTANCE TO OXIDATION TO MAXIMISE
SERVICE LIFE.
TESTS OF OIL TO ASSESS ITS CONDITION AND SUGGEST
CORRECTIVE ACTION
TESTS
RECOMMENDED
BRIEF EXPLANATION
BREAKDOWN VOLTAGE
(IEC60156)
MEASURES THE OILS ABILITY TO
WITHSTAND ELECTRICAL STRESS
WATER CONTENT (IEC
60814)
ACIDITY
(NEUTRALIZATION
FACTOR) (IEC 62021-1)
WATER ACCELERATES THE
DETERIORATION OF BOTH THE
INSULATING OIL AND THE PAPER
INSULATION, LIBERATING MORE
WATER IN THE PROCESS.
ORIGINATES FROM OIL
DECOMPOSITION. SOMETIMES
ALSO FROM ATMOSHPHERIC
CONTAMINATION.
TESTS OF OIL TO ASSESS ITS CONDITION AND SUGGEST
CORRECTIVE ACTION
TESTS
RECOMMENDED
BRIEF EXPLANATION
INTERFACIAL
TENSION
(ASTMD971-99A)
THE INTERFACIAL TENSION (IFT)
MEASURES THE TENSION AT THE
INTERFACE BETWEEN TWO LIQUID (OIL
AND WATER) WHICH DO NOT MIX AND IS
EXPRESSED IN DYNE/CM OR mN/m.
DEFINITE RELATIONSHIP WITH IFT AND
ACIDITY. INCREASE IN ACIDITY CAUSES
DROP IN IFT. RATIO OF IFT/ACIDITY GIVES
QUALITY INDEX SYSTEM. AND A NEW OIL
CAN HAVE A QI OF >1500(45/.03)
DIELECTRIC
DISSIPATION
FACTOR OR
RESISTIVITY
(IEC60247)
THE DISSIPATION TEST MEASURES THE
LEAKAGE CURRENT THROUGH AN OIL.
REVEALS PRESENCE OF CONTAMINATION
AND PRESENCE OF MOISTURE RESIN,
VARNISH, FOREIGN CONTAMINANTS.ETC
TESTS OF OIL TO ASSESS ITS CONDITION AND SUGGEST
CORRECTIVE ACTION
TESTS
RECOMMENDED
DISSOLVED GAS
ANALYSIS (DGA)
IEC60567
GASES ANALYSED :HYDROGEN - H2
METHANE – CH4
ETHANE – C2H6
ETHYLENE –C2H4
ACETYLENE – C2H2
CARBON MONOXIDE – CO
CARBON DIOXIDE – CO2
NITROGEN – N2
OXYGEN – O2
BRIEF EXPLANATION
ADVANCE WARNING OF DEVELOPING
FAULTS.
A MEANS FOR CONVENIENTLY
SCHEDULING
REPAIRS.
 MONITOR THE RATE OF FAULT
DEVELOPMENT
ORIGIN OF GASES
PARTIAL DISCHARGE – MAJOR GAS
HYDROGEN AND MINOR GAS IS
METHANE
THERMAL FAULTS - < 300 DEG –
METHANE AND ETHANE, > 300 DEG :ETHYLENE MORE THE TEMP. MORE IS
THE PRODUCTION OF ETHYLENE.
ARCHING – HIGH ENERGY DISCHARGE.
MAJOR GAS – ACETYLENE.
DISSOLVED GAS ANALYSIS
Transformer Chemistry Services method of interpretation
is based upon :
• Key gases : CSUS values (Age compensated)
• BS 5800/IEC 599 ratios (providing the Total Combustible
Gases present are above 300 ppm)
• Rogers Ratio’s
• Trend (Production rates of gases) Morgan-Schaffer
Tables
• Total Combustible Gas Production Rates
TDCG(c57.104-1991)
• Total Combustible Gas Westinghouse Guidelines
• Age of transformer.
• History of transformer (Repaired, degasses, etc).
GAS CONCENTRATION LIMITS OF TRANSFORMER (IEEE)
GAS
NORMAL
CAUTION
HYDROGEN
- H2
<100PPM
100-700PPM
>700PPM
ARCHING CORONA
METHANE –
CH4
<120 PPM
120 – 400
PPM
>400PPM
SPARKING
ETHANE –
C 2 H6
<65 PPM
65-100 PPM
>100PPM
LOCAL OVERHEATING
ETHYLENE –
C 2 H4
<50PPM
50-100 PPM
>100PPM
SEVERE OVERHEATING
ACETYLENE
– C2H2
<2 PPM
2-5 PPM
>5 PPM
ARCHING
CARBON
MONOXIDE
– CO
<350PPM
350-570PPM
>570PPM
SEVERE
OVERLOADING
CARBON
DIOXIDE –
CO2
<2500PPM
2500-10000
PPM
>10000PPM
SEVERE
OVERLOADING
<700PPM
7001900PPM
>1900PPM
TCG
WARNING INTERPRETATION
TOTAL DISSOLVABLE COMBUSTABLE GAS.
Total Combustible
Gases
Recommended Action
0-500 PPM
NORMAL AGING
ANALYZE AGAIN IN 6-12 MONTHS
501 to 1200 PPM
DECOMPOSITION MAYBE IN EXCESS OF
NORMAL AGING
1201 to 2500 PPM
MORE THAN NORMAL DECOMPOSITION
ANALYZE IN 1 MONTH
2500 PPM and
above
MAKE WEEKLY ANALYSIS TO DETERMINE
GAS PRODUCTION RATES
CONTACT MANUFACTURER
TESTS OF OIL TO ASSESS ITS CONDITION AND SUGGEST
CORRECTIVE ACTION
TESTS
RECOMMENDED
FURAN ANALYSIS
(IEC61619)
BRIEF EXPLANATION
ANALYSIS OF THE CONDITION OF
PAPER INSULATION. DIRECT TEST OF
PAPER IS THE TENSILE STRENGTH OR
DEGREE OF POLIMERISATION (DP).
INDIRECT METHOD IS FURAN ANLYSIS
WHICH MEASURES THE QUANTITY OF
2- FURALDEHYDE (IN PPM) IN OIL IS
DIRECTLY RELATED TO THE DP OF THE
PAPER INSIDE THE TRANSFORMER.
ANALYSIS OF THE DP VALUES
DP
RANGE
REMARKS
<200
TEST INDICATES EXTENSIVE PAPER DEGRADATION
EXCEEDING THE CRITICAL POINT. STRONGLY
RECOMMEND THAT THE TRANSFORMER BE TAKEN OUT
OF SERVICE IMMEDIATELY AND VISUALLY
INSPECTED.
200250
THE PAPER IS NEAR OR AT THE CRITICAL CONDITION.
RECOMMENDED THAT THE TRANSFORMER BE TAKEN
OUT OF SERVICE AS SOON AS POSSIBLE AND
THOROUGHLY INSPECTED. PAPER SAMPLES CAN BE
TAKEN FOR DIRECT DP TESTING.
260 350
THE PAPER IS APPROACHING THE CRITICAL CONDITION.
SUGGEST INSPECTION BE SCHEDULED AND/OR
RE-SAMPLE WITHIN 1 YEAR TO REASSESS CONDITION.
ANALYSIS OF THE DP VALUES
DP
RANGE
REMARKS
360450
THE PAPER IS STARTING TO APPROACH THE CRITICAL
CONDITION. SUGGEST A RE-SAMPLE IN 1-2 YEARS TIME.
460600
SIGNIFICANT PAPER DETERIORATION BUT STILL WELL
AWAY FROM THE CRITICAL POINT
610900
MILD TO MINIMAL PAPER AGEING.
>900
NO DETECTABLE PAPER DEGRADATION
RECOMMENDED LIMITS OF INSULATING OILS (OLD) AS PER
IS1866 - 2000
TESTS
TRANSFORMER
VOLTAGE RATING
LIMITS
INTERFACIAL
TENSION (N/m)
FOR ALL VOLTAGE
LEVELS
0.015(Min)
NEUTRALIZATION
VALUE MG OF
KOH/G
FOR ALL VOLTAGE
LEVELS
0.3 (MAX)
BREAKDOWN
VOLTAGE
ABOVE 170KV
BETWEEN 72.5 – 170KV
BELOW 72.5KV
50(MIN)
40(MIN)
30(MIN)
DIELECTRIC
DISSIPATION
FACTOR @90°C
ABOVE 170KV
BELOW 170KV
0.2(MAX)
1.0(MAX)
RECOMMENDED LIMITS OF INSULATING OILS (OLD) AS PER
IS1866 - 2000
TESTS
TRANSFORMER
VOLTAGE RATING
LIMITS
RESISTIVITY X
1012, OHM-CM@
@90°C
FOR ALL VOLTAGES
0.1(MIN)
WATER CONTENT
PPM
ABOVE 170KV
BETWEEN 72.5 – 170KV
BELOW 72.5KV
20(MAX)
40(MAX)
NO FREE
MOISTURE
SEDIMENT AND
SLUDGE
FOR ALL VOLTAGES
NIL
RECOMMENDED LIMITS OF UNUSED INSULATING OILS AS PER
IS1866 - 2000
PROPERTIES
HIGHEST VOLTAGE OF EQUIPMENT (KV)
<72.5KV
72.5KV –
170KV
>170KV
APPEARANCE
CLEAR FREE FROM SEDIMENT AND
SUSPENDED MATTER
DENSITY
@29.5°C(G/C
M) MAX
0.89
0.89
0.89
VISCOSITY
@27°C(CST)
MAX
27
27
27
FLASH POINT
°C (MIN)
140
140
140
POUR POINT
°C (MAX)
-6
-6
-6
RECOMMENDED LIMITS OF UNUSED INSULATING OILS AS PER
IS1866 - 2000
PROPERTIES
HIGHEST VOLTAGE OF EQUIPMENT (KV)
<72.5KV
72.5KV –
170KV
>170KV
NEUTRALIZAT
ION VALUE
MG OF
KOH/G (MAX)
0.03
0.03
0.03
WATER
CONTENT
PPM
(MAX)
20
15
10
INTERFACIAL
TENSION
(N/m) Min
.035
.035
.035
RECOMMENDED LIMITS OF UNUSED INSULATING OILS AS PER
IS1866 - 2000
PROPERTIES
HIGHEST VOLTAGE OF EQUIPMENT (KV)
<72.5KV
72.5KV –
170KV
>170KV
DIELECTRIC
DISSIPATION
FACTOR
@90°C (MAX)
0.015
0.015
0.010
RESISTIVITY
X 1012, OHMCM@ @90°C
(MIN)
6
6
6
BREAKDOWN
VOLTAGE
(Min)
40
50
60
MEASUREMENTS MODES FOR TAN DELTA
 GST MODE :- GROUNDED SPECIMEN TEST
IS REFFERED TO AS MEASUREMENT OF AN
INSULATION SAMPLE THAT HAS ONE OF ITS
TERMINAL GROUNDED.
GSTg- MODE :- GSTg MODE MEANS
GROUNDED SPECIMEN TEST WITH GUARDING
UST MODE :- THE UNGROUNDED
SPECIMEN TEST IS REFERRED TO AS THE
TEST OF AN INSULATION SAMPLE THAT IS NOT
GROUNDED.
OVERALL POWER FACTOR AND CAPACITANCE
OVERALL POWER FACTOR AND CAPACITANCE GST
MODE ICH + ICHL
OVERALL POWER FACTOR AND CAPACITANCE GSTG-A
MODE. ( CAPACITANCE CH)
OVERALL POWER FACTOR AND CAPACITANCE UST-A
(CAPACITANCE CHL)
OVERALL POWER FACTOR AND CAPACITANCE
MEASUREMENTS OF HV SIDE
MEASUREME
NT
TEST
MODE
SWEEP
V TEST
FREQ.
ICH+ICH
L
GST
NONE
50 HZ
ICH
GSTg A
NONE
50 HZ
ICH(f)
GSTg A
FREQUEN
CY
50 HZ
TO
400 HZ
ICHL
UST-A
50 HZ
ICHL(f)
UST-A
50 HZ
TO
400 HZ
WATT
LOSSE
S
CAP
MEAS
PF
MEAS.
MEASUREMENTS FOR TAN DELTA AT DIFFERENT
FREQUENCY
OVERALL POWER FACTOR AND CAPACITANCE LV SIDE
(CALCULATION OF CL)
OVERALL POWER FACTOR AND CAPACITANCE GST
MODE ICL + ICHL
OVERALL POWER FACTOR AND CAPACITANCE GSTG-A
MODE. ( CAPACITANCE CL)
OVERALL POWER FACTOR AND CAPACITANCE UST-A
(CAPACITANCE CL)
OVERALL POWER FACTOR AND CAPACITANCE
MEASUREMENTS OF LV SIDE
MEASUREME
NT
TEST
MODE
SWEEP
V TEST
FREQ.
ICL+ICL
H
GST
NONE
50 HZ
ICL
GSTg NONE
-A
50 HZ
ICL(f)
GSTg FREQU
-A
ENCY
50 HZ
TO
400 HZ
ICLH
USTA
50 HZ
ICLH(f)
USTA
50 HZ
TO
400 HZ
WATT
LOSSE
S
CAP
MEAS
PF
MEAS.
CONDENSER TYPE BUSHINGS
CONDENSER TYPE BUSHINGS
MERITS OF CONDENSER BUSHING
MEASUREMENT OF TANDELTA OF BUSHING C1
MEASUREMENT OF TANDELTA OF BUSHING C1
MEASUREMENT OF TANDELTA OF BUSHING C2
MEASUREMENT OF TANDELTA OF BUSHING C2
RANGE OF DISSIPATION FACTOR
ITEMS
TYPICAL POWER FACTOR VALUES
@ 20°C OF 400KV GRADE
NEW
OLD
WARNING/ALA
RM LIMIT
POWER
0.2TRANSFORMERS, 0.4%
OIL INSULATED
0.30.5%
>0.5%
BUSHING
0.30.5%
>0.5%
0.20.3%
REACTOR PROTECTION SINGLE LINE DIAGRAM
REACTOR PROTECTION LEGENDS
ELEMENTS
DESCRIPTION
49
Reactor Thermal Overload Protection
63
PRV
71G
Gas Accumulator Protection Buchholz
71Q
Low Oil Level
80Q
Oil Flow Low Indicator
87P
Phase Differential Protection
REF
Restricted Earth Fault Protection
50P
Phase Instantaneous O/C Protection
51P
Phase Time O/C Protection
50N
Neutral Instantaneous O/C Protection
51N
Neutral Time O/C Protection
67G
Zero Sequence Voltage Polarized ground
directional O/C
SHUNT REACTOR PROTECTION SCHEME
 CT AND VT
 PHASE CT PROVIDES INFORMATION FROM
PHASE SIDE, NCT PROVIDES INFORMATION
FROM NEUTRAL SIDE AND GROUND CT IS
SINGLE PHASE CONNECTED BETWEEN NEUTRAL
AND GROUND.
 GROUND CT MEASURES 3i0 ACCURATELY AND
HELPS IN DETECTING LOW CURRENT FAULTS
LIKE TURN TO TURN ETC.
TRANSFORMER PROTECTION
TRANSFORMER PROTECTION