Transformer Protection Final

5/20/2018 Transformer Protection Final

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PROTECTION EQUIPMENTThe definitions that follow are generally used in relation to power

system protection:

1. Protection System: a complete arrangement of protection

equipment and other devices required to achieve a specified

function based on a protection principal (IEC 60255-20)

2. Protection Equipment: a collection of protection devices (relays,

fuses, etc.). Excluded are devices such as CTs, CBs, Contactors, etc.

3. Protection Scheme: a collection of protection equipment providing adefined function and including all equipment required to make the

scheme work (i.e. relays, CTs, CBs, batteries, etc.)

Relays may be classified according to the technology used:

a.electromechanicalb. static

c. digital

d. numerical


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Faults on the Transformer

Transformer faults are generally classified into five

categories:

winding and terminal faults

core faults

Tank and transformer accessory faults

Onload tap changer faults

Abnormal operating conditions

Sustained or uncleared external faults

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TRANSFORMER PROTECTIONOVERVIEW

Fault Type Protection Used

Primary winding Phase-phase fault Differential; Overcurrent

Primary winding Phase-earth fault Differential; Overcurrent

Secondary winding Phase-phase fault Differential Secondary winding

Phase-earth fault Differential; Restricted Earth Fault

Interturn Fault Differential, Buchholz

Core Fault Differential, Buchholz

MainTank EarthFault Differential, Buchholz; Overfluxing Overfluxing

Overheating Thermal

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NORMS OF PROTECTION FOR EHV CLASS POWERTRANSFORMERS

Voltage ratio &

capacity

HV Side LV SIDE COMMON RELAYS

132/33/11 KV

66/11 KV T/F BELOW 8

MVA CAPACITY

3 O/C AND

1 E/F RELAY

2O/C AND

1E/F RELAY

DIFFERENTIAL, REF, OT,WT, BUCHHOLZ FOR

MAIN

132/33/11 KV

66/11 KV T/F UP TO25/31.5/50 MVA

CAPACITY

3 O/C AND

1 E/F RELAY

2O/C AND

1E/F RELAY

DIFFERENTIAL WITH HIGH SET, REF, OT,WT

FOR HV AND LV WINDING, BUCHHOLZ FORMAIN AND OLTC,PRV FOR MAIN AND OLTC,

MOLG

220/132 KV 50/100

MVA

220/66 KV 50/100

MVA CAPACITY T/F

3 O/C AND

1 E/F

RELAY(DIR)

2O/C AND

1E/F RELAY


FOR HV AND LV WINDING, BUCHHOLZ FOR

MAIN AND OLTC,PRV FOR MAIN AND OLTC,

OVERFLUX AND OVERLOAD ALARM, MOLG

400/220 KV T/F 315

MVA AND ABOVE

3 O/C

DIR.AND

1E/F DIR.

3 O/C

DIR.AND 1E/F

DIR.


FOR HV AND LV WINDING, BUCHHOLZ FOR

MAIN AND OLTC,PRV FOR MAIN AND OLTC,

OVERFLUX AND OVERLOAD ALARM, MOLG


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ANSI NUMBERS OF RELAYS AND THEIR FUNCTIONNAME OF RELAY ANSI

NUMBER

PURPOSE OF THE RELAY

O/C AND E/F

RELAYIDMTL( NON

DIRECTIONAL)

IDMTL(

DIRECTIONAL)

DEFINITE TIME

RELAY OR

INSTANTANEOUS

RELAY

51 RNB67 CX/NX

50 RNB

These relays are used for protection of transmission lines/transformers. 3 O/C relays are

used on HV side of the T/F and 2O/C and 1 E/F relays are used on LV side of the T/F andfeeder panels.

IDMTL: A relay in which the time delay varies inversely with the characteristic quantity up

to a certain value, after which the time delay becomes substantially independent.

INSTANTANEOUS: A relay which operates and resets with no intentional time delay. They

are used for faults close to the source when the fault current is very high.

DIFFERENTIAL

RELAY

87L(

LINE)

87T( T/F)

87G(

GEN.)

Differential protection is a very reliable method of protecting generators, transformers,

buses, and transmission lines from the effects of internal faults. These relays compares the

two quantities generally current.

RESTRICTED

EARTH FAULT

64 REF is provided on the transformer where the winding is Y connected. It takes care of earth

fault in a restricted zone which includes T/F winding, LA CT/NCT and their auxiliary wiring.The relay is fed through 3 no. T/F CT(REF core) and concerned NCT.

NON DIRECTIONAL

E/F

64N During balanced conditions, current through the neutral is 0. If there is an unbalanced fault

(one phase or two phase to ground fault) the current through neutral is no longer zero.

Presence of current in the neutral indicates that there is a fault present. Ground current

relays can be set to quite low values to detect this current and operate.


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NEUTRAL

DISPLACEMENT

RELAY

64N Failure of capacitor unit in a bank will cause the voltage to rise on the

remaining units in the series group which contains the failed unit. Any

additional fail of the unit will cause the voltage to rise still higher which can

damage the healthy unit. To avoid such damage NDR is used which is fed

through open delta connection of the RVT.

DC SUPERVISION

RELAY

80

ABCD

It makes a continuous supervision of DC supply in the C&R panel, and in case of

its operation initiates alarm contacts to the AC buzzer.

VOLTAGE

SELECTION RELAY

75AB

C

Provided on 66/132/220 KV panels where double bus arrangement exist. It

selects the bus PT supply to which the feeder is connected through auxiliary

contacts of isolator and feeds to various protection schemes, metering etc.

MASTER

RELAY/TRIPPING

RELAY

86 It is used to extend tripping command to the circuit breaker and operates

whenever receive command from other protection relays, also acts as blocking

relay for breaker operation. Used for extending inter tripping commands.

BUS BAR

DIFFERENTIAL

RELAY

87M

87B

87C

Used to isolate the faulty bus by tripping all the circuits connected to the faulty

bus in case of any fault on that bus through high speed tripping relay:96

TRIP CIRCUIT

SUPERVISION RELAY

95AB

C

The trip circuit extends beyond the relay enclosure and passes through more

components, such as fuses, links, relay contacts, auxiliary switch contacts

and so on, and in some cases through a considerable amount of circuit wiring

with intermediate terminal boards. The no. of TCSR depends upon the no. of

tripping coils used in the breaker.

UNDER VOLTAGE

RELAY

27 Under voltage and over voltage relays initiate switching of capacitor banks or

stepping of up or down of a tap changer, in case voltage exceeds or falls below

the preset values. Generally these relays are used on capacitor bank panels.

Over voltage relays are used to trip the circuit breaker in case the voltageexceeds the preset limit value generally 110% of the normal value. Over

OVER VOLTAGE

RELAY

59


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OVER CURRENT

ALARM

51L It is a relay that initiates in case the current exceeds the preset values.

Generally used on 220 KV line and T/F C&R panels.

FUSE FAILURE RELAY 97 Operates on failure of PT/VT fuse/fuses. Monitor the fuses on the

secondary side of 11 kV metering PT. The failure of any of secondary fuse

on the 11 kV PT is accompanied by visual & audio indication.

ALARMCANCELLATION

74 Accepts the alarm initiated by another relays.

AC/DC CHANGE OVER 30X Without flag relay used for change over of AC and DC supply. It is provided

on all the panels. The indications are normally connected to AC supply and

in case of its failure the relay operates and the indications automatically

get connected to DC supply.

UNDER FREQUENCY 81 Operates when the frequency falls/exceeds below/above preset values.

DISTANCE

PROTECTION

MAIN-1

MAIN-II

21

21P

21Q

Impedance relays are used whenever over current relays does not provide

adequate protection. They function even if the short circuit current is

relatively low. The speed of operation is independent of current

magnitude. Measures the apparent impedance between the relay and the

short-circuit. The apparent impedance is measured by computing the ratio

between voltage and current at the relay. These relays, which are almostalways directional, are typically employed on transmission lines where

impedance is both predictable and constant.

BUS BAR PROTN. TRIP

RELAY

96.1,

96.2

Used to isolate the faulty bus by tripping all the circuit breakers connected

to the faulty bus.

OVER FLUX RELAY 99 Generally installed on 220 KV C&R Panels of 220/132/66 KV T/F and

operates if the V/f exceeds a preset value, to avoid saturation of magneticcircuits, and to avoid excessive core/iron losses in the T/F. generally its


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POLE DISCREPANCY

RELAY

62 It is provided on 220 KV and above circuit breakers to ensure that

all the poles of the CB close/open simultaneously. In case one or

two fail to open/close then this relay operates and trips the pole

which remain closed.

LBB RELAY 50 LBB It operates in case any breaker fail to trip on fault then all the

other breakers connected to this bus will get the trip commandthrough this relay.

AUXILLIARY RELAY 30 AX These relays are used to extend tripping commands such as

BT/OTT/WTT/PRVT etc to the master/tripping relay.

AUTO RECLOSER 152X A scheme of delay auto reclose will be provided to return circuits

to service after transient line faults.

VOLTAGE

UNBALANCE RELAY

60 Operates whenever unbalance in the voltage of all the three

phases exceeds a preset value.

BUCHHOLZ RELAY 63A

63T

These relays are sensitive to the generation of gases inside the

main tank OLTC tank due to incipient faults.

PRV RELAY This relay is installed on the transformer main tank and OLTC tank

to safe guard the power transformer due to sudden high pressureof gases developed.

CTD This is relay installed in the breaker and causes tripping of the

breaker in case of DC fails or voltage level goes below preset value.

ANTI PUMPING

RELAY

94 Used to avoid successive closing tripping closing and so on like

pumping action of the breaker during fault condition for avoidingdamage to the coil, breaker and other equipment.


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Fault in Power Transformer

External factors Fault in down stream

Lightning/ Switching

Magnetic inrush

Internal factors Insulation failure

Atmospheric conditions

Partial discharges

Carona

Fault in Tap changer

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Transformer protections are provided:

a. Against effects of faults in the system to which the transformer is

connected.b. Against effects of faults arising in the transformer.

Protections against faults in the system:

a. Short Circuits

b. High Voltage, high frequency disturbance

c. Pure Earth Faults

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Internal faults:

Incipient Faults Poor joints/connection

Core faults

Coolant failure(logged oilflow) results in gas liberation.

Bad load sharing betweenT/Fs.

Initially, such faults are of

minor nature but slowlymight develop into majorfaults. Such faults are nottraceable at the windingterminals by unbalance in

voltage or current.

Heavy faults Multi phase & Phase to

earth faults.

Short circuiting between

turns in HV & LV windings

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Name plate details Make

Sr No

Frequency Capacity

Voltage

Current

Phase

% Impedence

Insulation Levels

Drawing No.

P O No.

Vector group

Types of Cooling

Quantity of Oil

Weight of Copper

Weight of Core

YOM 13


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Principles of Protection System

Principles used are:

1. Overheating/Temperature and pressure

2. Over Current

3. Un-restricted earth faults

4. Restricted earth-faults

5. Percentage bias differential protection

6. Gas detection due to incipient faults

7. Low oil level

8 Pressure relief (high overload/arcing)

9. Over fluxing, Under frequency10. Tank earth current detection

11. Lightning/Over Voltage Surges

12. Tap changer problems

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Transformer Protection

Percentage bias differential protection

High impedance circulating current protection

(Restricted earth fault protection)

Over fluxing protection

Back up O/C & E/F

Over load alarm

Protection & monitors built into transformer

(Buchholz relay , OSR relay , Winding & Oil

temperature indicators ,Pressure relief deviceand Oil level indicator)


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Percentage Differential Protection

This Scheme is used for Protection of T/F against internal short circuits.

It is not capable of detecting incipient faults.

The relay settings for T/F protection are kept higher than that foralternators.

The typical values for T/F may be 40 % for operating coil & 10 % forrestraining coil respectively.

Because a T/F is provided with OLTC. The CT ratio cannot be changedwith varying T/F ratio of T/F. so, for taps other than nominal, an out ofbalance current flows through the operating coil of the relay duringload and external fault condition.

When a T/f is on no-load, there is no-load current in the relay. So, itssetting should be greater than no-load current.

To supply the matching current in the operating winding of the relay,the CTs which are on the star side of the T/F are connected in delta &vice versa.

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Percentage Differential Protection for Star-deltaconnected Transformer

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Restricted Earth fault Protection

REF protection is used to supplement the differential protection,

particularly where star neutral of the T/F is grounded through a

neutral grounding resistor to limit the earth fault current. REF

protection provides increased coverage to star winding against earth

faults.

It is provided between the main CT and corresponding NCT.

It is an instantaneous relay.

The relay used in REF protection is of high impedance type to make

scheme stable for external faults.

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Restricted Earth fault Protection

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This condition arises during abnormal operating conditions:

Heavy voltage fluctuations at lower frequency conditions.

This condition is experienced by the T/F during heavy power

swings, cascade tripping of generator sets & HT line in the

Grid, interstate system separation conditions & due to AVR

malfunctioning during start-up or shutting down in case of

Generator Transformer.


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It works on the principal of Voltage/Frequency

Acc to EMF eqn, V1=4.4 f m N1 OR

m = k V1/f

Usually 10 % of over fluxing can be allowed without damage.

If V1/f exceeds 1.1, overfluxing protection operates.

Over fluxing does not require high speed tripping & hence

instantaneous operation is undesirable when momentary

disturbances occur, but the T/F should be isolated in one or two

minutes if the over fluxing persists.


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V/fVn/fn

1.1 1.2 1.25 1.3 1.4

Duration ofwithstand

limit (minutes)

Cont. 2 1 0.5 0


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MAGNETIC INRUSH

The phenomenon of magnetizing inrush is a transient

condition, which occurs primarily when a transformer isenergized.

It is not a fault condition and therefore does not necessitatethe operation of protection, which on the contrary must

remain stable during the inrush transient, which is a majorfactor that is to be taken care of in the design of transformerprotection.

Magnetising inrush wave has the following harmonics1. 2nd harmonic63 %2. 3rd harmonic27 %3. 4th harmonic5%4. 5th harmonic4%


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O/C & E/F Protection

IDMT O/C elements on delta and star side, primarily serve as

back up protection against downstream short circuits and aretime co-ordinated with downstream O/C protections.

The high set instantaneous O/C elements are provided to detectsevere terminal short circuits and quickly isolate the transformer.

The relay is connected in high impedence mode with a seriesstabilizing resistor.


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O/C & E/F Protection

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Overheating Protection

Protection is based on measurement of winding temperature which is

measured by thermal image technique. Thermal sensing element is placed in small pockets located near the

top T/F tank in the hot oil.

A CT is employed on LV side to supply current to local heater.

Winding temperature high alarm/trip is provided through mercuryswitches in the winding temperature indicators.

By thermo-meters, mercury switches heat sensing silicon resistanceare also used for sensing the temperature rise.

The max safe overloading is that which does not overheat the winding.

Temperature of 55 deg C above ambient of 50 deg C is generallyprovided for tripping.

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Thermal Over heating

Winding Temperature

Sensors in 2 places

Over loads/other abnormalconditions cause higher temp.

Used to switch on fans

Alarms and trips above setvalues.

Oil Temperature sensors

Switch on fans

Alarm/Trip are the threefunctions associated with.

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Winding Temperature alarm & Trippings

The Transformer is provided with a W.T.I which contains a Liquid

which expands itself when heated along with Tr.Oil, kept in aPocket & heated by WTI C.T Currents. Change of Volume activatesa Temperature Indicator suitably calibrated. These Indicators arecompensated for atmospheric Temperature Changes.

Normally it has 04 Contacts

Contact-1 : Fan Start ( ex. 60C)

Contact-2 : Pump Start ( ex. 70C)

Contact-3 : WTI Alarm ( ex. 85C)

Contact-4 : WTI Trip ( ex. 90C)


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Oil Temperature alarm & Trippings

The Transformer is provided with a O.T.I which containsa Liquid which expands itself when heated along withTransformer Oil & Change of Volume activates aTemperature Indicator suitably calibrated. These

Indicators are compensated for atmosphericTemperature Changes.

Normally it has 02 Contacts

Contact-1 : OTI Alarm : 85 deg

Contact-2 : OTI Trip : 90 deg


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INFRARED TEMPERATUREMEASUREMENTS

A thermo vision apparatus with resolution better than 0.1 degreeC is used for checking of the temperature distribution on atransformer enclosure (tank and cover) and external auxiliaries

Temperature rise above ambient Recommendation

0-10 deg C Little probability of any damage

11-39 deg C Inspect for damage in next Mtc. S/D 40-75 deg . Check for probable damage at anearliest possible opportunity

Above 76 deg C Critical problem, take S/D forimmediate repairs


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LATEST DIAGNOSTIC TECHNIQUES

Frequency Response Analysis (FRA)

PD measurement and acoustic localisation offaults

Furan analysis in oil (HPLC chromatography)

On-line dissipation factor monitoring of

H.V. bushings

Polarisation spectrum or Recovery Voltage(RVM) measurement


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FREQUENCY RESPONSE ANALYSIS

Mechanical movement of winding mayoccur during transportation shocks or SC

forces or shrinkage due to ageing and mayresult in dielectric failure of transformer

Such winding displacement may not bedetected by DGA, winding resistance or C

&Tan measurementTechniques available to detect suchchanges are FRA or Vibration Measurement


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FRA primarily used for detection of

deformation/movements of winding

FRA is also carried out periodically for detection

of deformation of smaller magnitude, when

transformer experiences several short circuit

forces

Helps in monitoring health and condition

assessment of transformers

FRA SIGNIFICANCE


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PRINCIPLE OF FRA

Both the applied impulse signal and the correspondingwinding responses are recorded using a high performancedigitizer and the results transformed into frequencydomain by Fast Fourier Transform calculation, a response

function is obtained which is dependent almost entirely onthe test object and is independent of applied signal andtest circuit. The changes in the test object can be moreclearly and consistently identified. The technique used forFRA analysis is a sweep frequency technique which issuitable for site use and has a superior signal to noiseperformance at high frequencies.


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FRA TEST SET-UP


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ADVANTAGES OF FRA

FRA detects winding movement, which cannot be

ascertained by any other commonly known test

FRA test is sensitive and very repeatable FRA is very powerful and effective tool and capable

of detecting a range of transformer faults, it is

nevertheless primarily a mechanical condition

assessment test and must be used in conjunctionwith other diagnostic tests if a complete picture of

the condition of the transformer is to be obtained.


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REASONS FOR TRANSFORMER OILDETERIORATIONPHYSICAL CONTAMINATION DUE TO MOISTURE ENTRY, FIBERS IN

INSULATION TAPE AND VARNISH IN THE PARMA WOOD, PRESSBOARDASSEMBLY.

CHEMICAL REACTION DUE TO OXIDATION OF HYDROCARBON ALONG WITH

VARIATION IN LOAD CONDITION AND TEMPERATURE OF THE OIL FORMS

ACIDS AND SLUDGE .

ELECTRICAL STRESS DUE TO ELECTRICAL FAULTS LIKE PARTIAL DISCHARGE,

ARCING, SPARKING INSIDE THE TRANSFORMER.

THERMAL STRESS DUE TO LOOSE CONTACTS IN WINDING LEADS, CORE

CLAMPING ETC.

AGEING OF TRANSFORMER OIL.

POOR QUALITY OF TRANSFORMER OIL

OIL QUALITY INDEX SYSTEM


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OIL QUALITY INDEX SYSTEMGOOD OILS NN

IFT

-0.00 To -0.10

-30.0 To -45.0

Pale Yellow

MIN: 300-1500

Excellent

PROPOSITION

OILS

NN

IFT

-0.05 To -.10

-27.1 To -29.9

Yellow

MIN : 271-600

GOOD

MARGINAL OILS NN

IFT

-0.11 To -0.10

-24.0 To 27.0

Bright Yellow

MIN : 160-318

A drop in IFT signal

the beginning of

sludge in solution.

BAD OILS NNIFT

-0.16 To -0.40-18.0 To -23.0

AMBERMIN : 45-159

Oil is not providingproper cooling &

winding protection

V. BAD OILS NN

IFT

-0.41 To -0.65

-14.0 To -17.9

BROWN

MIN : 22-24

Sludge has already

been deposited in &

on T/F parts inalmost 100 % of

these units.

Insulation damaged

& reduced cooling

efficiency with high

operating tempcharacteristics.

EXTREMELY

BAD OIL

NN

IFT

-0.66 To -1.50

-9.0 To -13.9

BARK BROWN

MIN 6-21

NN

(NeutralizationNo.)

IFT (Inter

Facialtension)

MIN : Myers Index No. = IFT/NN


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FACTORS THAT DETERIORATES THEMATERIALS

Moisture

Oxygen

Temperature

40

EFFECT OF THE ABOVE FACTORS


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EFFECT OF THE ABOVE FACTORS

Moisture: Absorbed from Atmospheric air during breathing. Reduces the BDV Oxygen

41

Oxygen : O2from atmospheric air while breathing and which

is present in oil react with cellulose in presence ofcopper as catalyst form organic acid which

dissolves in oil to form sludge. Restrict oil circulation

Reduces heat transfer leading to increases thetemperature


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EFFECT OF THE ABOVE FACTORScont.

- Temperature :

Cellulose free sugar glucose, H2O, CO, CO2+ Celluloseof reduced chain length.

Reduces the BDV

Deteriorate the cellulous insulation.

Reduces Tensile strength

The various types of cooling methods used for power transformer


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The various types of cooling methods used for power transformerare given below (IS 2026: 1962)

CoolingCooling method Oil circulation Winding temperature rise limit

ON Natural Air-natural 550C

ONAF Natural Air-blast/Forced 550C

OFAN Forced Air-natural 600C

OFAF Forced Air-natural 600C

OFW Forced Water(Forced) 650C

Top oil temperature measured by thermometer for all types of cooling is 400c

(Ambient temperature)

RECOMMENDED LIMITS OF T/F OILS


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RECOMMENDED LIMITS OF T/F OILS

NAME OF

CHARATERISTICS

UNUSED MINERAL OIL IN-SERVICE T/F OIL

CATEGORY

OF POWER

T/F

RECOMMEND

ED LIMITS (AS

per IS: 1866-2000)

CATEGORY OF

POWER T/F

RECOMMENDED

LIMITS (AS per IS:

1866-2000)

ELECTRIC

BREAKDOWN(KV)

BELOW 72.5

KV

72.5 KV To

170 KV

Above 170 KV

40 KV (Min)

50 KV (Min)

60 KV (Min)

Up to 72.5 KV

72.5 KV To 170

KV

Above 170 KV

30 KV (Min)

40 KV (Min)

50 KV (Min)

Specific Resistance at

90C

All Voltages 6X10*12 Ohm-

Cm (Min)

All Voltages 0.1X10*12 Ohm-Cm

(Min)

WATER CONTENT BELOW 72.5

KV

72.5 KV To

170 KVAbove 170 KV

20 ppm (max)

15 ppm (max)

10 ppm (max)

BELOW 72.5 KV

72.5 KV To 170

KV

Above 170 KV

No free moist at room

temp

40 ppm

20 ppm

DIELECTRIC

DISSIPITATION

FACTOR at 90C

UP to 170 KV

Above 170 KV

0.015 (Max)

0.010 (max)

UP to 170 KV

Above 170 KV

1.0 (Max)

0.2 (max)

NEUTRALIZATION All Voltages 0.03 mg KOH/g All Voltages 0.3 mg KOH/g


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DISSOLVED GAS ANALYSIS

The inflammable gases dissolved in the transformer oil are

mainly hydrocarbon gases (methane CH4, Ethane C2H6, Ethylene

C2H4, Acetylene C2H2, Propane, Hydrogen, Carbon Monoxide and

carbon dioxide). With the help of dissolved gas analysis equipment

the concentration of these gases in PPM can be known and can becross checked with the IS standard. Also with the help of Rogers

ratio method, the type of probable incipient fault can be judged and

corrective action can be taken in advance to prevent failure of the

transformer.


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Buchholz relay

Incipient faultGas accumulation is slow-top floatalarm.

Heavy fault insideDecomposition high- heavy surge-bottom floattrip.

Simulate condition 50-100 ms

Normal time 200 ms.

Fault below the oil levels only detected.

Gas Detection


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a. Buchholz relay protection

b. Pressure relief valves / switches (for heavy internal faults)

Buchholz Protection

This is for two types of faults inside the transformer.

a. For incipient faults because of

1. Core bolt insulation failure

2. Short circuit in laminations

3. Local over heating because of clogging of oil

4. Excess ingress of air in oil system

5. Loss of oil due to heavy leakage

6. Uneven load sharing between two transformers in parallel

causing overheating due to circulating current.

47

These generate gases causing operation of upper float and

energises the alarm circuits.


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Why DGA

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1. DIRECTLY DETECTS THE GASES EVOLVED DUETO DECOMPOSITION OF OIL, INSULATING

MATERIALS LIKE PAPER, PRESS BOARD ETC.

2. EFFECTIVE AND POWERFUL DIAGNOSTIC TOOLFOR MONITORING THE DEVELOPING INCIPIENT

FAULT.

3. DETECTION OF FAULT FROM THE

INTERPRETATION OF THE TEST RESULTS GIVESEARLY WARNING OF ANY DEVELOPING FAULT.

4. THE KEY GAS INDICATES THE TYPE OF FAULT

DEVELOPING INSIDE THE TRANSFORMER.


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ADVANTAGES OF DGA MONITORING

49

DGA MONITORING MINIMISES OUTAGE OF EQUIPMENT.

DGA HELPS IN REDUCING THE REPAIR COST.

DGA HELPS TO IMPROVE LIFE SPAN OF THE EQUIPMENT.

DGA IMPROVES THE OPERATING CONDITION OF THE EQUIPME

DGA IMPROVES THE PLANT EFFICIENCY.


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GASES LIBERATED DURING OILDECOMPOSITION

1. METHANE CH4

2. ETHANE C2H

6

3. ETHYLENE C2H4

4. ACETYLENE C2H25. CARBON MONOXIDE CO

6. CARBONDIOXIDE CO2

7. HYDROGEN H2

8. OXYGEN O29. NITROGEN N2


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TEMPERATURES AT WHICH GASES EVOLVE

METHANE (CH4) > 1200C

HYDROGEN (H2) > 1500C

ETHANE (C2H6) > 2500C

ETHYLENE (C2H4) > 3500C

ACETYLENE (C2H2) > 7000C


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FAULTS ASSOCIATED WITH DIFFERENT GAS

OIL OVERHEATING : C2H4, C2H6, CH4

TRACES OF ACETYLENE WITH SMALL QUANTITY OF

HYDROGEN

OVERHEATED CELLULOSE : C02 & CO

LARGE QUANTITY OF CO2AND CO EVOLVED FROM

OVERHEATED CELLULOSE.

CH4AND C2H4FORMS IF FAULT INVOLVES OIL

IMPREGNATED STRUCTURE


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55/72SAMPLING OF OIL - PROCEDURE


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THE CONTAINER FOR SAMPLING SHALL BE STAINLESS STEEL OR GLASS

SAMPLING BOTTLE WITH STOP VALVES ON BOTH ENDS.

THE COLOUR OF THE BOTTLE SHALL BE BROWN PREFERABLY.

FREQUENCY OF SAMPLING FOR DGA

FOR NEW TRANSFORMER :

FIRST SAMPLE: : BEFORE ENERGIZING NEW T/F,

SECOND SAMPLE : AFTER ONE MONTH OF SERVICE

THIRD SAMPLE : ONCE IN A YEAR IF NO ABNORMALITY.

FOR REPAIRED TRANSFORMER :

FIRST SAMPLE : BEFORE ENERGIZING REPAIRED T/F

SECOND SAMPLE : AFTER THREE MONTHS AND

THIRD SAMPLE : ONWARDS ONCE IN A YEAR IF NO

ABNORMALITY.


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CODES & STANDARDS FOR DGA

IS 943 1979

IS 10593 1983IEC 599 1978

FOR OIL SAMPLINGIS: 9497


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DISSOLVED GAS ANALYSIS

Brief Overview of Various Interpretation Technique for DGA Data

Individual Fault Gases Acceptable Norms

Total Dissolved Combustible Gas (TDCG) limits

IEC 599 Method

IEEE Standard C: 57.104/1995Key Gas Method

Ratio Methods

Trend Analysis

INTERPRETATION OF DGA RESULTS:


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KEY GAS METHODS: The permissible concentration of Dissolved Gases As perIEEE.

All gas concentration are in ppm

GAS LESS THAN 4 YRS

in Service

4-10 years in

Service

More than 10 years

in service

Hydrogen 100-150 200-300 200-300

Methane 50-70 100-150 200-300

Acetylene 20-30 30-50 100-150

Ethylene 100-150 150-200 200-400

Ethane 30-50 100-150 800-1000

Carbon Monoxide 200-300 400-500 600-700

Carbon Dioxide 3000-3500 4000-5000 9000-12000

Recommendations if Values of the Fault Gases ExceedsPermissible Limits


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Permissible Limits

GAS Permissible

Limits

Value Obtained

by DGA

Recommendations Remarks

A. TRANSFORMER under 4 Years of Service

ETHANE C2H6 30-50 50-150 Monitoring > 150- View Seriously

Ethylene C2H4 100-150 150-300 Monitoring > 300- View Seriously

Acetylene C2H2 20-30 30-70 Monitoring > 70- Immediate

inspectionB. TRANSFORMER Between 4-10 Years of Service

ETHANE C2H6 100-150 150-500 Monitoring > 500- View Seriously


Acetylene

C2H2

30-50 50-70 Monitoring > 70- Immediate

inspection

C. TRANSFORMER Above 10 Years of Service

ETHANE C2H6 800-1500 Upto 800 Monitoring > 800- View Seriously


Acetylene

C2H2

100-150 70-100 Monitoring > 100- Immediate

inspection

T f Di i U i T t l Di l d C b tibl G


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Transformer Diagnosis Using Total Dissolved Combustible GasConcentrations.

STATUS H2 CH4 C2H2 C2H4 C2H6 CO CO2 TDCG

Condition

1

100 120 35 50 65 350 2500 720

Condition

2

101-700 121-400 36-50 51-100 66-100 351-570 2500-

4000

721-

1920

Condition

3

701-1800 401-

1000

51-80 101-

200

101-

150

571-

1400

4001-

10000

1921-

4630

Condition

4

>1800 >1000 >80 >200 >150 >1400 >10000 > 4630

Condition 1: Total dissolved combustible gas (TDCG) below this levelindicates the transformer is operating satisfactorily


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indicates the transformer is operating satisfactorily.

Condition 2: TDCG within this range indicates greater than normalcombustible gas level. Any individual combustible gas

exceeding specified levels in table 4 should haveadditional investigation. A fault may be present. TakeDGA samples as recommended by HVPNL authorities.

Condition 3: TDCG within this range indicates a high level ofdecomposition cellulose insulation and/or oil. Any

individual combustible gas exceeding specified levels intable 4 should have additional investigation. A fault orfaults are probably present.

Condition 4: TDCG within this range indicates excessivedecomposition of cellulose insulation and/or oil.Continued operation could result in failure of thetransformer.



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Rogers Ratio METHODS: The admissible gas concentration limits may can bemisleading. Sometimes faults can be detected with lesser concentrations.

Code Range of

Ratios

C2H2/C2

H4

CH4/H

2

C2H4/C2

H6

3

0

1

1

2

1

0

2

2

0

0

1

2

Case Fault Type Problem Found

0 No Fault 0 0 0 Normal Aging

1 Low EnergyPartial

Discharge

1 1 0 Electric discharge in bubbles, caused byinsulation voids or high moisture in oil.

2 High Energy

Partial

Discharge

1 1 0 Same as above


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Case Fault Type C2H2

/C2H

4

CH4/

H2

C2H4

/C2H

6

Problem Found

4 High Energy

Discharges,

arcing

1 0 2 Discharges (arcing) with power; arcing B/D of

oil between winding or coils, or between coils

& ground, or OLTC arcing across the contacts

during switching with the oil leaking into the

main tank.

5 Thermal fault

Transformer Protection Final

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