1transformer construction ,testing & maintenance

8/10/2019 1transformer Construction ,Testing & Maintenance

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TRANSFORMER CONSTRUCTION ,

TESTING AND M&R


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Need of transformer

Transformer Transformer

consumer

low voltage high low

high voltage voltage voltage

Power Station

Generation voltage 11 KV

Transmission Voltage 66 KV/132KV

Distribution Voltage 11 KV & below

Transformer - To step-up or step-down the voltage

and transfer power from one a.c. voltage to

another a.c. voltage at the same frequency.


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Power transformer


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Transformer :

A transformer is a static apparatus used for the

transferring power from one circuit to the other

without change in frequency.

It can raise or lower the voltage with a

corresponding decrease or increase in current.

The purpose of transformers is to transfer electricalenergy from systems of one voltage V1 to systems

of another voltage V2.


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Basic requirements of Transformer

Voltage Ratio

MVA Rating & cooling

Vector group

OLTC

Percentage Impedance

No load loss

Load loss


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Parts of transformer

Main tank

Radiators

Conservator

Tap changer

Silica breathers

Buchholz relay

MOG

PRD

OTI

WTI


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Tests on Transformer:

1. Oil Test

2. Voltage Ratio Test

3. Current Ratio Test

4. Vector Group Test

5. Magnetic Balance Test

6. Magnetizing current Test

7. IR Test


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Successful Electrical

Maintenance

PREVENTION is BETTER than CURE

MAINTENANCE is notthe REPAIR work

PREVENTIVE MAINTENANCE

REPAIR or BREAKDOWN MAINTENANCE.


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Maintenance Consists of..

Routine Inspection

Testing

Cleaning & Adjustments


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Key to SUCCESS is. DETECT INCIPIENT TROUBLE by-

- ROUTINE TESTS

- VISUAL

- AURAL

- TOUCH

- SMELL

- INSTRUMENTS


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Distribution Transformer Maintenance

Transformer Body

Inspect For Rust & Leakage

RustClean & Repaint

LeakageInvestigate

- Leaking Joint to beRectified

-Replace Gaskets


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BREATHER

Silica-gel Dehydrating Breather

- Colour of Crystals

- Saturated CrystalsPink

Need ReactivationBlueOil seal .


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Explosion Vent

Inspect DiaphragmAir tight joint withGaskets.

Gaskets

Shrink During Service

Check tightness of Bolts

Replace Leaking Gasket.


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Transformer Insulation is the Root of the Problem

-

Oil & Cellulose Paper Interact.

- By products decompose Insulation System

- By-products : Increase Moisture Level

: Create EnvironmentOverheat

-Fails


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Transformer Oil Functions. First : Serves as INSULATION to withstand the high

voltages present inside the transformer.

Second : It functions as the Heat Transfer medium todissipate Heat Generated within the transformerwinding.

Oil must maintain Good Electrical properties whileResisting Thermal Degradation & oxidation.


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Transformer - Oil

Just Like a car Transformer needsREGULAR

Oil Conditioning

to rid it of Harmful Contaminants

That could SHORTEN its life.


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Transformer Oil

Deterioration of Oil

1. Oxidation

2. In grace of Moisture

3. Solid Impurities4. Contamination


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Transformer Oil Even with Best Maintenance Procedures

Chemical Reaction continues.

RemedyAnnual Oil Testing

Oil Testing uncovers several Potential Problems

Sample Contains Evidence ofSoluble contaminants

- Dielectric contaminant- Acid Materials


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Testing of Oil Benefits Tests indicate the Interior-Condition of

oil.

Prevents Un-scheduled outages.

Determines Trends, if any.


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Tests on Oils

1. Physical AppearanceColour

- Clarity

- Odour2. Electric Strength Test

3. Crackle Test

4. Acidity Test5. Insulation Resistance Test


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OEM Recommendations..1. Hourly

2. Daily

3. Monthly

4. Six Monthly5. Yearly

6. Five Yearly

LoadAmps

Temperature

Voltage Breather

Oil Level

Gen. Observations Oil Testing

Overall Inspection


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BSES Practices. Load readings taken once in 6 months.

Silica gel replaced before/after

mansoon.

Oil Testing once in 12 months.

Oil Filtered if B.D.V. is low.

Oil Replaced if acidity beyond 1 mgkOH/gm.


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ISO Norms. Transformer Load Readings and

Inspection = Every Six Months.

Transformer Oil sampling for B.D.V.Measurement = Every Year.

Transformer Oil Filteration & Oil Testing

= Every 4 Years.


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DISTRIBUTION TRANSFORMERS FAILURE RATE IN BSES

Year Total in Units % FailureSystem Failed

1996-97 3056 70 2.29

1997-98 3229 65 2.01

1998-99 3405 40 1.17

1999-2000 3583 35 0.98


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Failuare Analysis Certain Makes have performed Badly.

Failuares more in recent make & specific

Batches.

75% of failuares at 9 years or less.

Faliare rate goes down as size increases.

Causes : Faults on L.T. side on U/G cables.

Higher Temperature.


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THANK YOU


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P.R.V. (Pressure Relief Valve)

When oil pressure in the tank rises above pre-

determine safe limit this valves operates.

This equipment is of spring loaded type. When It

operates the part of @ 150mm dia of PRV up lifts and

allow in side tank pressure to drop.

Saves transformer to buckle or damage further.

Gives indication of valve operation by rising the flag.

Operates a micro switch that disconnect the

transformer from live circuit.


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M.O.G. (Magnetic Oil Gauge).

This is used with conservator

It's purpose is to indicate the oil level in theconservator vessel and to give alarm when the

oil level falls below a threshold limit.


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SILICA GEL BREATHERS

Moisture is an enemy of transformer insulationsystem

Whenever oil contracts transformer breaths inthrough breathers.

With air lot of moisture which is present in the airalso go inside.

The silica gel present in the breathers absorbmoisture and only dry air goes inside.

After a certain level of absorption the color ofsilica gel crystals change from blue to pink whenthese particles should be recharged or replaced.


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OTI (Oil Temperature Indicator)

This consists of a bulb dipped in a oil cup on thecover of the transformer tank cover. This bulb isconnected to the meter placed in the marshalling

box meant for housing the equipment electricalcontrols by means of a capillary.

This indicates the temperature of the top part of

the transformer oil which is the hottest part.

It gives alarms/trip contacts when temperatureexceeds than set values.


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WTI (Winding temperature indicator) Similar in construction to the OTI.

A current transformer injects current into theheating coil of the equipment giving the extra

temperature gradient over the top oil temp. whichnormally a winding is expected to have.

It gives alarms/trip contacts when temperatureexceeds than set values.

It also gives start/stop contacts for fans & pumpsto start and stop when temperature exceeds.


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Oil test:

Collect samples of oil from main tank bottom,

Conservator, radiator and tap changer.

Oil breakdown test should withstand 60KV for 1min. with 2.5mm gap. If the value is lower,

advice further dry out till this value is achieved,

before proceeding further.

Collect 5 liters of oil from the tank send to a

standard laboratory like ERDA for moisture

content, resistivity testing.


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ERDA detail testing of oil

No. Test Requirement Obtained

value

Remarks

1 Electric strength Min. 60 KV

2 Water Content, ppm Max. 15

3 Neutralization value mg KOH/g of oil Max. 0.03

4 Sediment & sludge % by wt. Non-detectable

5 Dielectric dissipation factor Max. 0.015

6 Specific resistance, Ohm.cm Min. 6x 10

7 Interfacial tension, mN/m Min. 35

8 Flash point , deg.C Min. 140 deg.C


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Voltage Ratio Test:Apply 3-Phase 400 Volts on HV side and measure

respective voltage on LV side.

Tap no. ry (V) yb (V) br (V) rn (V) yn (V) bn (V)

1 95.9 95.4 94.5 55.3 54.6 54.4

9 105.3 104.4 105.5 61.0 60.0 59.9

17 117.1 116.0 117.2 67.9 66.3 66.5


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Current Ratio Test:

Apply 3-Phase 400 Volts on HV side keeping R-Y-B

phases shorted on LV side.

Measure the current on HV and LV side starting

from 1sttap to last tap.

Tap no. R (A) Y (A) B (A) ryb (A)

1 6.4 6.0 6.5

9 8.1 7.8 8.2 32.5

17 10.7 10.4 10.8


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Vector Group Test:

Short R-ph of HV & r-ph of LV.

Apply 3 phase, 440 V on HV side

Measure respective voltage as under:

rR

by

n

B Y

Ry: 67 V Bb: 346 V RY: 408 V

Rb: 68 V By: 347 V YB: 410 V

Yy: 400V Yb: 415V BR: 415 V


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Magnetic Balance Test:

Apply 415 Volts on LV side between phase &

neutral.Measure voltage on respective LV

terminals as under.

LVWinding

rn yn bn

rn= 407 407 334 78

yn= 407 205 407 201

bn =407 79 332 407

Apply 415 Volts on two terminals of HV andmeasure voltage on HV terminals as under:

HVWinding

RY YB BR

RY=424 424 194 222

YB=424 392 424 18.9

BR=424 381 36.2 424


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Magnetizing Current Test:

Apply 3-Phase 400 volts on HV side with LV

open.Measure the current on HV side

Apply 3-Phase 400 volts on LV side with HV open.Measure the current on LV side

Voltage Applied Current measured

RY440 V R11.11mA

YB440 V Y07.62 mA

BR440 V B10.95 mA


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Insulation Resistance Test:

15 seconds 60 seconds 60/15seconds (PI)

HT to Earth 190 x 5 260 X 5 1.4

LT to Earth 200 X 5 330 X 5 1.6

HT to Earth 270 X 5 400 X 5 1.5

CORE to Earth

Dryness of insulation is determined by

measuring insulation resistance of winding.

It should be taken by 2.5 KV motorized Megger.

IR of 60sec/IR of 15sec (PI) should be greater

than 1.2 for good insulation.


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Differential Stability Test

Out Zone Testing : Apply 400 volts 3 Phase on HV

side keeping shorting after CTs for 11 KV Incomer.

Type of fault R Phase Y Phase B Phase REF Relay

HV LV OP. HV LV OP. HV LV OP.

RN

YN

BN

RY

YB

BR

RYB


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Type of fault R Phase Y Phase B Phase REF Relay

HV LV OP. HV LV OP. HV LV OP.

RN

YN

BN

RY

YB

BR

RYB

Differential Stability Test

In Zone Testing : Apply 400 volts 3-Phase on HV side

keeping shorting for 11 KV Incomer before CTs.


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Voltage transformation ratio:

Primary winding & Secondary winding

E1 = 4.44* f *N1* m

E2= 4.44* f *N2* m

E2 / E 1 = N1/ N2

Where N1 = Primary Turns

N2 = Secondary Turns

m = FluxF = frequency

In our system we have different Voltage ratio

transformers: 220/132KV, 132/66KV, 132/33KV,

66/11KV, 33/11KV, 11/0.44KV.


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% Impedance Voltage: The Impedance voltage is defined as that voltage

having the rated frequency which must be

applied to the primary side of a transformer so

that the rated current Ir flows when secondaryterminals are shorted.

The rated impedance voltage is usually stated as

a percentage of the voltage rating Ur of the

winding to which the voltage is applied.

% Impedance = (Applied voltage/Rated

voltage)x100


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MVA rating & cooling: It defines the total output capacity of the

transformer in MVA at the given cooling like

ONAN, ONAF and OFAF.

Our 12.5MVA and 20MVA, 66/11kv transformersare with ONAN cooling.

Our 30MVA,132/11kv and 50MVA,132/33KV

transformers are with ONAN/ONAF Cooling.

Our 65MVA &100MVA, 132/66kv and

150MVA,220/132kv transformers are with

ONAF/ONAF/OFAF Cooling


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Vector group: Yna0 for 220/132kv,132/66kv interconnecting

transformers. DYn1 for 132/11kv, 66/11kv and 33/11kv

distribution transformers.Ynyn0 for 132/33kv

transformers. The vector group denotes the way the windings are

connected and the phase position of their respective

voltage vectors.

To identify the phase angle the vector of High voltage

winding is taken as a reference. The number multipliedby 30denotes the angle by which the vector of the LV

winding lags that of the HV winding.


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It add or subtract turns from the main coil of HVwinding while RAISE or LOWER command isgiven and maintains constant voltage on LV side.

Generally housed inside the transformer tank andoperation through local box/remote panel.

These are the standard products.

Selection is based on Winding connection,

current rating,range in terms of % of winding,number of steps, independent operation, paralleloperation.

OLTC: on load tap changer


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It is basically the core material loss.

Core is the magnetic circuit through which fluxflows.The material used for core making is called

CRGO i.e cold rolled grain oriented steel.

It is the power loss that transformer consumeswhen charged at Normal voltage and at No load.

It should be minimum as possible to reduce T & D

loss of the system. We insist manufactures to use prime grade

imported first quality CRGO material formanufacturing the core of the transformer.

No load loss


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It is basically the copper loss.

It is the power consumed by the transformerwhen on load.

It will change at different load and tap position ofthe transformer.

load loss


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Transformer tank

Bell type tank :A tank in

which the cover is not flat inshape and the tank and cover

junction is near the bottom of

the tank .Tank is in two parts

top tank and bottom tank

Conventional tank:A tankin which the cover is flat and

tank to cover junction is at thetop of the tank.


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Radiators

Tank mounted :Directly mounted on the tankwall, if the size and numbers of radiators is small.

Separately mounted:Mounted separately on A

OR H frame structure and connected with maintank through oil headers.

Selection of radiator banks mounting and its

positioning depends on MVA rating, type of cooling

like ONAF,ONAF and site space availability.

Arrangement need to be specified at the time

tendering for total L X B X H dimensions of

transformer, tank mounted,separately mounted,

HV side, LV side, on left side and right side.


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Buchholz relay

Because of any internal fault heat is generated whichdecomposes oil and insulation material resulting inthe formation of gases.

These gases rise up and while approaching theconservator passes through the buchholz relay.

Transformer

Tank

Alarm

Conservator


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