ERECTION,OPERATION

AND MAINTENANCE OF

DISTRIBUTION AND

POWER TRANSFORMERS

RECEIPT OF TRANSFORMER AT SITE

(1) Dispatched with Core & Windings fitted in the Tank

and filled with Nitrogen upto 0.15kg/cm.sq or filled

with Insulating Oil above Windings.

(2) If filled with Nitrogen check Nitrogen Pressure and if

less than 0.2 kg/cm.sq, replenish it upto 0.15kg/cm.sq.

(3) All other parts such as Conservator Tank, Bushings

(HV&LV),Radiators,Accessories,Protective devises

are dispatched in separate packages.

(4) Keep the following Tools & Materials ready:

Steel Slings of various capacities,Hydraulic

Lifting Jacks, Wooden Sleepers, Wooden

Packing Pieces, A Mobile Crane etc.

(5) Before Commencing Erection, Nitrogen must be

slowly released and the tank flushed with Dry

Oxygen or filled with Insulating Oil slowly to

avoid contact of winding with Atmospheric Air.

Precautions to be taken Long Storage of the Transformer is not

recommended. Store Oil Drums properly Assembly should commence from Bottom Core & Windings should be filled with Oil as soon

as Nitrogen is expelled. Choose a Dry Day when weather is clear Care to be taken so as to not to allow any foreign

object falling into the Transformer Blanking Plates should be removed when we are

ready for assembly.

Blanking Plates should also be stored safely after

work is completed.

The Person working with Tools used inside

Transformer must tie it by cotton tape

No coins,pens,watches etc should be used by the

persons working on the Transformer

Gaskets must be tightened properly ie Not too much

or too less

New Gaskets must be used for Bushings Radiators,etc

Ensure tightness of all the connections inside and

proper Locking

Transformer Oil must be pumped from the Drums to the Storage Tank through Purifier

Leave about 20mm oil at the bottom of each drum.It may be impure.

Pumping in Oil must be through the Drain valve. Also Draining. Leave one Opening at the Top Open to release Entry Air.

Before Commencing Oil filling and purification,Clean the top of oil drums,flush hoses,Oil Tank,and the Filter also.

Use Gasket Paste when fixing them Once the Joint is detached, use fresh Gaskets

Gasket Paste shall be applied uniformly before

pressing onto a metal surface.

All the Bushings must be thoroughly cleaned

especially the Bottom Portion before assembly.

Bushing Shields must be fitted to the Bushing before

assembly.

Storage of Gasket is very important.No rolling,folding

Oil Pockets for OTI & WTI must be filled with Oil

before fitting the Sensing Bulbs

For Transformers rated below 66kv Vacuuming to be

done at below 380mmHg

For Transformers above 66kv Vacuuming to be done at 759mm of Hg

Equalizer pipe connections must be connected between the Main Tank and OLTC to avoid Buckling of OLTC

The Bursting Head of Explosion Vent to be removed and a Blanking Plate fixed before vacuuming the Transformer.

The Required vacuuming shall be for two

hours. Replace Vent Explosion Head Oil may be injected through the Bottom Drain Valve

into the Transformer.No Rubber Hoses should be

used for filling

Hot Oil Circulation to be done to expel moisture

from the insulation at 60-70deg

Cooler Connections must be open for Hot Oil

Circulation.

Circulation must be continued for about a week until

No Moisture drops are seen from the Filter

Vacuum exhaust and IR Values improve.

The Status of Drying should also confirm the BDV of

Oil at 60kv/mt and moisture content at 15ppm.

Fix the Arcing Horns for the Bushings (for 66kv and

above)and adjust

Connect the Transformer Body earth

Release any Trapped Air from the Bushings, Top

Cover etc

Connect the Heating Coils of the WTI to BCT

Terminals.

Precautions to be taken when working in a Live Substation …

Do not work too close to another Live equipment

Before working Take a Permit. All sources of power

must be disconnected

A safety earth to be provided wherever necessary

The Area of work should be surrounded with a

Safety Ribbon and all the staff clearly educated

Trained and qualified persons must carry out

inspection and enforce safety

All the Testing devices must be handled by Qualified

persons

A Safety Sargent should be posted at the work site

who should deenergize if needed

If it is necessary to work inside the Tank, Dry Air

must be allowed as the Oil is pumped out .

Dry Air supply must be slowly allowed inside from

the Bottom and allowed to go out from the top by

keeping one Blanking Plate Open should a person

work inside

Another Person should always stay outside

watching the person inside

Electrical Tools should be used with plug pins only

Warning Boards should be Exhibited

Before energizing the Equipment ensure that all the

working personnel have come out and all the Earths

provided for safe working are removed.

ERECTION,O&M OF 3 Phase 11KV/433V DISTRIBUTION TRANSFORMERS

Read the Manufacturers Instructions carefully.

The Transformer should be commissioned only after

completing all the assembly and Pre-commissioning

Tests.

You can refer to the following ISS as necessary:

IS1180 (Out door Dist Tr), IS 2026 (Power Tr), IS

2099 (HV Porcelain Bushing), IS 3347 (Out Door

Bushings),IS 7421(LV Bushings), IS 335 (New Ins

Oil), IS 6600 (Loading Guide for Tr), IS12444 (Cu

Wire Rods), IS 5 (Color for Ready Mix Paints)

Mobile Crane or other approved devices should be used.

For Loading and Unloading the Transformer the Two Lifting Lugs provided on the sides of the Transformer should only be used.

Adequate care must be taken when shifting the Tr to site from Storage Yard

The Transformer is dispatched in complete shape with the Tank, core & Windings, Conservator, Vent, HV & LV Bushings, assembled. Visual Inspection must be done before erection to detect any damages, leakage of oil,cracks.

Pre commissioning Site Tests such as Megger

Test, Ratio Test, Excitation Current, Winding

Continuity,Oil sample Testing for Dielectric and

Acidity to be done.

Oil is inflammable.Take care not to bring naked

flame or light such as during welding near the

Transformer

Fibrous cleaning material should not be used.

Use cloth

Equipment for Erection, Testing and Equipment for Erection, Testing and CommissioningCommissioning

AC Voltmeters (0-600V)- 2 nos

Ratio Test Meter

Megger ( 500volts and 1000 volts or 2000volts)

Electric Oil Pump / Hand Pump

Spanners Single/Double Edged, Ring, complete Set

Screw Drivers, assorted sizes, Tester

Knife, Tailor’s scissors

MS Bolts & Nuts, Washers (plain and spring)

Cotton cloth for cleaning

Paint Brush with Paint as required

Oil Sampling Bottle and Testing Kit

Transformer should be erected on a Level

Foundation

The Foundation must be properly designed. Sufficient clearances between phases and Ground

shooed be maintained For Indoor Installation, good ventilation must exist Examine Standard Fittings like Rating Plate, Oil

Gauge,Lifting Lugs, Conservator,Vent, etc

Bi Metallic Connectors to be used in HV & LV

connections

Transformer Earthing Terminals must be earthed.

(Note CRGO core is connected to clamping frame

which is connected to the Transformer Tank)

Lifting Lugs provided on the Tank Cover is meant

for Lifting Core & Windings. Nor for Lifting Tank

Check for Drain valve, Sampling Valve, Oil Level

Gauge, Breather, Vent, Bottom Channel (Mounting),

Corrugated Fins for Radiator.

Pre Commissioning TestsPre Commissioning Tests Ratio TestRatio Test : : This can be done by using a Turns Ratio Tester or by

using Two Volt meters. Applying a 3 phase 415 volts supply on HV side,

measure the Voltages on the LV side between phases and also

between phase and Ground on every Tap. Check Continuity of winding at every tap. Ensure that the voltages as measured is as per the

Rating plate and within the Tolerance permitted. Alternatively , the Turns Ratio can be directly

checked at various Taps by Ratio Tester

Insulation ResistanceInsulation Resistance :: The IR values should be checked with a 500

Volts megger for LV winding and a 1000/2000 volts megger for HV winding

IR values between Phase to Ground on all HV and Lv windings must be taken . Also between HV & LV . The Temperature at the time of test must be noted

The Megger Leads must be continuous-no joints permitted

No external Lines,LAs must be connected to the Transformer

The HV & LV Bushings must be thoroughly cleaned.

The Safe IR Values as per CBI&P 11kv and 33kv are :

Voltage.. Minimum safe IR in Mega Ohms at a Temperature of

° C 30 40 50 60 11kv 400 200 100 50 33kv 500 250 125 65 Temp 1.98 3.95 7.85 15.85

Correction

Precautions :Precautions :

Any IR Value taken must be Reduced to 20 deg C. All the values must be taken with the same instrument,temperature recorded with the same meter.

NoteNote that for every 10 °C rise in Temperature the IR value gets Halved.

TAN DELTATAN DELTA (Dielectric Dissipation Factor) on the (Dielectric Dissipation Factor) on the Windings:Windings: This Test indicates the Dielectric Loss Factor.

The Power Factor of the Insulation is directly related

to the moisture absorbed by the winding.

The higher the Power Factor,the greater is the moisture

absorption. The ideal would be 0.01* 10 to the power

of

Magnetizing Current :Magnetizing Current :

A 3 phase 415 Volts supply is applied to the

Primary and the Magnetizing current measured in

all the phases.

These values must be equal and confirm to the

Manufacturers Test Certificates.

Oil Tests:Oil Tests:

The BDV of Insulating Oil must withstand 30 kv per

minute when Tested with a Kit having 12mm spheres

and a 2.5 mm Gap

The same will hold good in respect of OLTC oil for

Power Transformers.

Oil samples from Top and Bottom have to be collected

for this Test.

General Checks :General Checks :

Air release must be done.

Drain /Top/ Sampling valves must be closed.Valve

if any between conservator and Tank must be open. Thermometer pockets must be filled with oil Transformer Neutral and Body Earth must be

connected to Earth. Oil Level is upto the mark. Breather is OK

After all the Tests as above are satisfactory:

Allow a settling time of Eight hours

Release Air from all the pockets

Energize the Transformer with LV Fuses in the

Distribution Box removed

Observe for any abnormal Hum or Sound

If OK, measure Voltage between phases and

between phase to Ground

After a few hours, take out the Transformer, check

and rectify any abnormality and put it back into

operation

MAINTENANCE OF TRANSFORMERS

Maintain a register indicating all the Precommissioning Test Results.

Any maintenance should be done after de-energisation of Transformer and Earthing

Physical inspection must be done periodically for any Oil Leakages, Loose HV & LV & Earth Connections, Clean the Body, Bushings etc,

During Over hauls, the core and winding must be lifted, windings inspected for displacement, core for any looseness, Tie Rods, clamps, bolts and nuts for tightness, Ducts for any blockage due to sludge and remedy.

Check the Load in Amperes on LV side Regularly

Check LT Voltages in all the phases

Check Oil level.Leakages.

Check for tightness of HV/LVConnections once in

six months.

Test the Oil for Dielectric Strength and Acidity once

in a year.

Overhaul the Transformer atleast once in Five

Years.

ERECTION,O&M OF 3PH,33/11KV ERECTION,O&M OF 3PH,33/11KV 3.15mva,5.0mva TRANSFORMERs3.15mva,5.0mva TRANSFORMERs

Read the Instruction Manual Carefully.

Read the complete set of Drawings .

Energize only after completing all the assembly

work,conducting all the Pre Commissioning Checks

and satisfactory pre commissioning Tests.

The Relevant Indian standards are:

IS 2026 (P1), (P2), (P3), (P4), (P5) on

Power Transformer General,Temp Rise,Dielectric

Tests, Terminal marking & Tapings, Bushings &

Clearances

Small capacity Transformers, after tests are

dispatched filled with Oil.Where the Conservator

and Radiators are of detachable type, Oil is filled

upto 140mm below Top Cover.

Use Lifting Lugs provided on the Top of the

Transformer on either side. Do not use the Lugs

provided on t he Top Cover. These are meant for

Lifting Core and Windings

Use Suitable Mobile Crane or approved Lifting

Devices for unloading/Loading

Packages containing Bushings & Accessories

must be carefully handled

In Lifting the Complete Transformers use such Lugs

to maintain balance and avoid Tilting

Before Unloading the Transformers received from

the Factory, Examine the parts for any missing,

damaged items;

Before commissioning, conduct Visual Checks,

Megger the Windings,Do the Ratio Test, Check

Winding Continuity,Excite with Low Voltage,

Conduct BDV of Oil

Do not store for Longer periods.All Blanking plates

etc must be preserved

A properly designed Foundation must be ready.

Oil Soak Pits are recommended.

Easy access around the Transformer.

Ensure Proper Clearance to Live Parts.

Cardon the Work place with Safety Ribbon and

Danger Boards.

Indoor Installations must be well ventillated.

Oil is inflammable.Take care when welding nearby.

Avoid assembly, as far as possible on a moist day.

Those having access to the interior of the Transformer must empty their pockets.

Workmen should tie the spanners they use, to their hands, with Strings(Cotton)

Clean every part with Oil or with Non fluffy cloth for wiping

The Transformer oil can be circulated to get a Temperature above ambient before accessing interiors.

Transformer should be erected on a Level Foundation

Oil Filteration to be Stopped When BDV is 40Kv/mt, WC-25ppm, Tan Delta 0.05(33kv)

Tools &Testing Equipment Required: Oil Test Kit Ratio Meter ( Two AC Volt Meters) Mobile Crane Vacuum Gauge Megger,Motor Operated,1.0kv/2.5Kv Hydraulic Jacks,Wooden Sleepers/ Packing wood,

spanners,tools. Stream Line Filter Slings, Shackles Chain Pulley Blocks/Tirfor Tarpaline,Ropes,Cleaning rags etc

Pre Commissioning Tests: Ratio Test Polarity Test Resistance of windings with Kelwin Bridge Insulation Resistance Magnetizing Current Oil Tests BDV/Acidity Erection, assembly must bear the safety clearances

when working in Live Stations Settings of all Current and Voltage Operated Relays

and Testing Operation of Alarms & Tripping

All Hv and LV connections must be tight

Ensure Proper Neutral and Body Earthing

Release Air from all Air Release Pockets

Ensure all Radiator Valves are Open

Ensure Drain Valves,Sample Valves,Top valves in

closed condition

Thermometer Pockets are filled with Oil

Oil Level is as required

Arcing Horns and Gaps set.

WTI CT connected to Compensating Heater.

Allow a settling Time of 24 hours

Release the Air from all the Pockets again

Energize the Transformer from HV.

Note for any abnormal Hum/ sound

Check the LV Voltages

Build up the Voltage in Steps and Record

Transformer should be switched off again after 24 hrs

Note any abnormal Temperature/vibration/ hum/

behaviour.Rectify.

Energise and Load the Transformer.

All the details to be recorded and informed to the

Manufacturer.

Check points for Large Transformers Keep a fire fighting equipment Ready Welding works must bear Fire Safety No smoking near the Transformer/Filter Earth all Control Cabinets,andMBs Ensure that the Tank has no pressure when Blanking

Plate is being opened Remove all loose parts on the Top cover Fasten tools to wrist. Empty Pockets. Utmost care at work site All components to be well cleaned.

Minimum Exposure of Core & Winding

Any object dropped inside despite all precautions must be brought out.

Assemble Turrets in Right places Calibrate OTI & WTI Check Oil Levels in Bushings Settings of OTI & WTI, Alarm/Trip Earth Resistance of Transformer Earth Hand operation of OLTC in all Taps Limit Switches Operation of OLTC Step By Step Operation of OLTC Local/Remote Operation of OLTC Tap Position Indicator

Out of Step Relay of Tap cahnger

Off Circuit Tap Changer Inter Lock

Analyse any Gas Collected in Buch Relay

Check Inter Trip operation

Trip Free Operation of HV& LV CBs.

Check Alarms /Trips as below:

Bucholtz, WTI ,OTI, Oil Flow,Water Flow,

Differential Pressure (Low Cooling),Fan fail,PR

Valve ,Diffl Relay,O/C Relay,E/F Relay,REF Relay.

Color of Gas Due to Decomposition of:

White Paper & Cloth

Yellow Wood

Grey Iron

Black Electric arc in Oil

Clear Air

When Gas Passed thro’ 5% Ag No3 sol:

White Precipitate turning Brown. Oil Decomposition

When Gas passed thro weak Ammonia solution, if the curdled precipitate turns to Dark Brown, indicates Paper,wood,cotton decomposition.

Vacuum Filtration of Oil should be done separately at 50-70 deg C,and shall be terminated on attaining parameters below:

upto 145kv 60kv/mt BDV, 20ppm WC upto 245kv 65kv/mt BDV, 15ppm WC upto 420kv 70kv/mt BDV, 15ppm WC

VACUUM TREATMENT

kv Evacuate&Hold Standing Time < 145 12Hrs 12Hrs

upto 420 24Hrs 48Hrs

> 420 36Hrs 120Hrs

No Electrical Test Permitted in Vacuum

Don’ts for Power Transformers. Don’ts for Power Transformers.

1.Don’t Energise without Investigation if it Trips

2.Don’t Re Energise without analysis of Gas if

Bucholtz Relay Acts

3.Don’t Re Energise without conducting all Pre

Commissioning Tests

4.Don’t Handle Off Ckt Tap changer when the

Transformer is in service

5.Don’t Energise the Transformer unless the Off Ckt

TC is Locked

6.Don’t Leave Tertiary Terminals un-protected.Connect

LAs

7.Don’t allow OTI/WTI to exceed 65 deg C.Filter Temp

beyond 70deg C.

8.Don’t Parallel Transformers unless conditions are

satisfied

9.Don’t Use Low Capacity Lifting Jacks

10. Don’t Move the Transformers with Bushings fitted

11.Over Load Transformer (limits as per IS 6600)

12.Change WTI/OTI settings fequently

13.Leave the Red Pointer(Max) behind the Black

Pointer(Instant)

14.Allow Loose Connections

15.Meddle with Protection Circuits

16.Allow Conservator Oil Level fall below 1/4 level

17.Allow Oil Level in Bushings fall .Top up immediately

18. Allow shalling Box and Control Cabinets unlocked.

19.Switch off Heater in MB off.

20.Allow Dirt Deposits on Bushings

21.Allow any Un-authorised Person near the Transformer.

22.Leave Ladders Un-Locked when Transformer is in

service

23.Allow Water Pressure more than Oil Pressure

24.Mix Oil unless it confirms to IS :335

25.Allow inferior Oil.Process it confirm IS

1866(BDV&WC).

26.Continue with Pink silica gel.Recondition or replace

27.Leave Secondary Terminals of CT Open

28.Store for Longer Periods

DO’S FOR POWER TRANSFORMERS :DO’S FOR POWER TRANSFORMERS :1.Check thouroughly when an Alarm /Trip occurs

2.Check Air Cell in Conservator

3.Attend Lakages of Oil in Bushings Immediately

4.Clean Petty Coats of Bushings occassionally

5.Check BDV& WC of Oil in Main tank and BDV for OLTC

6.Check Oil Seal in Breather,top up if reqd

7.Check Oil for Acdity

8.Tighten Inspection cover Bolts & Nuts Uniformly when reqd

9.Clean Relay Alarm Contacts,their operation,accuracy etc

10.Periodically check Protection Circuits

11.Pointers of all the gauges for free movement

12.Clean the Oil Conservator thoroughly before Erection

13.Check Bucholtz Relay Operation

14.Check Painting and touch up if necessary

15.Check WTI/OTI Oil Pockets and replenish Oil if need

be

16.Release Air through Vent of Divertor before Energising

17.Check Oil Level in Divertor

18.Check Gear Box Oil Level,Top up

19.Replace worn out OLTC contacts as required

20.Check all Bearings and Lubricate

21.Keep the Equalising Pipe Valve between the Tank & OLTC if provided, open during Vacuuming and Oil filling.

22.Connect a N2 Cylinder and keep the valve open, maintain a positive pressre through the Automatic (or Manual) regulator

23.Fill the Transformer with Oil at the Earliest.

24 .Check Door Seals of Marshalling Box and Replace

25. Check whether the Main Tank can withstand Vacuum and if so the Safe Limit.(For EHV class 1.00 Torr)

NoteNote: Radiators and Conservator Tanks cannot Radiators and Conservator Tanks cannot withstand Vacuum withstand Vacuum

MAINTENANCE SCHEDULES (For Power Transformers)

Hourly :

Winding Temperature, Oil Temperature, Ambient

Temperature

Load in Amps (Min/Max),Voltages(Min/Max)

Daily :

Oil Level in Conservator,OLTC,Bushings,Oil Leakages

Monthly :

Breather functioning,Silica gel,Oil seal,Cleanliness of

Bushings,Loose Connections(Hot Spots)

Quarterly : Cleanliness of Bushings,Oil Leakages,BDV and WC of Oil Functioning of Cooler Fans , Oil Pumps Check OLTC Operation-Step by Step,Limit Switch,Brakes. Check Breather functioning and Oil Leaks

Annual : Transformer Oil Tests Alarms and Trips OTI & WTI Pockets Earth Connection (Neutral and Body) Oil Leaks Fan Motors Oil Pumps Oil Levels Divertor servicing

Divertor Switch Oil-Replacement Bushing Connectors for Tightness Test Bucholtz Relay by Air Injection Test Pressure Relief Trip Tan Delta for Bushings and Winding

Major Over HaulMajor Over Haul

(Once in 7 to 10 Years)(Once in 7 to 10 Years)

Detailed examination of the winding,Core, Oil,

Bushings,Protective Devices,Selector& Divertor

Switches etc.

Permissible BDV & WC

Below 72.5kv … 40kv/mt, 25ppm

72.5 to 145kv … 50kv/mt, 20ppm

145 & above … 60kv/mt,15ppm

(The Gap in Testing Kit to be 2,5mm)

IR values at 30deg C for Power & Distribution Transformers:

33/11kv 11kv /440V HV to Earth 250MΏ 200MΏ

LV to Earth 200MΏ 100MΏ

HV to LV 250MΏ 200MΏ

Single Phase Amorphous Metal

TransformerSuitable for : Direct Pole Mounting

Plinth Mounting

Temperature Rise (Above Ambient):

45 deg C by Winding

40 deg C by Oil

Rating :

11kv/240V,4.5% Z,75kv(Peak) BIL,copper

Core:

Shell Type, made of Amorphous Metal (Iron-

Boron-Silicon )Alloy properly annealed,after

winding the core

Windings: Core-LV-HV

Inter Layer Insulation- Epoxy resin Bonded Kraft Paper

Transformer Oil : As per IS 335 (latest)

Tests Before Commissioning :

(1) Physical- Tank Denting

Damaged fittings

Cracked/Broken Bushings

Loose Bolts & Nuts

Loose Earthing

HV Studs Loose/Bent

(moisture may ingress)

(2)Electrical Tests:

a) IR Test (with 1000v Megger)Meggering should be done for one minute

Minimum IR Values at 20deg C:

HV to LV Earthed…500 Mega Ohms

LV to HV Earthed…500 Mega Ohms

HV to LV………….. 500 Mega Ohms

( Temperature Corrections to be applied)

b) Ratio : Apply 220 Volts between primary and measure the

secondary voltage. Should confirm with Rating Plate.

Maintenance ScheduleMaintenance ScheduleMonthly:

(1) Visual Inspection

(2) Check Connections for Tightness

Quarterly :

(1) Visual Inspection

(2) Check Bushings for Cracks/Damages/ Dirt and Clean.

Annual :

(1) Oil Tests for BDV, Acidity & Top Up

(2) Check Earth Resistance

(3) Visual Inspection

Once in Two Years :(Applicable for Transformers without Conservator):

Internal Inspection,Oil Filteration, Tests as required.

Once in Five Years :

Complete Overhaul ( lifting of core & windings,

examining,tightening, replacement of Oil, Gaskets,

Tightening of connections,Checking Earth Connections,

Drying, Pre commissioning Tests,checks)

Completely Self Protected TransformerCompletely Self Protected Transformer

(CSP)(CSP)

(1) It is Hermetically Sealed, Maintenance Free, High

Performance

(2) Does not allow consumer to meddle

(3) Takes care of Over Loads

(4) Allows Emergency Controls

(5) Signals Over Loads

(6) Less Failure Rate (1 to 2% as compared to 20% in

conventional)

Has Internal Expulsion Fuse for HV side

The above mounted inside the HV Bushing in

series with HV Winding.

The Fuse designe do meet 12 times full load current

for 10 milli seconds

LT circuit Breaker is Mounted inside the Tank to

protect against Over Loads

Bimetallic strips utilised to indicate the Over Load

and trip the LT CB

Latching can be adjusted from outside to trip the CB

for higher Loads if it is necessary to close the CB

immediately after it trips to meet over Loads

Magnetic Trips: are provided as an additonal

feature in some CBs to provide Instantaneous

Trip.

Coordination of HV Fuse and LV CB are designed

for Bolted Faults.

LAs are directly mounted on the HV Bushings to

minimise lead lengths

Transformer is completely sealed with Nitrogen

cushion (55% of the Oil volume is provided for

N2 Cushion to allow Expansion Contraction due

to loads and temperature variations)

Maintenance : Clean Body & Bushings Check Bushings for Cracks Check HV&LV Connections for Tightness Check Earth Connection Check Earth Resistance Check for Over Loads Check IR values Overhaul once in 5 years

(Precautions and Pre- Commissioning checks/Tests on

par with other Transformers except Oil tests etc. )

Amorphous Metal Transformers

Iron-Boron-Silicone Metal Alloy is particularly

Excellent

It has Crystalline Structure

When it is melted and suddenly cool ed at a faster

rate, it solidifies without Crystalisation

maintaining Amorphous state.

Advantage: Low Core Losses

Sub-stations Bus Bars : In Door….. Generally Rigid Out Door .. Rigid/ Strung Single Bus Double Bus Double Bus with Transfer Bus Mesh or One & Half Breaker System Material Could be Copper / Alluminium ACSR Conductor/ Al.pipe Sizes normally used: 33kv 19/0.064 cm 40mm dia 132kv 37/0.168 cm 80mm dia 220kv 61/0.157 cm 80mmdia Circuit Breakers : Bulk Oil Circuit Breakers

Minimum Oil Circuit Breakers Air Blast Circuit Breakers Vacuum Circuit Breakers Air Circuit Breakers Vacuum Contactors SF6 Circuit Breakers Disconnecting Switches: Horizontal Break- Centre Post Rotating Two Post Rotating Single Break / Double Break Vertical Break- Single Break Pento Graph. Current Transformers : Live Tank Dead Tank

Indoor (Cast resin) Out Door Core Type (Bushing CT) Ratio Changing can be on the Primary or

the Secondary winding Number of secondary cores could be upto

Five as : (1) Metering (2) Protection (3) Differential (for Gen & Trs) (4) Bus Bar (Main) (5) Bus Bar (Check)

Lightning Arrestors: Zinc Oxide Gap with Blow out Magnetic coil Potential Transformers : Electro Magnetic Transformers Capacitor Voltage Transformers In general there will be two Cores (1) 110 volts (2) 110/sq root of 3 Structures : Steel Cement Concrete

Tele Metering : Wave Trap CVT with LMU Auxiliaries : Earthing Cables : Power / LT /HT Control AC / DC PT / CT PVC / XLPE / Fire Resistant Single / Multi core Copper / Alluminium Stranded / single

Oil handling System Illumination ( Normal & Emergency) Compressed Air system Over Head Crane Fire Protection Sand/water/CO2 / Foam/ Mulsifier system DC Supply : Single / Double Battery AC Supply : Main / Alternate Emergency Diesel generator Double Feed Communication

Safety Clearances : Normal Highest BIL Ph-Ph Ph- E KV KV cm cm 33 36 170 32 40 132 145 550 115 135 650 138 160 220 245 900 196 230 1050 230 270 Sectional Clearnce: 33kv 36kv 300 132kv 145kv 350 220kv 245kv 430

Lightning Arrestors : High Voltages are caused due to : (1) System Instabilty (2) System Fault (3) Switching Discharges (4) Lightning The Duration of HV could be : A few Micro Seconds to A Few Seconds The High Voltage Range could be : 1.15 to 4 times the Normal operating

Voltage.

ZINC OXIDE LAs: Comprises ZnO Poly crystalline element Housed in a hollow insulator No,size depends on syst Kv, Energy class Each Grain has 1 to 10 ohm cm Grain Boundary has 10 to the power of 10 The Resistance of the Inter Granular Layer

decreases at a threshold value Discharge takes place and normalized The Resistance is Non-Linear. There are thre types of Regions/Zones:

(1) Pre-Break down: , 1ma per Cm cube (2) Break Down stage (3) Up Return: > 1000A per Cm Cube Some Important Parameters: (1) COV : Normal Voltage/sq rt of 3 (2)MCOV : Max sys KV/sq.rt.of 3 Rated KV = Max sys kv * GFF GFF (Ground Fault Factor)= 0.8 for Solidly earthed system 1.0 for In effectively Earthed system LA Discharges: 5kA to 20kA.

Pressure Relief Diaphragm is provided as an Exhaust Vent to safe guard equipments

Las designed on Pollution Levels : Light…………….16mm Medium…………20mm Heavy…………..25mm V.Heavy………..31mm Other Conditions: Ambient : +50C to -10C Altitude : Above 1000mts Location : Corrosive / Polluted /Smoke Chemical / Contamination

Abnormal Frequency : < 48HZ or > 62HZ Abnormal Conditions: Earth Quake Vibrations High Wind Icing Health Monitoring : (1) Check Surge Counter Rading regularly (2) Check Leakage Currents (3) Check Exhaust Vents

Current Transformers: The Primary winding is generally a Bar. It could consist of two or more turns to save copper of the

secondary winding There is no appreciable voltage drop in the Primary. A Heavy current could flow through the Primary at a High

Voltage. The current in the Primary is not dependant on the Load

on the secondary. A number of secondary windings (called cores,Ring

Type) are placed around the Primary. The Classification could be for metering,protection etc The Iron Core must be of Low Loss, Low Reluctance

type must have Low Flux Density (about 1000 lines /sq cm). MUMETAL (Ni-Fe Alloy) is used as Core.

The Ratio changing could be on Primary or Secondary.

If you have a 300/1 CT, and if the Primary is a single turn Bar, the secondary will have 300 turns. If you want a 150/1 Ratio, the secondary must have a tap at 150 turns.

Alternately, if the primary has two turns, if the two turns are connected in series,by Ratio Changing Links, the rated 1 amp current corresponding to 150 A flows in the secondary.

If the Primary is connected in parallel, the secondary will still deliver 1 amp but will correspond to 300 A primary current.

Generally a Current Transformer in the EHV class is hermetically sealed,and is Maintenance Free (Internal)

Important : The Secondary Winding should not be left open or loose.Dangerously High Voltages will appear.

CLEARANCES : (mm) voltge For equipment ph-ph spacing in rating ph-ph ph-E Isol Sw (AH) 11kv 460 310 610 920 33kv 920 610 760 1220 132kv 1700 1020 2140 3050

Clearances in Strung Bus (mm) KV Rating Space between conductors 220 4500 132 3600 33 2200 11 1300 Ground Clearances : (mm) 220 kv 5500 132kv 4600 33kv 3700 11kv 3700

Sectional Clearances (mm): KV Rating Clearance 220 4300 132 3500 33 2800 11 2600 Clearance for Indoor Equipment (mm): KV Rating Ph- Ph Ph - Earth 33 356 222 11 127 76 (when conductors, bus are not insulated)

Gap Settings:(mm) Sys KV Tr Bushg gap TT Gap GapAT 33 none none none 132 800 735 760 220 1140 1120 1120