7110100 Transformers

8/9/2019 7110100 Transformers

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Welcome to a session on the

study of Transformers , usedin the sub transmission anddistribution systems and alsoin the EHV Network


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Transformers

1. Principles2. Types3. Materials used in Construction

4. Magnetic Circuit5. Winding & Insulation6. Voltage Regulation7. Cooling Arrangements

8. Auxiliaries9. Loading Life10. Protection11. Case Studies


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Transformer Components

Core Windings HV / LV / Tertiary

Insulating Oil

Tapping Switch

Coolers : Fans / oil pumps / Radiators Bushings HV / LV / Tertiary / Neutral

Terminals

Tank : Main / Conservator (+ OLTC)

Gas Relay : Bucholtz (main / OLTC) / Surge

Over Pressure Relief / Vent Diaphragm Control Cabinet : OLTC M / R / A

Cooler M / A

Temperature Indicators : Oil / Winding


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Types of Transformers

Distribution

Power : HV / EHV class, Single or Three phase

: Two / Three windings: Auto

Oil filled / Dry (Resin cast) / Gas Filled

Completely self protected (CSP) Pole Mounted, Single phase

Pad Mounted 3ph unit


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Transformers Principles

Transfers power from one circuit to another without change infrequency

Works on mutual induction principle Comprises of two or more coils linked to a magnetic circuit

Has one primary and one or more secondary windings Receives power at one voltage on the primary Delivers power on the secondary at another voltage Some formulae : e1 = -L * di / dt

V1 = N1 * d / dt(Counter emf is equal and opposite to applied volts) E1 = 4.44 * f * N1 * d / dt E2 = 4.44 * f * N2 * Bmax * A V1/ V2 = I2 / I1 = NI / N2 = K (Turns ratio)


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Transformer on no load

I0 = ( I2 + Iw2)

Transformer on load :N2 * I2 = N1 * I2 I2 = N2 / N1 * I2 = K * I2

(I2 is the load component of primary current)

Transformer with magnetic leakageZ1 = R1sq + X1sqZ2 = R2sq + X2sqV1 = E1 + I1 * Z1E2 = V2 + I2 * Z2

Percentage Regulation = (V2 V2 / v2 ) * 100 Losses : CoreCopper

% Efficiency = { [ Input losses] / Input }* 100


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Conditions of maximum efficiency occurs when

Copper Loss = Iron Loss

In general a transformertransformer is the most efficientequipment (95%)

In higher ranges 99% efficiency is achievable

Auto transformersAuto transformers are used when the transformation

ratio differs slightly from unity Copper saved in auto transformer = K * wt of copper in2winding Tr

Teritiary :1. Can be unloaded / loaded

2. Delta winding ensures stability

3. Provides path for third harmonic currents

4. Rating depends on usage


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Materials used in construction

70 % of the materials used are : Core Winding Insulating Oil.

Core : In India, distribution Transformers manufactured use 0.27M4grade CRGO.

However, the core losses can be reduced by using other superiorgrades as below: H1 B 0.27 : Reduction 16% H1 B 0.23 : 24% Laser Grade ZD MH-0.23 : 38%

Slitting of core must be done carefully otherwise it adversely affects

quality. MITRING (at 45) reduces Reluctance and reduces core loss. Annealing is to be done at 760 to 845C to

Reduce mechanical stress Prevent contamination Enhance insulation of lamination coating


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Core :

Provides a high permeability closed path for Magnetic Flux

Supports the winding

Consists of laminated sheet steel (CRGO,Amorphous) Limbs - members, Yoke- Horizontal member

Core laminations are very thin, coated with insulation /varnish.

Types: Core - Generally used in India,Europe,USSR. Two , Three , FiveLimbed core construction

Shell- Generally preferred in USA. In this the winding is encircled by Core.The Flux in the vertical limb divides into two paths into the Horizontalyokes.The Cross Section of the Limb is twice that of the Yoke.

Core Type: Most of the Power Transformers are of this type.

In Three Limbed core, the cross section of Limb and Yoke are equal.With

Large Transformers the height of the Transformer should be kept withinTransportation limits.

Five Limbed core used in such cases. The Top and Bottom yokes are madewithin 58% of the area of the principal Limb and the two return Limbs areabout 45% of the Principal Limb.

This gives the choice of weight,Height, and Losses.


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The Core is made up of a number of Laminations

The vertical members carrying winding is called Leg / Limb

The Horizontal member at the Top and Bottom is called Yoke.

The Limbs and the Yokes are stacked separately. In construction, the Laminations of the Limb and Yoke are interleaved.

A long lamination of the Limb is matched with a short lamination of the Yokein one Layer. In the next layer, a short lamination of the Yoke is matchedwith a long layer of the Limb. This method of alternate long and shortlaminations form a stack in interleaved construction.

Where the lamination of the Limb and the Yoke meet, the joint could be Butttype or Mitred

In the Butt joint, the flux changing from vertical to Horizontal experiencesReluctance.

In Mitred Joint, normally used with CRGO, the laminations are so placed theLines of flux flow in the direction of rolling in the Limbs/ Yokes/ Joints.

This reduces the Reluctance of the Magnetic path, Iron Losses andMagnetizing current.

No Load Loss is reduced by 12%

Excitation Current reduced by 25%


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The Limb and the Yoke are made of a number of laminations in steps.

Each Step comprises of a some number of laminations of equal width.

The width of the central step is Maximum and that at the circumference isMinimum.

The cross section of the Limb and Yoke is nearly circular.

The Mitred joint could be at 45 or 35 to 55 degrees. But the former reduceswastage.

CLAMPING :

Ensures mechanical rigidity and provides required magnetic characteristic. Top and Bottom yokes are clamped by steel sections using number ofinsulated yoke studs.

These studs do not pass through the core but held between steel sections.

Of late, Glass fibre bands, are wound over the Limbs tightly with the desired

Tension and heat treated This utilizes the CRGO much better.


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Clamping methods without using Core Studs :

(1) Glass Fibre Insulated Steel tape are tightened by insulating Laminatedwood or Reinforced synthetic Resin Bolt.

(2) Resin impregnated glass fibre bands are used for tightening the coreassembly and baked at high temperatures.

(3) The core is temporarily taped . A closely fitting SRBP cylinder is insertedto insulate the limb and clamp the laminations.

These methods can be used for Yokes also.

Hysteresis Loss : The Alternating voltage applied to a winding producesalternating Magnetic Flux and the crystals of the silicone steel are subjectedto such fluxes and result in spending of the energy. This loss is calledHysteresis.

Eddy Current Loss : When the Alternating Magnetic flux flows through thecore, a small emf is induced in the core which causes a circulating

current.This is called eddy Current and the energy as Eddy Current Loss. Magnetostriction: or generally called as Humming, is an inherent property of

Laminated steel and is due to the elongation and contraction of thelaminations. Higher silicone content will reduce the Hum but the laminationwill be very brittle.


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Winding MaterialCopper - Usually in 8 16mm rods is drawn to therequired sizes and then insulated with paper etc..

Annealing is done for softening and stress releaving in electrically heatedannealing plant under vacuum upto 400-500C.

After 48hrs when the temperature reaches ambient, the vacuum is slowlyreleased and the material is transferred to Insulation section.


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WINDINGS

Types:

Distributed Spiral Helical Continuous disc Transposed Interleaved Disc Shield Layer

Distributed Winding:

Used for HV windings where current does not exceed 20 amps. Used in small capacity Distribution Transformers.

Circular cross section Conductor is used. The coils are joined in series spaced with Blocks which provide

Insulation and Cooling.


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SPIRAL :

Used up to 33kv,for Low Currents.Strip conductors are used. Wound closely on Bakelite/Press Board cylinders without cooling

ducts.

Multi layer windings are provided with cooling ducts betweenlayers No transposition is necessary as the lengths are identical. Normally used on Flat side. But some times wound on edge.The

width and thickness of the strip must be adequate to preventtwisting.

Helical :Used for low Voltage, High Currents

A number of conductors are used in parallel to form one turn. The turns are wound axially, in helix. Each turn is separated from the other by a duct. Could be Single/Double /Multi Layer Winding Transposition is done to reduce Eddy Losses. Each Conductor within a coil is not of the same length, does not

embrace same Flux, has different impedances, hence circulatingcurrents, and therefore Transposed.


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Continuous Disc : Used for voltages between 33 and 132kv only for medium currents. Coil comprises of a number of Sections placed axially.

Cooling Duct is provided between each section. Each section has Flat Coil , has more than one Turn. Each turn may have more than one conductor (up to 5)Interleaved Disc :

Used above 145kv

Interleaving makes the coil to with stand higher impulse voltagesShielded Layer : Used up to 132kv in star connected transformers with graded

insulation Consists of a number of concentric spiral coils. These are arranged in layers Layers are graded longest at neutral and shortest at line end Layers are arranged in concentric shields Layers are separated by cooling ducts This ensures uniform distributed voltages


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VOLTAGE REGULATION

Offcircuit Tap changer

On load Tap changer(voltage is changed by changing the number of turns in the primary or

secondary windings)

Offcircuit Tap changer :

1. Cheapest

2. Transformer has to be taken out of service

3. Operating crank is provided outside the transformer

4. Tap positions are indicated

5. Interlocks are provided6. An insulating shaft is connected to the moving contact

7. Fixed contacts are provided on terminals

8. Switching sequence


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On load Tap changer :

1. Can be used on load2. Transition resistors / reactors used

3. Transition reactors are used only in USA4. Transition resistors are used in Diverter Switches5. Selector switch , inside the main transformer selects the tap6. Switching sequence as per transparency

Tap changing operation takes about 40 80 ms Single / double transition resistors can be used Single / double compartments can be used Selector and diverter switches are located in one compartment in

single compartment type Double compartment type has separate compartments for

selector and diverter Tapping is provided on the neutral end of the HV winding OLTC is provided on the neutral end of the HV windings Tap changer could be one three phase or three single phase units


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Reversing switch helps in wider voltage regulation The diverter switch has make before break contacts The tap selector contact are arranged in two circles ,one even ,the

other odd taps Main Bucholtz relay takes care of the selector faults OLTC surge relay takes care of faults in the diverter Taps can be controlled manually / from remote / auto Tap changing is done step by step

Some transformers have tripping scheme if the time exceeds Limit switches cut off the motor supply at the end of the taps Insertion of operating crank disconnects A/C supply to the motor In manual operation turning the crank further at the end taps

disengages the drive shaft of the motor The motor drive has all the electrical and mechanical interlocks ,

auxiliary switches , raise/lower push buttons , contactors, terminalblocks etc

A/C supply is automatically cut off to the control circuit if the pushbutton is held depressed during the tap changing operation


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Dry Type Transformer: It is one in which the insulating medium surrounding the windings is

Gas or Compound.

It does not use the insulating oil.

The windings are coated with Varnish / Resin under vacuum.

Vacuum impregnated with varnish protects winding against Moistureand Contaminants.

They are Single / Three Phase They could be ventilated / Non Ventilated / Sealed.

It could be Indoor / outdoor

It is Non Inflammable

It has improved Thermal capability

It is applicable in areas Exposed to Fumes/ Vapour/ Dust/ Steam/Salt Spray/ Dripping of water / Snow.


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Completely Self Protected Transformer ( CSP):

Has a Protective Link in the primary Has a Circuit Breaker in the secondary

Has an External la for surge protection

The Transformer is protected against Over loads

It is protected against surges

It protects the external system from external faults

The LT breaker is provided inside the tank

An indication is received when the LT CB operates HV Link is mounted separately inside the Tank.


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Amorphous Metal Distribution Transformers:

AM comprises of Iron, Boron, and Silicone alloy in the form of Molten alloy.

It is rapidly cooled such that it cannot form crystals. It is non crystalline andhas random molecular structure. Hence Hysterisis Loss is Low.

The Metal is extremely hard. But the sheets are very thin. Hence Eddy Current Loss is Low. It has Low Hysterisis Loss.

In respect of AM, when the AC supply is applied, the alternating magneticfield applied to the metal causes the atoms reorient and return to the sameoriginal orientation. In CRGO , a friction is caused resulting in heat andexcessive loss.

AMT uses 70 to 80% less energy than the CRGO.

The space factor for CRGO is 96% where as for AMT it is 80%. SpaceFactor is defined as the ratio of Core Cross section to the Area available forthe core.Lower the Space Factor, cores have to be larger and heavier.

The initial cost of a AMT is therefore 25 to 30 % more than a CRGO However, the TOC is considered ie when the loss is capitalized, the AMDT

is economical and beneficial to the user since it makes available additionalpower, the pollution levels are low, and also helps in demand sidemanagement of the load.


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CONSERVATOR TANK:

It is an Expansion Vessel

It maintains oil in the Transformer above a Minimum Level It has a Magnetic Oil Level Guage.

It can give an alarm if the oil level falls below the limit

A portion of the Tank is separated for use with OLTC.

This usually has oil level indicators

Main Conservator Tank can have a Bellow It has an oil filling provision

It has an oil drain valve

Provision is there for connecting a Breather.

HIGH voltage Bushings:

Types: Porcelain Condenser

Porcelain Bushing usage is limited to 36kv

They are not used in EHV as they are Bulky.


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A Current carrying conductor passes through a PorcelainInsulator filled with Insulating Oil, Air, or Gas.

Condenser Type Bushings:

The Insulation wall thickness is made of a number ofcapacitors by concentric conducting cylinders. Better capability to with stand Electric Stress. The Aluminium foil used in the condenser is

automatically earthed through a Cap provided at theBottom

This helps in measuring the Tan delta Valuesperiodically to keep an eye on the insulation

The Core Insulation is :SRBP- Synthetic Resin Bonded Paper

OIP - Oil Impregnated PaperRIP - Resin Impregnated Paper Resin Coated Paper/ Kraft Paper/ Crepe Kraft Paper

are used for making core for the above It is Hermetically Sealed.


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SILICA GEL BREATHER:

Prevents Moisture Ingress Connected to Conservator Tank

Silica Gel is Blue when Dry; Pink when moist Oil Seal provides a Trap for Moisture before passing thro Silica

Gel

COOLING:

The Oil serves the dual purpose of insulating medium and

coolant .The Heat generated inside a Transformer is dissipated to theAtmosphere orWater through the insulating oil.

The dissipation ofheat is necessary to control the temperaturewithin limits of the class of insulation used.

This ensures longer life and less thermal degradation of

insulation.For distribution Transformers, sufficient number ofcoolingtubes are provided to limit the temperature rise.

For Power Transformers , both natural and Forced Air / oil /water cooled methods are adopted.


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By Forced cooling the Transformer capacity can be increasedby more than 50%.

Types of Cooling : ONAN- Oil Natural, Air Natural

ONAF - Oil Natural, Air Forced OFAF - Oil Forced, Air Forced

OFWF- Oil Forced, Water Cooled.

The Power Transformer will have normally two sets of Radiator

Banks. Heat is directly dissipated to atmosphere through theRadiator Banks in ONAN system.

In ONAF, the capacity of the Transformer (ie ONAN rating) canbe increased to its full designed capacity by Installing cooler

fans below or on the surface of the Radiators to take away theHeat at a faster rate.

In OFAF, the Oil is forced and the Air is also forced. Thedisadvantage is that, the Transformer does not have a ONANrating. Auxiliary supply must always be ensured.


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Radiators can be of two types :

(1) Tank Mounted- For Small Capacity Trs

(2) Separately Mounted- for Large Trs Normally standby Fan for each Bank and

Standby Oil pump is specified to take care of

failure of any Fan/ Pump. In OFWF cooling system, the pressure of Oil inthe cooler is maintained above the pressure ofwater.The inlet temperature of Water should notexceed 30C. A complete standby water cooleris maintained which must be designed to betaken into service when desired. Normally

Hydro station Gen Trs have this cooling


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Flow indicators with alarm is provided forOFWF cooling.

Alarm is provided for Cooler Fan failure/ ACauxiliary supply failure

Cooler fans and Oil pumps are controlled bywinding Temperature MercurySwitches.Insulating Oil Reqmt:

1.0/lt per kva for Trs from 400 to1600kva

0.6lts per kva for Trs from 1600 to 80000kva

0.5lts per kva for Trs above 80000kva


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PROTECTIVE DEVISES:1. BUCHOLTZ RELAY:

It is a Main Gas Operated Protection It is mounted on the pipe between the Conservator & Main tank.

The above pipe is inclined at an Angle of 3 to 7 degrees

It has a Float and two sets of Mercury Switches

It has a Glass Guage marked in CC

The Relay can give an Alarm or Send a Trip signal for Isolation Gas Trapped inside operates the above contacts.

It has an Air Release Cock and a Drain cock

A Stop Valve Provided helps in Testing the Relay

The Relay should be periodically Tested by Air Injection

2. TEMPERATURE INDICATORS:(1) For Oil Temperature

(2) For Winding Temperature


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They Indicate Maximum Temperature, Resettable They indicateCurrent Temperature.

Each Indicator has Two sets of mercury switches operated byTemperature to Give Alarm and Trip.

The WTI has additional Switch for Auto Operation of CoolingEquipment.

The Coolers can automatically Start or Stop.

It has a Temperature sensing Bulb placed in a Socket on the Top ofthe Transformer tank.

Two capillary Tubes connect the Bulb and the Instrument

One Capillary Tube connects the Operating Bellow

The second one connects a compensating Bellow

The Operating System is filled with a Liquid which changes its

Volume as the Temperature varies The Compensating Bellow acts on the Operating bellow to

compensate the Ambient Temperature.

The Bellow expands or Contracts as the Temperature varies


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This is Transmitted to the Temperature Indicator.

The Graduated Temperature settings can be adjusted

OTI & WTI work on the same principle but in WTI anadditional bellow heated by a resistor in the WTI CTcircuit which represents the actual winding temperatureis provided.

As It is not possible to measure the WindingTemperature directly,it is measured by Thermal imaging

Temperature variations in the heating element isproportional to the Winding temperature.

It simulates the increase in the winding Temperature

over Oil Temperature. This indicates the Hot Spot Temperature.


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Transformer under Short Circuit :

Electro Magnetic Forces are caused by Short Circuits both External andInternal

These Result in : (1) Radial Force (2) Axial Force Radial Force Squeezes Inner Winding and Bursts External windings Axial Forces Tend to Bend the Conductor Axially

Some Methods adopted by the Manufacturers for strengthening the

Transformers against Short Circuits are :

(1) Providing adequate Radial supports for Inner Coils(2) Pre Compressed Insulating Material used to prevent Shrinkage

when in service.(3) Proper Processing and Pre Stressing to obtain Dimensional

Stability.Hydraulic Dampers/ springs provided in certain cases to absorb Impact

due to Short Circuit.


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Conditions for parallel operation1.Same voltage ratio

2.Same % impedance

3.Same polarity and phase sequence

4.Same Vector Group

Item 1 must be satisfied to a close value.

Item 2 deviation results in unequal loading.

Item 3 must be satisfied absolutelyItem 4 depends on Groups and Time indices

There are several Groups and Time indices as below :


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There are several Groups and Time indices as below :

Group-1 Time indices 0, 4, 8 ex: Dd0, Yy0, Dz0

Group-2 6, 10, 2 ex: Dd6, Yy6, Dz6

Group-3 1, 5 ex: Dy5, Yd5, Yz5 Group-4 7, 11 ex: Dy11, Yd11, Yz11

Transformers in Group 1&2 can be paralleled in their ownGroup.

Transformers in Group 3 & 4 can be interconnected for paralleloperation .

Normally the Vector Group of Distribution Transformers areDy11.

The Power Transformers are Yy0, Yd1 (GeneratorTransformer), Dy1, Dy11

It is possible to have different Vector Groups but the mostcommonly used connections are the Dy11,YY0, Yd1.


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LIFE OF A TRANSFORMER:

Being static, a Transformer designed and constructed as per

standards almost Never Dies, it is Killed by improper maintenanceand abnormal stresses.

Failure is therefore due to Temperature, Moisture over a period oftime and due to operational stresses (electro magnetic, thermal,mechanical beyond the strength of the parts which have deteriorated

over a period of time. It is not possible to precisely determine the life of a Transformer.There is no scientific basis on which the Risk of continuing theTransformer in service exists.

However deterioration of Insulation, due to various factors especially

the temperature is the one causing failure The table shown gives the loading capabilities of a transformer at aplace like Hyderabad based on several historic data.

SPECIFICATIONS


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SPECIFICATIONS

1. Mandatory2. Supplimentary

3. Additional

Mandatory :

KVA RatingVoltage ratio

System earthingNumber of phasesFrequencyTapping / SwitchVector groupService conditions indoor / outdoor

Terminations Cable / outdoorCoolingWinding materialAltitudeMounting


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Supplementary :

No load current and losses Load losses

Percentage impedance

Temperature rise over ambient

Basic insulation level

System fault level

Specific requirement of fittings / accessories

Provision of additional Neutral

MVO / WTI / OTI / MB

Limitation of flux density Protection HV fuses / LV MCB

Size of ACSR cable

Dimensional limitation if any

Evaluation of cost by loss capitalization


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Additional Specifications

Core constructions Core lamination Stray loss Tank thickness

Gasket quality Explosion vent (Bursting pressure) PR Valve External / Internal clearances Noise level Foundation details Track guage Space


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Some Interesting News:

Toshiba Electric co. started manufacturing Transformer in 1894

The Transformers produced by them are:

1909 44 kv, 4.5 MVA 1917 110 kv 13.2 MVA bank

1939 220 kv, 80 MVA

1961 330 kv, 300 MVA

1968 525 kv, 1200 MVA

1982 765 kv, 805.5 MVA

1985 515 kv,1260 MVA

1988 765 kv, 1650 MVA

Adopts a mitre core joint To withstand short circuits, A pressure is applied with a Hydraulic

jack on thick annular insulating plate on the top, a perfectly dried precompressed press board is used.All insulation pretreated.


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To prevent large Leakage flux, a magnetic shield is provided on theinner surface of he tank, and clamp surface opposite of the coil. Alsoslits are provided on core leg clamping plates and so on inorder to

reduce stray losses to prevent overheating.Also Non Magnetic steelis used in the vicinity of large current leads.

Normally 3 phase Transformers, three cores are used.

Some times, for large capacity transformers, four or Five cores areused in order to reduce the Transport height.

Interleaved Disc wiinding, Continuous Disk Winding, Helical coilWinding are generally used.

Partial Discharges can take place due to :

(1) Incompletely dried insulation

(2) Voids in insulation

(3) Floating Material (4) Edged Electrodes

(5) Dust, Foreign Matter

(6) Concentrated electric stress applied to oil gap


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To prevent all these, the assembly must be made in Dust proofenvironment, proper drying and oil treatment methods adopted,for avoiding voids oil impregnable glue is to be used on insulation

like paper, press board.Tests that are done before Commissioning:

1. Insulation Resistance and the corresponding Temperature

2. Turns Ratio in all taps

3. Impedance Test

4. Short Circuit Test

5. Vector Group Test

6. Core Insulation Test

7. Polarity Test

8. Core balance Test9. Dielectric test on Oil.

10. No Load current Test


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7110100 Transformers

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