94647184 Transformers Stdm

24 May 2012 PMI Revision 00 1

Transformers


TRANSFORMERS


Presentation outline

Need

Working Principle

Types of Transformers

Construction Features

Transformers Accessories

Major Transformers in Power Plants

Transformer Losses

Condition Monitoring of Transformers


WHY TRANSFORMER

• TO REDUCE OR INCREASE VOLTAGE/CURRENT IN AC

SYSTEM

• TO OPTIMISE COST OF BULK TRANSMISSION OF

POWER FROM GENERATORS TO CONSUMERS

• ENABLES SAFE SUPPLY VOLTAGE TO CONSUMERS

• ISOLATION OF TWO SYSTEMS FOR VOLTAGE

REGULATION

• MATCHING SOURCE AND LOAD IMPEDANCES FOR

MAXIMUM POWER TRANSFER IN ELECTRONIC AND

CONTROL CIRCUITARY


IEC 60076-1

A static piece of apparatus with two or more windings

which, by electromagnetic induction, transforms a

system of alternating voltage and current into another

system of voltage and current usually of different

values and at the same frequency for the purpose of

transmitting electrical power.

IEEE C57.12.80-2002

A static device consisting of a winding, or two or more

coupled windings with or without a magnetic core for

introducing mutual coupling between electrical

circuits.

Definition of Transformer as per Stds


Working Principle • If a time-varying voltage is applied to the primary winding

of turns, a current will flow in it producing a magnetomotive force (MMF). Just as an electromotive force (EMF) drives current around an electric circuit, so MMF tries to drive magnetic flux through a magnetic circuit. The primary MMF produces a varying magnetic flux in the core, and, with an open circuit secondary winding, induces a back electromotive force (EMF). In accordance with Faraday's law of induction, the voltage induced across the primary winding is proportional to the rate of change of flux:

http://en.wikipedia.org/wiki/Magnetomotive_force



http://en.wikipedia.org/wiki/Electromotive_force



http://en.wikipedia.org/wiki/Magnetic_circuit



http://en.wikipedia.org/wiki/Magnetic_flux






http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction








Working Principle

Vp/Vs = Np/Ns where

• Vp and Vs are the voltages across the primary winding and secondary winding,

• Np and Ns are the numbers of turns in the primary winding and secondary winding,

• dΦP / dt and dΦS / dt are the derivatives of the flux with respect to time of the primary and secondary windings.




What are the Types of

Transformers

Power transformers : Used in transmission

network of higher voltages, deployed for step-up

and step down transformer application (400 kV,

200 kV, 132 kV, 66 kV, 33kV,22kV)

Distribution transformers: Used for lower

voltage distribution networks as a means to end

user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V,

230V)


Classifications

Transformers are adapted to numerous engineering applications and may be classified in many ways:

• By power level:

(from fraction of a volt-ampere(VA) to over a thousand MVA),

• By application:

(power supply, impedance matching, circuit isolation),

• By frequency range:

(power, audio, radio frequency(RF))

• By voltage class:

(a few volts to about 765 kilovolts)


Classifications

• By cooling type:

(air cooled, oil filled, fan cooled, water cooled (Natural/ Forced) etc.)

• Connection :

Single phase, Star / star, Star delta etc

• Construction :

Core Type Shell Type

• By RATIO of the number of turns in the coils

Step-up The secondary has more turns than the primary.

Step-down The secondary has fewer turns than the primary.


Classifications

• Isolating

• Intended to transform from one voltage to the same voltage. The two coils have approximately equal numbers of turns, although often there is a slight difference in the number of turns, in order to compensate for losses (otherwise the output voltage would be a little less than, rather than the same as, the input voltage).

• Variable

• The primary and secondary have an adjustable number of turns which can be selected without reconnecting the transformer.


COMPONENTS

• Current conductors – Primary & secondary Windings

• Magnetic flux conductor - Laminated Steel Core

• Insulation

• Cooling

• Protection

• Supporting accessories


CORE EARTHING

• Core earthing:

• With the exception of individual laminations and

core bolts, all internal parts of the transformer

require earthing

• Due care is taken for earthing system to avoid

multiple paths which may initiate partial discharges

because of circulating currents inducing relatively

high voltages across high impedences of an arc

path.


BUSHING

• BUSHING:

• The high voltage connection pass from the winding to terminal bushings. Bushings up to 36KV class, 3150 amps are normally of plain porcelain and oil communicating type. Higher current rated bushings and bushings of 52 KV and above are of oil impregnated paper condenser type. Th oil inside the condenser bushing will not be communicating with the oil inside the transformer. Oil level gauge is provided on the expansion chamber of the condenser bushing.


Core

-The “conductor” for the flux

-The skeleton for mechanical rigidity of the active

part

-The core, an unbroken path for magnetic flux

CRGO or Cold Rolled Grain Oriented Steel is available

in various grades (generally called M3, M4, M5 & M6).

COMPONENTS - CORE


Core

• In all types of transformers, the core is constructed of sheet steel lamination to provide continuous magnetic path with a minimum of air gap. The steel used should have high permeability and a low hysterisis loss at the usual operating flux densities. The eddy currents loss is minimised by laminating the core with the laminations being insulating from each other by a high coat of core plate varnish or by an oxide layer on the surface. The thickness of laminations vary from 0.30 mm to 0.5mm


Types

The transformers are of two general types

distinguished from each other by the manner in

which the primary and secondary coils are placed

around the laminated steel core. They are

(a) shell type and

(b) core type.



Transformer Construction


CORE Type Transformer


SHELL Type Transformer


Wound core


Stacked core


Transformer Insulation

• Minor insulation Like inter turn insulation, is achieved using cellulogic paper.

• Major insulation Between primary and secondary, phase to phase and inner coil to core. This is achieved by Bakelite, wooden blocks, cellulogic paper cylinders.

• Transformer Oil: derivative or petroleum crude. This has good dielectric strength.

also a good cooling medium and absorbs heat from the windings in transformer.


Transformer Insulation

• mineral oil has a flash point of 140°C and 160°C fire

point. This also 'can Sustain the combustion with its

own energy, once it catches fire. Thus this is unsuitable

for the transformer located indoors.

• The indoor transformers are filled with a synthetic liquid

known as silicate liquid. This is fire assistant and has

flash point well above 300°C.

24 May 2012 28

Transformer Oil


• NAPTHANIC BASE OILS GENERALLY HAVE HIGHER

RESISTIVITY VALUES WHEN COMPARED TO

PARAFFINIC BASE OILS AND HAVE BETTER OXIDATION

STABILITY.

• EQUALLY GOOD PARAMETERS CAN BE ACHIEVED

WITH PARAFFINIC BASE OILS ALSO, WHEN PROPERLY

REFINED.

• OIL PARAMETERS ARE IMPORTANT. BASE OF OIL IS

NOT IMPORTANT(NONE OF THE STANDARDS SPECIFY

THE BASE OF OIL)

USE OF TRANSFORMER OIL WITH CRUDE BASE AS NAPTHANIC/ PARAFFINIC


• PRESENT NTPC SPEC. IS IN LINE WITH IS:335 (1993).

• SUGGESTED IS TO HAVE OIL WITH LOW VISCOSITY

AS COMPARED TO PRESENTLY BEING USED FOR

BETTER COOLING AND FOR BETTER OIL FLOW.

• BHEL IS IN THE PROCESS OF FURTHER UPGRADING

THE TRANSFORMER OIL PARAMETERS FOR HIGHER

VOLTAGE CLASS TRANSFORMERS TO HAVE BETTER

STABILITY OF OIL CHARACTERISTICS.

USE OF TRANSFORMER OIL WITH CRUDE BASE AS NAPTHANIC/ PARAFFINIC


Tap Changer(s)-(On load/Off load)

• Tank

• Radiators

• cooling fans, oil pumps, oil to water heat exchangers

(Cooling ONAN / ONAF/ OFAF/ OFWF external

coolers)

• Bushings

• Buchholz Relay/Oil Surge Relay

• Temperature Indicators- WTI , OTI

• Oil Level Indicators

• Pressure Relief Device

• Marshalling Box/Control cubicle

• Oil Preservation Systems: Conservators (gas sealed,

Bellows/membrane sealed) Breathers

Thermo siphon Filters

Accessories & Auxiliaries



RADIATORS

24 May 2012 34

Buchholz Relay


Buchholz's Relay

• This has two Floats, one of them with surge catching baffle and

gas collecting space at top. This is mounted in the connecting pipe line between conservator and main tank.This is the most dependable protection for a given transformer.

• Gas evolution at a slow rate, that is associated with minor faults inside the transformers gives rise to the operation of top float whose contacts are wired for alarm. There is a glass window with marking to read the volume of gas collected in the relay. Any major fault in transformer creates a surge and the surge element in the relay trips the transformer. Size of the relay varies with oil volume in the transformer and the mounting angle also is specified for proper operation of the relay.



• Alarm element Operates When a specified volume of

gas gets collected in Chamber during

Broken down core bolt insulation

Shorted Laminations

Bad Contacts

Overheating of winding parts

• Trip element Operates by Oil surge in the event of

serious fault

Short Circuit between Winding Phases or

within Windings

Puncture of Bushing

BUCHHOLZ PROTECTION



Off Load Tap Changer



Conservator

• Conservator With the variation of temperature there is corresponding variation in the oil volume. To account for this, an expansion vessel called conservator is added to the transformer with a connecting pipe to the main tank. In smaller transformers this vessel is open to atmosphere through dehydrating breathers (to keep the air dry). In larger transformers, an air bag is mounted inside the conservator with the inside of bag open to atmosphere through the breathers and the outside surface of the bag in contact with the oil surface.


Buchholtz relay


SILICA GEL BREATHER

conservator


Breather

• Both transformer oil and cellulosic paper are highly hygroscopic. Paper being more hygroscopic than the mineral oil The moisture, if not excluded from the oil surface in conservator, thus will find its way finally into the paper insulation and causes reduction insulation strength of transformer. To minimise this the conservator is allowed to breathe only through the silicagel column, which absorbs the moisture in air before it enters the-conservator air surface.


•The Drycol Breather (A) operates by drawing moist, warm air (B) from an air space or from outside via the air inlet tube (C), and returning it to the conservator (D) as cold dry air. A series of thermoelectric modules are used to alternately cool and heat the units vertical central duct (E) in an automatic continuously repeated cycle, extracting moisture as frost and ice, and melting it to escape via a drain tube (F).

http://www.power-technology.com/contractors/switchgear/alstomtd/


Winding / Oil Temperature Indicator


Pressure Relief Device/Expansion vent

• Transformers tank is a pressure vessel as the

inside pressure can group steeply whenever

there is a fault in the windings and the

surrounding oil is suddenly vaporized. Tanks as

such are tested for a pressure with stand

capacity of 0.35 Kg/ cm". To prevent bursting of

the tank, these tanks are in addition provided with

expansion vents with a thin diaphragm made of

bakelite/copper/glass at the end. In present day

transformers, pressure relief devices are

replacing the expansion vents. These are similar

to safety valves on boilers (spring loaded).

24 May 2012 48

Pressure Relief Device( PRD)


Temperature Indicators

• Most of the transformer (small transformers have

only OTI) are provided with indicators that displace oil temperature and winding temperature. There are thermometers pockets provided in the tank top cover which hold the sensing bulls in them. Oil temperature measured is that of the top oil, where as the winding temperature measurement is indirect. This is done by adding the temperature rise

• due to the heat produced in a heater coil (known as image coil) when a current proportional to that flowing in windings is passed in it to that or top oil. For proper functioning or OTI & WTI it is essential to keep the thermometers pocket clean and filled with oil.


Cooling of Transformers

• Heat is produced in the windings due to current flowing in

the conductors (I- R) and in the core on account of eddy

currents and hysteresis losses.

Types: ONAN, ONAF, OFAF, OFWF

• Air Cooled: In small dry type transformer heat is dissipated

directly to the atmosphere.

• Oil Cooled: In oil immersed transformers heat is dissipated

by thermo-syphon system action. The oil serves as the

medium for transferring the heat produced inside the

transformer to the outside atmosphere. Based on Thermo-

syphon principle.


Cooling of Transformers

• Forced Oil forced Air: As the size of the transformer becomes large, the rate of oil circulating by thermo syphon action becomes insufficient to dissipate all the heat produced and an artificial means of increasing the circulation have to be adopted; namely forced oil circulation by electric pumps, providing large radiators with forced air draft, cooling by electric fans which are automatically switched on and off depending upon the loading of transformer. In very large transformers special coolers with water circulation may have to be employed.


Different transformers in a power

Plant

• Generator Transformer (GT)

Station Transformer (ST)

Unit Auxiliary Transformer (UAT)

Excitation Transformer

Neutral Grounding Transformer

Auxiliary transformers

Tie / Auto transformer


Generator Transformer

• Generator Transformer The generator is connected to this transformer by means of isolated bus ducts. This transformer is used to step up the generating voltage of around 15KV to grid voltage. This transformer is generally provided with OFAF cooling. It is also provided with off circuit/on load taps on the high voltage side. This transformer has elaborate cooling system consisting of number of oil pumps and cooling fans apart from various accessories.


SPECIFICATION

STAGE -1 GT-Vindhyachal

RATED O/P 250MVA

RATED VOLT. (HV) 420KV

RATED VOLT. (LV) 15.75KV

RATED CURRENT(HV) 344A

RATED CURRENT(LV) 9175A

VECTOR GROUP Ynd11


Unit Auxiliary Transformer

• The UAT draws its input from the main bus-duct

connecting generator to the generator Transformer. The total

KVA capacity of unit auxiliary transformer required can be

determined by assuming 0.85 power factor and 0.9 efficiency

for total auxiliary motor load. It is safe and desirable to

provide about 20% excess capacity than circulate so as to

provide for miscellaneous auxiliaries and possible increase

in auxiliary load. With higher unit ratings and higher steam

conditions, the auxiliary power required also increases and

limitations imposed by the switchgear voltages available,

indicate the maximum size of unit auxiliary transformer which

can be used.


UAT

• For large units, it has become necessary to use more than one auxiliary transformer. In selecting a unit auxiliary transformer, care has to be taken that the percentage impedences of the transformers for the proposed unit should satisfy the following conflicting requirements.

• The maximum short circuit currents on auxiliary bus should be limited within the maximum switchgear rating available,

• The maximum permissible voltage dip while starting the largest single auxiliary motor, usually boiler feed pump, shall remain within acceptable limits.


GENERATOR

7UA 7UB

SA SB

GT (3X200 MVA)

Unit Tx 21/11 K.V. (50 MVA)

IN THE EVENT OF UNIT TRIPPING AUTO CHANGEOVER FROM UT TO STATION WILL TAKE PLACE


Station Transformers

• The station transformer is required to feed power to the auxiliaries during start ups. This transformer is normally rated for the initial auxiliary load requirements of unit. In typical cases, this load is of the order of 60% of the load at full generating capacity. But in large stations where more than one units are operating, the station transformers should have sufficient capacity to start two units at a time in addition to feeding the common auxiliaries. It is also provided with on load tap changer to cater to the fluctuating voltage of the grid.



Instrument Transformers

• . Step down values to safe levels for measurement

• . Potential Transformers

• . Also called voltage transformers

• . Standard output 110V

• . Current Transformers

• . Standard output of 1 or 5 amps

• . Metering and relaying standards

• . Can produce high voltages if open circuited


Rectifier transformers

• Output of the main transformer is connected to the

rectifier unit.


Transformer connections :

- Delta/Star…used in Generating stations for Step-up.

- Star/Delta…used in Receiving stations for Step-

down

* All GTs are Delta/Star connected.

All Tie T/Fs are Star/Star connected.


STAR / DELTA connection

STAR / STAR connection


STAR DELTA


Transformer losses

1. Load loss (or copper loss)

2. No load loss (or iron loss)

• The total transformer loss, PTOTAL, at any load

level can then be calculated from:

PTOTAL = PNO-LOAD+ (% Load)2 x PFULL LOAD



Transformer Loss vs. Load


TRANSFORMER LOSSES

Winding resistance

Current flowing through the windings causes resistive heating of the conductors (I2 R loss). At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.

• Magnetostriction

Magnetic flux in the core causes it to physically expand and contract slightly with the alternating magnetic field, an effect known as magnetostriction. This in turn causes losses due to frictional heating in susceptible ferromagnetic cores.

http://en.wikipedia.org/wiki/Skin_effect



http://en.wikipedia.org/wiki/Proximity_effect



http://en.wikipedia.org/wiki/Magnetostriction

http://en.wikipedia.org/wiki/Ferromagnetic


TRANSFORMER LOSSES

• Stray losses

Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive objects, such as the transformer's support structure, and be converted to heat.

http://en.wikipedia.org/wiki/Leakage_flux




Copper Losses

24 May 2012 71

On-Line

Monitoring


ON LINE MONITORING EQUIPMENT ON TRANSFORMER

1. DGA to be monitored on all transformers periodically. Periodicity

depends on the nature of evaluation of gases.

2. HYDRANE directly mounted on the transformer and in contact with

transformer oil.

The use of this device is to detect the change in fault gases, to

monitor their evolution. As an alarm is triggered oil is sampled

from the transformer to evaluate the nature and severity of the

fault.


7. ON LINE MONITORING EQUIPMENT ON TRANSFORMER Contd.

3. Acoustic PD measurement.

In case DGA results indicate the presence of high discharges

Acoustic PD measurement is done.

Acoustic PD detector consists of sensor which can sense sound

vibrations produced due to occurrence of discharges in oil.

The sensor has pre-amplifier and filter circuit to eliminate

environmental noise

4 Infrared thermography

With the help of infrared thermo-vision camera, the thermo-image

of transformer is prepared

This scanning helps in detecting overheated zones, loose &

corroded connection.


• CONDITION MONITORING OF TRANSFORMER

• Since Regular Internal Inspection of the transformers is not possible

• Only "Diagnostic tests" mentioned below are possible:

• Dissolved Gas Analysis (DGA) of insulating oil

• - the most significant hydrocarbons (gases) generated due to decomposition of oil are:

• Hydrogen (h2)

• Methane (ch4)

• Ethane (c2h6)

• Ethylane (c2h4)

• Acctelene (c2h2)

• Due to decomposition of cellulose: co and co2 are formed

• Partial Discharge (PD) Tests.

• Sonic and Ultrasonic detection

• Infrared Thermographic inspection


OFF LINE MONITORING

1. Frequency Response Analysis

• Short circuit forces could move the winding.

• Can also change winding inductance and capacitance.

• Conventional tests are not sensitive to winding movement.

• Such changes can be effectively detected by FRA

Method

• Sweep frequency input is given in the winding.

• Input & response recorded.

• FFT analysis of response to obtain the finger print of the winding.

• These finger prints are compared periodically to assess the condition of winding.


2. Recovery voltages measurement

It indicates the content of water in the insulation.

PRINCIPLE

•Sample of discharged insulation

•Charge with DC voltage and note time

•Sample to be short circuited for predetermined period of time

( approximately half of the charged time)

•Open the terminal and measure voltage.

3. Tan- & Capacitance by standard bridge.


4. Resistance & magnetizing current.

5. Maintenance schedule call for daily , quarterly, yearly, two

yearly maintenance requirement. Over and above after 7 to

10 years, thorough overhauling and inspection of core-coil

assembly by lifting tank top cover / core-coil assembly

needed

In overhauling sludge deposited on coil is removed and coil

assembly which may become loose is tightened. All other

clampings & cleats etc. are also tightened


THANK YOU


• IEC-International Electrotechnical Commission

• IEEE-Institute of Electrical and Electronics

Engineers

Documents

94647184 Transformers Stdm