transformers stdm

September 8, 2014 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 OPTIMISE COST OF BULK TRANSMISSION OF POWER FROM GENERATORS TO CONSUMERS

• REDUCTION IN TRANSMISSION LOSS

• TO REDUCE OR INCREASE VOLTAGE IN AC SYSTEM

• ENABLES SAFE SUPPLY VOLTAGE TO CONSUMERS

• ISOLATION OF TWO SYSTEMS FOR VOLTAGE REGULATION

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 C57.12.80-2002A 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

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, 110 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)

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:

Working Principle

Vp/Vs = Np/Nswhere

• 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.

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.) By purpose:

(distribution, rectifier, arc furnace, amplifier output, etc.). 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.

Connection : Single phase, Star / star, Star delta etc

Construction : Core Type Shell Type

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.

• •Current conductors – Primary & Secondary Windings•Magnetic flux conductor - Laminated Steel Core and also

Insulation •Cooling •Protection •Supporting accessories

Basic Components of a Transformer

-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

• 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

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

• Thus 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.

September 8, 2014 26

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) BreathersThermo siphon Filters

Accessories & Auxiliaries

RADIATORS

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 or 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 thermoelectr ic modules are used to alternately cool and heat the units vert ical central duct (E) in an automatic continuously repeated cycle, extracting moisture as frost and ice, and melt ing it to escape via a drain tube (F).

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).

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.

• 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

GT (3X200 MVA)

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

IN THE EVENT OF UNIT TRIPPING AUTO CHANGEOVERFROM 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 120V• . 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 PLOAD

Transformer Loss vs. Load

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.

• 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.

Copper Losses

Core Losses

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 TRANSFORMERContd.

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.

OFF LINE MONITORING1. 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 FRAMethod• 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-δ & 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

transformers stdm

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