A

Training Report

On

CONDITION MONITORING OF TRANSFORMERConducted by Ashish Ray (07-Feb-2012)

Prepared By Submitted to

PARTH PATEL ASHOK KUMAR SINGH

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INDEX Page No.

1. Basics of Transformer…………………………………………………………........ 4. Working principle………………………………………………………………… 4. Construction………………………………………………………………………… 4. Accessories & their functions……………………………………………….. 7.

2. Different Types of Tests………………………………………………………………. 11. DGA Test……………………………………………………………………………… 11. Transformer Oil Testing……………………………………………………….. 13. Furanic Compound Test………………………………………………………… 15. Insulation Resistance & Polarization Index Test……………………. 17. Step Voltage Test………………………………………………………………….. 18.

3. Transformer Protection……………………………………………………………….. 19. Overload Capability ……………………………………………………………… 19. Buchholz Protection……………………………………………………………… 20.

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Basics Of Transformer:-

Working Principle : A transformer is static (or stationary) piece of apparatus which:

Transfers electric power from one circuit to another. It does so without a change in frequency. The principle is based on mutual induction between two

circuits linked by a common magnetic flux. Basically a transformer consists o f a :

A primary coil or winding. A secondary coil or winding. A core that supports the coils or the windings.

Transformer Construction:

Main constructional elements of Transformers are –

A) Magnetic circuit : Core & clamping structure B) Electric Circuit : Winding,Insulation, Bracing devices. C) Terminals :Tapping, Tapping switches, Terminal Insulator, Leads ,Bushings D) Tank : Oil, Cooling devices, conservator, piping,Breather E) Protective Circuit : Buchholz relay, WTI, OTI, Oil surge relay, & Monitoring pressure relief device, MOG

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A) Magnetic Circuit :

The core provides closed path for flux. It is made up of CRGO insulated laminations. (CRGO has iron loss of about 1.3 W / Kg at 1.6 Tesla )

B) Electric Circuit:

Winding, insulation & bracing are constructional parts of electrical circuit of transformers. This is the most vulnerable part of transformer because of direct association with power system. Must be designed to withstand voltage stress resulting from system fault, transient over voltage and thermal stresses (lightening or switching surges).

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Insulation: Commonly used material are -

Paper or press board. Oil is used as insulating medium. Insulating varnish applied to make coil mechanically strong.

C) Terminal:

Leads : Connection of winding are (copper rod or bus bar) taken to bushing.

Bushing : Up to 33 kV porcelain bushing are used. Above 33 kV, condenser & oil filled terminal bushings are used.

D) Transformer tank :

Cooling : Small transformers are air cooled whereas large transformers are provided with oil or oil & air cooling .

The transformer tank is designed to withstand full vacuum.

Types of Cooling:

Air insulated air cooled AN Air Natural AF Forced Air Cooling ANAN Natural Air cooling inside and outside transformer

Oil immersed air cooled ONAN Natural Oil Circulation. Natural Air flow ONAF Natural oil and forced air flow OFAN Forced Oil & Natural Air Circulation OFAF Forced Oil and Forced Air Circulation

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Oil immersed & water cooled

ONWF Natural Oil, water (internal) cooler OFWF Forced oil, water (external) cooler

E) Protective devices : Various protective devices mounted on transformer are as

follows:

Bucchholz Relay : Gas actuated relay, Transformer Internal Fault.

WTI/OTI : Provided for alarm and / OR trip against over load

PR Device : To release internal pressure generated in the transformer during fault.

MOG : Alarm when oil level is low

Oil Surge Relay : To release actual pressure generated during fault in OLTC

Accessories & Their Functions:-

Terminal & Bushing : Type of Bushing, terminal is selected depending on voltage, currents & operating conditions. Porcelain, condenser type of oil filled bushings are used as per requirement.

Cable Boxes : Cable boxes are primarily designed for receiving and protecting cable ends and ensure effective sealing of cable against ingress of moisture.

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Conservator : It is provided to accommodate change in oil volume caused due to change in loads or ambient conditions.

Breather : Whenever there is change in ambient temperature or load , there is a change in oil temperature and hence the volume of oil. Increase in volume causes the air above oil level in conservator to be pushed out and decrease causes air to be drawn in. Thus the transformer breathes.

When air is breathed in, moisture and dust from atmosphere is sucked in .

Silica gel crystals absorb moisture. Color of silica gel is blue when dry and turns pink when absorbs moisture.

Oil cup at the bottom is filled with oil which acts as coarse filter and removes dust form outside air.

Magnetic Oil Level gauge:

This is a dial instrument operated by magnetic coupling from a float on oil surface.

It is normally fitted with contact to give alarm for low oil level.

Oil temperature indicator:

Bourdon tube with a pointer arrangement mounted in a case comprising of a reading dial and a glass cover. There is a

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temperature sensing bulb which communicates to the bourdon tube through the armored capillary.

Winding temperature indicator:

It comprises of following:

WTI pot :mounted at top of transformer tank. Oil in pot is temp. of top oil.

Imae coil:Heater coil and develops additional hear raising temperature of oil incide heater coil.

WTI CT:

Mounted on one of the line lead with secondary connected to image coil

WTI: The bulb of the WTI is immersed in oil inside image col .Temperature of this oil is dependent on top oil temperature and load on transformer.

Buchholz Relay : Gas & oil operated relay detects formation of gas or development of sudden pressure inside the oil of transformer.

Any electrical fault inside the transformer is accompanied by evolution of gas.

Pressure Relief Device: This is provided to relieve the internal pressure in the event of major fault within the transformer.

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Tapping Switch: To maintain secondary voltages reasonably constant at load end when incoming voltage and/or load on transformer changes, it is necessary to change the voltage ratio (I.e, turns ratio of the winding) of the transformer.

This is achieved by changing the number of turns ( HT Side) by operating a switch called as tapping switch.

Depending on the requirement, off circuit or on load tap changer is installed in the transformer.

Radiators :

The function of radiator is to limit the temperature of oil and winding by dissipating heat that is generated due to losses within transformer while in service.

When transformer is in operation warm oil rises and enters the radiator from the top valve cools and then descend to enter the bottom of the tank.

Other Accessories: Inspection cover, jacking lugs, Earthing terminals, Rating Plate, Filter valve, Drain Valve, Terminal marking, Rollers etc.

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Different types of Tests:-

DGA Test :- Insulating materials within transformer breakdown to liberate

gases. The identity of these gases indicate the type of fault and the rate

of gas generation indicate the severity of fault. Causes of fault gases can be divided into three categories:

Corona or partial discharge Pyrolisis or thermal heating Arcingo Arcing is the most severe fault (intensity of energy that is

dissipated per unit time per unit volume of the fault) ,less with heating, least with corona.

o One of the most important technique to indicate the health of a transformer.

Sampling And Labeling Procedure : Dry Weather, avoid contamination Clean, dry, leak proof glass or stainless steel container. Equipment operating normally Take safety precautions. Sample bottle must be full without any air trap completely

sealed and should be properly labeled.

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Condition Monitoring of Transformer Oil :-

Role of transformer oil: It is used as coolant. It is used as insulating material.

Reasons for deterioration of transformer oil:

Physical contamination:

o Release of fibrous impurities by paper, pressboard, wood and cotton tapes in contact with oil for longer period at elevated temperatures.

o Due to dissolution of varnish .

o Due to foreign matters like dust, metallic particles and other solid impurities

o Due to moisture

Effects:

o Life is reduced by high sludge formation.

o Electrical properties of insulating oil get disturbed due to conductivity of suspended particles.

Chemical deterioration:

o It is due to oxidation.

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Effects of oxidation:o Results in acids, sludge .Acid attack solid insulation and

metal. Sludge causes poor thermal conduction and mechanical hindrance to proper oil circulation.

Contamination of gases:

o Gases are present in oil due to following:

o Those which dissolve in the oil from atmosphere.

o Those which are generated inside due to thermal decomposition of oil, decomposition of oil by arcing.

Effects of gases:

o The ignition of inflammable gases can be causes by corona occurring in th air space or arcing.

Transformer Oil Testing :-

Physical condition of the oil : Color ,clarity and odour gives information regarding quality of oil and presence of certain contaminants in oil.

Electrical strength : Important parameter as used as insulating medium. This test gives conductive contaminants and moisture present in oil.

Water content : Reveals total water content ,leak or cellulosic deterioration

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Specific resistance (Resistivity) : This test provides a measure of the total soluble contaminants and ageing products .It is numerically equal to the resistance between opposite faces of a centimeter cue of the oil and is expressed as ohm com.

Dissipation factor : This test provides a measure of the total soluble contaminants and ageing products.

Neutralisation value : This test gives acid present in the oil. It is the no. of milligrams of potassium hydroxide required to neutralise completely the acids produced in one gram of oil.

Interfacial tension test : This test provides a measure of sludge and polar component present in oil. It is expressed as molecular attractive force between the molecules of water and oil at oil-water interface.

Flash point : Sudden drop in flashpoint is indicative of unsafe working condition of transformer.

Foranic Compound Test :-

Life of transformer is dependent on life of solid insulation and life limit is determined by thermal degradation of winding paper.

Kraft insulation paper is used as solid insulation.

When oil soaked paper is damaged by heat, some oil soluble compound are released into oil called furans.

Paper is made of cellulose consisting of log chains of glucose rings joined by glycosidic bonds.

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During degradation bonds are broken and glucose rings are opened. Glucose is unstable which further degrades which are more stable and oil soluble called furans.

Furanic Compounds: The most commonly found furanic derivative is

2-furfuraldehyde(2 FAL) and other derivatives are:

o 2- furfuryl alcohol (2 FOL) , o 2- Acetyl furan (2 ACF), o 5- hydroxymethyl furfuraldehyde,(5HMF) o 5 methyl furfuraldehyde(5MEF).

Furans And Gases:o Cellulose degradation=Glucose+H2O+CO+CO2+Organic acids.o With DGA and furan test extent of paper damage can be seen.o CO and CO2 are determined by DGA and are considered as

level indicator for cellulosic degradation.o In case of severe localised paper damage ,high furans and high

gas content can be seen.o In case general heating slow building of furans without

necessarily seeing an increase of gas content.

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Insulation Resistance And Polarization Index Test :-

These tests are performed to verify state of dryness if insulation. These tests are intended to check overall

cleanliness ,dryness ,localized defects and general condition of insulation system.

When DC voltage is applied across insulation, the current flows is the resultant of three currents: Capacitive charging current Absorption current Leakage current

Capacitance leakage current : The current lasts for a few seconds as DC voltage is applied and

drops out after the insulation is charged to its full voltage. The time depends on the size and capacitance of the test object. Larger time for larger capacitance objects.

Absorption leakage current: It is caused by polarisation of molecules within dielectric

material. In low capacitance equipments the current is high for first few

seconds and decrease slowly to nearly zero. In high capacitance equipment or wet and contaminated

insulation ,there will be no decrease of absorption current for long time.

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Conduction or leakage current This is the current that normally flows through the

insulation ,between conductors or conductors to ground. It increases quickly and becomes stable. This current increases as insulation deteriorates and becomes

predominant after absorption current vanishes. It is steady and time independent .Hence the important current

for measuring insulation resistance.

Step Voltage Test :-

The DC voltage is applied in various steps and in each step the leakage current is noted. Step duration:60 seconds.

The variation of this leakage current (or IR) with test voltage gives the condition of insulation.

If insulation is dry, clean and with out physical damages shall show the same value at all voltage levels.

If insulation value decreases at higher voltage levels ,may be due to dirt, moisture, cracking, aging etc.

The application of increased voltage creates electrical stresses on internal insulation cracks. This can reveal aging and physical damage in relatively clean and dry insulation which would have not been apparent at lower voltages.

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Transformer Protection :-

Over load Capability :- Working life of transformer – dependent on life of insulation Rate of deterioration of insulation – increases with increasing

winding temperature q W

Winding temperature – dependent on loading Transformer has substantial over load capability IF q W < 80 deg.c – use of life negligible If transformer is operated @ 104 deg. For every hour of operation

= 2 hrs of life is lost. If transformer is operated @ 116 deg. For every hour of operation

8 hrs of life is lost Rate of using transformer life doubled for every temperature

increase of 6 deg.c

Criteria For Overload Capability :- For normal duty cycle, current shall not exceed 150% I RAT

For emergency duty, current can exceed 150% I RAT provided associated cables, switch gear, tap changers, bushings etc. are suitable rated.

Under no circumstances,

q Winding shall exceed 140 deg.c

q Oil shall exceed 115 deg.c.

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Buchholz Protection :- Relay installed in the pipe line between transformer tank and

conservator

Two Floats : Upper float = responds to slow accumulation of gas due to mild

or incipient faults – for alarm Lower float (Vane) – responds to oil surge caused by major

internal faults – for trip

Relay mounting precautions : Gas shall freely pass up the pipe work Extra turbulence shall not be induced in oil stream Relay shall be mounted on straight run of pipe line which

should slope from transformer to conservator at an angle of 5 deg.

Operating time: 100 to 200 milliseconds Petcock provided on top of housing to draw accumulated gas

for analysis Gas actuated relay Popularly used in all countries except USA Used to detect incipient faults which may lead to major damage

if allowed to continue Some Examples:

Hot spots on the core due to short in lamination insulation Core bolt insulation failure Faulty joints

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Inter turn faults Loss of oil due to leakage

Depends for its operation on the fact that most internal faults generate gases.

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