Cooling,Maintenance,Diagnostics& Testing of transformers

S S CHOPADE

Why Transformer Cooling?

Heat is one of the most common destroyers of Transformers

Only 100C above the transformer rating will cut transformer life by 50%

What are the basic types of transformer coolings?

“A” indicates air, “FA” indicates Forced air, “FA” indicates Forced air, “O” indicates oil,“FO” indicates Forced oil.

Liquid-Immersed Transformer Cooling System

Class OA: Oil-Immersed, self-cooled.

◦ Transformer windings and core are immersed in some type of oil and are self-cooled by natural circulation of air around the outside enclosure.

◦ Fins or radiators may attached to the enclosure to aid ◦ Fins or radiators may attached to the enclosure to aid in cooling

Dry Type Transformer Cooling SystemClass AA are ventilated self cooled transformers

Class AFA transformers are self cooled (A) & additionally cooled by forced circulation of air (FA)

Class AA/FA are ventilated, self cooled.In addition, they have fans providing additional forced-air cooling.Fan may be wired to start auto when temp. reaches a pre-set value.These transformer generally have a dual load rating, one for AA and larger load These transformer generally have a dual load rating, one for AA and larger load rating for FA.

Class OA/FA: Liquid-immersed, self cooled/forced air cooled.

Same as OA above, with the addition of fans.

Fans are usually mounted on radiators.

The transformer typically has two load ratings, one with the fans off (OA) and a larger rating with fans operating (FA). operating (FA).

Fans may be wired to start automatically at a pre-set temperature.

Class OA/FA/FA: Liquid-immersed, self-cooled/forced air -cooled.

Same as OA/FA above with an additional set of fans. There typically will be three load ratings corresponding to each increment of cooling. Increased ratings are obtained by increasing cooling air over portions of the cooling surfaces.

Typically there are radiators attached to the tank to aid in Typically there are radiators attached to the tank to aid in cooling.

The two group of fans may be wired to start automatically at pre-set level temperature increases. There are no oil pumps. Oil flow through the transformer windings is by the natural principle of convection(heat rising)

Liquid-immersed, Air cooled/Forced Liquid-cooled.

There are two classes in this section

1. Class OA/FA/FOA: Liquid-immersed, self-cooled/forced air-cooled/forced liquid, and forced air-cooled.

Windings and core are immersed in some type of oil. The transformer has self-cooling (OA) natural ventilation, forced air-cooling FA (fans), and forced oil-cooling (pumps) with air-cooling FA (fans), and forced oil-cooling (pumps) with additional forced air-cooling (FOA) (more fans).

The transformer has three load ratings corresponding to each cooling step.

Class OA/FOA/FAO: liquid-immersed, self-cooled/forced oil, and forced air-cooled/forced oil, and forced air-cooled.

Cooling controls are arranged to start only part of the oil pumps and part of the fans for the first load rating/temperature increase, part of the fans for the first load rating/temperature increase, and the remaining pumps and fans for the second load rating increase.

INSPECTIONS OF OIL-FILLED TRANSFORMER

A transformer maintenance program must be based on thorough routine inspections.

These inspections must be in addition to normal daily/weekly data gathering trips to check oil levels and temperatures. Some monitoring may be done remotely using SCADA systems, but this can never substitute for thorough systems, but this can never substitute for thorough inspections by competent maintenance or operations people.

Testing Testing and Maintenance of Highand Maintenance of High--Voltage Bushings. Voltage Bushings.

Caution:

Do not test a bushing while it's in its wood shipping crate, or while it is lying on wood. Wood is not as good an insulator as porcelain and will cause the readings to be inaccurate.

Keep the test results as a baseline record to compare with future tests.Keep the test results as a baseline record to compare with future tests.

If the oil level is low and there is no visible external leak, there may be an internal leak around the lower seal into the transformer tank.

If possible, re-fill the bushing with the same oil and carefully monitor the level and the volume it takes to fill the bushing to the proper level.

If it takes more than one quarter, make plans to replace the bushing.

The bushing must be sent to the factory for repair or it may be junked; it cannot be repaired in the field.cannot be repaired in the field.

Corona (air ionization) may be visible at tops of bushings at night, especially during periods of rain, mist, fog, or high humidity.

At the top, corona is considered normal; however, as a bushing becomes more and more contaminated, corona will creep lower and lower.

If the bushing is not cleaned, flashover will occur when corona near the grounded transformer top.

If corona seems to be lower than the top of the bushing, clean the bushing as quickly as possible.

Oil Preservation Sealing SystemsOil Preservation Sealing Systems

The purpose of sealing systems is to prevent air and moisture from contaminating oil and cellulose insulation.

Sealing systems are designed to prevent oil inside the transformer from coming into contact with air.

Air contains moisture, which causes sludging and an abundant supply of oxygen.supply of oxygen.

Oxygen in combination with moisture causes greatly accelerated deterioration of the cellulose.

This oxygen-moisture combination will greatly reduce service life of the transformer.

GasketsGaskets

Gaskets have several important jobs in sealing systems.

A gasket must create a seal and hold it over a long period of time.

It must be impervious and not contaminate the insulating fluid or gas above the fluid.

It should be easily removed and replaced.

It must be elastic enough to flow into imperfections on the sealing surfaces.

It must withstand high and low temperatures and remain resilient enough to hold the seal even with joint movement from expansion, contraction, and vibration.

Transformer Oil Functions. Transformer oils perform at least four functions for the transformer.

◦ provides insulation

◦ provides cooling

◦ helps extinguish arcs.

◦ Oil also dissolves gases generated by oil degradation, moisture and gas from cellulose insulation, deterioration, gases and moisture

Transformer Oils

Oil also dissolves gases generated by oil degradation, moisture and gas from cellulose insulation, deterioration, gases and moisture from atmosphere to which oil is exposed to.

• Close observation of dissolved gases in the oil, and other oil properties, provides the most valuable information about transformer health.

Looking for trends by comparing information and understanding its meaning, is the most important transformer diagnostic tool.

Key Gas MethodKey Gas Method

key gas analysis method for interpreting dissolved gasses is set forth in IEEE

key gases formed by degradation of oil and paper insulation are

hydrogen (h2),

methane (ch4),

ethane (c h ), ethane (c2 h6),

ethylene (c2 h4),

acetylene (c2 h2),

carbon monoxide (co), and

oxygen (o2).

Except carbon monoxide and oxygen, all other gases are formed from the degradation of the oil itself.

◦ carbon monoxide, carbon dioxide (co2), and oxygen are formed from degradation of cellulose (paper) insulation.

◦ carbon dioxide, oxygen, nitrogen (n2 ), and moisture can also be absorbed from the air if there is a oil/air interface, or if there is a leak in the tank.

Additional transformer problems are listed below; there Additional transformer problems are listed below; there are many others.are many others.

Gases are generated by normal operation and aging, mostly H2 and CO with some CH4.

Operating transformers at sustained overload will generate combustible gases.

Problems with cooling systems, discussed in an earlier section, can cause overheating.

A blocked oil duct inside the transformer can cause local overheating, generating gases.

Additional transformer problems are listed below; there Additional transformer problems are listed below; there are many others.are many others. An oil directing baffle loose inside the transformer causes mis-direction of

cooling oil.

Oil circulating pump problems (bearing wear, impeller loose or worn) can cause transformer cooling problems.

Oil level is too low this will not be visible if the level indicator is inoperative.

Sludge in the transformer and cooling system.

Circulating stray currents may occur in the core, structure, and/or tank. Circulating stray currents may occur in the core, structure, and/or tank.

An unintentional core ground may cause heating by providing a path for stray currents.

A hot-spot can be caused by a bad connection in the leads or by a poor contact in the tap changer.

A hot-spot may also be caused by discharges of static electrical charges that build up on shields or core and structures which are not properly grounded.

Additional transformer problems are listed below; there are many others…………….

Hot-spots may be caused by electrical arcing between windings and ground, between windings of different potential, or in areas of different potential on the same winding, due to deteriorated or damaged insulation.

14. Windings and insulation can be damaged by faults downstream (through faults), causing large current surges through the windings. (through faults), causing large current surges through the windings. Through faults cause extreme magnetic and physical forces that can distort and loosen windings and wedges. The result may be arcing in the transformer, beginning at the time of the fault, or the insulation may be weakened and arcing develop later.

Dissolved Gas Analysis Detection Limits.Dissolved Gas Analysis Detection Limits.

Hydrogen (H2) about 5 ppm

Methane (CH4) about 1 ppm

Acetylene (C2H2) about 1 to 2 ppm

Ethylene (C2H4) about 1 ppm

Ethane (C H ) about 1 ppm Ethane (C2H6) about 1 ppm

Carbon monoxide (CO) and carbon dioxide (CO2) about 25 ppm

Oxygen (O2 ) and nitrogen (N2) about 50 ppm

These ratios and the resultant fault indications are based on large numbers of DGAs and transformer failures and what was discovered after the failures.

properties properties of of XmerXmer OilOil

Viscosity

This determines the rate of cooling and varies with temp

Insulating property

This serves one of the basic purpose of using oil

Flash point (Temp at which oil vapour at oil surface ignites)

A flash point of not less than 160oC is usually demanded for safety reasonsA flash point of not less than 160 C is usually demanded for safety reasons

Fire point (temp at which oil ignites )

Again A fire point of not less than 200oC or above 25 % higher than flashpoint is usually demanded.

Purity

Must be free from sulfur to avoid corrosion of metal surface & accelerationof formation of sludge

3 Acid Number.3 Acid Number.

Acid number (acidity) is the amount of potassium hydroxide (KOH) in milligrams (mg) that it takes to neutralize the acid in 1 gram (gm) of transformer oil. The higher the acid number, the more acid is in the oil.

It is recommended that the oil be reclaimed when it reaches 0.20 mg KOH/gm

Transformer testingTransformer testing

Factory tests

Polarity test

No load loss test

Load loss test, short ckt test impedance calculation

Dielectric test

Routine tests

CT testing (ratio,polarity & knee point if specified)

Ratio test

Winding resistance measurementDielectric test

Separate source voltage withstand

Induced overvoltage test

Temp rise test

Lightening impulse test

measurement

Short ckt test

Vector group test

Magnetising current measurement

Magnetic balance test

Oil deteriorationOil deterioration Major causes of oil deterioration are

Oil in contact with air subjects to oxidation reactions ,Temp & presence of catalysts (Solid iron,copper etc) accelerates it , as a result oil darkens & its acidity increases

Simultaneously its electrical properties such as dielectric dissipation factor(DDF) & resistivityfalls considerably

Ultimate effect can be formation of sludge too

In some cases material used in the construction of oil may decompose to deteriorate the qualityof oil.of oil.

Following contaminant may be found in oil in service

• Water

• Sediment (insoluble oxides,degradation product of solid or liquid insulating materials, Fibers)

• precipitable sludge (Contaminants , Oil deterioration products)

• Polar substances ( Oil soluble compound resulting from oxidation of oil)

• Acids (Formed by CO2,CO & moisture in oil)

• Dissolved gases

• Light hydrocarbon (Formed by oil under influence of heat, Energy faults)• Light hydrocarbon (Formed by oil under influence of heat, Energy faults)

Tests on oil sampleTests on oil sample

Visual inspection (Colour & Odour)

Cloudiness in oil may be due to suspended moisture or sediments such as iron-oxide or sludge

Dark brown coloured oil may indicate the presence of dissolved bituminous compound

Green coloured oil may indicate the presence of dissolved copper compounds & a rapid deterioration of oil may be expectedcompounds & a rapid deterioration of oil may be expected

Acrid acid smell indicates the presence of volatile acids which can cause corrosion

Electric strength test

A typical laboratory test set up is required for the test in which oil issubjected to high voltage with a fixed gap between electrode & voltage isincreased till oil breaks down( a detail method is given in IS 6792-1972)

permissible values for 2.5 mm gap

upto 72.5 kV 30 kV min

72.5 to 145 kV 40 kV min

Above 145 kV 50 kV min

Water content test Water content test

Presence of water in the oil may be detected by crackle test however exactPPM of water can be determined by titration method in laboratory.

Permissible values are

below 145 kV 35 ppm

above 145 kV 25 ppm

Sludge test

This is again a laboratory test which determines sludge & sediment in the oil bymethod of precipitation of oil sample with n-heptanesolution.

The precipitated matter is then dissolved in the mixture of toluene, acetone andalcohol at 50oC till no more dissolution is there.

The precipitated quantity is termed as sludge and the dissolved one representssediments in the oil

Permissible values are absolutely no sediments or precipitable sludge should betracedtraced

Dielectric dissipation factor test Permissible value for DDF( tan ) at 90oC is0.005 (max)

Specific resistance test Permissible value for resistivity is 0.1 X 1012 ohm-cm(min) at 90oC

Maintenance schedule Maintenance schedule -- hourlyhourly

Oil & Wdg temp check

Check against reasonable rise

Shutdown Xmer & investigate the matter

Voltages & Load monitoring

Check against the rated parameters

Take the corrective action with the help of tap changerTake the corrective action with the help of tap changer

Daily Maintenance scheduleDaily Maintenance schedule

Oil level in Xmer & OLTC

Check against normal level

Top up & investigate for the leakage

Breather

Check color of active agent

if silicagel is pink , change by spare charge & old charge may be reactivated forif silicagel is pink , change by spare charge & old charge may be reactivated forfurther use

Quarterly Maintenance ScheduleQuarterly Maintenance Schedule

Bushing

Examine cracks & dirt deposited

Clean or replace

Oil in Xmer or OLTC

Check for dielectric strength & water content

Take suitable action to restore quality of oilTake suitable action to restore quality of oil

Fan & Pumps

Check bearing, examine contacts, check manual & auto P/U ckt.

Replace faulty components

Half Yearly Maintenance ScheduleHalf Yearly Maintenance Schedule

Oil cooler

Test for pressure

OLTC & driving gear

Check for all moving parts contacts,brake mechanism

Yearly Maintenance ScheduleYearly Maintenance Schedule

Insulation resistance

Compare with the values at the time of commissioning

Process if required

Cable boxes

Check for sealing arrangement for cable boxes,moisture condensate if any in air filled cable boxif any in air filled cable box

Replace gasket if leaking

Drying out Drying out

Drying out is done when IR values or Moisture contents are not comparable with the standard value.

It is done by circulation of hot oil through streamline filter - a machine incorporating oil heater & vacuum chamber

It is preferable to lag or blanket Xmer tank to prevent loss of heat, radiation of from radiators may be prevented by lowering the oil level below the top inlet or by operating shut off valves if provided.the top inlet or by operating shut off valves if provided.

Oil is drawn from bottom & let into the Xmer from top to remove settled impurities. At about 8 - 10 Hrs cycle is reversed

Oil temperature should be of the order 60oC.In no case it is to be exceeded beyond 65oC & circulation to be continued till IR & PPM is satisfactory.

Drying out curve

Oil

Tem

p in

o CIR

val

ue in

M o

hm

Time in Hrs

Oil

Tem

p in

IR

val

ue in

M o

hm

Moisture coming out