11. Transformer Introduction and Maintenance

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Typical Voltages


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Typical Voltages Single Phase Power

Single-phase distribution is forlow intensity loads, typicallywhen loads are mostlyelectronic, lighting, or heating,

with few large electric motors.A single-phase motor needsadditional circuits for starting,and such motors areuncommon above 10 or 20 kW

in rating.2 wires: 1 phase line andneutral

RMS = 0.707 * (V or I)(Root Mean Square)is a way of expressing ACvoltage or current as functionally equivalent to DC


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Typical Voltages Three Phase Power

Three-phase power is used forall large scale distributionsystems.

A continuous series of three

overlapping AC cycles offset by120 degrees means that three-phase motors run moresmoothly and do not requirebrushes, start capacitors, andare easy to reverse as needed.

4 wires: 3 phase lines andneutral

Three-phase power delivers 1.73times higher power than that ofsingle phase with the same lineamperage.

50% more conductor = 73% more

power


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Power Quality Problems

- Swell- RMS voltage exceeds the nominal voltage

- Sag RMS voltage is below the nominal voltage

- Flicker(ie. Voltage flicker) Variations in RMS voltage

- Spikes/Surges Brief increases in voltage

- Undervoltage Insufficient power or brownout

- Overvoltage Voltage rise above 110% for > 1 min.

- Frequency Variations

- Harmonics Variation in the voltage wave shape


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Power and Distribution Transformers- Power transformers

Liquid filled loosely divided into 3 size categories:

1. Small power under 10 MVA2. Medium power 10 to 50 MVA3. Large power above 50 MVA.

- Distribution transformers Reduces voltage to 120/240 volts single phase for residential

customers Reduces voltages to 480/277 or 208Y/120 for commercial or

industry Sizes range from 5 KVA to 2500 KVA Single phase and three phase Pad mounted or pole mounted Dry-type and liquid filled


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Transformer Insulating Methods

Dry Type insulation systems: Varnish Polyester

Silicone Epoxy

Liquid filled insulation systems:

Oil

Silicone Hybrid (R-Temp, etc.)

The most important factor in optimizing a transformers lifeexpectancy is the quality of its insulation system over time.

In power transformers, the voltage difference between parts of theprimary and secondary windings can be quite large. Layers of

insulation are inserted between layers of windings to preventarcing.

The oil used in power transformers is susceptible to moisture andits insulating value is seriously reduced when even small amountsof water are present.


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Transformer Cooling Class Designations

Dry-Type transformers use a 3 letter designation.

The first letter designates the type of cooling surrounding the windings.

A- for air

G- for a gas other than air

The second letter designates the method of circulating the coolingmedium through the windings.

N- for natural convection

F- for forced circulation

The third letter will designates the method of removing the heat fromthe cooling medium.

N- for natural circulation of outside air (ventilated type)

F- for forced circulation of outside air (ventilated type)

C- for natural convection to outside air (enclosed type)

P- for forced circulation of outside air (enclosed type)


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Dry Type Examples:

ANN- Air filled natural convection with open ventilated enclosure

through which the ambient air readily enters and leaves theenclosure.

GNC- Gas-filled unit cooled by natural convection of the gas withinthe enclosure and by natural convection of ambient air outside the

enclosure.

AFC- Air-filled cooled by forced fan circulation of air within theenclosure and by natural convection of ambient air over the outsideof the enclosure.

GNP- Gas-filled unit cooled by natural convection of gas within theenclosure and by forced flow of ambient air over the outside of theenclosure.


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Liquid-Filled transformers use a 4 letter designation .


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Liquid Filled Examples:

ONAN- Oil filled natural cooling

ONAF- Oil-filled forced-air cooling

ODAF- Oil-filled forced-directed oil forced-air cooling

LFWF- Flame-retardant liquid filled forced-liquid water cooling.

LFAN- Flame-retardant liquid filled forced-liquid natural cooling


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Basic Impulse Insulation Levels (BIL)

The ability of a transformer to withstand voltage surges.

Outdoor (and indoor) electrical distribution systems are subject tolightning surges. Even if the lightning strikes the line some distancefrom the transformer, voltage surges can travel down the line and intothe transformer.

When high voltage switches and circuit breakers are opened andclosed they also can create voltage surges. Both types of surges havesteep wave fronts and can be very damaging to electrical equipment .

To minimize the effects of these surges, the electrical system isprotected by lighting arresters, but they do not completely eliminate thesurge from reaching the transformer.


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Basic Impulse Insulation Levels (BIL)

Switchgear and insulators typically include a BIL rating.

For example, a 15kV power transformer is typically rated 95kV BIL,but 110kV BIL is also an available option.

The BIL ratings should be coordinated with lightning/surge protectionand expected transient overvoltages.


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Percent Impedance (%Z)

Percentage impedancerepresents the amount of normal ratedprimary voltage that must be applied to the transformer to producefull rated load current when the secondary winding is short circuited.It determines the maximum value of current that will flow under fault

conditions.In a three phase system, if a phase faults to earth, the voltageapplied to each phase of the transformer is no longer in balance;fluxes in the windings no longer oppose. This leads to morecomplex calculations requiring the use zero sequence impedances(Z0)

Z% = Impedance Voltage x 100Rated Voltage


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Station TransformerType of Transformer Oil Immersed Operation of Transformer Full Load

Rated Power 3150 kVA Ref Standard IEC-60076

Nominal Primary Voltage 13.8 kV Phase 3

Nominal Secon. Voltage 480/277 V Frequency 60 Hz

Vector Group Dyn11

Impedance % at 75C Winding

Temp

8% (Subject to IEC

tol)Type of Cooling ONAN Type of Insulation Fluid Biodegradable Oil

Temperature RiseWinding 60C Neutral Grounding Solid

Top of Oil55C

Maximum Sound Level 65 dB

Lifting Lugs Yes Flux Densit y @ Rated Voltage


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Insulation Liquid/Oil- Oil acts as insulation and coolant

- Tap-up oil must be same as original

- Oil exposed to air can be contaminated- Oil Handling equipment must be clean

- Sampling container should be glass with glass stopper.

- Cloudiness of oil is due to moisture or contamination- Water Content


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Station Transformer Accessories

Liquid Level Indicator ORTO 124A . . . 2

Liquid Temperature Indicator ORTO 0255 2F . . . 2Winding Temperature Indicator ORTO 0265 3A . . . 2Thermal Winding Protector ORTO 0852 ICP . . . 2Pressure Relief Device ORTO 510 TCFN . . . 2

Pressure Vacuum Indicator ORTO 0305 T . . . 2Rapid Pressure Rise Relay QUALITROL 900-010-

03 CS-40305 . . 3Current transformer TM2000


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Transformer MaintenanceFor long service life, transformers are required to receive

reasonable amount of maintenance.

The main objective of maintenance program is to prevent

failure and maintain the transformer in good condition.

Maintenance program shall include:

Regular monitoring (Operators)

Periodic inspections (Maintenance Staff) Regular Testing and Reconditioning (Maintenance Staff)

Documenting all records (Operator & Maintenance Staff)


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Periodic Inspection1. In-service inspections on liquid immersed transformers

are a combination of three things: Current operating information

Testing of aspects that can be performed with the

transformer energized Visual inspection of components

In-service inspections are performed to give an overallindication of the transformers condition, and to identify

any problems that need immediate or future attention.


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The three items can be further broken into individualinspections as follows:

- Current and Voltage Readings

- Temperature Readings- Liquid Level Indicators- Pressure/Vacuum Gauges- Insulating Oil Field Tests

- Gauges and Alarms- Paralleling Circuit- Tap Changers and Controls- Dehydrating Breathers

- Cabling System- Fault Gas Monitor- Visual Inspection/Leaks


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2. Out-of-service inspections on liquid immersedtransformers In-Service Inspection Items Insulation Resistance

Gauges and Alarms

Cooling Equipment Tap Changer


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ANALYZING MAINTENANCE DATAThe maintenance data in transformer records should be analyzed forseveral reasons.

To find out if there are problems that need immediate attention.

To find out if there are any minor problems that need attention in thefuture.

To find out if there are any indications of potential problems.

Dielectr ic Breakdown Strength

This is the most common field and laboratory test for transformer oil. Thistest will only detect free water, dirt, and conductive particles. Generalacceptable values are at least 35 kV for new oils, and 25 kV for usedoils.


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Acidity

This is the second most common field test for transformer oil.Transformer oil becomes acidic due to oil oxidation. Sludge is theeventual by-product of acidic oil.

Acidity can be measured in the field using a spot test. The spot test ismade by applying drops of an acidity buffer solution to a piece of filterpaper, and allowing it to soak in. Two drops of oil are then added to thesame place, followed by one drop of an indicating solution. Thiscombination causes a color change that is compared to a standard color

chart. The ratings are as follows: Green and yellow - 0.3 mg koh/gm More green than yellow - less than 0.3 mg More yellow than green - more than 0.3 mg Orange or brown - high acidity

Blue - alkaline

There is no definite cutoff value between good or bad for this test. Thevalue is used in trend analysis to establish an overall rate of oildeterioration.


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Color

The oil color comparison test is not useful by itself. The results are onlysignificant if there is a marked change from one test to another. Thistests consists of comparing a sample of oil against a series of standardcolored disk filters, in a specially built comparator.

Moisture Content

Wet transformer insulation is one of the most harmful conditions for atransformer. It also is difficult to measure, because most of the moisturein a transformer is absorbed by cellulose winding insulation. It does notstay in the oil. The tests for moisture content find the ppm (parts permillion) water content of the oil. This value is then used to approximate

how wet the insulation is. The critical value for this test is 50 ppm. Whenan oil sample has this concentration of water, the paper or celluloseinsulation is almost always wet.


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Visual Examination

This test simply evaluates the visual condition of the oil. A sample isdrawn and checked for cloudiness, particles of insulation, carbon, orother suspended materials. Good oil should be clear and sparkling.Cloudiness shows the possibility of moisture, carbon, and/or sludge.

Power Factor

The power factor test on oil is considered a negative screening test. Itmeasures the leakage current through an oil. This is a measure ofcontamination or deterioration. The drawback of the power factor test isthat it is not specific in what it detects. It does show that polar materials

are present, other tests must be made to find out what they are.

The power factor of new oil is normally 0.05% or less. When the powerfactor exceeds 0.5%, more investigation should be done.


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Gas-In-Oil Analysis

This test is a dissolved gas analysis. It involves taking an oil sample,removing the gases from the sample, and then determining their

concentration. The type of gases present and their concentration canshow various problems with the transformer.

Electrical Test Data

There are three common electrical tests performed:

Insulation Resistance Insulation Power Factor

Transformer Turns Ratio


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Insulation Resistance

Insulation resistance tests generally refer to tests performed with amegohmmeter (megger).

Several factors can cause large fluctuations in megger readings.

Equipment temperature

Duration of the test External leakage due to dirty insulators and bushings

These variables must be controlled to prevent getting confusing test

data.

Four different tests can be performed with a megger:

Second Spot Test

Dielectric Absorption Ratio Polarization Index Step Voltage Test

The most common test for transformers is the Polarization Index


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Insulation Power Factor

Insulation power factor gives an indication of the quality of the insulation.

Power factor readings of 0.03 percent are not unusual under idealconditions. However, values this low are not normally obtained.

In-service oil-filled transformers can have power factors as high as twopercent and still be considered acceptable.

Insulation power factors above two percent should be looked at more forproblems.

Transformer Turns Ratio

Transformer turns ratio tests are performed to identify short circuited

turns, wrong tap settings, errors in turn count, mislabeled terminals, andfailure in tap changers.

The turns ratio that is measured should agree with the ratio calculatedwithin 0.5 percent.

11. Transformer Introduction and Maintenance

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