NETA Dry Type Power Transformer

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SUBSTATION COMMISSIONING

COURSE

MODULE SIX

COMMISSIONING

DRY-TYPE POWER

TRANSFORMERS

Written by:

Electrical Industry Training Centre of Alberta 4234 – 93 Street

Edmonton, Alberta, Canada Phone: (780) 462-5729 Fax: (780) 437-0248

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TABLE OF CONTENT

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Introduction

This module will detail the requirements for performing electrical and mechanical tests on the MV dry-type power transformers. The purpose of the tests, testing requirements and procedures are presented. Future module will be written on the liquid-immersed transformers.

This module will introduce the NETA acceptance testing procedures for

transformer comprising of mechanical and visual inspections, electrical testing and test data analysis. General guidelines for acceptance testing will be presented. When equipment specific instructions are required the equipment

manufacturers/manuals should be consulted.

An understanding on the theory of operations, functions, types and ratings are discussed and this information will be useful when performing acceptance tests. The discussion will be applicable to the substation medium voltage class power transformers. Generation step-up transformers and overhead type distribution transformers are excluded from this discussion.

Objectives

By the end of this module the participants will have the basic skills to perform acceptance testing on dry-type transformers, conduct visual and mechanical inspections, perform insulation resistance tests, polarizing index test, applied voltage tests, winding resistance tests, ratio test, polarity test, power factor test, capacitance and dissipation test and completing the inspection / test forms and conducting an assessment of the test data.

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1. North American Dry-type Transformer Standards

Power transformers are used to convert and/or isolate power from one voltage level to other voltage level(s). Power transformers are classified as having a rating

greater that 501 kVA for three-phase transformer and larger than 168 kVA for single-phase transformer as per NETA standards and design for step-down operation.

1.1 Canadian Standards

Dry-type transformers for use in Canada must comply with Canadian standards. The CSA standards contain information on applications and testing requirements. These standards include but are not limited to:

• CAN/CSA-C2 Single Phase and Three Phase Distribution Transformers

• CAN/CSA-C88 Power transformers and Reactors

• CAN/CSA-C227.3 Low Profile, Single Phase, Dead Front Pad-mounted Distribution Transformers

• CAN/CSA-C227.4 Three Phase, Dead Front Pad Mounted, Distribution Transformers

• CSA C9-02 Dry-Type Transformer

• CSA C802.2-06 Minimum Efficiency Values for Dry-type Transformers

CSA C9-02 is the predominant standard for the design requirements of dry-type transformers and reference the IEEE C57.12.91 for the testing requirement that is not specifically noted within the document and also reference IEEE C57.96 for the loading guidelines.

When transformers are not made to Canadian standards, the manufacturer must declare that the unit has the equivalent safety performance as one made to Canadian standards.

The Canadian CAN/CSA-C802.2 standard is an efficiency standard for dry-type transformers that imposes maximum losses for dry-type single-phase and three-phase self-contained units or components of larger assemblies, 60 Hz, ANN, rated 15 to 833 kVA for single phase and 15 to 7500 kVA for three phase.

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Testing based on CSA standards.

Liquid filed transformers shall be tested as per IEEE C57.12.90 as specified in CAN/CSA C88-M90 for Power Transformers and Reactors.

Dry type transformers shall be tested as per IEEE C57.12.91 as specified in CSA C9-2002 for Dry Type Transformer.

1.2 US Standards

The ANSI/IEEE standards which contain information on design and testing requirements include but are not limited to:

• IEEE C57.12.00 Standard General Requirements for Liquid- Immerse Distribution, Power and Regulating Transformers

• IEEE C57.12.01 Standard General requirements for Dry-Type Distribution and Power Transformers Including Those with Solid-cast and/or Resin Encapsulated Windings

• IEEE C57.12.90 Standard Test Code for Liquid-Immersed Distribution, Power and Regulating Transformers

• IEEE C57.12.91 Standard Test Code for Dry-Type Distribution and Power Transformers

NEMA standard publication TR1-2000 for Transformers, Regulators and Reactors has adopted the majority of the ANSI/IEEE C57 families of transformer standards which then becomes part of the NEMA standard.

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2. Nameplate Data

Industry standards require critical information to be plainly marked, in a permanent manner on the nameplate(s).

Some of the more important information are:

• Manufacturer’s name, trade name or other recognized symbols of identification

• Serial, catalogue, style, model or other identifying designation

• Winding temperature class

• Rated temperature rise in degree Celcius

• Cooling classification

• Rated high and low voltages

• Tap voltages either as actual voltages or as a percentage of the rated voltages

• Rated Frequency

• Rated kVA capacity

• Number of phases unless clearly indicated on the connection diagram

• Percent impedance at actual rated ambient temperature plus rated temperature rise in degrees Celcius

• Weight

• Connection diagram or equivalent information

• Vector diagram for polyphase transformers

• Terminal markings

• Enclosure number type if other then general purpose type 2.1 Serial number

The serial number is required any time the manufacturer must be contacted for information or parts. It should be recorded on all transformer inspections and tests record.

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2.2 Winding Temperature Class

There are five temperature classes which indicates the maximum permissible temperature rise on the windings at rated voltage and kVA output. These are based on rated ambient temperature between -30 ºC to 30 ºC and that the units be

installed at an elevation above sea level and below 1000 meters in elevation. Table 1: Transformer Winding Insulation System, °C

Temperature class Average winding rise measured by rise of resistance * Winding hottest-spot rise Class 130 75 90 Class150 90 110 Class180 115 140 Class200 130 160 Class220 150 180

* Higher average winding temperature rises by resistance may be permitted if the manufacturer provides thermal-design test data acceptable to the purchaser supporting that the temperature limits of the insulation are not exceeded.

Note: The average winding rise and hottest spot are based on an average daily ambient

temperature of 30°C with a maximum ambient of 40 °C at an elevation not exceeding 1000m and above sea level.

2.3 Temperature rise

Temperature rise is the maximum allowable temperature difference between the winding temperature at the rated ambient temperature at the rated voltage and kVA output that the transformer can operate. Temperature rise limits are also specified to other parts of the transformer.

Transformer temperature rise limits shall not be exceeded under the following conditions:

• For transformers with primary taps, delivery rated kVA at rated output voltage with the primary energized on the lowest tap,

• For any tap delivery rated output kVA at a lagging power factor of 80% or higher, with 105% output voltage,

• Energized at 110% output voltage, at no-load,

• Operating with approximately sinusoidal load current with a harmonic content less the .05 per unit.

Metallic parts in contact or adjacent to the insulation shall not attain a temperature in excess of the figures in column 1 of Table 1.

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Exposed metallic parts shall not exceed a temperature rise of 65 °C. Field terminations shall not exceed a temperature rise of 55 °C.

2.4 kVA Rating

Then power rating of a transformer is the output that can be delivered for the time specified, at rated secondary voltage and frequency without exceeding the

temperature rise limits for the transformer design.

Whenever a transformer is provided with taps and connected on taps above or below rated voltage, the capacity shall be the full rated kVA of the transformer.

Table 2: Standard kVA Ratings for Dry-Type Transformers Single phase Three-phase Single phase Three-phase

2 6 333 1000 3 9 500 1500 5 15 667 2000 10 30 833 2500 15 45 1000 3000 25 75 1250 3750 37 112 1667 5000 50 150 2500 7500 75 225 3333 0pen-ended 100 300 5000 --- 450 open-ended 167 500 250 750

Preferred self-cooled kVA capacities are shown in Table 2.

Preferred forced-cooled ratings are 133% of the self-cooled ratings.

Full voltage taps are full-capacity taps and provisioned with two 2.5% taps above and below rated voltage at rated kVA. Exceptions are for low voltage transformers and units where the purchaser has specified the tap ratings.

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2.5 Voltage Rating

The voltage rating is the operating voltage for the windings and associated tap positions.

Table 3: Preferred Voltage Ratings for Dry-type Transformers

Primary Voltage Secondary Voltage

Single-phase Three-phase Single-phase Three-phase

120 / 240 480 120 / 240 208 Y / 120 480 600 240 / 480 480 Y / 277 2,400 2,400 480 600 Y / 347 4,160 4,160 600 4160 Y / 2400 7,200 12,470 2,400 8,000 13,860 4,160 12,470 24,940 13,860 27,600 14,400 34,500 16,000 46,000 20,000 24,900 34,500 46,000 Notes:

1) If a three-phase transformer is to be applied in a wye-wye configuration, the supplier shall be so advised at the time of ordering.

2) If a three-phase transformer is manufactured for application in a wye-delta configuration, the supplier shall include a precautionary note on the transformer: “DO NOT CONNECT

PRIMARY SIDE TO SYSTEM NEUTRAL OR TO GROUND”.

3) Single-phase transformers with a voltage class of 2.5 kV and below shall be insulated for possible use within a three-phase wye-connected bank.

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2.6 Cooling Class

Cooling classifications are identified by three-letter designations according to the cooling characteristics.

The first letter designates the cooling medium surrounding the winding:

• A – for air

• G – for gas other than air

The second letter designates the method of circulating the cooling medium through the winding:

• N – for natural convection

• F – for forced circulation

The third letter designates the manner of removing the heat from the cooling medium:

Ventilated type:

• N – for natural circulation of outside air that is in contact with the windings

• F – for forced circulation of outside air that is in contact with the windings Enclosed type:

• C – for natural convection of outside air that is not in contact with the winding

• P – for forced circulation of outside air that is in not in contact with the windings

Cooling class examples:

ANN: Air-filled natural convection with open-ventilated enclosure through which the ambient air readily enters and leaves the enclosure

GNC: Gas-filled unit cooled by natural convection of the gas within the enclosure and by natural convection of ambient air outside the enclosure AFN: Air-filled cooled by forced circulation of the air within the open-ventilated enclosure and by natural convection of ambient air outside the enclosure

GNP: gas-filled unit cooled by natural convection of the gas within the enclosure and forced flow of ambient air over the outside of the enclosure

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ANC / ANP: Air-filled unit cooled by natural convection of the air within the enclosure and by natural convection of ambient air outside the enclosure and emergency operation at higher kVA cooled by natural convection of the air within the enclosure and by forced flow of ambient air over the outside of the enclosure

2.7 Polarity Markings

Polarity is a designation of the relative instantaneous directions of current in the leads of a transformer. The primary and secondary leads are said to have the same polarity on each half cycle. The current enters an indentified or marked primary lead and leaves the similarly identified or marked secondary lead in the same direction as if the two leads formed a continuous circuit.

The relative lead polarities are indicated by identification marks on the primary and secondary leads of like polarity or by other appropriate identification.

All single-phase transformers are of subtractive polarity unless otherwise specified: 2.8 Winding Designations

The windings are distinguished from one another as follows:

• The high voltage winding designated as HV or H and the low voltage winding designated as LV or X, for a two winding transformer

• Transformers with more than two winding shall have designations as H, X , Y and Z

• The highest voltage winding are designated as HV or H

• The other windings are designated in order of decreasing voltage as X, Y and Z

• If two or more windings have the same voltage rating and different kVA, the higher kVA winding are designated in order of sequence of X, Y and Z

• If two or more windings have the same kVA and voltage rating, the designation of the windings can be arbitrarily assigned

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2.9 Terminal markings

External terminals are identified from one another with a capital letter followed by a subscript number.

For a three-phase 2 winding transformer, the terminals of the H windings would be marked H1, H2 and H3 and the terminals of the X winding would be marked X1, X2

and X3.

The highest voltage winding are identified in phase sequence order by H1, H2, H3

and the other winding in order of voltage rating by X1, X2, X3 and Y1, Y2, Y3, etc.

The neutral terminals of a three-phase transformer are marked by the subscript 0 (e.g. H0, X0, etc). A neutral terminal common to two or more windings is marked

with the combination of the appropriate winding letters followed by the subscript 0 (e.g. H0X0 as for autotransformers).

The grounded terminal of a two-terminal transformer is designated with the subscript 2 and the undgrounded terminal is designated with the subscript 1.

2.10 Vector Diagrams

Vector diagram shows the angular displacement between phases as shown by the transformer terminal markings and is related to its winding diagram.

The vector lines of the diagram represent induced voltages using the recognized counter-clockwise direction of rotation for phase sequence. The vector

representing any phase voltage of a given winding is drawn parallel to the other phase voltages of the same winding that it is associated with.

2.11 Angular displacement between voltages of windings for three-phase transformers

The angular displacement between high-voltage and low-voltage phase voltages of three-phase transformers with Δ–Δ or Y–Y connections is 0°.

The angular displacement between high-voltage and low-voltage phase voltages of three-phase transformers with Y–Δ or Δ–Y connections is 30°, with the low

voltage lagging the high voltage by 30° (or high voltage leads the low voltage by 30°). Refer to Figure 1.

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The angular displacement of polyphase transformers is the time angle, expressed in degrees, between the line-to-neutral voltage terminal (H1) and the line-to-neutral voltage of the corresponding identified low-voltage terminal (X1).

Figure 1 - Phase relation of terminal designation for three-phase transformers.

Winding Diagram

The winding diagram shows the internal winding connections for the various tap positions and external bushing connection. Winding diagrams are normally incorporated into the vector diagram.

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2.12 Percent Impedance (%Z)

The percent impedance is the percent voltage required to circulate rated current flow through one winding when another winding is short-circuited at the rated voltage tap at rated frequency. %Z is related to the short circuit capacity of the transformer during short circuit conditions.

For a two winding transformer with a 5% impedance, it would require 5% input voltage applied on the high voltage winding to draw 100% rated current on the secondary winding when the secondary winding is short-circuited. If 100% rated voltage is applied to the high voltage winding, approximately 20X rated current would flow in the secondary winding when the secondary winding is short-circuited.

Table: 4 Impedance Levels

Based kVA Minimum Impedance, % 0 – 150 Manufacturer’s standard 151 – 300 4 301 – 600 5 601 – 2,500 6 2,501 – 5,000 6.5 5,001 – 7,500 7.5 7,501 – 10,000 8.5 Above 10,000 9.5 Notes:

a. The impedance of a two-winding transformer shall not vary from the guaranteed value by more that ± 7.5%

b. The impedance of a transformer having three or more windings or having zig-zag windings shall not vary from the guaranteed value by more than ± 10%

c. The impedance of an auto-transformer shall not vary from the guaranteed value by more than ± 10%

d. The difference of impedances between transformers of the same design shall not exceed 10% of the guaranteed values

e. Differences of impedance between auto-transformers of the same design shall not exceed 10% of the guaranteed values

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2.13 Basic Impulse Insulation Level

The line terminals of a winding are assigned a basic lightning impulse insulation level (BIL) to indicate the factory dielectric tests that these terminals are capable of withstanding.

The impulse level is the crest value of the impulse voltage during a lightning strike that the transformer is required to withstand without failure. The impulse level is a momentary withstand rating. The BIL ratings are shown in Table 5

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Table 5: Insulation Voltage Class and Dielectric Tests for Dry-type Transformer* Insulation Voltage

Class, kV

Nominal Voltage Applied Potential Test, kV

BIL, full and chopped-wave, kV crest Chopped-wave minimum time to flashover, µs 1.2 1,200 and less 4 --- ---2.5 2,400 10 20 1.0 5.0 4,160/2,400 4,160 4,800 12 30 1.0 8.7 8,320Y/4,800 7,200 8,320 19 45 1.25 15.0 12,470/7,200 13,860Y/8,000 12,470 13,860 31 60 1.5 18.0 24,940 GrdY/14,400 27,600 GrdY/16,000 34 95 1.6 25.0 35,500 GrdY/19,900 24,940Y/14,400 24,940 27,600Y/16,000 27,600 40 125 2.0 34.5 34,500 34,500Y/19,900 46,000 GrdY/26,600 50 150 2.25

* Higher voltage rating is available when specified Notes:

1) Applied potential test levels are for application to windings that do not have reduced neutral terminal insulation. 2) Equipment having a voltage rating between listed values shall be tested at the higher values.

3) Transformer terminals that may be subjected to transient overvoltages exceeding 80% of their BIL should be protected by surge arresters.

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3. Safety Consideration 3.1 High Voltage Safety

Many of the tests involve the use of high voltage test equipment; testing should be performed by qualified personnel familiar with the test set operations and the hazards associated with the tests.

Refer to Module 2 for Safety Working Practices and Guidelines.

Refer to IEEE Standard 510 – 1983, Recommended Practice for Safety in High Voltage & High Power Testing.

3.2 Electrostatic Charge

After any high potential voltage is removed, an electrical charge may be retained by the insulating system. Failure to discharge the residual electrostatic charge could result in an electrical shock. Always ground the last test point(s) before moving the test leads.

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4. Electrical Testing

Acceptance testing of transformers should be made after installation and prior to energizing a new transformer to determine the serviceability and to record data for future comparison.

Electrical test

Electrical tests are required before energizing the transformer. The required basic acceptance tests are:

• Insulation resistance test

• Applied-voltage test

• Ratio test.

Additional tests can also be performed to provide a more comprehensive testing regiment as required and to provide a set baseline data for comparison in future testing. Other Additional tests are:

• Polarizing index test

• Winding Resistance test

• Polarity test

• Power factor (dissipation factor) test

• Core Insulation est

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4.1 Insulation Resistance Test

The insulation resistance test is of value for future comparison and also for determining if the transformer is to be subjected to the applied voltage test. The winding insulation resistance test is a DC high voltage test used to determine the dryness of winding insulation system. The test measures the insulation resistance from individual windings to ground and/or between individual windings.

The measurement values are subject to wide variation in design, temperature, dryness and cleanliness of the parts. This makes it difficult to set minimum acceptable insulation resistance values that are realistic for wide variety of

insulation systems that are in use and performing satisfactorily. If a transformer is known to be wet or if it has been subjected to unusually damp conditions, it should be dried before the application of the applied voltage test. Low readings can

sometimes be brought up by cleaning or drying the apparatus.

The insulation resistance test should be performed at a transformer temperature as close as possible or at 20 °C. Test conducted at other temperature should be

corrected 20°C with the use of temperature correcting factor.

The test equipment is calibrated to read in Megohm and commonly know as a HV Megger. Typical maximum test set voltage values may be 1kV, 5kV or 15kV. The 30kV Megger is also available.

Duration of the test voltage shall be 1 minute.

In the absence of manufacture’s recommended values, the following readings may be used. Refer to Table 6.

Table 6: Transformer Insulation Resistance Acceptance Testing Winding Insulation Class, kV Insulation Resistance, MΩ*

1.2 600

2.5 1000

5.0 1500

8.7 2000

15 3000

* Normally dried transformers may be expected to have readings 5 to 10 times the above minimum values.

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Note:

Table 6 was sourced from IEEE C57-94-1982 Recommended Practice for

Installation, Operation and Maintenance of Dry-type General Purpose Distribution and Power Transformer. Table 6 differs from NETA Table 100.5 figures for

transformer Insulation Resistance Acceptance Testing values. There is no industry consensus for satisfactory values.

Other references noted a general rule of thumb for acceptable insulation values at 1MΩ per 1kV of nameplate rating plus 1MΩ.

Note:

Under no condition should the test be made while the transformer is under vacuum. Note:

The significance of values of insulation resistance test requires some interpretation depending on design, dryness and cleanliness of the insulation involved. It is

recommended that the insulation resistance values be measured during periodic maintenance shutdown and trended. Large variation in the trended values should be investigated for cause.

Note:

Insulation resistance may vary with applied voltage and any comparison should be made with the same measurements at the same voltage and as close as possible to the same equipment temperature and humidity as practically possible.

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4.1.1 Insulation Resistance Test Procedure:

1. Isolate the equipment, apply working grounds to all incoming and outgoing cables and disconnect all incoming and outgoing cables from the transformer bushing terminals connections. Disconnected cables should have sufficient clearance from the switchgear terminals greater that the phase spacing distance. Use nylon rope to hold cable away from incoming and outgoing terminals as required.

2. Ensure the transformer tank and core is grounded.

3. Disconnect all lightning arresters, fan system, meter or low voltage control systems that are connected to the transformer winding.

4. Short circuit all winding terminals of the same voltage level together.

5. Perform a 1 minute resistance measurements between each winding group to

the other windings and ground.

6. Remove all shorting leads after completion of all test.

Table 7: Insulation Resistance Test Connections for Two Winding Transformer Test No. Single-phase transformer Three-phase transformer

1 High voltage winding to low voltage winding and ground

High voltage winding to low voltage winding and ground

2 High voltage winding to low voltage winding

High voltage winding to low voltage winding

3 High Voltage winding to ground

High voltage winding to ground with low voltage winding to guard

4 Low Voltage winding to high voltage winding and ground

Low voltage winding to high voltage winding and ground

5 Low voltage winding to ground

Low voltage winding to ground and high voltage winding to guard

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4.2 Applied-Voltage Test

The applied-voltage (high-potential dielectric withstand) test is a 60 Hz, AC high voltage test used to verify the integrity of the insulation system between the winding being tested to all other windings and to ground. The test is conducted at the specified voltage level for 1 minute.

Field dielectric test are conducted at 75% of the factory voltage test levels of the test values listed in Table 8. When field test are made on a periodic basis, it is recommended that the test voltages be limited to 65% of factory test.

Note: If dc test equipment is used, the test voltage should not exceed the factory rms test voltage. Test equipment should be of the full-wave bridge design with ripple content should be less than 1%.4

4.2.1 Voltage Rate of Rise

The voltage should be brought up gradually to full value in not more than 15 seconds and held for 1 minute. It should then be gradually reduce in not more that 5 seconds.3_{This is a stark contrast to the circuit breaker’s vacuum bottle integrity}

test at which the voltage rate of rise is much lower where a fast rising voltage may cause a vacuum dielectric breakdown.

3 _{IEEE Standard C57.12.91-2001}

Test Code for Dry-type Distribution and Power Transformers 4 _{ANSI/IEEE C57.94-1982}

Recommended Practice for Installation, Application, Operation and Maintenance of Dry-type General Purpose Distribution and Power Transformers

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4.2.2 Testing Guidelines

1. A winding-to-winding and winding-to-ground applied voltage test shall be made in accordance with Table 8 on Δ and Y-connected windings when the neutral is ungrounded. This suggests that ungrounded Y winding are fully insulated to the line terminal values.

2. For internally solidly grounded-Y windings;

a) A line-terminal-to-ground test voltage shall be induced from another winding. This test voltage shall be twice the operating line-terminal-to-neutral voltage, with the line-terminal-to-neutral grounded;

b) A phase-to-phase test voltage shall be induced from another winding, which will develop twice the operating phase-to-phase voltage between line

terminals.

c) Twice the rated turn-to-turn voltage shall be developed in each winding. Note: Voltage levels may have to be reduced such that no winding need be

tested above its specified level to meet the test requirements of another winding.

3. The voltage should be started at one-quarter or less of the full value and be brought up gradually in not more than 15 seconds. After being held for the specified duration, it should be gradually reduced in not les than 5 seconds to one-quarter of the maximum value and then turned off.

Caution:

It is important to recognize that the neutral terminal of a wye-connected

transformer that is designed for a grounded wye connection may have an insulation level lower than that of the line terminal. The insulation level of the neutral end of the winding may differ from the insulation level of the highest voltage neutral terminal. In such case, the dielectric test on the neutral shall be determined by lower insulation value of the neutral terminal or the neutral end of the winding.

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Table 8: Insulation Voltage Class and Dielectric tests for Dry-type Transformers (Factory Test Levels)

Insulation Voltage Class, kV

Nominal Transformer Voltage

Applied Potential Test, kV 1.2 1,200 and less 4 2.5 2,400 10 5.0 4,160 / 2,400 4,160 4,800 12 8.7 8,320Y / 4,800 7,200 8,320 19 15.0 12,470 / 7,200 13,860Y / 8,000 12,470 13,860 31 18.0 24,940 GrdY / 14,400 27,600 GrdY / 16,000 34 25.0 35,500 GrdY / 19,900 24,940Y / 14,400 24,940 27,600Y / 16,000 27,600 40 34.5 34,500 34,500Y / 19,900 46,000 GrdY / 26,600 50 Notes:

1) A single voltage of 4160 represents a delta-connected 3-phase transformer for

connection to a 3-wire system or a single-phase transformer for connection to a 2-wire ungrounded system. A voltage such as 4160 GrdY / 2400 represents a wye-connected 3-phase transformer having an effectively grounded neutral for connection to a 4-wire multigrounded system. It also represents a 2400 V single-phase transformer with an effectively grounded neutral end in the 2400 V winding. I may be connected line-to-ground or as part of a 3-phase bank on a 4160 V 4-wire multiline-to-grounded system. A voltage such as 4160Y / 2400 represents a 3-phase transformer with a fully insulated neutral for connection to a 416 V 4-wire system.

2) Power frequency dielectric withstand test levels are for application to windings that do not have reduced neutral terminal insulation.

3) Equipment having a voltage rating between listed values shall be tested at the higher level.

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Figure 1: Applied Voltage Test of the HV winding to LV winding and Ground

Figure 2: Applied Voltage Test of the LV winding to HV winding and Ground

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4.2.3 Applied Potential Test Procedure

(Two winding Δ-Y transformer with 100% neutral insulation level)

1. Isolate the equipment, apply working grounds to all incoming and outgoing

cables and disconnect all incoming, outgoing cables from the transformer bushing terminals connections. Disconnected cables should have sufficient clearance from the switchgear terminals greater that the phase spacing distance. Use nylon rope to hold cable away from incoming and outgoing terminals as required.

2. Isolate the neutral bushing connection from transformer grounding bar. 3. Short-circuit all high voltage bushing terminals together.

4. Short-circuit all low voltage bushing terminals and the neutral bushing terminal together.

5. Perform a high-voltage winding to low-voltage winding and ground test, at

75% of the specified value in Table 5 for the high-voltage winding insulation class level.

Refer to Figure 1.

6. Attain the test voltage level at a constant rate of rise, to achieve the test voltage level at the end of 15 seconds.

7. Hold the test voltage for 1 minute. This is a go-no-go test.

8. Reduce the voltage to zero at a constant rate of decline, no faster than 5

seconds.

9. Perform a low-voltage winding to high-voltage winding and ground test, at

75% of the specified value in Table 5 for the low-voltage winding insulation class level.

Refer to Figure 2.

10. Repeat steps 6 through 8.

11.Remove all shorting jumpers after all tests are completed. Reconnect neutral

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4.3 Turns Ratio Test

The turns ratio test is an AC low voltage test which determines the ratio of the high voltage winding to all other windings at no-load. The turns ratio test is performed on all taps of every winding.

The Transformer Turns Ratio tester (TTR) is used to measure the turns ratio between the windings.. Voltage is applied on the H marked leads and measured of the X marked lead by the test set. Ratio measurements are conducted on all tap positions and calculated by dividing the induced voltage reading into the applied voltage value

When ratio tests are being made on three-phase transformers, the ratio is taken on one phase at a time with a three-phase TTR until the ratio measurements of all three phases are completed. Measured ratio variations should be within .5% of the nameplate markings.

Some TTR can perform transformer ratio measurement and also assess if on-load tap changer contacts are making satisfactorily during its transition from one tap position to the next position.

Figure 3: Three-phase TTR Connection diagram.

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4.3.1 Turns Ratio Test Procedure

2. Connect the H designated three-phase test lead with the military style

connector at one end to the mating connection on the test set marked with an H. Ensure that the connector’s index notch lines up properly.

3. Connect the X designated three-phase test of lead military style connector at

one end to the mating connection on the test set marked with an X. Ensure that the connector’s index notch lines up properly.

4. Connect the H1, H2, H3 designated test lead to the corresponding H1, H2,

H3 transformer terminal / bushing. Connect the H0 test lead if H0 terminal/bushing is present.

Refer to Figure 3.

5. Connect the X1, X2, X3 designated test leads to the corresponding X1, X2,

X3 transformer terminals / bushings. Connect the X0 test lead if X0 terminal/bushing is present.

6. Perform turns ratio measurements for all tap positions.

7. Confirm that the measured ratios is within .5% of the calculated ratios. Note:

Transformers that have wye connections but do not have the neutral of the wye brought out shall be tested for ratio with three-phase power supply. Any inequality in the magnetizing characteristics of the three phases will then result in a shift of the neutral and thereby cause unequal phase voltages. When such inequality is found, the connection should be changed, either to a delta or to a wye connection, and the line voltages measured. When these are found to be equal to each other and the proper values (1.732 times the phase voltages when connected in wye), the ratio is correct.

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4.3 Polarizing Index Test

The Polarizing Index (PI) test is an extension of the winding insulation resistance test. Two insulation resistance measurements are taken, one reading at 1 minute and the second reading at 10 minute.

Polarisation Index = R10 / R1

Where:

R10 = megohms insulation resistance

at 10 minutes

R1 = megohms insulation resistance at 1 minute

The 10 minute resistance is divided by the 1 minute resistance to give the PI value. The PI indicates the relative dryness and level of moisture ingress into the

insulation.

A PI of winding to winding and winding to ground should be determined. A PI below 2 calls for further investigation.

Readings at every minute intervals are recorded for graphing purposes. A falling off of the insulation value near the testing time could indicate insulation problems.

Figure 4: Example of Time Dependent Insulation Readings for Normal and Poor Insulation.

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4.3.1 Polarizing Index Test Procedure

Note: The polarizing index tests should be combined with the insulation resistance test by extending the required test time to 10 minutes. This will minimize testing time when both tests are combine versus performing two separate tests.

2. Ensure the transformer tank and core is grounded.

3. Disconnect all lightning arresters, fan system, meter or low voltage control systems that are connected to the transformer winding.

4. Short circuit all winding terminals of the same voltage level together.

5. Perform a 10 minute resistance measurements between each winding

group to the other windings and ground. Record all data points required at the various intervals;

Every 15 seconds for the first minute, Every 30 seconds up to 4 minutes,

Every 1 minute up to 10 minutes test time.

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4.4 Winding Resistance Test

The winding resistance test is a low voltage constant DC current test used to measure the ohmic values of the individual transformer windings. The ohmic values can be used for:

- Calculations of the I2_{R component of conductor losses.}

- Calculation of winding temperature at the end of a temperature test cycle. - As a base for assessing possible damage in the field.

Regardless of the configuration either wye or delta, the winding resistance

measurements are made phase-to-phase and comparisons are made to determine if the readings are comparable.

If all readings are within 1% of each other then they are acceptable.

The purpose of the test is to test for gross differences between the windings and for checking if there are open circuits in the connections. Measuring the resistance of the windings assures that the connections are correct and the resistance

measurement indicates that there are no severe mismatches or open. Operating Principles

A DC current is passed through the transformer winding and an internal standard current shunt in the test set. After both DC voltage drops are measured they are ratioed and the display is read as resistance on the front panel meter.

Measurement of the voltage drop across the winding uses the standard formula for a voltage drop across an inductor, where:

VL = IL x R + (L di/dt) = voltage across the transformer winding

IL = DC current through transformer winding

R = resistance of the transformer winding L = inductance of the transformer

With zero ripple voltage, then di/dt is zero and the term L di/dt becomes zero.

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Caution:

When a DC current is flowing in an inductive element, opening the circuit will be generate high voltages during kickback from the collapsing magnetic field in the coil. DO NOT open the circuit during testing

When terminating the test, wait until the “charged” indicator on the test set goes off before removing any leads. If it takes 30 second for the winding to charged, it will take a longer time for the winding to discharge.

When transferring leads from one winding to another, the same relative polarity should be maintained with regard to the measuring leads and the transformer terminals.

Limitations

The transformer resistance test has several limitations which should be recognized when performing the test and interpreting test data:

The information obtained from winding resistance measurements on delta

connected windings is somewhat limited. Measuring from the corners of a closed delta the circuit is two windings in series and in parallel with the third winding. The resistance of the transformer's winding can limit the effectiveness of the test in detecting problems. The lower the resistance of a winding the more sensitive the test is with respect to detecting high resistance problems. Windings with high DC resistance will mask problems.

The detection of shorted turns is not possible in all situations. Often shorted turns at rated AC voltage cannot be detected with the DC test. If the fault is a carbon path through the turn to turn insulation it is a dead short at operating potentials. However, at test potential, 30 V DC, the carbon path may be a high resistance parallel path and have no influence on the measured resistance.

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4.4.1 Winding Resistance Test Procedure

2. Select the correct winding metal type (aluminum or copper) on the test set if

provided. Resistivity of copper and aluminum is different.

3. For delta or wye connected windings, connect +ve DC current source lead to the H1 (or X1) terminal and the –ve DC current source lead to the H2 (or X2) terminal.

4. Connect the +ve DC voltage sensing lead to the H1 (or X1) terminal closer

toward the winding and the –ve DC voltage sensing lead to the H2 (or X2) terminal closer toward the winding.

5. Perform winding measurement for the H1-H2 ( or X1-X2) winding. 6. Repeat step 3 and 4 for H2-H3 (or X2-X3) and H3-H1 (or X3-X1) while

maintaining the relative polarity of the test leads.

7. For Wye connected windings, connect +ve DC current source lead to the H1 (or X1) terminal and the –ve DC current source lead to the H0 (or X0)

terminal.

8. Connect the +ve DC voltage sensing lead to the H1 (or X1) terminal closer toward the winding and the –ve DC voltage sensing lead to the H0 (or X0) terminal closer toward the winding.

9. Perform winding measurement for the H1-H0 ( or X1-X0) winding. 10. Repeat step 7 and for H2-H0 (or X2-X0) and H3-H0 (or X3-X0) while

maintaining the relative polarity of the test leads.

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4.6 Power Factor Test

The power factor test is a maintenance test used to determine the insulation system dielectric power loss by measuring the power angle between an applied AC voltage and the resultant current. Power factor is defined as the ratio of the power

dissipated divided by the input volt-ampere multiplied by 100%. This test may be required to be performed during the acceptance testing stage to establish a baseline reading for future test comparison.

Figure 5: Insulation System Equivalent Circuit and Power Factor Vector Diagram

Dielectric losses in and power factor can be calculated by: Watts = E x IT x Cosine Ө

Power factor = Cosine Ө = Watts / (E x IT)

PF test is performed for detecting insulation deterioration or degradation usually caused by moisture, carbonization or other forms of contaminants of the winding and bushing. Winding distortions results in a change in winding capacitance and short-circuited and partially short-circuited turns result in abnormally high excitation current.

Types of transformers that are normally subjected to the power factor test are: - Two winding transformers

- Three winding transformer - Auto-transformers

- Instrument transformers

Other test names synonymous with the PF test are Dielectric loss angle, dissipation factor test, tan delta or Doble test.

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General conditions required for testing transformers are:

1. The unit must be de-energized and isolated from the power system including the neutral connection from ground.

2. Transformer enclosure must be properly grounded and applicable to when testing spare units.

3. All terminal of each winding are short-circuited together including the neutral terminals. This will minimize the effect of winding inductance during testing.

4. LTC should be set of neutral if it has arrester-type elements that are not effectively short circuited in the neutral position.

The power factor test typically applies a test voltage less than the stress working levels of the equipment. Refer to Table 6.

Table 6 Recommended Power factor Test Voltage for Dry-type Power Transformer connected in delta and ungrounded-wye

Winding Rating Line-to-line, kV

Test Voltage

Line-to-ground, kV

Above 14.4 2 and 10

12 to 14.4 2, 10 and at operating line-to-ground voltage

5.04 to8.72 2 and 5

2.4 to 4.8 2

Below 2.4 1

Note: Transformer windings provided with neutral insulation rating which is less than the line insulation rating should be tested below the neutral insulation rating level.

The required tests are noted in Table 7 and its connection is shown in Figure 6 and Figure 7. The difference between the high-voltage winding tests and the low-voltage winding tests are the placement of the test leads and the test low-voltage levels. Test 3 and test 8 of Table 7 should generate the same value as they both measure the same capacitance between the windings.

Table 7 Power Factor test connection for two winding dry-type transformer Test Number Test Mode Energized Winding

Ground Guard UST Measure

1 GST High Low --- --- CH+CHL

2 GST High --- Low --- CH

3 UST High --- --- Low CHL

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4 Calculate Test 1 minus Test 2 CHL

5 GST Low High CL+CHL

6 GST Low High CL

7 UST Low High CHL

8 Calculate Test 5 minus Test 6 CHL

Figure 6: Power Factor High Voltage Winding Test Connection

Figure 7: Power Factor Low Voltage Winding Test Connection

Caution: Always ground to the previously energized terminal with a grounding stick before making any connection changes to bleed off any electrical charge that may be present. Leave the grounding connected until connection changes is completed and before the start of the next test.

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4.6.1 Power Factor Test Procedure (Two winding dry-type transformer)

1. Isolate the equipment, apply working grounds to all incoming and outgoing cables and disconnect all incoming and outgoing cables from the transformer bushing terminals. Disconnected cables should have sufficient clearance from the switchgear terminals greater that the phase spacing distance. Use nylon rope to hold cable away from incoming and outgoing terminals as required.

2. Isolate the neutral bushing connection if applicable from the transformer

grounding bar.

3. Short-circuit all high voltage bushing terminals together.

5. Connect the power factor test set.

Refer to Table 7 for the test measuring mode and associated test number. 6. Apply the specified test voltage levels as noted in Table 2.

7. Record PF and watts loss values.

8. Repeat step 5 to 7 until all tests are completed

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4.7 Capacitance and Dissipation Factor Test

The capacitance and dissipation factor test is an AC low voltage maintenance test and is very similar to the power factor test. The test as it is termed, measures the capacitance and dissipation factor (or loss factor) of the transformer insulation system. This test may be required to be performed during the acceptance testing stage to establish a baseline reading for future test comparison.

While the transformer preparation is identical to the power factor test procedure, there is no requiremnts to make connection changes once the initial test set connections are made. High-voltage winding and low-voltage winding test set connection changes are made through a selector switch provided on the test set which effect the test set connections akin to Table 7.

Winding capacitance and dissipation factor test values are obtained by balancing a null meter for each variable at every the measured variable selector switch

positions.

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4.7.1 Capacitance and Dissipation Factor Test Procedure (Two winding dry-type transformer)

2. Isolate the neutral bushing connection if applicable from the transformer grounding bar.

3. Short-circuit all high voltage bushing terminals together.

5. Connect the capacitance and dissipation factor test set. Refer to Figure 8.

6. Record the capacitance and dissipation factor values once the null meter is

balance for both phasing position. Record values for the five test-variable selector switch position.

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4.8 Phasing Test

The vector diagram of any three phase transformer which identifies the angular displacement and phase sequence can be verified by connecting the H1 and X1 leads together and then exciting the high-voltage windings at a suitably low voltage with a know phase sequence from the three phase source. Voltage measurements are taken between various pairs of lead and comparing them for their relative magnitude with the aid of the corresponding vector diagrams.

This section is limited to the two-winding transformer having a phase displacement of 0 and 30 degrees. Six-phase unit are excluded from this discussion but can be referenced in IEEE C57.12.91 – 2001 Standard Test Code for Dry-type

Distribution and Power Transformer. Refer to Table 8.

4.8.1 Phasing Test Procedure

2. Isolate the neutral bushing connection if applicable from the transformer grounding bar.

3. Connect H1 and X1 terminals.

4. Apply a low voltage having A-B-C phase sequence to H1, H2 and H3

terminals respectively. Select an applied voltage to effect proper measurement.

5. Measure the voltages between the various terminals as shown in Table 8 for the respective winding group.

6. Ensure the voltage check measurements conforms to the conditions as

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Table 8 Three-phase Transformer Vector Diagram and Terminal Markings Group 1 having 0 degree angular displacement

Angular Displacements Measurement Connection Measurement Checks Connect: Measure: Check: H1 to X1 H1-H2, H2-X2, H2,X3, H3-X2, H3-X3 H2-H3 = H3-X2, H2-X2 < H1-H2, H2-X2 < H2-X3, H2-X2 = H1-X3 Page 41 of 55

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Table 8 Three-phase Transformer Vector Diagram and Terminal Markings Group 2 having 30 degree angular displacement

Angular Displacements Measurement Connection Measurement Checks Connect: Measure: Check: H1 to X1 H1-H3, H2-X2, H2-X3, H3-X2, H3-X3 H3-X2 = H3-X3 H3-X2 < H1-H3 H2-X2 < H2-X3 H2-X2 < H1-H3

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5. NETA Large Dry-type, Air Cooled Transformer Acceptance Testing Procedure

Note: The NETA transformer acceptance testing procedures applicable to the MV large dry-type class has been recompiled with minor revisions.

Refer to ANSI/NETA ATS-2009, Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems.

Note: Many of the listed tests in the NETA standard are optional while optionally indicate tests may be required to be performed; as such, those notes have been excluded in this document. Any personnel responsible for performing acceptance testing should determine which specific tests are required for each specific projects.

5.1 Visual and Mechanical Inspection

1. Compare equipment nameplate data with drawings and specifications. 2. Inspect physical and mechanical condition.

3. Inspect anchorage, alignment, and grounding.

4. Verify that resilient mounts are free and that any shipping brackets / bolts have been removed.

5. Verify the unit is clean.

6. Verify that control and alarm settings on temperature indicators are as specified. 7. Verify that cooling fans operate and that fan motors have correct overcurrent

protection.

8. Inspect bolted electrical connections for high resistance using one or more of the following methods:

a. Use of a low-resistance ohmmeter.

b. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 2 of module 2 or Table 100.12 of NETA ATS-2009.

c. Perform thermographic.

9. Perform specific inspections and mechanical tests as recommended by the manufacturer.

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10.Verify that as-left tap connections are as specified. 11.Verify the presence of surge arresters.

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5.2 Electrical Tests

1. Perform insulation-resistance tests to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data apply 2500 Vdc for

winding rating up to 5000 Vac and apply 5000 Vdc for winding rating over 5001 Vac.

2. Perform power-factor or dissipation-factor tests on all windings in accordance with the test equipment manufacturer’s published data.

3. Perform a power-factor or dissipation-factor tip-up test on windings greater than 2.5 kV.

4. Perform turns-ratio tests at all tap positions. 5. Perform an excitation-current test on each phase.

6. Measure the winding resistance at each tap connection.

7. Measure core insulation resistance at 500 Vdc to determnine if the core is insulated.

8. Perform an applied voltage test on all high and low-voltage windings-to-ground. See ANSI/IEEE C57.12.91 sections 10.2 and 10.9.

9. Verify correct secondary voltage, phase-to-phase and phase-to-neutral, after energization and prior to loading.

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5.3 Test Values

5.3.1 Test Values – Visual and Mechanical

1. Control and alarm settings on temperature indicators shall operate within manufacturer’s recommendations for specified settings.

2. Cooling fans should be operational.

3. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.

4. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data or table 2 of module 2 or Table 100.12 of NETA ATS-2009

5. Results of the thermographic survey shall be in accordance with Section 9. 6. Tap connections shall be left as found unless otherwise specified.

5.3.2 Test Values – Electrical

1. Compare bolted connection resistance values to values of similar connections. Investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.

2. Minimum insulation-resistance values of transformer insulation shall be in accordance with manufacturer’s published data. In the absence of

manufacturer’s published data, use Table100.5 or refer to section 4.1, Table 6.

Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated. The polarization index shall not be less than 1.0.

3. CH and CL power-factor or dissipation-factor values will vary due to support insulators and bus work utilized on dry transformers. The following shall be expected on CHL power factors:

Power transformers: 2.0 percent or less Distribution transformers: 5.0 percent or less

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Consult transformer manufacturer’s or test equipment manufacturer’s data for additional information.

4. Power-factor or dissipation-factor tip-up exceeding 1.0 percent shall be investigated.

5. Turns-ratio test results shall not deviate more than one-half percent from either the adjacent coils or the calculated ratio.

6. The typical excitation current test data pattern for a three-legged core transformer is two similar current readings and one lower current reading. 7. Temperature-corrected winding-resistance values shall compare within

one percent of previously-obtained results.

8. Core insulation-resistance values shall not be less than one megohm at 500 volts dc.

9. AC dielectric withstand test voltage shall not exceed 75 percent of factory test voltage for one minute duration. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the

dielectric withstand test, the test specimen is considered to have passed the test. 10.Phase-to-phase and phase-to-neutral secondary voltages shall be in agreement

with nameplate data.

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6. Test Set Operational Manual

Three-phase Transformer Turns Ration Test Set Digital Low Resistance Ohmmeter

Power Factor Test Set

Capacitance and Dissipation Factor Test Set 10 kV Insulation Resistance Test Set

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7. Test Forms

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CUSTOMER DATA

Name: EITCA Date (mm/dd/yy):

Address: 4234 – 93 STREET City, Province: EDMONTON ALTA

Location: INDOOR Address: 4234 – 93 STREET

Substation: TRAINING LAB Postal code: T6E 5P5

Panel ID: Phone: 780-462-5729

Equipment ID: Contact person: MR. R. MATTHEWS

Tested by:

EQUIPMENT NAMEPLATE DATA

Make: HV Winding rating: kV

Type: LV Winding rating: kV

Load Rating: kVA BIL Rating; HV / LV / kV

Impedance: %

Number of taps: Temperature rise: / ºC

Manufactured Date: Cooling Class:

HV Winding material Total weight:

LV Winding material Winding configuration: /

VISUAL / MECHANICAL INSPECTION

Physical condition: Primary winding connection: Bushing Integrity:

Cleanliness: Sec. winding connection: Winding condition:

Anchorage: Core temperature sensors: Fan /fan motor condition:

Ground bus connections: Core grounding strap: Temp. controller integrity:

Comments:

INSULATION RESISTANCE TEST

H WINDINGS L WINDINGS

H to L & Gnd MΩ L to H & Gnd MΩ

H to Gnd MΩ L to Gnd MΩ

H to LV MΩ --- ---

---Test Voltage kV Test Voltage kV

Test Duration sec Test Duration sec

Temperature: °C Humidity: %

Test Equipment: Calibration Date:

Serial Number: Test Equipment ID:

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WINDING RATIO TEST T A P HV WINDING RATING CALC RATIO H1-H3 / X1-X0 H2-H1 / X2-X0 H3-H2 / X3-X0 MEAS RATIO

DIFF EXCIT RATIO DIFF EXCIT RATIO DIFF EXCIT

% mA % mA % mA 1 V 2 V 3 V 4 V 5 V

As found tap conn / rating: As left tap conn. / rating:

Serial number: Test Equipment ID:

Comments:

APPLIED VOLTAGE TEST

H WINDINGS L WINDINGS

H to L & Gnd MΩ L to H & Gnd MΩ

Test Voltage kV Test Voltage kV

Test Duration sec Test Duration sec

Comments:

WINDING RESISTANCE TEST

T A P

HIGH VOLTAGE WINDING LOW VOLTAGE WINDING

H1-H2 H2-H3 H2-H1 X1-X0 X2-X0 X3-X0

mΩ mΩ mΩ mΩ mΩ mΩ

1 --- ---

---2 LOW VOLTAGE WINDING

3 X1-X2 X2-X3 X3-X1

4 mΩ mΩ mΩ

5 --- ---

---As found tap conn / rating: As left tap conn. / rating:

Comments:

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POLARIZING INDEX TEST / INSULATION RESISTANCE TEST

H to L & Gnd L to H & Gnd

TIME RESITANCE TIME RESITANCE

15 sec GΩ 15 sec GΩ 30 sec GΩ 30 sec GΩ 45 sec GΩ 45 sec GΩ 1 min GΩ 1 min GΩ 1.5 min GΩ 1.5 min GΩ 2 min GΩ 2 min GΩ 2.5 min GΩ 2.5 min GΩ 3 min GΩ 3 min GΩ 3.5 min GΩ 3.5 min GΩ 4 min GΩ 4 min GΩ 5 min GΩ 5 min GΩ 6 min GΩ 6 min GΩ 7 min GΩ 7 min GΩ 8 min GΩ 8 min GΩ 9 min GΩ 9 min GΩ 10 min GΩ 10 min GΩ

Test Voltage: Vdc Test Voltage: Vdc

P.I. Ratio P.I. Ratio:

Temperature: °C Humidity: % %

Comments:

CORE INSULATION RESISTANCE TEST

Test Voltage: 500 Vdc Core Ground Resistance: MΩ

Comments:

CAPACITANCE / DISIPATION TEST

TEST

CAPACITANCE DISSIPATION

MULTI

pF READINGDIAL

VALUE RANGE DIAL

READING VALUE 20 ºC CORR pF % CHL + CHG CHG CHL CLG CLH + CLG

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VECTOR DIAGRAM

PHASING TEST (H1 CONNECTED TO X1)

Measured voltage Voltage Relationship Voltage Checks

---H1-H3: V H3-X2 = H3-X3

---H2-X2: V H3-X2 < H1-H3

---H2-X3: V H2-X2 < H2-X3

---H3-X2: V H2-X2 < H1-H3

---H3-X3: V --- ---

---Applied Voltage: V Phase Sequence ---

---Comments:

TSTING NOTES and OBSERVATIONS

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8. Transformer Test Form References and Suggested Reading 1. CSA Standard C9-02

Dry-Type Transformers

Missisauga, Ontario, L4W-5N6, CANADA ISBN 1-55397-063-2

2. IEEE Standard C57.12.01-2005

General Requirements for Dry-type Distribution and Power Transformer, Including Those with Solid-Cast and/or Resin Encapsulated Windings © 2006 by the Institute of Elelctrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA

Print: ISBN 0-7381-4880-6 PDF: ISBN 0-7381-4881-4 3. IEEE Standard C57.12.91-2001

Test Code for Dry-type Distribution and Power Transformers

Print: ISBN 0-7381-2734-5 PDF: ISBN 0-7381-2735-3 4. ANSI/IEEE C57.94-1982

Recommended Practice for Installation, Application, Operation and Maintenance of Dry-type General Purpose Distribution and Power Transformers

5. ANSI/NETA ATS 2009 American National Standard

Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems

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References and Suggested Reading 6. Doble Test Procedures

85 Walnut Street, Watertown, Massachusetts, 02472-4037, USA PN 500-0397