Transformer Testing & Maintenance Fundamentals

Transformer Testing & Maintenance Fundamentals1 AVO Training Institute, Inc. 2018

Moderatorn Ron SpataroAVO Training Institute Marketing Manager2

Q&An Send us yourquestions andcomments duringthe presentation3

Today’s PresenterMike CarterAVO Training Institute, Senior Instructor andCurriculum Advisor4

Definition of a Transformers The transformer is a static electric device consistingessentially of two or more windings wound on thesame core Electromagnetic induction transforms electric energyfrom one circuit to another circuit such that thefrequency of the energy remains unchanged while thevoltage and current usually change5

Transformer Testing and Maintenance Standards Manufacturer’s Instruction Manuals: It is important to follow the recommendationscontained in the manufacturer’s published data. Many of the details of a complete and effective testingprocedure can be obtained from this source.6

Transformer Testing and Maintenance Standards NETA: The InterNational Electrical Testing Association(NETA) is an accredited standards developer for theAmerican National Standards Institute (ANSI). NETA defines the standards by which electricalequipment is deemed safe and reliable. NETA is the leading source of specifications,procedures, testing, and requirements. NETA is not only for commissioning new equipmentbut for testing the reliability and performance ofexisting equipment.7

Transformer Testing and Maintenance Standards Specifications and publications: ANSI/NETA MTS: The standard for maintenancetesting specifications for electrical power equipmentand systems. ANSI/NETA ATS: The standard for the acceptancetesting specifications for electrical power equipmentsystems.8

Transformers, Dry Type, Air-Cooled, Largen Dry Type, Air Cooled, Large Transformer9

Transformers, Dry Type, Air-Cooled, Largen Transformers, Dry Type, Air-Cooled, Largen This category consists of power transformers withwindings rated higher than 600 volts and low-voltagetransformers larger than 167 kVA single-phase or 500 kVAthree-phase.n Visual and Mechanical Inspectionn 1. Inspect physical and mechanical condition includingevidence of moisture and corona.n 2. Inspect anchorage, alignment, and grounding.10

Transformers, Dry Type, Air-Cooled, Largen Visual and Mechanical Inspectionn 3. Prior to cleaning the unit, perform as-found tests, ifrequired.n 4. Clean the unit.n 5. (Optional) Verify that control and alarm settings ontemperature indicators are as specified.n 6. Verify that cooling fans operate correctly.11

Transformers, Dry Type, Air-Cooled, Largen Visual and Mechanical Inspectionn 7. Inspect bolted electrical connections for high resistanceusing one or more of the following methods:n 1. Use of a low-resistance ohmmeter.n 2. Verify tightness of accessible bolted electricalconnections by calibrated torque-wrench method inaccordance with manufacturer’s published data or NETATable 100.12.n 3. Perform a thermographic survey.12

Transformers, Dry Type, Air-Cooled, Largen Visual and Mechanical Inspectionn 8. Perform specific inspections and mechanical tests asrecommended by the manufacturer.n 9. Perform as-left tests.n 10. Verify that as-left tap connections are as specified.n 11. Verify the presence of surge arresters.n Electrical Testsn 1. Perform resistance measurements through boltedconnections with a low-resistance ohmmeter .13

Transformers, Dry Type, Air-Cooled, Largen Electrical Testsn 2. Perform insulation-resistance tests winding-to-windingand each winding-to-ground. Apply voltage in accordancewith manufacturer’s published data. In the absence ofmanufacturer’s published data, use NETA Table 100.5.Calculate polarization index.n 3. Perform insulation power-factor or dissipation-factortests on all windings in accordance with the test equipmentmanufacturer’s published data.n 4. (Optional) Perform a power-factor or dissipation-factortip-up test on windings rated greater than 2.5 kV.14

Transformers, Dry Type, Air-Cooled, Low-Voltage, LargeANSI/NETA MTS 2015 Table 100.5Transformer Insulation Resistance15

Transformers, Dry Type, Air-Cooled, Largen Electrical Testsn 5. Perform turns-ratio tests at the designated tap position.n 6. Perform an excitation-current test on each phase.n 7. (Optional) Measure the resistance of each winding atthe designated tap position.n 8. Measure core insulation resistance at 500 volts dc if thecore is insulated and if the core ground strap is removable.16

Transformers, Dry Type, Air-Cooled, Low-Voltage, Largen Test Values – ElectricalTypical TTR Test17

Transformers, Dry Type, Air-Cooled, Largen Electrical Testsn 9. (Optional) Perform an applied voltage test on all highand low-voltage windings-to-ground. See ANSI/IEEEC57.12.91, Sections 10.2 and 10.9.n 10. Verify correct secondary voltage phase-to-phase andphase-to-neutral after energization and prior to loading.n 11. Test surge arresters. (Two of the most common teststo perform in the field on surge arresters are the powerfactor test and infrared analysis.)18

Transformers, Dry Type, Air-Cooled, Largen Test Values – Visual and Mechanicaln 1. Alarm, control, and trip circuits from temperature andlevel indicators as well as pressure relief device and faultpressure relay should operate within manufacturer’srecommendations for their specified settings.n 2. Cooling fans and/or pumps should operate.19

Transformers, Dry Type, Air-Cooled, Largen Test Values – Visual and Mechanicaln 3. Compare bolted connection resistance values to valuesof similar connections. Investigate values which deviatefrom those of similar bolted connections by more than 50percent of the lowest value.n 4. Bolt-torque levels should be in accordance withmanufacturer’s published data.20

Transformers, Dry Type, Air-Cooled, Largen Test Values – Visual and Mechanicaln 5. Results of the thermographic survey shall be inaccordance with Section 9.n 6. Tap connections shall be left as found unless otherwisespecified.n Test Values – Electricaln 1. Compare bolted connection resistance values to valuesof similar connections. Investigate values which deviatefrom those of similar bolted connections by more than 50percent of the lowest value.21

Transformers, Dry Type, Air-Cooled, Largen Test Values – Electricaln 2. Minimum insulation-resistance values of transformerinsulation should be in accordance with manufacturer’spublished data. In the absence of manufacturer’spublished data, use Table 100.5. Values of insulationresistance less than this table or manufacturer’srecommendations should be investigated. The polarizationindex shall be compared to previously obtained results andshould not be less than 1.0.22

Transformers, Dry Type, Air-Cooled, Largen Test Values – Electricaln 3. CH and CL power-factor or dissipation-factor values willvary due to support insulators and bus work utilized on drytransformers. The following should be expected on CHLpower factors: Power transformers: 2.0 percent or less,Distribution transformers: 5.0 percent or less. Consulttransformer manufacturer’s or test equipmentmanufacturer’s data for additional information.23

Transformers, Dry Type, Air-Cooled, Largen Test Values – Electricaln 4. Power-factor or dissipation-factor tip-up exceeding 1.0percent should be investigated.n 5. Turns-ratio test results should not deviate more thanone-half percent from either the adjacent coils or thecalculated ratio.n 6. The typical excitation current test data pattern for athree-legged core transformer is two similar currentreadings and one lower current reading.24

Transformers, Dry Type, Air-Cooled, Largen Test Values – Electricaln 7. Temperature-corrected winding-resistance valuesshould compare within one percent of previously-obtainedresults.n 8. Core insulation-resistance values should be comparableto previously-obtained results but not less than onemegohm at 500 volts dc.25

Transformers, Dry Type, Air-Cooled, Largen Test Values – Electricaln 9. AC dielectric withstand test voltage shall not exceed 65percent of factory test voltage for one-minute duration. DCdielectric withstand test voltage shall not exceed 100percent of the ac rms test voltage specified in ANSIC57.12.91, Section 10.2 for one-minute duration. If noevidence of distress or insulation failure is observed by theend of the total time of voltage application during thedielectric withstand voltage test, the test specimen isconsidered to have passed the test.26

Transformers, Dry Type, Air-Cooled, Largen Test Values – Electricaln 10. Phase-to-phase and phase-to-neutral secondaryvoltages should be in agreement with nameplate data.n 11. Test results for surge arresters shall be in accordancewith Section 7.19.27

Transformers, Liquid-Filledn Liquid Filled Transformer28

Transformers, Liquid-Filledn Visual and Mechanical Inspection1. Inspect physical and mechanical condition.2. Inspect anchorage, alignment, and grounding.3. Verify the presence of PCB labeling.4. Prior to cleaning the unit, perform as-found tests, ifrequired.n 5. Clean bushings and control cabinets.nnnn29

Transformers, Liquid-Filledn Visual and Mechanical Inspectionn 6. (Optional) Verify operation of alarm, control, and tripcircuits from temperature and level indicators, pressurerelief device, and fault pressure relayn 7. Verify that cooling fans and/or pumps operate correctly.30

Transformers, Liquid-Filledn Visual and Mechanical Inspectionn 8. Inspect bolted electrical connections for high resistanceusing one or more of the following methods:n 1. Use of a low-resistance ohmmeter.n 2. Verify tightness of accessible bolted electricalconnections by calibrated torque-wrench method.n 3. Perform a thermographic survey.31

Transformers, Liquid-Filledn Visual and Mechanical Inspectionn 9. Verify correct liquid level in tanks and bushings.n 10. Verify that positive pressure is maintained on gasblanketed transformers.n 11. Perform inspections and mechanical tests asrecommended by the manufacturer.n 12. Test load tap-changer.32

Transformers, Liquid-Filledn Visual and Mechanical Inspectionn 13. Verify the presence of transformer surge arresters.n 14. Perform as-left tests.n 15. Verify de-energized tap-changer position is left asspecified.33

Transformers, Liquid-Filledn Electrical Testsn 1. Perform resistance measurements through boltedconnections with a low-resistance ohmmeter.n 2. Perform insulation-resistance tests, winding-to-windingand each winding-to-ground. Apply voltage in accordancewith manufacturer’s published data. In the absence ofmanufacturer’s published data, use Table 100.5. Calculatepolarization index.34

Transformers, Liquid-Filledn Electrical Testsn 3. Perform turns-ratio tests at the designated tap position.n 4. Perform insulation power-factor or dissipation-factortests on all windings in accordance with test equipmentmanufacturer’s published data.n 5. Perform power-factor or dissipation-factor tests on eachbushing equipped with a power-factor/capacitance tap.n In the absence of a power-factor/capacitance tap, performhot-collar tests. These tests shall be in accordance withthe test equipment manufacturer’s published data.35

Transformers, Liquid-Filledn Electrical Testsn 6. Perform excitation-current tests in accordance with thetest equipment manufacturer’s published data.n 7. Measure the resistance of each winding at thedesignated tap position.n 8. (Optional) If the core ground strap is accessible, removeand measure the core insulation resistance at 500 volts dc.n 9. (Optional) Measure the percentage of oxygen in the gasblanket.36

Transformers, Liquid-Filledn Electrical Testsn 10. Remove a sample of insulating liquid in accordancewith ASTM D 923. The sample shall be tested for thefollowing.n 1. Dielectric breakdown voltage: ASTM D 877 and/orASTM D 1816n 2. Acid neutralization number: ANSI/ASTM D 974n 3. (Optional) Specific gravity: ANSI/ASTM D 1298n 4. Interfacial tension: ANSI/ASTM D 971 or ANSI/ASTMD 228537