Inspection and Test procedures for Instrument Transformers

Inspection and Test procedures for Instrument Transformers (on photo: Ritz Instrument Transformers)

Procedures to follow:

1. Visual and Mechanical Inspection
2. Electrical Tests:
   1. Electrical Tests – Current Transformers
   2. Electrical Tests – Voltage Transformers
   3. Electrical Tests – Coupling-Capacitor Voltage Transformers
   4. Electrical Tests – High-Accuracy Instrument Transformers (Reserved)
3. Test Values:
   1. Test Values: Visual and Mechanical
   2. Test Values: Current Transformers – Electrical
   3. Test Values:Voltage Transformers – Electrical
   4. Test Values: Coupling Capacitor Voltage Transformers
   5. Test Values: High-Accuracy Instrument Transformers (Reserved)
4. Tables (100.5, 100.9 and 100.12)

1. Visual and Mechanical Inspection

1. Compare equipment nameplate datawith drawings and specifications.
2. Inspect physical and mechanical condition.
3. Verify correct connection of transformers with system requirements.
4. Verify that adequate clearances exist between primary and secondary circuit wiring.
5. Verify the unit is clean.
6. Inspect bolted electrical connections for high resistance using one or more of the following methods:
   1. Use of low-resistance ohmmeter in accordance with Section 2.1 and 2.2.
   2. Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data or Table 100.12.
   3. Perform thermographic survey.
7. Verify that all required grounding and shorting connections provide contact.
8. Verify correct operation of transformer withdrawal mechanism and grounding operation.
9. Verify correct primary and secondary fuse sizes for voltage transformers.
10. Verify appropriate lubrication on moving current-carrying parts and on moving and sliding surfaces.

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2. Electrical Tests

2.1 Electrical Tests – Current Transformers

1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 1.
2. Perform insulation-resistance test of each current transformer and its secondary wiring with respect to ground at 1000 volts dc for one minute.For units with solid-state components that cannot tolerate the applied voltage, follow manufacturer’s recommendations.
3. Perform a polarity test of each current transformer in accordance with ANSI/IEEE C57.13.1.
4. Perform a ratio-verification test using the voltage or current method in accordance with ANSI/IEEE C57.13.1.
5. Perform an excitation test on transformers used for relaying applications in accordance with ANSI/IEEE C57.13.1.
6. Measure current circuit burdens at transformer terminals in accordance with ANSI/IEEE C57.13.1.
7. When applicable, perform insulation-resistance tests on the primary winding with the secondary grounded. Test voltages shall be inaccordance with Table 100.5.
8. When applicable, perform dielectric withstand tests on the primary winding with the secondary grounded. Test voltages shall be inaccordance with Table 100.9.
9. Perform power-factor or dissipation-factortests in accordance with test equipment manufacturer’s published data.
10. Verify that current transformer secondary circuits are grounded and have only one grounding point in accordance with ANSI/IEEE C57.13.3. That grounding point should be located as specified by the engineer in the project drawings.

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2.2 Electrical Tests – Voltage Transformers

1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 1.
2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Test voltages shall be applied for one minute in accordance with Table 100.5. For units with solid-state components that cannottolerate the applied voltage, follow manufacturer’s recommendations.
3. Perform a polarity test on each transformer to verify the polarity marks or H1- X1 relationship as applicable.
4. Perform a turns-ratio test on all tap positions.
5. Measure voltage circuit burdens at transformer terminals.
6. ** Perform a dielectric withstand test on the primary windings with the secondary windings connected to ground. The dielectric voltage shall be in accordance with Table 100.9. The test voltage shall be applied for one minute.
7. Perform power-factor or dissipation-factortests in accordance with test equipment manufacturer’s published data.
8. Verify that voltage transformer secondary circuits are grounded and have only one grounding point in accordance with ANSI/IEEE C57.13.3. The grounding point should be located as specified by the engineer in the project drawings.

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2.3 Electrical Tests – Coupling-Capacitor Voltage Transformers

1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 1.
2. Perform insulation-resistance tests winding-to-winding and each winding-to-ground. Test voltages shall be applied for one minute in accordance with Table 100.5. For units with solid-state components that cannottolerate the applied voltage, follow manufacturer’s recommendations.
3. Perform a polarity test on each transformer to verify the polarity marking. See ANSI/IEEE C93.1 for standard polarity marking.
4. Perform a turns-ratio test on all tap positions, if applicable.
5. Measure voltage circuit burdens at transformer terminals.
6. ** Perform a dielectric withstand test on the primary windings with the secondary windings connected to ground. The dielectric voltage shall be in accordance with Table 100.9. The test voltage shall be applied for one minute.
7. Measure capacitance of capacitor sections.

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2.4 Electrical Tests – High-Accuracy Instrument Transformers (Reserved)

3. Test Values

3.1 Test Values – Visual and Mechanical

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. (1.6.1)
2. Bolt-torque levels shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12.
3. Results of the thermographic survey.
4. Perform power-factor or dissipation-factortests in accordance with test equipment manufacturer’s published data.
5. Verify that the coupling-capacitor voltage transformer circuits are grounded and have only one grounding point in accordance with ANSI/IEEE C57.13.3. That grounding point should be located as specified by the engineer in the project drawings.

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3.2 Test Values: Current Transformers – 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. Insulation-resistance values of instrument transformers shall not be less than values shown in Table 100.5.
3. Polarity results shall agree with transformer markings.
4. Ratio errors shall be in accordance with C57.13.
5. Excitation results shall match the curve supplied by the manufacturer or be in accordance with ANSI C57.13.1.
6. Measured burdens shall be comparedto instrument transformer ratings.
7. Insulation-resistance values of instrumenttransformers shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5.
8. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectricwithstand test, the primary winding is considered to have passed the test.
9. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.
10. Test results shall indicate that the circuits have only one grounding point.

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3.3 Test Values: Voltage Transformers – 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. Insulation-resistance values of instrument transformers shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5.
3. Polarity results shall agree with transformer markings.
4. Ratio errors shall be in accordance with C57.13.
5. Measured burdens shall be comparedto instrument transformer ratings.
6. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the dielectricwithstand test, the primary windings are considered to have passed the test.
7. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.
8. Test results shall indicate that the circuits are grounded at only one point.

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3.4 Test Values: Coupling Capacitor Voltage Transformers

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. Insulation-resistance values of instrument transformers shall not be less than values shown in Table 100.5.
3. Polarity results shall agree with transformer markings.
4. Ratio errors shall be in accordance with C57.13.
5. Measured burdens shall be comparedto instrument transformer ratings.
6. 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.
7. Capacitance of capacitor sections of coupling-capacitor voltage transformers shall be in accordance with manufacturer’s published data.
8. Power-factor or dissipation-factor values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use test equipment manufacturer’s published data.
9. Test results shall indicate that the circuits are grounded at only one point.

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3.5 Test Values: High-Accuracy Instrument Transformers (Reserved)

TABLE 100.5

Transformer Insulation Resistance Acceptance Testing

Table 100.5 – Transformer Insulation Resistance Acceptance Testing

In the absence of consensus standards, the NETA Standards Review Council suggests the above representative values.

NOTE: Since insulation resistance depends on insulation rating (kV) and winding capacity (kVA), values obtained should be compared to manufacturer’s published data.

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TABLE 100.9

Instrument Transformer Dielectric Tests Field Acceptance

Table 100.9 – Instrument Transformer Dielectric Tests Field Acceptance

Table 100.9 is derived from Paragraph 8.8.2 and Tables 2 of ANSI/IEEE C57.13-1993, Standard Requirements for Instrument Transformers.

+ Periodic dc potential tests are not recommended for transformers rated higher than 34.5 kV.
* DC potential tests are not recommended for transformers rated higher than 200 kV BIL. DC tests may prove beneficial as a reference for future testing. In such cases the test direct voltage shall not exceed the original factory test rms alternating voltages.

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TABLE 100.12

US Standard Fasteners – Bolt-Torque Values for Electrical Connections

Table 100.12.1 – Heat-Treated Steel – Cadmium or Zinc Plated

Table 100.12.2 – Silicon Bronze Fasteners

Table 100.12.3 – Aluminum Alloy Fasteners

Table 100.12.4 – Stainless Steel Fasteners

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Reference: Standard for acceptance testing specifications for electrical power equipment and systems – American National Standards Institute