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Medium Voltage Cables
Operating Voltage greater than 600V

High Potential Testing

  • High potential (hipot) cable system testing can be performed using DC, AC or Very Low Frequency AC (also known as VLF) test voltage.
  • Hipot testing involves applying an overvoltage to the cable system for ashort duration to verify the dielectric integrity of the system (cable, splices, and terminations)
  • In most cases hipot testingis applied as a pass/fail or go/no-go test. If the cable system does not fail during the test, it is considered to have passed the test and can be placed into or back into service.
Low Voltage Switch with integrated GFI relay
NETA Test Procedure

NETA ATS

7.3.3 Cables, Medium- and High-Voltage

A. Visual and Mechanical Inspection:
  1. Compare cable data with drawings and specifications.
  2. Inspect exposed sections of cables for physical damage
  3. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.3.3.B.1.
    2. Verify tightness of accessible bolted electrical connections by calibrated torquewrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform a thermographic survey in accordance with Section 9.
  4. Inspect compression-applied connectors for correct cable match and indentation.
  5. Inspect shield grounding, cable supports, and terminations.
  6. Verify that visible cable bends meet or exceed ICEA and manufacturer’s minimum published bending radius.
  7. *Inspect fireproofing in common cable areas.
  8. If cables are terminated through window-type current transformers, inspect to verify that neutral and ground conductors are correctly placed and that shields are correctly terminated for operation of protective devices.
  9. Inspect for correct identification and arrangements.
  10. Inspect cable jacket and insulation condition.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 7.3.3.A.3.1.
  2. Perform an insulation-resistance test individually on each conductor and each shield with all other conductors and shields grounded. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.
  3. Perform a shield-continuity test on each power cable.
  4. Perform cable time domain reflectometer (TDR) measurements on each conductor.
  5. In accordance with ICEA, IEC, IEEE and other power cable consensus standards, testing can be performed by means of direct current, power frequency alternating current, very low frequency alternating current, or damped alternating current (DAC). These sources may be used to perform insulation-withstand tests, and baseline diagnostic tests such as partial discharge analysis, and power factor or dissipation factor. The selection shall be made after an evaluation of the available test methods and a review of the installed cable system. Some of the available test methods are listed below.
  6. 5.1 Dielectric Withstand
    1. Direct current (dc) dielectric withstand voltage
    2. Very low frequency (VLF) dielectric withstand voltage
    3. Power frequency (50/60 Hz) dielectric withstand voltage
    4. Damped alternating current (DAC) voltage
  7. 5.2. Baseline Diagnostic Tests
    1. Power factor/ dissipation factor (tan delta)
      1. Power frequency (50/60 Hz)
      2. Very low frequency (VLF)
    2. DC insulation resistance
    3. Partial discharge
      1. Online (50/60 Hz)
      2. Off line
        1. Power Frequency (50/60 Hz)
        2. Very low frequency (VLF)
C. 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. (7.3.3.A.3.1)
  2. Bolt-torque levels should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12. (7.3.3.A.3.2)
  3. Results of the thermographic survey shall be in accordance with Section 9. (7.3.3.A.3.3)
  4. The minimum bend radius to which insulated cables may be bent for permanent training shall be in accordance with Table 100.22. (7.3.3.A.6)
D. 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. Insulation-resistance values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated.
  3. Shielding shall exhibit continuity. Investigate resistance values in excess of ten ohms per 1000 feet of cable.
  4. TDR graphical measurements should clearly identify the cable length and characteristic should be consistent with other phases.
  5. 5.1 Based on the test methodology chosen, refer to applicable standards or manufacturer’s literature for acceptable values.
  6. 5.2 ased on the test methodology chosen, refer to applicable standards or manufacturer’s literature for acceptable values.

NETA MTS

7.3.3 Shielded Cables, Medium- and High-Voltage

A. Visual and Mechanical Inspection:
  1. Inspect exposed sections of cables for physical damage and evidence of overheating and corona.
  2. Inspect terminations and splices for physical damage, evidence of overheating, and corona.
  3. Inspect bolted electrical connections for high resistance using one or more of the following methods:
    1. Use of a low-resistance ohmmeter in accordance with Section 7.3.3.B.1.
    2. Verify tightness of accessible bolted electrical connections by calibrated torquewrench method in accordance with manufacturer’s published data or Table 100.12.
    3. Perform a thermographic survey in accordance with Section 9.
  4. Inspect compression-applied connectors for correct cable match and indentation.
  5. Inspect shield grounding, cable support.
  6. Verify that visible cable bends meet or exceed ICEA and manufacturer’s minimum published bending radius.
  7. *Inspect fireproofing in common cable areas.
  8. If cables are terminated through window-type current transformers, inspect to verify that neutral and ground conductors are correctly placed and that shields are correctly terminated for operation of protective devices.
B. Electrical Tests:
  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 7.3.3.A.3.1.
  2. Perform an insulation-resistance test individually on each conductor and each shield with all other conductors and shields grounded. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.
  3. Perform a shield-continuity test on each power cable.
  4. Due to the various cable testing methods commercially available, the following section does not denote “optional” or “required” tests. It is only after careful analysis of all circuit parameters between the testing entity and the cable owner that a preferred testing method should be selected.
  5. In accordance with ICEA, IEC, IEEE and other power cable consensus standards, testing can be performed by means of direct current, power frequency alternating current, very low frequency alternating current, or damped alternating current (DAC). These sources may be used to perform insulation-withstand tests, and baseline diagnostic tests such as partial discharge analysis, and power factor or dissipation factor. The selection shall be made after an evaluation of the available test methods and a review of the installed cable system. Some of the available test methods are listed below.
  6. 4.1 Dielectric Withstand
    1. Direct current (dc) dielectric withstand voltage
    2. Very low frequency (VLF) dielectric withstand voltage
    3. Power frequency (50/60 Hz) dielectric withstand voltage
  7. 4.2 Diagnostic Tests
    1. Power factor/ dissipation factor (tan delta)
      1. Power frequency (50/60 Hz)
      2. Very low frequency (VLF)
      3. Damped-alternating current (20 to 500 Hz)
    2. DC insulation resistance
    3. Partial discharge
      1. Online (50/60 Hz)
      2. Off line
        1. Power Frequency (50/60 Hz)
        2. Very low frequency (VLF)
C. 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. (7.3.3.A.3.1)
  2. Bolt-torque levels should be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.12. (7.3.3.A.3.2)
  3. Results of the thermographic survey shall be in accordance with Section 9. (7.3.3.A.3.3)
  4. The minimum bend radius to which insulated cables may be bent for permanent training shall be in accordance with Table 100.22. (7.3.3.A.6)
D. 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. Insulation-resistance values shall be in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.1.Values of insulation resistance less than this table or manufacturer’s recommendations should be investigated.
  3. Shielding shall exhibit continuity. Investigate resistance values in excess of ten ohms per 1000 feet of cable.
  4. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application during the test, the test specimen is considered to have passed the test.
  5. Based on the test methodology chosen, refer to applicable standards or manufacturer’s literature for acceptable values.
NETA ATS / MTS
TABLE 100.5
Neta Table 100.12
Neta Table 100.12
NETA ATS / MTS
Neta Table 100.22
Insulation Resistance
DESCRIPTION:

This test is performed at or above rated voltage to determine if there are low resistance paths to ground or between winding to winding as a result of winding insulation deterioration.

The test measurement values are affected by variables such as temperature, humidity, test voltage, and size of transformer. This test should be conducted before and after repair or when maintenance is performed.

The test data should be recorded for future comparative purposes. The test values should be normalized to 20°C for comparison purposes

PURPOSE:

The IR test is of value for future comparative purposes and also for determining the suitability of the transformer of energizing or application of the high-potential (hi-pot) test. Insulation-resistance tests should be performed on winding-to-winding and each winding-to-ground. Apply voltage in accordance with manufacturer’s published data. In the absence of manufacturer’s published data, use Table 100.5. Calculate polarization index

PROCEDURE:
Test Connections
  1. High-voltage winding to low-voltage winding and to ground
  2. High-voltage winding to ground
  3. Low-voltage winding to high-voltage winding and to ground
  4. Low-voltage winding to ground
  5. High-voltage winding to low-voltage winding
Test Duration : 1 minute

Megohmmeter reading should be maintained for a period of 1 min.
Make the following readings for two-winding transformers:

TEST RESULTS:
Insulation Resistance test:

Acceptable test values are shown below on NETA Table 100.5

  • This test is performed at or above rated voltage to determine if there are low resistance paths to ground or between winding to winding as a result of winding insulation deterioration.
  • The test measurement values are affected by variables such as temperature, humidity, test voltage, and size of transformer. The test values should be normalized to 20°C for comparison purposes.
  • The test data should be recorded for future comparative purposes.
Polarization Index (PI) test:
Test Duration : 10 minute

This is an extension of the IR test. In this test, the two IR measurements are taken, the fi rst reading at 1 min and the second reading at 10 min.

Then the ratio of the 10 min reading to 1 min reading is calculated to give the PI dielectric absorption value. A PI of winding-to-winding and winding-to-ground should be determined.

A PI below 2 is indicative of insulation deterioration and cause for further investigation.
NETA ATS TABLE 100.5
Insulation Resistance Test Values
Electrical Apparatus and Systems Other Than Rotating Machinery
Neta Table 100.5
NETA MTS TABLE 100.5
Insulation Resistance Test Values
Electrical Apparatus and Systems Other Than Rotating Machinery
Neta Table 100.5
NETA ATS TABLE 100.14
Insulation Resistance Conversion Factors (20° C)
Neta Table 100.5
Dieletric Withstand Test Methods
Medium Voltage Cable
VLF (Very Low Frequency)
Low Voltage, less than 600V
Background
  1. The purpose of a withstand test is to verify the integrity of the cable under test. If the test cable has a defect severe enough at the withstand test voltage, an electrical tree will initiate and grow in the insulation.
  2. The IEEE/EPRI/CEA and other world engineering bodies recommended test level for solid dielectric cable is up to three times (3Vo) line to ground voltage for 15 + minutes.
  3. VLF testing is used for any application requiring AC testing of high capacitance loads. The major application is for testing solid dielectric cable (per IEEE400.2), followed by testing large rotating machinery (per IEEE 433-1974), and occasionally for testing large insulators, arrestors, and the like.
Test
Frequency
0.1 Hz
  1. Most VLF units produce a sine wave output.
  2. Xc (capacitive reactance) = 1/2πfC. A 10,000’ 15 kV cable has approximately 1uF of capacitance. The capacitive reactance at 60 Hz is 2650 ohms. To apply the IEEE recommended 22kV test voltage, it would require a power supply rated for 8.3 amps, or 183kVA. Obviously not practical for field use.
  3. At 0.1 Hz, the capacitive reactance is 1.6 megohms. The same 22kV would draw only 14mA, or only .302kVA, or 600 times less than at 60 Hz. At 0.01 Hz, a cable 6000 times longer can be tested than at 60 Hz.
Test Voltages
0.1 Hz
Non-
Destructive
Test
  1. VLF hipoting is not destructive
  2. VLF does not cause degradation of good insulation and does not lead to premature failures like with DC voltage testing.
  3. Although VLF id a non-destructive test, failures can happen during testing if existing cable defects are present, like water trees and splice defects, break through during the test.
Test Results
  1. VLF is not a diagnostic test. It is an AC stress test.
  2. VLF test results are pass or fail. Unlike a DC-Hpot test, leakage current values are not recorder during test
  3. Cables should be able to hol 2-3 time nominal violtage for a pecified amount of time
  4. A replacement plan to repair or replace a cable should should be in place, if the cable fails during test
DC Hipot
Background
  1. IEEE 400 standard, no longer recommend DC hipot testing for maintenance testing of field aged XLP or EPR cables.
  2. IEEE 400 XLP and EPR cables may not provide meaningful information, and in fact may cause damage.
  3. ICEA standards do not recommend testing XLP or EPR cables after they have been in service for 5 years.
Test Voltages
0.1 Hz
What Is Tan δ, Or Tan Delta?

Tan Delta, also called Loss Angle or Dissipation Factor testing, is a diagnostic method of testing cables to determine the quality of the cable insulation. This is done to try to predict the remaining life expectancy and in order to prioritize cable replacement and/or injection. It is also useful for determining what other tests may be worthwhile.

What Is Tan δ, Or Tan Delta?

If the insulation of a cable is free from defects, like water trees, electrical trees, moisture and air pockets, etc., the cable approaches the properties of a perfect capacitor. It is very similar to a parallel plate capacitor with the conductor and the neutral being the two plates separated by the insulation material.

In a perfect capacitor, the voltage and current are phase shifted 90 degrees and the current through the insulation is capacitive. If there are impurities in the insulation, like those mentioned above, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. It is no longer a perfect capacitor. The current and voltage will no longer be shifted 90 degrees. It will be something less than 90 degrees. The extent to which the phase shift is less than 90 degrees is indicative of the level of insulation contamination, hence quality/reliability. This “Loss Angle” is measured and analyzed.

Below is a representation of a cable. The tangent of the angle δ is measured. This will indicate the level of resistance in the insulation. By measuring IR/IC (opposite over adjacent – the tangent), we can determine the quality of the cable insulation. In a perfect cable, the angle would be nearly zero. An increasing angle indicates an increase in the resistive current through the insulation, meaning contamination. The greater the angle, the worse the cable.

How Long A Cable Can I Test?

That depends on the AC voltage source used. The standard VLF unit from High Voltage, Inc. can test 3-4 miles of cable: one model can test 30 miles. It is generally advantageous to test shorter lengths rather than a long cable, because the shorter the section of cable that is tested, the more precise we can be in determining where the cable is good or bad.

Can The Test Find The Locations Of Cable Defects?

No. Tan delta tests the cable from point A to point B and gives an assessment of the insulation quality between those points. A determination can then be made if, and when, to replace or enhance the cable. For any value of tan delta, there could be many minor defects or a few major defects: it cannot discriminate. When you are tan delta testing, you are only determining how good a cable is between two points. Again, it’s not a faultfinding tool. It is a tool to permit a utility to make educated decisions regarding cable replacement

This assumes the cable being tested is in conduit and entire lengths will be replaced. In direct buried situations, a better test is to use the VLF unit as an AC hipot and apply the IEEE recommended 3 times normal voltage for at least 30 minutes. Any serious defect within the insulation or accessories may fail: the reason for the stress test. Find the fault, repair it, and move on.

How Long Does The Whole Test Take?

The test itself can take less than twenty minutes, depending upon the settings of the instrument and the number of different test voltage levels used. It is only necessary to capture a few cycles of the voltage and current waveform to make the analysis. At 0.1 Hz, the period of the sine wave is 10 seconds, so it takes 20 – 30 seconds for a reading to be made. At .02 Hz, the period is 50 seconds, requiring perhaps 3 minutes of test time at each voltage setting.

Isn’t This The Same As A Power Factor Test?

Not quite, although it essentially provides the same qualitative assessment as a power factor test. With power factor, the cosine of the angle between the voltage and current is measured, yielding the power factor. With tan delta, we are measuring the tangent of the complimentary angle, and it is measured in radians, not degrees as power factor is done. For slight angles, the tan delta readings will be the same as power factor. As the angle, hence loss, increases, the tan delta numbers and the power factor numbers will not be the same.

Are There Any Limitations To Using Tan Delta Testing?

Since we are measuring the loss angle of an insulating material, and making an analysis about the test results possibly based on historical data, it is not advisable to test a cable length that contains more than one type of cable. Different cables have different loss characteristics. It is not a good practice to test a cable length of XLPE insulation spliced to an EPR or PILC cable. The only way in which this is meaningful is when many tests are done on the same cable length over time and the results are carefully trended.

AC Hipot Testing

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