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Testing and Commissioning of MV/HV Cables
Example of asbestos paper insulation wrap on high-voltage cable inside an underground cable vault. Several layers of the soft and friable insulation are wrapped around the cable in long, wide strips. Originally, pure white, the discoloration is from sediment mud after formerly being submerged in the once flooded vault; some water leakage is still present.

1. 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 1.2 above.
    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 a thermographic survey.
      (NOTE: Remove all necessary covers prior to thermographic inspection. Use appropriate caution, safety devices, and personal protective equipment.)
  4. Inspect compression-applied connectors for correct cable match and indentation.
  5. Inspect shield grounding, cablesupports, 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.

** Optional test

2. Electrical Tests

  1. Perform resistance measurements through bolted connections with a low-resistance ohmmeter, if applicable, in accordance with Section 1.1.
  2. Perform an insulation-resistance test individually on each conductor 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. 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, or very low frequency alternating current. These sources may be used to perform insulation-withstand tests, and baseline diagnostic tests suchas 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 ofthe available test methods are listed below:
    .

    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
    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. Off-line partial discharge:
        1. Power frequency (50/60 Hz)
        2. Very low frequency (VLF)

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.
  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.
  3. Results of the thermographic survey.
    (NOTE: Remove all necessary covers prior to thermographic inspection. Use appropriate caution, safety devices, and personal protective equipment.)
  4. The minimum bend radius to which insulated cables may be bent for permanent training shall be in accordance with Table 100.22.

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. 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 dielectric withstand 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.

Tables

Table 100.12.1

Bolt-Torque Values for Electrical Connections

– US Standard Fasteners (a)
– Heat-Treated Steel – Cadmium or Zinc Plated (b)

Bolt-Torque Values for Electrical Connections
Table 100.12.1 – Bolt-Torque Values for Electrical Connections

a) Consult manufacturer for equipment supplied with metric fasteners.
b) Table is based on national coarse thread pitch.


Table 100.12.2

– US Standard Fasteners (a)
– Silicon Bronze Fasteners (b, c)

Torque (Pound-Feet)

Torque (Pound-Feet)
Torque (Pound-Feet)

a) Consult manufacturer for equipment supplied with metric fasteners.
b) Table is based on national coarse thread pitch.
c) This table is based on bronze alloy bolts having a minimum tensile strength of 70,000 pounds per square inch.


Table 100.12.3

– US Standard Fasteners (a)
– Aluminum Alloy Fasteners (b, c)

Torque (Pound-Feet)

Torque (Pound-Feet) - Aluminum Alloy Fasteners
Torque (Pound-Feet) – Aluminum Alloy Fasteners

a) Consult manufacturer for equipment supplied with metric fasteners.
b) Table is based on national coarse thread pitch.
c) This table is based on aluminum alloy bolts having a minimum tensile strength of 55,000 pounds per
square inch.


Table 100.12.4

– US Standard Fasteners (a)
– Stainless Steel Fasteners (b, c)

Torque (Pound-Feet)

Torque (Pound-Feet) - Stainless Steel Fasteners
Torque (Pound-Feet) – Stainless Steel Fasteners

a) Consult manufacturer for equipment supplied with metric fasteners.
b) Table is based on national coarse thread pitch.
c) This table is to be used for the following hardware types:

  • Bolts, cap screws, nuts, flat washers, locknuts (18-8 alloy)
  • Belleville washers (302 alloy).

Tables in 100.12 are compiled from Penn-Union Catalogue and Square D Company, Anderson Products Division, General Catalog: Class 3910 Distribution Technical Data, Class 3930 Reference Data Substation Connector Products.


Table 100.1

Insulation Resistance Test Values Electrical Apparatus and Systems

Insulation Resistance Test Values Electrical Apparatus and Systems
Table 100.1 – Insulation Resistance Test Values Electrical Apparatus and Systems

In the absence of consensus standards dealing with insulation-resistance tests, the Standards Review Council suggests the above representative values. Test results are dependent on the temperature of the insulating material and the humidity of the surrounding environment at the time of the test.

Insulation-resistance test data may be used to establish a trending pattern. Deviations from the baseline information permit evaluation of the insulation.


Table 100.22

Minimum Radii for Power Cable

Single and Multiple Conductor Cables with Interlocked Armor, Smooth or Corrugated Aluminum Sheath or Lead Sheath

Minimum Radii for Power Cable
Table 100.22 – Minimum Radii for Power Cable

ANSI/ICEA S-93-639/NEMA WC 74-2000, 5-46 kV Shielded Power Cable for Use in the Transmission and Distribution of Electric Energy, Appendix I – Recommended Bending Radii for Cables and Table I1 – Minimum Radii for Power Cable.

a. 12 x individual shielded conductor diameter, or 7 x overall cable diameter, whichever is greater.

Resource: STANDARD FOR ACCEPTANCE TESTING SPECIFICATIONS for Electrical Power Equipment and Systems (NETA 2009)

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

9 Comments


  1. Syed Kumail Abid
    Jan 17, 2016

    Our client has required us to perform tests to prove that metallic screen of 11kV cables are capable of sustaining the specified earth fault current . Can you please elaborate this . Who will perform these test? How can these tests can be performed.


    • Stephen MacGregor
      Nov 15, 2016

      Hello Syed Kumail Abid, I didn’t see any replies on your question but I’d be grateful to know what advice you were given. It is my understanding that the metallic screen on 11kV cables is not there to carry fault current. The screens are to control the electrical stress produced under load. Whilst it is true that the screen must be connected to earth in order to function correctly, it is not designed to carry any significant load. Its only other purpose is to act as an electrical protection device guarding against punctures of the cable. This would produce a localised short circuit between conductor and the earthed screen, which should immediately operate the protection device.


  2. Mustapha Adewuyi
    Jan 26, 2015

    Good job,thanks


  3. S L Ali
    Nov 19, 2014

    Thank you…..
    good job..this is a great way of having info at your fingertips.


  4. Jack Nottidge
    Jul 11, 2014

    Great article as always. Take a look at our website or contact Thorne and Derrick for a wide range of products related to MV/HV cables and cable joints.


  5. Shakeel Ahmed
    Jun 20, 2014

    Dear
    I tested a Mv cable (5kv) first i applied insulation test on 5kv result was 218 Gega ohm.when VLF applied .test failed on 4kv .i am worry about megger test .i am thinking megger test is not reliable .please suggest ,
    best regards


  6. lee
    Dec 10, 2013

    good job..this is a great way of having info at your fingertips


  7. Felix
    Jan 30, 2013

    Thank You :)


  8. Anup kumar meena
    Jan 17, 2013

    thanks for this

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