Specification for Transformers in MV Distribution System



This specification intended to supply, install, test and commission mineral oil immersed 1500 KVA, 3 phase indoor transformers, fitted with cable boxes, bolts, and all necessary accessories. Transformers shall be indoor type, oil filled, two winding, four off load capacity, 2.5% taps, two above and two below high voltage rating in compliance with IEC 76 or BS 171.

Specification for Transformers in MV Distribution System

Transformer data

  1. Winding connection: Primary (HV) delta, secondary (LV) star with solidly grounded brought out neutral, vector group DYN 11.
  2. KVA rating: As shown on the contract drawing.
  3. Frequency: 60 Hz.
  4. Rated voltage: Primary 13.8 KV ± 2.5%, ± 5% secondary, 380 V at full load.
  5. Terminal arrangement: HV cable box for cross linked polyethylene 3 core cable/LV air insulated box for connection of LV cables.
  6. Transformers shall be designed for continuous full load operation at 55 deg. C average ambient temperature and be cooled by natural oil and natural air convection (ONAN). It  shall be equipped with pressure relief valves, dial type thermometers, lifting lugs, jack pads, tank filling and emptying valves and earthing bolts.
  7. Transformers shall have an oil level sight indicator.
  8. Transformers shall have a thermistor temperature sensor embedded within the windings.  The thermistor shall be compatible with the temperature monitoring relay detailed elsewhere within this Specification and on the drawings.
  9. Transformers over 1 MVA in rating shall be fitted with a Buchholtz relay and conservator.  The Buchholtz relay shall operate on both rate and quantity of gassing.

LV Distribution Switch Board

The contractor shall supply, install and commission the low voltage cable tails from the transformer to the main low voltage switch boards detailed elsewhere within this Specification and on the drawings.

Installation Requirements for Transformers

The contractor shall prepare the concrete base, assemble, install, field test and adjust for final operation each distribution substation.

1. Inspection and Tests

The contractor shall assemble and install the transformer, LV connection and accessories as shown on the drawings.  All necessary terminal connectors shall be furnished.

The contractor shall furnish all bolts, nuts, wedge washers, lock washers and other materials required to mount the equipment and assemble the weatherproof enclosure.  Operating mechanisms shall be installed and tested in accordance with manufacturer’s instructions or as directed by the Architect/Engineer.  HV switch gear, transformer and LV distribution gear shall be connected to grounding system.

The high voltage units shall be tested in accordance with the requirements of specifications and procedures for this type of equipment.  Tests shall include, but not to be limited to the following: Switch board and control panel testing in manufacturer’s works and on site:

Every switch board and control panel shall be tested fully prior to its delivery, with the following procedures:

  1. Mechanical torque tests on all bus bar connections and other fixings.
  2. Tighten all electrical connections.
  3. Carry out a visual inspection of the equipment.
  4. Installation resistance tests at 1000 V, phase to phase, phase to neutral, phase and neutral to earth.
  5. Apply test voltages for 2 minutes on each of the following:
    1. Phase to phase.
    2. Phase to neutral.
    3. All conductors to earth.
    4. The contractor shall test all equipment using Dc injection equipment and digital meters. 
    5. All conductors not under test shall be bonded to earth. 
    6. The contractor shall record all leakages current of all tests of submission to the supervising officer.
  6. As item 4.
  7. Conductance tests for each switch.
  8. Functional tests where applicable.
  9. Care shall be taken at the time of high voltage impulse testing and Megger insulation resistance testing to ensure that no items or equipment damaged by the high voltage.  Any items of equipment susceptible to the high voltage shall be disconnected prior to testing.
  10. The high voltage impulse test voltage shall be 2E+2KV where “E” equals the line (phase to phase) voltage, and shall be applied for a minimum of 2 minutes.  The conductors not subjected to the tests shall be earthed for the duration of the tests.
  11. On site the contractor shall carry out the tests as detailed above, and shall issue a certificate following completion.
  12. The contractor shall allow for the provision of barrier and screens and a suitable number of qualified personnel to carry out the testing in accordance with the Health and Safety at Works Act.
  13. In addition, a sample of the oil within each item of High Voltage switch gear will be taken and analyzed after filling for contamination.

The following tests shall be made on each transformer by the contractor before energizing.

2. Summary of Transformer tests:

  1. Transformer - routine tests.
  2. Temperature rise tests.
  3. Magnetic circuit - routine tests.
  4. Tanks - routine tests and type tests.
  5. Cooling plant - routine and type tests.

3. Transformers - Routine tests

All transformers shall be subject to the following routine tests:

1.    Measurement of winding resistance.

2.    Ratio, polarity and phase relation ship.

3.    Impedance voltage.

4.    Load loss.

5.    No load loss and no load current.

6.    Insulation resistance.

7.    Noise level tests to NEMA standards.

8.    Voltage withstand in accordance with BS specifications.

4. Temperature rise tests

The test shall be in accordance with BS 171 and shall be carried out on one transformer of each size and type.  Temperature rise tests shall be conducted on the tapping corresponding to the maximum losses.

5. Magnetic circuit - routine tests

Each core completely assembled is to be tested for one minute at 2000 volts AC between core bolts, side plates, structural steel works and core at the core and coils stag.  After the transformer is tanked and completely assembled, a further test is to be applied between the core and the earth structural steel work to prove that the core is earthed through the removable link, at one point only.

6. Tranformer Oil Tanks

The tests specified in BS 171 Section 35.1 Oil leakage - All tanks, conservators and oil filled compartments which are subjected in service or during maintenance to oil pressure are to withstand without leakage.  A hydraulic pressure test equal to 69 KN/m2 or the normal pressure plus 34 KN/m2 which ever is the greatest, for 24 hours during which time no leakage or oil ingress into normally oil free spaces shall occur.

Unless type tests certificates can be produced for tests carried out on similar equipment, the following tests are to be included for tanks, conservators and pressure relief devices.

1.    Vacuum tests:    The equipment is to withstand a vacuum of 500 mm (20 inches) of mercury (10psig) less than atmospheric pressure, when empty of oil.  The permanent deflection of plates or stiffness on removal of vacuum is not to exceed the following values:

  1. Length of plate: Less than 1300 mm (50"); Permanent deflection: 3.17 mm (1/8")
  2. Length of plate: 1300 mm to 2500 mm (50" to 100"); Permanent deflection: 9.5 mm (3/8")
  3. Length of plate: Greater than 2500 mm (100"); Permanent deflection: 12.7 mm (1/2")

2.    Pressure tests:  The equipment is to withstand a pressure corresponding to 69 KN/m2 or the normal pressure plus 34 KN/m2 whichever is the greater.  The permanent deflection of plates or stiffeners on removal of pressure is not to exceed the value stated in respect of the vacuum test in the preceding paragraph.

7. Cooling plant

Coolers are to be pressure tested as specified in the above section. Site tests of existing transformers and new transformers following installation: All existing transformers that are modified in any way shall be tested with the following:

  1. Measurement of winding resistance.
  2. Ratio, polarity and phase relationship, to ensure the accuracy of the termination identification.
  3. No load loss and no load current.
  4. Insulation resistance.
  5. Oil contamination after changing oil.

The LV distribution board shall be tested by the contractor in accordance with established procedures for this type of equipment.  Tests shall include, but not necessarily limited to the following:

 Switch board and control panel testing in manufacturers works and on site: Any switch board and control panel shall be tested fully prior to its delivery, with the following procedures:

  1. Mechanical torque tests on all bus bar connections and other fixings.
  2. Tighten all electrical connections.
  3. Carry out a visual inspection of the equipment.
  4. Insulation resistance tests at 1000 Volt, phase to phase, phase to neutral, phase and neutral to earth.
  5. Apply test voltage for 2 minutes on each of the following:
    • Phase to phase.
    • Phase to neutral.
    • All conductors to earth.

The contractor shall test all equipment using DC injection equipment and digital meters.  All conductors not under test shall be bonded to earth.  The contractor shall record all leakages current of all tests for submission to the supervising officer.

6.    Conductance tests for each switch.

7.    Functional tests where applicable.

Care shall be taken at the time of high voltage impulse testing and Megger insulation resistance testing to ensure that no items of equipment are damaged by the high voltage.  Any items of equipment susceptible to the high voltage shall be disconnected prior to testing.

The high voltage impulse test voltage shall be 2E + 2 KV where “E” equals the line (phase to phase) voltage, and shall be applied for minimum of 2 minutes.  The conductors not subjected to the tests shall be earthed for the duration of the test.

On site the contractor shall carry out the tests as detailed above, and shall issue a certificate following completion. The contractor shall allow for the provision of barrier and screens and a suitable number of qualified personnel to carry out the testing in accordance with the Health and safety at Works Act.

In addition, a sample of the oil within each item of high voltage switch gear will be taken and analyzed after filling for contamination. The contractor’s work shall not be considered complete until all equipment as stated above has been installed and tested and the results of the tests have been accepted by the Architect/Engineer.

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