Type Tests for above 33KV up to 220KV XLPE Copper Cable
Tests on Conductor
- Annealing Test
- Resistance Test
Tests on Insulation
3. Measurement of Thickness of Insulation
4. Measurement of tensile strength and elongation at break
5. Thermal ageing in air oven
6. Hot set test
7. Shrinkage test
8. Void and contaminants test
Tests on semiconductive layers of the insulation
9. Resistivity test on semi-conducting layer
Tests on metallic screen
10. Tests for copper wire
11. Tests for copper tap
Tests on Metallic Sheath
12. Measurement of thickness of the metallic sheath
Tests on Armours
13. Measurement of diameter for circular armours & measurement of width and thickness of strip armours,
14. Tensile strength and elongation at break of armour,
15. Wrapping test on armour,
16. Resistivity test on armour.
Tests on Outer Sheath
17. Measurement of thickness of outer sheath
18. Tensile strength and elongation at break of outer sheath
19. Thermal ageing in air oven of outer sheath
20. Loss of mass test
21. Heat shock test
22. Hot deformation test
23. Shrinkage test
24. Thermal stability test
Tests on Complete Cable Specimen
25. Flammability test
27. Water tightness test
28. Thermal ageing on entire cable specimen
29. Bending test followed by partial discharge test
30. Load cycle test followed by partial discharge test
31. Tan delta measurement with capacitance measurement at ambient temperature
32. Impulse Voltage withstand test
33. Power Frequency withstand test
Routine Tests on XLPE HV Cables
- Measurement of conductor resistance
- Measurement of partial discharge
- High voltage withstand test
Acceptance Tests on XLPE HV Copper Cables
- Conductor resistance test
- Annealing test
- Measurement of dimension of insulation
- Hot set test on insulation
- Void and contaminants test
- Test for thickness of metallic sheath
- Partial discharge test
- High voltage withstand test
- Measurement of capacitance
Type Tests
Here, it is to be noted that copper used for underground or overground XLPE cables is annealed copper. But the aluminium used for the same purpose is hard-drawn aluminium.Therefore, for copper conductors annealing test is mandatory, but for aluminium conductors, tensile test and wrapping test are essential.
Annealing Test on Copper Conductor
This test is performed in accordance with IS:10810 (Part-1). A strand of the copper conductor is taken. The length of the copper strand may be 250 mm plus the length required for gripping the strand from two ends
The test specimen that is the copper strand is fixed between the two heads of a tensile machine. The heads of the machine have gripping arrangement for holding the specimen copper strand. The length of the copper strand between two grip heads of the machine is normally 250 mm. The marking is provided at both ends of strand just after the grip head of the machine. Now, tensile load is applied on the strand by means of the tensile machine. The load is gradually increased until the strand is broken or fractured. The rate of elongation i.e. the rate of relative movement of the grips should not be more than 100 mm/minute.
After the strand is broken, the both parts of the specimen strand are taken out from the machine. Now, the broken parts of the specimen are fitted together and the elongated length of the strand is measured by a scale. The initial length of the elongated strand is 250 mm. The final length as measured is L (say). Therefore, the elongation will be Le = L – 250 mm.
Routine Tests
There are mainly two routine tests performed on an XLPE cables.
1. High Voltage Withstand Test on Insulation,
2. Partial Discharge Test on Insulation.
Procedure for High Voltage Routine Tests
High voltage test is performed on the full drum length of the cable. Here, we need to apply test voltage between conductor and metallic sheath. We apply this test voltage for 30 minutes. As per standards, the test voltage shall be 2.5 times of rated phase to ground voltage of the XLPE cable. If breakdown of the insulation occurs during this period, the specimen cable is considered as passed the test. Suppose the XLPE cable under test is 76/132KV cable. Therefore, the phase to ground voltage is 76KV. Now, 2.5 times of 76KV is 190KV. So, the test voltage for 76/132KV cable will be 190KV and that shall be applied for 30 minutes.
Procedure for Partial Discharge Tests
Here, to be noted that the partial discharge test is performed on full drum length of the cable. Here, the test voltage is also applied between conductors and metallic sheath. The test voltage is gradually increased up to 1.73 times of the rated phase to ground voltage. Then hold it for 10 seconds and after that, it shall be reduced to 1.5 times of the rated phase to ground voltage and hold it for for 1 minute. At this reduced voltage level, the partial discharge is measured. It must be less than 10 pC. For type test this limit should be less than 5 pC.
For example, if a cable is rated 76/132 kV, the phase to ground voltage is 76 kV. Hence, 1.5 × 76 = 114 kV is the test voltage for the partial discharge test.
Tensile Test on Hard Drawn Aluminum Strand
Aluminum is soft metal. So, it can not be used as copper after annealing. Although annealing makes aluminum more conductive but annealing also makes it softer. Copper is harder metal and annealing also does reduce the hardness of copper but still it stays much hard to serve the purpose of mechanical strength of the conductor. If annealed aluminum is used for conductor of a cable, it may not sustain the mechanical force applied during laying in trenches or laying underground.
A hard drawn aluminum strand in the conductor of a cable demands tensile test. Here, normally we take a length of 250 mm of a strand of the conductor. Also we take some extra length for gripping both ends of strand in a tensile machine. The exact distance between two grip heads of the machine shall be 250 mm. Now mark on the strand by a marker just before the tip of the grips.
Also, we need to measure the diameter of the conductor for circular strand for calculating its crosssectional area. For shaped strand length and weight of the strand are to be measured for calculating the cross sectional area of the shaped strand.
Now, the tensile load is gradually increased and the rate of separation of two grips of the machine must not be more than 100 mm per minute. Now, wait for the moment when the strand gets ruptured due to applied force.
Dividing the recorded tensile strength at breaking of strand by the cross sectional area of the strand gives us the tensile strength per unit cross sectional area of the strand and we have to compare it with specified values given in the standards.
Wrapping Test on Hard Drawn Aluminum Strand
This test does not require any tool and machinery. Just we need to take a strand of conductor and one portion of that specimen strand has to be wrapped over the other portion of the strand. We need to wrap it as tightly as possible. This confirms the diameter of each turn is equal to the diameter of the strand itself. The wrapping may be done by free hand. The total number of turn is 8. Now, unwrap 6 nos of turn and rewrap again tightly. Then we need to examine whether any crake developed on the rewrapped turns or not. If there is no crake developed, the strand is passed for the wrapping test.