Testing of Cables

High voltage power cables have proved quite useful, especially in the case of HV d.c. transmission. Underground distribution using cables not only adds to the aesthetic looks of a metropolitan city but it provides better environments and more reliable supply to the consumers.

Preparation of Cable Sample

The cable sample has to be carefully prepared for performing various tests, especially electrical tests. This is essential to avoid any excessive leakage or end flashovers which otherwise may occur during testing and hence may give wrong information regarding the quality of cables.

The length of the sample cable varies between 50 cm to 10 m. The terminations are usually made by shielding the ends of the cable with stress shields to relieve the ends from excessive-high electrical stresses. A cable is subjected to the following tests:

1. Bending test.
2. Loading cycle test.
3. Thermal stability test.
4. Dielectric thermal resistance test.
5. Life expectancy test.
6. Dielectric power factor test.
7. Power frequency withstand voltage test.
8. Impulse withstand voltage test.
9. Partial discharge test.

1. Bending Test

The cable is bent around a cylinder of a specified diameter to make one complete turn. It is then unwound and rewound in the opposite direction. The cycle is to be repeated three times. It is to be noted that a voltage test should be made before and after the bending test.

2. Loading Cycle Test

A test loop, consisting of cable and its accessories is subjected to 20 load cycles with a minimum conductor temperature of 5°C over the design value, and the cable is energized to 1.5 times the working voltage. The cable should not show any sign of damage.

3. Thermal Stability Test

After the test as at (2), the cable is energized with a voltage 1.5 times the working voltage for a cable of 132 kV rating (the multiplying factor decreases with increases in operating voltage), and the loading current is so adjusted that the temperature of the core of the cable is 5°C higher than its specified permissible temperature. The current should be maintained at this value for six hours.

4. Dielectric Thermal Resistance Test

The ratio of the temperature difference between the core and sheath of the cable and the heat flow from the cable gives the thermal resistance of the sample of the cable. It should be within the limits specified in the specifications.

5. Life Expectancy Test

To estimate the life of a cable, an accelerated life test is carried out by subjecting the cable to a voltage stress higher than the normal working stress. It has been observed that the relation between the expected life of the cable in hours and the voltage stress is given by

g = K ÷ n√t

Where K is a constant which depends on the material and n is the life index depending again on the material.

6. Dielectric Power Factor Test

High Voltage Schering Bridge is used to perform a dielectric power factor test on the cable sample. The power factor is measured for different values of voltages e.g. 0.5, 1.0, 1.5, and 2.0 times the rated operating voltages.

The maximum value of power factor at normal working voltage does not exceed a specified value (usually 0.01) at a series of temperatures ranging from 15°C to 65°C.

The difference in the power factor between the rated voltage and 1.5 times the rated voltage and the rated voltage and twice the rated voltage does not exceed a specified value.

Sometimes the source is not able to supply the charging current required by the test cable, a suitable choke in series with the test cable helps in tiding over the situation.

7. Power Frequency Withstand Voltage Test

Cables are tested for power frequency AC and DC voltages. During manufacture the entire cable is passed through a higher voltage test and the rated voltage to check the continuity of the cable.

As a routine test, the cable is subjected to a voltage 2.5 times the working voltage for 10 min without damaging the insulation of the cable.

HVDC of 1.8 times the rated DC voltage of negative polarity for 30 min. is applied and the cable is said to have withstood the test if no insulation failure takes place.

8. Impulse Withstand Voltage Test

The test cable is subjected to 10 positive and 10 negative impulse voltages of magnitude as specified in the specification, the cable should withstand 5 applications without any damage. Usually, after the impulse test, the power frequency dielectric power factor test is carried out to ensure that no failure occurs during the impulse test.

9. Partial Discharge Test

Partial discharge measurement of cables is very important as it gives an indication of the expected life of the cable and it gives the location of fault, if any, in the cable. When a cable is subjected to high voltage and if there is a void in the cable, the void breaks down and a discharge takes place. As a result, there is a sudden dip in voltage in the form of an impulse.

This impulse travels along the cable. The duration between the normal pulse and the discharge pulse is measured on the oscilloscope and this distance gives the location of the void from the test end of the cable. However, the shape of the pulse gives the nature and intensity of the discharge.

To scan the entire length of the cable against voids or other imperfections, it is passed through a tube of insulating material filled with distilled water.

Four electrodes, two at the end and two in the middle of the tube are arranged. The middle electrodes are located at a stipulated distance and these are energized with high voltage. The two end electrodes and cable conductor are grounded.

As the cable is passed between the middle electrodes if a discharge is seen on the oscilloscope, a defect in this part of the cable is stipulated and hence this part of the cable is removed from the rest of the cable.