GOOD CABLE INSULATION
When voltage is impressed across any insulation system, some current leaks into, through, and around the insulation. When testing with dc highvoltage, capacitive charging current, insulation absorption current, insulation leakage current, and by-pass current are all present to some degree. For the purposes of this document on cable fault locating, only leakage current through the insulation will be considered.
For shielded cable, insulation is used to limit current leakage between the phase conductor and ground or between two conductors of differing potential. As long as the leakage current does not exceed a specific design limit, the cable is judged good and is able to deliver electrical energy to a load efficiently.
Cable insulation may be considered good when leakage current is negligible but since there is no perfect insulator even good insulation allows some small amount of leakage current measured in microamperes. See Figure 1.
The electrical equivalent circuit of a good run of cable is shown in Figure 2. If the insulation were perfect, the parallel resistance RP would not exist and the insulation would appear as strictly capacitance. Since no insulation is perfect, the parallel or insulation resistance exists. This is the resistance measured during a test using a Megger® Insulation Tester. Current flowing through this resistance is measured when performing a dc hipot test as shown in Figure 1. The combined inductance (L),series resistance (RS), capacitance (C) and parallel resistance (RP) as shown in Figure 2 is defined as the characteristic impedance (Z0) of the cable.
WHEN CABLE INSULATION IS BAD
When the magnitude of the leakage current
exceeds the design limit, the cable will no longer deliver energy efficiently. See Figure 3.
Why A Cable Becomes Bad
All insulation deteriorates naturally with age,
especially when exposed to elevated temperature due to high loading and even when it is not physically damaged. In this case, there is a distributed flow of leakage current during a test or while energized. Many substances such as water, oil and
chemicals can contaminate and shorten the life of insulation and cause serious problems. Cross-linked polyethylene (XLPE) insulation is subject to a condition termed treeing. It has been found that the presence of moisture containing contaminants, irregular surfaces or protrusions into the insulation plus electrical stress provides the proper environment for inception and growth of these trees
within the polyethylene material. Testing indicates that the ac breakdown strength of these treed cables is dramatically reduced. Damage caused by lightning, fire, or overheating may require replacement of the cable to restore service
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