Insulation Fundamentals

The fundamental understanding of cable insulation properties forms the foundation for assessment of cable operability. These same fundamentals provide the basis for evaluating whether various electrical and physical tests and measurements are meaningful, cost-effective, and warranted.

They are a basis for evaluation of present or conventional cable test practices against the critical properties of concern for:

Cable operability

Life extension

Retention of the original environmental qualification, and

The adequacy of environmental qualification.

General Properties of Insulation

The electrical properties of concern for cable insulations are dielectric loss properties (resistivity, insulation resistance, dielectric constant and permittivity) and dielectric endurance properties (dielectric strength, breakdown strength, and ability to withstand corona attack).

Although these properties are important for higher voltage and other specialty applications, many of them lose their importance for the low-voltage cabling used in thermal/nuclear power plants. It is demonstrated that the significance of mechanical and thermal properties depend upon the application of the cable.

Insulation resistance measurements are commonly used to evaluate insulation systems.

For shielded cable, insulation resistance is directly related to the volume resistivity of the cable. For unshielded cable, the insulation resistance has a complex relationship to volume and surface resistivity because there is no shield for a return path.

Good Cable Insulation

When voltage is impressed across any insulation system, some current leaks into, through, and around the insulation. When testing with DC high-voltage, capacitive charging current, insulation absorption current, insulation leakage current, and by-pass current are all present to some degree.

For the purposes of this article 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 RPwould 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.

Cable Faults Described

When at some local point in a cable, insulation has deteriorated to a degree that a breakdown occurs allowing a surge of current to ground, the cable is referred to as a faulted cable and the position of maximum leakage may be considered a catastrophic insulation failure.

See Figure 4.

At this location the insulation or parallel resistance has been drastically reduced and a spark gap has developed. See Figure 5.

Occasionally a series fault shown in Figure 6 can develop due to a blown open phase conductor caused by high fault current, a dig-in or a failed splice.

The Ugly Cable Insulation

In the matter of fact, there is no ugly cable insulation. It can be either good or bad. Every condition between is considered as bad.

Hipot Cable Testing Part-1 (VIDEO)

Hipot Cable Testing Part-2 (VIDEO)

Resources: Power Plant Practices to Ensure Cable Operability – Electric Power Research Institute; Fault Finding Solutions – Megger