Protect Medium-Voltage Cable Insulation from Failure
Cable designed to be discharge-resistant may be the best way to protect medium-voltage cable insulation from failure and to ensure system reliability.
Design engineers must ensure the reliability of solid-dielectric power cable against failures caused by partial discharge (PD), which can be quite a challenging task. Engineers can guard against PD by designing cable to be either discharge-free or discharge-resistant. Designing to the discharge-resistant standard leads to far greater overall system reliability.
What is PD?
PD, or corona, electrically ages cable, at voids where the deterioration begins. PD occurs when an electric field inside the void exceeds the breakdown threshold of the gas in the space. Electrical breakdown pulses occur in microscopic voids, and contaminants fracture the insulation, resulting in points of high stress.
The discharge transfers high energy through ion bombardment onto the surface of the insulation, breaking molecular bonds and degrading the void’s internal surfaces by chemical reactions between the polymer and the ionization by-products. This ionization process usually occurs over a small portion of the insulation, and the rest continues to work.
PD can also exist in gaps or voids between the insulation and stress-control shields due to improper handling or external damage during installation or operation, as well as in voids and gaps in splices and termination regions due to inferior workmanship and poor fit.
While the existence of PD does not typically cause a catastrophic event, such as a cable failure, immediately, it can harm the cable insulation in the long run and may lead to premature cable failure.
Two Philosophies for Partial Discharge
There are two different philosophies for dealing with PD in power cables.
One approach, used by most major medium-voltage cable manufacturers, is to try to eliminate the partial discharge in the manufacturing process, which results in what’s called discharge-free insulation. Because PD occurs in voids, the elimination (discharge-free) approach attempts to ensure that the manufacturing process removes all dischargeable voids. Visual examination with optical microscopy, or, more commonly, factory PD testing, is used to test the cable and label it discharge-free.
However, it is important to understand that the practical limit of current technology means that no cable can be manufactured to be 100% void-free throughout its life. Over time, all cable deteriorates and breaks down. So cable labeled discharge-free when first manufactured is actually cable with potentially damaging defects and with less ability to resist the corona effect over time, which lead to breakdowns and electrical failure.
The other approach, used by Kerite, is to build partial-discharge immunity into the cable insulation by using materials specifically formulated to resist PD-initiated degradation, which results in discharge-resistant cable. Kerite’s proprietary ethylene propylene/ethylene propylene diene monomer (EPM/EPDM)-based compounds are formulated, mixed, and extruded in-house with the express goal of resistance to degradation caused by PD.
The discharge-resistant approach recognizes that undetectable but dischargeable voids exist in all cable, and that cable reliability depends upon the insulation’s ability to operate in the presence of PD. Designs using the discharge-resistant approach focus on material development, including the use of tools to enhance the EPR (ethylene propylene rubber) compound ingredients and build in the discharge-resistant characteristics (immunity) that will make the cable insulation extremely long-lasting. PD is intentionally introduced and reactions recorded to evaluate the PD resistance of different materials, since not all materials exhibit the same resistance to partial discharge. Figure 1 shows the types of materials in the specially formulated compound that build-in discharge resistance.
Kerite’s philosophy is that PD resistance should be a basic requirement for all insulating materials used in power cable. For example, Kerite uses evaluation methods to introduce discharges on the exterior surface of the insulation and measure how different materials perform.
This focus on building PD immunity by making cable that is discharge-resistant is believed to be one of the main contributors for Kerite’s superior field performance, demonstrated by more than a century of empirical evidence and a proven track record. EPR-based cables show no signs of partial discharge over extended periods of time, effectively providing a lifetime guarantee that there will be no electrical failure due to insulation deterioration.
No Cables Are Truly Discharge-Free
Over the past few decades, most cable manufacturers have emphasized the development of new manufacturing technologies designed to reduce voids by making material improvements, including removing contamination in compounds. Together with more stringent factory tests using advanced PD detectors, these new-generation cables have come a long way in providing longer life by reducing manufacturing defects that are known to cause PD.
As manufacturing processes have improved, standards for the amount of PD allowed have gone down dramatically, but a certain amount is still allowed – AEIC and ICEA standards currently allow a maximum of 5 Picocoulumbs (pC). The fact of the matter is that no cables are truly discharge-free and current factory partial-discharge testing fails to detect potentially damaging defects. These failures cause aging mechanisms that cannot be eliminated practically. While cable reliability may not be of the same magnitude of concern as 20 years ago, it still matters greatly. Buying premium cable is still critical for ensuring a system’s overall reliability.
Testing for partial discharge
Tests to evaluate the effect of PD on cable insulation help locate and measure the sites of PD in cable. A typical material test utilized for this is the cylindrical electrode method (ASTM D 2275), which simulates PD sites on insulation surfaces.
The cylindrical electrode test, shown in Figure 2, is a more refined test that can be better controlled and used to directly measure the degrading effect of PD. The test voltage is selected to produce stable discharge intensity at the surface and degradation in a reasonable amount of time.
Erosion channels appear in most insulation materials and virtually all dielectric failure occurs in these eroded channels. Figure 3 shows the average depth of the erosion channels. It is clear that there are significant performance differences, both between EPR and PE families and among EPR compounds.
As demonstrated in the graph, excellent discharge resistance is practically achievable through compounding. Despite the variability of test results due to varying environmental factors and dielectric strength of the insulations, commercial EPRs show superior partial discharge resistance over cross-linked polyethylene (XLPE), and Kerite-formulated EPR compounds show superior partial discharge resistance over other EPR compounds. In this example, the Kerite formulation is 100 times more discharge-resistant than the next best cable.
While manufacturers of so-called discharge-free cable use such testing to demonstrate that the cable has no corona, over time voids appear as gas dissipates, and other mechanisms lead to more and more corona, PD discharge, electrical treeing, and eventually even electrical failure.
Keep in mind that production tests are done in the factory in a controlled and clean environment. Then the cable is shipped and has to be installed. Often installers are not kind to cable during installation; particularly at the interface between materials, they may be “torturing” the cable, which creates potential PD sites.
With eroding insulation, existing voids get bigger and bigger, and there are higher levels of discharge. As the cable ages, the gases that fill the voids dissipate, and moisture migrates in, changing the insulation compounds and washing out protective ingredients. In addition, the cable must be spliced and terminated, so workmanship becomes an issue. The end result is that the area of PD in splice or termination becomes a critical area and a weak link in the whole system.
PD activity may develop during the service life of cables regardless of their discharge-free design. At some point, cable originally tested and deemed discharge-free may fail, whereas Kerite’s formulation, with its built-in immunity to PD, is not degrading and will not fail.
So, what’s the final takeaway on the benefit of discharge-resistant over discharge-free cable insulation? The presence of PD in service aged cables is a foregone conclusion. Therefore, it follows that selection of materials that better tolerate PD activity over the cable’s life adds a welcome comfort margin to any reliability assessment.
This article was contributed by Kerite.