Reviewing cable life, and cable life expectations

By Michael Joseph, American Wire Group

The topic of medium voltage (MV) cable life is invariably discussed when considering the reliability of medium voltage power cable systems, such as those utilized to provide the distribution backbone of renewable energy installations.  The answer that is typically received is that MV power cables are designed to have a life expectancy in operation of 40 years.

In practice, cable system operators have found that non-insignificant populations of MV cable can remain in operation well beyond the 40-year expectation. Exploration of the factors that affect cable life and expected performance and moving towards the proactive elimination of these negative stressors on site through best-practice manufacturing processes and robust quality control in the factory, combined with good installation and commissioning practices in the field, can achieve MV power cable reliability well beyond the 40-year figure.

It is important to keep in mind that the 40-year life expectancy is a statistically built figure based on accelerated cable life testing. The entirety of the 40-year figure is expanded upon in 1956, when Jack Crowds stated, “Half the [cable] samples (in a test) would fail by the 40th year.” [1] This concept leaves for a significant population that is expected to perform well beyond the 40-year mark. 

A study published in 2015 [2], examined three 15 kV-rated MV cable populations that were subjected to steady-state service conditions. Cables were removed at several points over a 25-year service interval and subjected to a retained ac breakdown strength test, where voltage is raised in discrete steps until a breakdown is achieved. It is interesting to observe that even though MV power cables see a sharp drop in the retained AC breakdown strength, there is very little change after 10 years of service. Ultimately, the retained breakdown strength after 25 years is still well above the nominal operating voltage stress level.

The question remains of what are the factors that eventually drop this breakdown strength below operating conditions and cause a cable failure.

Understanding the construction of MV power cables is key to identifying the stressors that contribute to the primary cable system failure mode. The insulation system, between the conductor and neutral, is what keeps the electricity in the conductor.

The conductor shield is in intimate contact with the conductor and provides a smooth transition to the insulation layer. The insulation layer, typically made from Tree-Retardant Cross-Linked Polyethylene (TR-XLPE), or ethylene propylene diene monomers (EPDM, sometimes referred to as ethylene propylene rubbers, or EPR) is the component that experiences the electrical stress under applied voltage. The insulation shield surrounds the insulation and works in tandem with the conductor shield to provide for a uniform electric field within the insulation layer.  Since the electrical stresses of the applied voltage acts directly on the insulation layer, it is easy to conclude that long-term performance is mostly dependent on the quality of the insulation layer.

The increase of cable life expectancy, including beyond the 40-year metric, depends on identification and elimination of the factors that contribute to the probability of failure of the cable insulation system. These factors can be grouped into three categories: 1) low-level stressors that affect the cable on a molecular level linked to chemical and dielectric behavior 2) duty cycle stressors linked to environmental and service conditions, and 3) discrete physical stressors which are linked to point defects in the insulation system.

The low-level stressors typically identified under steady state AC voltage include thermal and oxidative behavior and water diffusion. However, these issues are explored and resolved during the material development and qualification stage.  The next category of duty cycle stressors includes electrical stress, thermal stresses, mechanical stresses, chemical stresses, and radiation (usually ultraviolet radiation) stresses. The use of appropriate jacket materials will eliminate the risk of radiation and chemical stresses. Cable qualification processes include thermo-mechanical testing which will demonstrate resistance to mechanical stresses. This leaves electric voltage stress and temperature stresses as the two primary duty cycle stressors that push cable to failure. In renewable applications vs. historical utility use cases, these stressors are even more critical as renewable systems are designed and sized to utilize almost 100 percent ampacity with discrete full-use and no-use cycles. It has already been demonstrated that cables, even after service aging, still retain a breakdown strength well in excess of the service voltage stress.  The remaining stressor, temperature, relies on the proper cable system design and selection of the correct cable conductor size for the specific calculation. Cables are designed for maximum operating temperatures of 90C to 105C, and if current carrying (ampacity) limits are respected during operation, these temperatures should not be exceeded.

The remaining category contributing to cable failures is discrete physical stressors. These are defects in the cable insulation that cause localized stress enhancements.  These localized stress enhancements, such as damage, voids, contaminations, localized overheating, and formation of concentrated moisture can raise the stress above the design breakdown strength of the bulk insulation material. This causes the insulation to break down into a carbonized formation called an electrical tree.  Electrical trees do not immediately cause a breakdown of the entire cable insulation system, but instead exhibit a behavior under voltage stress called partial discharge (PD). This PD activity and electrical tree defect effect is cumulative and will progress the cable to failure throughout the serviceable life. These defects are introduced during manufacturing, handling, or during accessory installation.  Realizing and exceeding expected cable life is dependent now more than ever on the elimination of these discrete physical stressors.

Robust factory quality control processes strive to eliminate manufacturing defects from entering the field. The remaining defects can be eliminated by good handling and installation practices.

However, reliability can only be assured by supporting good factory quality control and handling process using a commissioning test program. A commissioning test program that includes 50/60 Hz (operation frequency) with a factory comparable sensitivity level is preferred to eliminate these stressors. In lieu of this type of quantitative PD testing, other qualitative tests such as withstand testing with a very low frequency (VLF) voltage source can be useful, even though it may not present the full picture of expected cable performance. Tests that do not include analytics or a high voltage component (soak test or insulation resistance testing) may be useful for ongoing maintenance monitoring but provide little useful feedback at initial commissioning.

The notion of 40-year cable life is a widely held belief in the wire and cable industry. The practical experience of cable system operators in utility applications, however, challenges this notion. This aligns with the statistical probability concept that built the 40-year cable life metric. A study of service-aged cable over the course of a 25-year period demonstrates that under steady-state service conditions, the retained breakdown strength of MV power cables can remain well above the service voltage. As cable failures still happen, the factors that affect power cable reliability were explored. The most critical category of discrete physical stresses within the most critical component of the insulation layer manifest as partial discharge generating sites. Now more than ever, as renewable sites push the cable systems to design limits of ampacity and temperature, it is through robust quality control, handling, installation, and commissioning processes that system operators can eliminate these stressors from the cable system and ensure reliability well beyond the 40-year expectation.

(1) W. Thue, 2002, “Underground Residential Distributions Systems”, IEEE/PES ICC Fall 2002 Meeting Minutes,
875-901

(2) P. Cox, 2015, “Alternating Current & Impulse Breakdown Performance of 24-Year-Old Field Aged EPR Insulated Installed on the MLGW Electric System”, IEEE/PES ICC Spring 2015 Meeting Minutes, 309-331

Michael Joseph is Engineering Director of American Wire Group (www.buyawg.com).

Q4 2023