What are the Tests and Maintenances That Should Be Done on MV (XLPE) Cables?

MV XLPE cables are critical infrastructure elements that enable the safe and controlled transportation of energy at medium voltage level. For this reason, the tests and maintenance that must be done on MV XLPE cables should not be seen only as post-failure operations. The main purpose is to reduce the risk of interruption and equipment damage by detecting early weaknesses that may develop in the cable body, outer sheath, headers, additional bell areas and shield-grounding scheme. Because in medium voltage cable systems, the problem occurs not only in the cable itself, but also in accessories and application details.

The first step in maintenance is always safety. If work is to be done on the cable, the relevant line or cell must be disabled in the appropriate maneuvering order, tension must be confirmed and field safety must be ensured. Since MV cable systems are often associated with a substation, MV cubicle, RMU, ring line or process feed, it is not enough to isolate the cable alone; The connected system side must also be secured. Screen connections and grounding arrangements should also be carefully considered, especially if work is to be done on the header and additional areas.

Visual inspection is the basis of maintenance. The route where the cable is laid, cable duct, tray, pipe passage or directly underground area should be examined as much as possible. The outer sheath is checked for crushes, cuts, friction marks, chemical effects, mechanical stress, rodent effect, suspicion of moisture ingress or incorrect fixation. Subsequent excavation, construction, additional installation or heavy equipment passages along the cable route should also be evaluated. Because outer sheath damage is often the first sign of the cable before it goes into electrical failure.

Headers and additional bells are the most sensitive areas in MV XLPE cable systems. Many faults in the field are not caused by the body of the cable, but by incorrect cap application, poor joint workmanship or errors in the shield connection. For this reason, head surfaces, sealing areas, stress control elements, heat shrink or cold shrink parts, connectors and mechanical protection components should be carefully examined during maintenance. If there is any trace of darkening, cracks, scar formation, surface contamination, discharge or local heating, these should not be considered as minor defects.

Screen and grounding scheme is one of the main safety elements in MV XLPE cable. If there is a continuity of the metal screen, screen grounding points, bonding structure, cross-bonding arrangement, its accuracy and whether the screen current path is uninterrupted should be checked. Loose shield connection, improper bonding or broken shield conductor; may adversely affect fault behavior, display voltages and system security. For this reason, the cable system should not be seen as just conductor and insulation.

Sheath testing is an important maintenance tool to understand the durability of the outer sheath. This test is used to evaluate whether the dielectric integrity of the outer sheath has been compromised. Sheath testing helps detect outer sheath damage at an early stage, especially in routes that are directly buried, pass through channels or are prone to mechanical risk. Damage to the outer sheath does not always cause an immediate power outage; However, over time, moisture ingress can pave the way for screen corrosion and larger malfunctions.

It is not right to rely on a single test in the maintenance approach to evaluate the isolation status. In MV XLPE cable systems, the VLF withstand test is one of the basic methods used to verify the resistance of the cable to a certain voltage level. In addition, tan delta measurement provides information about the general aging and loss behavior of the insulation. If the cable defect is at an early stage, partial discharge measurement can be a much more powerful diagnostic tool in understanding the location and character of weak points. For this reason, test selection in MV cable maintenance should be made with condition assessment logic rather than pass-fail logic.

Tan delta assessment is especially valuable for monitoring overall insulation health. When there are inhomogeneous deteriorations in insulation, water tree-like aging effects, or increased dielectric loss, tan delta results can provide meaningful data to the maintenance plan. It alone does not explain everything; However, when used together with VLF and other observations, it gives a strong idea about the general condition of the cable.

Partial discharge measurement is a powerful diagnostic method, especially for header, joint bell and local insulation defects. Because most of the MV XLPE cable failures are caused by point weaknesses. Situations such as workmanship defects, contamination, gaps, screen cutout defects, or accessory assembly problems may produce PD behavior. For this reason, PD measurement provides a serious advantage in critical feeds, recurrent malfunctions or suspicious situations after a new installation.

The insulation resistance approach can be used as a rapid preliminary assessment in some sites; However, it alone is not sufficient to decide on medium voltage XLPE cable systems. This test can help detect serious defects, especially at the rough level, but in the modern condition assessment approach, it should be considered together with more detailed methods for header, additional bell and insulation health. The actual condition of the cable system often cannot be reduced to just one megger value.

Thermal inspection is very valuable in MV cable systems, especially at the points where headers, terminations, cable lugs, RMU or cell connections and cable shield grounding are located. The entire cable trunk often cannot be viewed visually; However, looseness or increased contact resistance in the header and connection areas can be detected early with a thermal camera. A temperature difference between similar phases is a strong sign that requires detailed examination at that point.

Cable route and mechanical protection arrangement should also be included in the maintenance plan. Cable trays, cable clamps, bending radii, wall penetrations, mechanical protectors, cable labels and warning signs along the route should be checked. Especially in underground applications, external effects such as loss of marking, risk of excavation near the surface or change of route may weaken cable security over time.

Maintenance for MV XLPE cables is not only work done after a malfunction; Commissioning and post-repair tests are also of great importance. Checking for cable damage before installation, verification of accessories after installation, and acceptance tests before the system is energized help catch many problems that may arise in the future. Control tests should not be neglected before recommissioning, especially on lines where headers or additional interventions have been made.

One of the most critical aspects of maintenance is recording and trend tracking. Sheath test results, tan delta data, PD measurement reports, thermal images, route observations and accessory controls should be stored regularly. Because MV cable faults are often more clearly understood not by a one-time measurement, but by changes over time. Repeated small deviations in the same line may be a harbinger of major failure in the future. In summary, the tests and maintenance that must be done on MV XLPE cables are; It consists of visual route inspection, header and additional bell checks, screen-grounding verification, sheath test for the outer sheath, VLF for insulation evaluation, tan delta and PD approach when necessary, as well as thermal and mechanical observations. If MV XLPE cable lines, header-joint applications, MV cubicle connections and maintenance plan will be evaluated together in your facility HV/MV testing, maintenance and repair with LV/MV/HV project design and consultancy services can support the technical decision process.