Correcting Adverse Conditions

3.2.1 Adverse Dry Conditions

Although dry cables are expected to have longer lives than wet cables, dry cable failures are possible if adverse conditions exist. These conditions can exist from the time of installation or can occur over the life of the plant. Care should be taken to resolve adverse conditions when they are recognized. A number of dry condition failures are described in Appendix A to provide insights regarding failures that could occur and the related circumstances. Dry failures are often caused by random factors such as manufacturing flaws and installation errors coupled with an adverse environment or condition. The following subsections describe the stressors and their effects.

3.2.2 Physical Stress

A number of physical conditions can adversely affect medium-voltage cable life, including over-bending, compression, cuts, and gouges. When a shielded cable is bent into too tight a bend radius, the insulation-to-shield interfaces can be disrupted, providing gaps where partial discharge (PD) can occur, which would lead to short life. Permanent compression of the shield and insulation system can cause elevated potential stress in the insulation or disruption of shields. Cuts and gouges, depending on their severity, can disturb shields or even cause elevated potential stress in the insulation. Tension or compression forces on cables due to routing and termination can result in mechanical damage (due to external vibration) or electrical discharge degradation (for nonshielded or single-point grounded cables). Tension can also result in failure at a cable connection point such as a splice or termination. If such conditions are identified, the stress should be resolved and repairs made as necessary, or the cable condition should be monitored through periodical diagnostic testing. At minimum, performing a damage evaluation

Understanding the Physical Condition of the System

is warranted. Discussions with the manufacturer of the cable might provide insights regarding the importance of the damage and any necessary corrective actions. Compression and damage to cables can occur at the dropouts from trays to local conduits. The cable should be protected from sharp edges of the conduit by a bell or other appropriate fitting, and padding might be necessary on the rungs of the tray where the cable drops out to preclude excessive load on the side of the cable.

3.2.3 Vertical Support

Medium-voltage cables with larger conductors are heavy and need appropriate support devices where long vertical drops occur. Improperly supported cable can be crushed at its top support, leading to high electrical stress or disrupted shields at the top of the vertical run. Care must be taken when following the National Electric Code requirements for supporting larger cables. The standard prescribes the same number of supports for any cable larger than 500 kcmil (250 mm²), but if the standard were followed literally, the supports would exceed the support manufacturer’s allowable weight per unit length for cables larger than 500 kcmil (250 mm²). Manufacturer’s literature should be consulted when determining vertical support requirements. If not properly supported, the weight of the cable can also pull on connections at the top of the run, possibly leading to failure. Long vertical runs should be supported by strain relief grips.

3.2.4 Adverse Environments

Given that medium-voltage cables run throughout the power plant, localized adverse

environments can affect them. These environments can be permanent or the effect of errors or failures in the plant. The most likely adverse environment is elevated temperature and radiant energy conditions that occur when thermal insulation is left off, displaced, or temporarily removed from adjacent high-energy piping. Another possible damaging environmental effect is hot water or steam leaking from a pipe or valve that impinges the cable. These conditions should be corrected as soon as they are observed.

3.2.4.1 Temperature-Related Aging

Many areas and rooms inside the power plant are relatively cool environments, less than 40°C (104°F). However, some areas that contain medium-voltage cables can have temperatures well in excess of 50°C (122°F), which could reduce the life of the cables. These areas must be identified and appropriately managed in accordance with the general guidance provided in this report.

3.2.4.2 Radiation-Related Aging

It is not expected that medium-voltage cables will be subject to radiation levels high enough to cause cable aging in conventional plants. The EPRI has developed reports explaining that higher doses of radiation change the physical properties of the cable. Increased insulation hardness and loss of elongation at a break of the insulation occur after severe aging. Medium-voltage cables identified to be subject to doses greater than 5 Mrd (50 kGy) per 40 years or less should be monitored in accordance with the recommendations in this report.

Understanding the Physical Condition of the System

3.2.4.3 High Conductor Temperature from Ohmic Heating

Medium-voltage cables can also be affected by long-term high currents due to loading errors or unbalanced magnetic circuits. Such conditions can be compounded at fire stops, where heat transfer is reduced, causing further elevation of temperature within the cable. If conductor temperatures are found to exceed the cable design rating, they must be evaluated, and corrective actions must be taken. To date, the dominant issues relating to ohmic heating have occurred on multiple-conductor cable due to magnetic or resistive imbalances among the individual phase conductors. Studies on heating are available in the Nuclear Energy Institute (NEI) white paper 06-05, “Medium Voltage Underground Cable” [11].

3.2.4.4 High-Resistance Connections

Improperly made splices and terminations can deteriorate from elevated temperatures due to high-resistance connections. Terminations that are separable and not properly reassembled (as verified by post-maintenance measurement of connection resistance or thermography (if accessible) are also candidates for thermal degradation over time due to high connection resistance. These conditions, if not identified and corrected, will thermally degrade the cable insulation or accessory over time. Identifying and correcting high-resistance connections according to the guidance in this report will help to limit this failure mode.

3.2.5 Surface Corona and Partial Discharge

Nonshielded cable can be subject to surface PD (corona) in the small gap adjacent to the location at which the cable touches a grounded metal surface. Corona discharges occur from ionization of the air gap between the cable and the grounded surface. The conductor voltage distributes across the insulation, jacket, and air gap, with a large portion of the voltage across the air. With a high voltage across a small gap, a voltage stress higher than the breakdown stress of the air occurs.

The gap discharges and the voltage redistributes across the insulation and jacket so that the discharge is stopped. However, each electron stream causes a small increment of damage to the polymer surface, resulting in erosion of the polymer. Over an extended period, these discharges can erode the surface of the cable’s jacket and continue to slowly reduce the dielectric strength of the insulation system, if not corrected. In such cases, the presence of corona discharge is often indicated by a white powder in the vicinity of the discharge. Corona attack can be identified by visual examination during maintenance when terminations and junction boxes are accessible. An example of corona discharge is shown in Figure 3-3.

Understanding the Physical Condition of the System

Figure 3-3

White Powder Indicates a Location of Corona Discharge Between a Cable and a Ground Cable in Close Proximity