Chapter 2. Answers to Key Questions

In this chapter of the guide, key questions were categorized in eight sections. These sections are:

• Useful Life of Power Cables
• Degree of Degradation
• Detect Degradation of Power Cables
• Factors Influencing the Lifespan
• Cost-Effective Methods to Extend the Life Span
• Smart Replacement Strategy
• Installation Concerns
• Cable Failure Process

2.1. Useful Life of Cables

The existing transit systems vary in age and size and can be classified as small, medium, and large. In this guide, different types of transit systems were investigated with various examples. The lifespan of cables is affected by electrical parameters such as jacketing, cable size, diameter, and the method of insulation, overhead/underground, operating voltage, and weathering. Several attempts have been made to determine the factors associated with the influence on the life cycle of wire and cable insulation and jacketing.

2.2. Degree of Degradation

One of the most prevalent causes of degradation in cables are changes in weather conditions, including air moisture, ground, and temperature. The research conducted by Hyvoonen (2008) predicts the insulation condition of MV cable insulation. Different electrical measurements and chemical analyses are tested to find out the most representative combination. A largescale test program was carried out on field-aged and oil-impregnated paper insulated cables.

2.3. Detecting Degradation of Cables

Based on the size structures and the type of power supply of various transit agencies, there are vast numbers of indicators used to test and verify the aging of cables. One of the most common techniques is cable monitoring (CM), and it detects damage and measure the extent of degradation in electric cable insulation. Factors considered in selecting appropriate CM inspection guidance include intrusiveness, cable characteristics, and degradation mechanisms. The commonly used condition monitoring techniques include visual inspection, compressive modulus, dielectric loss measurement, insulation resistance, polarization index, AC voltage withstands test, partial discharge test, DC high voltage test, step voltage test, and a time-domain reflectometry test.

2.4. Factors Influencing the Lifespan

Cables are critical assets for transit systems and cable lifetime is generally estimated from the temperature change and associated lifetime reduction. However, determining lifetime reduction is intricate due to the complex physics-of-failure (PoF) degradation mechanism of the cable. This is further complicated with the different sources of uncertainty that affect the cable lifetime estimation. Generally, simplified, or deterministic PoF models are adopted, resulting in non-accurate decision-making under uncertainty. In contrast, the integration of uncertainties leads to a probabilistic decision-making process impacting the flexibility to adopt decisions.

2.5. Cost-Effective Methods to Extend the Life Span

Physical and visual inspections are the most common methods of detecting cable failures. The faults determined with the physical and visual inspections are generally at an advanced stage of the failure. Inspections are generally used to detect faults in overhead cables while having limited ability to detect faults in underground cables (Gouveia, 2014). Bloom et al. (2006) outlined a general decision model that enables utilities to generate business cases for asset management policies, with a specific application to underground distribution cables.

2.6. Smart Replacement Strategy

Monitoring strategies are crucial to extend the life of cables. Bicen (2017) presented a trend-based method to estimate the remaining service life of underground cables in which acceleration, and cumulative loss of life (LOL) can be calculated continuously in small time intervals in the model. The main novelty of this method is to determine the remaining service life of the underground cables considering the LOL trend on an annual basis. Electrical insulation failures occur as a result of aging caused by the presence of stresses. It is critical to determine the stress values to define the loading conditions. The other important issue is checking the insulation conditions.

2.7. Installation Concerns

Cable failure is a rising concern among utilities, particularly underground cables. They are harder to access and more costly to replace, as their installation cost is ten times higher than overhead lines. The expected higher failure rates in a cable’s life cycle occur in premature failures or aging failures. This curve is known as the “bathtub” curve, because of its shape. The explanation for early or premature failures is mostly with bad installation or cable defects from manufacturing or transport. Aging results from cable operation and exposure to aging mechanisms. Generally, premature failures are not considered assuming that this part of the cable lifetime is lived in the manufacture’s lab or detected during installation and testing procedures.

2.8. Cable Failures Process

Failures of cable systems are disruptive, expensive, hazardous, and result in loss of vital evidence. Cables, joints, terminations, and connectors that are properly installed and have not been subjected to mechanical forces, moisture, or extreme temperatures have a predictable long service lifetime. Cable life is mainly determined by the aging of the cable insulation. The cables can fail from any combination of electrical, mechanical, and thermal factors.

2.9. Conclusions

In this chapter answers to key questions are given. Several research findings determined the lifetime and the replacement time spent in replacing underground cables. The estimated lifetime of the cables is around 35 to 40 years based on the experimental research results. To detect the failure in the cables, there are various test methods available. Visual inspections are performed in manholes and taps, and hi-pot, meggering and thumb tests are run in certain periods to determine and fix the cable faults. In addition, the Blavier test can be performed when there is a ground fault in one cable and there are no other faulted cables. The Blavier test is the measurement of resistance from the end of one segment or the other when there is no sound cable. This method is considered a cost-effective test method used for cables to identify what may need replacement. Although the tests are relatively straight forward, repair or replacement can be challenging and complex work and may need more equipment to detect faults in traction cables. Fast and effective cable failure detection is critical and reduces the higher risks. The key features for installing underground cables are the distance between the cables, insulation size, depth of burial, ground thermal resistivity, temperature, and the cable support system. The guide has reviewed the parameters to be taken care of during the installation of underground cables. In addition, the parameters that are required to test the cable resistivity based on aging have been summarized thoroughly. New insulation materials brought a new aspect to cable service life. In addition, condition assessment and failure prediction are important for a longer cable service life. Even the failure mechanisms are different in various cases, and as such monitoring is very important in cable service life. Table 2.1. presents the summary of the Literature Review.

Table 2.1. A Summary for Literature Review