What is the expected life of insulated cables

Update:11-05-2020
Summary:

Deterioration of cable insulation during its normal ope […]

Deterioration of cable insulation during its normal operation is a major concern. Usually, electric cables receive less periodic maintenance compared to the other electric components, although they are subjected to several environmental conditions during operation such as high temperature and oxidative atmospheres.In this study, a standardized accelerated thermal ageing technique was used, with the application of the Arrhenius model. This technique is commonly used in accelerated life testing to establish a lifetime-stress relationship and estimate cable lifetime. Two types of Cross-Linked Polyethylene material working at elevated temperatures between 95 and 105 °C were selected for testing. In such accelerated ageing processes, it is required for the insulation to reach a degradation level, which is considered the end of life for the material under evaluation. The end of life criteria (also called endpoint) is defined as a percentage reduction of elongation at break, which is considered in this study to be 50% retention of elongation at break.

 

Aluminum Foil Mylar

Thermal ageing was carried out according to the BS 7870-2 standard, while elongation at break was evaluated at several ageing stages. The uncertainty in the measurement was estimated. The short-term data points determined by ageing treatment is represented graphically in the Arrhenius plot. The extrapolation of such data was used to predict the long-term performance and estimate the cable lifetime. The lifetime for XLPE is expected to be between 40 and 60 years at 90 °C rated operating temperature. Experimental findings of this study show an estimated cable lifetime between 7 and 30 years for rated operating temperatures between 95 and 105 °C.Deterioration of cable insulation during its normal operation has been a major concern for utilities. Cross Linked Polyethylene  was invented in 1963, in the General Electric Research Laboratory, to create a crosslinked material with better mechanical and thermal properties than Polyethylene, which can stand an operating temperature of 90 °C .

 

There have been attempts to increase the operating temperature of conventional high voltage transmission cables to more than 90 °C, in order to prevent thermal degradation for XLPE dielectric. Efforts were exerted to estimate the insulation lifetime and study the ageing process that may lead to a breakdown. Chemical changes were observed when an XLPE cable with water-barrier design was thermally aged. It was found that crystallinity was increased after the application of thermal stress. Exposing a flame-retardant cross-linked polyethylene to thermal ageing at 100, 135 and 155 °C for 800–2000 h was found to affect its performance progressively. Ageing was found to happen through three stages, in the first stage cross linking increased in the material and caused a reduction in conduction current and the imaginary part of complex permittivity. In the second stage, oxidative degradation resulted and became dominant which resulted in an increase in conduction current and the real and imaginary permittivity. In the last stage, the conduction current stabilized but the complex permittivity continued to increase .

 

Insulation degradation was investigated by applying different stresses, including electrical, thermal, and mechanical stresses. To achieve this, the insulations are exposed to high stresses for a specific time, until a degradation level endpoint is reached. Ageing under wet conditions was also considered for particular insulations after it was found that humidity is a main source of water trees in .Since ageing of insulating materials under normal operating conditions would take long durations, and as it is inconvenient to get aged samples from the field to do the required testing and assessment, accelerated ageing techniques have been implemented by the researchers to simulate the stresses and environmental conditions that the cables are most likely to encounter while in service.This work aims to estimate the lifetime of XLPE cable working at elevated temperatures by applying the Arrhenius model on thermally aged samples. This model is commonly used in accelerated life testing to assess the lifetime-stress relationship and estimate the service life of a product. This procedure follows a thermal ageing protocol, according to the British Standard.