Carrying out the insulation condition measurement and analysis is important for the safe operation of the submarine cable. In this study, the high‐voltage frequency dielectric response is used to identify the ageing state of the AC 500 kV cross‐linked polyethylene (XLPE) submarine cable. The complex capacitance and tanδ in the low‐frequency area for the new, thermal ageing, electro‐thermal ageing cable all increase with the testing voltage increased from 200 to 2000 V. It is founded that the dielectric response at high voltage is more likely to detect the ageing differences. Based on the carbonyl index and the dielectric loss of the XLPE specimens at different position of the insulation layer, the non‐uniform ageing phenomenon of the cable insulation is presented for the thermal ageing and electro‐thermal ageing cable. The loss quantity parameter and its changing behaviour with testing voltage based on the frequency dielectric response could be used to identify the ageing state. The comparison of the modified Cole–Cole model parameter under 200 and 2000 V also indicates that the higher voltage dielectric response is helpful to identify the ageing differences.
Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced more than 20 years ago and has been thoroughly evaluated and proven in a number of research projects and field tests with good results. DFR data in combination with mathematical modeling of the oilpaper insulation is proven as an excellent tool for understanding insulation properties e.g. moisture content in cellulose insulation and temperature dependence of the insulation system. The model used to describe the insulation system inside transformers and bushings is called an XY-model. The model works well for most transformers but there are sometimes measurements showing an irregular response and not the expected shape for oil-paper insulation. In those cases, the moisture analysis becomes more difficult and sometimes almost impossible. One example is "frequency hump" phenomena sometimes observed in DFR measurements. A model of the internal creep currents from e.g. contamination has been developed and by applying it in moisture assessment software, better interpretation is obtained. Simulations are complemented with measurements on an actual XY-cell with a conductive layer on the surface of the insulation. Results support the simulation model.
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