Experience of CPRI in very low frequency tan delta testing and condition monitoring of medium voltage polymeric cables

Power cables form vital components in a power distribution network. Synthetic dielectric extruded cables are extensively used for distribution of power in the medium voltage range in various industries, power plants, cities and towns. The distribution class of power cable networks represent a large capital investment for electrical utilities and hence reliability of cables is at most important to avoid loss of revenue resulting from premature failure. In this era of reformation and liberalization of electricity markets, the power utilities are faced with new challenges of providing uninterrupted quality power supply with minimum cost. Many cable networks operating in various industries, petrochemical plants, process industries and urban areas are 20–30 years old and shall have to operate closer to their maximum limits due to various economic considerations. As a result of this, electrical utilities are now faced with decisions to maintain, refurbish, repair or replace the cables in the network. To facilitate making such vital decisions the utilities are often confronted with the questions like How to assess state and condition ofthe cables? What is the criterion to be adopted? In this endeavour; the utilities are looking for appropriate diagnostic test methods to determine state and condition of the cables.

In recent years, Very Low Frequency (VLF) dissipation factor testing has been proved to be effective for condition assessment of extruded cables. Central Power Research Institute (CPRl) has conducted VLF tan delta test on service aged cables to assess extent of ageing and on new cables to generate commissioning test data for future condition monitoring. The paper presents the experience of CPRl in VLF dissipation factor testing on the medium voltage XLPE cables. The base value of VLF tan delta and its variation with test voltage have been found to be sensitive to the level of ageing and presence of impurities in the bulk of the XLPE insulation.