Location of the Weak Part in the Underground Power Cables

The right-of-way problem associated with overhead transmission lines has resulted in a large share of electric power distribution & transmission being facilitated by underground power cables in our country and worldwide. Most of the underground cables in service all over the world had been installed long back and it has been observed that the frequency of failures for a large proportion of them has increased exponentially. The reasons for these failures are mainly electro-thermal ageing, excessive partial discharge (PD), and mechanical damage. Going by the statistical data provided by some power utilities, it is found that in metropolitan cities like Mumbai, during summer there is an average of three cable failures per day. These numbers are even direr and alarming in other older metropolitan cities like Tokyo, New York, and other parts of the world, where cables are the sole medium to transmit power. These failures apart from having a huge impact on the economic system also cause hindrance to social and business aspects regarding the utility sector. In cable, there is no such reliable technique which is available to localize all these deformities. Therefore, it is high time to develop technology or devices capable to identify aged/weak portions in the cable. The weak cable portions are portions that are prone to failure due to reasons such as ageing, excessive temperature rise, high moisture content, heavy PD activity, etc, which cause changes in material properties.

The aforementioned reasons cause changes in impedance on the particular weaker sections of the cable. These changes reflect in frequency response analysis, as each of the above-mentioned deformities has distinct characteristics with respect to change in frequencies. This encourages us to use the Sweep Frequency Response Analysis method (SFRA) in the cable to locate the weak sections.

SFRA is a widely used technique for locating faults and condition monitoring in transformers, but to date, there is some rare literature available for the application of SFRA in power cables. The biggest problem in the cable is that there is no manufacturer signature graph available, unlike transformers, there is no signature graph presents for cable. I have found out an analytical technique by which the signature impedance graph of the healthy cable can be generated, and when this impedance graph was compared with that of the faulty cable impedance graph the exact position of the fault was predicted . Hence making the SFRA test feasible for power cable. The above-mentioned method was also patented which was under review

Fig. Voltage gain of different peak points in a SFRA graph for a healthy cable along with faults at different parts of the cable

Fig. Resonating frequencies of different peak points in a SFRA graph for a healthy cable along with faults at different location in the cable