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Need to Keep Your Electric Power "On"? Call a Cable Doctor

Ruslan Papazyan
Electrical Engineering
KTH (Royal Institute of Technology), Sweden
October 2003

Recently, millions of people in the US and Canada realized the importance of electricity after getting trapped in dark subways and elevators. One way to prevent events like this is to keep all the equipment supplying electricity in good "health". This could be done by diagnosing the equipments' condition and then, if necessary, repairing or replacing it. I am studying methods that help diagnose power cables. My aim is to determine if they have an "illness", pinpoint where exactly it is, and then recommend a treatment. So it is somewhat like being a medical doctor; it's just that the patient is a cable.

To make sure that power stays on, we need to keep all elements that provide us with electricity in good condition. This includes the parts involved in the production of electricity, its transmission to our cities and, finally, its distribution to each user. Out of these components it has been found that high voltage cables are one of the most troublesome links, causing a significant amount of breakdowns. The problem is compounded because there is a considerable quantity of cables in use - in Sweden alone their total length exceeds 60,000 km. In addition, replacing a faulty cable can be rather expensive, costing up to 1,000,000 SKr ($130,000) per kilometer.

There are lots of reasons for a cable to fail: it could result from mechanical damage, a natural disaster (i.e. thunderstorms), or it could be that the cable has aged so much that it's no longer able to work properly. It's also particularly important to know where exactly the faulty spot is since cables can be up to several kilometers long and their total replacement is a time-consuming and costly procedure. Replacing a cable can be avoided if we know where exactly to look for the potentially weak spot and change this section only.

The method that I am developing helps determine if the cable has aged at a specific place along its length and decide where exactly that place is. The way to pinpoint this spot is by using a technique that resembles the use of radar for finding airplanes and ships. For this purpose I send an electrical signal along the cable; when the signal reaches the aged spot it reflects back. I can then detect this reflected signal, and, based on the time it takes the signal to come back, I estimate the distance to the spot. Moreover, the "strength" of the signal indicates how badly deteriorated the spot is. To go even further, I can use the shape of the reflected signal to estimate what is the cause of the aging. To collect information on what the reflected signal should look like, I conduct experiments where I deliberately cause a cable to deteriorate and measure the reflections. Then, when I measure an unknown cable, I know what types of reflected signals corresponds to what specific kinds of faults.

I have already tried this method on several occasions where companies have requested to diagnose the condition of their cables. During these tests, it was possible to detect the condition of the cable and on one occasion I found that there was a specific spot, which showed more severe signs of aging. Later, the company confirmed that they had previously experienced problems in the same section that had lead to a breakdown of the cable.

The immediate reward of being able to use your knowledge to understand the symptoms of your "patient" and recommend a cure is highly gratifying. Such results make me believe that this localization method is a promising way to help estimate the condition of power cables. This would lead to significant savings for the owner of the cable and what is more, to a more reliable supply of electricity.