Unique Approach To An Old Problem

New Tecnology Helps BC Hydro Locate Heat Resistance

By Michael MacMillan

BC. Hydro is combining new computer technology with fibre optic cables to improve the performance of its underground transmission lines.

Officials are hoping the new system will offer them 10 to 15 per cent greater efficiency from heavily-loaded underground circuits. If successful, it could eliminate nearly $10 million in costs.

Known as Distributed Temperature Sensing (DTS), the technology was developed by York Sensors, a private U.K.-based company. Together, B.C. Hydro, and its research and development affiliate Powertech Labs Inc., are conducting extensive engineering studies with DTS to increase the capacity of the most heavily-loaded sections of its seven kilometer underground 230 kV transmission system in the Greater Vancouver area. The system is one of the longest underground runs in North America.

Work is being carried out in collaboration with the U.S.-based Electric Power Research Institute (EPRI).

Forty years of urban development and subsequent load growth have put B.C. Hydro's system under such strain that some of the circuits periodically operate at their thermal limit. The section chosen for application of DTS runs 3.5 kilometers from a terminal near Burnaby Lake to the Newel Substation. It is a heavily-used section, providing power to customers in the south Burnaby, southeast Vancouver and New Westminster areas.

Overcoming underground cable impedance has long been a priority for electric utilities. Resistance and reactance both hamper the flow of electricity, but only resistance causes heating, which results from the dissipation of electricity within the system. Causes of heating are wide-ranging; from the inherent resistance of the wire itself, to the magnetic properties of the cores in power transformers and even the inductors or the insulating (or dielectric) properties of the materials in capacitors.

Although it is a concern in overhead transmission wires, heating is especially problematic in underground cable systems. Once heat is generated beneath the soil, which is a poor conductor of heat, it has nowhere to go.

Cable designers are well aware of this problem, and as such they have designed cables to handle a certain amount of heat. The maximum temperature a cable can withstand is referred to as its "thermal limit." If the temperature exceeds this limit, equipment in the system begins to break down. Not surprisingly, utilities across North America are currently looking for ways to reduce resistance heating and its associated risks which, together with other impedance factors, waste an estimated seven per cent of all electricity generated in the U.S. annually.

B.C. Hydro is counting on DTS to help solve this problem. Although it takes advantage of the latest technology, DTS operates on a simple premise. It uses advanced opto-electronic equipment to launch a laser light down conventional communications grade fibre optic cables. The cable is either installed in the same duct as the cables, or in a spare duct next to them. A component of the light that is reflected back to the source is temperature dependent.

"By using very high speed measurements and computer processing, it is possible to determine the temperature all along the fiber with an accuracy of about one degree Celsius, and to within two meters over distances up to 10 kilometers," explained Allen MacPhail, a specialist engineer with B.C. Hydro's system development division.

Until recently, there existed no means by which to accurately measure temperature along the cable in order to determine where heating was becoming a problem. Particularly for older circuits, the location and extent of these "hotspots" are virtually undetectable. Classic cable ampacity calculations therefore apply very conservative factors of safety to ensure overheating does not occur, the theory being that it is better to be safe than sorry.

But if the true nature of the thermal profile along an entire cable route could be positively identified, steps could then be taken to improve conductivity in and around the affected sections. This would allow operators to adjust the conservative factors of safety without compromising reliability. Enter DTS.

"Normally the classical methods to determine ampacity makes a lot of assumptions about the environment in which the cable is sitting in," said Sudhakar Cherukupali, senior research engineer at Powertech Labs. "There is a safety factor that's built-in that makes conservative assumptions about the conditions of the soil, so you're not really utilizing the full capability of the cable."

Acquiring DTS came only after an extended international shopping trip. Powertech carefully studied several other systems before choosing it. A favourable cost-analysis study, Cherukupali said, made it attractive to management. They were also impressed with the system's track record, the experience of its creators and the fact that DTS allows users to access information remotely. From an engineer's standpoint, Cherukupali was also attracted to its innovative style.

"It's an aging system, and we wanted to utilize the asset to the maximum possible without jeopardizing performance. This technology seemed to be a novel way to do that."

DTS could soon provide engineers with the ability to improve their cables with surgeon-like precision. To demonstrate his point, Cherukupali explained how three months into the project, he and his colleagues discovered that a particular section of the cable that lies directly underneath a major roadway is much hotter than in other locations. Knowing the precise location means that B.C. Hydro, if they choose to, could excavate the cable at the location, and refill the site with material of higher conductivity, thereby easing the stress on the system.

But he cautions that calculating the total potential ampacity is still tricky business. Data must first be interpreted before it can be acted upon. Test results, Cherukupali said, will be carefully analyzed before any conclusions are made. DTS may be an effective tool, but it is by no means a panacea.

"You don't take one spot measurement today and say that's your hotspot, because these things can change depending on whether it's summer or winter, or frost depths," he added.

In fact, Powertech Labs intends to take a more than a year's worth of information before acting on test data. This, he said, will ensure that he and his colleagues are acting upon a reliable set of data. Once this process is complete, Powertech will mount the fiber cable directly adjacent to the hotspot, to get an even more accurate reading of the temperature.

What also has the Powertech team excited is the potential. for DTS. He is confident that it can be used in other applications at B.C. Hydro. But there is a lot of work to be done before Powertech can entertain other possibilities. Even when the DTS data is ready, it must be weighed against B.C. Hydro's larger plan to update their provincial transmission and distribution system. Considered in this light, Cherukupali's work is just one step in continuing process.

"So there is still a fair bit of work to be done. It's still on-going." ET