Vancouver Island is Nestled Tightly Along the Lower Southwestern Coast of British Columbia, Canada. The island is 460 km (286 miles) long and 130 km (80 miles) wide at its widest point. Two parallel 525-kV circuits form a vital part of the transmission system supplying most of the load on Vancouver Island from mainland British Columbia. A 9-km (5.6-mile) continuous section crosses Malaspina Strait between Cape Cockburn Terminal (CCB) on Nelson Island and Texada East Terminal (TXE); the remaining 30-km (18.6-mile) continuous section crosses the Strait of Georgia between Texada West Terminal (TXW) and Nile Creek Terminal (NCT) on Vancouver Island. Installed in 1984, the circuits are each capable of transmitting 1200 MW (1410 A) with shunt reactive compensation.

There are 12 single-core 525-kV self-contained fluid-filled cables that were supplied by Prysmian Cables and Systems (Milan, Italy) and Nexans Norway (Oslo, Norway). Six of the submarine cables cross Malaspina Strait and the other six cables cross the Strait of Georgia. The maximum water depth of the crossings is about 400 m (1312 ft).

BC Hydro (Vancouver) owns the submarine cable assets. British Columbia Transmission Corp. (BCTC; Vancouver) is responsible for planning, managing and operating the transmission system.

CIRCUIT FEATURES

The cables in the straits of Georgia and Malaspina were placed on the seabed near the shoreline where the water depth is less than 20 m (66 ft), and cable was then buried in trenches approximately 1 m (3 ft) deep. In the intertidal zone, each cable was installed in a concrete chase, together with two 100-mm-diameter polyethylene pipes, buried 1.5 m to 2 m (5 ft to 6.6 ft) deep. These cables are forced-cooled with an external closed-loop pipe arrangement to eliminate the need for land-sea splices. The chase was filled with high thermal conductivity weak-mix concrete and covered with a concrete slab. The installation of this high-voltage cable system was a major achievement in the mid-1980s, and it remains the highest-capacity ac submarine system in the world. The operating experience has been excellent.

Vancouver Island is also served by a HVDC transmission system, a portion of which is nearing its end-of-life expectancy. Given this expected loss of firm capacity, coupled with increased load growth, planning and regulatory approval has been underway for some time to construct a 230-kV ac cable system between the mainland and Vancouver Island. To reduce BCTC's risks in serving the island, it was decided several years ago to explore the possibility of uprating the 525-kV cable system using real-time temperature monitoring and dynamic thermal circuit-rating technology. The concept was to create better models of actual cable capacity (and optimistically, higher ratings) for planning and operating the system, rather than relying on original design ratings.

APPLIED RESEARCH

In the mid-1990s, in collaboration with the Electric Power Research Institute (EPRI; Palo Alto, California, U.S.), a research and development project was completed to demonstrate fibre-optic distributed temperature sensing (DTS) applications to identify thermal bottlenecks in BC Hydro's existing underground transmission cables. This work demonstrated benefits such as increased cable capacity and avoidance of potentially damaging overloads. In fact, the technology is credited with deferring a major capital cable-replacement project in the Vancouver area by two years.

As an extension of this research and development effort, one of the suppliers of the 525-kV Vancouver Island cable was contracted to confirm the feasibility of inserting a 1.4-mm-diameter stainless-steel tube containing optical fibres (fibre in metal tube [FIMT]) into the conductor fluid ducts at the shore-ends of the cable route, which are the thermal-limiting sections. This prototype system was carefully evaluated for performance with hydraulic pressures of 2700 kPa and temperatures of -26°C to 82°C (-15°F to 180°F) at the terminations, and its feasibility was then confirmed in tests at the supplier's test facility.

Following this successful research component, a capital project was implemented to insert optical fibres into 12 cable terminations located at two cable-terminal stations (identified as NCT and CCB on the map on page 38). Fibres were inserted into the conductor over approximately 100 m (328 ft) from the top of the terminations. This was sufficient to reach into the intertidal zones to:

• Study the limiting shore-end thermal characteristics of the submarine cables to determine a more precise system thermal rating, including investigation of shore-end forced-cooling system effectiveness

• Provide conductor real-time temperature monitoring to allow system operators to operate the system at higher capacities

• Modify the shore-end cooling system and controls, as necessary, to optimise cable use and performance.

FIELD INSTALLATION AND CALIBRATION

Two open-ended multimode fibres in a 1.8-mm stainless-steel tube (FIMT cable) were provided with suitable fluid seal fittings and inserted into the hollow cable cores. Considerable effort and precautions were taken to control the hydraulic system pressure from the remote pressurizing plants and to minimize the fluid flow during fibre insertion, so as to not jeopardize the hydraulic system integrity. This larger diameter (1.88-mm) FIMT cable was chosen for the final installation to improve the tube rigidity and ease of installation on-site.

Twelve 525-kV optical down links containing four multimode fibres were attached to the 525-kV bus and optical fibres appropriately spliced to the FIMT fibre cable. Electric-field analyses were performed to help design a special 525-kV high-voltage, corona-free optical-cable splice case. Additional optical-fibre cable was then routed from the base of the downlink to the NCT and CCB control buildings to interface with the DTS systems provided by J-Power Systems (Tokyo, Japan).

In all installations, 0.5-m (1.6-ft)-diameter loops were placed in the corona shields of the cable terminations to allow temperature calibration and confirmation of the measured temperatures by the DTS unit. These loops provide a reference point to continuously compare and confirm the accuracy of the temperature measurements outside and inside the power cable. The DTS unit was previously calibrated at the factory, but temperature calibration was repeated on-site because of the different optical fibres used to confirm data accuracy, reliability and stability at the sites.