Distributed fibre-optic temperature sensing is used to uprate the Cancouver Island 525 kV submarine cable system
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.
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.
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.
Originally, there were no current transformers at any of the cable terminals, so there was no local data available on the actual cable conductor current. It is noteworthy that the 30-km (19-mile) section has a significant component of reactive charging current in addition to the active load current. To obtain accurate information on actual cable currents flowing in the central conductor and armour, for comparison with measured temperatures, special 600-V-class, flexible, electromagnetic current transformers were retrofitted around the cables, beneath the terminations and around connections to the cable armour clamps. Resistance temperature devices (RTDs) were placed at predetermined locations on the cable surfaces to record cable armour temperature and on the cooling pipes to record water inlet and outlet temperatures and ambient soil temperatures at depths of 1 m, 1.5 m and 2 m. New data-acquisition systems were installed at NCT and CCB stations to archive, display and provide modem access to computers in the engineering office. A system also was installed to send conductor temperatures and cable currents to BCTC's System Control Centre.
The DTS software archives temperature data and allows post-processing of archived data to help track, for example, the maximum temperature in the fluid duct as a function of time of day. This data also can be exported for use with other relevant cable data to track trends. The installed system also has the ability to alarm or warn the system operator if the cable-conductor temperature reaches a preset threshold.
CONDUCTOR TEMPERATURE PROFILES
The figure above shows some temperature profiles obtained from the DTS system. The left end of each profile represents the point where the DTS cable exits the DTS unit in the control building, thereby displaying the room temperatures. Approximately 50 m (164 ft) thereafter, the DTS cable leaves the control room in a plastic duct, and at the 125-m (410-ft) mark, the optical cable enters the conductor core.
Daily observation of the temperature profile showed that at a distance of about 150 m (492 ft), the conductor temperature rose and fell in a cyclical manner, even if the load-current changes were minimal. After careful examination, it was clear that this change coincided with the tides — during low tide, the temperature increased and during high tide, the temperature decreased.
In this same graph, the yellow horizontal line at 40°C (104°F) and the red line at 60°C (140°F) have been set to trigger temperature warning and alarm signals. The horizontal sections 5°C (41°F) between the 80 m to 130 m (262 ft to 427 ft) for cable G1 to G3 and between 150 m to 190 m (492 ft to 624 ft) for cables G4 to G6 represents the FIMT in the cable termination corona shield displaying ambient air temperature.
A plot of data acquired during September through October 2005 at the NCT station shows the cable currents (armour and conductor), soil temperatures, coolant flow conditions and coolant temperatures. Data was monitored under three conditions, namely natural-coolant convection, forced-coolant circulation with chillers off and forced-coolant circulation with chillers on. In the original design, the chiller is only activated when one of the following conditions occurs:
One of the soil RTDs reaches a temperature of 8.5°C (47.3°F) or higher and the coolant to the chiller is above 32°C (89.6°F).
One or more armour RTDs reaches 50°C (122°F) and the coolant to the chiller is above 32°C.
The coolant temperature reaches 35°C (95°F) or higher.
HIGHLIGHTS FROM DATA
The information gathered from the installation provided extremely useful input for determining new cable-capacity ratings, for example:
There is variability in measured currents between phases on one circuit at one cable site.
Impedance differences because of cable-construction differences caused one supplier's cables to carry more current than the other. However, the conductor temperatures on the more lightly loaded cables have been consistently higher and should be used to determine the governing current ratings.
Longer response times for armour temperature measurements with changing load, as compared to conductor temperature measurements, confirm the significant time sensitivity and importance of real-time conductor temperature data in comparison to the armour temperature data, especially during emergency loading periods.
Conductor temperatures in all cases seem to follow a downward trend with the soil temperature at the 1.5-m depth, as expected, over the monitored period.
Over a two-year period, the soil temperatures measured at a 1.5-m depth have shown a summer maximum of 20°C (68°F) and a winter minimum of 6°C (43°F). The minimum is lower than previously assumed.
The daily load factors at NCT were found to be approximately 0.9 to 0.93, while those at CCB were about 0.7 to 0.76 over the monitored period with a considerable component of charging current. This is due to the differences in the charging current components at each end. The daily load factor is expected to decrease at higher loads.
CAPACITY UP-RATING BASED ON FINDINGS
Based on a detailed analysis of operating information derived from the Vancouver Island project — as well as the expected load profiles, cable-design parameters and thermal modelling results — revised winter ratings were developed during 2006 for each cable circuit and are now in effect:
|Winter rating||Original rating||Revised rating||Increase|
|Short-term||1200 MW||1440 MW||20%|
This represents a significant increase in transmission capacity at a modest cost.
Work is ongoing toward developing a full dynamic cable rating system based on accurate thermal models and incorporating the needs of the system operators. This task is much simpler with the knowledge of real-time conductor temperatures, although the forced-cooling system still needs to be properly modelled.
Clearly, the ability to measure and not infer a conductor temperature provides additional opportunities to optimise transfer capability, especially under emergency conditions. Studies are underway to explore whether there are more untapped margins for improved steady-state or short-term ratings, while being cognizant of any limitations of the hydraulic system. Measurements are planned with progressively increasing load tests (both at peak winter and summer conditions), as well as under various cooling conditions. Also, the effect of actual shore-end soil temperatures on circuit ratings will be investigated. This is expected to improve the knowledge on cable thermal ratings throughout the year and not restricted to winter and summer ratings.
The Vancouver Island project provides tangible benefits by safely increasing the power-transfer limits of existing assets and corridors, while allowing for deferment of new transmission circuits and other costly system upgrades. It is the first operational application in the world where cable-conductor real-time temperature information has been used to uprate a 525-kVac submarine cable system.
The higher ratings achieved from this project will allow BCTC to meet increasing customer loads on Vancouver Island, pending completion of a new 230-kV cable circuit. The technology allows an increased use of a critical, large-value asset and gives the utility confidence that it will continue to operate safely and reliably. It also supports the utility in making informed planning and operating decisions through better knowledge of asset performance.
Sudhakar Cherukupalli joined BC Hydro R&D, which later became Powertech Labs, after completing his Ph.D. at the University of British Columbia. He is currently a team leader in transmission cable design and has extensive experience in design, installation and testing of cables and accessories, as well as application of optical sensors in power systems. Cherukupalli has worked on research projects for BC Hydro and British Columbia Transmission Corp. Strategic R&D, EPRI and the Canadian Electricity Association, and has authored and coauthored more than 30 technical publications. He is the Canadian National Representative of CIGRÉ D1-Emerging Technologies and a professional engineer in the province of British Columbia. Sudhakar.Cherukupalli@bchydro.com
Allen MacPhail has a bachelor's degree in applied science (EE) from the University of British Columbia. At BC Hydro, he has specialized in transmission cable engineering and has experience from feasibility studies to the commissioning and maintenance of cable systems up to 525 kVac and ±450 kVdc. He is a CIGRÉ participant and past chair of the Association of Edison Illuminating Companies' task group for developing the CS9 specification for extruded insulation cable systems from 46 kV to 345 kVac. Since retiring in 2006, MacPhail has formed Cabletricity Connections, specialising in power-cable application engineering. A.Macphail@ieee.org
Ross Nelson has a bachelor's degree in applied science (EE) from the University of British Columbia and is a professional engineer in the province of British Columbia. His experience with BC Hydro includes various electrical equipment-related positions in quality control and inspection, stations equipment planning applications, loss evaluation studies, power-quality management and revenue metering services. Nelson has been a project manager for HV Power Cable Projects and has led numerous underground and submarine cable projects. Ross.Nelson@bchydro.com
Joseph Jue recently retired as a specialist engineer with British Columbia Transmission Corp. He was responsible for the asset management, maintenance and operation of the underground and submarine cable system. email@example.com
Jim Gurney manages British Columbia Transmission Corp.'s research and development program. He served on the IEEE Standards Board from 1998 to 2002, was vice chair in 2002 and now serves on the Canadian National Committee of the International Electrotechnical Commission. Gurney graduated from the University of British Columbia with a bachelor's degree in applied science (EE) and is a professional engineer in the province of British Columbia. firstname.lastname@example.org