In February 2015, BC Hydro introduced a 500-kV overhead transmission line project to connect its Nicola substation to its Meridian substation. For the most part, the project was carried out as planned: The construction of civil, electrical, protection and control (P&C) in the stations was complete and the line was nearly complete. The testing and commissioning of 500-kV equipment as well as P&C systems both were approved and accepted.
However, when the Meridian station and transmission line were ready to be energized, the project team discovered a major issue in the transmission line phasing arrangement. The last transmission structure was in A-C-B configuration instead of the required A-B-C configuration. Because of the miscommunication among the utility’s disciplines, the station design team did not realize an additional phasing rotation was required for the Meridian station until after construction had been completed. The lines and Meridian station could not be energized as scheduled in the middle of December 2015 until the phasing arrangement was rotated from A-C-B to A-B-C.
Line and Substation
The Meridian substation is a 500/230/12-kV substation that supplies power to commercial, industrial and residential customers in addition to other substations in both the Greater Vancouver and Vancouver Island regions in the British Columbia province of Canada. This substation also supplies electric power to the United States by way of the Ingledown substation.
The 500-kV line from the Nicola substation is connected to the Meridian substation’s 500-kV main bus through a gantry tower, disconnect switch and two 500-kV bays, which consist of Siemens hybrid breakers. The 500-kV hybrid circuit breakers are SF6-insulated, metal-enclosed circuit breakers with gas-insulated compartments. The last transmission line terminal structure is about 85 m (279 ft) from the gantry tower situated inside the Meridian station fence.
For 500-kV lines, the basic impulse level (BIL) requirement is 1800 kV; therefore, phase-to-phase clearance must meet a minimum of 5200 mm (17 ft) in BC Hydro standards.
To fix the phase-rotation issue, the design team had to consider several options. If the utility opted to redesign and reconnect phases B and C, the P&C system would require extensive and expensive design changes along with a lengthy construction, testing and commissioning process. The project would not be completed on schedule in December 2015.
In the original construction, four conductor bundles of 2303-kcmil conductors with the full tension were converted to two conductor bundles of 2303-kcmil slack span between the last transmission terminal structure and gantry tower. There was insufficient phase-to-phase and phase-to-ground clearances in this span to loop one of the phases (phase B or phase C) under another phase (phase C or phase B). All the transmission structures were built up at the time this phasing mismatch was discovered. The cost of redesigning and reconstructing transmission structures and overhead lines could be more than CAD$10 million.
Therefore, the design team sought a simple, safe, time- and cost-effective way to rotate phases B and C to meet the project energization date of mid-December 2015. Four options for correcting the phase conflict were proposed. The leading option chosen was to rotate phases on the gantry tower, which would not require any modifications to the transmission structures or P&C systems.
The design included adding one 500-kV suspension insulator underhung on the existing gantry tower to transfer phase B to the middle position. The plan for phase C was to use 152-mm (6-inch) tubular bus, 500-kV post insulators, steel supports and foundations to bring phase C from the middle position to the outside position. Because of BC Hydro’s limits of approach requirement, four bollards were added beside the road to prevent the vehicle from driving too close to phase C’s 500-kV bus work.
The 500-kV line uses phases that consist of two conductor bundles of 2303-kcmil bare aluminum stranded conductors (ASC) and spacers. One underhung suspension insulator was used to hold 2303-kcmil conductors to ensure the conductors for phase B would create the minimum phase-to-phase 5200-mm clearance.
Civil engineering studied the existing gantry tower structural strength to ensure the tower could handle the extra loads of one suspension insulator, twin bare conductors, spacers and fittings, plus wind and ice loads. The design was finalized and accepted, materials were ordered, and the construction crew started the installation in November 2015.
When the construction crew modified the bus work of phases B and C according to the design, they discovered two issues:
• The phase B main blade had a corona ball on the top that would not enable the switch to close fully when a 90-degree aluminum terminal connector was installed on the terminal pad. The solution for this conflict was to use a 500-kV-rated tee connector welded with one 2303-kcmil conductor individually (two tee connectors and two conductors are required here), and then the weldment was smoothly ground so the phase B main blade could fully close.
• After all civil and electrical components were installed — because of the 2303-kcmil conductors’ stiffness and allowable tensile force on the terminal connectors — 4810 mm (16 ft) was the maximum clearance the installation crew could achieve. Although this would meet the IEEE Standard 1427 for 3765 mm (12 ft) of clearance, it would not meet BC Hydro’s required minimum clearance of 5200 mm. However, with the approval of electrical technical and functional managers, the line and Meridian station were put into service on time in Dec. 14, 2015.
To remediate the clearance issue, an outage request was approved in 2017 so the station design team could fix the phase-to-phase clearance deficiency and achieve BC Hydro’s required minimum clearance of 5200 mm. This time, two 500-kV suspension insulators were underhung on the gantry tower to lift the 2303-kcmil conductors higher, which created more space between phases B and C. A new 3-D model was created that shows a sphere with the center of phase C’s bus, and the radius of 5200 mm indicates the required minimum clearance exists around phase C.
In the new design, the total cable length was increased, so the cable’s deflection had to be recalculated. According to the structural calculation, the governing deflection of the 2303-kcmil conductors in between two underhung insulators and the conductors themselves was ±100 mm (±4 inches) maximum.
Based on the new 3-D model and calculation, there was a 1300-mm (4.3-ft) distance between the sphere of 5200 mm of phase C’s rigid bus and the lowest sag of the 2303-kcmil conductors in phase B. The new calculated phase-to-phase clearance was as follows: with 100-mm deflection, 5200 + 1300 – 100 = 6400 mm (21 ft) > 5200 mm.
The gantry tower is a 500-kV lattice deadend tower inside the Meridian station. The external loads acting on the tower were the transmission line pull or strain insulator load, including shield wires at the top of the spire, wind effect, ice, earthquake, and construction and maintenance load. Point loads also were applied at the joints of the bottom truss girder where the line for strain insulators are located to account for any cable or equipment loads that may be hung or supported in the future. Analysis also covered the required angles of applications, thus accounting for all the 3-D directions. The STAAD.Pro 3-D structural analysis and design software program was used for the analysis and design.
Two underhung insulators, which were used for rotation, turned out to be lighter in weight than the point load provided for the future. The insulators were hooked to the bottom truss girder of the tower and free to rotate in most directions, thus minimizing the longitudinal and horizontal load components on the joints. Overall, the new solution did not introduce any additional load to the existing gantry tower.
Software IEC 865 was used to analyze phase B conductors’ mechanical strength under 40-kA single line-to-ground fault current, considering 1.27-mm (0.5-inch) ice loads, -30°C (-22°F) temperature and 385-Pa wind loads. The results showed the phase B maximum horizontal displacement is 0.05 m (2 inches) under a 40-kA fault current condition. There were no main conductor clashes or any subconductor clashes.
On Sept. 1, 2017, the BC Hydro crew completed construction for the new design. The new phase B-to-phase C clearance is 5741 mm (19 ft), which is greater than 5200 mm. Thus, the rotation of phase B and phase C at the gantry tower was completed successfully. The cost of rotation was minimal compared to redesigning P&C systems or rotating the phasing of transmission lines.
Many lessons were learned from this project, but the following four are at the top of the list:
• Communication and cross-disciplinary checking with overhead transmission lines are critical to both overhead transmission and substation design, especially accuracy of the phasing arrangement.
• When performing electrical design for 500-kV connections, include equipment’s corona rings and balls into the calculations.
• What drawings show versus the real installation are different, especially for extra high voltage.
• Accuracy and details of 500-kV equipment are very important in drafting. ♦
Melody H. Xiao () is a senior electrical engineer with BC Hydro for 13 years. She has 30 years of expertise on design, construction and project management of substations (12 kV to 500 kV), commercial and industrial buildings, and plant lighting and power supplies. She has been leading substations electrical design teams for the major projects such as Site C since 2012. She is registered professional engineer with Engineers and Geoscientists BC.
Marian Balea is an electrical engineering team lead with more than 30 years of experience, encompassing system planning and design, design and project management, and operation and maintenance of an electric system in a heavy industrial environment. He has also participated in transmission stations electrical equipment specifications, procurement and design standards development. He also has experience in coordinating large groups of contractors and consultants. He holds a MsC degree in electrical power engineering from Technical University of Iasi, Romania, and is a registered professional engineer.
Romeo del Barrio is a senior engineer in civil/structural stations design with BC Hydro. He has 30 years of experience in design and construction of structures in the energy and power generation, oil and gas, process, manufacturing, pulp and paper, and commercial building sectors. He has been involved in the civil and structural design of 12-kV to 500-kV substations with BC Hydro since 2008. He is a registered professional engineer with Engineers and Geoscientists BC.