The southern terminus of the critical Pacific HVDC Intertie has been upgraded with the construction of a new ±500 kV DC, 3100 MW HVDC converter station. The converter station is the world’s largest, and will help to ensure reliable power sharing across the western USA.

Christer Eriksson, ABB

In March 2005, ABB joined the Los Angeles Department of Water and Power (LADWP), Southern California Edison and the cities of Burbank, Glendale and Pasadena in a rededication ceremony at the Sylmar HVDC (high voltage direct current) converter station. The ceremony recognised the completion of a $118 million, 30-month modernization project to upgrade the southern terminus of the 1362 km-long Pacific DC Intertie – one of the world’s longest and highest rated transmission lines.

The modernization project involved refurbishing the 3100 MW HVDC converter station with high efficiency, state-of-the-art thyristor valves to replace the older mercury arc valves, and to consolidate the facility into one location. In addition to increasing the reliability and lifespan of the system, the upgrades will reduce operation and maintenance costs, increase energy efficiency and incorporate lessons learned from major earthquakes.

Figure 1. The $118 million, 30-month modernization project saw the upgrade of the southern terminus of the 1362 km Pacific DC Intertie – one of the world’s longest and highest rated transmission lines
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“This facility plays a key role in providing additional energy resources to the greater Los Angeles area,” said Mayor Jim Hahn at the rededication ceremony. “The people of Los Angeles benefit from this source of clean, efficient and reliable power.”

The Pacific Intertie

For over 30 years, electricity consumers on the US West Coast have benefited from a power link that allows large amounts of power to be transmitted between the Pacific Northwest and the Pacific Southwest. The high voltage transmission lines that make this power-sharing possible are called collectively, the Pacific Intertie.

The Intertie includes three AC lines and one HVDC line. Together, they comprise the largest single electricity transmission programme in the United States. The Intertie is capable of transmitting up to 7900-4800 MW on AC and 3100 MW on DC.

Because large amounts of Northwest hydropower can be transmitted reliably to the Southwest, less power has been generated at fossil fuel power plants – an important benefit in a part of the country with air pollution problems. Conversely, because the Northwest is able to import power from California, particularly over-night when demand is low, water can be reserved in reservoirs that otherwise would be used to make electricity. Typically, power is shipped north in winter, when Northwest hydropower is less available, and south in the summer, when southern California needs more energy due to heat waves.

Figure 2. The Intertie is crucial to power sharing across the coast
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The initial intertie consisted of two 500 kV AC lines and one ±400 kV DC line linking the state of Oregon with Los Angeles city. The northern end of the DC line is at the Bonneville Power Administration’s Celilo converter station, which is just south of the Dalles Dam about 145 km east of Portland. The southern end is 1362 km away at the Sylmar converter station on the northern outskirts of Los Angeles. Sylmar is owned by five utilities, Southern California Edison Company, the City of Glendale, the City of Pasadena, the City of Burbank and Los Angeles Department of Water and Power, also acting as the managing and operating agent.

First contract

ABB has been heavily involved in the Pacific HVDC Intertie from the first contract in 1965, when it was responsible for the manufacturing, supply and commissioning of the converter (mercury-arc) valves for a 1440 MW, ±400 kV transmission scheme. Although the technology was already well established, the contract nevertheless represented a challenge to both the owners and the suppliers, since the line voltage, the line length and the line current were greater than for any previous HVDC project, making it the most complex yet.

The transmission scheme entered operation in 1970, but shortly after (in 1971), the San Fernando earthquake devastated the Sylmar converter station. The station building was severely damaged, as was much of the equipment, and it was not until 1973 that the station was rebuilt and operation could be restored. After a few years of operation, the owners decided to make use of the inherent overload capacity of the equipment and raise the transmission rating to 2000 A and 1600 MW.

Figure 3. The use of advanced thyristor technology has enabled converters to be designed for a total of 3100 MW and to be installed in a building that was designed originally for 1100 MW
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Eventually, it was agreed to raise the line voltage of the Pacific Intertie to 500 kV to help secure the long term supply of power to the city of Los Angeles. This led to the creation of the Pacific Intertie Upgrade (PIU) project, commissioned in 1985, for which ABB received the order. Essentially, the PIU consisted of putting a 100 kV six-pulse thyristor converter in series with the three mercury arc converters in each pole.

In 1985 another contract was awarded to ABB for the extension of the Pacific Intertie transmission rating from 2000 to 3100 MW. This project was called the Pacific Intertie Expansion (PIE). New 1100 MW converter stations had to be installed in parallel with the existing stations and the current in the DC line was raised to 3100 A. This time ABB delivered the converter stations for the terminals at both Celilo and Sylmar. At Sylmar it was also necessary to build the new terminal, Sylmar East, near the first Sylmar site. The PIE was commissioned in 1989.

Sylmar modernization

Due to age, reliability studies and damage sustained during the 1994 Northridge earthquake, the Sylmar converter station partners decided to place a major order with ABB for the Sylmar modernization project. This involved refurbishment of the ±500 kV, 3100 MW HVDC transmission system to upgrade outdated technology, as well as the consolidation of the facility onto the Sylmar East facility, on the east side of the Interstate 5 Freeway. Benefits of the creation of the single bipolar ±500 kV converter station include:

  • Improved reliability and availability in transmitting hydropower and renewable energy from the Pacific Northwest to the Los Angeles area.
  • Reduced operation and maintenance costs.
  • Improved seismic design of buildings and equipment to meet or exceed current codes and standards.
  • Innovative use of existing buildings and equipment to reduce first cost of the new terminal.

New technology and innovations, such as the use of new seismic resistant designs, have more than doubled the life of this vital north-south energy corridor. State-of-the-art HVDC converter transformers, unique in size and design, and thyristor valves that convert power from direct current (DC) to alternating current (AC) or vice versa, are just a few examples of the technological advances on display in this terminal. The thyristors are semi-conductor devices, acting as lightning-speed, fully automated switches that turn the three-phase AC power on or off 60 times per second to form DC power. The thyristor valves have replaced the original mercury arc valve converters that required much more maintenance to operate correctly.

One of the more challenging aspects of the Sylmar modernization project was the relatively short construction period – just nine months from de-energization of Sylmar East until commercial operation. ABB and LADWP were able to complete the project on budget and eight days ahead of schedule.

Challenging logistics

The use of ABB’s advanced thyristor technology has enabled converters to be designed for a total of 3100 MW and to be installed in a building that was designed originally for 1100 MW – an increase of almost 300 per cent. The station is now the world’s largest converter station.

Furthermore, Sylmar houses seven of the world’s largest converter transformers (including a spare unit). Manufactured at the ABB transformer facility in Ludvika, Sweden, each single-phase, three-winding 621 MVA unit weighs in at 554 tonnes. The task of moving these units from the factory to Sylmar converter station emerged as perhaps the most challenging logistics issue in the entire project. Each unit had to make the journey from Ludvika, Sweden via rail, ship, rail and road before arriving at the final location some 45 days later. The planning going into the move of each of the units was a truly remarkable achievement.

“There were many complex aspects of the work that had to be performed in a short period of time to minimize outages. We also had the challenge of handling large, heavy equipment, such as seven converter transformers,” said LADWP general manager Ron Deaton. “Weighing 500 tonnes, these transformers are among the largest in the world. They were shipped on special vessels from Ludvika, Sweden, to Long Beach harbour and brought to the site on specialized transport railcars.”