Distributed Energy

Solving Renewable Transmission Constraints with Water Infrastructure

One of the largest hurdles to delivering substantial amounts of renewable energy is transmission constraints. Los Angeles’ current in-basin transmission infrastructure is based largely on delivering power from coastal power plants to city loads. However, renewable energy sources are often far removed from established transmission lines. One solution is delivering renewable power to the coast so existing transmission pathways can be used. Water corridors may be the key.

Los Angeles is in the unenviable position of creating a new transmission network to disseminate renewable power throughout the country’s second-largest metropolis or finding a method to deliver the power to its coastal generation plants so existing transmission pathways can be used. This article investigates one approach to achieving the latter option by taking advantage of the city’s unique arrangement of having a large power utility and large water utility controlled by a single entity, the Los Angeles Department of Water and Power (LADWP). Water corridors may become power corridors, giving “Water and Power” a new meaning.

Getting Power from the Desert to the City

One of LADWP’s largest existing and potential renewable energy hubs lies in the Mojave Desert, north of its service territory at its Barren Ridge Switching Station, which has transmission continuing up to the southern Owens Valley. To take advantage of some of Mojave’s renewable resources in time to meet growing renewable portfolio standard (RPS) mandates, there is a need for a new, feasible and timely transmission path to bolster the existing double-circuit transmission line that currently exists from the Owens Valley to the Barren Ridge substation. The intent of this article is to present a potential pathway for a new high-voltage direct current (HVDC) cable installation from the Owens Valley, through the Mojave Desert, to one of LADWP’s coastal power plants—likely, the northernmost plant, Scattergood Generating Station (SGS). HVDC transmission is proposed because of the distance and the need to go underground within portions of the city.

1. This map shows a proposed path for routing high-voltage direct current (HVDC) cable from Owens Valley, California, to Los Angeles Department of Water and Power’s (LADWP’s) Scattergood Generating Station. Other possible lines connecting to Southern California Edison (SCE), Pacific Gas and Electric (PGE), San Diego Gas and Electric (SDG&E), other generating stations (GSs), and a receiving station (RS) are also shown. Routes use various right-of-way (ROW) corridors and sea paths. Source: Robert D. Castro

LADWP owns the Los Angeles Aqueduct (Aqueduct), which carries a large amount of water 233 miles from the Owens Valley to the Aqueduct termination point at the Van Norman Reservoir, close to LADWP’s Sylmar Converter Station. Because a portion of the Aqueduct is open, the water delivered to Van Norman is considered raw and is fully treated prior to use. One method of getting power to the city would involve laying a 500-MW HVDC cable in the center of the existing Aqueduct from the Owens Valley to its Van Norman termination point (Figure 1). HVDC cables are regularly used in the ocean, which proves cables could feasibly be placed in the Aqueduct. Furthermore, the solution would not require any additional water treatment processes.

The dual-use nature of the Aqueduct should not require extensive environmental evaluation, making the task much easier and faster. The Aqueduct has an existing right-of-way (ROW), which is secured and patrolled on a regular basis. Environmental permitting of a new HVDC transmission line, designed and constructed in the traditional manner, traversing the 233 miles from Owens Valley to SGS would normally take about three years. The existing ROW and security arrangements could expedite the California Environmental Quality Act process required for line construction by at least 50%.

The Van Norman Reservoir is 30 miles from SGS, but LADWP has a number of water trunk lines that could be used as underground conduits to traverse the majority of that distance. Under this scenario, perhaps as little as 10 miles of new underground conduit would need to be installed to connect the line from the Owens Valley to SGS. Using the existing Aqueduct ROW for the line would avoid a majority of the 12 to 15 years it typically takes for the environmental, permitting, design, and construction of a new 260-mile HVDC transmission line of similar overhead/underground capacity. This accelerated timeline would positively impact the likelihood of meeting any reasonable timeline for accelerated RPS mandates.

Routing Through Available Resources

While the line path to the coastal plants can be successfully addressed in this fashion, a number of constraints remain. There are additional costs to route the line and to convert it back to alternating current (AC) for intra-city transmission. However, these costs would be significantly less than the cost of alternative methods, such as installing HVDC cable in traditional towers (outside the city) to underground (within the city). Table 1 provides comparative costs for HVDC delivery to the coastal plants. In addition, with sufficient resources, it could be possible to complete the proposed project within five years, which compares favorably to the 12- to 15-year traditional timeframe.

Table 1. Comparative costs of HVDC transmission. Source: Robert D. Castro

Conceptually, once the line reached the SGS, additional HVDC cables, and associated converter stations, would need to be installed to connect the coastal plants to each other. Using such a method would mean one path to the coast could feed all three coastal plants. This scenario envisions the line initially connecting to the SGS converter station as a current-controlled rectifier to control the back electromagnetic field. It would then connect as a parallel path via a new HVDC cable in the ocean to a Harbor Generating Station (HGS) converter station as a current-controlled inverter. From HGS, the line would finally connect via a new HVDC cable as a parallel path to a Haynes Generating Station converter station as a voltage-controlled inverter. The goal would be for the line to reach one of the coastal plants as an injection point, and then connect to the other coastal plants through HVDC sea cable (Figure 2).

2. HVDC cable, similar to the one shown here, is used in many subsea applications, such as for transmitting offshore wind energy to shore. Source: Robert D. Castro

This design may align with LADWP’s tentative plans to convert two existing 287-kV transmission lines from Hoover Power Plant to HVDC, which would connect to LADWP’s Receiving Station B, located in the center of the city. Again, if overhead conversion is accelerated and LADWP transitions to underground HVDC from Receiving Station B through a water system corridor, then an HVDC corridor terminating at one of LADWP’s coastal plants could result.

In a similar fashion, existing overhead high-voltage transmission system ROWs could be made dual-use. LADWP currently has several plans to convert land underneath transmission lines to stormwater capture basins, with the water captured used to replenish the city aquifer.

Like water-only utilities, LADWP needs to ensure that the reliability of water delivery to its customers is not compromised. One of the most failure-prone elements of a transmission cable is the junction points adjoining two cable segments. Having the 233-mile Aqueduct out of service to attend to failures would not be acceptable. One solution could be to have the cable joints located outside of the Aqueduct, so as to allow servicing or replacement without hindering the water flow.

Focus on Safety

Safety is a primary issue that needs to be addressed in a thorough and complete fashion. The water flowing in the Aqueduct serves as an excellent grounding mechanism when the Aqueduct is in use. However, additional grounding would be needed on the Aqueduct to ensure safety during times when the line is energized and water is not flowing. Preliminary studies indicate that grounding at the cable joints, proposed to be out of the Aqueduct, would be sufficient to address concerns.

A new class of maintenance worker, electric/water maintenance worker (EWM), would need to be created, with technicians trained in both electrical maintenance and water maintenance tasks, to ensure safe operation and maintenance of both the Aqueduct and the line, whether energized or not. Because this combination of water infrastructure and high-voltage transmission has not been attempted before anywhere in the world, new operating protocols, safety manuals, operating orders, and extensive training would need to be developed, maintained, and enforced, so that the emphasis on safety remains a top priority.

Once the line entered the city trunk lines, additional measures would need to be instituted to ensure safety for trunk line workers and city residents. The design could expose workers to dangerous levels of power in energized lines, which would require additional operating protocols and training for EWM workers. Ideally, the impact would be limited to those relatively few water trunk lines that the cable would reside in. Most of those trunk lines are eight feet in diameter, so the relatively small cable bundle of around 12 inches would not impede water flow. Preliminary studies indicate that the existing water infrastructure provides adequate grounding for city residents near the line, but additional studies would need to be completed to provide a high level of assurance to residents.

Coastal Injection Points

The dual-use nature of the line could be expanded to include a dark fiber path, traversing the same path as the line. Communication companies may opt to install terminal equipment at various locations to provide fiberoptic access, including in areas neighboring the coastal plants. In addition, if fiberoptic cable were inserted into the HVDC cable between conductors, the fiberoptic cable could also be used as a very precise mechanism for locating potential faults in the line. Breaks in fiberoptic cables result in strong reflections that can be more precisely located than those in power cables, making the addition of fiberoptic beneficial in locating faults, as well as potentially providing an additional revenue stream.

The coastal plants have typically served as injection points for dispatchable power and voltage support. Adoption of high RPS requirements would likely curtail many of these functions. However, LADWP could prepare for such eventualities by inserting clutches into its gas turbine units. The clutches could separate the prime mover and heat generating functions of the coastal plants, while allowing them to function as synchronous condensers. The expansion connecting the coastal plants would allow for power and voltage support to be injected into multiple points on the city’s grid, and the HVDC configuration would allow for absolute control of real and reactive power flow, giving a sense of dispatchability missing from most high-RPS scenarios.

Having coastal interconnection points would also allow LADWP, and other utilities that have coastal facilities, prepare to accept power from offshore renewable resources, such as offshore wind, offshore solar, ocean wave generation, marine current technology, tidal power, and potentially other resources. Coastal energy storage facilities along the Western seaboard would also become more accessible with the expansion. Fast trending battery energy storage systems, as well as rapidly developing coastal seawater pumped storage units, along the coast would add another level of reliability and dispatchability to help firm and shape high-RPS systems.

New Corridors for HVDC

This approach, leveraging existing waterways to lay high-voltage cables, may be unique to LADWP in that it has a potential need for an HVDC cable pathway to the city, while also owning the water infrastructure that is in place. Other water utilities may be unlikely to agree to such an arrangement from a local power utility without first having a successful model to follow. Should this model prove effective, the California Aqueduct and the Colorado River Aqueduct may be other candidates for an HVDC cable pathway. Success could provide a viable mechanism to further the perennially predicted elimination of new overhead transmission throughout California and the U.S.

The challenge is to maximize generation, transmission, and distribution within Los Angeles by utilizing land LADWP owns. Some traditional steps toward this end include putting solar panels on water system properties, including buildings, ROWs, and, potentially, even floating panels on reservoirs. These steps could result in an aggregate of 100 MW to 200 MW of additional capacity. Whereas, if water system ROWs are used to connect existing coastal plants with ocean transmission, LADWP’s transmission system would expand its ability to bring in renewable resources from great distances at the levels of 1,000 MW, 2,000 MW, or even 3,000 MW, and distribute it throughout the city. This coastal network could be expanded northward to Southern California Edison (SCE) and Pacific Gas and Electric, and southward to SCE’s San Onofre Substation and further, bypassing the current landlocked status of LADWP’s local grid and providing opportunities to connect LADWP to all the major California utilities through the ocean, and even to Mexico.

The expansion would increase LADWP’s high-voltage transmission from its existing Pacific HVDC Intertie and California 500-kV Intertie, to include the proposed Aqueduct HVDC Intertie and the Pacific Ocean HVDC Intertie. The proposed Pacific Ocean HVDC Intertie has a successful recent local model to follow in the 53-mile, 400-MW, Trans Bay Cable in San Francisco, completed in November 2010. The proposed expansion would result in LADWP’s coastal plants becoming its new interconnection point to its local transmission grid through the coast.

The realization of the expansion, and its logical extension to other aqueducts and large water trunk lines, may preclude the need for new transmission lines in the southwestern U.S. For example, the Pacific Ocean HVDC Intertie to SCE could allow energy to flow through the 2,500-MW corridor once filled by the now-dormant San Onofre nuclear power plant.

The dual-use nature of the Aqueduct to trunk lines for water and HVDC could be extended to other water infrastructure. Like the Aqueduct ROW, existing sewer, flood control, and water treatment plants all have ROWs secured from public access, which could be used as dual-use ROWs, as long as appropriate safety protocols were incorporated. The heightened security of these inherently protected areas makes them attractive as potential power paths. ■

Daniel Scorza is power engineering manager with LADWP and Robert D. Castro is a lecturer at the University of Southern California.

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