HVDC Technology: DC Overlay on an AC System

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by R. Austria, K. Dartawan, M. Elfayoumy, M. Gutierrez, R. Tapia

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“Will a 2,000 MW HVDC line transfer 2,000 MW?”

The answer, which we’ll try to explain in this blog, is “plus or minus” if the DC line is being built to overlay an existing AC system. In such a situation, the DC line may continually carry 2,000 MW but the incremental transfer will not necessarily equal 2,000 MW.

Pacific_intertie_geographic_mapOverlaying a DC line on an existing AC system is not new. One example that has been in operation since 1984 is the Pacific DC Intertie (also called Path 65) which connects the Pacific Northwest to Los Angeles and parallels the AC system of the WECC. (See diagram at right, source: Bonneville Power Administration).

More recently, several projects have been placed into service or proposed that add DC in parallel to an existing AC grid. Whereas the traditional application for DC was to interconnect separate or asynchronous grids, these applications all have terminals of the DC line on the same synchronous network. Some examples:

  • The Cross Sound Cable Project parallels the New England and New York AC systems.
  • The Neptune Underwater HVDC Project parallels New York and PJM systems.
  • Proposed Zephyr and Chinook HVDC lines that would connect southeast Wyoming and south central Montana, respectively, to the Eldorado Valley south of Las Vegas, paralleling the existing WECC system.
  • The Trans Bay Cable Project proposes to construct a 53-mile undersea HVDC link between San Francisco’s City Center electrical power grid and a Pacific Gas & Electric substation near Pittsburg, California.

Several new factors contribute to increasing popularity of this type of application (overlaying an AC system) for DC lines, including:

  • The further development of higher capacity voltage source converter (VSC) DC technology. VSC tends to be cheaper than the traditional current source converter (CSC) technology, and also has a smaller footprint, ideal for urban applications.
  • Submarine DC capacity has improved to be comparable to that of AC submarine cables
  • Widening locational prices make it economically feasible to bring cheap power into a high price areas.

Case Study No. 1

We’ll address the premise of this blog — that a 2,000 MW DC line does not necessarily represent a 2,000 MW transfer increase — with two case studies.

In the first case study, a proposed HVDC line from Albany, NY to New York City comprising of two 1000-MW HVDC bipoles would bring ostensibly cheaper upstate energy to the New York City load area, characterized by high locational energy prices. The bipoles would overlay an existing 345 and 230 kV network, and specifically impact several key AC interfaces in the New York grid: Upstate New York – ConEd (UPNY-ConEd), Upstate New York – Southeast New York (UPNY-SENY) and New York City Cables (NYC Cable).

A detailed study of the impact of the proposed HVDC line on the New York system determined the following changes in transfer capacity for various operating conditions and type of limit (Note that we are adding a 2,000 MW DC line and so so would expect to see an average of 2,000 MW incremental capacity):

INTERFACE UPNY-ConEd Closed CONDITIONSummer Peak Thermal Emergency MW INCREMENT2648
UPNY-ConEd Closed Summer Peak Thermal Normal 2522
UPNY-ConEd Closed Winter Peak Thermal Normal 2147
UPNY-ConEd Closed Summer Peak Voltage Transfer 1392
UPNY-ConEd Open Summer Peak Thermal Emergency 2648
UPNY-ConEd Open Summer Peak Thermal Normal 2523
UPNY-ConEd Open Winter Peak Thermal Normal 2147
UPNY-ConEd Open Summer Peak Voltage Transfer 1887
UPNY-SENY Closed Summer Peak Thermal Emergency 2157
UPNY-SENY Closed Summer Peak Thermal Normal 2605
UPNY-SENY Closed Winter Peak Thermal Normal 1801
UPNY-SENY Open Summer Peak Thermal Emergency 2158
UPNY-SENY Open Summer Peak Thermal Normal 2606
UPNY-SENY Open Winter Peak Thermal Normal 2101
Average 2239
NYC Cable Summer Peak Thermal Emergency 1625
NYC Cable Summer Peak Thermal Normal 1625
NYC Cable Winter Peak Thermal Normal 1409
NYC Cable Summer Peak Voltage Transfer 1450
Average 1527

(Source: “Transmission and the Reliability of the New York State Bulk Power System, Part I: Thermal Transfer Limit Analysis,” by R. Tapia, M. Elfayoumy and R. Clayton, September, 2004, Power System Conference and Exhibition, New York, New York.)

Across the main interfaces from upstate NY to NYC (UPNY-Coned and UPNY-SENY), there is an average increase in transfer capacity of 112% of the bipoles’ capacity. Since these interfaces are thermally limited, the increase is even higher considering only thermal constraints, up to 117% of bipole capacity. The reason that we don’t seem to see a one-to-one compensation between the added DC capacity and incremental interface capacity is that there is a significant offloading of underlying lower voltage lines that eliminates the impact of certain limiting contingencies.

Coming closer into the New York City load pocket, the incremental capacity averages only 76% (1527 MW of 2000 MW) of the bipole capacity. This is the result of an interesting locational feature: as more power is dispatched upstate, the particular power plants that are ramped down or taken offline in the load pocket causes local congestions. One could view this in another way: the bipole results in certain must-run dispatch patterns in the load pocket.

In summary, for the first case study:

  • The DC line helps the transfer capacity close to the sending or inverter terminal and along the transfer path (in this case upstate NY) by allowing 2,340 MW average incremental transfer for a 2,000 MW HVDC line.
  • Because of congestion in the receiving or inverter terminal, only an average of 76% of the DC line capacity can be transferred to this area.

So, the proposed 2000 MW line, increases transfer capacity in the vicinity of the sending by an average of 2340 MW but delivers only an average of 1527 MW at the receiving end of the line. The reasons for the variations: elimination of certain limiting contingencies, and need for must-run generation at the load pocket.

For our next blog on this topic …

… we’ll discuss a second case study for a proposed DC line in Texas. Please subscribe in order not to miss the next posting.

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