“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.

Whereas, power plants using renewable energy sources were not too long ago considered exotic, today they are the new face of energy — the wind mill replacing the smokestack as the symbol of electric power generation. Spurred by governmental incentives, renewable energy sources are rapidly changing the nature and composition of power systems. They are still a fraction of the overall energy portfolio, but the renewables’ level of penetration of energy markets is growing. In most US RTOs and power pools, the queue for interconnection projects is dominated by renewables, primarily wind farms.

As an increasing number of wind turbines are connected to the power system, more and more wind farm interconnection studies are requested. Usually a wind farm consists of tens of wind turbines and the interconnecting cables. The wind turbines are mostly the same type for each wind farm, but the cables interconnecting these wind turbines vary in length, capacity and configuration, depending on the farm design, terrain, easements, etc.

Top 3 Reasons

Here are the top 3 reasons a transmission analyst may need to avoid modeling each turbine and each cable in the wind farm for the interconnection study, including:

(A serialized and expanded version of this article can be found here)

As an increasing number of wind turbines are connected to the power system, more and more wind farm interconnection studies are requested. Usually a wind farm consists of  tens of wind turbines and cables. The wind turbines are mostly the same type in one particular wind farm, but the cables interconnecting these wind turbines vary in length, capacity and configuration. A transmission analyst may need to avoid modeling each turbine and each cable in the wind farm for the interconnection study for one or more of several possible reasons:

1. It is laborious to setup the detailed model. For example, a 300 MW wind farm would comprise of 200 1.5-MW wind turbines interconnected at a distribution level voltage such as 34.5 kV in a feeder network similar to that of a suburban housing development. The simulation software for power flow, short circuit or stability analysis may not accommodate carrying detailed models for all the existing and proposed wind farms. To consider the dimensions, take the case of a system with some 5000 MW of installed wind capacity. Detailed modeling of the wind farms would require about 4000 turbine models, 5000 additional nodes and the same number of additional branches in the database.
2. The detailed model requires representation of distribution level feeder circuits that increase the “spread” of branch impedances in the power flow model. “Spread” here refers to the range of impedances included in the database. (see further discussion about spread or diversity in the article, “Converging the Power Flow”). Too much spread can lead to difficulties in solving, or converging, the power flow.

In view of all the above reasons, it may be sufficient to aggregate groups of wind turbines into equivalents that capture their net impact on the transmission system.

Speculation over conversion of ac lines to dc goes back almost as far as HVDC itself. It’s an idea easily quashed by the fact that you pay for terminals rated at total DC MW and gain only the incremental capability over the ac case. Furthermore dc conversion idled a third of the ac transmission asset. And even if you’d cleared those hurdles, engineers were reluctant to take full advantage of dc’s control capability – a necessary step to make full use of dc’s capability in an ac system context. In any case construction of new ac lines were a vastly less inexpensive way to expand transmission capability.

So what’s new?

• New transmission is extremely expensive or can’t be built at all.
• FACTS, reconductoring, temperature monitoring are helping stretch ac capability but within limits  and often at a high cost in losses.
• Communication and control technology has made huge advances. Wide Area Management Systems (WAMS), coupled with very sophisticated FACTS options, are polite to talk about.
• An HVDC configuration has been introduced which makes full thermal use of all three phase positions of an ac line.

It’s time to look again at the conversion prospect; perhaps with a mind-set that considers dc as the ultimate FACTS device –  one that controls flow and boosts transfer capability of existing, in-place assets by up to 4:1.

There are major challenges facing transmission planning today.  William Hogan (Harvard) says that “markets should produce better choices than … the central planner.”  Of the many methods for transmission planning, the emerging one seems to be the “wait-and-see” method — wait and see how the generation resources locate so as to relieve transmission constraints.  Many have pointed out that the prevalence of interconnection studies are today’s version of transmission planning.

Two initial points.  (1) Until we have a reliable energy source that is compact, safe and portable, we will be dependent on centralized power stations that require transmission to deliver to customers.  (2) Transmission requires infrastructure that, like roads, will be in our environment for many years.    These two facts imply the need for transmission planning in the form that looks at optimal use and allocation over a period covering an appreciable lifetime of transmission equipment.

One of the ideas proposed for developing a transmission super highway is to construct a number of DC lines to overlay the existing AC system. The motivation comes from DC’s inherent capability to overcome voltage and stability issues for transfers over long distances, making the network electrically “smaller.” (There is also an economic motivation that is important to recognize, but which we will not treat in this article.) How would such a massive development change the nature of a power system? What issues do system planners have to consider? In a more global view, what would investors and regulatory agencies need to take into account in funding and approving this type of project?

Voltage uprating is a technology that is actually a combination of techniques known for many years by folks in the line design area, for instance from the Towers, Poles and Conductors (TP&C) Line Design Subcommittee of the IEEE.  It is a possible planning alternative to techniques which aim to increase the current capacity of transmission lines, known collectively as current uprating.  Both voltage and current uprating share the common objective of utilizing the existing right-of-way and structures to transfer more power.

From a  planning perspective, voltage uprating is an option that works alongside such others as converting AC lines to DC (including the new “tripole” concept), and increasing phase order, which makes it a candidate for planners looking to make the most out of existing infrastructure.  So, the $64 dollar question is: “Why isn’t it planned for more often?”

Mahmoud K. Elfayoumy, Ramon R. Tapia and Roger Clayton, “Transmission and the Reliability of the New York State Bulk Power System, Part I: Thermal Transfer Limit Analysis”

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Dag Reppen, “Increasing Utilization of the Transmission Grid Requires New Reliability Criteria and Comprehensive Reliability Assessment,” PMAPS, 2004.

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