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The In-Between Voltage State

Among steady-state techniques, one of the most common methods for evaluating voltage stability is through the venerable P-V curve, also known as the “knee” curve.  This method is used to identify the real power, or megawatt, margin to the point when the transmission grid is no longer able to support voltages, a state the industry has referred to as “voltage collapse.”  Many analysts have long suspected that the P-V curve at best provided an approximate indicator, and that there was a lot more it was not capturing.  Recent indications from near voltage collapse events seem to confirm that there is indeed a lot more, which may perhaps require a re-thinking of the whole process of using steady-state methods for voltage analysis.

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Open Source or Proprietary Data: the Model Dilemma

Data creep is an ugly name for a common practice, that of adding special models to community databases.  This is no less prevalent in databases of interconnected grids.  In the Eastern Interconnection, a planning database will contain on the order of 70,000 buses and 30,000 dynamic models, representing everything from Florida to the Texas panhandle, from Idaho to New Brunswick, at voltage levels from 34.5 to 765 kV. Whether we need all the data for any specific analysis is moot, however, frequently studies will carry the full dataset regardless that the focus is on localized
phenomena. Equivalents or reduced models were necessary in the past when computers had memory restrictions and low speed performance. Nowadays, reduced models may be justified for the sake of simplicity and convenience as long as accuracy is not compromised.

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Transmission Planning – Returning to Fundamentals

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.

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Overlaying DC Transmission On an AC System

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?

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Resource Planning for Island Networks

In an island network, where the main resources are local generation, the simplest form of resource planning criteria is, perhaps, the loss of the largest unit with another unit out on maintenance. As the number of units, and load, increase, the criteria may take the form of a percent reserve on peak demand. Probabilistic forms of the criteria would specify an allowable energy not served or maximum loss of load expectation, such as one day in ten years.

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Why not Voltage Uprating as a Planning Option?

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?”

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