Online calculator prevents grid overload, enhances reliability

Stephen Lee


The 17 security coordinators in the Eastern Interconnection, which covers the eastern southern and midwest areas of the United States, received an early Christmas present last fall: delivery of a new online service that promises to make their lives easier, while enhancing wide area reliability in their half of the nation. Called the Interchange Distribution Calculator (IDC), this new data processing and communication service came online in October. While it has not yet seen action during peak load summer months, the system has already proven itself to be effective.

A flood of transactions

Commissioned in January 1999 by the North American Electric Reliability Council (NERC), IDC addresses the problem of the growing number and complexity of bulk power transactions clogging the nation`s high-voltage transmission system. It also accounts for the cross-boundary nature of many of these transactions-often involving parties in different control areas and security regions. Buyers and sellers transact a range of deals for energy and then arrange firm and non-firm transmission, the latter including hourly, daily, weekly, or monthly contracts. Each of these types of transactions carries a priority (e.g., hourly non-firm is the lowest priority, and firm is the highest priority). Hence, at the top of each hour, new transactions may be scheduled, old ones may be completed, and of course, the availability of generators as well as the load level fluctuates. In the past, this dynamic situation has forced some security coordinators to simply curtail all transactions when security was threatened.

The problem was that no organized way existed of recommending the coordinated reshuffling of transactions when needed-curtailing lower priority transactions, for example-to maintain adequate system security. Security coordinators were forced to implement transmission loading relief (TLR)-actually curtailing various transactions for the good of reliability-without an efficient way of deciding how to do this. As the number of transactions rose, the need for a way to address this problem had become almost essential.

IDC fits the bill. NERC hired EPRI to act as project manager and to coordinate efforts by NERC staff and the NERC IDC Working Group, currently chaired by Jack Kerr of Virginia Power, to develop the online IDC operation service. It essentially acts as an expert advisor on curtailment priorities, given a range of input data on system status and transactions. A recent example of its actual use illustrates the system`s value.

IDC in action

On Jan. 27, 2000, the security coordinator in a particular area faced a contingency overload situation. Operators observed that if they were to lose one 765-kV transmission line, this loss would overload a specific 345-kV line-here called the limited flow gate. In this case, the overload would cause a voltage stability problem, which is a difficulty of increasing concern as summer approaches. The security coordinator used IDC to determine which transactions to curtail among the many that were being conducted. The analysis also accounted for new transactions scheduled to begin in the next hour, as well as the priorities of these transactions. The security coordinator declared a level of TLR (there are six levels of such relief), curtailed transactions based on IDC analysis, and ensured continued secure operation. NERC`s web site contains a list of TLR procedure logs, provided within 72 hours after the fact, for each use of TLR (www.nerc. com/~filez/logs.html).

The late January application of IDC also demonstrated how the system can be used in two discrete modes. Phil Hoffer, an engineer in Transmission Operations for American Electric Power (AEP), explains that IDC can help “if you`re already in an overloaded situation by suggesting which transactions to immediately curtail to maintain reliability.” IDC can also “help you determine what will happen at the beginning of each hour, as additional transactions come on.”

According to David Zwergel, engineer in Operations Planning at Allegheny Power, the event was one of several recent cases in which IDC helped maintain system security. “In the near future, the IDC will enable security coordinators to reallocate and reload transactions in an orderly manner, while respecting the priority on flow gates.”

Designed to be quite robust, IDC`s calculations are based on an examination of more than 1,000 interchange transactions at any time and their impact on over 700 transmission facilities. The IDC engine also considers the topology (i.e., location and service status) of power lines, transformers and generators when computing a list of transactions to be curtailed.

Providing information to market

Zwergel, who acted as chairman of the NERC IDC Working Group when IDC was designed, emphasizes another key feature-the information it provides the market. “The IDC enables security coordinators to provide current TLR levels to a web site, where any user can see the status of various flow gates-which ones are limiting, and how deep into TLR we are.” Market participants can use this knowledge to arrange other transactions that avoid the limited flow gates, providing what Zwergel calls “a form of passive redispatch.”

Of course, IDC does not operate alone. Rather, it is a key component in a larger system that includes “e-tagging,” in which electronic tags containing information on each transaction are created and sent to all participants in the deal. The e-tag system came online almost simultaneously with IDC for the Eastern Interconnection.

Ultimately, EPRI and others envision a larger “transaction reservation and flow-based congestion management system.” Such a system would integrate IDC; the existing Open Access Same-time Information System (OASIS), which serves as an online hub for transmission information; and NERCnet, a network NERC envisions to tie together a variety of systems aimed at maintaining reliability (see figure).

“The IDC offers substantial additional capabilities now to the electric power industry,” explains NERC project manager Lou Leffler, “and will be enhanced over time to serve all segments of the industry.” For example, in addition to IDC`s online use, a study mode capability for this tool is now being planned. n

Stephen Lee,, is Area Manager of Grid Operations and Planning at EPRI (Electric Power Research Institute).

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Fiber optics advance to high voltage metering

Steve Dolling, NxtPhase Corp.

The electric power industry is going through a major restructuring at the same time as it is dealing with an aging infrastructure. Increasingly, independent power producers and transmission providers are turning to optical measurement systems for reliable and effective measurement of their high value energy flows.

First explored in the 1970s and trialed in the 1980s, optical sensing technology finally became commercially viable in the late 1990s. Significant advances in optical sensing technology are moving the electric power industry into the next phase of its development.

Optical voltage and current transducers offer a number of advantages over conventional instrument transformers. Highly accurate, these sensors provide excellent galvanic isolation, and do not exhibit saturation and ferroresonance effects common to iron core devices. Typically, they reduce the weight by an order of magnitude and are often used in “zero footprint” installations mounted in unique ways on existing structures. The benefits of these sensors are many, including:

– Higher accuracy;

– Wider bandwidth;

– Greater dynamic range;

– Smaller and lighter with more application flexibility;

– No mechanism for violent failure;

– Possibility to use the same device for revenue metering and protection;

– Combinations of current and voltage sensor in the same column;

– Greater personnel safety; and

– No oil-cellulose or sulfur hexafluoride (SF6) insulation.

At a recent EPRI Optical Sensor Systems Workshop in Atlanta, Harley Gilleland, conference technical coordinator, gathered a large group of experienced users who were keenly aware of the significance of the technology. According to Gilleland, “There are clear advantages in high voltage revenue metering installations today. It is only a matter of time before optical sensor systems are widely used for all transmission level metering, relaying and power quality applications.”

Optical voltage transducers

Optical voltage sensors (see photo) use a variety of methods to measure voltage. In general, they use electric field sensors that operate via an electro-optic effect. However, the challenge is deriving an accurate voltage measurement with electric field sensors given the variable geometry of the conductor relative to ground and sensitivity to external disturbances to the electric field. Most optical voltage sensors use optical fibers for the transmission of light and apply the entire voltage across the sensing element using SF6 gas for insulation.

Technology has progressed, and today, an optical voltage transducer (NXVT) produced by NxtPhase Corp. has pushed optical sensing technology one step further. NXVT combines the benefits of optical sensing technology with additional benefits to the user. It uses multiple miniature electric field sensors inside a high quality composite polymer post insulator to measure voltage with revenue metering accuracy. The electrodes remain widely separated, resulting in a safe design with low dielectric stresses and no need for SF6 gas as an insulator.

Since the successful demonstration of the prototype concept NXVT at Powertech Labs, Surrey, British Columbia, Canada, in 1999, NxtPhase has developed a field-ready prototype. The first installation of a three-phase, 230 kV, 0.3 percent accuracy class metering NXVT system is scheduled at BC Hydro in April 2000 with commercial units available in September. The voltage sensors actually share the column with the other half of NxtPhase`s technology-the NXCT optical current transducer.

Optical current transducers

Over the past 15 years, optical current sensors have received significant attention by a number of research groups around the world. Considered to be the next generation of high voltage measurement devices, these sensors will replace the industry`s existing iron-core current transformers. Optical current sensors have significant advantages. They are made of non-conductive material and are lightweight, which allows for much simpler insulation and mounting designs. In addition, optical sensors provide a much larger dynamic range and frequency response than iron-core current transducers.

Optical current sensors work via the Faraday effect. Current flowing in a conductor induces a magnetic field, which, through the Faraday effect, rotates the plane of polarization of light traveling in a sensing path encircling the conductor. If the light is uniformly sensitive to the magnetic field all along the sensing path, the rotation of the plane of polarization of the light is directly proportional to the current flowing through the conductor. Therefore, measurement of this rotation yields a measurement of the current.

The optical current transducer being developed by NxtPhase (NXCT) is an offshoot from Honeywell`s fiber optic gyro program. The NXCT works on the principle that the magnetic field, rather than rotating a linearly polarized light wave, changes the velocities of circularly polarized light waves within a sensing fiber wound around the current-carrying conductor. “It`s easier to accurately measure changes in light velocity than changes in polarization state. Chief among these reasons is that by using a velocity measurement scheme, we do not need to construct the sensing region from annealed fiber, which is brittle and difficult to work with in a production environment,” said Dr. Jim Blake, director of R&D at NxtPhase.

NXCT has a wide dynamic range. For example, one unit can measure currents from 0.1A to 100,000A. Prototype sensors have been installed in a generation application with accuracies as good as 0.03 percent. It is ideally suited for the most demanding metering and protection applications.

Honeywell has produced fiber optic gyros for a variety of navigation applications since 1992. Early on, Honeywell realized that this technology, with only minor modifications, could be applied to the field of current sensing, and a program to diversify into this area was maintained by Honeywell for several years. Recently, Honeywell joined with Carmanah Engineering, Vancouver, British Columbia, Canada, to launch NxtPhase with the goal to commercialize both the voltage and current sensing technologies.

Power is increasingly measured and sold several times over between the point of generation and end use. The value of energy flowing through new metering points is at an all time high and will continue to increase as the power industry restructures. New technologies are critical enablers of new markets, and optical sensing technology with its inherent technical and cost advantages, will play a key role as the industry evolves.

Steve Dolling is NxtPhase`s director of sales and marketing. He may be contacted at 604-215-9822, or e-mail Additional information is available at

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