<Fundamentals of the Electric Power Transmission System(233)
The electric power system in the United States contains three interrelated elements: the generating facilities that produce the power; the transmission network that conducts the flow of power from the points of generation to the points of distribution; and the distribution system that delivers the electric power to the consumers. The transmission network is the integrating medium of the power supply system providing the electrical connections between the many geographically separated parts of the electric power generating and distribution systems.
The electric transmission network is unlike any other mode of transportation. The flow of electricity is virtually instantaneous, changing magnitude and direction as conditions on the power system dictate.(234) Electricity distributes itself along paths of least resistance that are determined by a complex electrical relationship involving the relative size, location, and distribution of generation resources, transmission line facilities, and centers of demand. All transmission paths share the power transfer, and the degree of sharing is determined by the relationship of the network components. The system consists of transmission and distribution lines, substations with voltage transformers, circuit breakers, and other equipment required to transmit power safely from generation sources to ultimate customers. Transmission voltage levels have increased with improvements in technology and in keeping with the growing demand for electricity.
The transmission system performs several essential functions simultaneously: (1) it supplies the physical means for delivering electricity from the generating sources to the load centers; (2) it integrates generating sources and load centers into a flexible and resilient whole; and (3) it interconnects the physical facilities with those of neighboring systems. Although transmission lines are often added to the network initially to meet a single specific requirement, once added they become an integrated part of the transmission network and their operation becomes interdependent with all the other elements of the network. Operating the system effectively requires significant planning and operational coordination of the generators and transmission facilities to: (1) achieve efficient use of all system facilities, (2) prevent overloading and failure, and (3) maintain adequate reserve transmission and generation capacity to ensure system reliability.
The Need for Coordination of System Operation
The current electrical system has developed in response to the regulations and economics of the electrical utility and nonutility sectors of the electric power industry, as well as to the changing technical factors that influence the generation, transmission, and distribution of electricity. Since the different electrical systems operate as a unified power grid (there are three in the contiguous United States) and the effects of power flows are not confined to contractual paths or apparent direct paths, handling the ever changing flow of electricity is a critical activity for system operators of the power grids. Large power transfers, for example, can change transmission line loadings hundreds of miles from the direct electrical path connecting the source and destination. Actions by individual utilities or nonutility generators (NUGs) can affect the operation of all the others on the system.
Control and Operation of Electric Systems
As electrical energy itself cannot be stored, power must be instantaneously available to end users at any time, in any amount at the proper voltage. As a result, severe demands are imposed on electrical equipment and the transmission network when meeting changing loads. Monitoring the flow of scheduled electricity, handling customer requirements, and coordinating trade among utilities are among the responsibilities of the dispatch center. A dispatch center can be operated independently of other electrical systems by a single utility; it can link two or more interconnected utilities, or even unify several power systems with combined load requirements and maintenance programs.
The operators of dispatch centers continually monitor load patterns to ensure that adequate electricity is available at all times. For most dispatch centers, it is the daily responsibility to (1) record the flow of electricity at the customer load centers and the entering and exiting amounts on its transmission lines, (2) watch the transmission connecting points for each interconnected electrical system, and (3) monitor the power flow from each generation plant. The dispatch center determines the power available from its system, balances the unit-generation marginal costs with buy-or-sell opportunities with other utilities, coordinates the bulk power transactions, examines what plants must be dispatched to avoid technical system problems or undue economic costs, and accounts for system power losses. It also projects demand requirements in order to determine how much generating capacity will be needed and when. These projections may be done hourly, daily, weekly, or at longer intervals.
One electrical operating entity (power pool, electric utility, State authority, and/or Federal utility) within a group of interconnected electrical systems takes responsibility for maintaining system frequency for that electrical geographic area, monitors the load, and ensures generation availability to meet load requirements. Some control centers within these control areas are highly computerized, automatically loading the generating facilities as needed and maintaining the system at the correct operating frequency. This is important because deviations in the scheduled power flows or from the standard system frequency can automatically cause compensating changes in the output at the generating plants. These deviations can mean there has been a loss or gain of a customer load, a plant or line has suffered a forced outage, or some plant or line has been returned to the system. Any of these changes can require some review or action by these control centers.
Stabilizing system frequency is made easier by coordination with other electric systems and by drawing from a larger base of on-line capability. Load changes are absorbed by all the electrical systems, and many of the increasing and decreasing load changes cancel out or offset each other, so that the effect on the entire interconnected electrical system is less than it would be on an isolated electric utility. Also, this integrated system frees each generating unit from the necessity to make continual large changes in production levels.
Integrating Nonutility Generators with the Bulk Electric System
Nonutility generation sources continue to be a growing portion of the U.S. electrical generation capacity. This role of NUGs reflects the changing structure of the electric supply system.
Integrating Nonutility Generators
NUGs present a challenge to the operators of the power grids because of their increasing numbers and growing contribution to wholesale generation. Matching customer load and generation for daily operations and future planning activities is becoming increasingly complex with the growing NUG role and increasing use of the transmission system. Electrical reliability concerns and the proper integration of NUGs into the supply system have become important issues.
The proper integration of NUGs into the electrical operations of interconnection and dispatching generation can be regarded as engineering problems for which technical solutions are available. However, there are institutional issues associated with the responsibility for serving customers and control of the electrical system. The increasing role of NUGs has altered the traditional view of participants in the electrical supply.
Utilities have three basic concerns involving the integration of NUGs with the bulk power system, relating primarily to the relationships of the NUGs, utilities, and customers:
Utilities, with the principal responsibility to operate the system, do not always have full operating control over the NUGs.
The perspectives of some utility industry organizations and NUG participants on these obligations may differ. Some of these differences can be attributed to positions of the organizations in the market, with respect to their cost structures and existing capacity. Some utilities welcome the opportunity for potential cost savings and diversity of supply options offered by NUGs, others are more concerned about operations and overall system reliability.
In response to such concerns, the North American Electric Reliability Council (NERC), which the utility industry charged to oversee the reliability of the bulk electric supply, has established guidelines on the minimum operating considerations that all utility and nonutility generators must follow to ensure the continued reliability of the system.(235)
Impact of Nonutility Generation on the Supply System
The electric utility industry and nonutility industry have worked together to safely and reliably interconnect NUGs. Many utilities are increasingly relying on NUG power as an important source of power. Several factors which utilities may not control can influence the operation of the overall system. The overall level of increase in NUG capacity is just one element. The size of individual facilities has a direct bearing on the potential system impacts; small facilities are less likely to have the same impacts as large ones. At the same time, the locations of individual projects, even small ones, can be critical. Where a facility is sited can affect transmission line loadings and substation equipment operation. Similarly, the timing of power production from a NUG facility can affect the balance of power flows on the system. Moreover, the availability and reliability of NUG power can influence the operation of the system and the requirements for reserve capacity.
The electrical supply system is operated within closely watched tolerances and can require complex and real-time balancing of generation and transmission facilities with fluctuating demand. The substantial interrelationships of all the system components--utility and nonutility--suggest that generation capacity that falls outside the direct control of system operators increases their operational and planning challenges, and may affect system reliability. The extent to which operation, size, location, timing, availability and reliability of NUG power production can be coordinated with system operators will determine the impacts of NUG integration on the bulk electricity supply system.
Technically, all of these factors exert both positive and negative influences on the electrical system, depending on site-specific conditions and timing of actions. For example, a NUG facility could be located specifically to help a utility avoid a transmission or distribution bottleneck. Proper integration of NUGs into the daily operational control and management of the electrical power grid is critical for capturing the benefits and minimizing the disadvantages for all entities connected to the grid.
Endnotes
233. Note that the description in this section describes a state prior to the issuance of Federal Energy Regulatory Commission Order 888 in April 1996.
234. The frequency of electric power supply in the United States is almost entirely 60 hertz (formerly cycles per second). The frequency of a system depends entirely upon the speed at which the supply generator is rotated by its prime mover. James Robert Eaton, Electric Power Transmission Systems. (Englewood Cliffs, New Jersey: Prentice-Hall, Inc., 1972), pp. 2-3.
235. North American Electric Reliability
Council, Integrating Nonutility Generators (Princeton, New Jersey,
January 1992). The specific guidelines now address both planning and operating
considerations, and apply to all utility and nonutility sources. The guidelines
address a range of needs, from specific design issues, to information needs,
and data exchange requirements. The guidelines also cover how the generation
sources would be brought on- and off-line during routine and emergency
conditions.
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