Switching in Electrical Transmission and Distribution Systems


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Electricity is transmitted between interconnections, but that is done by converting to direct current DC and then back to AC. Controlling the dynamic behavior of this interconnected transmission system presents an engineering and operational challenge. Demand for electricity is constantly changing as millions of consumers turn on and off appliances and industrial equipment. The generation of and demand for electricity are balanced regionally by about balancing authorities to ensure that voltage and frequency are maintained within narrow limits typically 5 percent for voltage and 0.

If more power is drawn from the grid than is being pumped into it, the frequency and voltage will decrease, and vice versa. If the voltage or frequency strays too far from its prescribed level, the resulting stresses can lead to system collapse and possibly damage to power system equipment. Most U. Since , electricity generation has risen from about 3 trillion kilowatt-hours kWh to about 4 trillion in Long-distance transmission has grown even faster for reliability and economic reasons, including new competitive wholesale markets for electricity, but few new transmission lines have been built to handle this growth.

Figure 9. However, in the late s, the restructuring and re-regulation of the U. The stress on the U. Ironically, new transmission lines also are the object of considerable public opposition even while the need for them is increased by opposition to generating stations. Reserve margin is the amount of transmission capacity available above the maximum power expected to be delivered over the system.

Some margin is necessary to allow for unexpected loads or outages on the system. The IOU data cover only 80 percent of the transmission system.

All investment is shown in dollars. Sources: — from EEI, ; — from Owens, The deficit of the late s is still affecting reliability; it has contributed to transmission bottlenecks and other transmission deficiencies throughout North America, even with the more recent upward trend in transmission expenditures since According to NERC, the transmission system is being operated at or near its physical limits more of the time Nevius, Stressed grids have less reserve margin for handling disturbances.

TLR is a measure of when scheduled transmission requests could not be accommodated. Transmission loading relief TLR is a sequence of actions taken to avoid or remedy potential reliability concerns associated with the transmission system. Calls for TLRs involve problems. The number of TLR events is not an outage measure; it is the number of times a congestion limit is reached.

Although this measure has been used to characterize transmission reliability, congestion limits can be reached purely for market reasons. Source: See www.

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Each event affected 50 million people. Today, essentially all large power-transmission equipment is imported from Europe and Japan. This could become a potentially. These may or may not result in transmission outages or outages to customers. Disturbances include electric service interruptions, unusual occurrences, demand and voltage reductions, public appeals, fuel supply problems, and acts of sabotage that can affect the reliability of the bulk electric systems. Source: Compiled from data in NERC, , , , , , , , , , , , , and Modernization is progressing much more rapidly abroad.

About 30 high-power HVDC projects are under construction in Europe, including many submarine cable connections to increase utilization of offshore wind power. Two-way metering is common in Europe because it helps to maximize the potential of rooftop photovoltaics, which are being heavily promoted in Germany and other countries. Although the United States has vast potential for wind and solar generation, there is no consensus or plan for how this power could be transmitted to load centers.

To meet these challenges, transmission. Two major issues contributed to this blackout: first, the operators did not know the system was in trouble; and second, there was poor communication between the utilities operating the transmission lines—First Energy and American Electric Power—and also between these utilities and the ISO responsible for the area the Midwest Independent System Operator. Adequate monitoring, communication, and dynamic reactive power support during the initial voltage sag could have helped to prevent lines from overloading, heating up, and sagging excessively.

Operators would have been better informed, and online real-time dynamic contingency analysis of potential system collapse would have helped operators stay aware of possible risks and actions to be taken in response. Finally, automatic actions could have been taken to island isolate portions of the system and prevent the ultimate cascading event which spread the localized outage across much of the northeast United States and Canada.

The system could also have been restored much more rapidly if a modern grid had been in place. Thus if business continues as usual, investment will focus on new construction to meet peak load growth, which is projected to increase. This view was presented repeatedly to the committee by industry representatives, including those representing Southern California Edison Co.

Distribution systems are in better condition. Reliability, for example, has increased steadily over the last 7 years—in part because these systems have to be enlarged to handle new consumers. Public utility commissions usually provide revenue incentives based on indexes, shown in Annex 9.

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A , that directly measure customer service reliability. Consequently, the distribution companies have improved or at least held steady their customer outage statistics. Growth provides the opportunity for distribution companies to introduce new and smarter technologies on a limited basis before undertaking a wider application. For example, a utility can introduce modern, smart technologies on a substation-by-substation basis as it is determined that portions of the distribution network need upgrading.

For example, the present system of local automatic controls overseen by human operators at regionally based control centers is not able to adequately foresee that disturbances in Cleveland can black out New York, Toronto, and Detroit, or that transmission outages in Switzerland can black out all of Italy. Modern communications and controls can move much faster to diagnose problems and bypass or isolate them. The same technology can provide cost benefits by maximizing power flows and integrating power from renewable energy sources.

New technology is an important part of the answer to the challenges facing the grid, but policy and regulatory changes will also be needed, particularly with.

Some lowering of this number may be possible with aggressive electricity end-use efficiency measures. The steady reliability of the distribution system does not contradict the increasing congestion and increasing number of system disturbances on the transmission system.


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Outages on the transmission system do not necessarily result in outages on the distribution system, as the transmission system is a network. This means that if one path is closed, there are alternative paths for power to flow to the consumer.

For example, the cost of not having power when it is needed is far greater to the user than the lost revenues to the utility that cannot provide it. Recognizing the value of a reliable, efficient, and flexible grid, and supporting the investments to make that possible, may require a national-level strategy.

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As discussed below in this chapter, expanding and modernizing distribution systems will require considerably more investment than for transmission systems. Much of the expansion will be noncontroversial because it will be required to meet growing loads and can be done without much impact on people who do not directly benefit from it. In addition, modernization of distribution can be achieved on a more limited basis than for transmission, which will require coordination across many systems.

Therefore, the emphasis in this chapter will be on transmission. They must also be expanded to meet future needs. The characteristics required to achieve these performance standards are as follows: Accommodates all generation and storage options. A modern transmission infrastructure would include emerging technologies such as large-scale variable power sources and advanced energy storage devices. For example, it could smooth the variability of power from remotely located intermittent renewable resources 11 and maintain reactive power 12 on the system.

The distribution system should be able to accommodate increasing amounts of distributed generation—often variable such as rooftop photovoltaic devices —and smaller-scale advanced energy storage devices. Enables wholesale power markets. In addition, the distribution system should enable the end user to participate in power markets by allowing self-generation opportunities. Is self-healing.

Motivates and includes the customer. The modern distribution system would empower customers to make end-use decisions that increase. All equipment, lines, and loads have inductances and capacitance that in an AC system take power during half of each cycle and deliver it back during the other half cycle; hence, they load the lines and equipment but do not deliver net power.

Otherwise, the grid can become seriously unbalanced. A major system collapse can occur when a system becomes unbalanced—for example, when a major line is lost and other lines become overloaded as more power flows through them. As these lines are shut down by protective devices, the disturbance can propagate throughout the system, leaving large areas without power.

It would allow for self-generation and storage as well as for customers to participate in an interactive mode by responding to price signals. Provides high power quality where needed. The highest power quality is not necessarily cost-effective for all users, so some may still need to provide additional sources of power, standby generation, or other devices that can ride through minor electrical disturbances on either the transmission or the distribution system.

Switching in Electrical Transmission and Distribution Systems Switching in Electrical Transmission and Distribution Systems
Switching in Electrical Transmission and Distribution Systems Switching in Electrical Transmission and Distribution Systems
Switching in Electrical Transmission and Distribution Systems Switching in Electrical Transmission and Distribution Systems
Switching in Electrical Transmission and Distribution Systems Switching in Electrical Transmission and Distribution Systems
Switching in Electrical Transmission and Distribution Systems Switching in Electrical Transmission and Distribution Systems
Switching in Electrical Transmission and Distribution Systems Switching in Electrical Transmission and Distribution Systems
Switching in Electrical Transmission and Distribution Systems Switching in Electrical Transmission and Distribution Systems
Switching in Electrical Transmission and Distribution Systems Switching in Electrical Transmission and Distribution Systems
Switching in Electrical Transmission and Distribution Systems Switching in Electrical Transmission and Distribution Systems

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