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Wednesday, April 1, 2015

Electric power transmission


From Wikipedia, the free encyclopedia


Two-circuit, single-voltage power transmission line; "Bundled" 4-ways

Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to electrical substations located near demand centers. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. Transmission lines, when interconnected with each other, become transmission networks. The combined transmission and distribution network is known as the "power grid" in North America, or just "the grid". In the United Kingdom, the network is known as the "National Grid".

A wide area synchronous grid, also known as an "interconnection" in North America, directly connects a large number of generators delivering AC power with the same relative frequency, to a large number of consumers. For example, there are four major interconnections in North America (the Western Interconnection, the Eastern Interconnection, the Quebec Interconnection and the Electric Reliability Council of Texas (ERCOT) grid), and one large grid for most of continental Europe.

The same relative frequency, but almost never the same relative phase as ac power interchange is a function of the phase difference between any two nodes in the network, and zero degrees difference means no power is interchanged; any phase difference up to 90 degrees is stable by the "equal area criteria"; any phase difference above 90 degrees is absolutely unstable; the interchange partners are responsible for maintaining frequency as close to 60.0000 Hz as is practical, and the phase differences between any two nodes significantly less than 90 degrees; should 90 degrees be exceeded, a system separation is executed, and remains separated until the trouble has been corrected.

Historically, transmission and distribution lines were owned by the same company, but starting in the 1990s, many countries have liberalized the regulation of the electricity market in ways that have led to the separation of the electricity transmission business from the distribution business.[1]

System

Most transmission lines are high-voltage three-phase alternating current (AC), although single phase AC is sometimes used in railway electrification systems. High-voltage direct-current (HVDC) technology is used for greater efficiency at very long distances (typically hundreds of miles (kilometers)), or in submarine power cables (typically longer than 30 miles (50 km)). HVDC links are also used to stabilize and control problems in large power distribution networks where sudden new loads or blackouts in one part of a network can otherwise result in synchronization problems and cascading failures.

Diagram of an electric power system; transmission system is in blue

Electricity is transmitted at high voltages (120 kV or above) to reduce the energy losses in long-distance transmission. Power is usually transmitted through overhead power lines. Underground power transmission has a significantly higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations.

A key limitation of electric power is that, with minor exceptions, electrical energy cannot be stored, and therefore must be generated as needed. A sophisticated control system is required to ensure electric generation very closely matches the demand. If the demand for power exceeds the supply, generation plant and transmission equipment can shut down, which in the worst case may lead to a major regional blackout, such as occurred in the US Northeast blackout of 1965, 1977, 2003, and other regional blackouts in 1996 and 2011. It is to reduce the risk of such a failure that electric transmission networks are interconnected into regional, national or continent wide networks thereby providing multiple redundant alternative routes for power to flow should such equipment failures occur. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure spare capacity is available should there be any such failure in another part of the network.

Overhead transmission


3-phase high-voltage lines in Washington State
Four-circuit, two-voltage power transmission line; "Bundled" 2-ways
A typical ACSR. The conductor consists of seven strands of steel surrounded by four layers of aluminium.

High-voltage overhead conductors are not covered by insulation. The conductor material is nearly always an aluminum alloy, made into several strands and possibly reinforced with steel strands. Copper was sometimes used for overhead transmission, but aluminum is lighter, yields only marginally reduced performance and costs much less. Overhead conductors are a commodity supplied by several companies worldwide. Improved conductor material and shapes are regularly used to allow increased capacity and modernize transmission circuits. Conductor sizes range from 12 mm2 (#6 American wire gauge) to 750 mm2 (1,590,000 circular mils area), with varying resistance and current-carrying capacity. Thicker wires would lead to a relatively small increase in capacity due to the skin effect, that causes most of the current to flow close to the surface of the wire. Because of this current limitation, multiple parallel cables (called bundle conductors) are used when higher capacity is needed. Bundle conductors are also used at high voltages to reduce energy loss caused by corona discharge.

Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages, such as 66 kV and 33 kV, are usually considered subtransmission voltages, but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 765 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages.

Since overhead transmission wires depend on air for insulation, the design of these lines requires minimum clearances to be observed to maintain safety. Adverse weather conditions, such as high wind and low temperatures, can lead to power outages. Wind speeds as low as 23 knots (43 km/h) can permit conductors to encroach operating clearances, resulting in a flashover and loss of supply.[2] Oscillatory motion of the physical line can be termed gallop or flutter depending on the frequency and amplitude of oscillation.

Underground transmission

Electric power can also be transmitted by underground power cables instead of overhead power lines. Underground cables take up less right-of-way than overhead lines, have lower visibility, and are less affected by bad weather. However, costs of insulated cable and excavation are much higher than overhead construction. Faults in buried transmission lines take longer to locate and repair. Underground lines are strictly limited by their thermal capacity, which permits less overload or re-rating than overhead lines. Long underground AC cables have significant capacitance, which may reduce their ability to provide useful power to loads beyond 50 miles. Long underground DC cables have no such issue and can run for thousands of miles.

History

New York City streets in 1890. Besides telegraph lines, multiple electric lines were required for each class of device requiring different voltages

In the early days of commercial electric power, transmission of electric power at the same voltage as used by lighting and mechanical loads restricted the distance between generating plant and consumers. In 1882, generation was with direct current (DC), which could not easily be increased in voltage for long-distance transmission. Different classes of loads (for example, lighting, fixed motors, and traction/railway systems) required different voltages, and so used different generators and circuits.[3][page needed]

Due to this specialization of lines and because transmission was inefficient for low-voltage high-current circuits, generators needed to be near their loads. It seemed, at the time, that the industry would develop into what is now known as a distributed generation system with large numbers of small generators located near their loads.[4]

In 1886, in Great Barrington, Massachusetts, a 1 kV alternating current (AC) distribution system was installed. That same year, AC power at 2 kV, transmitted 30 km, was installed at Cerchi, Italy. At an AIEE meeting on May 16, 1888, Nikola Tesla delivered a lecture entitled A New System of Alternating Current Motors and Transformers, describing the equipment which allowed efficient generation and use of polyphase alternating currents. The transformer, and Tesla's polyphase and single-phase induction motors, were essential for a combined AC distribution system for both lighting and machinery. Ownership of the rights to the Tesla patents was a key advantage to the Westinghouse Company in offering a complete alternating current power system for both lighting and power.

Regarded as one of the most influential electrical innovations, the universal system used transformers to step-up voltage from generators to high-voltage transmission lines, and then to step-down voltage to local distribution circuits or industrial customers.[3] By a suitable choice of utility frequency, both lighting and motor loads could be served. Rotary converters and later mercury-arc valves and other rectifier equipment allowed DC to be provided where needed. Generating stations and loads using different frequencies could be interconnected using rotary converters. By using common generating plants for every type of load, important economies of scale were achieved, lower overall capital investment was required, load factor on each plant was increased allowing for higher efficiency, a lower cost for the consumer and increased overall use of electric power.

Nikola Tesla's alternating current polyphase generators on display at the 1893 World's Fair in Chicago. Tesla's polyphase innovations revolutionized transmission

By allowing multiple generating plants to be interconnected over a wide area, electricity production cost was reduced. The most efficient available plants could be used to supply the varying loads during the day. Reliability was improved and capital investment cost was reduced, since stand-by generating capacity could be shared over many more customers and a wider geographic area. Remote and low-cost sources of energy, such as hydroelectric power or mine-mouth coal, could be exploited to lower energy production cost.[3]

The first transmission of three-phase alternating current using high voltage took place in 1891 during the international electricity exhibition in Frankfurt. A 25 kV transmission line, approximately 175 km long, connected Lauffen on the Neckar and Frankfurt.

Voltages used for electric power transmission increased throughout the 20th century. By 1914, fifty-five transmission systems each operating at more than 70 kV were in service. The highest voltage then used was 150 kV.[5]

The rapid industrialization in the 20th century made electrical transmission lines and grids a critical infrastructure item in most industrialized nations. The interconnection of local generation plants and small distribution networks was greatly spurred by the requirements of World War I, with large electrical generating plants built by governments to provide power to munitions factories. Later these generating plants were connected to supply civil loads through long-distance transmission.[6]

Bulk power transmission


A transmission substation decreases the voltage of incoming electricity, allowing it to connect from long distance high voltage transmission, to local lower voltage distribution. It also reroutes power to other transmission lines that serve local markets. This is the PacifiCorp Hale Substation, Orem, Utah, USA

Engineers design transmission networks to transport the energy as efficiently as feasible, while at the same time taking into account economic factors, network safety and redundancy. These networks use components such as power lines, cables, circuit breakers, switches and transformers. The transmission network is usually administered on a regional basis by an entity such as a regional transmission organization or transmission system operator.

Transmission efficiency is greatly improved by devices that increase the voltage, (and thereby proportionately reduce the current) in the line conductors, thus allowing power to be transmitted with acceptable losses. The reduced current flowing through the line reduces the heating losses in the conductors. According to Joule's Law, energy losses are directly proportional to the square of the current. Thus, reducing the current by a factor of two will lower the energy lost to conductor resistance by a factor of four for any given size of conductor.

The optimum size of a conductor for a given voltage and current can be estimated by Kelvin's law for conductor size, which states that the size is at its optimum when the annual cost of energy wasted in the resistance is equal to the annual capital charges of providing the conductor. At times of lower interest rates, Kelvin's law indicates that thicker wires are optimal; while, when metals are expensive, thinner conductors are indicated: however, power lines are designed for long-term use, so Kelvin's law has to be used in conjunction with long-term estimates of the price of copper and aluminum as well as interest rates for capital.

The increase in voltage is achieved in AC circuits by using a step-up transformer. HVDC systems require relatively costly conversion equipment which may be economically justified for particular projects such as submarine cables and longer distance high capacity point to point transmission. HVDC is necessary for the import and export of energy between grid systems that are not synchronized with each other.

A transmission grid is a network of power stations, transmission lines, and substations. Energy is usually transmitted within a grid with three-phase AC. Single-phase AC is used only for distribution to end users since it is not usable for large polyphase induction motors. In the 19th century, two-phase transmission was used but required either four wires or three wires with unequal currents. Higher order phase systems require more than three wires, but deliver little or no benefit.

The price of electric power station capacity is high, and electric demand is variable, so it is often cheaper to import some portion of the needed power than to generate it locally. Because loads are often regionally correlated (hot weather in the Southwest portion of the US might cause many people to use air conditioners), electric power often comes from distant sources. Because of the economic benefits of load sharing between regions, wide area transmission grids now span countries and even continents. The web of interconnections between power producers and consumers should enable power to flow, even if some links are inoperative.

The unvarying (or slowly varying over many hours) portion of the electric demand is known as the base load and is generally served by large facilities (which are more efficient due to economies of scale) with fixed costs for fuel and operation. Such facilities are nuclear, coal-fired or hydroelectric, while other energy sources such as concentrated solar thermal and geothermal power have the potential to provide base load power. Renewable energy sources, such as solar photovoltaics, wind, wave, and tidal, are, due to their intermittency, not considered as supplying "base load" but will still add power to the grid. The remaining or 'peak' power demand, is supplied by peaking power plants, which are typically smaller, faster-responding, and higher cost sources, such as combined cycle or combustion turbine plants fueled by natural gas.

Long-distance transmission of electricity (thousands of kilometers) is cheap and efficient, with costs of US$0.005–0.02/kWh (compared to annual averaged large producer costs of US$0.01–0.025/kWh, retail rates upwards of US$0.10/kWh, and multiples of retail for instantaneous suppliers at unpredicted highest demand moments).[7] Thus distant suppliers can be cheaper than local sources (e.g., New York often buys over 1000 MW of electricity from Canada).[8] Multiple local sources (even if more expensive and infrequently used) can make the transmission grid more fault tolerant to weather and other disasters that can disconnect distant suppliers.

A high-power electrical transmission tower, 230 kV, double-circuit, also double-bundled

Long-distance transmission allows remote renewable energy resources to be used to displace fossil fuel consumption. Hydro and wind sources cannot be moved closer to populous cities, and solar costs are lowest in remote areas where local power needs are minimal. Connection costs alone can determine whether any particular renewable alternative is economically sensible. Costs can be prohibitive for transmission lines, but various proposals for massive infrastructure investment in high capacity, very long distance super grid transmission networks could be recovered with modest usage fees.

Grid input

At the power stations, the power is produced at a relatively low voltage between about 2.3 kV and 30 kV, depending on the size of the unit. The generator terminal voltage is then stepped up by the power station transformer to a higher voltage (115 kV to 765 kV AC, varying by the transmission system and by country) for transmission over long distances.

Losses

Transmitting electricity at high voltage reduces the fraction of energy lost to resistance, which varies depending on the specific conductors, the current flowing, and the length of the transmission line. For example, a 100 mile 765 kV line carrying 1000 MW of power can have losses of 1.1% to 0.5%. A 345 kV line carrying the same load across the same distance has losses of 4.2%.[9] For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the I2R losses by a factor of 100, provided the same sized conductors are used in both cases. Even if the conductor size (cross-sectional area) is reduced 10-fold to match the lower current, the I2R losses are still reduced 10-fold. Long-distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV. At extremely high voltages, more than 2,000 kV exists between conductor and ground, corona discharge losses are so large that they can offset the lower resistive losses in the line conductors. Measures to reduce corona losses include conductors having larger diameters; often hollow to save weight,[10] or bundles of two or more conductors.

Transmission and distribution losses in the USA were estimated at 6.6% in 1997[11] and 6.5% in 2007.[11] By using underground DC transmission, these losses can be cut in half.[citation needed] Underground cables can be larger diameter because they do not have the constraint of light weight that overhead conductors have. In general, losses are estimated from the discrepancy between power produced (as reported by power plants) and power sold to the end customers; the difference between what is produced and what is consumed constitute transmission and distribution losses, assuming no theft of utility occurs.

As of 1980, the longest cost-effective distance for direct-current transmission was determined to be 7,000 km (4,300 mi). For alternating current it was 4,000 km (2,500 mi), though all transmission lines in use today are substantially shorter than this.[7]

In any alternating current transmission line, the inductance and capacitance of the conductors can be significant. Currents that flow solely in 'reaction' to these properties of the circuit, (which together with the resistance define the impedance) constitute reactive power flow, which transmits no 'real' power to the load. These reactive currents, however, are very real and cause extra heating losses in the transmission circuit. The ratio of 'real' power (transmitted to the load) to 'apparent' power (sum of 'real' and 'reactive') is the power factor. As reactive current increases, the reactive power increases and the power factor decreases. For transmission systems with low power factor, losses are higher than for systems with high power factor. Utilities add capacitor banks, reactors and other components (such as phase-shifting transformers; static VAR compensators; physical transposition of the phase conductors; and flexible AC transmission systems, FACTS) throughout the system to compensate for the reactive power flow and reduce the losses in power transmission and stabilize system voltages. These measures are collectively called 'reactive support'.

Subtransmission


danger subtransmission line above

subtransmission line, wood H-frame construction

Subtransmission is part of an electric power transmission system that runs at relatively lower voltages. It is uneconomical to connect all distribution substations to the high main transmission voltage, because the equipment is larger and more expensive. Typically, only larger substations connect with this high voltage. It is stepped down and sent to smaller substations in towns and neighborhoods. Subtransmission circuits are usually arranged in loops so that a single line failure does not cut off service to a large number of customers for more than a short time. Loops can be "normally closed", where loss of one circuit should result in no interruption, or "normally open" where substations can switch to a backup supply. While subtransmission circuits are usually carried on overhead lines, in urban areas buried cable may be used. The lower-voltage subtransmission lines use less right-of-way and simpler structures; it is much more feasible to put them underground where needed. Higher-voltage lines require more space and are usually above-ground since putting them underground is very expensive.

There is no fixed cutoff between subtransmission and transmission, or subtransmission and distribution. The voltage ranges overlap somewhat. Voltages of 69 kV, 115 kV and 138 kV are often used for subtransmission in North America. As power systems evolved, voltages formerly used for transmission were used for subtransmission, and subtransmission voltages became distribution voltages. Like transmission, subtransmission moves relatively large amounts of power, and like distribution, subtransmission covers an area instead of just point to point.[12]

Transmission grid exit

At the substations, transformers reduce the voltage to a lower level for distribution to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (33 kV to 132 kV) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage (varying by country and customer requirements—see Mains electricity by country).

High-voltage direct current

High-voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is to be transmitted over very long distances, the power lost in AC transmission becomes appreciable and it is less expensive to use direct current instead of alternating current. For a very long transmission line, these lower losses (and reduced construction cost of a DC line) can offset the additional cost of the required converter stations at each end.
HVDC is also used for submarine cables because over about 30 kilometres (19 mi) lengths AC cannot be supplied[citation needed]. In these cases special high-voltage cables for DC are used. Submarine HVDC systems are often used to connect the electricity grids of islands, for example, between Great Britain and mainland Europe, between Great Britain and Ireland, between Tasmania and the Australian mainland, and between the North and South Islands of New Zealand. Submarine connections up to 600 kilometres (370 mi) in length are presently in use.

HVDC links can be used to control problems in the grid with AC electricity flow. The power transmitted by an AC line increases as the phase angle between source end voltage and destination ends increases, but too large a phase angle will allow the systems at either end of the line to fall out of step. Since the power flow in a DC link is controlled independently of the phases of the AC networks at either end of the link, this phase angle limit does not exist, and a DC link is always able to transfer its full rated power. A DC link therefore stabilizes the AC grid at either end, since power flow and phase angle can then be controlled independently.

As an example, to adjust the flow of AC power on a hypothetical line between Seattle and Boston would require adjustment of the relative phase of the two regional electrical grids. This is an everyday occurrence in AC systems, but one that can become disrupted when AC system components fail and place unexpected loads on the remaining working grid system. With an HVDC line instead, such an interconnection would: (1) Convert AC in Seattle into HVDC; (2) Use HVDC for the 3,000 miles of cross-country transmission; and (3) Convert the HVDC to locally synchronized AC in Boston, (and possibly in other cooperating cities along the transmission route). Such a system could be less prone to failure if parts of it were suddenly shut down. One example of a long DC transmission line is the Pacific DC Intertie located in the Western United States.

Capacity

The amount of power that can be sent over a transmission line is limited. The origins of the limits vary depending on the length of the line. For a short line, the heating of conductors due to line losses sets a thermal limit. If too much current is drawn, conductors may sag too close to the ground, or conductors and equipment may be damaged by overheating. For intermediate-length lines on the order of 100 km (62 mi), the limit is set by the voltage drop in the line. For longer AC lines, system stability sets the limit to the power that can be transferred. Approximately, the power flowing over an AC line is proportional to the cosine of the phase angle of the voltage and current at the receiving and transmitting ends. Since this angle varies depending on system loading and generation, it is undesirable for the angle to approach 90 degrees. Very approximately, the allowable product of line length and maximum load is proportional to the square of the system voltage. Series capacitors or phase-shifting transformers are used on long lines to improve stability. High-voltage direct current lines are restricted only by thermal and voltage drop limits, since the phase angle is not material to their operation.

Up to now, it has been almost impossible to foresee the temperature distribution along the cable route, so that the maximum applicable current load was usually set as a compromise between understanding of operation conditions and risk minimization. The availability of industrial distributed temperature sensing (DTS) systems that measure in real time temperatures all along the cable is a first step in monitoring the transmission system capacity. This monitoring solution is based on using passive optical fibers as temperature sensors, either integrated directly inside a high voltage cable or mounted externally on the cable insulation. A solution for overhead lines is also available. In this case the optical fiber is integrated into the core of a phase wire of overhead transmission lines (OPPC). The integrated Dynamic Cable Rating (DCR) or also called Real Time Thermal Rating (RTTR) solution enables not only to continuously monitor the temperature of a high voltage cable circuit in real time, but to safely utilize the existing network capacity to its maximum. Furthermore it provides the ability to the operator to predict the behavior of the transmission system upon major changes made to its initial operating conditions.

Control

To ensure safe and predictable operation the components of the transmission system are controlled with generators, switches, circuit breakers and loads. The voltage, power, frequency, load factor, and reliability capabilities of the transmission system are designed to provide cost effective performance for the customers.

Load balancing

The transmission system provides for base load and peak load capability, with safety and fault tolerance margins.
The peak load times vary by region largely due to the industry mix. In very hot and very cold climates home air conditioning and heating loads have an effect on the overall load. They are typically highest in the late afternoon in the hottest part of the year and in mid-mornings and mid-evenings in the coldest part of the year. This makes the power requirements vary by the season and the time of day. Distribution system designs always take the base load and the peak load into consideration.

The transmission system usually does not have a large buffering capability to match the loads with the generation.
Thus generation has to be kept matched to the load, to prevent overloading failures of the generation equipment.

Multiple sources and loads can be connected to the transmission system and they must be controlled to provide orderly transfer of power. In centralized power generation, only local control of generation is necessary, and it involves synchronization of the generation units, to prevent large transients and overload conditions.

In distributed power generation the generators are geographically distributed and the process to bring them online and offline must be carefully controlled. The load control signals can either be sent on separate lines or on the power lines themselves. Voltage and frequency can be used as signalling mechanisms to balance the loads.

In voltage signaling, the variation of voltage is used to increase generation. The power added by any system increases as the line voltage decreases. This arrangement is stable in principle. Voltage-based regulation is complex to use in mesh networks, since the individual components and setpoints would need to be reconfigured every time a new generator is added to the mesh.

In frequency signaling, the generating units match the frequency of the power transmission system. In droop speed control, if the frequency decreases, the power is increased. (The drop in line frequency is an indication that the increased load is causing the generators to slow down.)

Wind turbines, vehicle-to-grid and other distributed storage and generation systems can be connected to the power grid, and interact with it to improve system operation.[citation needed]

Failure protection

Under excess load conditions, the system can be designed to fail gracefully rather than all at once. Brownouts occur when the supply power drops below the demand. Blackouts occur when the supply fails completely.

Rolling blackouts (also called load shedding) are intentionally engineered electrical power outages, used to distribute insufficient power when the demand for electricity exceeds the supply.

Communications

Operators of long transmission lines require reliable communications for control of the power grid and, often, associated generation and distribution facilities. Fault-sensing protective relays at each end of the line must communicate to monitor the flow of power into and out of the protected line section so that faulted conductors or equipment can be quickly de-energized and the balance of the system restored. Protection of the transmission line from short circuits and other faults is usually so critical that common carrier telecommunications are insufficiently reliable, and in remote areas a common carrier may not be available. Communication systems associated with a transmission project may use:
Rarely, and for short distances, a utility will use pilot-wires strung along the transmission line path. Leased circuits from common carriers are not preferred since availability is not under control of the electric power transmission organization.

Transmission lines can also be used to carry data: this is called power-line carrier, or PLC. PLC signals can be easily received with a radio for the long wave range.

Optical fibers can be included in the stranded conductors of a transmission line, in the overhead shield wires. These cables are known as optical ground wire (OPGW). Sometimes a standalone cable is used, all-dielectric self-supporting (ADSS) cable, attached to the transmission line cross arms.

Some jurisdictions, such as Minnesota, prohibit energy transmission companies from selling surplus communication bandwidth or acting as a telecommunications common carrier. Where the regulatory structure permits, the utility can sell capacity in extra dark fibers to a common carrier, providing another revenue stream.

Electricity market reform

Some regulators regard electric transmission to be a natural monopoly[13][14] and there are moves in many countries to separately regulate transmission (see electricity market).
Spain was the first country to establish a regional transmission organization. In that country, transmission operations and market operations are controlled by separate companies. The transmission system operator is Red Eléctrica de España (REE) and the wholesale electricity market operator is Operador del Mercado Ibérico de Energía – Polo Español, S.A. (OMEL) [1]. Spain's transmission system is interconnected with those of France, Portugal, and Morocco.

In the United States and parts of Canada, electrical transmission companies operate independently of generation and distribution companies.

Cost of electric power transmission

The cost of high voltage electricity transmission (as opposed to the costs of electric power distribution) is comparatively low, compared to all other costs arising in a consumer's electricity bill. In the UK, transmission costs are about 0.2p/kWh compared to a delivered domestic price of around 10p/kWh.[15]

Research evaluates the level of capital expenditure in the electric power T&D equipment market will be worth $128.9bn in 2011.[16]

Merchant transmission

Merchant transmission is an arrangement where a third party constructs and operates electric transmission lines through the franchise area of an unrelated utility.

Operating merchant transmission projects in the United States include the Cross Sound Cable from Shoreham, New York to New Haven, Connecticut, Neptune RTS Transmission Line from Sayreville, N.J., to Newbridge, N.Y, and Path 15 in California. Additional projects are in development or have been proposed throughout the United States, including the Lake Erie Connector, an underwater transmission line proposed by ITC Holdings Corp., connecting Ontario to load serving entities in the PJM Interconnection region. [17]

There is only one unregulated or market interconnector in Australia: Basslink between Tasmania and Victoria. Two DC links originally implemented as market interconnectors, Directlink and Murraylink, have been converted to regulated interconnectors. NEMMCO

A major barrier to wider adoption of merchant transmission is the difficulty in identifying who benefits from the facility so that the beneficiaries will pay the toll. Also, it is difficult for a merchant transmission line to compete when the alternative transmission lines are subsidized by other utility businesses.[18]

Health concerns

Some large studies, including a large United States study, have failed to find any link between living near power lines and developing any sickness or diseases, such as cancer. A 1997 study found that it did not matter how close one was to a power line or a sub-station, there was no increased risk of cancer or illness.[19]
The mainstream scientific evidence suggests that low-power, low-frequency, electromagnetic radiation associated with household currents and high transmission power lines does not constitute a short or long term health hazard. Some studies, however, have found statistical correlations between various diseases and living or working near power lines. No adverse health effects have been substantiated for people not living close to powerlines.[20]

There are established biological effects for acute high level exposure to magnetic fields well above 100 µT (1 G). In a residential setting, there is "limited evidence of carcinogenicity in humans and less than sufficient evidence for carcinogenicity in experimental animals", in particular, childhood leukemia, associated with average exposure to residential power-frequency magnetic field above 0.3 µT (3 mG) to 0.4 µT (4 mG). These levels exceed average residential power-frequency magnetic fields in homes, which are about 0.07 µT (0.7 mG) in Europe and 0.11 µT (1.1 mG) in North America.[21][22]

The Earths natural geomagnetic field strength varies over the surface of the planet between 0.035 mT - 0.07 mT (35 µT - 70 µT or 0.35 G - 0.7 G) while the International Standard for the continuous exposure limit is set at 40 mT (40,000 µT or 400 G) for the general public.[21]

Tree Growth Regulator and Herbicide Control Methods may be used in transmission line right of ways[23] which may have health effects.

United States government policy

Historically, local governments have exercised authority over the grid and have significant disincentives to encourage actions that would benefit states other than their own. Localities with cheap electricity have a disincentive to encourage making interstate commerce in electricity trading easier, since other regions will be able to compete for local energy and drive up rates. For example, some regulators in Maine do not wish to address congestion problems because the congestion serves to keep Maine rates low.[24] Further, vocal local constituencies can block or slow permitting by pointing to visual impact, environmental, and perceived health concerns. In the US, generation is growing four times faster than transmission, but big transmission upgrades require the coordination of multiple states, a multitude of interlocking permits, and cooperation between a significant portion of the 500 companies that own the grid. From a policy perspective, the control of the grid is balkanized, and even former energy secretary Bill Richardson refers to it as a third world grid. There have been efforts in the EU and US to confront the problem. The US national security interest in significantly growing transmission capacity drove passage of the 2005 energy act giving the Department of Energy the authority to approve transmission if states refuse to act. However, soon after the Department of Energy used its power to designate two National Interest Electric Transmission Corridors, 14 senators signed a letter stating the DOE was being too aggressive.[25]

Special transmission

Grids for railways

In some countries where electric locomotives or electric multiple units run on low frequency AC power, there are separate single phase traction power networks operated by the railways. Prime example are the countries of Europe, which utilize the older AC technology based on 16 2/3 Hz.

Superconducting cables

High-temperature superconductors (HTS) promise to revolutionize power distribution by providing lossless transmission of electrical power. The development of superconductors with transition temperatures higher than the boiling point of liquid nitrogen has made the concept of superconducting power lines commercially feasible, at least for high-load applications.[26] It has been estimated that the waste would be halved using this method, since the necessary refrigeration equipment would consume about half the power saved by the elimination of the majority of resistive losses. Some companies such as Consolidated Edison and American Superconductor have already begun commercial production of such systems.[27] In one hypothetical future system called a SuperGrid, the cost of cooling would be eliminated by coupling the transmission line with a liquid hydrogen pipeline.

Superconducting cables are particularly suited to high load density areas such as the business district of large cities, where purchase of an easement for cables would be very costly.[28]

HTS transmission lines[29]
Location Length (km) Voltage (kV) Capacity (GW) Date
Carrollton, Georgia 2000
Albany, New York[30] 0.35 34.5 0.048 2006
Long Island[31] 0.6 130 0.574 2008
Tres Amigas 5 Proposed 2013
Manhattan: Project Hydra Proposed 2014
Essen, Germany[32][33] 1 10 0.04 2014

Single wire earth return

Single-wire earth return (SWER) or single wire ground return is a single-wire transmission line for supplying single-phase electrical power for an electrical grid to remote areas at low cost. It is principally used for rural electrification, but also finds use for larger isolated loads such as water pumps. Single wire earth return is also used for HVDC over submarine power cables.

Wireless power transmission

Both Nikola Tesla and Hidetsugu Yagi attempted to devise systems for large scale wireless power transmission in the late 1800s and early 1900s, with no commercial success.
In November 2009, LaserMotive won the NASA 2009 Power Beaming Challenge by powering a cable climber 1 km vertically using a ground-based laser transmitter. The system produced up to 1 kW of power at the receiver end. In August 2010, NASA contracted with private companies to pursue the design of laser power beaming systems to power low earth orbit satellites and to launch rockets using laser power beams.

Wireless power transmission has been studied for transmission of power from solar power satellites to the earth. A high power array of microwave or laser transmitters would beam power to a rectenna. Major engineering and economic challenges face any solar power satellite project.

Security of control systems

The Federal government of the United States admits that the power grid is susceptible to cyber-warfare.[34][35] The United States Department of Homeland Security works with industry to identify vulnerabilities and to help industry enhance the security of control system networks, the federal government is also working to ensure that security is built in as the U.S. develops the next generation of 'smart grid' networks.[36]

Records

World Wireless System


From Wikipedia, the free encyclopedia


The Wardenclyffe Power Plant intended by Nikola Tesla to be a "World Wireless" telecommunications facility.

The World Wireless System was a turn of the 19th century proposed communication and electrical power delivery system designed by the inventor Nikola Tesla based on his theories of using the Earth and/or the Earths atmosphere as an electrical conductor. Tesla claimed this system would allow for "the transmission of electric energy without wires" on a global scale[1] as well as point-to-point wireless telecommunications. He made public statements citing various schemes to accomplish this from the mid-1890s on. By the end of 1900 Tesla had convinced banker J. P. Morgan to finance the construction of a wireless station (eventually sited at Wardenclyffe) based on his ideas intended to transmit messages across the Atlantic to England and to ships at sea. Almost as soon as the contract was signed Tesla decided to scale up the facility and add his ideas of wireless power transmission to better compete with Guglielmo Marconi's radio based telegraph system. Morgan refused to fund the changes and, when no additional investment could be found, the project at Wardenclyffe was abandoned in 1906, never to become operational.
During this period Tesla filed numerous patents on the basic functions of his system including transformer designs, transmissions schemes, tuning circuits, and methods of signaling. He also described his plan to have 30 Wardenclyffe style towers around the world that would be tied into existing telephone and telegraph systems. Tesla would continue to elaborate to the press and in his writings for the next few decades on his system's capability's and how it was superior to radio based systems.

Despite Tesla's claims that he had "carried on practical experiments in wireless transmission"[2] there is no evidence Tesla ever transmitted power beyond laboratory-scale distances and most modern scientific opinion is that his wireless power scheme would not have worked.

History


Illustration of Tesla in 1891 showing two exhausted tubes illuminated by a rapidly alternating electrostatic field created between two metallic sheets.[3][4][5]

Nikola Tesla sits in front of a spiral coil from a high-voltage transformer at his East Houston St., New York laboratory in 1896

Origins

Tesla's idea's for a world wireless system grew out of experiments he begun in the early 1890s after he learned of Hertz's experiments with electromagnetic waves using coils and spark gaps.[6][7] He duplicated those experiments and went on to develop various alternator apparatus and developed his own very large air-gapped coil, known now as a Tesla coil.[8][9] Tesla's primary interest in wireless phenomenon was as a power distribution system, early on pursuing wireless lighting.[10] From 1891 on Tesla was delivering lectures including "Experiments with Alternate Currents of High Potential and High Frequency" in 1892 in London and in Paris and went on to demonstrate "wireless lighting"[11] in 1893[12] including lighting Geissler tubes wirelessly.

One-wire transmission

The first experiment was the operation of light and motive devices connected by a single wire to one terminal of a high frequency induction coil, performed during the 1891 New York City lecture at Columbia College. While a single terminal incandescent lamp connected to one of an induction coil’s secondary terminals does not form a closed circuit “in the ordinary acceptance of the term”[13] the circuit is closed in the sense that a return path is established back to the secondary by capacitive coupling or 'displacement current'. This is due to the lamp’s filament or refractory button capacitance relative to the coil’s free terminal and environment; the free terminal also has capacitance relative to the lamp and environment.

Wireless transmission

The second result demonstrated how energy can be made to go through space without any connecting wires. The wireless energy transmission effect involves the creation of an electric field between two metal plates, each being connected to one terminal of an induction coil’s secondary winding. A gas discharge tube) was used as a means of detecting the presence of the transmitted energy. Some demonstrations involved lighting of two partially evacuated tubes in an alternating electrostatic field while held in the hand of the experimenter.[14]

In his wireless transmission lectures Tesla proposed the technology could include the telecommunication of information.

Development


Drawing from the 1897 U.S. Patent 649,621, "Apparatus for Transmission of Electrical Energy" showing Tesla's idea of transmitting electrical energy through the upper atmosphere via balloons tethered at 30,000 feet.

Tesla (like many scientists of that time[15]) thought, even if radio waves existed, they would probably only travel in straight lines making them useless for long range transmission. He theorized that transmitting electrical signals any distance would have to use the planet Earth or some other medium to overcome this limitation.[16] By the end of 1895 he was making statements to the press on the possibility that "the earth's electrical charge can be distributed, and thereby electrical waves efficiently transmitted to any distance without the use of cables or wires" to transmit "intelligible signals" and "motive power".[17] In April 11, 1896 Tesla made another statement that he believed that "messages might be conducted to all parts of the globe simultaneously" using electric waves "propagated through the atmosphere and even the ether beyond".[18] In September of 1897 he applied for a patent[19] on a wireless power transmission scheme consisting of transmitting power between two tethered balloons maintained at 30,000 feet, an altitude where he thought a highly conductive layer would exist.

Between 1895 - 1898 Tesla constructed a large coil in his New York City lab called a magnifying transmitter to test his earth conduction theories.[20] In 1899 Tesla conducted large scale experiments at Colorado Springs, Colorado. From his measurements there he concluded the Earth was "literally alive with electrical vibrations." He noted that lightning strikes seemed to show the Earth was indeed a big conductor with the waves of energy from each strike going from one side of the Earth to the other. At Colorado Springs he also constructed a large magnifying transmitter measuring fifty-one feet (15.5 m) in diameter which could develop a working potential estimated at 3.5 million to 4 million volts and was capable of producing electrical discharges exceeding one hundred feet (30 m) in length.[21] With it he tested earth conduction and lit un-connected electric lights outside his lab in a demonstration of wireless power transmission at relatively short ranges.

In Colorado Springs, a tuned coil in resonance with a transmitter illuminates a light near the bottom of the picture.[22] Tesla did not disclose how far away the transmitter was.[22][23]

After Tesla returned to New York City from Colorado Springs in 1900 he sought venture capitalists to fund what he thought was revolutionary wireless communication and electric power delivery system using the Earth as the conductor. At the end of 1900 he gained the attention of financier J. P. Morgan who agreed to fund a pilot project (later to become the Wardenclyffe project) which, based on Tesla's theories, would be capable of transmitting messages, telephony, and even facsimile images across the Atlantic to England and to ships at sea. Morgan was to receive a controlling share in the company as well as half of all the patent income. Tesla's decision in July 1901 to scale up the facility and add his ideas of wireless power transmission to better compete with Guglielmo Marconi's new radio based telegraph system was met with Morgan's refusal to fund the changes.

Construction on the Wardenclyffe "wireless plant" in Shoreham started towards the end of 1901 and continued for the next 3 years. The plant included a Stanford White designed 94 by 94 ft (29 by 29 m) brick building, a wood-framed tower 186 feet (57 m) tall with a 68 feet (21 m) in diameter "cupola" on top, and a 120 feet (37 m) shaft sunk into the ground with sixteen iron pipes driven 300 feet (94.4 m) horizontal from the shaft in order for the machine, in Tesla's words, "to have a grip on the earth so the whole of this globe can quiver."[24][25] Funding problems continued to plague Wardenclyffe and by 1905-1906 most of the site's activity had to be shut down.

Elements


Tesla basic patent drawing for the grounded resonant transformer used at Wardenclyffe
U.S. Patent 1,119,732

Through the latter part of the 1890s and during the construction of Wardenclyffe Tesla applied for many patents covering the elements that would make up his wireless system. The system Tesla came up with was based on electrical conduction with an electrical charge being conducted through the ground and being returned through the air.[26] It consisted of a grounded Tesla coil as a resonance transformer transmitter that he theorized would be able to create a displacement of Earth's electric charge by alternately charging and discharging the oscillator's elevated terminal. This would work in conjunction with a second coil at a distant location with the grounded helical resonator of that Tesla Coil and an elevated terminal used in receive mode.[27][28][29] Tesla believed that the placement of a grounded resonance transformer at another point on the Earths surface in the roll of a receiver tuned to the same frequency as the transmitter would allow electric current flowing through the Earth between the two. He also believed waves of electric current from the sending tower would reflect back from the far side of the globe, resulting in amplified stationary waves of electric current that he could direct to any point on the globe, localizing power delivery directly to the receiving station. The other part of his system was electricity returned via "an equivalent electric displacement""[30] in the atmosphere via a charged conductive upper layer that Tesla thought existed,[26] a theory dating back to an 1872 idea for a proposed wireless power system by Mahlon Loomis.[31] The current could be used at the receiver to drive electrical devices.[28]

Tesla told a friend his plans included the building of more than thirty transmission-reception stations constructed near major population centers around the world[32] with Wardenclyffe being the first. If plans had moved forward without interruption the Long Island prototype would have been followed by a second plant built in the British Isles, perhaps on the west coast of Scotland near Glasgow. Each of these facilities would include a large magnifying transmitter of a design loosely based upon the apparatus assembled at the Colorado Springs experimental station in 1899.[33]

Claimed applications


1925 artist's conception of what Nikola Tesla's wireless power transmission system might look like in the future, powering aircraft and lighting the city in the background.

Tesla description of his wireless transmission ideas in 1895 included its humanitarian uses in bringing electricity to remote parts of the world and fostering greater communications amongst nations.[34] His June, 1900 Century Magazine article "The Problem of Increasing Human Energy" included photographs of his Colorado Springs experiments and his thoughts on how Hertz experiments were not "an experimental verification of the poetical conceptions of Maxwell". He elaborated on his system of using the Earth as the medium for communication and power delivery. He wrote how he could set up communication at any distance. He noted his system would localize electrical delivery to a point on the globe via stationary waves. He also noted this same process could be used to locate objects such as icebergs or ships at sea.

In 1909 Tesla stated:
"It will soon be possible, for instance, for a business man in New York to dictate instructions and have them appear instantly in type in London or elsewhere. He will be able to call up from his desk and talk with any telephone subscriber in the world. It will only be necessary to carry an inexpensive instrument not bigger than a watch, which will enable its bearer to hear anywhere on sea or land for distances of thousands of miles. One may listen or transmit speech or song to the uttermost parts of the world."[35][36]
Tesla also thought that electrical current flowing through the charged layer in the atmosphere would make it glow, providing night time lighting for cities and shipping lanes.[31]

Tesla elaborated on his world wireless system in his 1919 Electrical Experimenter article titled "The True Wireless", detailing its ability for long range communication and putting forward his views that radio based communication could not possibly work as theorized.[37][38]

Feasibility

Although Tesla demonstrated wireless power transmission at Colorado Springs, lighting electric lights mounted outside the building where he had his large experimental coil,[39] and would make claims afterwards that he had "carried on practical experiments in wireless transmission"[40] he did not scientifically test his theories. He believed he had achieved Earth resonance at Colorado Springs which, according to his theory, would work at any distance.[41] There is no evidence Tesla ever transmitted power beyond these laboratory-scale distances[23][42][43][44][45][46][47][48][49] and most modern scientific opinion is that his wireless power scheme would not have worked.[42][43][47][49][50][51][52][53][54]

Related patents

  • SYSTEM OF ELECTRIC LIGHTING, April 25, 1891, U.S. Patent 454,622, June 23, 1891.
  • MEANS FOR GENERATING ELECTRIC CURRENTS, August 2, 1893, U.S. Patent 514,168, February 6, 1894.
  • ELECTRICAL TRANSFORMER, March 20, 1897, U.S. Patent 593,138, November 2, 1897.
  • METHOD AND APPARATUS FOR CONTROLLING MECHANISM OF MOVING VESSEL OR VEHICLES, July 1, 1898, U.S. Patent 613,809 November 8, 1898.
  • SYSTEM OF TRANSMISSION OF ELECTRICAL ENERGY, September 2, 1897, U.S. Patent 645,576, March 20, 1900.
  • APPARATUS FOR TRANSMISSION OF ELECTRICAL ENERGY, September 2, 1897, U.S. Patent 649,621, May 15, 1900.
  • METHOD OF INTENSIFYING AND UTILIZING EFFECTS TRANSMITTED THROUGH NATURAL MEDIA, June 24, 1899, U.S. Patent 685,953, November 5, 1901.
  • METHOD OF UTILIZING EFFECTS TRANSMITTED THROUGH NATURAL MEDIA, August 1, 1899, U.S. Patent 685,954, November 5, 1901.
  • APPARATUS FOR UTILIZING EFFECTS TRANSMITTED FROM A DISTANCE TO A RECEIVING DEVICE THROUGH NATURAL MEDIA, June 24, 1899, U.S. Patent 685,955, November 5, 1901.
  • APPARATUS FOR UTILIZING EFFECTS TRANSMITTED THROUGH NATURAL MEDIA, March 21, 1900, U.S. Patent 685,956, November 5, 1901.
  • METHOD OF SIGNALING, July 16, 1900, U.S. Patent 723,188, March 17, 1903.
  • SYSTEM OF SIGNALING, July 16, 1900, U.S. Patent 725,605, April 14, 1903.
  • ART OF TRANSMITTING ELECTRICAL ENERGY THROUGH THE NATURAL MEDIUMS, May 16, 1900, U.S. Patent 787,412, April 18, 1905.
  • ART OF TRANSMITTING ELECTRICAL ENERGY THROUGH THE NATURAL MEDIUMS, April 17, 1906, Canadian Patent 142,352, August 13, 1912.
  • APPARATUS FOR TRANSMITTING ELECTRICAL ENERGY, January 18, 1902, U.S. Patent 1,119,732, December 1, 1914.

Operator (computer programming)

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Operator_(computer_programmin...