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Electrical Energy Systems

Electric power transmission

From Wikipedia, the free encyclopedia

 
TrnsmnLine7
Electric power transmission is one process in the transmitting of electricity to consumers. The term refers to the bulk transfer of electrical power from place to place. Typically, power transmission is between the power plant and a substation near a populated area. This is distinct from electricity distribution, which is concerned with the delivery from the substation to the consumers. Due to the large amount of power involved, transmission normally takes place at high voltage (66 kV or above). Electricity is usually transmitted over long distance through overhead power transmission lines (such as those in the photo on the right). Underground power transmission is used only in densely populated areas (such as large cities) because of the high cost of installation and maintenance.

A power transmission system is sometimes referred to colloquially as a "grid"; however, for reasons of economy, the network is rarely a true grid. Redundant paths and lines are provided so that power can be routed from any power plant to any load center, through a variety of routes, based on the economics of the transmission path and the cost of power. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line, which, due to system stability considerations, may be less than the physical or thermal limit of the line. Deregulation of electricity companies in many countries has led to renewed interest in reliable economic design of transmission networks. The separation of transmission and generation functions is one of the factors that contributed to the 2003 North America blackout.

 

AC power transmission

AC power transmission is the transmission of electric power by alternating current. Usually transmission lines use three phTrnsmnLine6ase AC current. In electric railways, single phase AC current is sometimes used in a railway electrification system. In urban areas, trains may be powered by DC at 600 volts or so on the "third rail."

Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages such as 66 kV and 33 kV areusually considered sub-transmission voltages but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 230 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages.

Due to costs, power lines that are not underground are not usually insulated. However, this makes accidents more likely.

 

History

In an AIEE Address, May 16, 1888, Nikola Tesla delivered a lecture entitled A New System of Alternating Current Motors andTransformers, describing the equipment which allowed efficient generation and use of alternating currents. Tesla's disclosures, in the form of patents, lectures and technical articles, are useful for understanding the history of the modern system of power transmission.

The first transmission of three-phase alternating current using high voltage took place in the year 1891 on the occasion of the international electricity exhibition in Frankfurt. In that year, a 25 kV transmission line, approximately 175 kilometres long, was built between Lauffen at the Neckar and Frankfurt.

The rapid industrialization in the 20th century made electrical transmission lines and grids a critical part of the economic infrTransmnLine4PGEastructure in most industrialized nations. Initially transmission lines were supported by porcelain pin-and-sleeve insulators similar to those used for telegraph and telephone lines. However, these reached a practical limit of 40 kV. In 1907 the invention of the disc insulator by Harold W. Buck of the Niagara Falls Power Corporation and Edward M. Hewlett of General Electric allowed practical insulators of any length to be constructed, which allowed the use of higher voltages. The first large scale hydroelectric generators in the USA(engineered and installed under the technical oversight of Nikola Tesla) were installed at Niagara Falls and provided electricity to Buffalo, New York via power transmission lines.

The first three-phase alternating current power transmission at 110 kV took place in 1912 between Lauchhammer and Riesa, Germany. On April 17, 1929 the first 220 kV line in Germany was completed, running from Brauweiler near Cologne, over Kelsterbach near Frankfurt, Rheinau near Mannheim, Ludwigsburg-Hoheneck near Austria. The masts of this line were designed for eventual upgrade to 380 kV. However the first transmission at 380 kV was erected in Germany on October 5, 1957 between the substations in Rommerskirchen and Ludwigsburg-Hoheneck. In 1967 the first extra-high-voltage transmission at 735 kV took place on a Hydro-Quιbec transmission line. In 1982 the first transmission at 1200kV took place in the Soviet Union.

 

Bulk power transmission

Engineers design transmission networks to transport the energy as efficiently as possible, 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.

Efficiency is improved by increasing the transmission voltage using a voltage step-up transformer, which has the effect of reducing the current in the conductors, whilst keeping the power transmitted equal to the power input. The reduced current flowing through the conductor reduces the losses in the conductor and since according to Ohms Law, the losses are proportional to the square of the current, halving theTrnsmnLine5PCHydrocurrent results in a four-fold decrease in transmission losses.

A transmission grid is a network of power stations, transmission circuits, and substations. Energy is usually transmitted within the grid with three-phase AC. DC systems suffer from the fact that voltage conversion is expensive (and so are only used for special high voltage links) while single phase AC links suffer from oscilations in the power transmitted (very bad for the smoothness of motors and generators) and the inability to directly generate a rotating magnetic field. Other phase orders of polyphase systems are possible but two phase (90 degree separation) still needs either 3 wires with unequal currents or 4 wires and higher phase order systems need more than 3 wires for marginal benefits.

The capital cost of electric power stations is so high, and electric demand is so variable, that it is often cheaper to import some portion of the variable load than to generate it locally. Because nearby loads are often correlated (hot weather in the Southwest portion of the United States might cause many people there to turn on their air conditioners), imported electricity must often come from far away. Because of the economics of load balancing, transmission grids now span across countries and even large portions of continents. The web of interconnections between power producers and consumers ensures that power can flow even if one link is disabled.

Long-distance transmission of electricity is almost always more expensive than the transportation of the fuels used to make that electricity. As a result, there is economic pressure to locate fuel-burning power plants near the population centers that they serve. The obvious exceptions arehydroelectric turbines -- high-pressure water-filled pipes being more expensive than electric wires. The unvarying portion of the electric demand is known as the "base load", and is generally served best by facilities with low variable costs but high fixed costs, like nuclear or large coal-fired powerplants.

 

Grid input

At the generating plants the energy is produced at a relatively low voltage of up to 30 kV (Grigsby, 2001, p. 4-4), then stepped up by the power station transformer to a higher voltage for transmission over long distances to grid exit points (substations).

 

Losses

It is necessary to transmit the electricity at high voltage to reduce the percentage of energy lost. For a given amount of power transmitted, aTrnsmnLine3Finlandhigher voltage reduces the current and thus the resistive losses in the conductor. Long distance transmission is typically done with overhead lines at voltages of 110 to 1200 kV. However, at extremely high voltages, more than 2000 kV between conductor and ground, corona discharge losses are so large that they can offset the lower heating loss in the line conductors.

Transmission and distribution losses in the USA were estimated at 7.2% in 1995 [1], and in the UK at 7.4% in 1998. [2]

In an alternating current transmission line, the inductance and capacitance of the line conductors can be significant. The currents thatflow in these components of transmission line impedance constitute reactive power, which transmits no energy to the load. Reactive current flow causes extra losses in the transmission circuit. The fraction of total energy flow (power) which is resistive (as opposed to reactive) power is the power factor. Utilities add capacitor banks and other components throughout the system—such as phase-shifting transformers, static VAr compensators, and flexible AC transmission systems (FACTS)—to control reactive power flow for reduction of losses and stabilization of system voltage.

 

Power Loss

Electrical power is invariably partially lost during transmission. This applies to short distances such as between components on a printed circuit board as well as to cross country high voltage lines. Loss power is proportional to the resistance of the wire and the square of the current.

Ploss = RI2

Because of this relationship, it is favourable to transmit energy with voltages as high as possible. This reduces the current and thus the power lost during transmission.

 

Grid exit

At the substations, transformers are again used to step the voltage down to a lower voltage for distribution to commercial and residential users. This distribution is accomplished with a combination of sub-transmission (33 kV to 115 kV, varying by country and customer requirements) and distribution (3.3 to 25 kV). Finally, at the point of use, the energy is transformed to low voltage (100 to 600 V, varying by country and customer requirements).

 

Communications

Operators of long transmission lines require reliable communications for control of the power grid and, often, associated generation and distribution facilities. Fault-sensing protection 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 deenergized 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 is insufficiently reliable. In remote areas a common carrier may not be available at all. Communication systems associated with a transmission project may use:

  • Microwaves
  • power line carrier
  • Optical fibres

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 longwave range.

Sometimes there are also communications cables using the transmission line structures. These are generally fibre optic cables. They are often integrated in the ground (or earth) conductor. Sometimes a standalone cable is used, which is commonly fixed to the upper crossbar. On the EnBW system in Germany, the communication cable can be suspended from the ground (earth) conductor or strung as a standalone cable.

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<110kV double circuit powerline of EnBW AG near Leonberg, Germany with an aerial cable mounted like a garland on the ground conductor
 
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 fibres" to a common carrier, providing another revenue stream for the line.

 

 

Electricity market reform

MV_TransmnLines1
 
< Transmission towers in the New Zealand countryside

Transmission is a natural monopoly and there are moves in many countries to separately regulate transmission (see New Zealand Electricity Market). In the USA the Federal Energy Regulatory Commission had issued a notice of proposed rulemaking setting out a proposed Standard Market Design (SMD) that would see the establishment of Regional Transmission Organizations (RTOs). The first RTO in North America is the Midwest Independent Transmission System Operator (MISO) [3]. MISO's authority covers parts of the transmission grid in the United States midwest and one province of Canada (through a coordination agreement with Manitoba Hydro). MISO also operates the wholesale power market in the United States portion of this area.

In July 2005, the new FERC chairman, Joseph Kelliher announced the end of SMD efforts because "the rulemaking had been overtaken by the voluntary formation of RTOs and ISOs" according to FERC.

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 Elιctrica de Espaρa (REE) [4] and the wholesale electricity market operator is Operador del Mercado Ibιrico de Energνa - Polo Espaρol, S.A. (OMEL) [5]. Spain's transmission system is interconnected with those of France, Portugal, and Morocco.


 

A brief introduction to Electrical Energy Systems in simple language

by George Christofi and Stella Vounioti

An Electrical Energy System is a complicated concept. In our homes we are used to turn the lights on or off with a press of a button and when the lights go off without our intention, we say: “there is an interruption of electricity supply”. Why is that? What is hiding behind all that?

The lamp lights on without twinkling or having any problems when we have a relatively stable voltage level (Volts), frequency (Hertz) and uninterruptible current flow (Amperes).

Every day we consume electrical energy, i.e. by turning on the lights, the television, the personal computer, the air conditioning system, or by using the refrigerator and the washing machine. The electrical energy to be produced can be sufficient and distributed to us, the consumers, if there are enough machines to produce this energy (electrical generator units). This is called Daily Generation Availability.

Around 7 o’ clock, when each one of us goes to work, many electrical appliances are set to work. As a consequence, there is an increasing demand of electrical energy, that reaches a peak at around noon and starts declining after midnight, when there is decreased need for consuming electrical energy.

Daily Generation_max dem

The amount of electrical energy that is produced for the whole of Cyprus every second is controlled by the Transmission System Operator, with the National Energy Control Centre acting as a tool for this constant surveillance .

ECC

At the National Energy Control Centre, electrical engineers constantly control the energy that is being generated by the Power Stations, and then transmitted to the areas controlled by the Cyprus Government and then distributed to the consumers, distinguished a home, commercial, or industrial use, as shown in the picture below:

Transmission System-diagram

If there are any faults, resulting to Unplanned Interruptions of Electricity Supply (e.g. by lightning), the engineers of the National Energy Control Centre locate them and supervise their repair, cooperating with technicians of the Electricity Authority of Cyprus, who travel to the geographical spot where the fault was reported and repair it. The isolation of the faulted part of the network can be done through remote operations from the National Energy Control Centre, using computers and suitable telecommunication equipment.

All the machines that are used for the generation and transmission of electrical energy need maintenance, like every other machine. Thus, a Maintenance Schedule is prepared for all the Government controlled areas of the Cyprus Republic, for maintaining equipment of the electricity system. To do so, such equipment is being isolated from the system to be maintained, without disrupting the normal electricity flow towards the consumers.

How is the electrical energy transmitted from the electricity power station to our homes?

The electrical energy is produced in the power stations and is converted through transformers at a high voltage (132kVolts), in order to reduce the heat losses during transmission. Then, it is transferred to the transmission substations and the voltage is stepped down to a medium voltage (22 or 11kVolts). After that, the energy is distributed to the distribution substations, it is stepped down to a low voltage (415/240 Volts) and distributed to the consumers through overhead lines or underground cables. This procedure is shown in the following diagram:

Electricity Network_image

1. Power Station

2. Transmission Substation that raises Voltage

3.High Voltage Transmission Line

4.Transmission Substation that lowers the High Voltage to Medium Voltage

 

5. Medium Voltage Distribution Line

6.Distribution Substation that lowers the Medium Voltage to Low Voltage

7.Medium Voltage Distribution Line

8.Electrical supply to consumers


In the grid, i.e. the electricity network, all the electrical energy that is produced needs to be consumed at the same time. Storage of electrical energy into other forms of energy is limited. Therefore, we need to ensure that all the electrical energy that each home, factory, or office requires has to be produced adequately at the moment is needed. This is called Balancing of Generation and Demand.

Real Time Snapshot of Generation on 12:45 26/07/06

On the above map, the High Voltage Lines that transmit energy throughout Cyprus are depicted. But will these Power Lines and Power Stations be adequate for the whole island for the next 10 years?

For this reason, the Transmission System Operator is preparing studies for the building of new or upgrading of old power lines and power stations so that in the next 10 years, when we press the switch in our house the lamp will continue to light up!

In addition, the Transmission System Operator is responsible for checking whether the new producers (new connections) are compatible with the technical characteristics needed for the whole system to work. Some of the new producers use Renewable Energy Sources to generate electrical energy, such as Wind Turbine Generators in Wind Parks, which produce energy using wind potential.

Wind park

Having in mind all of the above, the Transmission and Distribution  System can be described as an “imaginary train” that never stops! During the train motion, people go on and off and at the same time wagons or machines get attached to it, so that as time goes by more people can jump in. If a wagon malfunctions, we use other wagons, to service the people that were using it, until we fix it. However, the train must never stop running! If it does, we have a general blackout! Using the same thinking, in a Transmission Distribution System we can connect new Power Stations, new consumers, and new Transmission and Distribution Lines and Substations for expansion of the Electricity System.

 
© 2017 Copyright Transmission System Operator, Cyprus