Scegli il punto di consegna e ritira quando vuoi Scopri come. Descriptions of energization, power quality, cable safety constraints and more, guide readers in cable planning and power network operations. They also provide technical and economical comparisons of a variety of cables and analysis methods, in order to examine the performance of AC power transmission systems.
A range of topics are covered, including: energization and de-energization phenomena of transmission lines; power quality; and cable safety constraints. It will enable readers to make performance comparisons between power transmission systems, which will be valuable for postgraduates, as well as engineers involved in power cable manufacturing or electrical transmission systems. He received his Dr. His main fields of research are multiconductor analysis, EHV-HV transmission lines and advanced matricial techniques for static and dynamic power system analysis.
Prodotti correlati. Power Electrical Systems. 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 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. At the power stations , the power is produced at a relatively low voltage between about 2. The 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.
Measures to reduce corona losses include conductors having larger diameters; often hollow to save weight,  or bundles of two or more conductors. Factors that affect the resistance, and thus loss, of conductors used in transmission and distribution lines include temperature, spiraling, and the skin effect. The resistance of a conductor increases with its temperature. Temperature changes in electric power lines can have a significant effect on power losses in the line.
Spiraling, which refers to the way stranded conductors spiral about the center, also contributes to increases in conductor resistance. The skin effect causes the effective resistance of a conductor to increase at higher alternating current frequencies. Corona and resistive losses can be estimated using a mathematical model. Transmission and distribution losses in the USA were estimated at 6. As of , the longest cost-effective distance for direct-current transmission was determined to be 7, kilometres 4, miles.
For alternating current it was 4, kilometres 2, miles , though all transmission lines in use today are substantially shorter than this. In any alternating current transmission line, the inductance and capacitance of the conductors can be significant. 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 the product of a circuit's voltage and current, without reference to phase angle 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 ; and flexible AC transmission systems , FACTS throughout the system help to compensate for the reactive power flow, reduce the losses in power transmission and stabilize system voltages. These measures are collectively called 'reactive support'. Current flowing through transmission lines induces a magnetic field that surrounds the lines of each phase and affects the inductance of the surrounding conductors of other phases.
The mutual inductance of the conductors is partially dependent on the physical orientation of the lines with respect to each other. Three-phase power transmission lines are conventionally strung with phases separated on different vertical levels. The mutual inductance seen by a conductor of the phase in the middle of the other two phases will be different than the inductance seen by the conductors on the top or bottom. An imbalanced inductance among the three conductors is problematic because it may result in the middle line carrying a disproportionate amount of the total power transmitted.
Similarly, an imbalanced load may occur if one line is consistently closest to the ground and operating at a lower impedance. Because of this phenomenon, conductors must be periodically transposed along the length of the transmission line so that each phase sees equal time in each relative position to balance out the mutual inductance seen by all three phases.
To accomplish this, line position is swapped at specially designed transposition towers at regular intervals along the length of the transmission line in various transposition schemes. 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. 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. At the substations , transformers reduce the voltage to a lower level for distribution to commercial and residential users.
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 power transmission allows for lesser resistive losses over long distances in the wiring. This efficiency of high voltage transmission allows for the transmission of a larger proportion of the generated power to the substations and in turn to the loads, translating to operational cost savings. As a consequence, the useful power used at the point of consumption is:. Assume now that a transformer converts high-voltage, low-current electricity transported by the wires into low-voltage, high-current electricity for use at the consumption point.
The useful power is then:.
Oftentimes, we are only interested in the terminal characteristics of the transmission line, which are the voltage and current at the sending and receiving ends. The line is assumed to be a reciprocal, symmetrical network, meaning that the receiving and sending labels can be switched with no consequence. The transmission matrix T also has the following properties:.
The parameters A , B , C , and D differ depending on how the desired model handles the line's resistance R , inductance L , capacitance C , and shunt parallel, leak conductance G. The four main models are the short line approximation, the medium line approximation, the long line approximation with distributed parameters , and the lossless line. In all models described, a capital letter such as R refers to the total quantity summed over the line and a lowercase letter such as c refers to the per-unit-length quantity.
The lossless line approximation is the least accurate model; it is often used on short lines when the inductance of the line is much greater than its resistance. For this approximation, the voltage and current are identical at the sending and receiving ends. The characteristic impedance is pure real, which means resistive for that impedance, and it is often called surge impedance for a lossless line. When lossless line is terminated by surge impedance, there is no voltage drop. Though the phase angles of voltage and current are rotated, the magnitudes of voltage and current remain constant along the length of the line.
For a short line, only a series impedance Z is considered, while C and G are ignored. The associated transition matrix for this approximation is therefore:. In this model, the series impedance and the shunt current leak conductance are considered, with half of the shunt conductance being placed at each end of the line. The analysis of the medium line brings one to the following result:. Series resistance and shunt conductance are considered as distributed parameters, meaning each differential length of the line has a corresponding differential resistance and shunt admittance. For the full development of this model, see the Telegrapher's equations.
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 long submarine cables where AC cannot be used because of the cable capacitance. 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:.
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Such a system could be less prone to failure if parts of it were suddenly shut down. 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 kilometres 62 miles , 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.
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This angle varies depending on system loading and generation. It is undesirable for the angle to approach 90 degrees, as the power flowing decreases but the resistive losses remain. 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. 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.
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.
Electric power transmission
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 locally distributed storage and generation systems can be connected to the power grid, and interact with it to improve system operation.
Internationally, the trend has been a slow move from a heavily centralized power system to a decentralized power system. The main draw of locally distributed generation systems which involve a number of new and innovative solutions is that they reduce transmission losses by leading to consumption of electricity closer to where it was produced. 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. 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.
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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. Some regulators regard electric transmission to be a natural monopoly   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. OMEL . Spain's transmission system is interconnected with those of France, Portugal, and Morocco. In regions of separation, transmission owners and generation owners continue to interact with each other as market participants with voting rights within their RTO. 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. Merchant transmission is an arrangement where a third party constructs and operates electric transmission lines through the franchise area of an unrelated incumbent utility. 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.
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. 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 incumbent utility businesses with a monopolized and regulated rate base. Some large studies, including a large study in the United States, have failed to find any link between living near power lines and developing any sickness or diseases, such as cancer. A 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. 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. The study's case number is too old to be listed as a case number in the commission's online database, DMM, and so the original study can be difficult to find. The study chose to utilize the electric field strength that was measured at the edge of an existing but newly built right-of-way on a kV transmission line from New York to Canada, 1.
The opinion also limited the voltage of all new transmission lines built in New York to kV. This study established a magnetic field interim standard of mG at the edge of the right-of-way using the winter-normal conductor rating. This later document can also be difficult to find on the NYSPSC's online database, since it predates the online database system.
As a comparison with everyday items, a hair dryer or electric blanket produces a mG - mG magnetic field. An electric razor can produce 2. Whereas electric fields can be shielded, magnetic fields cannot be shielded, but are usually minimized by optimizing the location of each phase of a circuit in cross-section. When a new transmission line is proposed, within the application to the applicable regulatory body usually a public utility commission , there is often an analysis of electric and magnetic field levels at the edge of rights-of-way.
These analyses are performed by a utility or by an electrical engineering consultant using modelling software. At least one state public utility commission has access to software developed by an engineer or engineers at the Bonneville Power Administration to analyze electric and magnetic fields at edge of rights-of-way for proposed transmission lines. Often, public utility commissions will not comment on any health impacts due to electric and magnetic fields and will refer information seekers to the state's affiliated department of health. 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.
These levels exceed average residential power-frequency magnetic fields in homes, which are about 0. The Earth's natural geomagnetic field strength varies over the surface of the planet between 0. Tree Growth Regulator and Herbicide Control Methods may be used in transmission line right of ways  which may have health effects.
It was originally established by Congress in as the Federal Power Commission and has since undergone multiple name and responsibility modifications. That which is not regulated by FERC, primarily electric power distribution and the retail sale of power, is under the jurisdiction of state authority. Two of the more notable U.
Order No. The legal and policy cornerstone of these rules is to remedy undue discrimination in access to the monopoly owned transmission wires that control whether and to whom electricity can be transported in interstate commerce. These tariffs allow any electricity generator to utilize the already existing power lines for the transmission of the power that they generate. EPAct gave FERC significant new responsibilities including but not limited to the enforcement of electric transmission reliability standards and the establishment of rate incentives to encourage investment in electric transmission.
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. 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 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 energy act giving the Department of Energy the authority to approve transmission if states refuse to act. 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.
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