Electric power transmission

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2022) (Learn how and when to remove this template message )

High-voltage power transmission allows for lesser resistive losses over long distances. This efficiency delivers a larger proportion of the generated power to the loads.

In a simplified model, the grid delivers electricity from an

${\displaystyle V}$

${\displaystyle P_{V}}$

${\displaystyle R}$

${\displaystyle R_{C}}$

If the resistances are

, because the same current ${\displaystyle I={\frac {V}{R+R_{C}}}}$ runs through the wire resistance and the powered device. As a consequence, the useful power (at the point of consumption) is:

${\displaystyle P_{R}=V_{2}\times I=V{\frac {R}{R+R_{C}}}\times {\frac {V}{R+R_{C}}}={\frac {R}{R+R_{C}}}\times {\frac {V^{2}}{R+R_{C}}}={\frac {R}{R+R_{C}}}P_{V}}$

Should an

ideal transformer

convert high-voltage, low-current electricity into low-voltage, high-current electricity with a voltage ratio of ${\displaystyle a}$ (i.e., the voltage is divided by ${\displaystyle a}$ and the current is multiplied by ${\displaystyle a}$ in the secondary branch, compared to the primary branch), then the circuit is again equivalent to a voltage divider, but the wires now have apparent resistance of only ${\displaystyle R_{C}/a^{2}}$. The useful power is then:

${\displaystyle P_{R}=V_{2}\times I_{2}={\frac {a^{2}R\times V^{2}}{(a^{2}R+R_{C})^{2}}}={\frac {a^{2}R}{a^{2}R+R_{C}}}P_{V}={\frac {R}{R+R_{C}/a^{2}}}P_{V}}$

For ${\displaystyle a>1}$ (i.e. conversion of high voltage to low voltage near the consumption point), a larger fraction of the generator's power is transmitted to the consumption point and a lesser fraction is lost to Joule heating.

Modeling

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2022) (Learn how and when to remove this template message)

The terminal characteristics of the transmission line are the voltage and current at the sending (S) and receiving (R) ends. The transmission line can be modeled as a black box and a 2 by 2 transmission matrix is used to model its behavior, as follows:

${\displaystyle {\begin{bmatrix}V_{\mathrm {S} }\\I_{\mathrm {S} }\\\end{bmatrix}}={\begin{bmatrix}A&B\\C&D\\\end{bmatrix}}{\begin{bmatrix}V_{\mathrm {R} }\\I_{\mathrm {R} }\\\end{bmatrix}}}$

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 has the properties:

• ${\displaystyle \det(T)=AD-BC=1}$
• ${\displaystyle A=D}$

The parameters A, B, C, and D differ depending on how the desired model handles the line's

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 such models, 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.

Lossless line

The lossless line approximation is the least accurate; it is typically used on short lines where the inductance is much greater than the 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. When a lossless line is terminated by surge impedance, the voltage does not drop. Though the phase angles of voltage and current are rotated, the magnitudes of voltage and current remain constant along the line. For load > SIL, the voltage drops from sending end and the line consumes VARs. For load < SIL, the voltage increases from the sending end, and the line generates VARs.

Short line

The short line approximation is normally used for lines shorter than 80 km (50 mi). There, only a series impedance Z is considered, while C and G are ignored. The final result is that A = D = 1 per unit, B = Z Ohms, and C = 0. The associated transition matrix for this approximation is therefore:

${\displaystyle {\begin{bmatrix}V_{\mathrm {S} }\\I_{\mathrm {S} }\\\end{bmatrix}}={\begin{bmatrix}1&Z\\0&1\\\end{bmatrix}}{\begin{bmatrix}V_{\mathrm {R} }\\I_{\mathrm {R} }\\\end{bmatrix}}}$

Medium line

The medium line approximation is used for lines running between 80 and 250 km (50 and 155 mi). The series impedance and the shunt (current leak) conductance are considered, placing half of the shunt conductance at each end of the line. This circuit is often referred to as a nominal π (pi) circuit because of the shape (π) that is taken on when leak conductance is placed on both sides of the circuit diagram. The analysis of the medium line produces:

${\displaystyle {\begin{aligned}A&=D=1+{\frac {GZ}{2}}{\text{ per unit}}\\B&=Z\Omega \\C&=G{\Big (}1+{\frac {GZ}{4}}{\Big )}S\end{aligned}}}$

Counterintuitive behaviors of medium-length transmission lines:

• voltage rise at no load or small current (Ferranti effect)
• receiving-end current can exceed sending-end current

Long line

The long line model is used when a higher degree of accuracy is needed or when the line under consideration is more than 250 km (160 mi) long. Series resistance and shunt conductance are considered to be distributed parameters, such that each differential length of the line has a corresponding differential series impedance and shunt admittance. The following result can be applied at any point along the transmission line, where ${\displaystyle \gamma }$ is the propagation constant.

${\displaystyle {\begin{aligned}A&=D=\cosh(\gamma x){\text{ per unit}}\\[3mm]B&=Z_{c}\sinh(\gamma x)\Omega \\[2mm]C&={\frac {1}{Z_{c}}}\sinh(\gamma x)S\end{aligned}}}$

To find the voltage and current at the end of the long line, ${\displaystyle x}$ should be replaced with ${\displaystyle l}$ (the line length) in all parameters of the transmission matrix. This model applies the Telegrapher's equations.

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 transmitted over very long distances, the power lost in AC transmission becomes appreciable and it is less expensive to use direct current instead. For a long transmission line, these lower losses (and reduced construction cost of a DC line) can offset the cost of the required converter stations at each end.

HVDC is used for long submarine cables where AC cannot be used because of cable capacitance.^[31] In these cases special high-voltage cables are used. Submarine HVDC systems are often used to interconnect the electricity grids of islands, for example, between Great Britain and continental Europe, between Great Britain and Ireland, between Tasmania and the Australian mainland, between the North and South Islands of New Zealand, between New Jersey and New York City, and between New Jersey and Long Island. Submarine connections up to 600 kilometres (370 mi) in length have been deployed.^[32]

HVDC links can be used to control grid problems. 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 allows the systems at either end to fall out of step. Since the power flow in a DC link is controlled independently of the phases of the AC networks that it connects, 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 grid. With an HVDC line instead, such an interconnection would:

• Convert AC in Seattle into HVDC;
• Use HVDC for the 3,000 miles (4,800 km) of cross-country transmission; and
• 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

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2022) (Learn how and when to remove this template message)

The amount of power that can be sent over a transmission line varies with the length of the line. The heating of short line 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 overheat. For intermediate-length lines on the order of 100 kilometres (62 miles), the limit is set by the

This angle varies depending on system loading. It is undesirable for the angle to approach 90 degrees, as the power flowing decreases while resistive losses remain. The product of line length and maximum load is approximately proportional to the square of the system voltage. Series capacitors or phase-shifting transformers are used on long lines to improve stability. HVDC lines are restricted only by thermal and voltage drop limits, since the phase angle is not material.

Understanding the temperature distribution along the cable route became possible with the introduction of

For overhead cables the fiber is integrated into the core of a phase wire. The integrated Dynamic Cable Rating (DCR)/Real Time Thermal Rating (RTTR) solution makes it possible to run the network to its maximum. It allows the operator to predict the behavior of the transmission system to reflect major changes to its initial operating conditions.

Control

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2022) (Learn how and when to remove this template message)

To ensure safe and predictable operation, system components 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.

Load balancing

The transmission system provides for base load and peak load capability, with margins for safety and fault tolerance. Peak load times vary by region largely due to the industry mix. In hot and cold climates home air conditioning and heating loads affect 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. Power requirements vary by season and 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 loads with generation. Thus generation has to be kept matched to the load, to prevent overloading 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. This involves

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 signaling mechanisms to balance the loads.

In voltage signaling, voltage is varied 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, virtual power plants, and other locally distributed storage and generation systems can interact with the grid to improve system operation. Internationally, a slow move from a centralized to decentralized power system has taken place. The main draw of locally distributed generation systems is that they reduce transmission losses by leading to consumption of electricity closer to where it was produced.^[33]

Failure protection

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

Rolling blackouts (also called load shedding) are intentionally engineered electrical power outages, used to distribute insufficient power to various loads in turn.

Communications

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2022) (Learn how and when to remove this template message)

Grid operators require reliable communications to manage the grid and associated generation and distribution facilities. Fault-sensing protective relays at each end of the line must communicate to monitor the flow of power 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, while in some remote areas no common carrier is available. Communication systems associated with a transmission project may use:

• Microwaves
• Power-line communication
• Optical fibers

Rarely, and for short distances, pilot-wires are strung along the transmission line path. Leased circuits from common carriers are not preferred since availability is not under control of the operator.

Transmission lines can be used to carry data: this is called power-line carrier, or power-line communication (PLC). PLC signals can be easily received with a radio in 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

Market structure

Electricity transmission is generally considered to be a natural monopoly, but one that is not inherently linked to generation.^[34][35][36] Many countries regulate transmission separately from generation.

Spain was the first country to establish a

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, New Jersey, to New Bridge, New York, 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.^[38]

Australia has one unregulated or market interconnector -

The cost of high voltage transmission is comparatively low, compared to all other costs constituting consumer electricity bills. In the UK, transmission costs are about 0.2 p per kWh compared to a delivered domestic price of around 10 p per kWh.[42]

The level of capital expenditure in the electric power T&D equipment market was estimated to be $128.9 bn in 2011.^[43]

Health concerns

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 failed to find any link between living near power lines and developing any sickness or diseases, such as cancer. A 1997 study reported no increased risk of cancer or illness from living near a transmission line.

Applications for a new transmission line typically include an analysis of electric and magnetic field levels at the edge of rights-of-way. Public utility commissions typically do not comment on health impacts.

Biological effects have been established for acute high level exposure to magnetic fields above 100 µT (1 G) (1,000 mG). In a residential setting, one study reported "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.^[49][50]

The Earth's natural geomagnetic field strength varies over the surface of the planet between 0.035 mT and 0.07 mT (35 µT – 70 µT or 350 mG – 700 mG) while the international standard for continuous exposure is set at 40 mT (400,000 mG or 400 G) for the general public.^[49]

Tree growth regulators and herbicides may be used in transmission line right of ways,^[51] which may have health effects.

Specialized transmission

Grids for railways

In some countries where electric locomotives or electric multiple units run on low frequency AC power, separate single phase traction power networks are operated by the railways. Prime examples are countries such as Austria, Germany and Switzerland that utilize AC technology based on 16 ²/₃ Hz. Norway and Sweden also use this frequency but use conversion from the 50 Hz public supply; Sweden has a 16 ²/₃ Hz traction grid but only for part of the system.

Superconducting cables

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 is costly.^[54]

Single-wire earth return

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2022) (Learn how and when to remove this template message)

Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line for supplying single-phase electrical power 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

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2022) (Learn how and when to remove this template message)

Both Nikola Tesla and Hidetsugu Yagi attempted to devise systems for large scale wireless power transmission in the late 1800s and early 1900s, without 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

. Major engineering and economic challenges face any solar power satellite project.

Security

The examples and perspective in this article may not represent a worldwide view of the subject. You may improve this article, discuss the issue on the talk page, or create a new article, as appropriate. (March 2013) (Learn how and when to remove this template message)

The

In June 2019, Russia conceded that it was "possible" its electrical grid is under cyber-attack by the United States.^[63] The New York Times reported that American hackers from the United States Cyber Command planted malware potentially capable of disrupting the Russian electrical grid.^[64]

Records

• Highest capacity system: 12 GW Zhundong–Wannan (准东-皖南)±1100 kV HVDC.^[65][66]
• Highest transmission voltage (AC):
  • planned: 1.20 MV (Ultra-High Voltage) on Wardha-Aurangabad line (India) – under construction. Initially will operate at 400 kV.^[67]
  • worldwide: 1.15 MV (Ultra-High Voltage) on
    Ekibastuz-Kokshetau line (Kazakhstan
    )
• Largest double-circuit transmission,
  Kita-Iwaki Powerline
  (Japan).
• Highest
  Yangtze River Crossing
  (China) (height: 345 m or 1,132 ft)
• Longest power line:
  Democratic Republic of Congo
  ) (length: 1,700 kilometres or 1,056 miles)
• Longest span of power line: 5,376 m (17,638 ft) at Ameralik Span (Greenland, Denmark)
• Longest submarine cables:
  • North Sea Link, (Norway/United Kingdom) – (length of submarine cable: 720 kilometres or 447 miles)
  • NorNed, North Sea (Norway/Netherlands) – (length of submarine cable: 580 kilometres or 360 miles)
  • Basslink, Bass Strait, (Australia) – (length of submarine cable: 290 kilometres or 180 miles, total length: 370.1 kilometres or 230 miles)
  • Baltic Cable, Baltic Sea (Germany/Sweden) – (length of submarine cable: 238 kilometres or 148 miles, HVDC length: 250 kilometres or 155 miles, total length: 262 kilometres or 163 miles)
• Longest underground cables:
  • Murraylink, Riverland/Sunraysia (Australia) – (length of underground cable: 170 kilometres or 106 miles)

See also

References