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Electric power transmission or "high voltage electric transmission" is the bulk transfer of electrical energy, from generating power plants to substations located near to population centers. This is distinct from the local wiring between high voltage substations and customers, which is typically referred to as electricity distribution. Transmission lines, when interconnected with each other, become high voltage transmission networks. In the US, these are typically referred to as "power grids" or sometimes simply as "the grid", while in the UK the network is known as the "national grid." North America has three major grids: The Western Interconnection; The Eastern Interconnection and the Electric Reliability Council of Texas (or ERCOT) grid.
Historically, transmission and distribution lines were owned by the same company, but over the last decade or so many countries have introduced market reforms that have led to the separation of the electricity transmission business from the distribution business.
Transmission lines mostly use three phase alternating current (AC), although single phase AC is sometimes used in railway electrification systems. High-voltage direct current (HVDC) technology is used only for very long distances (typically greater than 400 miles, or 600 km); submarine power cables (typically longer than 30 miles, or 50 km); or for connecting two AC networks that are not synchronized.
Electricity is transmitted at high voltages (110 kV or above) to reduce the energy lost 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 in the distribution of electricity is that, with minor exceptions, electrical energy cannot be stored, and therefore it must be generated as it is needed. A sophisticated system of control is therefore required to ensure electric generation very closely matches the demand. If supply and demand are not in balance, generation plants and transmission equipment can shut down which, in the worst cases, can lead to a major regional blackout, such as occurred in California and the US Northwest in 1996 and in the US Northeast in 1965, 1977 and 2003. To reduce the risk of such failures, electric transmission networks are interconnected into regional, national or continental wide networks thereby providing multiple redundant alternate routes for power to flow should (weather or equipment) failures occur. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line which is mostly 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.
High Voltage DC technology is an effective and technical solution for the transmission of power over very long distances. This system is an economical solution to connect grids of different frequencies or asynchronous grids, too. This technology has some better qualities compared to AC power transmission and is the only alternative to high voltage AC transmission.
Although the AC power transmission technology performs generation and distribution of energy significantly well, some jobs cannot be performed with high efficiency and low cost. The DC technology is comparatively beneficial in various fields such as power transmission over long distances, power transmission between asynchronous grids, power transmission through cables, and carrying additional power without increasing the short circuit ratio.
Economic Benefits of High Voltage DC Technology
The total cost of stabilization of power transmission for very long distance is divided into five parts. These parts are tower cost, land cost, terminal cost, transmission line cost, and capitalized cost.
The DC power transmission system reduces the cost of various parts. The reductions in the cost of power transmission due to this system are as follows.
â€¢ Actually, the economic benefits are mainly dependent on the size of the tower. The size of tower required for this system is only 30% as great compared to AC power transmission.
â€¢ Due to the requirement of smaller size towers, this system requires 50% less land compared to the AC power transmission. The smaller land requirement enhances its economical benefits.
â€¢ The transmission line cost required for this system is 33% less than the cost required for the AC transmission system. This is because the DC transmission system requires a shorter break-even-distance.
Technical Benefits of High Voltage DC Technology
The main technical feature of the DC transmission systems is high controllability when compared to AC power transmission. The other features of the system are as follows.
Benefit in load flow
In the DC transmission system the load flow is totally controlled by the operators present on both sides. However, in the AC transmission system, the load flow is uncontrolled and depends on the actual network conditions.
Benefit in peak supply
DC transmission systems have inherent overload capability; rather they work actively for the peak load. On the other hand, the overload capability the AC power system is very low and not controlled by any means.
AC network connection stabilization
The AC network connection stabilization at the ends of the power grid is comparatively easier and more accurate in the DC power transmission system.
Current Applications of High Voltage DC Technology
The DC transmission system has various applications, but here are described the three most important applications of this system.
â€¢ Submarine cable connections
The DC transmission system is best for cable transmission because of its symmetrical monopole configuration. Different cable designs are used worldwide based on this DC transmission. This system is used in such areas where submarine cables are required to connect to the main grid such as for energy platforms, offshore wind farms, island connections, and urban in-feeds.
â€¢ Back-to-back transmission
The DC transmission system is very beneficial for back-to-back transmission in many aspects such as it doesn't increase the short circuit power and restricts the spread of cascading disturbances.
â€¢ Overhead line transmission
The most economical part of a DC transmission system is to use it in overhead lines. The overhead lines using this system require narrow transmission corridors, which decreases the cost.
The high voltage DC power system also has some other benefits such as an increase in stability of the AC system and parallel AC lines. Actually, this system provides benefits mainly for long distances as it minimizes power loss during transmission. This power system is best for seabed power transmission and was used in the shared 1961 IFA power grid system of England and France. The AC power transmission system is limited in various areas, whereas the DC power transmission system has various technical and economic benefits.
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This is done in order to minimise power losses in the power distribution network due to the resistance of the transmitting cables. It should be noted that for a given cable resistance, voltage drop, and thus power dissipated in the cable and not available to use, is directly related to the current flow through the conductor.
According to Ohm's Law: P = I2 Ã- R, that is power (in this case, power lost) is equal to current squared times resistance. To deliver power, it takes amps and volts. If you raise the volts, you can reduce the amps and still get the same power. If you reduce the amps, you lower the losses. Did you notice the squared term in the formula? That means if you reduce the current 10 times lower, your losses go down to one one-hundredth of what they would have been.
This is a huge issue for the utilities. Every kW lost is one they cannot collect money for, yet they still have to pay for fuel to generate it, they have to size the generator bigger to supply it, and they have to size the transmission system to carry it. There are other good reasons too (see below), but minimizing line loss is the $main$ one. A few transmission systems have been designed at 1.2 million volts. The utilities would have billion-volt systems if they could figure out how to do it.
A major reason is that, to carry the same amount of power, if the transmission voltage is made higher, then, even though a thinner cable has a higher resistance for a given length, the cables can be made thinner and lighter in weight.
Use of a higher transmission voltage saves a tremendous amount of money in many ways. For example for the expensive material used for the cables (often a steel multi-strand core wound with an outer skin of copper, aluminium, or similar good conducting wires) and for the weight and costs of construction and erection of the towers that carry the cables across the countryside.
To carry 400 kV (= 400 kilovolts = 400 thousand Volts) the steel towers have to be taller and the porcelain insulators have to be longer than they would have to be for cables carrying lower voltages but the cost of making the towers taller and the insulators longer is far less than the cost of the extra weight of the much thicker cables that would be needed to carry the same power at a lower voltage. *** (See Note below for more explanation)
There are many other costs which have to be reckoned when deciding what voltage to use for long-distance power distribution. For example the high cost of the massive power transformers and big switching stations that have to be included in the power distribution network; the power that is lost from the cables - radiated to the surrounding air as heat - because of the electrical resistance of the materials from which the cables are made.
The above answer just gives a very simplified overview of the kinds of things a skilled power transmission engineer has to work with and calculate when designing a new power transmission network.
Long-Distance Electricity Transmission
Electricity transmission, often underappreciated and occasionally maligned, is an essential part of an economy with high energy demands and even more crucial in a carbon-constrained world.
Renewable energy resources vary in strength from place to place. The Western US and the Great Plains have some of the strongest on-land renewable energy sources (sun, wind, and geothermal); the Great Lakes and offshore locations in both Atlantic and Pacific Oceans have some very high quality wind resources.
Demand for energy is not concentrated where renewable energy is most economically harvested or available at appreciable strength, so using existing high voltage transmission and building new transmission will be part of a sustainable clean energy system. Even within regions with favorable renewable resources, people have tended to settle where the wind and sun are not quite as intense. The expense of putting transmission underground has historically been many times that of conventional high voltage lines so is only feasible in dense urban areas for short distances. On the other hand, transmission corridors are narrow and carry in them power for many millions of people: the high voltage line called the Pacific Intertie, for instance, can carry enough electricity for 5 million homes in a right of way approximately 50 feet wide.
Besides the compactness of a high-voltage line, one of the benefits of long-distance high voltage transmission is how little power is lost, even when sent several thousand miles. Transmission and distribution losses in the US grid are currently around 7% of total system power. If desert power plants were connected with load centers on the East Coast by high-voltage DC lines, just 15% of the power would be lost in transmission from end to end. Furthermore, high voltage DC lines or HVDC, have few of the electromagnetic fields that some people fear have negative health effects.
Both high-voltage AC (HVAC) and HVDC have their places in an efficient electrical distribution system. HVAC is less expensive to build for short distances and it is easier to tap into along the way allowing for more of a branch-like distribution structure. HVDC is better suited for long distance point-to-point transmission because of its lower losses and narrower footprint per unit power.
The building of new long distance transmission is key if we are to keep our twin commitments both to shielding the planet from the effects of fossil fuel combustion and to an urban, suburban and rural lifestyle that is heavily dependent upon powered devices. An effective transmission system that is connected to centers of demand and the strongest supply of renewable energy will have a lighter footprint, be less expensive, and be more quickly built than a more distributed energy system that may emerge a few decades hence. Figuring out the routes and exact technical specifications of such a system should be carried out with the utmost in transparency, spirit of compromise, and sense of urgency given the high cost of delay and lack of inclusiveness.
California through RETI and Texas through CREZ have started such a process to work out long-term state plans for siting and building transmission that will enable a clean energy future. A Southwest-wide extension of state-by-state initiatives will create a basis for a stable regional and eventually national low-carbon grid.