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90-648: The system consists of a high-voltage direct current (HVDC) back-to-back station with a transmission rate of 660 MW and DC voltage of 180 kV at 1 Railroad Avenue ( 40°49′56″N 74°00′44″W  /  40.83222°N 74.01222°W  / 40.83222; -74.01222 ) in Ridgefield, which is connected by a 230 kV line with the nearby substation. From the static inverter plant the three-phase AC line to Consolidated Edison 's W. 49th Street substation at 40°45′57″N 73°59′45″W  /  40.76583°N 73.99583°W  / 40.76583; -73.99583 starts,

180-409: A > 1. By the law of conservation of energy , apparent , real and reactive power are each conserved in the input and output: where S {\displaystyle S} is apparent power and I {\displaystyle I} is current . Combining Eq. 3 & Eq. 4 with this endnote gives the ideal transformer identity : where L {\displaystyle L}

270-497: A vector product , decreases. Since DC power has no phase, the phase shift cannot occur in the DC case. HVDC transmission may also be selected for other technical benefits. HVDC can transfer power between separate AC networks. HVDC power flow between separate AC systems can be automatically controlled to support either network during transient conditions, but without the risk that a major power-system collapse in one network will lead to

360-554: A 12 kV DC transmission line, which also served to convert 40 Hz generation to serve 60 Hz loads, at Mechanicville, New York . In 1941, a 60 MW, ±200 kV, 115 km (71 mi) buried cable link, known as the Elbe-Project , was designed for the city of Berlin using mercury arc valves but, owing to the collapse of the German government in 1945 , the project was never completed. The nominal justification for

450-403: A DC component flowing in the windings. A saturable reactor exploits saturation of the core to control alternating current. Knowledge of leakage inductance is also useful when transformers are operated in parallel. It can be shown that if the percent impedance and associated winding leakage reactance-to-resistance ( X / R ) ratio of two transformers were the same, the transformers would share

540-464: A changing magnetic flux encircled by the coil. Transformers are used to change AC voltage levels, such transformers being termed step-up or step-down type to increase or decrease voltage level, respectively. Transformers can also be used to provide galvanic isolation between circuits as well as to couple stages of signal-processing circuits. Since the invention of the first constant-potential transformer in 1885, transformers have become essential for

630-452: A circuit that is effectively an ultra-high-voltage motor drive. More recent installations, including HVDC PLUS and HVDC MaxSine, are based on variants of a converter called a Modular Multilevel Converter (MMC). Multilevel converters have the advantage that they allow harmonic filtering equipment to be reduced or eliminated altogether. By way of comparison, AC harmonic filters of typical line-commutated converter stations cover nearly half of

720-452: A collapse in the second. The controllability feature is also useful where control of energy trading is needed. Specific applications where HVDC transmission technology provides benefits include: Long undersea or underground high-voltage cables have a high electrical capacitance compared with overhead transmission lines since the live conductors within the cable are surrounded by a relatively thin layer of insulation (the dielectric ), and

810-410: A distance of 200 kilometres (120 mi), including 10 kilometres (6.2 mi) of underground cable. This system used eight series-connected generators with dual commutators for a total voltage of 150 kV between the positive and negative poles, and operated from c. 1906 until 1936. Fifteen Thury systems were in operation by 1913. Other Thury systems operating at up to 100 kV DC worked into

900-468: A given transmission line to operate with a constant HVDC voltage that is approximately the same as the peak AC voltage for which it is designed and insulated. The power delivered in an AC system is defined by the root mean square (RMS) of an AC voltage, but RMS is only about 71% of the peak voltage. Therefore, if the HVDC line can operate continuously with an HVDC voltage that is the same as the peak voltage of

990-446: A high resistance when conducting, wasting energy and generating heat in normal operation. The ABB breaker combines semiconductor and mechanical breakers to produce a hybrid breaker with both a fast break time and a low resistance in normal operation. Generally, vendors of HVDC systems, such as GE Vernova , Siemens and ABB , do not specify pricing details of particular projects; such costs are typically proprietary information between

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1080-670: A large transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. Transformers may require protective relays to protect the transformer from overvoltage at higher than rated frequency. One example is in traction transformers used for electric multiple unit and high-speed train service operating across regions with different electrical standards. The converter equipment and traction transformers have to accommodate different input frequencies and voltage (ranging from as high as 50 Hz down to 16.7 Hz and rated up to 25 kV). At much higher frequencies

1170-460: A metal sheath. The geometry is that of a long coaxial capacitor . The total capacitance increases with the length of the cable. This capacitance is in a parallel circuit with the load. Where alternating current is used for cable transmission, additional current must flow in the cable to charge this cable capacitance. Another way to look at this is to realize, that such capacitance causes a phase shift between voltage and current, and thus decrease of

1260-405: A much lesser extent. Nevertheless, for a long AC overhead transmission line, the current flowing just to charge the line capacitance can be significant, and this reduces the capability of the line to carry useful current to the load at the remote end. Another factor that reduces the useful current-carrying ability of AC lines is the skin effect , which causes a nonuniform distribution of current over

1350-493: A nameplate that indicate the phase relationships between their terminals. This may be in the form of a phasor diagram, or using an alpha-numeric code to show the type of internal connection (wye or delta) for each winding. The EMF of a transformer at a given flux increases with frequency. By operating at higher frequencies, transformers can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve

1440-432: A number of approximations. Analysis may be simplified by assuming that magnetizing branch impedance is relatively high and relocating the branch to the left of the primary impedances. This introduces error but allows combination of primary and referred secondary resistances and reactance by simple summation as two series impedances. Transformer equivalent circuit impedance and transformer ratio parameters can be derived from

1530-433: A permeability many times that of free space and the core thus serves to greatly reduce the magnetizing current and confine the flux to a path which closely couples the windings. Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy current losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires. Later designs constructed

1620-482: A result of a civil war in Mozambique . The transmission voltage of ±533 kV was the highest in the world at the time. Line-commutated converters have some limitations in their use for HVDC systems. This results from requiring a period of reverse voltage to affect the turn off. An attempt to address these limitations is the capacitor-commutated converter (CCC). The CCC has series capacitors inserted into

1710-401: A similar concept HVDC PLUS ( Power Link Universal System ) and Alstom call their product based upon this technology HVDC MaxSine . They have extended the use of HVDC down to blocks as small as a few tens of megawatts and overhead lines as short as a few dozen kilometers. There are several different variants of VSC technology: most installations built until 2012 use pulse-width modulation in

1800-413: A three-phase bridge rectifier known as a six-pulse bridge , containing six electronic switches, each connecting one of the three phases to one of the two DC rails. A complete switching element is usually referred to as a valve , irrespective of its construction. However, with a phase change only every 60°, considerable harmonic distortion is produced at both the DC and AC terminals when this arrangement

1890-468: A transformer design to limit the short-circuit current it will supply. Leaky transformers may be used to supply loads that exhibit negative resistance , such as electric arcs , mercury- and sodium- vapor lamps and neon signs or for safely handling loads that become periodically short-circuited such as electric arc welders . Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers in circuits that have

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1980-454: A valve becoming positive (at which point the valve would start to conduct if it were made from diodes) and the thyristors being turned on. The DC output voltage of the converter steadily becomes less positive as the firing angle is increased: firing angles of up to 90° correspond to rectification and result in positive DC voltages, while firing angles above 90° correspond to inversion and result in negative DC voltages. The practical upper limit for

2070-422: A varying magnetic flux in the transformer core, which is also encircled by the secondary winding. This varying flux at the secondary winding induces a varying electromotive force or voltage in the secondary winding. This electromagnetic induction phenomenon is the basis of transformer action and, in accordance with Lenz's law , the secondary current so produced creates a flux equal and opposite to that produced by

2160-462: A wider power transmission grid to another. Changes in load that would cause portions of an AC network to become unsynchronized and to separate, would not similarly affect a DC link, and the power flow through the DC link would tend to stabilize the AC network. The magnitude and direction of power flow through a DC link can be directly controlled and changed as needed to support the AC networks at either end of

2250-440: Is also known as line-commutated converter (LCC) HVDC. On March 15, 1979, a 1920 MW thyristor based direct current connection between Cabora Bassa and Johannesburg (1,410 km; 880 mi) was energized. The conversion equipment was built in 1974 by Allgemeine Elektricitäts-Gesellschaft AG (AEG) , and Brown, Boveri & Cie (BBC) and Siemens were partners in the project. Service interruptions of several years were

2340-519: Is at the expense of flux density at saturation. For instance, ferrite saturation occurs at a substantially lower flux density than laminated iron. Large power transformers are vulnerable to insulation failure due to transient voltages with high-frequency components, such as caused in switching or by lightning. Transformer energy losses are dominated by winding and core losses. Transformers' efficiency tends to improve with increasing transformer capacity. The efficiency of typical distribution transformers

2430-402: Is between about 98 and 99 percent. As transformer losses vary with load, it is often useful to tabulate no-load loss , full-load loss, half-load loss, and so on. Hysteresis and eddy current losses are constant at all load levels and dominate at no load, while winding loss increases as load increases. The no-load loss can be significant, so that even an idle transformer constitutes a drain on

2520-475: Is complex (especially with line commutated converters), as is expanding existing schemes to multi-terminal systems. Controlling power flow in a multi-terminal DC system requires good communication between all the terminals; power flow must be actively regulated by the converter control system instead of relying on the inherent impedance and phase angle properties of an AC transmission line. Multi-terminal systems are therefore rare. As of 2012 only two are in service:

2610-425: Is given by the universal EMF equation: A dot convention is often used in transformer circuit diagrams, nameplates or terminal markings to define the relative polarity of transformer windings. Positively increasing instantaneous current entering the primary winding's 'dot' end induces positive polarity voltage exiting the secondary winding's 'dot' end. Three-phase transformers used in electric power systems will have

2700-539: Is in its whole length implemented as underground or submarine cable, buried 10 feet (3.0 m) in non-navigable and 15 feet (4.6 m) in navigable sections. After travelling through the Edgewater Tunnel , the cable enters the Hudson River at Edgewater, New Jersey and runs along the eastern side of the river parallel to the shore until piers 92 and 94, where it enters Manhattan . Construction of

2790-467: Is left over from £750M for the installed works. Add another £200–300M for the other works depending on additional onshore works required. An April 2010 announcement for a 2,000 MW, 64 km (40 mi) line between Spain and France is estimated at €700 million. This includes the cost of a tunnel through the Pyrenees. At the heart of an HVDC converter station , the equipment that performs

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2880-625: Is no potential difference. DC will never cross zero volts and never self-extinguish, so arc distance and duration is far greater with DC than the same voltage AC. This means some mechanism must be included in the circuit breaker to force current to zero and extinguish the arc, otherwise arcing and contact wear would be too great to allow reliable switching. In November 2012, ABB announced the first ultrafast HVDC circuit breaker. Mechanical circuit breakers are too slow for use in HVDC grids, although they have been used for years in other applications. Conversely, semiconductor breakers are fast enough but have

2970-421: Is rarely attempted; the 'real' transformer model's equivalent circuit shown below does not include parasitic capacitance. However, the capacitance effect can be measured by comparing open-circuit inductance, i.e. the inductance of a primary winding when the secondary circuit is open, to a short-circuit inductance when the secondary winding is shorted. The ideal transformer model assumes that all flux generated by

3060-531: Is referred to as the rectifier and the station that is operating with power flow from DC to AC is referred to as the inverter . Early HVDC systems used electromechanical conversion (the Thury system) but all HVDC systems built since the 1940s have used electronic converters. Electronic converters for HVDC are divided into two main categories: Most of the HVDC systems in operation today are based on line-commutated converters (LCCs). The basic LCC configuration uses

3150-405: Is the instantaneous voltage , N {\displaystyle N} is the number of turns in a winding, dΦ/dt is the derivative of the magnetic flux Φ through one turn of the winding over time ( t ), and subscripts P and S denotes primary and secondary. Combining the ratio of eq. 1 & eq. 2: where for a step-up transformer a < 1 and for a step-down transformer

3240-542: Is used, the cable capacitance is charged only when the cable is first energized or if the voltage level changes; there is no additional current required. DC powered cables are limited only by their temperature rise and Ohm's law . Although some leakage current flows through the dielectric insulator , this effect is also present in AC systems and is small compared to the cable's rated current. The capacitive effect of long underground or undersea cables in AC transmission applications also applies to AC overhead lines, although to

3330-422: Is used. An enhancement of this arrangement uses 12 valves in a twelve-pulse bridge . The AC is split into two separate three-phase supplies before transformation. One of the sets of supplies is then configured to have a star (wye) secondary, and the other a delta secondary, establishing a 30° phase difference between the two sets of three phases. With twelve valves connecting each of the two sets of three phases to

3420-419: Is winding self-inductance. By Ohm's law and ideal transformer identity: An ideal transformer is linear , lossless and perfectly coupled . Perfect coupling implies infinitely high core magnetic permeability and winding inductance and zero net magnetomotive force (i.e. i p n p  −  i s n s  = 0). A varying current in the transformer's primary winding creates

3510-592: The Quebec – New England Transmission between Radisson, Sandy Pond, and Nicolet and the Sardinia–;mainland Italy link which was modified in 1989 to also provide power to the island of Corsica . HVDC circuit breakers are difficult to build because of arcing : under AC, the voltage inverts and in doing so crosses zero volts dozens of times a second. An AC arc will self-extinguish at one of these zero-crossing points because there cannot be an arc where there

3600-468: The magnetizing branch of the model. Core losses are caused mostly by hysteresis and eddy current effects in the core and are proportional to the square of the core flux for operation at a given frequency. The finite permeability core requires a magnetizing current I M to maintain mutual flux in the core. Magnetizing current is in phase with the flux, the relationship between the two being non-linear due to saturation effects. However, all impedances of

3690-464: The transmission , distribution , and utilization of alternating current electric power. A wide range of transformer designs is encountered in electronic and electric power applications. Transformers range in size from RF transformers less than a cubic centimeter in volume, to units weighing hundreds of tons used to interconnect the power grid . Ideal transformer equations By Faraday's law of induction: where V {\displaystyle V}

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3780-776: The 1930s in Sweden ( ASEA ) and in Germany . Early commercial installations included one in the Soviet Union in 1951 between Moscow and Kashira , and a 100 kV, 20 MW system between Gotland and mainland Sweden in 1954. Before the Chinese project of 2019, the longest HVDC link in the world was the Rio Madeira link in Brazil , which consists of two bipoles of ±600 kV, 3150 MW each, connecting Porto Velho in

3870-516: The 1930s, but the rotating machinery required high maintenance and had high energy loss. Various other electromechanical devices were tested during the first half of the 20th century with little commercial success. One technique attempted for conversion of direct current from a high transmission voltage to lower utilization voltage was to charge series-connected batteries , then reconnect the batteries in parallel to serve distribution loads. While at least two commercial installations were tried around

3960-493: The AC equivalent line, then for a given current (where HVDC current is the same as the RMS current in the AC line), the power transmission capability when operating with HVDC is approximately 40% higher than the capability when operating with AC. Because HVDC allows power transmission between unsynchronized AC distribution systems, it can help increase system stability, by preventing cascading failures from propagating from one part of

4050-479: The AC line connections. CCC has remained only a niche application because of the advent of voltage-source converters (VSCs) which more directly address turn-off issues. Widely used in motor drives since the 1980s, voltage-source converters (VSCs) started to appear in HVDC in 1997 with the experimental Hellsjön–Grängesberg project in Sweden. By the end of 2011, this technology had captured a significant proportion of

4140-432: The DC link. The disadvantages of HVDC are in conversion, switching, control, availability, and maintenance. HVDC is less reliable and has lower availability than alternating current (AC) systems, mainly due to the extra conversion equipment. Single-pole systems have availability of about 98.5%, with about a third of the downtime unscheduled due to faults. Fault-tolerant bipole systems provide high availability for 50% of

4230-592: The HVDC market. The development of higher rated insulated-gate bipolar transistors (IGBTs), gate turn-off thyristors (GTOs), and integrated gate-commutated thyristors (IGCTs), has made HVDC systems more economical and reliable. This is because modern IGBTs incorporate a short-circuit failure mode, wherein should an IGBT fail, it is mechanically shorted. Therefore, modern VSC HVDC converter stations are designed with sufficient redundancy to guarantee operation over their entire service lives. The manufacturer ABB Group calls this concept HVDC Light , while Siemens calls

4320-634: The commissioning of replacement thyristor converters. The development of thyristor valves for HVDC began in the late 1960s. The first complete HVDC scheme based on thyristor was the Eel River scheme in Canada, which was built by General Electric and went into service in 1972. Since 1977, new HVDC systems have used solid-state devices , in most cases thyristors . Like mercury arc valves, thyristors require connection to an external AC circuit in HVDC applications to turn them on and off. HVDC using thyristors

4410-436: The conversion between AC and DC is referred to as the converter . Almost all HVDC converters are inherently capable of converting from AC to DC ( rectification ) and from DC to AC ( inversion ), although in many HVDC systems, the system as a whole is optimized for power flow in only one direction. Irrespective of how the converter itself is designed, the station that is operating (at a given time) with power flow from AC to DC

4500-402: The converter station area. With time, voltage-source converter systems will probably replace all installed simple thyristor-based systems, including the highest DC power transmission applications. A long-distance, point-to-point HVDC transmission scheme generally has lower overall investment cost and lower losses than an equivalent AC transmission scheme. Although HVDC conversion equipment at

4590-456: The core, the transformer is core form; when windings are surrounded by the core, the transformer is shell form. Shell form design may be more prevalent than core form design for distribution transformer applications due to the relative ease in stacking the core around winding coils. Core form design tends to, as a general rule, be more economical, and therefore more prevalent, than shell form design for high voltage power transformer applications at

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4680-400: The corresponding current ratio. The load impedance referred to the primary circuit is equal to the turns ratio squared times the secondary circuit load impedance. The ideal transformer model neglects many basic linear aspects of real transformers, including unavoidable losses and inefficiencies. (a) Core losses, collectively called magnetizing current losses, consisting of (b) Unlike

4770-428: The cross-sectional area of the conductor. Transmission line conductors operating with direct current suffer from neither constraint. Therefore, for the same conductor losses (or heating effect), a given conductor can carry more power to the load when operating with HVDC than AC. Finally, depending upon the environmental conditions and the performance of overhead line insulation operating with HVDC, it may be possible for

4860-440: The electrical supply. Designing energy efficient transformers for lower loss requires a larger core, good-quality silicon steel , or even amorphous steel for the core and thicker wire, increasing initial cost. The choice of construction represents a trade-off between initial cost and operating cost. Transformer losses arise from: Closed-core transformers are constructed in 'core form' or 'shell form'. When windings surround

4950-407: The energy lost as heat in the wires is directly proportional to the square of the current ( energy lost as heat = current 2 ⋅ resistance ⋅ time ) , {\textstyle ({\text{energy lost as heat}}={\text{current}}^{2}\cdot {\text{resistance}}\cdot {\text{time}}),} using half the current at double the voltage reduces

5040-448: The equivalent circuit shown are by definition linear and such non-linearity effects are not typically reflected in transformer equivalent circuits. With sinusoidal supply, core flux lags the induced EMF by 90°. With open-circuited secondary winding, magnetizing branch current I 0 equals transformer no-load current. The resulting model, though sometimes termed 'exact' equivalent circuit based on linearity assumptions, retains

5130-427: The firing angle is about 150–160° because above this, the valve would have insufficient turnoff time. Early LCC systems used mercury-arc valves , which were rugged but required high maintenance. Because of this, many mercury-arc HVDC systems were built with bypass switchgear across each six-pulse bridge so that the HVDC scheme could be operated in six-pulse mode for short maintenance periods. The last mercury arc system

5220-424: The following series loop impedances of the model: In normal course of circuit equivalence transformation, R S and X S are in practice usually referred to the primary side by multiplying these impedances by the turns ratio squared, ( N P / N S )  = a . Core loss and reactance is represented by the following shunt leg impedances of the model: R C and X M are collectively termed

5310-461: The following tests: open-circuit test , short-circuit test , winding resistance test, and transformer ratio test. If the flux in the core is purely sinusoidal , the relationship for either winding between its rms voltage E rms of the winding, and the supply frequency f , number of turns N , core cross-sectional area A in m and peak magnetic flux density B peak in Wb/m or T (tesla)

5400-412: The ideal model, the windings in a real transformer have non-zero resistances and inductances associated with: (c) similar to an inductor , parasitic capacitance and self-resonance phenomenon due to the electric field distribution. Three kinds of parasitic capacitance are usually considered and the closed-loop equations are provided Inclusion of capacitance into the transformer model is complicated, and

5490-745: The last mercury arc HVDC system (the Nelson River Bipole 1 system in Manitoba , Canada) having been put into service in stages between 1972 and 1977. Since then, all mercury arc systems have been either shut down or converted to use solid-state devices. The last HVDC system to use mercury arc valves was the Inter-Island HVDC link between the North and South Islands of New Zealand, which used them on one of its two poles. The mercury arc valves were decommissioned on 1 August 2012, ahead of

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5580-445: The limitations of early electric traction motors . Consequently, the transformers used to step-down the high overhead line voltages were much larger and heavier for the same power rating than those required for the higher frequencies. Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current. At a lower frequency, the magnetizing current will increase. Operation of

5670-678: The line losses by a factor of 4. While energy lost in transmission can also be reduced by decreasing the resistance by increasing the conductor size, larger conductors are heavier and more expensive. High voltage cannot readily be used for lighting or motors, so transmission-level voltages must be reduced for end-use equipment. Transformers are used to change the voltage levels in alternating current (AC) transmission circuits, but cannot pass DC current. Transformers made AC voltage changes practical, and AC generators were more efficient than those using DC. These advantages led to early low-voltage DC transmission systems being supplanted by AC systems around

5760-662: The link capacity, but availability of the full capacity is about 97% to 98%. The required converter stations are expensive and have limited overload capacity. At smaller transmission distances, the losses in the converter stations may be bigger than in an AC transmission line for the same distance. The cost of the converters may not be offset by reductions in line construction cost and power line loss. Operating an HVDC scheme requires many spare parts to be kept, often exclusively for one system, as HVDC systems are less standardized than AC systems and technology changes more quickly. In contrast to AC systems, realizing multi-terminal systems

5850-499: The link started in May 2011 and was completed in June 2013. The cable was completely removed and replaced in 2017. The Ariadne , a 130-meter cable ship , performed the submarine cable replacement. High-voltage direct current A high-voltage direct current ( HVDC ) electric power transmission system uses direct current (DC) for electric power transmission, in contrast with

5940-455: The load power in proportion to their respective ratings. However, the impedance tolerances of commercial transformers are significant. Also, the impedance and X/R ratio of different capacity transformers tends to vary. Referring to the diagram, a practical transformer's physical behavior may be represented by an equivalent circuit model, which can incorporate an ideal transformer. Winding joule losses and leakage reactance are represented by

6030-662: The lower end of their voltage and power rating ranges (less than or equal to, nominally, 230 kV or 75 MVA). At higher voltage and power ratings, shell form transformers tend to be more prevalent. Shell form design tends to be preferred for extra-high voltage and higher MVA applications because, though more labor-intensive to manufacture, shell form transformers are characterized as having inherently better kVA-to-weight ratio, better short-circuit strength characteristics and higher immunity to transit damage. Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel . The steel has

6120-462: The more common alternating current (AC) transmission systems. Most HVDC links use voltages between 100 kV and 800 kV. HVDC lines are commonly used for long-distance power transmission, since they require fewer conductors and incur less power loss than equivalent AC lines. HVDC also allows power transmission between AC transmission systems that are not synchronized . Since the power flow through an HVDC link can be controlled independently of

6210-450: The phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. HVDC also allows the transfer of power between grid systems running at different frequencies, such as 50 and 60 Hz. This improves the stability and economy of each grid, by allowing the exchange of power between previously incompatible networks. The modern form of HVDC transmission uses technology developed extensively in

6300-444: The power supply. It is not directly a power loss, but results in inferior voltage regulation , causing the secondary voltage not to be directly proportional to the primary voltage, particularly under heavy load. Transformers are therefore normally designed to have very low leakage inductance. In some applications increased leakage is desired, and long magnetic paths, air gaps, or magnetic bypass shunts may deliberately be introduced in

6390-401: The primary winding links all the turns of every winding, including itself. In practice, some flux traverses paths that take it outside the windings. Such flux is termed leakage flux , and results in leakage inductance in series with the mutually coupled transformer windings. Leakage flux results in energy being alternately stored in and discharged from the magnetic fields with each cycle of

6480-439: The primary winding. The windings are wound around a core of infinitely high magnetic permeability so that all of the magnetic flux passes through both the primary and secondary windings. With a voltage source connected to the primary winding and a load connected to the secondary winding, the transformer currents flow in the indicated directions and the core magnetomotive force cancels to zero. According to Faraday's law , since

6570-542: The project was that, during wartime, a buried cable would be less conspicuous as a bombing target. The equipment was moved to the Soviet Union and was put into service there as the Moscow–Kashira HVDC system. The Moscow–Kashira system and the 1954 connection by Uno Lamm 's group at ASEA between the mainland of Sweden and the island of Gotland marked the beginning of the modern era of HVDC transmission. Mercury arc valves were common in systems designed up to 1972,

6660-436: The same impedance. However, properties such as core loss and conductor skin effect also increase with frequency. Aircraft and military equipment employ 400 Hz power supplies which reduce core and winding weight. Conversely, frequencies used for some railway electrification systems were much lower (e.g. 16.7 Hz and 25 Hz) than normal utility frequencies (50–60 Hz) for historical reasons concerned mainly with

6750-423: The same magnetic flux passes through both the primary and secondary windings in an ideal transformer, a voltage is induced in each winding proportional to its number of turns. The transformer winding voltage ratio is equal to the winding turns ratio. An ideal transformer is a reasonable approximation for a typical commercial transformer, with voltage ratio and winding turns ratio both being inversely proportional to

6840-402: The same voltage. This is because direct current transfers only active power and thus causes lower losses than alternating current, which transfers both active and reactive power . In other words, transmitting electric AC power over long distances inevitably results in a phase shift between voltage and current. Because of this phase shift the effective Power=Current*Voltage, where * designates

6930-521: The selection. However, some practitioners have provided some information: For an 8 GW 40 km (25 mi) link laid under the English Channel , the following are approximate primary equipment costs for a 2000 MW 500 kV bipolar conventional HVDC link (excluding way-leaving , on-shore reinforcement works, consenting, engineering, insurance, etc.) So for an 8 GW capacity between Britain and France in four links, little

7020-781: The state of Rondônia to the São Paulo area with a length of more than 2,500 km (1,600 mi). High voltage is used for electric power transmission to reduce the energy lost in the resistance of the wires. For a given quantity of power transmitted, doubling the voltage will deliver the same power at only half the current: power = ( voltage ) ⋅ ( current ) = ( 2 ⋅ voltage ) ⋅ ( 1 2 ⋅ current ) {\displaystyle {\text{power}}=({\text{voltage}})\cdot ({\text{current}})=(2\cdot {\text{voltage}})\cdot ({\tfrac {1}{2}}\cdot {\text{current}})} Since

7110-414: The supplier and the client. Costs vary widely depending on the specifics of the project (such as power rating, circuit length, overhead vs. cabled route, land costs, site seismology, and AC network improvements required at either terminal). A detailed analysis of DC vs. AC transmission costs may be required in situations where there is no obvious technical advantage to DC, and economical reasoning alone drives

7200-538: The terminal stations is costly, the total DC transmission-line costs over long distances are lower than for an AC line of the same distance. HVDC requires less conductor per unit distance than an AC line, as there is no need to support three phases and there is no skin effect . AC systems use a higher peak voltage for the same power, increasing insulator costs. Depending on voltage level and construction details, HVDC transmission losses are quoted at 3.5% per 1,000 km (620 mi), about 50% less than AC (6.7%) lines at

7290-637: The transformer core size required drops dramatically: a physically small transformer can handle power levels that would require a massive iron core at mains frequency. The development of switching power semiconductor devices made switch-mode power supplies viable, to generate a high frequency, then change the voltage level with a small transformer. Transformers for higher frequency applications such as SMPS typically use core materials with much lower hysteresis and eddy-current losses than those for 50/60 Hz. Primary examples are iron-powder and ferrite cores. The lower frequency-dependant losses of these cores often

7380-415: The transformer produces a varying magnetic flux in the transformer's core, which induces a varying electromotive force (EMF) across any other coils wound around the same core. Electrical energy can be transferred between separate coils without a metallic (conductive) connection between the two circuits. Faraday's law of induction , discovered in 1831, describes the induced voltage effect in any coil due to

7470-435: The transmitted power, which is a vector product of voltage and current. Additional energy losses also occur as a result of dielectric losses in the cable insulation. For a sufficiently long AC cable, the entire current-carrying ability of the conductor would be needed to supply the charging current alone. This cable capacitance issue limits the length and power-carrying ability of AC power cables. However, if direct current

7560-519: The turn of the 20th century, the technique was not generally useful owing to the limited capacity of batteries, difficulties in switching between series and parallel configurations, and the inherent energy inefficiency of a battery charge/discharge cycle. First proposed in 1914, the grid controlled mercury-arc valve became available during the period 1920 to 1940 for the rectifier and inverter functions associated with DC transmission. Starting in 1932, General Electric tested mercury-vapor valves and

7650-452: The turn of the 20th century. Practical conversion of current between AC and DC became possible with the development of power electronics devices such as mercury-arc valves and, starting in the 1970s, power semiconductor devices including thyristors , integrated gate-commutated thyristors (IGCTs), MOS-controlled thyristors (MCTs) and insulated-gate bipolar transistors (IGBT). The first long-distance transmission of electric power

7740-400: The two DC rails, there is a phase change every 30°, and harmonics are considerably reduced. For this reason, the twelve-pulse system has become standard on most line-commutated converter HVDC systems built since the 1970s. With line commutated converters, the converter has only one degree of freedom – the firing angle , which represents the time delay between the voltage across

7830-405: The voltage across the valve is evenly shared between the thyristors. The thyristor plus its grading circuits and other auxiliary equipment is known as a thyristor level . Transformer In electrical engineering , a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits . A varying current in any coil of

7920-485: The voltage. Each set was insulated from electrical ground and driven by insulated shafts from a prime mover . The transmission line was operated in a constant-current mode, with up to 5,000 volts across each machine, some machines having double commutators to reduce the voltage on each commutator. This system transmitted 630 kW at 14 kV DC over a distance of 120 kilometres (75 mi). The Moutiers–Lyon system transmitted 8,600 kW of hydroelectric power

8010-591: Was demonstrated using direct current in 1882 at Miesbach-Munich Power Transmission , but only 1.5 kW was transmitted. An early method of HVDC transmission was developed by the Swiss engineer René Thury and his method, the Thury system, was put into practice by 1889 in Italy by the Acquedotto De Ferrari-Galliera company. This system used series-connected motor-generator sets to increase

8100-642: Was shut down in 2012. The thyristor valve was first used in HVDC systems in 1972. The thyristor is a solid-state semiconductor device similar to the diode , but with an extra control terminal that is used to switch the device on at a particular instant during the AC cycle. Because the voltages in HVDC systems, up to 800 kV in some cases, far exceed the breakdown voltages of the thyristors used, HVDC thyristor valves are built using large numbers of thyristors in series. Additional passive components such as grading capacitors and resistors need to be connected in parallel with each thyristor in order to ensure that

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