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61-457: High capacity transmission such as current technology 800 kV high voltage direct current lines would span the country, linking to local electric utilities and distantly located bulk power generation facilities. The national backbone would be intelligent in a manner similar to local smart grid clusters. As local electricity networks are upgraded to smart grids , interactions with the national backbone can become more coordinated. Examples given of

122-497: 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

183-554: A virtual power generation cluster, a local electric utility system, or a grid energy storage facility. According to advocates, a national system would do for the US energy economy what Dwight Eisenhower 's Interstate Highway System did for efficient distribution of products. According to scientists such as James E. Hansen , construction of such a national grid is essential to any strategy to combat global warming . According to energy independence advocates such as T. Boone Pickens , it

244-554: 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

305-452: 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

366-452: 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

427-410: 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

488-468: 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

549-446: 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

610-460: 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

671-405: 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

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732-482: 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

793-401: 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

854-413: 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

915-454: 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

976-462: 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

1037-440: 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

1098-475: 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:

1159-583: Is essential for the U.S. economy and its strategic energy interests. The "Unified Smart Grid" is promoted by Alliance for Climate Protection ( Repower America program) and Al Gore . Gore's cost estimate is $ 400 billion and would be recovered by transmission tariffs. The need for a national bulk transmission grid is detailed in T. Boone Pickens 's energy independence plan . 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

1220-467: 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

1281-625: 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

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1342-531: 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

1403-542: 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

1464-422: 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

1525-592: 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

1586-776: 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

1647-516: 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

1708-530: The 60s. The Pacific Intertie carries up to 3.1 GW on two 500 kV overhead lines. The 1200 km Quebec to New England HVDC line with 2 GW of capacity was constructed in the 1980s. A 5 GW 800 kV system was constructed along the southern provinces of China at a cost of €300 million in 2010. The Unified Smart Grid proposal is more than a collection of point to point interconnections between regional systems with some communications intelligence. The topology conceptually has many smart grid access points that could form

1769-493: 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

1830-479: 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

1891-432: 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

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1952-592: 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

2013-521: The city. The closest to Moscow where it was visually confirmed that Wagner troops had reached was Krasnoye , Lipetsk Oblast. Within the framework of administrative divisions , Kashira serves as the administrative center of Kashirsky District . As an administrative division, it is, together with five rural localities , incorporated within Kashirsky District as the Town of Kashira . As

2074-634: 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

2135-436: 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

2196-402: 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

2257-428: 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

2318-407: 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

2379-427: 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

2440-534: The image of Zilant , a heraldic symbol of Kazan . Town status was granted to Kashira in 1777. Kashira's southern suburbs were entered in by Germany on 24 November to 17 December 1941 and was a massacre site of Poles and Jews relating to the Katyn massacre . It is reported that, during the Wagner Group rebellion , Kashira was the closest Wagner troops got to Moscow. Kashira is 95 kilometres (59 miles) south of

2501-670: The kinds of coordination are that hydropower from the northwest can be dispatched if wind is expected to temporarily subside in the Dakotas . Discretionary air conditioning in California can be turned on if strong winds are blowing in Texas. Long distance interconnections are not new. The 1400 km Pacific DC Intertie between Los Angeles and the Pacific northwest was proposed in the 1930s and initiated by President John F. Kennedy in

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2562-745: 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

2623-678: 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

2684-662: 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

2745-462: 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

2806-450: 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

2867-542: 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,

2928-402: 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

2989-521: 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

3050-781: 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

3111-414: 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

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3172-538: 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

3233-435: 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

3294-519: 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

3355-452: 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

3416-400: 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

3477-631: 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 . Kashira Kashira ( Russian : Каши́ра ) is a town and the administrative center of Kashirsky District in Moscow Oblast , located on the Oka River 115 kilometers (71 mi) south of Moscow . Population: 41,870 ( 2010 Census ) ; 40,898 ( 2002 Census ) ; 44,110 ( 1989 Soviet census ) . It

3538-485: 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

3599-591: 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

3660-657: Was first mentioned in 1356 as the village of Koshira ( Кошира ) named after the Koshira River (today's Kashirka River ). However, 1619 is considered Kashira's foundation year, when the town was transferred from the left bank of the Oka to the right bank some 5 kilometers (3.1 mi) upstream and rebuilt after it was badly damaged by the Crimean Tatars in 1592 and 1596. The town was once home to exiled Kazan Khan Ghabdellatif . The coat of arms of Kashira contains

3721-642: 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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