power

HIGH VOLTAGE DIRECT CURRENT HVDC or high-voltage, direct current electric power transmission systems contrast with the more common alternating-current systems as a means for the bulk transmission of electrical power. The modern form of HVDC transmission uses technology developed extensively in the 1930s in Sweden at ASEA. Early commercial installations include one in the USSR in 1951 between Moscow and Kashira, and a 10-20 MW system in Gotland, Sweden in 1954 ------------------------------------------- History of HVDC transmission An early method of high-voltage DC transmission was developed by the Swiss engineer Rene Thury [2]. This system used series-connected motor-generator sets to increase voltage. Each set was insulated from ground and driven by insulated shafts from a prime mover. The line was operated in constant current mode, with up to 5000 volts on each machine, some machines having double commutators to reduce the voltage on each commutator. An early example of this system was installed in 1889 in Italy by the Society Acquedotto de Ferrari-Galliera. This system transmitted 630 kW at 14 kV DC over a distance of 120 km[3]. Other Thury systems operating at up to 100 kV DC operated up until 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[4]. The grid controlled mercury arc valve became available for power transmission during the period 1920 to 1940. In 1941 a 60 MW, +/- 200 kV, 115 km buried cable link was designed for the city of Berlin using mercury arc valves (Elbe-Project), but owing to the collapse of the German government in 1945 the project was never completed[5]. The nominal justification for 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 [6]. Introduction of the fully-static mercury arc valve to commercial service in 1954 marked the beginning of the modern era of HVDC transmission. Mercury arc valves were common in systems designed up to 1975, but since then, HVDC systems use only solid-state devices. ------------------------------------------- Advantages of HVDC over AC transmission In a number of applications the advantages of HVDC makes it the preferred option over AC transmission. Examples include: * Undersea cables, where high capacitance causes additional AC losses. (e.g. 250 km Baltic Cable between Sweden and Germany[7]). * Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for example, in remote areas. * Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install. * Allowing power transmission between unsynchronised AC distribution systems. * Reducing the profile of wiring and pylons for a given power transmission capacity. * Connecting remote generating plant to the distribution grid, for example Nelson River Bipole. * Stabilizing a predominantly AC power-grid, without increasing maximum prospective short circuit current. * Reducing corona losses (due to higher voltage peaks) for HVAC transmission lines of similar power * Reducing line cost since HVDC transmission requires less conductor (i.e. 2 conductors one is positive another is negative) Long undersea cables have a high capacitance. While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional I2R power losses when the cable is carrying AC. In addition, AC power is lost to dielectric losses. HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. This voltage determines the insulation thickness and conductor spacing. This allows existing transmission line corridors to be used to carry more power into an area of high power consumption, which can lower costs. ------------------------------------ Disadvantages The required static inverters are expensive and cannot be overloaded very much. At smaller transmission distances the losses in the static inverters may be bigger than in an AC powerline, and the cost of the inverters may not be offset by reductions in line construction cost. In contrast to AC systems, realizing multiterminal systems is complex, as is expanding existing schemes to multiterminal systems. Controlling power flow in a multiterminal DC system requires good communication between all the terminals; power flow must be actively regulated by the control system instead of by the inherent properties of the transmission line.