Mercury arc valve - Citizendia

A mercury arc valve (mercury vapor rectifier) is a type of electrical rectifier which converts alternating current into direct current. A rectifier is an electrical device that converts Alternating current (AC to Direct current (DC a process known as rectification. Direct current ( DC) is the unidirectional flow of Electric charge. Rectifiers of this type were used in electric motor power supplies for industry, in electric railways, streetcars and diesel-electric locomotives. Railway electrification supplies electrical energy to railway Locomotives and Multiple units so they can operate without having a Reciprocating A number of vehicles use a diesel-electric Powertrain for providing locomotion. A locomotive is a railway Vehicle that provides the motive power for a Train. They also found use in static inverter stations and as rectifiers for high-voltage direct current power transmission. An inverter is an electrical or electro-mechanical device that converts Direct current (DC to Alternating current (AC the resulting AC can be at any required voltage HVDC or high-voltage Direct current Electric power transmission systems contrast with the more common Alternating current systems as a means Mercury arc rectifiers were invented by Peter Cooper Hewitt in 1902 and further developed throughout the 1920s and 1930s by researchers in both Europe and North America. Peter Cooper Hewitt ( May 5, 1861 - August 25, 1921) was an American electrical engineer who demonstrated the mercury-vapor lamp Year 1902 ( MCMII) was a Common year starting on Wednesday (link will display calendar of the Gregorian calendar (or a Common year starting The 1920s is sometimes referred to as the " Jazz Age " or the " Roaring Twenties " when speaking about the United States and Canada The 1930s were described as an abrupt shift to more radical and conservative lifestyles as countries were struggling to find a solution to the Great Depression. Before the advent of solid-state devices, mercury arc rectifiers were one of the more efficient rectifiers (more efficient mechanical rectifiers had reliability issues). By the 1970s, the development of high-voltage solid state devices made the mercury arc rectifier obsolete even in high-voltage DC applications. This article is about the Decade 1970-1979 For the Year 1970 see 1970. Solid-state Electronic components devices and systems are based entirely on the Semiconductor, such as Transistors Microprocessor chips and Direct current ( DC) is the unidirectional flow of Electric charge.

Glass bulb mercury arc rectifier from the 1940s

1 Use
2 Design
3 Function
4 Others
5 Environmental hazard
6 External links
7 References
8 See also

Use

Mercury arc valves were used until the 1960s for the production of high voltage direct current. The 1960s decade refers to the years from the beginning of 1960 to the end of 1969 Applications included power supply for streetcars and electric railways, variable-voltage power supplies for large radio transmitters, and static inverters. Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. Static inverter stations were used to provide DC power to legacy Edison-style DC power grids in urban centers until the 1950s. In the "War of Currents" era (sometimes "War of the Currents" or "Battle of Currents") in the late 1880s George Westinghouse The 1950s Decade refers to the years of 1950 to 1959 inclusive

Since 1960 mercury arc valves have been increasingly replaced by silicon rectifiers and thyristors. Year 1960 ( MCMLX) was a Leap year starting on Friday (link will display full calendar of the Gregorian calendar. Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 The thyristor is a solid-state Semiconductor device with four layers of alternating N and P-type material The last domain of the mercury arc valves was for high-voltage direct current transmission until 1975. HVDC or high-voltage Direct current Electric power transmission systems contrast with the more common Alternating current systems as a means Year 1975 ( MCMLXXV) was a Common year starting on Wednesday (link will display full calendar of the Gregorian calendar.

They are still used in some South African mines (2006), and until 21 September 2007, on Pole 1 of the HVDC Inter-Island link between the North and South Islands of New Zealand. The Inter-Island HVDC link (designation BEN-HAY A) is the high voltage direct current connection between the two main islands of New Zealand, put into New Zealand is an Island country in the south-western Pacific Ocean comprising two main landmasses (the North Island and the South Island Prior to this date, the original mercury arc rectifiers had seen over 40 years of near continuous service. Half of the original system, still using the original valves, may be brought back into action to secure the power supply into the North Island during winter months. As of February, 2008, the only fully operational HVDC system using mercury arc valves is Pole 1 of the HVDC Vancouver Island link. The HVDC Vancouver Island is the name for HVDC interconnection between the Vancouver Island Terminal (VIT near North Cowichan British Columbia on Vancouver Pole 1 of the Vancouver Island system went into service in early 1968, and is still providing reliable service.

Mercury arc valve operating in 2006

Design

A glass envelope mercury arc rectifier valve

One type of mercury vapour electric rectifier consists of an evacuated glass bulb, with a pool of liquid mercury sitting in the bottom as the cathode. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device Over it curves the glass bulb, which condenses mercury evaporated in the course of operation of the device. The glass envelope has one or more arms with graphite rods as anodes. The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. An anode is an Electrode through which Electric current flows into a polarized electrical device Their number depends on the application. If direct current is to be produced from single-phase alternating current, then two anodes are used, each connected to the outer ends of a centre-tapped transformer secondary winding. With three-phase alternating current three or six anodes are used, to provide a smoother direct current. Six-phase operation can improve the efficiency of the transformer as well as providing smoother DC, by enabling two anodes to conduct simultaneously. During operation, the arc transfers to the anodes at the highest positive potential (with respect to the cathode). Design of the arms and envelope is intended to prevent an arc from forming between the anodes; such a condition is called "backfire" and is a critical factor in the design of mercury arc rectifiers.

Glass envelope rectifiers can produce hundreds of kilowatts of direct-current power in a single unit. A 6-phase rectifier rated 150 amperes has a glass envelope approximately 600 mm (24 inches) high by 300 mm (12 inches) outside diameter. These rectifiers will contain several pounds of liquid mercury. The large size of the envelope is required due to the low thermal conductivity of glass. Mercury vapor in the upper part of the envelope must give up heat through the glass envelope to condense and return to the cathode pool.

The current-carrying capacity of a rectifier is limited in part by the size of the wires fused into the glass envelope for connection of the anodes and cathode. Development of high-current rectifiers required leadwire materials and glass with very similar coefficients of thermal expansion in order to prevent leakage of air into the envelope.

For larger valves, a metal tank with ceramic insulators for the electrodes is used, sometimes with a vacuum pump system to counteract slight leakage of air into the tank around imperfect seals. The design patented by Uno Lamm of ASEA is one example of this type which includes grading electrodes between the anode and cathode to prevent backfire. August Uno Lamm ( May 22 1904 &ndash June 1 1989) was a Swedish electrical engineer and inventor sometimes called "The Father ASEA ( Allmänna Svenska Elektriska Aktiebolaget) was a Swedish industry company Metal-tank rectifiers were built with ratings of 2000 A and 125 kV per unit.

Both glass and metal envelope rectifiers may have control grids inserted between the anode and cathode. This allows the conduction of the rectifier to be controlled, for example to delay the instant at which the arc transfers to the anode on the alternating current waveform, thereby giving control of the mean output voltage produced by the rectifier. Such grid-controlled valves are an essential part of a static inverter.

The temperature of the envelope must be carefully controlled, since the working pressure within the envelope is set by the coolest spot on the enclosure wall. A typical design maintains temperature at 40 degrees Celsius and a mercury vapor pressure of 7 millipascals.

Function

Principle

Operation of the rectifier relies on an electrical arc discharge between electrodes in a sealed envelope containing mercury vapour. An electric arc is an Electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive A pool of liquid mercury acts as a self-renewing cathode that does not deteriorate with time. The mercury emits electrons freely, whereas the carbon anodes emit very few electrons even when heated, thus rectifying action occurs.

Once an arc is formed, electrons are emitted from the surface of the pool, causing shock-ionisation of mercury vapour along the path towards the anodes. The mercury ions are attracted towards the cathode, and the resulting ionic bombardment of the pool maintains the temperature of the 'emission spot', so long as a current of a few amperes continues.

The mercury ions emit light at characteristic wavelengths, the relative intensities of which are determined by the pressure of the vapour. At the low pressure within a rectifier, the light appears pale blue-violet and contains much ultraviolet light. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays

The cathode is connected to the DC load, which in turn is connected to the center tap of an AC transformer, which always remains at zero potential. A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device In electronics a center tap is a connection made to a point half way along a winding of a Transformer or Inductor, or along the element of a Resistor or A transformer is a device that transfers Electrical energy from one circuit to another through inductively coupled Electrical conductors For each AC phase, a wire from each of the two end taps is connected to an anode "arm" on the mercury arc rectifier. An anode is an Electrode through which Electric current flows into a polarized electrical device As the voltage on each anode goes positive, it will begin to conduct through the mercury vapour to the cathode. As the anodes of each AC phase are fed from opposite ends of the transformer winding, one will be positive N volts, and the other negative N volts, and thus a current will always be maintained from one or more positive anodes to the cathode.

Single-phase mercury arc rectifiers were infrequently used because every time the AC voltage dropped to zero the arc would be extinguished. The direct current produced by a single-phase rectifier contains a varying component (ripple) at twice the power supply frequency, which was undesirable in many applications for DC. The utility frequency ( American English) or mains frequency ( British English) is the Frequency at which Alternating current (AC is The solution was to use 2, 3 or even 6 phase AC power supplies so that the rectified current would maintain a more constant voltage level. A polyphase system is a means of distributing Alternating current electrical power. Polyphase rectfiers also balanced the load on the phases of a polyphase supply system, which is desirable for reasons of system performance and economy.

Starting

A conventional mercury arc rectifier is started by a brief high-voltage arc within the rectifier, between the cathode pool and a starting electrode. By one of a number of means, the starting electrode is brought into contact with the pool and allowed to pass current through an inductive circuit. The contact with the pool is then broken, resulting in a high emf.

The momentary contact between the starting electrode and the pool may be achieved by allowing an external electromagnet to pull the electrode into contact with the pool; the electromagnet can also serve as the starting inductance. An electromagnet is a type of Magnet in which the Magnetic field is produced by the flow of an electric current. Alternatively, the electromagnet may be arranged to tip the bulb of a small rectifier, just enough to allow mercury from the pool to reach the starting electrode. An alternative system provides a narrow neck of mercury between two pools, and by passing a very high current at negligible voltage through the neck, displaces it by magnetostriction, thus opening the circuit.

Excitation

Since momentary interruptions or reductions of output current may cause the cathode spot to extinguish, many rectifiers incorporate a small local circuit to maintain an arc whenever the plant is in use. Typically, a two or three phase supply of a few amperes passes through small 'excitation' anodes. A magnetically-shunted transformer of a few hundred VA rating is commonly used to provide this supply.

Others

A 150 kV mercury arc valve at Manitoba Hydro's Radisson converter station, August 2003

The largest ever mercury arc rectifiers were used until 2004 at the Nelson River Bipole high-voltage DC power transmission project. The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. Founded in 1961 Manitoba Hydro is the electric power and natural gas utility in the province of Manitoba. The Nelson River Bipole is a historic Electric power transmission system of two high voltage direct current lines in Manitoba, operated by Manitoba

Special types of mercury arc rectifiers are the Ignitron and the Excitron. An ignitron is a type of controlled Rectifier dating from the 1930s

In 1919 the book "Cyclopedia of Telephony & Telegraphy Vol. Year 1919 ( MCMXIX) was a Common year starting on Wednesday (link will display the full calendar of the Gregorian calendar (or a Common 1" by Kempster Miller and others described an amplifier for telephone signals that used a magnetic field to modulate an arc in a mercury rectifier tube. Generally an amplifier or simply amp, is any device that changes usually increases the amplitude of a signal. Basic principle A traditional landline telephone system also known as "plain old telephone service" (POTS, commonly handles both signaling and audio information In Telecommunications, modulation is the process of varying a periodic Waveform, i This pre-dated the application of the vacuum tube to amplification of audio signals but was never commercially important. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall.

Environmental hazard

Mercury compounds are toxic, highly persistent in the environment, and present a danger to humans and the environment. The use of large quantities of mercury in fragile glass envelopes presents a hazard of potential release of mercury to the environment should the glass bulb be broken. Some HVDC static inverter stations have required expensive clean-up to eliminate traces of mercury emitted from the station over its service life. Steel tank rectifiers frequently required vacuum pumps which continually emitted small amounts of mercury vapor.

External links

References

A. H. Howatson, An Introduction to Gas Discharges, Pergamon Press, Oxford, 1965 - especially Chapter 8.
H. Rissik, Mercury-Arc Current Convertors, Pitman. 1941.
The Project Gutenberg EBook of Cyclopedia of Telephony & Telegraphy Vol. 1by Kempster Miller, George Patterson, Charles Thom, Robert Millikan, Samuel McMeen

Contents

Use