Mercury arc valve

A mercury arc valve (mercury vapor rectifier) is a type of electrical rectifier which converts alternating current into direct current. Rectifiers of this type were used in electric motor power supplies for industry, in electric railways, streetcars and diesel-electric locomotives. They also found use in static inverter stations and as rectifiers for high-voltage direct current power transmission. 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. Before the advent of solid-state devices, mercury arc rectifiers were the most efficient form of conversion from alternating to direct current. By the 1970s, the development of high-voltage solid state devices made the mercury arc rectifier obsolete even in high-voltage DC applications.

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Use

Mercury arc valves were used until the 1960s for the production of high voltage direct current. Applications included power supply for streetcars and electric railways, variable-voltage power supplies for large radio transmitters, and static inverters. Static inverter stations were used to provide DC power to legacy Edison-style DC power grids in urban centers until the 1950s.

Since 1960 mercury arc valves were increasingly replaced by silicon rectifiers and thyristors. The last domain of the mercury arc valves was for high-voltage direct current transmission until 1975.

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. Prior to this date, the original mercury arc rectifiers had seen over 40 years of near continuous service.

Design

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. 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. 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 (24inches) high by 300 mm (12inches) 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. 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. 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 to flow.

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.

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. 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

  The largest ever mercury arc rectifiers were used until 2004 at the Nelson River Bipole high-voltage DC power transmission project.

Special types of mercury arc rectifiers are the Ignitron and the Excitron.

In 1919 the book "Cyclopedia of Telephony & Telegraphy Vol. 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. This pre-dated the application of the vacuum tube to amplification of audio signals but was never commercially important.

Environmental hazard

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. Mercury compounds are toxic, highly persistent in the environment, and present a danger to humans and the environment.

References

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

See also

• Ignitron
• Thyratron