A mercury arc valve is a type of electrical rectifier which converts alternating current into direct current. They were used in electric motor power supplies for industry, in electric railways, streetcars and diesel-electric locomotives, and in static inverter stations in electric power transmission. Mercury arc rectifiers were invented by Peter Cooper Hewitt in 1902 and further developed throughout the 1920's and 1930's by researchers in both Europe and North America. Mercury arc rectifiers were the most efficient form of conversion from alternating to direct current before the advent of solid-state devices. By the 1970's, the development of high-voltage solid-state devices made the mercury arc rectifier obsolete even in high-voltage DC applications.

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. Small mercury arc rectifiers were used in the power supplies for vacuum tube (valve) electronic equiment such as power amplifiers and transmitters. 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.

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 a glass bulb, which condenses mercury evaporated in the course of operation of the device. An ignition electrode, moved by a solenoid is mounted just above the mercury pool. This evaporate some mercury and make the rectifier ready for operation. The glass envelope has one or more arms with graphite electrodes as anodes. Their number depends on the application. If direct current from alternating current is to be produced, then two "arms" are used. With three-phase alternating current three or six anodes are used, to provide a smoother direct current. With a three-phase supply, six "arms" (anodes)may be used, with the current shifted in phase by 60° with large inductive coils. During operation, the arc transfers to the anode 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 up to hundreds of kilowatts of direct-current power in a single valve. A 6-arm rectifier rated 150 amperes has a glass envelope approximately 600 mm high by 300 mm 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, to prevent leakage of air into the envelope.

For larger valves, a metal tank with ceramic insulators for the electrodes is used. Uno Lamm of ASEA patented the design of high-voltage rectifiers with metal tanks. These will also have grading electrodes between the anode and the cathode to prevent backfire. Metal-tank rectifiers were built with ratings of 2000 A and 125 kV per valve. Metal-tank valves may have vacuum pump systems to counteract slight leakage of air into the tank around imperfect seals.

Both glass and metal envelope rectifiers may have control grids inserted between the anode and cathode. This permits the conduction of the rectifier to be controlled. The instant at which the arc transfers to the anode can be delayed on the alternating current waveform, allowing control of the voltage produced by the rectifier. 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 C and a mercury vapor pressure of 7 millipascals.

Since the rectifier only passes one-half of the alternating current waveform, two rectifers are connected in opposite polarity so that both portions of the alternating supply are converted to direct current. The resulting circuit is a bridge rectifier.

Function

Principle

Operation of the rectifier relies on an electrical arc struck between electrodes in a low-pressure envelope. The pool of liquid mercury acts as a self-renewing cathode that is very rugged and that does not deteroriate with time. The voltage drop in an arc is much lower than the voltage drop of a thermionic gas diode, and current density is not limited by the electron emission ability of the cathode.

Since electron emission is less likely from the anode electrodes than from the mercury pool cathode, current will only flow in one direction through the rectifier.

Starting

A mercury vapour electric rectifier is started in a similar fashion to a fluorescent lamp. This takes place by means of one or more of the upper electrodes immediately over the mercury pool. The starting electrodes vaporizes liquid mercury to fill the rectifier envelope with mercury vapor, making the rectifier ready for operation. When the rectifier is running, it produces sufficient heat to maintain the vapor level, and so the arc discharge does not go out The starting electrodes are then no longer needed. Since very low DC current may not sustain the arc, for some applications an excitation anode is provided to ensure that the arc is not extinguished.

When mercury atoms are electrically excited, the characteristic mercury light emission can be seen, pulsing at the same frequency as the current (see fluorescent tube); and a a moving luminous spot can be seen at the surface of the mercury pool cathode (though viewing is not recommended, as mercury emissions are high in ultraviolet light). This spot marks the point at which the current leaves the mercury.

Others

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

Special types of mercury arc rectifiers are the thyratron, the Ignitron and the Excitron. For charging of secondary batteries, "Tungar" tubes were used, which were able to supply the necessary current with the relatively small charging voltages (e.g. 12 V).

Environmental hazard

The use of large quantities of mercury in fragile glass envelopes presents a hazard of release of mercury to the environment. Some HVDC static inverter stations have required expensive clean-up to eliminate traces of mercury emitted from the station over its service life. Early models of high-voltage valve 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 envirionment.

External links

ABB page on the history of high voltage direct current transmission

The Tube Collector Virtual Museum. Description of mercury arc rectifiers and further links, including photographs

References

A. H. Howatson, "An Introduction to Gas Discharges", Pergamon Press, Oxford, 1965 - especially Chapter 8.