Enormous currents and voltages are involved in modern electrical equipment and exceptionally strong and efficient apparatus, such as switchgear, circuit-breakers, reactors and transformers, must be used to keep these powerful forces in harness
MERCURY ARC RECTIFIER used for converting alternating into direct current. The rectifiers are built up in metal containers, fitted with a complete water-cooling system for the anodes and pumps for continuous evacuation. As the metal shells are “alive”, they are mounted on insulators and the whole unit is guarded by railings.
THE single factor that, probably more than any other, led to the remarkable progress in heavy electrical engineering during the twentieth century was the substitution of the three-phase turbo-alternator for the engine-driven alternator. A tenfold increase became practicable in speeds of rotation and generating capacity for a given floor area. Generating units increased in size to 25,000 kW and upwards to over 100,000 kW, generating up to 11,000 volts and, in some instances, up to 33,000 volts. The dream of rationalization of electric supply became a reality.
Not least among the problems which faced the electrical engineer during this rapid march of progress was the development of switchgear capable of controlling the enormously increased currents and voltages, and the design of transformers to convert these high voltages to the extra high voltage of the main transmission network of the Grid system of interconnected supply.
The struggle, which still goes on, is analogous to the classic example of the armour-piercing shell and the armour-plate manufacturers. As the size of units increases and the generating voltage rises, the problems of insulation and safe operation of switchgear and transformers become increasingly difficult of solution. They carry with them the necessity for a vast increase in scientific research on insulating materials and methods of insulation, coupled with careful investigation of the severe static stresses that became evident at higher operating voltages. The effect of the surge or transient voltage and current rise at short circuit is enormously enhanced, causing devastating effects unless held in check.
It is not easy to illustrate what is meant by a surge, but some notion of it may be gathered by visualizing a busy road which crosses a river by a swing bridge. As long as the bridge is closed the stream of traffic flows steadily across it. When the bridge is opened the traffic is stopped much in the same way as an opened switch cuts off electric current. As soon as the bridge is closed again the halted accumulated traffic hastens across it, imposing a sudden heavier load on it than does the normal traffic. A short circuit may be similarly likened to the traffic across the bridge being abnormally increased by users of other bridges up and down stream suddenly deciding that the swing bridge is their easiest way across, and all making for it. The higher voltages of to-day may be taken as represented by heavy, fast motor vehicles crossing the bridge instead of the old light, slow horse-drawn ones.
Small wonder, then, with modern high-voltage practice, that the simple knife, or hinged, switches mounted in a row of slate cubicles, which formed the main switchboard of a power station at the beginning of this century, have developed into the massive totally enclosed metal-clad structures of the present day. Often they are grouped in separate buildings and operated from control rooms fitted with illuminated boards bearing a mimic diagram of all circuits, with push buttons for the various operations.
To safeguard electrical machinery and the men operating it, there are, incorporated in the circuits, devices that automatically open the appropriate switches at once if dangerous conditions arise. A breakdown in electrical supply may leave connected to the line valuable electrical machinery that would be ruined if the current were suddenly restored; so an automatic release trips the circuit-breaker immediately current fails. Overload releases also play their part when powerful current surges occur due to a short circuit or any other cause. Various mechanical and electrical interlocks are provided, in addition, to prevent the improper operation of a switch, and to ensure that the switch is “dead” before it is opened.
The safeguarding of the costly large generating units and transformers entails great responsibility, as any mishap involves risk of fire and may destroy many thousand pounds’ worth of property, apart from the serious economic loss caused by any breakdown of supply. Each switch is therefore provided with means for completely isolating it from the line when necessary for examination or repair. Furthermore, each switch is completely isolated from its neighbour by fireproof barriers of masonry or of metal that is thoroughly earthed.
The general trend of power station switchgear design is towards sectionalization, to ensure isolation and to prevent any breakdown that may occur from affecting neighbouring gear.
The major switching operations are carried out, as far as is possible, at the generating voltage or power station side of the transformers, circuit interruption being effected on the extra high voltage or transmission line side of the transformers by means of special fuses and long air-break switches built up on porcelain insulators. Extra high-voltage circuit-breakers of large capacity must, however, be provided to isolate the main transformers where bulk supply is given or taken from the overhead transmission lines.
The three-phase oil circuit-breaker, now generally adopted for all main switching operations, comprises three pairs of insulated contacts rigidly mounted on the main framework of the switch. These are connected so as to form a “break” in each of the three phases of the circuit. The circuit is closed by energizing a powerful electrical solenoid which lifts three metal bars on an insulating framework simultaneously to bridge the three pairs of fixed contacts. Fixed and moving contacts are completely immersed in a removable tank filled with special insulating oil. The old knife switch used only one fixed contact, the other pole being the hinge of the moving blade.
The moving contacts are lifted, by the solenoid, against the action of powerful springs and make a rubbing contact of large area with the fixed terminals. In the closed position they are caught and held by a latch. To open the switch this latch is released. The powerful springs then force the moving contact bars down at high speed and break each phase at two points, one at either end of each bar. No advantage has been found in increasing the number of breaks to more than two, because of the added complications and doubtful advantages. On the contrary, modern design is tending towards the use of a single break only.
The closing of a switch is not as simple a matter as it may seem at first sight. Large electro-magnetic forces resist the closing as the contacts approach each other and add to the load imposed on the lifting solenoid by the powerful opening springs. In a large switch the total load on the solenoid may amount to as much as three or four tons. The opening of the switch also brings into play electrostatic stresses that tend to collapse the fixed contacts, which generally take the form of spring-loaded sections or fingers that embrace the moving contact bars. The mechanical construction of these must be robust.
The dielectric properties of the insulating oil surrounding the contacts are of material value in quenching the arc which inevitably follows a break in the circuit. The oil, however, suffers some degree of carbonization from the intense heat generated by arcing. This heat also generates inflammable gases in the oil that create heavy stresses on the structure of the oil tank and the switch.
Metal-Clad Switches
These considerations are particularly serious on the incidence of a short circuit, which brings into action automatic relays that trip the switch and open the circuit. A severe short circuit may necessitate an immediate renewal or thorough cleansing of the whole of the oil in the tank, amounting to 500 gallons or more. Even in ordinary service periodic purification of the oil by means of a centrifugal separator is essential to maintain its dielectric strength. The fixed and moving contacts also tend to suffer deterioration by burning and must be fitted with arcing pieces that can be easily renewed when necessary. Arcing may be seen on a minute scale on the spring contact of the ordinary domestic electric bell, a tiny spark being observable every time the contact is broken by vibration of the bell hammer.
In earlier designs the circuit-breakers were mounted on wheeled frameworks at a sufficient height to permit of the oil tanks being lowered to expose the contacts, the six electrical connexions being brought out to plugs at the back of the switch. Each switch was housed in a separate cubicle and plugged into insulated sockets at the back of the cubicle that gave connexion to the busbars behind and to the three-core cable of the circuit. At any time when the switch was open it could be drawn out as a complete unit from its cubicle and taken to any convenient spot for examination.
This truck-type cubicle switchgear was largely superseded in modern power stations by the so-called metal-clad switches, each of which is a self-contained unit. These structures can be installed in any required number alongside one another and bolted together to form a continuous switchboard, the separate lengths of busbar being coupled by links into three long bars. A duplicate set of busbars is often provided to give flexibility in operation. A pair of horizontal runways at the front carry the circuit-breaker, which plugs into insulated sockets in the fixed frame. A hand-operated device enables the circuit-breaker to be racked out on the runway, completely isolating it for examination without removing it from the runway.
THREE-PHASE OIL CIRCUIT-BREAKER, of 132,000 volts. This comprises three pairs of insulated contacts immersed in tanks filled with special insulating oil. The dielectric properties of the oil serve to quench the arc caused by a break in the circuit.
As the necessity for still larger breaking capacities became evident, the increasing size of the metal-clad circuit-breaker made it impracticable to provide for its movement. In the larger switches it is now incorporated into the fixed metal-clad structure and provided with knife switches to isolate it when necessary for inspection. This has also facilitated greater isolation of the phases in the circuit-breaker and of the busbars and cable runways from the circuit-breaker, masonry walls being sometimes added for the latter purpose. As a further precaution, the busbars and isolating switches, both of substantial mechanical construction, are completely immersed in insulating oil of high dielectric value.
The criterion of a modern power switch lies in its breaking capacity, expressed in kilovoltamperes (kVA). This must be sufficient to cope successfully with any short circuit that may occur in the particular system. Typical breaking capacities of switchgear in modern large power stations vary from 1,000,000 to 1,500,000 kVA.
The alternating current to be interrupted consists of alternate positive and negative waves of current. In passing from positive to negative there is a transient point of zero current. It is only at this instant that the electric arc, which forms at the opening of a switch, can be finally broken. The quenching of the arc must be effected before the next wave of “recovery voltage” has time to re-establish the arc.
A short circuit can be interrupted and the arc quenched at the point of zero current only if, at that instant, the insulation value or dielectric strength between the contacts is being restored more rapidly than the rise of recovery voltage across the contacts. The conducting medium of the arc consists of hydrogen and other gases produced by the intense heat of the arc and rendered conducting by electrical and thermal stresses. The problem is to get rid of this conducting medium at the instant of zero current. The difficulty of doing this is vastly increased by the fact that at short circuit high-frequency oscillations are superimposed on the circuit, varying from a few hundred oscillations a second to perhaps one or two hundred thousand a second, according to the electrical constants of the adjoining circuits. These extremely rapid oscillations create a recovery voltage to maintain the arc that may rise at the rate of several thousand volts per millionth of a second, so there is no time to be lost in re-establishing a dielectric insulating barrier between the contacts.
Arc-Splitters
Various forms of explosion pot have been devised for the contacts of heavy switchgear to assist in breaking the arc. A typical example is the cross-jet pot, in which the moving contacts consist of round plugs forced up into fixed sockets that offer a large contact surface when the plug is home. Surrounding each socket is a cylindrical explosion pot, within which contact is broken. The
pots and contacts are completely immersed under oil in a tank, as before.
When contact is broken inside the pot the rapid expansion of the gases formed by the arc drives the body of oil within the pot along a side passage and projects it in a powerful cooling stream directly across the path of the arc, thus driving the conducting gases carrying the arc against the edge of horizontal plates called arc-splitters. This forcible interposition of a cool dielectric medium between the contacts succeeds in quenching the arc at about the third or fourth “zero point” of current, which in effect is almost instantaneous action.
The grave fire risk due to the presence of large quantities of oil in the switchgear — a single circuit-breaker may have as much as 1,000 gallons in its tanks, busbars and isolating switch chambers — is considerably reduced by the use of explosion pots. These preclude the possibility of the arc jumping to the sides of the oil tank and so enable a much smaller tank with less oil to be used. A further reduction of oil can also be effected by the use of “condenser” type busbars, which require to be immersed in oil at their junction boxes only.
37,500-KVA THREE-PHASE POWER TRANSFORMER, 6,800 volts to 33,000 volts, installed at Belfast Corporation Power Station. Above the transformer tank is the oil conservator tank through which the transformer “breathes” to prevent entry of moisture or dirt. On the left are the cooling radiators for removing the heat generated in the oil.
A much greater reduction in oil quantity is effected in some high-voltage switchgear by the use of a single break only in each circuit, fitted with an explosion pot This enables a still smaller oil tank to be used and appears, particularly at voltages above, say, 20,000 volts, to be almost as efficient as a double break on short circuit.
In all such high-voltage switchgear, indoor and outdoor the three phases of the circuit are completely isolated in separate metal chambers. In outdoor switchgear the two high-voltage contacts of each phase are brought out through long porcelain insulators and air-break isolating switches to the extra high-tension over-line.
The incidence of a current surge, due to a short circuit, or other cause may well be regarded as a sudden attack or raid on the electrical supply system that brings into play all the local defence forces, such as relay, circuit breakers, fuses and the like, that protect the plant It is desirable to limit the disturbance they create, as far as possible, to the immediate vicinity of the fault, and thereby avoid the tripping of relays farther afield. This may be done to some extent by inserting reactors in the circuits at suitable spots.
Those who are acquainted with “chokes” in radio sets will readily understand the principle of the reactor, which consists merely of a helical winding interposed in the circuit. While this coil will pass the ordinary current, its self-induction offers a high resistance to large current surges and checks their progress. In short, it acts much as a spiral spring would act in receiving and absorbing a heavy shock. Unlike a plain resistance, it does not. destroy the energy, but in some measure gives it back to the circuit and rejects the surplus in the form of heat.
The windings of reactors for power circuits consist of large-section cables or conductors evenly wound into a loose coil with air space all round each turn. At points round the circumference of the coil concrete supports embrace the separate turns and hold them rigidly in place. An iron core, if used, would increase the self-inductance of the coil, but would tend to disturb the normal working of the circuit. Considerable heat is created when surges arrive, so ample room for ventilation is necessary round each turn and round the complete coil. Bare conductors would be best for getting rid of the heat, but, as the coil is in effect an electro-magnet when in use and may pick up any stray iron accidentally left near it, the conductor is generally lightly insulated.
In addition to the practicability of generation at high voltage, a second even more important factor in the choice of alternating current for electrical generation lies in the ease with which the generating voltage can be raised to any level to give economical transmission for long distances.
The function of the transformer is the conversion of electrical energy at low voltage into electrical energy at high voltage, or vice versa. In all such conversions there is inevitably some slight loss of power and, as the power in question is a compound of voltage and quantity of current, it follows that if, say, 3,300 amperes of current at 6,000 volts are raised by a transformer to a pressure of 66,000 volts, there will be only somewhat less than 330 amperes at the higher voltage. This balance of power between input and output is fortunately inherent in the transformer principle, so that any demand for more current at the higher voltage automatically causes the low-voltage current to rise relatively.
The advantage of such a transformation lies in the fact that the smaller current can be carried economically by a much smaller section of wire than would be required for the original current. This point is of vital importance in such a transmission system as that of the British Grid, which requires some three or four thousand miles of circuits and, in spite of the high voltages used, needs wire of from ⅜ in. to ⅞ in. diameter for economical working. At the far end of the transmission line any desired low voltage can be re-established by simply stepping down.
The principle on which a transformer works can best be understood by considering first an electro-magnet consisting of a layer of insulated wire wound spirally round a cylindrical bar of iron. When a current is passed through the wire a magnetic field is set up and a magnetic “flux” flows round the coil. The effect is vastly increased by the presence of iron, steel or any alloy of high magnetic permeability in the vicinity of the coil, as the flux flows much more easily in such materials than it does in air. As the iron core forms part of the magnetic path it becomes a magnet whose polarity will be reversed if the current is reversed indirection. Thus, if an alternating current is passed through the wire the magnetic polarity of the bar will be reversed with each reversal in direction of the current. The substance of the iron core is affected by rapid magnetic reversals of this kind, which cause
internal “friction” in the molecules of the iron and loss of energy in the form of heat.
If the coil of wire were wound on one side of an iron link or ring, that would increase the strength of the magnetic field appreciably, as it would then have a closed path of high magnetic permeability through the iron. If a second coil of insulated wire were wound round the opposite side of the ring an electrical current would be induced in this second coil with every change in strength or polarity of the magnetic field in the link.
An alternating current m the first or “primary” winding would create an alternating magnetic field in the link that in turn would induce an alternating current in the second or “secondary” winding.
This was the type of transformer as originally conceived by Michael Faraday in 1831, a direct current being used in the primary with make and break to give the necessary changes in the magnetic field. A stronger effect was obtained by winding the secondary coil on top of the primary coil and surrounding them closely with an iron path for the magnetic flux. The development of the modern power transformer from this early example has passed through many phases. Special alloys have been produced of high permeability for the magnetic path; special methods of insulation and forms of winding have been devised; the energy loss in the form of heat has been the subject of much anxious investigation with a view to securing maximum efficiency; and the resulting temperature rise has been confined to reasonable limits by various methods of cooling of coils and cores. The ultimate aim throughout is to produce the strongest practicable magnetic field round the secondary winding, coupled with thorough insulation of all windings and ample mechanical strength under the stress of short circuit conditions and surge impulses.
The resulting transformer design is inevitably a compromise between conflicting conditions. On the one hand, theoretical efficiency and rigidity of construction ask for the closest possible winding of the coils against each other, and against the iron, without air spaces. On the other hand, considerations of insulation and of removal of heat demand open spacing and a cooling medium round the wires and cores. The cooling medium used is oil, the entire transformer being submerged in a tank of high-grade insulating oil. External circulating tubes or large radiators outside the oil tank are connected to the top and bottom of the rank to help in removing heat from the oil, the process often being further assisted by air-blast fans or water cooling systems to take the heat from the radiators. In large transformers the cooling arrangements necessarily occupy a considerable amount of space round the transformer tank.
Transformer designs may be classified as either core type or shell type, the former being in more common use for large units. The steel cores, being liable to disturbing effects from eddy currents, are made non-conducting by building them up of countless sheets of thin sheet steel stampings, each sheet backed with an insulating paper, the pile being solidly clamped and fastened with insulated bolts to form a solid block. In the shell type the core stampings are interleaved round the windings to form an almost complete steel enclosure, whereas in the three-phase core type transformer the three separate cores, with their respective windings for each phase, are interconnected only by massive steel yokes across the tops and across the bottoms of the cores.
In a typical three-phase core type transformer the coils are generally made of copper strip, wound on edge and heavily insulated by special processes, the end turns (which are particularly liable to attack from transients) receiving special attention. The strip is bent to shape on formers in small sections which, after insulation, are electrically welded together, the complete coils being slipped over the cores before the top yoke is bolted into position.
To prevent the helical coils from acting in a similar manner to reactors, which is not their function, the turns are specially wound to prevent self-induction. By their very nature, however, they need to be robust to withstand the surges caused by lightning and faults. Spacing strips of insulation are fitted between turns and windings, and between windings and core, to bring the cooling oil into contact, as far as possible, with ail surfaces. The oil must be maintained in an absolutely pure condition by frequent centrifugal separation to remove moisture and impurities.
TESTING A SIX-PHASE MERCURY ARC RECTIFIER BULB. Rectifiers of this type were first devised by Cooper Hewitt in 1903. The rectifier is fitted with a cathode and a series of anodes under vacuum in a glass vessel.
Some flexibility in the supply voltage is called for in most systems of distribution and, to meet this, tappings are brought out from the transformer windings and connected to special tapping switches that enable small increases or decreases of voltage to be effected without breaking any of the circuits. Important potential consumers for any scheme of electric supply in Great Britain are the railway companies, many suburban lines having already been changed over from steam to electric working. The most suitable form of electric supply for the operation of railways has been the subject of considerable debate and experimental trials in many countries. So far, the suburban lines of Great Britain, with one or two exceptions, have been operated with motors using direct current at 600 volts.
The customary method of electric railway operation is to supply alternating current in bulk at high voltage to substations along the line, where transformers are installed in conjunction with apparatus for conversion from alternating to direct current. Of recent years the choice of conversion apparatus has been narrowed down to the relative merits of rotary converters and of mercury arc rectifiers.
The rotary converter, originally designed in 1890, consists in effect of a direct-current generator fitted with alternating current sliprings connected to suitable points in the armature. The machine has the advantage of being reversible in use, that is, alternating current fed into the sliprings can be taken out as direct current from the armature at the other end of the machine, or, alternatively, surplus direct current at the armature can be passed back to the A.C. supply. In this it is superior to the mercury rectifier, which needs a separate machine for each direction of working.
The mercury arc rectifier was a later arrival devised by Peter Cooper Hewitt in 1903. Because of the novel principles involved it required a much longer period of development to find its true value, but it now stands as a serious rival to the rotary types of machine.
Resemblance to Radio Valve
In principle the mercury arc rectifier bears some similarity to a radio valve, being fitted with a cathode and a series of anodes mounted inside a glass or metal container that is completely evacuated of air. In the same way as the radio valve, the mercury rectifier first took shape as a glass vessel enclosing a vacuum, the electrical connexions being sealed through the glass. Of recent years, however, powerful radio valves for broadcasting, as well as mercury arc rectifiers, have been built up with metal containers and continuously evacuated, by special pumps. Unlike the radio valve, the mercury rectifier has no so-called “space charge” and requires only some 17 to 30 volts drop in the arc to enable it to pass large quantities of direct current derived from both positive and negative impulses of the alternating current supply. The cathode is connected to a pool of mercury in the bottom of the chamber with a large number of anodes mounted above, each of which is coupled to the several phase connexions from the secondary of the alternating current transformer. In operation a column of mercury vapour is formed that carries the arc of current from the cathode to each anode in rapid sequence, as their potentials fluctuate with the alternating supply. In effect a rotating arc is formed that races round the various anodes from the cathode. Continuous water cooling of the bowl and of the anodes, and so forth, is essential and several special fittings are included in the design to ensure trouble-free operation with freedom from backfires. As the entire metal shell of the rectifier is “alive”, it is mounted on insulators and guarded at a safe distance by railings to prevent close approach when in operation.
TRANSFORMER FOR THE BRITISH GRID, which carries high voltages for three or four thousand miles in wire varying in diameter from ⅜ in. to ⅞ in. This transformer steps down voltage from 132,000 to 11,000 and has a rating of 30,000 kVA.
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