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AC ARC FURNACE TRANSFORMERS
This section deals with the special features of transformers used for the provision of supplies to alternating current electric arc furnaces. Although, to the author's knowledge, there are no direct current electric arc furnaces in the UK in the early 2000s, there are some in Europe and a few in Japan. Direct current furnaces have a higher efficiency and feed back less disturbance into the sup ply system.
Arc furnaces utilize the heating effect of an electric arc to melt the contents of the furnace. They are characterized by very high currents, perhaps up to 200 kA, at relatively modest voltages, between, say, about 200 and 1000 V. Because of the power required by the furnace, which might be between 10 and 100 MVA and occasionally even up to 200 MVA or higher, and the nature of the load, the transformers will need to take their supply from a strong HV system. In the UK it has been found appropriate to provide a supply to furnaces of up to around 120 MVA at 33 or 66 kV derived from a supply which is usually dedicated solely to supplying a number of arc furnaces and having a direct connection to a grid bulk supply point so that the disturbances created on the network will be maintained at an acceptable level. For furnaces rated at 60 MVA and above the bulk supply point needs to be associated at least with the 275 kV system. The relatively modest 33 or 66 kV HV voltage for the furnace transformer has the benefit of making the insulation level of the HV windings and the tapchangers considerably lower and therefore less expensive than they might otherwise have been.
In some less developed countries it is occasionally possible that a hydro electric scheme might supply a smelter, or a number of smelters and little more.
In such locations the arc furnace transformers may operate directly from the hydro transmission voltage of, say, 220 kV but it is more likely there will be some intermediate voltage, possibly 33 kV provided specifically for supplying the arc furnace transformers so as to simplify the design of these. It should be noted that wherever bulk supplies transformers are associated with the furnace transformers, these bulk supplies transformers will be subjected to similar adverse loading conditions to those imposed on the furnace transformers themselves, so that many of the design considerations regarding furnace transformers described below will, to a considerable extent, also apply to the associated bulk supplies transformers.
The principle of the AC arc furnace is shown in FIG. 43. The load cycles of furnaces vary widely, depending on their size and the metallurgical requirements. Many furnaces have load cycles falling within the range 3-8 hours.
The first part of the cycle consists of the melt-down period when the solid charge is melted and the main energy input is required. The latter part of the cycle is the refining period; in this the energy supplied has only to make good the heat losses. The melt-down period is characterized by heavy current fluctuations caused by arc instability and movement of the charge (FIG. 44). In the refining period fluctuations are much smaller because all the charge is molten.
The severity of the current fluctuations during the melt-down period is governed to varying extents by the electromagnetic design of the furnace and its transformer and by the type of charge. Individual current excursions several times larger than the furnace nameplate rating are possible. The fineness of the charge has an important bearing on these fluctuations. For example, finely shredded steel scrap causes much smaller fluctuations than does a charge consisting of large irregular pieces. The main causes of the fluctuations are the movement of the arcs due to the changing electromagnetic-field conditions, and in some cases their extinction and restriking, and also by the short-circuiting of the graphite electrodes by movement of parts of the charge.
Deciding on the continuous rating of the furnace transformer requires a detailed study of the operating cycle. Although in arriving at a suitable rating the effects of the current surges which occur during the first part of the loading cycle must not be overlooked, advantage may be taken of the two distinct phases of the cycle so that it is possible to utilize some overload capacity to meet the peak loadings which occur during the melt-down phase on the basis that these will be balanced by the reduced loading during the refining stage. At the beginning of the melt cycle the instantaneous loading of the transformer may be up to twice its continuous rating.
Because of the high currents required from the LV windings, the constructional problems of furnace transformers are similar to those of large rectifier transformers described in the previous section, except that they are compounded by the rapidly fluctuating nature of the load. The LV turn cross-section needs to be very large and the number of turns required is few. The phase current can be reduced by a factor of _3 by connecting this winding in delta. In order to bring out the leads, the LV must be the outer winding and it becomes impracticable to produce plain helical outer windings having a large cross-section and a small number of turns; the helix angle would be very large, making it difficult to obtain good electromagnetic balance, wasting a large amount of space at each end of the winding; and the winding itself would possess very little 'capstan effect' to resist the outward bursting forces experienced during current surges and short circuits.
A high degree of short-circuit strength is particularly important in a furnace transformer in view of the nature of the load. Designing the transformer to have a fairly high impedance assists in limiting the magnitude of the cur rent surges and minimizes their effects on the supply network, however too high a value reduces the furnace short-circuit power which increases its cycle time. The combined impedance of transformer and furnace may be as high as 50 percent on rating and the major portion of this will be provided by the transformer. Nevertheless the windings will be repeatedly subjected to severe mechanical shocks during the melt-down period so that their bracing and structural supports must be exceedingly robust. All winding spacers and end support blocks must be positively keyed in position to ensure that the constant buffeting that they receive does not cause them to become loosened.
Radial support for the windings must be provided by substantial pressboard or synthetic-resin-bonded paper (s.r.b.p.) cylinders; the winding conductors them selves will probably be of silver bearing copper, work hardened by the winding process to provide maximum strength and rigidity. Core frames must also be of substantial section, usually having extended and reinforced coil support plates which in turn support heavy laminated-wood winding end platforms in order to provide maximum rigidity and to withstand axial short-circuit forces with the minimum of deflection. Tie bars connecting top and bottom yokes will probably be provided beneath the inner winding so as to be well out of the way of stray fluxes created by the high currents in the LV winding.
To enable the requirements of good electromagnetic balance and high mechanical strength to be met, the LV winding is usually constructed from a number of parallel disc pairs. For example, if the winding is required to have 16 turns of 100 strands in parallel, then this may be formed by stacking, say, 50 disc pairs -- pairs because both start and finish must be at the outer surface -- axially along the winding length. Each disc pair will then contain 16 turns wound with two strands in parallel. If the strand size is, say, 15 x 5 mm, this will have a cross-section of approximately 72 mm^2, allowing for radiused corners, then 100 parallel strands provides a total conductor cross-section of 7200 mm^2. A current density of 3.2 A/mm^2 gives a total current carrying capacity of 23.04 kA which for a winding voltage of 1000 V, delta connected, is equivalent to a three-phase rating of about 40 MVA.
The other characteristic of the arc furnace which compounds the transformer designer's problems is that the voltage drop in the furnace varies greatly during the changing stages of the melt. To strike the arc and maintain it at the initial melt-down stage requires a very much greater voltage than that necessary during the refining stage when obtaining equilibrium requires an accurate control of the furnace current. Close control of LV voltage is therefore important and, in view of the very high current in this winding, this must be achieved by means of tappings on the HV winding.
Again, in order to maintain optimum electromagnetic balance of the transformer windings so as to minimize out of balance forces, these tappings will best be contained in a separate layer which will probably be of the multistart helical type placed inside the HV winding. This arrangement has the benefit of enabling optimum balance to be maintained regardless of the number of tappings in circuit and also allows the tapping leads to be relatively easily taken from the top and bottom ends of the winding. In this location the radial short-circuit forces will be directed inwards towards the core so that a helical arrangement has adequate mechanical strength.
As explained in Section 4.6, control of LV voltage by means of HV tappings leads to variation of flux density and, in view of the wide range of voltage variation necessary, there will be considerable flux density variation. Using a modern grain oriented steel, it will be necessary to design for a maximum nominal flux density of 1.9 Tesla under any supply voltage conditions, which may mean up to 10 percent high, so that, at nominal supply voltage, a limiting flux density of about 1.72 Tesla must be assumed. If this flux density equates to maximum LV output voltage, then in order to provide a minimum voltage of, say, 50 percent of this, the lowest flux density will need to be as low as 0.86 Tesla.
In addition, in order to produce the required degree of current control, a large number of very small tapping steps must be provided. This presents practical problems; there are limits to the number of tappings which can be provided in a multistart arrangement beneath the HV winding, and also most commercial tapchangers have a maximum of about 18 steps, 19 positions, anyway.
One solution is to provide two transformers: a regulating transformer and a step-down transformer. The former will probably be auto-connected. It will operate at a nominally constant flux density, allowing only a 10 percent margin for supply voltage variation, and will be controlled by a line-end tapchanger to provide, say, eight tapping steps from 100 to 50 percent of the supply volt age. The line-end tapchanger will not need to be particularly special provided the supply voltage is no more than 66 kV, in fact, any tapchanger designed for 66 kV delta-connected operation will be suitable. The output from the regulating transformer will then supply the tapped HV winding of the step-down transformer via a second tapchanger which will provide the intermediate fine tapping steps. The number of these steps will depend only on the maximum number of steps that this tapchanger can accommodate and the number of tapping leads that it is considered economic to bring out from beneath the HV winding. Since this transformer will be subjected to a widely varying supply voltage from the regulating transformer, it will operate at a widely varying flux density.
Notwithstanding the above comments concerning tapchanger requirements, it should not be overlooked that the duty of the tapchangers associated with arc furnace transformers is a very demanding one, both in terms of tapchanging duty and number of operations, which will be considerably greater than that of the tapchanger of a normal system transformer. The tapchanger must be very conservatively rated therefore to ensure that it is capable of repeated operation at any condition of load or overload and it needs to be frequently maintained to ensure that it remains capable of this duty.
FIG. 45 shows the core and windings of an 85 MVA 34.5/1.2-0.78 kV AC electric arc furnace transformer having a tapchanger at the line end of each phase. The HV winding, which is the tapped winding, is next to the core and the leads to the helically interleaved tapping windings can be seen emerging at the bottom of each leg from beneath the LV windings.