A transformer supplying an arc furnace has to deliver unusually high currents over a wide range of voltage. Power ratings between 50 and 100 MVA are quite common now with the secondary currents of more than 50 kA.

Furnace transformers have special features for handling very high currents compared to conventional transformers. Since the LV winding current is high, its voltage is better controlled by providing taps on the HV winding.

Arc Furnace Transformers

An arc furnace has three electrodes connected to the secondary terminals of the furnace transformer which has to be specially designed to withstand frequent short circuits on the secondary side.

Currents drawn in the arc furnace are characterized by wide fluctuations and unbalanced conditions, which lead to problems of voltage drops, harmonics, etc. These effects can be mitigated by supplying furnaces directly from a high voltage transmission line having high capacity (i.e., an adequate short-circuit level at the supply point) through a furnace transformer.

In such a case, when the voltage ratio is high, suitable measures should be taken for protecting the secondary winding against the electrostatically transferred voltages from the high voltage primary winding. These measures are connection of a surge arrester, or a capacitor, between the secondary terminals and a ground and placement of an electrostatic shield between the primary and secondary windings.

The leakage reactance of the furnace transformer affects the furnace operation since it is added to the reactance of the high current connections between the transformer secondary terminals and electrode tips. The higher the reactance, the lower the useful service currents are, thereby reducing the efficiency of the operation.

Hence, the leakage reactance needs to be kept as small as practically possible with due consideration to the short-circuit withstand of windings and clamping/support structures. Also, a certain minimum value of reactance is required in the furnace circuit to stabilize arcs.

In large furnace installations, the low voltage connections usually provide the necessary reactance. For smaller installations, a reactor may have to be added in series with the primary winding to give a sufficient reactance value for the stability.

The series reactor, which may be housed in the tank of the furnace transformer, is usually provided with taps so that the reactance value can be varied for an optimum performance. Hence, depending on the rating of furnace installation and its inherent reactance, the leakage reactance of the furnace transformer has to be judiciously selected to meet the stability and efficiency requirements.

Although the core-type construction is common, the shell-type construction is also used because one can get a desired low impedance value by suitably interleaving the primary and secondary windings.

The furnace transformers are provided with a separate regulating (tap) winding. The variation of the percentage reactance over the entire tapping range depends on the disposition of the windings.

The melting process of a furnace requires initially more power to break down and melt the furnace charge. The power required afterward for refining the molten metal is lower. The variable power input requirement is achieved by varying the supply voltage to the electric arc furnace over a wide range continuously using an OLTC (On Load Tap Changer). Its use is essential where temporary interruptions in supply for changing taps is not desirable.

Since the regulation required is generally fine, an OLTC with a large number of steps is required. Due to frequent operations, its oil quality should be regularly checked. It is preferable to place the OLTC in a separate compartment so that its maintenance can be carried out without having to lower the oil to an extent that windings are exposed.

The commonly used arrangements for the voltage regulation are shown in Figure 1.

Arrangement (a), which consists of taps at the neutral end of the primary winding, is used for low rating furnace transformers (5 to 10 MVA). The cost of the OLTC is minimum due to lower voltage and current values (the primary voltage may be of the order of 33 or 66 kV).

The disadvantage of this arrangement is that the step voltage is not constant throughout the range of voltage regulation. For a fixed primary voltage, when the tap position is changed for varying the secondary voltage the voltage per turn changes, which results in nonuniform step variations in the secondary voltage from one tap to another.

Arrangement (b), used for larger furnace applications, eliminates the disadvantage of the previous arrangement. A separate autotransformer is used for the voltage regulation. The step voltage is uniform throughout since the voltage per turn is independent of the tap position for a given input voltage applied to the primary winding of the autotransformer which may be supplied directly from a system at 66 or 132 kV.

The OLTC voltage class is higher than that of arrangement (a) and three single-phase tap changers may have to be used. Also, the autotransformer and the furnace transformer are usually housed in separate tanks, thereby increasing the cost and size of the total system.

The most popular arrangement used for medium and large power furnace applications consists of a furnace transformer with a booster arrangement as shown in Figure 1(c).

 The booster transformer on the output side boosts or bucks the fixed secondary voltage of the main transformer. The primary winding of the booster transformer is supplied from the tap winding of the main transformer, and the supply voltage is selected such that it results in the least onerous operating conditions for the OLTC. Hence, the OLTC cost is low in this arrangement.

Also, the variation in the secondary voltage is the same from one tap position to another throughout the range of regulation. Usually, the main and booster transformers are placed in the same tank minimizing the length of connections between the secondary windings of the two transformers. The amount of structural steel required is also reduced.

The booster transformer rating is much smaller than that of the main transformer, being sufficient for the regulation purpose only. Although the diameters of the two cores are different, the magnetic circuits of the two transformers have generally the same center-to-center distance and equal window heights to facilitate the connections between their secondary windings.

If one wants to reduce the core material content, the center-to-center distance of the booster transformer can be lower, but the connections become complicated.

Since the currents of the secondary windings of the main and booster transformers are equal, the same conductor type and size are generally used for both windings.

Also, the two windings are often connected in a figure eight fashion (Figure 2) avoiding extra connections between them. A special arrangement is required to lift the two core-winding assemblies simultaneously.

Since the current carried by the secondary windings is quite high, a continuously transposed cable (CTC) conductor is used which minimizes the eddy losses, gets rid of the transposition problems, and improves the winding space factor. The material of structural parts supporting high current terminations and tank parts in the vicinity of high current fields should be nonmagnetic steel.

Suitable modifications in magnetic clearances and the tank material eliminated the hot spots. The secondary winding of a furnace transformer is made up of a number of parallel coils arranged vertically and connected by vertical copper bars.

A goand-return arrangement is used for the input and output connections (placed close to each other) reducing the magnetic field and associated stray losses in the nearby structural parts. A delta-connected secondary winding is preferable since the current to be carried by it is reduced. Many times, both the ends of each phase of the secondary winding are brought out through the terminals and the delta connections are made at the furnace (i.e., the connections are automatically formed by the metallic charge in the furnace).

This minimizes the inductive voltage drops in the leads and can achieve a better phase balance between the electrode currents. Due to heavy connections, some unbalance may exist which has to be minimized by some specific arrangements.

The secondary winding terminals are usually located on the vertical side of the tank (instead of the top cover) resulting in lower lengths of the connections, stray losses, and the transformer cost. The LV (secondary) winding is invariably the outermost winding and the HV (primary) winding can be placed next to the core.

In such a case, the regulating (tap) winding is between the HV and LV windings. Such a disposition of windings reduces variations in the percentage impedance as the tap position is changed from the minimum to the maximum in variable flux designs.

Small furnace transformers are naturally cooled with radiators. For large ratings and where there are space restrictions, forced oil cooling with an oil-to-water heat exchanger can be used. The oil pressure is always maintained higher than the water pressure (so that the water does not leak into the oil if a leakage problem develops).

The LV terminations may be in the form of U-shaped copper tubes of certain inside and outside diameters so that they can be water cooled from inside. These copper tubes can be cooled by oil as well.