Industrial Furnace Transformer - Part Two

In the previous topic; Industrial Furnace Transformer Part One, I state that the industrial transformers include the following types: 

  1. Furnace Transformers.
  2. Convertor Transformers.
while Furnace transformers can roughly be divided into two groups:
  1. AC Furnaces Transformer.
  2. DC Furnaces Transformer.

and AC Furnaces Transformer in turn subdivided to:
  1. Arc Furnace Transformer.
  2. Reduction Transformer.
I explained the Arc furnace transformer in this past topic and today i will continue explaining this type and the other types; Reduction Transformer & DC Furnaces Transformer as follows.

AC Electric Arc Furnace Transformers

Arc Furnace operation principles:see fig.1

The arc furnace, where scrap materials are charged, has three electrodes connected to the secondary terminals of the furnace transformer.

The arc forms between the charged material and the electrode, the charge is heated both by current passing through the charge and by the radiant energy evolved by the arc. The electrodes are automatically raised and lowered by a positioning system, which may use either electric winch hoists or hydraulic cylinders.

Usually in combination with an arc furnace, there is a Ladle furnace (LF) which used to maintain the temperature of liquid steel during processing after tapping from EAF or to change the alloy composition. Unlike an arc furnace, a ladle furnace does not have a tilting or scrap charging mechanism.

Voltage regulation and tap changing:

The Voltage regulating system maintains approximately constant current and power input during the melting of the charge

The commonly used arrangements for the voltage regulation

Arrangement (a)


It 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 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 a non-uniform change in the secondary voltage from one tap to another).

Arrangement (b)

It is 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 tap position for a fixed input voltage applied to the primary of autotransformer. The autotransformer, which may be supplied directly from a system at 66 or 132 kV, reduces the voltage down to the level of the primary winding of the furnace transformer. 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 furnace transformer are usually housed in separate tanks thereby increasing the cost and size of the total system.

Arrangement (c)


It is The most popular arrangement used for medium and large power furnace applications is the furnace transformer with a booster arrangement, 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 quite low in this arrangement.

Also, the variation of secondary voltage is 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 both the transformers. The amount of structural steel required is also reduced.

2- Reduction furnace transformers

Reduction furnace transformers are mainly used in the metallurgical industry. Production of ferroalloys is an important application. The major alloys are ferro-silicon, Silicon metal and ferro-manganese. Other significant alloys are ferro-nickel and ferro-chrome. Reduction furnaces are also used in the production of non ferrous metals such as copper, nickel, tin, lead and zinc. Reduction furnaces are also used in the production of calcium carbide. The mist common design for reduction furnace transformers is single phase.

DC furnace transformers

DC Electric Arc Furnace Transformers 
DC furnace transformers are also used to melt scrap metal (steel) three phase is the most common design. Since direct current is used in the melting process the transformer is installed in combination with a rectifier, which feeds the furnace.

Direct current furnaces have a higher efficiency and feedback less disturbance into the supply system.

DC arc furnaces are used, with a single electrode in the roof and the current return through a conductive bottom lining or conductive pins in the base. The advantage of DC is lower electrode consumption per ton of steel produced, since only one electrode is used, as well as less electrical harmonics and other similar problems. The size of DC arc furnaces is limited by the current carrying capacity of available electrodes, and the maximum allowable voltage. Maintenance of the conductive furnace hearth is a bottleneck in extended operation of a DC arc furnace.

In the next Topic, I will continue expalining the Convertor Transformers. so, please keep following.


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