Industrial Furnace Transformers

In our course EP-3 for Transformers, I mentioned that Transformers have many different types according to Application, one of these applications was the Power Transformers which have sub-types as follows:

1- Generator step-up transformers.
2- Step-down transformers.
3- System intertie transformers:

• Phase Shifting Transformers.
• HDVC Transformers.

4- Industrial transformers.
5- Traction transformers.


I explained each one of power transformers types in previous topics; just follow the link for each type to land on its topic today i will continue explaining additional types of power transformers which will be the industrial transformers as follows.


Industrial transformers

Industrial Transformers

Transformers play an important role in several industries, especially in metallurgical plants and processes as in primary aluminum, copper, zinc, chlorine, and carbon industries, or for large industrial drives. And while production capacities are growing on a grand scale in order to satisfy worldwide demand, higher voltages and currents need to be supplied by more and more powerful industrial transformers which include the following types:

1. Furnace Transformers.
2. Convertor Transformers.

First: Furnace Transformers

furnace transformer is a transformer where a furnace melts charged material through an electric arc. Transformers of these types are used in steel melting and metallurgical industry.

Types of  Furnace Transformers

Furnace transformers can roughly be divided into two groups:

1. AC Furnaces Transformer.
2. DC Furnaces Transformer.

AC Furnaces Transformer in turn subdivided to:

• Arc Furnace Transformer 
• Reduction Transformer 

1- Arc Furnace Transformer

Arc Furnace Transformer

Technical features:

• Ratings up to 250 MVA.
• Wide Low Secondary voltage from 80 V up to 1,500 V.
• High secondary current.
• Electrode current for steel up to 120 kA depending upon the MVA rating.
• Electrode current for ferroalloy up to 180 Ka depending upon the MVA rating.
• Made both a single and three phase (Arc furnace transformers are normally designed as three phase units).
• Designed as shell type or core type with a few exceptions (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).

Usage:

Arc furnaces transformers are used in the process of melting scrap metal inside an arc furnace.

Arc Furnace transformer design:

The furnace transformer has to be specially designed to meet the following:

• Withstand frequent short circuits on the secondary side.
• Minimize voltage drop, harmonics and other effects resulting from wide fluctuations and unbalanced conditions for Currents drawn in the arc. 

These effects can be mitigated by supplying furnaces directly from a high voltage transmission line having high capacity (adequate fault level at the supply point) through a furnace transformer.

• Minimizing the Leakage Reactance of the arc furnace transformer since it gets added to the Reactance of the high current connections between the transformer secondary terminals and the electrode tips. 

fig(1): Electric-Arc-Furnace series-Reactor Connection

However, a certain minimum value of reactance is required in the furnace circuit to stabilize arc. 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 sufficient reactance value for the stability (see fig.1). These series reactors, which may be housed in the tank of furnace transformer, are 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. Some arrangements must be taken for protecting the secondary winding against the transferred voltages from the high voltage primary winding by the following: 

• Connection of a surge arrester or capacitor between the secondary terminals and ground.
• Placement of electrostatic shield between the primary and secondary windings.

The secondary winding of a furnace transformer is made up of a number of parallel coils arranged vertically and connected by vertical copper bars. The goand-return arrangement is used for 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 (get 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

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 water does not leak into oil if a leakage problem develops).

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



In the next Topic, I will continue expalining the Furnace transformers types. so, please keep following.