HOW FURNACE TRANSFORMER WORKS
What are Furnace Transformers?
Furnace transformers supply power to electric furnaces of the induction, resistance, open-arc, and submerged-arc types. The secondary voltage are low, occasionally less than 100 V, but generally several hundred volts.
Sizes range from a few kVA to over 150 MVA, with secondary currents over 100,000 A. High currents are obtained by parallel connection of many winding sections.
Current is collected by internal bus bars and brought through the transformer cover by the bus bars or by high-current bushings.
The power input to the furnace is controlled by adjusting the output voltage of the furnace transformer.
Optimum performance of the furnace may require adjustment of the secondary voltage over a range of 3:1 or more. This may by accomplished by a regulating transformer between the highvoltage power source and a fixed-ratio furnace transformer.
More frequently, regulation is obtained by taps in the high-voltage winding. In addition to taps in the high-voltage winding, a delta-Y switch in the high-voltage winding is often used to extend the range of voltage by an additional ratio of 1.73.
Motor-operated off-load tap changers are usual, but occasionally on-load tap-changing equipment is justified by the saving in melt time and reduced breaker maintenance.
The load-tapchanging duty is more severe than on the usual power transformer, not only with respect to frequency of operation but also because of the extreme range, which results in large kVA increments per tap.
Circuit reactance furnishes current stability for ac arc furnaces. In the larger sizes, the inherent impedance of the transformer and its associated secondary conductors is sufficient for adequate stability.
This is not generally true for smaller arc furnaces. Consequently, it is customary in furnace transformers rated 7500 kVA and below to include a reactor in the tank with the transformer.