Transient behavior of an Aluminum pot-line

OVER A PERIOD of several years a number of transformers and induction regulators supplying energy to aluminum pot-lines and similar installations have been damaged by insulation failures, apparently resulting from abnormal overvoltages. The transformers used for these applications generally step down from 13.8 kv to a secondary voltage in the range 600 to 900 volts. A basic impulse level of at least 40 kv is normal for the secondary winding. The fact that insulation failures occur on these windings is an indication of the severity of the voltage transients, and clearly shows that they are out of the range of normal switching transients. However, in many cases it is difficult to see how such voltage surges could invade the low-voltage system from the high-voltage side of the power transformers. This points to some other means of surge generation in the low-voltage circuit, and a likely cause is current suppression.