LT/LV & HIGH VOLTAGE TESTING TRANSFORMER

The uses of electric are furnace has become increasingly common in recent years, particularly in the chemical and metallurgical industries. Such furnaces require large current at low and variable voltage, and demand special considerations in transformer design. Where possible, a shell design is used as the windings on the low – voltage side can be sectionalized with greater uniformity of impedance. A three-phase furnace requiring a single turn per secondary phase, however, is of the core type. Similar considerations apply to elector – chemical loads.

The usual parallel connections of the secondary coils demand on even distribution of the leakage reactance on both primary and secondary sites: otherwise circulating currents may flow between parallel – connected sections. The secondary is frequently constructed of sheet – type single turns, keep thin to avoid excessive eddy – currents. The turns are stamped to shape from copper sheet. The phases are led out with the connections arranged for minimum leakage. The reactance of leads may be seriously large, particularly if the transformer is some distance from the furnace. Voltage regulation is provided by tapping the high – voltage side, to avoid complicated side, to avoid complicated heavy – current connections. The tappings must be arranged symmetrically to avoid unbalanced mechanical forces in case of a short circuit – a not infrequent occurrence.

At present H.V. testing technique requires four kinds of H.V. supply – Industrial frequency alternating voltages up to 1000 – 2000 KV., or less : It is most common, being applied to routine tests of materials, machines and apparatus, and for use in connection with scheming bridge measurements.

Constant direct voltages : It is employed almost exclusively for the testing of cables on site for the production of X-rays, and feeding the anodes of high- speed cathode – ray oscillographs. High frequency generators

High frequency alternative voltages up to about 1000kv. At about 100-200Kc/s. : These are required for testing at ratio frequencies, and for investigation on porcelain insulators. Surge or impulse voltages up to about 1000 KV or more & of duration a few micro/Millie – seconds : It is required for the experimental investigation of transient disturbances on transmission lines due to lighting switching etc., particularly surge voltages of steep wave – font. All these supplies involve the use of transformers.