Negative Booster Transformer

a special-purpose transformer used in contact systems. The electromagnetic field from a single-phase alternating current flowing through the system’s wires has an effect on aerial and cable communication lines, on metal pipelines, and on other service or communication lines located near electrified railroads. Negative booster transformers are designed to reduce this effect.

The currents in a contact system produce by mutual induction electromotive forces (emf’s) in neighboring service and communication lines; these emf’s affect the normal operation of the lines and are hazardous for maintenance personnel. The currents in the rails, which flow in the opposite direction to the currents in the contact system, reduce the harmful influence of the latter since they produce induced emf’s with the opposite sign—that is, they have a protective shielding effect. The rails are not insulated from the earth and therefore a portion of the current, which is called leakage current, is shunted into the earth as if the earth were a conductor connected in parallel with the rails. As a result, the currents in the rails are substantially decreased; the leakage currents, which flow at a great depth, produce almost no shielding effect. In order to increase the shielding effect, the primary winding of a negative booster transformer is connected in series with the contact system, while the transformer’s secondary winding is connected in series with the rails. The transformer ratio of the negative booster transformer is unity; the secondary winding is forced to maintain its current equal to that of the primary winding, with the result that less current passes into the earth and the shielding effect is increased.

To enhance the effectiveness of the negative booster transformer, an additional return conductor connected in parallel with the rails is suspended from the supports of the contact system, and the secondary winding of the negative booster transformer is connected in series with it. A shielding effect twice as great as that for the first method of connection is thereby produced. Negative booster transformers are installed usually at distances of 1.5 to 3 km (when connected to the rails) and 3 to 6 km (when connected to an additional conductor).