Voltage transformers and current transformers associated with switchgear

The adoption, in recent years, of layer windings and the introduction of cold-rolled silicon steels have effected a marked increase of impulse strength together with a reduction of size of electromagnetic voltage transformers. The development of synthetic resins as insulants for electromagnetic transformers, although in its early stages, shows promise that they will replace other insulants for voltages up to 33 kV. Graphs are given for estimating the output of capacitor-type transformers, and it is concluded that they are uneconomical below 110 kV. Economically they are very attractive when used in conjunction with high-capacitance-type extra-high-voltage current transformers which have recently been developed. The transient performance of current transformers is examined, and it is concluded that protective systems should either impose a very small burden or permit some saturation. The burden can often be reduced by rating the secondary winding at 1 amp, and this is recommended where the protective apparatus is not adjacent to the transformers. The main objection to such a rating lies in the high voltages which may be induced in the winding; these are discussed. Various power-system operations and protection requirements are considered in order to determine the most suitable location for current transformers and voltage transformers relative to the associated switchgear.