Protective Current transformers (Refer TABLE-II) are specified by (1) a burden (2) accuracy class and (3) accuracy limiting factor. In other words the errors of the protective current transformers at the rated full loaded current as well as at a current given by the accuracy limit factor times the full load current are specified. There exists a class of protective current transformers for special purpose application known as class PS CTs. These are mainly intended for use in applications where the required characteristics of the current transformers cannot be conveniently expressed in terms used for class 5P, 10P and 15P current transformers. The knee point voltage, exciting current and secondary winding resistance are so dependent on the protective gear involved. The manufacturers of protection gear stipulates the requirements in terms of minimum knee point voltage and maximum exciting current. The current transformers of class PS type are of low reactance type since the core is made up of jointless ring type (spirally wound toroidal type) cores and secondary turns are substantially evenly distributed along the whole length of the magnetic circuit. PS class current transformers are characterised by

(1) Normal ratio

(2) Knee point voltage (KPV)

(3) Exciting current at KPV or a stated percentage

(4) Secondary resistance at 750 C.

CONSTRUCTIONAL FEATURES

Current Transformer can be of Ring type, Rectangular type, or Resin moulded type. High quality cores are used for the construction. Computer aided designs are used for all ratings to improve quality and to satisfy customer requirements

1. Tape Wound Current Transformer (TP)
   These are ring type Current Transformers with PVC tape insulation.
2. Resin Cast Current Transformer
   Resin Cast Current Transformers are with dry insulation moulded with Epoxy Resin.
   • Window Low Tension (WL)
     These are Current Transformers which have opening in the center to accommodate a primary conductor running through it.
   • Wound Primary (AR)
     Current Transformers having a primary winding of more than one turn wound over the core and with primary and secondary terminals.
   • Primary Turns Through Centre
     A Current Transformer for Lower Ratios – Turns through center to be provided by the end user.
3. Interposing CTs
   These are used when the ratio of transformers is high. It is also used to correct phase displacement for differential protection of transformers.
4. Summation CTs
   Summation current transformers are suitable for the summation of several synchronized alternating currents with similar phases but with differing load phase shifts. It is also possible to have the summation of currents with varied nominal voltages of similar phase positions.

Core Balance CTs
These are Zero sequence CTs used for earth leakage and earth fault protection. The three-core cable or three single cores of a three-phase system passes through the inner diameter of the CT.