Current instrument transformers 35..800 kV

Measuring high-voltage current transformers (CTs) of the IOSK series are designed to transmit a signal of measuring information to measuring instruments, protection, automation, signaling and control of AC electrical circuits from 35 to 800 kV with a frequency of 50 or 60 Hz. The principle of operation of current transformers is based on the phenomenon of electromagnetic induction. HV instrument transformers are designed for a service life of at least 30 years (and even more with proper operation) without any maintenance. The oil is hermetically sealed from the air by a stainless steel bellows assembly and all external parts are made of corrosion resistant material.

By design, reference-type instrument transformers with top windings in a metal case. The high-voltage windings are housed in a metal case mounted on top of the transformer's insulating column, and oil is used as the internal insulating medium. Current transformers (CT) can have up to eight secondary, measuring and protective windings, for various loads and accuracy classes. The primary winding is connected to the inputs passing through the aluminum housing, the change in the number of turns of the primary winding is carried out using special metal jumpers. The secondary terminals of the windings are located in a terminal box located at the base of the transformer. To prevent unauthorized access to the secondary windings, the cover of the terminal box located inside the terminal box is sealed.


  • Rated voltage from 35 to 800 k
  • Rated current up to 5000 A
  • Rated short-time thermal current (Ith) up to 63 kA/1 or 3 sec.
  • Rated dynamic withstand current (Idyn) up to 160 kA
  • Number of secondary windings up to 8
  • Winding accuracy class for measurement 0.2; 0.2S; 0.5; 0.5S; 1; 3;
  • Winding accuracy class for protection 5P;10P
  • Switching the transformation ratio on primary or secondary outputs.
  • Oil expansion with stainless steel bellows
  • External insulation porcelain or polymer
  • Oil level indicator
  • Ambient conditions: temperature from -60° to +55°
  • Compliance with IEC and IEEE standards or equivalent standards
  • Possibility of manufacturing transformers for direct (DC) voltage.


  • Special protection accuracy classes: TPS, TPX, TPY, TPZ, PR
  • Affordable High and Extended Range Accuracy (ERCT) Design
  • Possibility of manufacturing transformers for direct (DC) voltage.
  • Consistent accuracy over the entire service life
  • Maintenance Free
  • Small amount of oil
  • High fire and explosion safety

The description contains the characteristics of transformers that are produced in series. By agreement with the customer, it is possible to manufacture non-standard transformers.