Current instrument transformer of bushing design CTT 12

The CTT 12 is a one-turn through-current instrument transformer of  bushing design, intended for measurement and protection purposes in MV indoor distribution systems with rated primary currents up to 5 000 A, and the highest voltage of the system of 12 kV.

The secondary current value is 5 A or 1 A, with the possibility of combination.

The CTT 12 is a one-turn through-current instrument transformer of  bushing design, intended for measurement and protection purposes in MV indoor distribution systems with rated primary currents up to 5 000 A, and the highest voltage of the system of 12 kV.

The secondary current value is 5 A or 1 A, with the possibility of combination.

The accuracy classes of the measuring circuits are 0.2; 0.2S; 0.5; 0.5S; 1 and 3, and those of the protection circuits are 5P and 10P. After consultations with the client the CT can be designed with special classes of accuracy of the PX, TPX and TPY type.

Requirements on the accuracy class of the CTT 12 transformers are met within 25% tpo 100% of the rated burden range. For burdens < 10 VA the accuracy class requirements are met within the range of 0 to 100 %.

The ultimate operating current is 120% of IN, and in mutual agreement between the manufacturer and the client also other current values may be offered, e.g.  200 % IN.

The secondary winding is wound on a magnetic core made from oriented alloys. 

The number of cores corresponds to the client´s wishes and to the required parameters, with maximum 3 cores.

All active parts of the transformer are embedded in an epoxy mixture which provides for the electric insulation and mechanical function.

The mounting surface of the transformer can be of any arrangement, for mounting on a base plate (Fig. 3) using four fixing screws.

The number and position of the primary conductors (copper busbar) is determined by the primary current amperage (IN ≤ 4000 A horizontally, IN > 4000 A vertically). The contacting is shown in Fig. 4.

It is recommended to connect the secondary outlets using cable lugs corresponding to the conductor cross section. The secondary terminal board is equipped with a sealable screw, and with by-packed connectors and screws for earthing or short-circuiting the winding (see the “Operating and assembly instructions”).

The CTT 12 transformers satisfied all tests required by ČSN EN 61869 – 2 standard, IEC 61869-2.

Highest voltage for equipment: 12 kV
Power frequency test voltage: 28 kV
Lightning impulse test voltage: 75 kV
Nominal primary current: 1000 - 5000 A
Nominal secondary current: 5 A or 1 A
Rated short-time thermal current, Ith: 50 kA
Nominal frequency: 50 Hz
Weight: 32 kg
Thermal insulation class: E
Operation conditions: 

operating temperature from -5°C to +40°C

Standard: ČSN EN 61869-2, IEC 61869-2