PS-Class [Special Purpose] Current Transformers:
Class PS transformers are the special purpose protective current transformers. These are used where the required characteristics of the current transformers cannot be conveniently expressed in terms of the ordinary protection current transformers like Class 5P, 10P and 15P current transformers.
The typical applications involve differential and restricted earth fault protection schemes.
PS-CLASS Protection CT
The knee point voltage is very important factor for designing this type of transformers.
Let us see what the Knee Point Voltage is.
Knee Point Voltage is the sinusoidal voltage of rated frequency applied to the secondary terminals of the current transformer, all other windings being open circuited, which when increased by 10 percent, causes the exciting current to increase by 50 percent.
The performance of the current transformer depends on the specified minimum knee point voltage.
Special Characteristics of PS-Class Current Transformers:
- The turns ratio shall be numerically same as the rated transformation ratio.
- Error in turns ratio shall not exceed ±25 percent.
- The knee point voltage shall not be less than the specified rated knee-point voltage.
- The exciting current shall not be greater than the maximum exciting current, at the rated knee point voltage, or at the specified fraction thereof.
- When specified, the resistance of the secondary winding corrected to 75oC, shall not be greater than the specified value.
For this type of CTs knee point voltage test must be done in addition to the routine and type tests.