PS Class CT is a special class CT used for differential protection of HT motor, generator, and transformer. The PS class CT is also called PX or X type or special class current transformer, and its specifications should be as per IS2705-part 4.
For differential protection of transformer, restricted earth fault protection, and other unit protection application, the CT used should not saturate at fault current and the CTs used in pairs at the incoming and outgoing side of the equipment must have the same knee point voltage. The current transformer manufacturer takes the following data from the user.
1. Secondary winding resistance
2. Excitation current Io- at knee point voltage( Vk/2 or Vk/4 etc.)
3. System Fault current
PS class CT Characteristics and Knee Point Voltage Formula
The knee point voltage of the CT is the secondary voltage of the CT beyond which the CT saturates. The knee point voltage of the CT is;
Knee Point Voltage( KPV ) = K x Fault Current x ( Rct + 2 Rl+ Rb)
Where, Rct = Secondary winding resistance
Rb = CT burden
Rl = Resistance of the connecting leads
The knee point voltage is dependent on the secondary resistance of CT and its burden. To have identical knee point voltage at specified excitation current, the resistance of wires connected from CTs to protection relay should be almost identical.
When the knee point voltage of CTs used at both sides of the equipment is the same, it will not give tripping for through fault and, the differential relay will operate only when there is a fault inside the equipment. Thus, PS class CT is a must for differential protection.
The main difference between the protection current transformer and PS (special protection) current transformer is that the protection current transformer does not have fixed knee point voltage, whereas the PS class CTs have identical knee point voltage at the same excitation current.
The special protection CTs have identical Characteristics. The accuracy limit factor of PS class CT is better than the protection transformer like 5P10,5P20.
The excitation current of the PS class CT for defined knee point voltage generally Vk/2 or Vk/4 should be less than or equal to as specified in the CT datasheet.