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 current transformer. The PS class CT should be according to 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 incoming and outgoing side of the equipment must have the same knee point voltage. The following specifications need to be furnished to CT manufacturer.
Secondary winding resistance
Excitation current Io- at knee point voltage( Vk/2 or Vk/4 etc.)
System Fault current
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 of the secondary resistance of CT and Ct burden. To have identical knee point voltage at specified excitation current, the resistance of wires connected from CTs to protection realy should be almost identical. when the knee point voltage of CTs used at both sides of the equipment to be protected is the same, it will not give tripping for through fault and, differential relay will operate only when there is fault inside the equipment.
The main difference between protection class P class CT and PS class CT is that the P class CT does not have fixed knee point voltage, whereas the PS class CTs have the identical knee point voltage at the same excitation current. The PS class CT have identical Characteristics. The accuracy limit factor of PS class CT is better than the protection class CT 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 CT data sheet.