What is PS Class CT?

Protection special class CT  'PS" or 'PX' is used for protection of the electrical equipment like motor, transformer, generator, and bus bar. The protection or P class current transformer is used for the protection of feeders. 

The central theme of any electrical protection scheme is that the current should not saturate at the time when fault current flows through the current transformer. Under CT saturation conditions, it behaves as an open circuit, and protection of the electrical equipment is not guaranteed. 

The protection class or 'P' class CT like 5P10, 5P20 is used for the protection of feeders. The knee point voltage of protection class CT  is more than the knee point voltage of the metering class CT. It means the protection class CT saturates at much higher CT secondary voltage.

For the protection of the alternator, generator, high tension motor and bus bar, protection zone, differential protection, or unit protection scheme is used. The unit protection scheme provides tripping to the breaker in case there is a fault within the equipment. For instance, the differential protection trip in the transformer feeder shows that there is a fault in the transformer. If the differential protection relay operates, it is essential to check the transformer before switching on the equipment again.

The differential protection scheme is designed by selecting the PS class CT. The PS class CT has a higher knee point voltage. The following parameters of the current transformer of CT used for the PS class must be defined.

• Knee Point Voltage Vk(KPV) Minimum - Volts

• Excitation Current(Maximum) at Vk/2 - Amperes

• Rct (secondary winding resistance) at 75 Degree Centigrade (In Ohm)

Specifications of the PS class CT are as given below.

The PS class CT used for the differential transformer must have identical magnetization characteristics. Any Aberration in the CT characteristics used on the primary and secondary side may lead to spurious tripping during the through fault condition.

What does 5P10 & 5P20 mean for CT?

Protection of the electrical network is paramount for ensuring the isolation of the faulty section in order to maintain uninterrupted power supply to other healthy electrical netwoks. The protection relay and the current transformer which measures the current and fed that current to the protection relay must be most reliable. The careful selection of the protection class CT is most important parameter to ensure no CT saturation at the time of fault. The protection class CT has more knee point saturation point as compared to metering class CT.

Protection class (P class) CT is connected to the protection relay that gives tripping command to circuit breaker at the time of fault condition. The protection scheme of feeder as given below.

At the time of the fault, the primary current of CT increases abnormally high and the core can get magnetized above its rated capacity and whatever fault current flowing in the circuit can’t be reflected in the secondary side of the CT. This phenomenon is known as the saturation of CT. If CT gets saturated at the time of the fault, the protection relay will not operate.

5P10 class CT: 

P stands for protection class. If the primary current is 10 times to the rated primary current of the CT, the CT will function perfectly, within the rated composite accuracy class 5 %.

5P20 class CT: 

P stands for protection class. If the primary current is 20 times to the rated primary current of the CT, the CT will function perfectly, within the rated composite accuracy class 5 %.

A CTR of 200/5 with 5P10 class will give error of 5 % if the primary current through the CT is 2000 ampere.

A CTR of 200/5 with 5P20 class will give error of 5 % if the primary current through the CT is 4000 ampere.