News - What is PT/CT?

PT is commonly known as voltage transformer in power industry and CT is the common name of the current transformer in power industry.

Voltage transformer (PT): it is the electrical equipment that changes the high voltage of power system into a certain standard low voltage (100V or 100 / √ 3V).

Potential transformer (PT, VT) is similar to transformer, which is used to change the voltage on the line. However, the purpose why transformer transforms voltage is to transmit electric energy. The capacity is very large, generally in kilovolt ampere or megavolt ampere as the calculation unit. The purpose why voltage transformer transforms voltage is mainly used to measure meters and power supply by relay protection devices,  measure the voltage, power and electric energy of the line, or to protect valuable equipment in the line when the line fails Therefore, the capacity of voltage transformer is very small, generally only a few volt ampere, dozens of volt ampere, and the maximum is not more than one thousand volt ampere.

Current transformer (CT): it is the electrical equipment that changes the current in high voltage system or the large current in low voltage system into a certain standard small current (5a or 1a).

Current transformer is an instrument which converts the large current in primary side into small current in secondary side according to the principle of electromagnetic induction. The current transformer is composed of closed core and winding. Its primary winding turns are very few, and it is connected in series in the circuit which needs to measure the current. Therefore, it often has all the current flowing through the line, and the secondary winding turns are more. It is connected in series in the measuring instrument and protection circuit. When the current transformer is working, its secondary circuit is always closed, so the impedance of the series coil of the measuring instrument and protection circuit is very small, and the working state of the current transformer is close to short circuit. The current transformer converts the large current at the primary side into the small current at the secondary side for measurement, and the secondary side cannot be opened circuit.