Current Transformer (CT)
1. CT is one special transformer used for current changing. This is enough insulation between primary and secondary winding of CT to ensure isolation between LV side and HV side. Change primary current to lower standard current, normally 5A or 1A, through primary and secondary winding number rate configuration, therefore, the size of instrument and relay will be reduced.
2. Functions of CT
a. Transmit information to measurement instrument or protection control device.
b. Isolate measurement and protection device from HV lines.
c. Instrument and protection relay will be of miniaturization and standardization.
3. If there is no load at secondary side during operation of CT, proceed reliable short-circuit, open circuit is forbidden. Because if CT is current source, secondary is open circuit and has no current, primary windings are used for excitation and core is saturated and high voltage appears to secondary wiring, there is threat to people’s safety and damage to device.
There are measures to prevent secondary open circuit:
a. Fuse is not allowed to be installed to secondary circuit.
b. Normally, no need switching for secondary circuit. If need switching, measures to prevent open circuit should be taken.
4. Classification of CT
a. According to usage: measurement, protection
b. According to kind: indoor and outdoor
c. According to insulation medium: oil, casting resin, dry, porcelain, gas insulated..
d. According to structure: pulling-through, strut type and busbar type