Role of Current Transformer and Notice
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Role of Current Transformer and Notice


The main role of current transformer is to transform the high value to the low value which is applicable to measuring meter and relay. The commonly-used secondary rated current is 5A.  

The current transformer’s primary winding is in series with electric circuit so the insulation level of winding should be consistent with the voltage stage of circuit. Its capacity meets the maximum load current passing through the electric circuit so that the temperature rising of primary winding does not exceed the limit.   

When the fault of short-circuit occurs, the current is many times higher than normal load current. Such a high current will generate great mechanic stress and thermal effects, which is harmful to the current transformer. Hence, when selecting the current transformer, the capacity and insulation level should meet the requirements. Moreover, it is necessary to calibrate the mechanical stress and thermal effect to make them not exceed the rated value of current transformer.        

Under any operating circumstance, open circuit is not allowed for secondary winding terminal of current transformer. or it will generate dangerous high voltage and cause personnel accidents. The reason is as follows: when the current transformer is in the open circuit of secondary winding, all current in the primary secondary is exciting current, resulting in an increase in flux density and high electromotive force. In addition, due to the big increase in flux density, the eddy current in the iron core and hysteresis loss go up and large quantities of heat energy are produced. The temperature rising of current transformer exceeds the limit. The high temperature does damage to the winding insulation which is near to magnetic conductor.   

Role of CT

The secondary winding circuit of current transformer must have one point grounding to protect the secondary equipment and personnel safety. However, the secondary circuit of current transformer should not have two-point grounding. Otherwise, it will cause relay protection malfunction.   

There are few turns of primary windings for current transformer. According to different needs, secondary winding of current transformer can be made into several independent coils.