Current Transformer and its working 2.pptx

A Current Transformer (CT) is used to measure the current of another circuit. CTs are used worldwide to monitor high-voltage lines across national power grids. A CT is designed to produce an alternating current in its secondary winding that is proportional to the current that it is measuring in its primary.

1. Current Transformer

2. Cur rent Transformer Construction BAR PRIMARY Called as toroid CT. Negligible leakage flux Low reactance Primary current should be high – to produce exciting ampere turns. Wound Primary CTs Primary and secondary winding are arranged concentrically. Secondary winding – inner winding. Cold rolled grain oriented steel Core is assembled by E,I,C or L shaped stampings. Primary windings of copper would primary CTs – edge wound copper strip. Secondary winding – Round section enameled copper wire.

3. Typical Protection Bar-Primary Current Transformer RELAY 1A ? 1000A ? 1000 turns sec. ? Insulation covered wire, giving inter-turn insulation & secondary to core insulation Generator, or system voltage source ‘Feeder’ or ‘Bus-bar’ forming 1 turn of primary circuit Insulation to stop flash-over from HV primary to core & secondary circuit Laminated ‘strip’ wound steel toroidal core

4. Potential transformers S ame as that of the ordinary power transformer VT Errors The error occurs, both in magnitude and in phase, of the measured value of the voltage. Ratio Error (Voltage Error) E rror due to a difference in the actual transformation ratio and the nominal (rated) transformation ratio Phase Angle Error The phase difference between the primary voltage and the reversed secondary phasors is the phase angle error of the VT. In order to keep the overall error within the specified limits of accuracy, the winding must be designed to have: ( i ) the internal resistance and reactance to an appropriate magnitude, and (ii) mininum magnetizing and loss components of the exciting current required by the core

5. Types of Voltage Transformers Two main basic types are available: Electromechanical VT`s Similar to a power transformer May not be economical above 132kV Capacitor VT`s (CVT) Used at high voltages Main difference is that CVT has a capacitor divider on the front end.