What is Current Transformer-Working principle

Instrument transformers are designed to measure high voltage and current in power system with standard but high accuracy, with low range ammeter and voltmeter. here we are going to discuss current transformer and its working.

current-transformer
Current Transformer

Current Transformers as the name suggests CT is designed to measure high-value current in the power system. The primary of the current transformer has few numbers of the turn of heavy wire, and other hands the secondary of the current transformer has many turns of fine wire.

In clam on CT (current transformer), the current-carrying conductor itself acts as a single turn primary winding. 

Below the clamp-on current transformer diagram shows that the current transformer is a step-up transformer as the voltage is stepped up and the current is stepped down for measurement.
current-transformer-diagram
Current Transformer Diagram
The low range ammeter is connected across the secondary winding of the current transformer because the internal resistance of an ammeter is negligible compared with the winding resistance of current transformers secondary. Therefor CT is designed to operate in under short circuit condition.
In current transformer magnetizing current is too small and flux density in the core is relatively low Also the core of the current transformer never saturate under normal operating condition. 

Why the secondary of the current transformer should not be left open circuit? 


If secondary left open, the current transformer may lose its calibration and give inaccurate readings.

The primary winding is still carrying current and no secondary current is present to counteract its mmf. 
The primary winding current acts like a magnetizing current and increase the flux in the core. Increase flux may saturate and magnetize the core when secondary of CT is closed again, the hysteresis loop may not be symmetrical around the origin but displaced in the direction of residual flux.

Also, the primary current produced large heat over the period of time and may destroy the insulation of winding and saturation of the current transformer. Result in excessively high voltage across secondary. 

A current transformer is designed like 100:5 this means that it has multiplying factor 20, if ammeter connected to 100:5 CT, when 100A current flow in primary secondary Registers 5A. The line current is 5x20 =100A, (2 x Multiplying factor) 

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