Current Transformers – Windings and Rated Insulation Levels

A current transformer is an instrument transformer in which the current ratio is within the specified limit. The primary winding is connected in series with the load and carries the load current to be measured. The secondary winding is connected to the measuring instrument s or relay.

Table of Contents

Primary Winding

Primary winding details of the current transformer are given below,

• The primary winding is made with high-quality Electrolytic Copper.
• Primary winding should be capable of carrying Rated Full load Current continuously and short circuit Fault Current for a specified time.
• Primary Current is normally 50, 100,150, 200, 300, 400, 600, 800, 1000, 2000 Ampere.
• Short Time Thermal Current: – The RMS value of the Primary Current which the CT will withstand for a Rated Time without suffering the harmful effect. 
• The values of Rated Time shall be 0.5, 1, 2, 3 Sec.
• As per specification CTs should have the following Rating.
• Short Time Thermal Current – 25 kA for 3 Sec.  – 50 KA for 1 Sec.
• Rated Dynamic Current: – The Peak Value of the Primary Current which a CT will withstand without any damage.  This Current will be 2.5 times Short Time Thermal Current.  As per specification, it is 125 kA.   

Secondary Winding:

Secondary winding details of the current transformer are given below,

• Ammeters, Energy Meters, Protective Relays are directly connected to the secondary of the CT.
• So its Rating should be Standard.
• Normally 1 Ampere and 5 Ampere are the standard ratings of the secondary winding of the CT.
• As per specification EHV CTs are having Four or Five Secondary Windings separately wound on its Cores.
• The secondary winding is with Electrolytic Copper Enameled or Double Cotton Covered.

Rated Insulation Levels:

Rated Insulation levels of CT are as follows

• The insulation of CT depends on the Rated voltage of the CT.
• Up to 11 kV Dry Type CTs are used. The insulation of dry-type CT is Epoxy Resin.
• For 66 kV and above Paper insulation is used. Transformer oil is used for insulation and cooling.
• Rated insulation level means the Power frequency and Impulse voltage withstand test voltages specified for the CT.