Primary Winding (High Current)
This winding is connected in series with the main circuit where the current is to be measured. In many CT designs, the main power conductor itself acts as the single-turn primary (bar-type or window-type CT).
A Current Transformer (CT) is a type of instrument transformer designed to produce an alternating current (AC) in its secondary winding that is precisely proportional to the AC current flowing in its primary winding.1
CTs are essential components in electrical power systems because they allow personnel to safely and accurately measure or monitor very large currents without having to directly handle high voltages or high currents.2
The CT works on the same basic principle as any transformer: electromagnetic induction.
This winding is connected in series with the main circuit where the current is to be measured. In many CT designs, the main power conductor itself acts as the single-turn primary (bar-type or window-type CT).
The high current flowing through the primary winding produces a proportional magnetic flux in the core.
This winding has many turns of fine wire and is connected to a low-impedance device such as an ammeter, wattmeter, protective relay, or energy meter.
The ratio of turns between the primary and secondary determines the current reduction. A standard CT is often designed so that when the rated primary current flows, the secondary current is a standard value such as 1 A or 5 A.
Example: A CT rated 100 / 5 A will deliver 5 A on its secondary when 100 A flows through its primary.
CTs serve two vital purposes in a power system:
• CTs scale down the large primary current to a small, measurable value such as 1A or 5A,
enabling standard low-current instruments to be used.
• They electrically isolate sensitive measuring equipment and control circuits from the
high voltage and high current of the main power line, ensuring operator safety.
• CTs provide input current to protective relays such as overcurrent or differential relays.
• When a fault occurs, the CT accurately feeds the surge current to the relay,
enabling rapid tripping of the circuit breaker to clear the fault.
A fundamental safety rule for CTs is that the secondary winding must never be left open-circuited while the primary winding is carrying current. If the secondary circuit is opened, the core becomes highly saturated, inducing extremely high voltages across the secondary terminals, which is dangerous to personnel and can destroy the CT and associated equipment. The secondary must always be short-circuited if the measuring instrument is removed.
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