Why PT and CT Terminals are Star Connected ?

A Potential transformer is actually an instrument transformer that is used in power systems to step down primary current voltages and secondary voltages from a higher level to lower secondary potential output voltage level. 

There is still a primary winding and a transformer high voltage primary and secondary winding.

The working principle of a potential transformer mainly depends on mutual induction. The primary and secondary winding of the transformer are electrically insulated from each other but magnetically interconnected through the minimal reluctance path of the core.

Potential transformer or voltage transformer gets used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. Commercially available relays and meters used for protection and metering, are designed for low voltage.

In Alternating Current power system two most important instrument transformers are frequently used, one is current transformer and voltage transformer or potential transformer. One interest thing is, both the transformers are star-connected.

In potential transformer the primary and secondary windings are star connected, because in star configuration, the line voltage is root 3 times of phase voltage, hence the voltage distribution to each winding is reduced or divided by 1.732 times. 

Due to that the insulation cost on the primary winding comes lesser. But in delta configuration the line voltage is equal to phase voltage, therefore we have to insulate the transformer to withstand up to 11kV.

PT’s primary winding one terminal is directly connected to the main supply another one is connected with the ground, refer the drawing, therefore any fault in the potential transformer the fault current directly goes to the ground, the fault current do not enter in to our power distribution.

Easy installation: in this, the PT primary terminal is connected with the line voltage and the secondary terminal is connected to the ground, therefore the installation procedure is simple, easy replacement and maintenance-free.

Why CT secondary terminals are star-connected?

A current transformer is a device used to produce an alternating current in its secondary, which is proportional to the AC current in its primary. 

This is primarily used when a current or voltage is too high to measure directly. 

This ratio is based on the primary conductor passing once through the transformer window.

A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). 

Current transformers are the current-sensing units of the power system and are used at generating stations, electrical substations, and in industrial and commercial electric power distribution.

The current transformers operating principle is based on the law of electromagnetic induction. 

With a certain number of turns, the voltage from the external network is supplied to the primary power winding and overcomes its total resistance. At the secondary winding source, simultaneously, a voltage drop is observed.

Star configuration is having the advantage of the line current is equal to phase current, therefore our protecting device measures the exact CT’s secondary current. It optimizes our system protection status.

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