Current Transformer (CT) – Construction and Working Principle

Construction

The potential transformer is constructed with a high-quality core working at a low flux value so that the magnetizing current is low. The transformer’s terminals have to be constructed so that the change in the voltage rating with load is minimum and the phase shift between the output and input voltage is also minimum.
The primary section has a great number of turns, and the secondary side has a much small number of them. The co-axial arrangement is applied in the potential transformer for reducing the leakage reactance. The insulation price is also decreased by dividing the primary section into particular parts, which minimized the insulation between the layers.

Connection

The potential transformer is arranged in parallel configuration with the circuit. The primary section of the device is directly attached to the power network, whose voltage should be monitored. The secondary terminals of the system are connected to the sensing instrument like the wattmeter, voltmeter, etc. The secondary section of the potential transformer is magnetically combined through the magnetic network of the primary windings.

Potential Transformer Connection
The main terminal of the device is rated for 400V to several thousand volts, and the secondary one is often rated for 400V. The rating of the primary voltage to the secondary one is known as the “transformation ratio or turn ratio”.

Types of Potential Transformer

The potential transformer is mainly categorized into two types, i.e., the conventional wound forms (electromagnetic form) and the capacitor voltage potential types.

Conventional Wound Potential Transformer

Conventional wound form is very expensive due to the demand of the insulations.

Capacitor Potential Transformer

Capacitor potential type is a junction of a capacitor potential divider and a magnetic part of a relatively small ratio. The circuit diagram of the capacitor types is presented in the figure below. The stack of great voltage capacitors from the potential divider, the capacitors of two sides become C₁ and C₂, and the Z is the burden.

Capacitor Potential Transformer Circuit Diagram
The voltage presented to the primary section of the intermediate transformer is usually of the order of 10kV. Both the intermediate transformer and the potential divider have the rating and insulation demand which are appropriate for economical construction.
The intermediate transformer should be of very low ratio error, and the phase angle provides the satisfactory performance of the complete set. The secondary terminal value is given by the formula presented below.