Summary:A current transformer (CT) is an electrical instrument that transforms electric current from one for...
A current transformer
(CT) is an electrical instrument that transforms electric current from one form to another. The basic parts of a CT are the magnetic core and secondary winding. The primary lining of the CT contains a wire with a current that passes through the center of the core. This wire creates a magnetic field, which drives the secondary winding. The secondary winding's current is proportional to the amount of current flowing through the core.
There are two basic types of current transformers: window-type and ring-type. Window-type CTs have an opening in the center, while ring-type CTs have a full-turn wound core. The primary terminals may be bar-type, or they may have rectangular pads. A ring-type CT is designed for outdoor use, and it uses a transformer oil or other suitable liquid as a fluid for the primary windings. Liquid-immersed CTs are often hermetically sealed.
Secondary output currents are proportional to the primary current and are often used for measuring primary current or powering instruments for measurement. Because of this, the secondary of a current transformer always has a 5A rating, allowing for the production of standardized current devices.
A current transformer is a type of instrument transformer. The secondary current is substantially proportional to the primary current and ideally varies in phase by zero degrees. While the basic construction of a current transformer is similar to that of an electrical power transformer, the principle of operation is different. Accurate measurement of the ratio between the primary and secondary currents is crucial in metering and indication. The primary and secondary windings are both connected to a measuring device or relay.
The operating principle of a current transformer is based on the law of electromagnetic induction. A voltage from an external network is applied to the primary power winding and overcomes its total resistance. This causes a magnetic flux to appear in a magnetic circuit around the coil. The direction of this magnetic flux depends on the type of magnetic material used in the core. If this magnetic flux is strong enough, the voltage is high enough to drive an electric motor.
A current transformer's polarity is determined by the direction in which its leads are wound. This polarity is essential when installing a CT and connecting it to protective relays or power meters. If the polarity is not properly installed, it may cause partial discharge. This can occur because of the aging of the solid insulation or due to contamination of the insulation fluid. It is also important to consider the load on the secondary circuit, as it influences the accuracy of the transformer's output.
The primary winding in a current transformer is generally made from a single flat turn of heavy-duty wire or a bus bar. The primary winding of a current transformer is connected in series with the conductor that is being measured. Some CTs also have a split core that allows the winding to open. The primary winding is also connected to the line load in series. It is possible to use multiple windings in a current transformer to avoid the drop in the output current of the power meter.