What Is a Current Transformer?

A current Transformer is a type of instrument transformer that can be used to measure power-frequency current. It has been designed to avoid the ferromagnetic effect of traditional electromagnetic CTs and to overcome their problems, such as linear saturation, ferromagnetic resonance, and insulation problems, achieving high measurement accuracy and good phase-frequency response.

It is an important component in the measuring and protection system of electric energy supply. Its function is to convert a large current value into a smaller readable value for safe and easy use by instruments and protective relays. The primary current conductor passes through the window or core of a current transformer and produces a magnetic flux that induces a voltage on the secondary winding. This voltage is proportional to the current passing through the primary current conductor and can be measured by a device connected in parallel to the secondary.

There are four typical types of current transformers: window, bushing, bar, and wound. In the first two types, the primary current conductor passes through a window or aperture in the core of the current transformer and is converted to a voltage by the secondary winding. The other two types have a core with one or more turns and the primary winding can either consist of a single turn that passes once through the aperture in the core (window or bushing type) or it can have a secondary winding with two or more turns, wound on the core together with the primary winding (bar or wound-type).

The accuracy class of a current transformer defines the permissible deviation in the secondary current from the calculated value. This is usually divided into metering and protection accuracy classes. The metering accuracy class includes the error limits for both the transformer ratio and phase difference, while the protection accuracy class does not include the limit for phase angle shifts between the primary and secondary currents.

Regardless of the current transformer type or accuracy class, the primary lead and the secondary leads should always be connected with their correct polarity. This is because the polarity of a current transformer determines whether the primary lead and the secondary lead are connected to the same or different points in the circuit. If the primary lead and the secondary lead are connected in opposite directions, it can cause serious damage to the circuit or the instrument that is being monitored.

During the design process, we can analyze the performance of the current transformer in terms of its ratio error and phase position by using an oscilloscope to record the output voltage. We can also compare the resulting waveform with the reference voltage from an actual instrument to verify the calibration of the designed CT. In addition, the AC signal of the program-controlled converter is applied to the primary and secondary leads of the designed CT, and its amplitude and phase are recorded to obtain experimental data for comparison. The resulting ratio error and phase difference are within acceptable limits.