A current transformer is a "meter" transformer that reduces a high value of current to a lower value. As the name suggests, instrument transformers are used to isolate instrumentation equipment from high voltage and current to facilitate the measurement of electricity.

　　Current transformers are widely used to measure current and monitor grid operation. The need for a current transformer is for two reasons: it isolates the protection system from high voltages and currents, thereby reducing the size and cost of the protection device; the output of the current transformer is standard (i.e. 1A or 5A) and no protection device is required, eg. relays with different operating values.

　　A current transformer is an instrument that uses the principle of electromagnetic induction to convert a large current on the primary side into a small current on the secondary side. A current transformer consists of a closed iron core and windings. Its primary winding has several turns, strung on the current line that needs to be measured.

　　Therefore, all the current of the line often flows, and the number of turns of the secondary winding is relatively large. It is connected in series with the measuring instrument and the protection circuit. When the current transformer is working, its secondary circuit is always in a closed state, so the impedance of the instrument series coil and the protection circuit is very small, and the working state of the current transformer is close to a short circuit. The current transformer converts the large current on the primary side into a small current on the secondary side for measurement, and the secondary side cannot be opened.