The power system often uses AC voltage and high current circuit to transmit power to end users, so we cannot measure the current or voltage directly by the instrument. The instrument transformer can reduce AC voltage or high current levels to where AC voltage or high current can be directly measured by the instrument. It can also provide power for relay protection and automatic device.
Instrument transformer is composed of current transformer (CT) and potential transformer (PT). CT and PT are connecting elements between primary system and secondary systems in the AC circuit. They are special transformers, but their working principle is basically the same as that of the common transformers. The primary winding of the potential transformer is connected in parallel with the primary system. It is equivalent to a transformer without a secondary load. The changes of the secondary load do not affect the corresponding voltage of the primary system.
Specific functions are as follows:
1. Converting high voltage and high current of primary circuit to low voltage (usu. 100V) and low current (5A, 1A, 0.5A) of the secondary circuit, the instrument transformers can help the standardization of the measuring instrument and protection device and thus reduce the cost of the secondary equipment.
2. Low voltage control cable can be used in the secondary circuit. The wiring is simple and the installation is convenient. Remote control and measurement can be realized.
3. The secondary circuit is not restricted by the primary one. The wiring is flexible, and the maintenance and debugging are convenient.
4. The secondary circuit is isolated from the primary high voltage parts, and the ground point can be set to the secondary to ensure the safety of the secondary equipment and of person.
Types of potential transformer:
- Structure: dry-type, potting-type, inflatable-type, or oil-immersed type;
- Phase: single phase or three phases;
- Winding: double-windings or triple-windings.