In order to transmit power, the power system often uses AC voltage and large current circuit to send power to end users, so it can't be measured directly by instrument. The function of instrument transformer is to reduce the AC voltage and the large current to the value that can be directly measured by instrument, and at the same time, it can provide power for relay protection and automatic device.
Instrument transformer is composed with current transformer (CT) and potential transformer (PT). It's a connecting element between primary system and secondary systems in the AC circuit. They all belong to special transformers, so the working principle is basically the same as that of transformers. The primary winding of potential transformer is parallel connected to primary system, and it is equivalent to a transformer without secondary load. The changes of their secondary load do not affect the corresponding voltage of the primary system.
Specific functions are as follows:
1. Converting the high voltage and large current of primary circuit to the standard low voltage (usu. 100V) and small current (5A, 1A, 0.5A) of secondary circuit, which can make the measuring instrument and protection device standardized, and reduce the structure and cost of the secondary equipment.
2. Low voltage control cable can be used in secondary circuit, and the wiring in the housing is simple and the installation is convenient, and 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 is isolated from the primary high voltage parts, and ground point can be set to the secondary to ensure the safety of the secondary equipment and person.
Types of potential transformer:
- Structures: dry-type, potting-type, inflatable-type and oil-immersed type;
- Phases: single- and three-phases;
- Windings: double- and triple-windings.