Here are Something About the CT Transformer You Should Know

The principle of the CT transformer

The current transformer is an instrument that converts the primary side large current into the secondary side small current according to the principle of electromagnetic induction.

It consists of a closed core and a winding and It has a small number of turns on the primary side winding, which is in the line of the current that needs to be measured.

The number of turns on the secondary side winding is relatively large and is connected in series in the measuring instrument and the protection circuit. When the current transformer is working, its secondary side circuit is always closed, so the impedance of the series coil of the measuring instrument and the protection circuit is small. The operating state of the current transformer is close to a short circuit.

Applications of the current transformer

The current transformer is an important secondary device in the power system. It is widely used in secondary circuits such as measurement and relay protection. In the case of high current or high voltage, we cannot directly measure the current of the circuit with an ammeter so it can only be measured by the secondary side of the current transformer.

It can be used not only in power system control but also in the precise current measurement. Fields like the industrial, medical, automotive and telecommunication systems can adopt the current transformer.

It is able to be applied to:

1. Extending the range of measuring instruments such as ammeter, energy meter, KVA meters, etc.
2. Different circulating current protection systems.
3. Distance protection in power transmission systems.
4. Over current fault protection.

Types of current transformers

Classified by insulating materials:

1. Gas insulated transformer: Main insulation consists of the gas.

Here is an example of the Sf6 Gas Insulated Transformer manufactured by Sihu.

It’s a kind of current transformer which can match the 35~1000kV high voltage switchgear. Its sparkling points are as followed.

1. Cast by epoxy resin with the flanges, having a good aging resistance.
2. Manufactured in a quite clean environment of stable temperature and humidity and then be dried and vacuumed, containing low moisture.
3. Packed by the plastic film, which can prevent the moisture.
4. Oil immersed current transformer: Insulated paper and insulating oil are used as insulation, and it is generally an outdoor type.
5. Dry current transformer: Dipped by ordinary insulating materials as insulation.
6. Castable current transformers: Current transformers molded from epoxy or other resin blends.

Classified by installation method:

1. Through-current transformer: current transformer used to pass through a panel or wall
2. Pillar current transformer: Installed on a plane or pillar, and also used as a current transformer for the circuit conductor pillar.
3. Casing current transformer: A current transformer that is directly fitted to an insulated bushing without a primary conductor or primary insulation.
4. Busbar type current transformer: a current transformer that is directly placed on the busbar.

Classified by working principle:

1. Electromagnetic current transformer: a current transformer that realizes current conversion according to the principle of electromagnetic induction
2. Electronic current transformer
3. The optical current transformer refers to an optical device used as a current sensor to be measured, and the optical device is composed of optical glass, all-fiber, and so on
4. Air core coil current transformer and it’s also known as Rogowski coil current transformer. The hollow coil is often made by uniformly winding the enameled wire on the annular skeleton. The skeleton is made of a non-ferromagnetic material such as plastic or ceramic. The relative magnetic permeability is the same as the relative magnetic permeability of the air. This is the hollow coil’s  distinctive feature compared to the iron core.
5. Core coil type low power current transformer (LPCT). It is a development of traditional electromagnetic current transformers. Due to the design of high-impedance resistors, the saturation properties are improved at very high primary currents, the measurement range is expanded, power consumption is reduced, and overcurrent, full-offset short-circuit current up to short-circuit current can be measured with high accuracy without saturation. Measurement and protection can share a core coil type low power current transformer whose output is a voltage signal.

Asides from the principle, application and the classification mentioned above, there is a lot of information on the ct transformer that are worth exploring.

Find out more by visiting www.gdshp.com. Located in China, it has been specializing in the field of instrument transformers for 30 years, providing OEM service.