Current Transformer (CT) is used to transform the high AC current to small easily manageable values. They're connected with the Panel Meter or Relay and they can help to measure the current or protect the equipments. Low voltage current transformers are manufactured as of two types for measuring CT.

• Measuring CT: Measuring current transformers are constructed to feed on other low voltage apparatus such as measuring instruments, relays, watt-hour meter (KW meter) and these type of current transformers are mainly used 0.5 and 1.0 class to transfer the current from highest rated current to rated secondary current.

• Protection CT: Protection current transformers are constructed to feed the protection relay. These type of current transformers are mainly used 5P. (Customer supplied when required.)

 

Selection of the Current Transformer

To select the current transformer correctly, the following points should be clarified:

• -The application (for measuring or protection).
• -The features of the working environment (Indoor or outdoor, operating temperature, air humidity etc….).
• -Operation voltage and frequency.
• -Range of the primary current (maximum and minimum of the current to be measured).
• -Dimension of the cable or Bus bar.
• -Data of the overload.
• -Short circuit current.
• -Specification of the measuring device associated with the current transformer (Accuracy, rated current, consumption etc…).
• -The diameter and length of the cable, the cable which is used to connect the current transformer and associated measuring device.

Power Losses of the CT

In the practical application, the power generated by the primary current should be equal or bigger than the power requirement of the associated measuring device plus the consumption of the connecting line.

Losses in the line, PL:

This is the power loss, through heat, generated by current through the resistance RL in the cable, in the transformer’s secondary circuit.

Factors to be taken into account:

Secondary current: PL = RL / 2

Cable diameter: RL is inversely proportional to the square of the diameter cable length: RL is proportional to the length of cabling (there and back).

Power: The nominal apparent power (VA) with a specified power factor, which was supplied by the Current Transformer, to the secondary current with the assigned current when it is connected to its nominal load. Sc (VA) = Zc (/sn)2 According to Standards, for apparent power greater than or equal to 5VA, the power factor is 0.8 inductive. For apparent power less than 5VA the power factor is considered to be one (unity).

Accuracy of A Current Transformer

The percentage of error, produced in a transformer, is established by IEC60044-1. In measurement transformer: 25% and 100% of nominal power. In protection transformers: 100% of nominal power.

Note: with…/1A transformers losses are reduced 25 times.

Saturated Condition of CT

The current transformer is saturated if the primary current, passing through the CT, is greater than the nominal rating of the CT.

The linearity of CT, between the primary and secondary sides decreases, so error increases. The saturation of the CT is inversely proportional to the load (FIG. 1).

The difference between measuring and protection current transformers is their behavior when an overload occurs on the primary side. Measuring CT is saturated when there is a primary current overload. In order to protect the equipment. On the secondary side, protection CT will not saturated until there is a very high current on the primary side, A class 5P15 protection transformer indicates that it has an accuracy rating of ±1% that it does not become saturated until the primary current reaches 15 times the nominal current rating of the CT.

In measuring transformers, the SAFETY FACTOR “FS” parameter indicates the excessive amperage on the primary side current in relation to the current sent to the measuring device on the secondary side.

Application Note

If the primary current is too small, to keep the same accuracy and output, we can add primary winding, but the rated turns ration should be the same, For example, if the primary current is 50A, we can use 100/5A current transformer with the primary current be turned twice which help to keep the same rated turns ratio (1:50 = 2:100).

Construction

CT consist of primary winding, secondary winding, magnetic core and insulated body. The high-grade silicon steel core is annealed, varnished then insulated with polycarbonate core caps. The secondary winding is toroidally wound by high precision semi-automatic machinery. For the tape wound ring type current transformer, the PEW coated windings are then covered with elephantite paper, varnished and double-tapped with PVS tapes. For the encapsulated type current transformer, the windings are enclosed in a compact and heat resistant split cap.

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