Current Transformer

A Current Transformer (CT) is an electric component that produced an alternating current in its secondary coil which is proportional to the AC in its primary coil. Current Transformers are known as instrument transformer, which are designed for measurement. Current Transformers using in an electrical line with high current or high voltage where current or voltage directly measurement is too risky. CT gives a lower amount of current with its secondary which is proportional to the current in the primary. This lower amount of current is suitable for measuring instruments. Current Transformers are broadly used for metering and protective relays in the electrical power industry.

A CT is applying where we need to measure line / Phase current. It is some time difficult to measure directly for high current or high voltage of the circuit. A CT provides an isolated lower current in its secondary coil to the ammeter for showing the measuring value. This lower current is proportional to the current in the primary circuit and it is suitable for measuring instruments. Current transformer widely used in electrical power industry for measuring, billing and protective relays.

Pic: Current Transformer (CT)

The construction of a Current Transformer consists of a primary winding, a core and a secondary winding. The major difference between a voltage transformer and current transformer is first one working with voltage and second one for current.

In the primary circuit of CT, Alternating Current (AC) produced an alternating magnetic field in the core. This magnetic field induces an alternating current in the secondary coil of CT. For a single turn in primary, the amount of current in secondary is equal to the current in the primary divided by the number of turns of the secondary.

If,

I_P = Current in Primary,

I_S = Current in Secondary,

N_P = Number of turns in Primary

N_S = Number of turns in Secondary, Then

I_S = I_P (N_P / N_S)

The current ratio used for specified the CT. This ratio is between primary current to secondary current. Standard rated secondary current of a CT normally 1 or 5 amperes. A 2000:5 CT ratio means it gives 5 amperes current at secondary coil when 2000 amperes current flowing in primary coil.

There are three basic types of current transformers: “wound”, “toroidal” and “bar”.

• Wound Current Transformers – This type of CTs primary coil is actually connected in series with the conductor which we want to measure.

• Toroidal Current transformers – Toroidal CT dose not contain a primary winding. There is a window or hole in the toroidal current transformer. Which current carrying line we need to measure, just pass it in this hole. Some of these have “split core” which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached.

• Bar-type Current Transformers – Bar-type current transformer using in the actual cable or bus-bar of the main circuit as the primary winding. This type of CTs mainly used in very high voltage line.

A bar-type current transformer which has 1 turn on its primary and 160 turns on its secondary is to be used with a standard range of ammeters that have an internal resistance of 0.2Ω’s. The ammeter is required to give a full scale deflection when the primary current is 800 Amps. Calculate the maximum secondary current and secondary voltage across the ammeter.

CAUTION:

Without any load (i.e. meters, relays, etc.) is connected to the Ct, we should be shorted the secondary terminal. Unless a very high voltage will be induced at the terminal and it will be damaged. If we compare CT with a Voltage transformer, CT is like a Step-up Transformer. So, when current is flowing in primary coil, it induced a very high voltage in secondary, may be kilovolts. In that case, when CT terminal is open, this high voltage damaged the insulation and CT is damaged for permanently.

Current Transformer Connections:

 3 Phase, 4 wire and 3 CT connection:

 3 Phase, 3 wire and 3 CT Connection:

 3 Phase, 3 wire and 2 CT Connection: