What is Instrument Transformers?

There are two categories

1) Metering

2) Protection type.

Metering Type - used in AC system for measurement of electrical quantities i.e. voltage, current, power, energy, power factor, frequency.

Protection Type- used with protective relays for protection of power system.

Basic function

To step down the AC System voltage and current. The voltage and current level of power system is very high.

It is very difficult and costly to design the measuring instruments for measurement of such high level voltage and current.

What are the advantages of Instrument Transformers?

*The large voltage and current of AC Power system can be measured by using small rating measuring instrument i.e. 5 A, 110-120 V.

By using the instrument transfomers,

• Cost reduction is achieved

• More ever the damaged/Faulty measuring instruments can be replaced easily.

• Provides electrical isolation between high voltage powercircuit and Secondary side - measuring/protective instruments and also assures the safety of operators.

• Several measuring instruments can be connected througha single transformer to power system.

• Due to low voltage and current level in measuring and protective circuit, there is low power consumption in measuring and protective circuits.

What are the Types of Instrument Transformers?

Instrument transformers are of two types-

Current Transformer (C.T.)

Potential Transformer (P.T.)

Current Transformer (C.T.)

Current transformer is used to step down the current of power system to a lower level to make it feasible to be measured by small rating Ammeter (i.e. SA ammeter). A typical connection diagram of a current transformer is shown in figure below.

• Primary of C.T. is having very few turns.

• Primary is connected in series with the power circuit. Therefore, sometimes it also called series transformer.

• The secondary is having large no. of turns. Secondary is connected directly to an ammeter. As the ammeter is having very small resistance. Hence, the secondary of current transformer operates almost in short circuited condition.

• One terminal of secondary is earthed to avoid the large voltage on secondary with respect to earth. Which in turns reduce the chances of insulation breakdown and also protect the operator against high voltage. More ever before disconnecting the ammeter, secondary is short circuited through a switch 'S' as shown in figure above to avoid the high voltage build up across the secondary.

What is Burden Rate?

• The secondary load of a current transformer is termed the "burden" to distinguish it from the primary load. The burden in a CT metering electrical network is largely resistive impedance presented to its secondary winding.

• Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA and 60 VA

• The most common cause of excess burden impedance is the conductor between the meter and the CT.

• When substation meters are located far from the meter cabinets, the excessive length of cable creates a large resistance. This problem can be reduced by using thicker cables and CTs with lower secondary currents (1 A), both of which will produce less voltage drop between the CT and its metering devices.