- Instrument Transformer Definition: An instrument transformer is a device that steps down high voltage and current from power systems to manageable levels for measurement and safety.
- Advantages: Instrument transformers standardize measurements, reduce equipment costs, and enhance operator safety by providing electrical isolation.
- Types of Instrument Transformers: The main types are Current Transformers (C.T.) and Potential Transformers (P.T.), each designed for specific measurement tasks.
- Safety and Functionality: These transformers include safety features such as grounding and operation under specific circuit conditions (short-circuited for C.T.s, open-circuited for P.T.s) to ensure accuracy and prevent accidents.
- Educational Resources: Books by authors like Bakshi and Morris provide additional information and technical insights into the use and application of instrument transformers.
What is Instrument Transformer?
An instrument transformer is used in AC systems to measurement of electrical quantities such as voltage, current, power, energy, power factor, and frequency. These transformers also play a critical role in protection of power system alongside protective relays.
Basic function of Instrument transformers is 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. Generally measuring instruments are designed for 5 A and 110 V.
The measurement of such very large electrical quantities, can be made possible by using the Instrument transformers with these small rating measuring instruments. Therefore these instrument transformers are very popular in modern power system.
Advantages of Instrument Transformers
- Large voltages and currents in AC power systems can be accurately measured using instruments with small ratings, such as 5 A and 110–120 V.
- By using the instrument transformers, measuring instruments can be standardized. Which results in reduction of cost of measuring instruments. More ever the damaged measuring instruments can be replaced easy with healthy standardized measuring instruments.
- Instrument transformers provide electrical isolation between high voltage power circuit and measuring instruments. Which reduces the electrical insulation requirement for measuring instruments and protective circuits and also assures the safety of operators.
- Several measuring instruments can be connected through a 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.
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. 5A ammeter). A typical connection diagram of a current transformer is shown in figure below.
Primary of C.T. is having very few turns. Sometimes bar primary is also used. 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.
Potential Transformer (P.T.)
Potential transformer is used to step down the voltage of power system to a lower level to make is feasible to be measured by small rating voltmeter i.e. 110 – 120 V voltmeter. A typical connection diagram of a potential transformer is showing figure below.
The primary of a P.T. has many turns and is connected across the power line, typically between a line and earth, which is why it’s often called a parallel transformer. The secondary, having fewer turns, connects directly to a voltmeter. Because the voltmeter has high resistance, the secondary operates in an almost open-circuited condition. Grounding one terminal of the secondary ensures operator safety by stabilizing the voltage relative to earth.
Difference between C.T. and P.T.
Few differences between C.T. and P.T. are listed below –
| Sl. No. | Current Transformer (C.T.) | Potential Transformer (P.T.) |
| 1 | Connected in series with power circuit. | Connected in Parallel with Power circuit. |
| 2 | Secondary is connected to Ammeter. | Secondary is connected to Voltmeter. |
| 3 | Secondary works almost in short circuited condition. | Secondary works almost in open circuited condition. |
| 4 | Primary current depends on power circuit current. | Primary current depends on secondary burden. |
| 5 | Primary current and excitation vary over wide range with change of power circuit current | Primary current and excitation variation are restricted to a small range. |
| 6 | One terminal of secondary is earthed to avoid the insulation break down. | One terminal of secondary can be earthed for Safety. |
| 7 | Secondary is never be open circuited. | Secondary can be used in open circuit condition. |
Some References books for Instrument Transformer
- Bakshi, U.A. ‘Electrical Measurements And Instrumentation”. Pune: Technical Publications, 2009. English.
- Cooper, Helfrick and. “Modern Electronic Instrumentation and Measurement Techniques”. Prentice-Hall of India, 1988.
- Golding, E.W. “Electrical Measurement and Measuring Instruments”,. Sir Issac Pitman and Sons , 1960.
- Jones, B.E. “Instrumentation Measurement and Feedback”. Tata McGraw-Hill, 1986.
- Morris, Alan S. “Measurement and Instrumentation Principles” . 2001.
- Sawhney, A. K. “Electrical and Electronic Measurement and Instrumentation”. Delhi: Dhanpat Rai Co. (P) Ltd., 2010. English.
