- 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 an AC measuring or protection transformer. It scales a high primary quantity down for measurement of electrical quantities such as voltage, current, power, energy, power factor and frequency. The same cores also feed protection of power system circuits and protective relays.
Instrument transformers step AC line voltage and current down to instrument ratings. Wiring a meter or relay across a high-voltage circuit directly is expensive and unsafe. IEC 61869 common secondaries are 1 A or 5 A for CTs and 100 V or 110 V for VTs. This teaching page uses 5 A and 110 V on measuring instruments.
Those small instruments then read the scaled secondary of the instrument transformers. IEC 61869-1 covers instrument transformers above 1 kV AC (or 1.5 kV DC).
Advantages of Instrument Transformers
- High AC line voltage and current can be measured with small-rating instruments. Teaching examples here use 5 A and 110 to 120 V; IEC also lists 1 A CT secondaries and 100 V VT secondaries.
- Meters and relays can be built to a few secondary ratings. A failed instrument is swapped for another of the same rating without redesigning insulation for the line voltage.
- The transformer isolates the high-voltage circuit from the instruments. That cuts the electrical insulation needed on meters and relays and keeps operators off the line potential, provided the secondary is earthed as specified.
- More than one meter or relay can share one transformer to power system if the total burden stays within the rated VA.
- Measuring and protection wiring runs at the low secondary voltage and current, so copper loss in that wiring is small compared with the primary circuit. Do not read that as a licence to overload the rated burden.
Types of Instrument Transformers
Instrument transformers are of two types:
- Current Transformer (C.T.)
- Potential Transformer (P.T.)
Current Transformer (C.T.)
A Current transformer scales line current down to a 1 A or 5 A secondary so a small ammeter or relay can read it. IEC 61869-2 is the current-transformer product standard. A typical connection is shown below.
The CT primary has few turns, or a bar through the window, and sits in series with the line, so it is sometimes called a series transformer. The secondary has many turns and feeds a low-impedance ammeter or relay, so it runs near short-circuit. One secondary terminal is earthed so the secondary cannot float at line voltage. Never open a CT secondary on load: the core can saturate and the open secondary can reach a dangerous voltage. Short the secondary with switch S before removing the ammeter, as the diagram shows.
Potential Transformer (P.T.)
A Potential transformer (voltage transformer in IEC 61869-3) scales line voltage down for a small-rating voltmeter, commonly 100 V or 110 V, with 120 V used on some systems. A typical potential transformer connection is shown below.
The VT primary has many turns and connects across the line, so it is called a parallel transformer. The secondary has few turns and feeds a high-impedance voltmeter, so it runs near open-circuit. One secondary terminal is earthed so the operator sees a defined voltage to earth.
Difference between C.T. and P.T.
Differences between C.T. and P.T.:
| Sl. No. | Current Transformer (C.T.) | Potential Transformer (P.T.) |
| 1 | Connected in series with the power circuit. | Connected in parallel with the power circuit. |
| 2 | Secondary is connected to an ammeter or current-coil. | Secondary is connected to a voltmeter or voltage-coil. |
| 3 | Secondary runs near short-circuit (never leave it open on load). | Secondary runs near open-circuit into a high-impedance burden. |
| 4 | Primary current is set by the line current (series). | Primary current depends on applied voltage and the secondary burden. |
| 5 | Primary current and excitation swing over a wide range as line current changes. | Primary current and excitation stay near rated voltage, within a small range. |
| 6 | One secondary terminal is earthed to limit voltage to earth. | One secondary terminal is earthed for operator safety. |
| 7 | Secondary must not be open-circuited on load. | Secondary may run open-circuit into a voltmeter (still earthed). |
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.
