Instrument Transformer and Uses

Fig. Working Principle of an Instrument Transformer

What is an Instrument Transformer and how does it work?

The Instrument transformers are the same as other conventional transformers, but different it is just high-accuracy class electrical devices. Instrument transformers are actually used to isolate or transform voltage or current levels for using metering or controlling signals. 

Operating the instruments for controlling the system or measuring the high-level voltage or current, the instrument transformers are used to decrease the voltage or current level to the measuring level for safely isolating secondary control circuitry from the high voltages or currents. 

Working Principle of Instrument Transformers

1. Electromagnetic Induction

• When alternating current flows through the primary winding, it produces a magnetic flux in the transformer core.
• This flux induces a voltage in the secondary winding according to Faraday’s law of electromagnetic induction.
• The induced voltage/current is proportional to the primary values but reduced to standardized levels (e.g., 5A or 1A for CTs, 110V for PTs).

2. Current Transformer (CT)

• Primary winding: carries the actual line current (often just one turn or conductor).
• Secondary winding: produces a scaled-down current (usually 5A or 1A).
• Principle: The secondary current is proportional to the primary current, maintaining the same phase relationship.

3. Potential Transformer (PT)

• Primary winding: connected across the high-voltage line.
• Secondary winding: delivers a reduced voltage (commonly 110V).
• Principle: The secondary voltage is proportional to the primary voltage, with minimal phase shift.

4. Isolation

• Instrument transformers provide electrical isolation between high-voltage circuits and measuring/protection devices.
• This ensures safety for operators and prevents damage to sensitive instruments.

🔑 Key Points

• Accuracy: Designed to minimize errors in ratio and phase angle.
• Standardization: Secondary values are kept within safe, standard ranges for instruments.
• Safety: Protects both equipment and personnel from direct exposure to high voltage/current.

Use of Instrument Transformer

Instrument transformers are essential components in electrical power systems, especially when dealing with high voltages and currents. Their primary role is to make measurements and ensure that protection is safe, accurate, and practical. Here’s a clear breakdown of their uses:

🔌 Main Uses of Instrument Transformers

1. Measurement

• Step down high voltage/current to safe, standardized levels.
• Allow meters (ammeters, voltmeters, wattmeters) to measure without being directly exposed to dangerous primary values.
• Ensure accuracy in readings across power generation, transmission, and distribution systems.

2. Protection

• Provide input to protective relays by converting large currents/voltages into manageable signals.
• Enable safe operation of circuit breakers and other protective devices.
• Prevent damage to equipment by isolating measuring/control circuits from high-energy primary circuits.

3. Safety & Isolation

• Electrically isolate measuring instruments and control devices from high-voltage lines.
• Protect operators and equipment from direct exposure to dangerous voltages.
• Reduce risk of accidents during maintenance and monitoring.

4. Standardization

• Convert primary values into standardized secondary values (e.g., 5A for current transformers, 110V for potential transformers).
• Simplify design and calibration of measuring instruments and relays.

5. Phase Shifting & Control

• Used in specialized applications like phase-shifting for power system studies.
• Help in synchronizing generators and managing load flow.

So, if you ask what an Instrument Transformer is, simply, we can say that Instrument Transformers are used in AC systems for the measurement of electrical quantities like voltage, current, power, energy, power factor, and frequency. Instrument transformers are also used with protective relays for the protection of the power system.

Instrument Transformer Connection Procedure

The high voltage or high current end of the circuit is connected to the primary winding of the instrument transformer, and the meter or relay is connected to the secondary circuit.

Types of Instrument Transformers

Instrument transformers are of two types –

1. Current Transformer (C.T.)
2. Potential Transformer (P.T.)

Read more details on the Current Transformer 

Current Transformer and Traditional Transformer

Read more details on the Current Transformer 

Voltage Transformer Primary & Secondary Connection

Difference 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	The secondary is connected to the ammeter.	The secondary is connected to the Voltmeter.
3	Secondary works are almost in short-circuited condition.	Secondary works are almost in an open-circuited condition.
4	The primary current depends on the power circuit current.	The primary current depends on the secondary burden.
5	Primary current and excitation vary over a wide range with the change of the power circuit current	Primary current and excitation variations are restricted to a small range.
6	One terminal of the secondary is earthed to avoid the insulation breakdown.	One terminal of the secondary can be earthed for safety.
7	The secondary is never open-circuited.	Secondary can be used in open circuit conditions.

You may know the details about the electrical transformer from the following articles:

 

1. Working Principle of Transformer;
2. Transformer Construction;
3. Core-type Transformers;
4. Shell-type Transformers;
5. Elementary Theory of an Ideal Transformer;
6. E.M.F. Equation of Transformer;
7. Voltage Transformation Ratio;
8. Transformer with losses but no Magnetic Leakage;
9. Transformer on No-load;
10. Transformer on Load;
11. Transformer with Winding Resistance but no Magnetic Leakage;
12. Equivalent Resistance;
13. Magnetic Leakage;
14. Transformer with Resistance and Leakage Reactance;
15. Simplified Diagram;
16. Total Approximate Voltage Drop in Transformer;
17. Exact Voltage Drop;
18. Equivalent Circuit Transformer Tests;
19. Open-circuit or No-load Test;
20. Separation of Core Losses;
21. Short-Circuit or Impedance Test;
22. Why Transformer Rating in KVA?;
23. Regulation of a Transformer;
24. Percentage Resistance, Reactance, and Impedance;
25. Kapp Regulation Diagram;
26. Sumpner or Back-to-back-Test;
    
27. The efficiency of a Transformer;
28. Condition for Maximum Efficiency;
29. Variation of Efficiency with Power Factor;
30. All-day Efficiency;
31. Auto-transformer;
32. Conversion of 2-Winding Transformer into Auto-transformer;
33. Parallel Operation of Single-phase Transformers;
34. Questions and Answers on Transformers;
35. Three-phase Transformers;
36. Three-phase Transformer Connections;
37. Star/Star or Y/Y Connection;
38. Delta-Delta or ∆/∆ Connection;
39. Wye/Delta or Y/ Connection;
40. Delta/Wye or ∆/Y Connection;
41. Open-Delta or V-V Connection;
42. Power Supplied by V-V Bank;
43. Scott Connection or T-T Connection;
44. Three-phase to Two-Phase Conversion and vice-versa;
45. Parallel Operation of 3-phase Transformers;
46. Instrument Transformers;
47. Current Transformers;
48. Potential or Voltage Transformers.