VOLTAGE TRANSFORMERS-VT USING

What is Voltage Transformer (VT)?

VT is the abbreviation Voltage Transformer, on the other way PT or Potential Transformer is a transformer that converts the voltage into the standard measurable level. 

This converted voltage is then proportionally transformed into the measuring or controlling primary voltage. 

VT operates on the same principle as a power transformer, the different only purpose of use. Unlike conventional transformers, voltage transformers have only one magnetic core attached to the secondary winding. Physically VT may be constructed in single pole or double pole.

Voltage transformers shall comply with the requirements of IEC 186 with amendments and supplements-

Instrument Transformers-CT&VT

Transformers that are used for instrumentation- like controlling, measuring, etc. are known as instrument transformers. Instrument transformers are mainly used to couple the main primary circuit and secondary controlling or measuring circuit.

Controlling or measuring devices are not capable to connect directly with high voltage or high-level current primary power lines. To reduce or step down the voltage or current level in standard ratio instrument transformers are used.

Instrument transformers are mainly two types- CT or Current Transformer and VT or Voltage Transformer. CT mainly step-down the high-level current into the low level and CT’s primary is connected in series with the monitored primary circuit. 

On the other hand, VT steps down the high voltage into low voltage, and VT’s primary is connected in parallel with the monitored primary circuit.

To know about CT in detail visit other post-CTCT (CURRENT TRANSFORMERS) IN ELECTRICAL DISTRIBUTION SYSTEM

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Capacitor Voltage Transformer- CVT

Capacitor Voltage Transformer (CVT) is an instrument transformer that is used in a power system to step down the extra high voltage (EHV) signals and provide a low voltage signal that is used for metering or operating a protective relay to control the circuit.

The basic components of the capacitor voltage transformer are shown in the figure.

A voltage or Ratio Error and Phase or Phase Angle Error of VT or PT

The difference between the ideal value and the actual value of the voltage is the voltage error or ratio error of the voltage transformer or potential transformer. The formula can be expressed as,

% Voltage error= { (V_p-K_T.V_s)/V_p}x100%

The angle between the primary system voltage V_p and the reversed secondary voltage vectors (K_T.V_s ) is known as phase error.

Where,

V_p - Primary terminal voltage,

V_s - Secondary terminal voltage,

K_T - Turns ratio = Numbers of primary turns/number of secondary turns,

Voltage Transformer Using Classes

Class 1 accuracy for protection/indicating instruments

Class 0.5 accuracy for integrating meters

Class 0.2 accuracy will be for tariff metering or acceptance efficiency testing.

Class 0.1 accuracy for generator low forward power interlock relays or as specified.

Voltage Transformer Ratings

The VA output rating shall be 50% in excess of the design requirements except for tariff metering voltage transformers which shall be at least 10% in excess of the design requirements.

For tariff metering voltage transformers the Contractor shall check the total installed secondary burden and if necessary shall install dummy burdens to achieve the calibrated accuracy.

Voltage Transformer Primary & Secondary Connection

The voltage transformer secondary circuit shall be earthed at one point only and metal cases shall be separately earthed. The transformer's core, where accessible, shall also be separately earthed.

All voltage transformers in the system at a given voltage level shall be earthed in the same manner.

Where it is required to earth the primary neutral of a metal-clad three-phase voltage transformer, the neutral earthing connection shall be insulated and brought out separately from the tan earthing connection. Means shall be provided to maintain the tank earthing connection while the voltage transformer is being withdrawn.

Where three single-phase voltage transformers are supplied for protection purposes, star-connected secondary windings shall have the star point formed by insulated connections and shall be earthed at a common point.

Where necessary for earth fault protection, voltage transformers shall be of five-limbed core construction.

Where possible primary windings shall be connected through fuses with current limiting features.

Secondary MCBs shall be provided as close as possible to each voltage transformer and labeled to show their function and phase color. The secondary circuits shall be monitored individually to detect and alarm individual fuse failures or MCB trips and to block protection operations if required.

Voltage transformers shall be designed so that saturation of their cores does not occur when 1.732 times normal voltage is applied to each winding.

Magnetization curves shall be submitted for approval for each type of voltage transformer.

The standard secondary voltage between phases shall be 110 volts unless special circumstances dictate otherwise, and are approved by the Engineer.

Secondary circuits from different voltage transformers, or separate windings of the same transformer, shall not be connected in parallel.

Voltage transformers shall be connected on the non-busbar side of circuit breakers unless otherwise approved by the Engineer.

 

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.