Premium Membership ♕

Save 50% on all EEP Academy courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles & guides.

Home / Technical Articles / Definitions of Voltage Transformer Parameters

VT Parameters

  1. Rated voltage factor
  2. Rated primary voltage (Up)
  3. Rated secondary voltage
  4. Accuracy power
  5. Accuracy class
  6. Voltage ratio error
  7. Phase or phase displacement error
  8. Rated thermal limiting output
Definitions of the Voltage Transformer Parameters
Definitions of the Voltage Transformer Parameters (on photo withdrawable voltage trasnformers 36kV in Schneider Electric’s DNF7 AIS MV Switchgear)

Introduction to VTs

The voltage transformer must comply with the network specifications, this is essential.

As with any device, the voltage transformer must meet requirements relating to the voltage, current and frequency. Voltage transformer specifications are only valid for normal conditions of use. A derating should be provided for in accordance with the ambient temperature and the altitude.

Rated voltage factor: this is the factor by which the rated primary voltage must be multiplied in order to determine the maximum voltage for which the transformer must attain the required levels of heating and accuracy. Go to Top ↑

The voltage factor is determined by the maximum operating voltage, which depends on the network earthing system and the way the VT’s primary winding is connected.

The voltage transformer must be able to withstand this maximum voltage for the time necessary to clear the fault (see Table 1 below).

Table 1 – Normal values of the rated voltage factor

Rated voltage factorRated timePrimary winding connection methodNetwork earthing system
1.2continuousphase to phaseany
1.2continuousbetween the neutral
point of a star
transformer and earth
1.2continuousphase to earthdirectly earthed neutral
1.530 seconds
1.2continuousphase to earthlimiting resistance earthing
with automatic earth fault
(tripping upon first fault)
1.930 seconds
1.2continuousphase to earthearthed neutral without
automatic earth fault clearance
(no  tripping upon first fault)
1.98 hours
1.2continuousphase to earthtuned limiting reactance
(or Petersen coil) earthing
without automatic earth fault
clearance (no tripping upon
first fault)
1.98 hours

Note: smaller time ratings are permissible by agreement between manufacturer and user.

Rated primary voltage (Up): Depending on their design, voltage transformers will be connected either:

  1. Between phase and earth (see Figure 1a); or
  2. Between phases (see Figure 1b).

Rated primary voltage formulae

Voltage transformer connections
Figure 1 – Voltage transformer connections

The voltage transformer must be suited to requirements relating to protection and measuring devices.

The foreseen application of the voltage transformer is used to determine the rated secondary voltage, the accuracy power, the accuracy class and the thermal power limit.

Rated secondary voltage: this is equal to 100 or 110 V for phase/phase VTs. For single-phase transformers designed to be connected between a phase and earth, the rated secondary voltage is divided by √3. Go to Top ↑

For example:

Rated secondary voltage formulae

Accuracy power: this is expressed in VA and it is the apparent power that the voltage transformer can supply to the secondary when it is connected under its rated primary voltage and connected to its accuracyload. It must not introduce an error in excess of the values guaranteed by the accuracy class. Go to Top ↑

The standardized values are: 10 – 15 – 25 – 30 – 50 – 75 – 100 – 150 – 200 – 300 – 400 – 500 – VA.

Accuracy class: this defines the guaranteed transformation ratio and phase error limits in specified power and voltage conditions. Go to Top ↑

Voltage ratio error: this is the error that the transformer introduces in the voltage measurement:

Voltage ratio error formulae

Phase or phase displacement error: this is the phase difference between the primary and secondary voltages and is expressed in minutes. Go to Top ↑

Rated thermal limiting output: this is the apparent power that the transformer can permanently supply at its rated secondary voltage without exceeding the heating limits stipulated in the standards.
Go to Top ↑

Accuracy class

Table 2 gives the accuracy class generally used in accordance with the corresponding application.

Table 2 – Accuracy class in accordance with the application for measuring VTs

ApplicationAccuracy class
Not used in industry0.1
Precision metering0.2
Usual metering0.5
Statistical metering and/or measurement1
Measurement not requiring high accuracy3

The accuracy class is guaranteed if the voltage is between 80 and 120% of the rated primary voltage and for any load between 25 and 100% of the rated accuracy power with an inductive power factor of 0.8.

The standardized IEC accuracy classes are: 0.1 – 0.2 – 0.5 – 1 – 3.

  1. Classes 0.1 and 0.2 are only used for laboratory devices.
  2. Classes 0.5 and 1 are used in the majority of cases.
  3. Class 3 is used very little.

Resource: Protection of Electrical Networks – Christophe Prévé (get this book from Amazon)

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Roger
    Jul 25, 2022

    What is the IEC standard which indicates accuracy classes for voltage transformers ?

  2. Mukesh
    Jan 29, 2019

    Hello Edvard,

    When Accuracy Class 1 Voltage transformer (for eg. 230V / 24V, 100VA) is suggested., does it mean that the output voltage shall not deviate more than 1%, when the burden is varied from 25% to 100% ?

    If yes,
    1) what about the voltage drop that happens across windings due to regulation ?

    2) If regulation is considered, & accordingly, the output voltage is set to rated output voltage at 100% rated burden.
    Then as the load is gradually decreased, the output voltage will tend to increase & may cross the limit of 1% accuracy. How does one overcome this condition ?

  3. Sarah
    Mar 01, 2017

    Hi, sir, plz I want to know more about the core of the voltage transformer, about the windings, the saturation curve, the full data, what can I do?

  4. Dana
    Nov 17, 2016

    Hi ,
    i want to ask about the connection of this voltage transformer :
    u1 : 31500/v3 v
    sek : a-n da-dn
    u2 : 100/v3 v 100/3 v
    va : 60 60
    cı : 0,5 3p
    a : 2 3.6
    the second seconder is open delta
    the first second is 0,5 for measuring
    could i connect the first seconder (star connection ) to protection relay and to m meter
    is that right if not could you explain to me the reason.
    thanks in advance
    Dana yossef

  5. Raghu
    Oct 12, 2015

    I have seen a VT with secondary winding having 0.5/3p. I didn’t understand how a single
    winding can possess two classes that to metering and protection for this winding what ratio error will reflect
    whether 0.5 (0.5%) or 3p(3%) . Can any one explain?

    • Sampath Purushothaman
      Mar 28, 2016

      0.5/3P- 0.5 It’s combined core.

      0.5 using for metering

      3p for protection

      Metering & protection combined core is possible in VT, but in CT it’s not possible. why think. & reply as per your knowledge. then i will explain

      • Vishnu
        Jul 16, 2021

        Please tell me why and some PT s having two class but only one winding! How it’s possible

  6. Francisco Alvarez
    May 12, 2015

    Good article. Thanks.

  7. Joe
    Apr 30, 2015

    Edvard , thank you so much you are the man as usual )
    I am reading all your articles

  8. Natalya
    Feb 13, 2014

    Sir what will be the effect if protection relay will be connected to broken delta of voltage transformer with accuracy class 3 (not 3P)? Normal operation of protection device is guaranteed in such scheme of connection or not? What about standards which regulate angular error in case of protection connection?

  9. JOSH
    Mar 17, 2013

    Sir what will be the effect if a delta-wye transformer (Vpri=400V, Vsec=230V) will be supplied by wye system input voltage of 400Volts.

    humble regards,


  10. Md Ebrahim Shah
    Mar 13, 2013

    although engineers try to make 100% error free transformer but maximum transformers cannot 100% error free

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

sixty four  −    =  sixty two

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge