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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)

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

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
any
1.2continuousphase to earthdirectly earthed neutral
1.530 seconds
1.2continuousphase to earthlimiting resistance earthing
with automatic earth fault
clearance
(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)

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

11 Comments


  1. 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


  2. Francisco Alvarez
    May 12, 2015

    Good article. Thanks.


  3. Joe
    Apr 30, 2015

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

  4. […] health of the apparatus, relay senses current through a current transformer (CT), voltage through a voltage transformer (VT). VT is also known as Potential Transformer […]


    • arnel
      Apr 09, 2015

      note that PT designation is obsolete. new IEEE designation is VT

  5. […] Supply generator 2. Supply transformer 3. Test transformer 4. Voltage transformer and measuring circuit 5. Filter 6. Measuring impedance 7. Selective switch 8. Measuring instrument […]


  6. 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?


  7. 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,

    JOSEPH


  8. Md Ebrahim Shah
    Mar 13, 2013

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

  9. […] should be very sinusoidal and at rated frequency.Normally, the measurements are made while the supply voltage is increased at equal intervals from 90% to 115% of the transformer rated voltage (Un) and this way […]

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