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Home / Technical Articles / Transformer differential protection (ANSI 87T)

Introduction to ANSI code 87 T

Transformer differential protection protects against short-circuits between turns of a winding and between windings that correspond to phase-to-phase or three-phase type short-circuits.

SIPROTEC 4 7UT6 Differential Protection Relay for Transformers
SIPROTEC 4 7UT6 Differential Protection Relay for Transformers – Connection of transformer differential protection with high impedance REF (I7) and neutral current measurement at I8

If there is no earthing connection at the transformer location point, this protection can also be used to protect against earth faults. If the earth fault current is limited by an impedance, it is generally not possible to set the current threshold to a value less than the limiting current.

The protection must be then carried out by a high impedance differential protection.

Transformer differential protection operates very quickly, roughly 30 ms, which allows any transformer deterioration in the event of a short-circuit between windings to be avoided.

Transformers cannot be differentially protected using high impedance differential protection for phase-to-phase short-circuit due to the natural differential currents that occur:

  1. The transformer inrush currents. The operating speed required means that a time delay longer than the duration of this current cannot be used (several tenths of a second);
  2. The action of the on-load tap changer causes a differential current.

The characteristics of transformer differential protection are related to the transformer specifications:

  1. Transformation ratio between the current entering Iin and the current leaving Iout ;
  2. Primary and secondary coupling method;
  3. Inrush current;
  4. Permanent magnetizing current.

The block diagram is shown in Figure 1 below.

Transformer differential protection block diagram
Figure 1 – Transformer differential protection block diagram

In order to prevent tripping upon occurrence of high fault currents of external origin, biased differential protection devices are used.

This is because of:

  • The differential current due to the on-load tap changer;
  • The current transformer measurement errors, as for pilot wire differential protection for cables or lines.

Protection is activated when:

Iin – Iout > K Iin +  Io       (see Figure 2).

Transformer differential protection tripping curve
Figure 2 – Transformer differential protection tripping curve

Problem relating to the transformation ratio and the coupling method

The primary and secondary currents have different amplitudes owing to the transformation ratio and different phases depending on the coupling method (delta-star transformer makes a phase displacement of 30°). Therefore, the current values measured must be readjusted so that the signals compared are equal during normal operation.

This is done using matching auxiliary transformers whose role is to balance the amplitudes and phases.

When one side of the transformer is star-connected with an earthed neutral, the matching transformers located on this side are delta-connected, so that the residual currents that would be detected upon occurrence of an earth fault outside the transformer are cleared.

Problem relating to the transformer inrush current

Transformer switching causes a very high transient current (from 8 to 15 In), which only flows through the primary winding and lasts several tenths of a second.

It is thus detected by the protection as a differential current and it lasts far longer than the protection operating time (30 ms). Detection based only on the difference between the transformer primary and secondary currents would cause the protection to be activated. Therefore, the protection must be able to distinguish between a differential current due to a fault and a differential inrush current.

Experience has shown that the inrush current wave contains at least 20% of second harmonic components (current at a frequency of 100 Hz), while this percentage is never higher than 5% upon occurrence of an overcurrent due to a fault inside the transformer.

The protection must therefore simply be locked when the percentage of second harmonic component in relation to the fundamental harmonic component (current at 50 Hz) is higher than 15%, i.e. I2/I1 > 15%.

Problem relating to the magnetizing current upon occurrence of an overvoltage of external origin

The magnetizing current constitutes a difference between the transformer primary and secondary currents (see section 6.1.1). It is therefore detected as a fault current by the differential protection even though it is not due to a fault.

In normal operating conditions, this magnetizing current is very low and does not reach the protection operating threshold.

However, when an overvoltage occurs outside the transformer, the magnetic material saturates (in general the transformers are dimensioned to be able to operate at saturation limit for the nominal supply voltage), and the magnetizing current value greatly increases. The protection operating threshold can therefore be reached.

Experience has shown that the magnetizing current due to the magnetic saturation has a high rate of fifth harmonic components (current at a frequency of 250 Hz).

To prevent spurious tripping upon occurrence of an overvoltage of external origin, there are two solutions:

  1. Detect a rise in voltage that locks the protection;
  2. Detect saturation using the presence of fifth harmonic current that locks the protection.

Transformer differential protection therefore requires fairly complex functions as it must be able to measure second and fifth harmonic currents or, in order to avoid measuring fifth harmonic currents, it must be able to detect overvoltages of external origin.

Resource: Protection of electrical networks – Christophe Prévé 

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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. Ullas A N
    Jun 01, 2022

    Actually for DY transformer YD CT connection is required, but that drawing bits shown YY connection CT, is that matching CT compensate that vector difference of YY CT connection?

  2. Mohammed Ali
    Apr 10, 2021

    What is bias current in differential relay. And our relay and 2nd Harmonic block.

  3. Saad Shakil
    Feb 06, 2021

    Quite informative article…

  4. Rey
    Jan 03, 2020

    It’s a good article!

  5. mohammad vadigard
    Jun 23, 2015

    i am Electrical engineer,at IRAN TABLO Co and I would like to now more about protection in distribution and transporting system

  6. Swapnil
    Aug 31, 2014

    Why the matching transformers are connected opposite connection than that of main transformer ? I read about residual current but If you can elaborate that’ll be really helpful.
    and thank you you’re awesome

    Jul 30, 2014

    Dear Sir,

    I just want to know if we fix the settings of over current relay for transformer protection as well as from inrush current. but if we don’t protect the inrush current then what happen. Please reply.

  8. komichi
    Jul 16, 2014

    Many thanks.

  9. vilas Deshpande
    Jul 11, 2014

    Need to about apps available .

  10. dyaa elsayed
    Jan 09, 2014

    great article very simple and very useful

  11. ather
    Sep 11, 2013

    what do we mean by BIASED INPUTS in differential relay?

    Feb 09, 2013


  13. Ingelec
    Oct 10, 2012

    good article

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