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 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

Introduction

Differential protection is a unit-type protection for a specified zone or piece of equipment. It is based on the fact that it is only in the case of faults internal to the zone that the differential current (difference between input and output currents) will be high.

However, the differential current can sometimes be substantial even without an internal fault.

This is due to certain characteristics of current transformers (different saturation levels, nonlinearities) measuring the input and output currents, and of the power transformer being protected.

With the exception of the inrush and overexcitation currents, most of the other problems, can be solved by means of the percent differential relay, which adds to the normal differential relay two restraining coils fed by the zone-through current, by proper choice of the resulting percent differential characteristic, and by proper connection of the current transformers on each side of the power transformer.

Percentage restraint differential protective relays have been in service for many years. Figure 1 shows a typical differential relay connection diagram. Differential elements compare an operating current with a restraining current.

The operating current (also called differential current), Id, can be obtained from the phasor sum of the currents entering the protected element:

Simple diagram connection for differential power transformer protection
Figure 1 – Simple diagram connection for differential power transformer protection

Id is proportional to the fault current for internal faults and approaches zero for any other operating (ideal) conditions.

There are different alternatives for obtaining the restraining current, IRT. The most common ones include the following:

Restraining current (Irt)

Where k is a compensation factor, usually taken as 1 or 0,5.

The differential relay generates a tripping signal if the differential current, Id, is greater than a percentage of the restraining current, IRT :

Restraining current (Irt)

Title:Transformer Differential Protection Scheme With Internal Faults Detection Algorithm Using Second Harmonics Restrain And Fifth Harmonics Blocking Logic – Ouahdi Dris, Farag. M. Elmareimi and Rekina Fouad
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Transformer Differential Protection Scheme With Internal Faults Detection Algorithm
Transformer Differential Protection Scheme With Internal Faults Detection Algorithm Using Second Harmonics Restrain And Fifth Harmonics Blocking Logic

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Page edited by E.C. (Google).

2 Comments


  1. milad
    Jan 31, 2016

    what’s year of this article?


  2. NAGENDRA V R N
    Mar 26, 2015

    Good info provided.

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