Siemens SIPROTEC Manual page 267

Line differential protection with distance protection

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Evaluation of Measurement Quantities

The earth fault differential protection compares the fundamental component of the current flowing in the star-

point connection, which is designated as 3I

the phase currents designated in the following as 3I

' acts as the tripping effect quantity. During a fault within the protected zone this current is always

Figure 2-134

When an earth fault occurs outside the protected zone, a zero sequence current also flows though the phase

current transformers. This is, on the primary side, in counter-phase with the starpoint current and has equal

magnitude. Therefore, both the magnitude of the currents and their phase relationship are evaluated for re-

straint purposes. The following is defined:

A tripping current

and a stabilisation or restraining current

where k is a stabilisation factor which will be explained below, at first we assume k = 1. I

ping effect quantity, I

To illustrate the effect, have a look at the three important operating conditions with ideal and adapted measured

SIPROTEC, 7SD5, Manual

C53000-G1176-C169-5, Release date 02.2011

Principle of restricted earth fault protection

counteracts this effect.

2.10 Restricted Earth Fault Protection (optional)

' in the following, with the fundamental component of the sum of

". Thus, the following applies (Figure 2-134):

produces the trip-