87GH Restricted Earth-Fault Protection – High-Impedance
Overview of Functions
High impedance restricted earth fault protection:
•
Detects earth faults in transformers, shunt reactors, neutral reactors or rotating machinery
•
Has high sensitivity to earth faults and so can detect faults near the neutral point
•
Can be part of an integrated overall differential protection
•
Detects earth faults in the protected zone
•
Is stable during external faults
Structure of the Function
The Restricted Earth Fault Protection – High-Impedance function has group dependent
settings. These settings can be different in each settings group – group numbers (Gn) 1 to 4.
This function corresponds to the logical node A87GHPTOC* in IEC 61850.
The function monitors the primary system differential current using the earth fault CT input e.g. CT4.
The zone of REF protection is defined by the position of the CTs and the transformer winding. REF protection
provides a low operate current (fault setting) for in zone earth faults and stability during external faults.
To achieve stability of the high impedance REF elements and limit overvoltages a series stabilizing resistor and
a non-linear resistor are wired into the scheme.
The operating voltage of the relay/stabilizing resistor combination is calculated taking into account the r.m.s.
value of the symmetrical component of the transformer through fault current.
The relay current setting is calculated taking into account the required operate level for in-zone earth faults
(fault setting).
Logic of the Function
[lo_7SR5_87GHlogicdiagram, 1, en_US]
Figure 5-54 Logic Diagram: 87GH Restricted Earth Fault Protection – High-Impedance
5.29
5.29.1
5.29.2
5.29.3
Protection and Automation Functions
5.29 87GH Restricted Earth-Fault Protection – High-Impedance
Reyrolle 7SR5, Overcurrent Protection Device, Device Manual 355
C53000-G7040-C014-1, Edition 11.2019
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