Instantaneous Earth Fault - Calculated; Overview Of Functions; Structure Of The Function - Siemens 7SR51 Device Manual

Protection and Automation Functions
5.15 50N Instantaneous Earth Fault – Calculated
5.15 50N Instantaneous Earth Fault – Calculated

Overview of Functions

5.15.1
Instantaneous/time delayed earth fault protection is used to provide:
•
Short circuit detection in electrical equipment
•
High speed protection where appropriate to its location in the power system network and/or network
impedances
•
Backup or emergency protection in addition to other protection functions or devices
•
Coordinated operation with other devices using time graded settings

Structure of the Function

5.15.2
The calculated instantaneous earth fault function element 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 A50NPTOC* in IEC 61850.
The function monitors the primary system earth current by calculating the sum of the 3 phase CT inputs e.g.
CT1/2/3.
The basic instantaneous/time delayed measured earth fault function (50N) is summarized below:
•
Measurement is based on fundamental frequency current
•
Elements operate with a definite time delayed (DTL) characteristic
•
Outputs are provided for general pickup and trip
The basic function is non-directional but can be directionalized. This requires 3 phase VT inputs e.g. VT1/2/3.
Each 50N element can be set to operate for forward current, reverse current or independently of current direc-
tion. Directionalized functionality uses the decision of the 67N element.
•
Provides information regarding flow direction of earth current for selective application of earth fault
protection
•
Ensures selective fault detection for parallel lines or transformers with infeed at one end
•
Ensures selective fault detection in cable runs with infeed at both ends or in lines connected to form ring
topologies
Where earth fault protection is directionalized correct operation is dependent on valid information being
received from the VT inputs, operation of each directionalized element will be affected if a VT fuse fails. When
a fuse failure is detected, 60VTS function, each element can be selected to either ignore the VT failure, switch
to non-directionalized operation (which does not require VT inputs) or be inhibited.
When installed in power transformer circuits earth currents may be measured by the function during switch in
of the transformer. Each element can be set to be inhibited when inrush current is detected, (see
sion Functions , section 81HB2 Inrush Current Detection).
Elements can be mapped to start 79 Automatic Reclosing by selection of the element as a 79 EF
Trigger within the 79 function element menus. For autoreclose sequences that require operation of instan-
taneous or time delayed protections to be inhibited this can be selected in settings menu.
234 Reyrolle 7SR5, Overcurrent Protection Device, Device Manual
C53000-G7040-C014-1, Edition 11.2019
6 Supervi-