51G Time-Delayed Earth Fault – Measured
Overview of Functions
Time-delayed earth fault protection is used to provide:
•
Detection of earth current in electrical equipment
•
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
The measured time-delayed 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
A51GPTOC*
in IEC 61850.
The function monitors the primary system current using the earth fault CT input e.g. CT4.
The basic time-delayed measured earth fault function (51G) is summarized below:
•
Measurement is based on either fundamental frequency or RMS current
•
A number of time vs current operate curves are selectable based on IEC and ANSI standards
•
Alternatively the 51G elements can 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 when the device is fitted with VT inputs.
Each 51G element can be set to operate for forward current, reverse current or independently of current direc-
tion. Directionalized functionality uses the decision of the 67G 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
, section 81HB2 Inrush Current Detection).
Elements can be mapped to start
79 Automatic Recloser
by selection of the element as a
79 PF
Trigger
within the 79 function element menus. For advanced autoreclose applications each element can be
selected to be inhibited during delayed autoreclose shots of a multi-shot sequence.
5.15
5.15.1
5.15.2
Protection and Automation Functions
5.15 51G Time-Delayed Earth Fault – Measured
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Reyrolle 7SR5, Transformer Protection Device, Device Manual
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