7SR18 Description Of Operation
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3.5.3 Time Delayed Measured Earth Fault Protection (51G)
Up to four time-delayed derived earth fault elements are provided in the relay (depending upon variant): -
51G-1, 51G-2, 51G-3 & 51G-4
51G-n Setting sets the pick-up current level.
A number of shaped characteristics are provided. An inverse definite minimum time (IDMT) characteristic is
selected from IEC and ANSI curves using 51G-n Char. A time multiplier is applied to the characteristic curves
using the 51G-n Time Mult setting. Alternatively, a definite time lag (DTL) can be chosen using 51G-n Char.
When DTL is selected the time multiplier is not applied and the 51G-n Delay (DTL) setting is used instead.
The 51-n Reset setting can apply a definite time delayed reset, or when the operation is configured as an IEC or
ANSI or user characteristic if the reset is selected as IEC/ANSI (DECAYING) reset the associated reset curve will
be used. The reset mode is significant where the characteristic has reset before issuing a trip output – see
‘Applications Guide’
A minimum operate time for the characteristic can be set using 51G-n Min. Operate Time setting.
A fixed additional operate time can be added to the characteristic using 51G-n Follower DTL setting.
Where directional elements are present the direction of operation can be set using 51G-n Dir. Control setting.
Directional logic is provided independently for each 51G-n element e.g. giving the option of using two elements
set to forward and two to reverse.
Operation of the time delayed measured earth fault elements can be inhibited from: -
Inhibit 51G-n A binary or virtual input.
79 E/F Inst Trips: 51G-n When ‘delayed’ trips only are allowed in the auto-reclose sequence
(79 E/F Prot’n Trip n = Delayed).
50-n Inrush Action: Block Operation of the inrush current detector function.
50N-n VTSAction: Inhibit Operation of the VT Supervision function
Figure 3.5.3-1 Logic Diagram: Measured Time Delayed Earth Fault Element (51G)
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