7SR210 & 7SR220 Description of Operation
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3.5.3 Time Delayed Sensitive Earth Fault Protection (51SEF)
Two time delayed sensitive earth fault elements are provided in the 7SR21 relay. Four elements are provided in
the 7SR22 relay e.g. giving the option of using two elements set to forward and two to reverse.
51SEF-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 51SEF-n Char. A time multiplier is applied to the characteristic curves
using the 51SEF-n Time Mult setting. Alternatively, a definite time lag (DTL) can be chosen using 51SEF-n Char.
When DTL is selected the time multiplier is not applied and the 51SEF-n Delay (DTL) setting is used instead.
The 51SEF-n Reset setting can apply a definite time delayed reset, or when configured as an ANSI
characteristic an ANSI (DECAYING) reset. If ANSI (DECAYING) reset is selected for an IEC characteristic, the
reset will be instantaneous. 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 51SEF-n Min. Operate Time setting.
A fixed additional operate time can be added to the characteristic using 51SEF-n Follower DTL setting.
Where directional elements are present the direction of operation can be set using 51SEF-n Dir. Control setting.
Directional logic is provided independently for each 51SEF-n element.
Operation of the time delayed earth fault elements can be inhibited from: -
Inhibit 51SEF-n A binary or virtual input, function key or remote data Comms
79 SEF Inst Trips: 51SEF-n When ‘delayed’ trips only are allowed in the auto-reclose sequence
(79 SEF Prot’n Trip n = Delayed).
51SEF-n VTSAction: Inhibit Operation of the VT Supervision function (7SR22).
Directional elements will not operate unless the zero sequence voltage (V
0
) is above the 67SEF Minimum
Voltage setting i.e. the residual voltage is greater than 3 times this setting and the phase is in the
Forward/Reverse operating range. If 67SEF Wattmetric is set to Enabled, the calculated residual real power
must be above the 67SEF Wattmetric Power setting. The residual power P
res
is equal to the wattmetric
component of 3V
0
I
SEF
and therefore the wattmetric component of 9V
0
I
0
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