Time Delayed Derived Earth Fault Protection (51N); Figure 3.3-2 Logic Diagram: Derived Instantaneous Earth Fault Element - Siemens 7SR21 Manual

Figure 3.3-2 Logic Diagram: Derived Instantaneous Earth Fault Element

3.3.3

Time Delayed Derived Earth Fault Protection (51N)

Two time delayed derived 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.
51N-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 51N-n Char. A time multiplier is applied to the characteristic curves
using the 51N-n Time Mult setting. Alternatively, a definite time lag delay (DTL) can be chosen using 51N-n
Char. When Delay (DTL) is selected the time multiplier is not applied and the 51N-n Delay (DTL) setting is used
instead.
The 51N-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 the 51N-n Min. Operate Time setting.
A fixed additional operate time can be added to the characteristic using the 51N-n Follower DTL setting.
Where directional elements are present the direction of operation can be set using 51N-n Dir. Control setting.
Directional logic is provided independently for each 51N-n element.
Operation of the time delayed earth fault elements can be inhibited from:
Inhibit 51N-n
79 E/F Inst Trips: 51N-n
51N-n Inrush Action: Inhibit
51N-n VTSAction: Inhibit
©2010 Siemens Protection Devices Limited
7SR21 & 7SR22 Argus Description of Operation
A binary or virtual input, or function key
When 'delayed' trips only are allowed in the auto-reclose sequence
(79 E/F Prot'n Trip n = Delayed).
Operation of the current inrush detector function.
Operation of the VT Supervision function (7SR22).
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