7SR242 Duobias Description Of Operation
©2010 Siemens Protection Devices Limited Chapter 1 Page 28 of 52
3.3.2 Time Delayed Derived Earth Fault Protection (51N)
Optionally two time delayed derived earth fault elements are provided, each can be selected to either winding 1 or
winding 2.
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, ANSI or user defined 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 or ANSI (DECAYING) reset. 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.
Operation of the time delayed earth fault elements can be inhibited from:
Inhibit 51N-n A binary or virtual input.
51N-n Inrush Action: Inhibit Operation of the inrush detector function.
General Pickup
51N-n
51N-n Setting
51N-n Charact
51N-n Time Mult
51N-n Min Operate Time
c
Wn-IL1
Wn-IL2
Wn-IL3
I
N
51N-n Delay (DTL)
51N-n Follower DTL
c
Pickup
trip
Inhibit 51N-n
&
51N-n Element
Enabled
Disabled
81HBL2
51N-n Inrush
Action
Off
Inhibit
51N-n Reset
&
Figure 3-8 Logic Diagram: Derived Time Delayed Earth Fault Protection
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