Time Delayed Overcurrent Protection (51) - Siemens Argus 7SR21 Technical Manual

7SR210 & 7SR220 Description of Operation
3.1.3

Time Delayed Overcurrent Protection (51)

Two time delayed overcurrent 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.
51-n Setting sets the pick-up current level. Where the voltage controlled overcurrent function (51VCO) is used
(7SR22 relays only) a multiplier is applied to this setting where the voltage drops below the setting VCO Setting,
see section 3.2.
A number of shaped characteristics are provided. An inverse definite minimum time (IDMT) characteristic is
selected from IEC and ANSI curves using 51-n Char. A time multiplier is applied to the characteristic curves using
the 51-n Time Mult setting. Alternatively, a definite time lag delay (DTL) can be chosen using 51-n Char. When
Delay (DTL) is selected the time multiplier is not applied and the 51-n Delay (DTL) setting is used instead.
The 51-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 51-n Min. Operate Time setting.
A fixed additional operate time can be added to the characteristic using 51-n Follower DTL setting.
Where directional elements are present the direction of operation can be set using 51-n Dir. Control setting.
Directional logic is provided independently for each 51-n element.
Operation of the time delayed overcurrent elements can be inhibited from: -
Inhibit 51-n
79 P/F Inst Trips: 51-n
51c
51-n Inrush Action: Inhibit
51-n VTSAction: Inhibit
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A binary input, virtual input, function key or remote data Comms
When 'delayed' trips only are allowed in the auto-reclose sequence
(79 P/F Prot'n Trip n = Delayed).
Activation of the cold load settings (see section 3.7).
Operation of the inrush current detector function.
Operation of the VT Supervision function (7SR22).
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