Time Delayed Overcurrent Protection (51) - Siemens 7SR224 Description & Operation

3.1.3

Time Delayed Overcurrent Protection (51)

Four elements are provided 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 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, ANSI or manufacturer specific 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 full list of operating
curves is given in section 2 – 'Settings, Configuration and Instruments Guide'. Operating curve
characteristics are illustrated in section 3 – 'Performance Specification'.
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.
For the directional elements 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
©2009 Siemens Protection Devices Limited
A binary or virtual input, or function key
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.
(1) Description of Operation 7SR224
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