7SR18 Description Of Operation
Unrestricted ©2018 Siemens Protection Devices Limited Page 37 of 78
3.3.4 Voltage Controlled Overcurrent (51V)
Voltage controlled overcurrent is only available in relays with four current inputs.
Each shaped overcurrent element 51-n Setting can be independently controlled by the level of measured
(control) input voltage.
For applied voltages above VCO Setting the 51-n element operates in accordance with its normal current setting
(see 3.1.3). For input Ph-Ph control voltages below VCO Setting a multiplier (51-n Multiplier) is applied to
reduce the 51-n pickup current setting.
51-n Multiplier is applied to each phase independently when its control phase-phase voltage falls below VCO
Setting. The voltage levels used for each phase over-current element are shown in the table below. Relays with a
Ph-N connection automatically calculate the correct Ph-Ph control voltage.
Current Element Control Voltage
I
L1
V
12
I
L2
V
23
I
L3
V
31
The Voltage Controlled Overcurrent function (51V) can be inhibited from:
VCO VTSAction: Inhibit Operation of the VT Supervision function.
Figure 3.3.4-1 Logic Diagram: Voltage Controlled Overcurrent Element
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