Siemens 7SR105 Rho - Current Protection: Undercurrent (37); Current Protection: Phase Overcurrent (50, 51); Instantaneous Overcurrent Protection (50); Figure 3-10 Logic Diagram: Undercurrent Detector (37)

7SR105 Rho Description of Operation
©2018 Siemens Protection Devices Chapter 1 Page 33 of 57
3.2 Current Protection: Undercurrent (37)
Two rms measuring under-current elements are provided.
Each phase has an independent level detector and current-timing element. Gn 37-n Setting sets the pick-up
current. An output is given after elapse of the Gn 37-n Delay setting.
Operation can be selected for any phase or all phases using Gn 37-n Start Option
Operation of the under-current elements can be inhibited from:
Inhibit 37-n A binary or virtual input
Gn 37-n U/C Guarded Operation of the undercurrent guard function
<
<
<
<
&
<
<
&
Inhibit 37-n
&
Enabled
Gn 37-n Element
Yes
Gn
37-
n U/
C
Guarded
Gn 37-n U/C Guard
Setting
I
A
I
B
I
C
Gn 37-n Setting
En
37
-
n
General Pickup
Gn
37-
n Delay
Gn 37
Start
Option
Figure 3-10 Logic Diagram: Undercurrent Detector (37)
3.3 Current Protection: Phase Overcurrent (50, 51)
All phase overcurrent elements have a common setting for the 50 elements and 51 elements to measure either
the fundamental frequency RMS or True RMS current:
True RMS current: 50 Measurement = RMS, 51 Measurement = RMS
Fundamental Frequency RMS current: 50 Measurement = Fundamental, 51 Measurement = Fundamental
3.3.1 Instantaneous Overcurrent Protection (50)
Two Instantaneous overcurrent elements are provided in the 7SR105 Rho Motor Protection Relay.
50-1, 50-2
Each instantaneous element (50-n) has independent settings. 50-n Setting for pick-up current and
50-n Delay follower time delay. The instantaneous elements have transient free operation.
Operation of the instantaneous overcurrent elements can be inhibited from:
Inhibit 50-n A binary or virtual input.