7SR210 & 7SR220 Description of Operation
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5.4 CT Supervision (60CTS & 60CTS-I)
The relay has two methods of detecting a CT failure, depending on the relay variant.
5.4.1 (60CTS-I) – 7SR21 & 7SR22
The current from each of the Phase Current Transformers is monitored. If one or two of the three input currents
falls below the CT supervision current setting CTS-I for more than 60CTS-I Delay then a CT failure output
(60CTS-I Operated) is given. If all three input currents fall below the setting, CT failure is not raised.
Operation of the CT supervision elements can be inhibited from: -
Inhibit 60CTS-I A binary input, virtual input or remote data Comms.
60CTS-I Delay
IL1
IL2
IL3
60CTS I
<
Any 2 phases
but not all 3
&
Inhibit 60CTS
60CTS-I Element
Enabled
Disabled
60CTS-I Operated
Inhibit 60CTS
60CTS-I Pick-UP
60CTS-I Operated
&
Figure 5.4.1-1 Logic Diagram: CT Supervision Function (60CTS-I) – 7SR21 & 7SR22
5.4.2 (60CTS) – 7SR22
Normally the presence of negative phase sequence (NPS) current in a power system is accompanied by NPS
voltage. The presence of NPS current without NPS voltage is used to indicate a current transformer failure.
The element has a setting for NPS current level 60CTS Inps and a setting for NPS voltage level 60CTS Vnps. If
the negative sequence current exceeds its setting while the negative sequence voltage is below its setting for
more than 60CTS Delay then a CT failure output (60CTS Operated) is given.
Operation of the CT supervision elements can be inhibited from: -
Inhibit 60CTS A binary input, virtual input or remote data Comms.
&
60CTS Delay
VL1
VL2
VL3
NPS
Filter
V2
60CTS Vnps
<
IL1
IL2
IL3
NPS
Filter
I2
60CTS Inps
>
&
Inhibit 60CTS
60CTS Element
Enabled
Disabled
&
60CTS Operated
Inhibit 60CTS
60CTS Pick-Up
60CTS Operated
Figure 5.4.2-1 Logic Diagram: CT Supervision Function (60CTS) – 7SR22
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