Current Transformer Supervision (60Cts) - Siemens 7SR21 Manual

5.2

Current Transformer Supervision (60CTS)

When a CT fails, the current levels seen by the protection become unbalanced. A large level of NPS current is
therefore detected - around 0.3 x In for one or two CT failures. However this condition would also occur for a
system fault. To differentiate between the two conditions, the element uses NPS voltage to restrain the CTS
algorithm as show in the accompanying table.
NPS Current
> Setting
> Setting
Table 5-2 Determination of CT Failure (1 or 2 Phases)
Following a CT Failure, there should be little or no NPS voltage. Perhaps 0.1 x Vn as a maximum.
Operation is subject to a time delay to prevent operation for transitory effects.
A 3-phase CT failure is considered so unlikely (these being independent units) that this condition is not tested for.
©2010 Siemens Protection Devices Limited
7SR21 & 7SR22 Argus Applications Guide
NPS Voltage
> Setting
< Setting
Decision
System Fault
CT Failure
Chapter 7 Page 35 of 39