7SR11 & 7SR12 Applications Guide
©2017 Siemens Protection Devices Limited Chapter 7 Page 43 of 48
5.2 Current Transformer Supervision
When a CT fails, the current levels seen by the protection become unbalanced, however this condition would also
occur during a system fault. Depending upon the relay model different methods are used to determine the
condition, depending upon the measured quantities available.
Current Transformer Supervision (60CTS – 7SR11)
Following a CT Failure, if one or two of the three phases falls below the CT supervision setting the element will
operate
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.
Current Transformer Supervision (60CTS – 7SR12)
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 shown in the accompanying table.
Table 5-1 Determination of VT 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.
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