Example 2 - Relay Replacement (Dyn1 Transformer) Using Existing Cts - Siemens 7SR242 Duobias Technical Manual

2.1.6 Example 2 – Relay Replacement (Dyn1 Transformer) Using Existing CTs
Figure 2-11 Relay Replacement
It is recommended to wire all line CTs in star when connecting to the 7SR24 relay. Where the 7SR24 is used to
replace an older biased differential relay the existing CTs will often be re-used. In this example it is recommended
that the existing LV line CTs connected in 'delta' are reconnected as 'star'.
Usually the interposing CTs associated with older schemes can be removed as both the vector group and current
magnitude compensation functions are carried out within the 7SR24. This requires ICT settings to be
programmed into the relay to correct for differences before the currents are applied to the differential algorithm.
When the relay is in balance the phase angle of the currents applied to each phase of the differential algorithm
will be in anti-phase.
Figure 2-12: AC Connections - Example 2
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