The transformer has a phase shift of zero. To prevent undesired tripping for external faults a zero sequence
shunt is required, this is implemented by selecting star/delta interposing CT settings. The Interposing CT
Connection setting on all sets of current inputs must be set to the same Yd setting e.g. all Yd1, -30° or all
Yd11, 30°.
Transformer with Primary Connections Crossed on Both Windings
Yd11 Transformer Connected as Yd9 (90°)
The phase-shift between the W1 and W2 primary systems may necessitate that primary connections to each
winding of the transformer have to be crossed. Figure 5-62 shows a typical arrangement where a Yd11 trans-
former is arranged to give a primary system phase-shift of 90° by crossing of its main connections. There are 2
optional methods that can be used to for setting the 87T function element.
•
Solution 1
Figure 5-62 shows W1 and W2 CT secondary wiring crossed over to replicating the crossovers on the
transformer primary connections:
[dw_7SR5_function87Tsettingsexample10, 1, en_US]
Figure 5-62
AC Connections: Yd9, 90° Transformer – Non-standard Secondary Connections
An advantage of the above is that the relay can be set to correspond to the vector group shown on the
main transformer rating plate i.e. Yd11, +30° simplifying installation. This approach is also applicable
where the transformer is used to reverse the system phase sequence.
NOTE
‘Non-standard’ secondary wiring connections are used. Relay instruments will indicate ’transformer’ quanti-
ties rather than system quantities.
Protection and Automation Functions
5.22 87T Transformer Differential Protection
Reyrolle 7SR5, Transformer Protection Device, Device Manual 325
C53000-G7040-C015-1, Edition 11.2019
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