Section 2: Protection Functions; Overall Differential Protection (87) - Siemens 7SR242 Duobias Technical Manual

Section 2: Protection Functions

This section provides guidance on the application and recommended settings of the 7SR24 protection functions.

2.1 Overall Differential Protection (87)

This section covers the transformer overall differential protections – the biased differential and high-set differential
elements. Transformer design limitations necessitate that the protection CTs are located on the line side of the
HV and LV windings, therefore the zone of differential protection covers both transformer windings.
The application of differential protection to transformers is complicated by:
The current magnitude change introduced by the transformer HV/LV turns ratio. The current ratio may also
be variable due to the presence of an On-Load-Tap-Changer (OLTC).
The transformer connections which may introduce a phase change between the currents flowing into each
winding of the transformer.
Magnetising inrush current which flows in only one winding of the transformer when energised.
Generally the procedure to calculate relay settings is carried out in the following order:
Figure 2-1 Procedure for calculating Overall Differential Protection Settings
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