Basler Electric BE1-CDS240 General Information - Manual (Page 12)

The tap adjustment factors can be used to eliminate the mismatch when the tap changer is at neutral.
But, when the tap changer is off of neutral, differential current will be measured.
BE1-CDS240 Solution
The percentage restraint characteristic of the differential function mentioned previously allows the relay to
accommodate a prescribed amount of mismatch. The differential current must exceed a percentage of the
through current as described by the slope characteristic setting of the differential protection function. Also,
the relay has four setting groups to allow the tap settings and other parameters to be changed
dynamically based upon operating conditions.
Problem 4: Phase Angle Shift
General
When differential protection is used in a transformer application, the transformer often introduces a phase
shift between the various current inputs to the zone of protection. An example is a transformer with a
delta- connected high side and a wye-connected low side as shown in Figure 1-4, Illustration A. The
currents in the phases connected to the delta side of the transformer are each made up of the
combination of the current flowing in two legs of the delta winding. On the other hand, the currents in the
phases connected to the wye side of the transformer are made up of the current in only one leg of the
wye winding. It can be seen that the primary currents flowing into the zone of protection when tap is
adjusted for magnitude mismatch still do not sum to zero as shown in Figure 1-4, Illustrations B and C.
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