Basler BE1-CDS240, Instruction Manual

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BE1-CDS240 Solution
There are two traditional solutions to this problem. The first is using delta phase-shift compensation on
the grounded side of the zone of protection to block the zero-sequence components from getting to the
differential protection. The second is removing the zero-sequence components from the currents using a
zero-sequence trap to prevent them from getting to the differential protection. The BE1-CDS240 provides
both solutions.
In the previous discussion on compensating for the phase shift on the primary system, the solution is to
combine the currents such that the currents seen by the differential are made up of the same components
at all terminals of the zone of protection. This solution has the added benefit of causing the currents to
match (after magnitude mismatch is eliminated by tap adjustment) under all situations of imbalance
including ground faults. That is, by using delta compensation on the wye side of the power transformer to
mimic the power transformer's delta connection, the zero-sequence currents are blocked from flowing to
the differential protection and circulate in the CT delta just as they circulate in the delta of the power
transformer on the delta side.
The BE1-CDS240 selects the proper phase shift compensation settings to not only provide the correct
phase shift but also to block zero-sequence currents as appropriate.
The second solution of inserting a zero-sequence trap is used in applications where there is a grounding
bank within the zone of protection that is not predicted by the transformer connection information
contained in the CT circuit setup parameters. In the example shown in Figure 1-7, the compensation logic
would require that the delta compensation be applied to the grounded wye winding. The grounding bank
on the delta side will cause the differential protection to operate for external ground faults. Figure 1-7
shows how this would be done with traditional differential relays.
C
B
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D2837-16
12-14-98
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Delta CT Connection
Wye/Delta (DAB)
Transformer with Ground
Bank on Delta Side
Wye CT Connection
GROUNDING
BANK
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1 2
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87T
87T
87T
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Ib-Io
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a
Figure 1-7. Traditional Zero-Sequence Trap for Application with Ground Banks
1-14 BE1-CDS240 General Information 9365200990 Rev F