Basler BE1-CDS220, Instruction Manual

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1-12 General Information BE1-CDS220
Problem 5: Zero Sequence Current Sources within the Zone Of Protection
General
A ground source (grounded transformer winding or zigzag grounding bank) within the zone of protection
can result in differential current being measured during ground imbalances. The most common example
of this is when the zone of protection is around a delta/grounded, wye transformer. If a ground fault or
neutral imbalance occurs on the power system external to the wye side zone of protection, the zero
sequence components of the current flow through the grounded neutral and are a component of the
current flowing out of the zone of protection. On the delta side, there is no path for the zero sequence
components to flow and they circulate inside the delta winding. The result is that this component of the
current is not seen entering the zone of protection on the delta side resulting in a differential current that
can cause the relay to operate.
BE1-CDS220 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-CDS220 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-CDS220 selects the proper phase shift compensation settings to not only provide the correct
phase shift, but to also 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.
This application is greatly simplified with the BE1-CDS220. The user can connect all CTs in wye and
specify that the delta transformer winding has a ground source. The BE1-CDS220 will apply delta
compensation to the wye winding to obtain phase shift and zero sequence compensation for that current
input. For the delta winding with ground source, it will numerically remove the zero sequence components
from that current input just as would traditionally be done with a zero sequence trap.
NOTE
The BE1-CDS220 relay uses transformer internal connection information to
determine the correct phase compensation to use. It is not possible to reliably
determine the phase compensation settings based simply upon phase angle shift
information because the phase shift from high to low side is dependent upon the
phase sequence of the power system phasors. That is, a power system with ABC
phase sequence will produce a different phase shift from high to low than a
power system with ACB phase sequence in the same transformer connection. By
specifying the transformer connections from the three-line diagram, the correct
phase compensation can be determined in all cases.