Basler BE1-CDS220, Instruction Manual

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BE1-CDS220 General Information 1-9
BE1-CDS220 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 1-4a. 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 Figures 1-4b and 1-4c.
BE1-CDS220 Solution
The input currents must be combined to mimic the way they are combined in the protected power
transformer so that the currents presented to the differential protection are made up of the same
components. Traditionally, this has been done by special connection of the CTs. Figure 1-5 shows the
transformer from Figure 1-4 with the CTs connected. The wye side CTs can be connected in delta such
that the differential protection is summing
I
A
with
I
a
-I
b
. The resulting two currents being seen by the
differential relay are now made up of the same components but exactly 180
°
out of phase with each
other. They will always sum to zero (after tap adjust for magnitude mismatch) under all conditions of
balance or unbalance except when there is a fault inside the zone of protection.
There are several drawbacks to the traditional solution:
•
The delta connection requires a dedicated set of CTs that generally cannot be used for other
purposes.
•
The delta connection is more difficult to test.
•
The delta connected CTs experience greater burden than wye connected CTs because the lead
burden must be multiplied by a factor of three in the CT performance calculation. See Appendix E
for more information on calculating CT performance.
The BE1-CDS220 relays support the traditional solution so that they may be used in retrofit/modernization
projects. However, in a numerical relay, it is possible to connect all of the CTs in wye as shown in Figure
1-6 so that the above mentioned drawbacks are not a consideration. The BE1-CDS relay can numerically
combine currents internally to accomplish a numerical delta if required for phase angle compensation.
The setup parameters for each of the current input circuits are described in Section 3, I/O Functions,
Power System Measurement Functions. The CT connection and the transformer connection are included
to allow the BE1-CDS220 relay to automatically determine the appropriate combination of currents to
present to the differential protection function.