arrangements up to two differential zones as shown in Section Two-zone busbar arrangments,
for three one-phase IEDs.
As an example, the necessary equipment for the summation type, busbar differential
protection for a single busbar station with up to 24 bays, is shown in Figure 82.
IED
with 1A
CT inputs
IB2 IX2IA2
L1 L2 L3 N
IA1 IB1 IX1
LA LB LX
A-bus
Summation CTs
Main CTs
CT1 CT2 . . . CT24
IEC06000127_2_en.vsd
L1 L2 L3 N L1 L2 L3 N
IEC06000127 V2 EN-US
Figure 82: Principle CT connections for the complete station
This summation type bus differential protection still has the same main CT requirements as
outlined in section "Rated equivalent secondary e.m.f. requirements". Some of these are:
• main CT ratio differences can be tolerated up to 10:1 (for example, 3000/5A CT can be
balanced against CT"s as low as 300/5)
• different main CT ratios are compensated numerically by a parameter setting
• main CT shall not saturate quicker than 2 ms ( refer to section
"Rated equivalent
secondary e.m.f. requirements" for detailed CT requirements regarding main CT knee-
point voltage)
However, due to the summation principle this type of busbar protection scheme has the
following limitations:
1MRK 505 370-UEN D Section 6
Differential protection
Busbar protection REB670 135
Application manual
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