ABB RELION 670 Series Applications Manual page 141

1MRK 505 337-UUS A
Main CTs
ANSI06000127 V2 EN-US
Figure 73: Principle CT connections for the complete station
This summation type bus differential protection still has the same main CT requirements as
outlined in section
• 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
e.m.f. requirements"
However, due to the summation principle this type of busbar protection scheme has the following
limitations:
Application manual
LA LB
IA1 IB1
52 52
A B C N A
IA2 IB2
CT1
"Rated equivalent secondary e.m.f.
for detailed CT requirements regarding main CT knee-point voltage)
LX
IX1
52
B C N A B
IX2
CT2 . . . CT24
IED
with 1A
CT inputs
requirements". Some of these are:
Differential protection
A-bus
C N
Summation CTs
ANSI06000127_2_en.vsd
"Rated equivalent secondary
Section 6
135