Current Transformer Requirements For Differential Protection - ABB RELION 620 Series Technical Manual

Section 4
Protection functions
4.6.4.6
478
P S
1 1
P
S
2
2
Main Bus
Reserve Bus
Common
Stabilizing
Main bus
resistor R
protection
s
3x required for three
GUID-247721CC-90C6-448F-9C30-F142714361D5 V1 EN
Figure 226: Differential protection on double busbar scheme with bus coupler
scheme (Single phase representation)

Current transformer requirements for differential protection

The sensitivity and the reliability of the protection depends on the characteristics of the
current transformers. The CTs must have an identical transformation ratio. It is
recommended that all the CTs have an identical constructions, that is, they have an equal
burden and characteristics and are of the same type, preferably from the same
manufacturing batch. If the CT characteristics and burden values are not equal,
calculations for each branch in the scheme should be performed separately and the worst-
case results should be used. In
branches are not equal, and hence, the maximum burden equal to 3.2 Ω should be used for
calculating the stabilized voltage.
P S
1 1
P
S
2
2
Voltage dependent
resistor R
u
Stabilizing
resistor R
phases
Figure 227, the CT winding resistance and the burden of the
1MAC504801-IB E
Main Bus
Reserve Bus
S P S
P
2 2 2
2
S P P S
1 1 1 1
Voltage dependent
resistor R
u
Reserve bus
s
protection
3x required for three
phases
620 series ANSI
Technical Manual