ABB Relion 650 Series Applications Manual page 72

Section 6
Differential protection
The protection is suitable for all voltage levels, and for several type of substation lay-outs, like
single busbar, single busbar with sectionalizer, double busbar, ring substation with up to three
zones.
3·87
ANSI05000774 V3 EN-US
Figure 26: Application of the high impedance differential function on busbar
Setting example
Either class TPS or PX CTs, according to IEC standards, are usually selected for the high impedance
differential protection. We suppose to use class PX CTs in the following example.
Basic data:
Current
transformer
ratio:
CT Class:
Secondary
resistance:
Table continues on next page
66
It is strongly recommended to use the highest tap of the CT whenever high
impedance protection is used. This helps in utilizing maximum CT capability,
minimize the current, thereby reducing the stability voltage limit. Another factor is
that during internal faults, the voltage developed across the selected tap is limited
by the non-linear resistor but in the unused taps, owing to auto-transformer
action, voltages much higher than design limits might be induced.
2000/5 A (Note: Must be the same at all locations)
C400 (At max tap of 2000/5)
0.5 Ohms (At 2000/5 Tap)
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1MRK 505 287-UUS B
ANSI05000774-3-en.vsd
Busbar protection REB650
Application manual