ABB Relion 670 series, Руководство по применению

Страница: 108 / 760

3·Id
IEC05000165-2-en.vsd
IEC05000165 V2 EN-US
Figure 36: The protection scheme utilizing the high impedance function for the T-feeder
Normally this scheme is set to achieve a sensitivity of around 20 percent of the used CT
primary rating so that a low ohmic value can be used for the series resistor.
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 secondary fault 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 induced may be much higher than
design limits.
Setting example
Basic data:
Current transformer ratio: 2000/1 A
CT Class: 20 VA 5P20
Secondary resistance: 6.2 ohms
Cable loop resistance:
<100 m 2.5 mm
2
(one way) gives 2
ˣ
0.8 ohm at 75° C
Max fault current: Equal to switchgear rated fault current 40 kA
Section 6 1MRK 505 343-UEN B
Differential protection
102
Application manual