Setting example
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.
Basic data:
Transformer rated current on HV winding: 250 A
Current transformer ratio: 600-300/5A A (Note: Must be the same at all locations)
CT Class: C200
Secondary resistance: 0.66 ohms
Cable loop resistance: <50 ft AWG10 (one way between the junction point
and the farthest CT) to be limited to approx. 0.05
Ohms at 75° C gives loop resistance 2 ˣ 0.05 = 0.1
Ohms
Max fault current: The maximum through fault current is limited by the
transformer reactance, use 15 ˣ rated current of the
transformer
Calculation:
( )
250
15 0.1 0.1 6.25
600 / 5
VR V> × × + =
EQUATION1775-ANSI V1 EN-US (Equation 26)
Select a setting of
TripPickup
=40 V.
The current transformerknee point voltage can roughly be calculated from the rated values.
Considering knee point voltage to be about 70% of the accuracy limit voltage.
(0.1 2) 100 210VkneeANSI V> + × =
EQUATION1770-ANSI V1 EN-US (Equation 27)
that is, greater than 2 ˣ
TripPickup
Check from the table of selected resistances the required series stabilizing resistor
value to use. Since this application requires high sensitivity, select R series= 100 ohm
which gives a current of 200 mA.
1MRK 502 071-UUS A Section 7
Differential protection
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062 175
Application manual
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