3·87
ANSI05000173-2-en.vsd
ANSI05000173 V2 EN
Figure 41: Application of the 1Ph High impedance differential protection HZPDIF
(87) function on an autotransformer
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. If a lower tap of the CT is used, 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 Irated
(on low voltage tap):
1150 A
Current transformer ratio: 1200/5 A (Note: Must be the same at all locations)
CT Class: C800 (At max tap of 1200/5)
Secondary resistance: 0.3 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 75deg C
gives loop resistance 2 · 0.05 = 0.1 Ohms
Max fault current: The maximum through fault current. Use 15 · Irated for power
transformer.
Section 3 1MRK504116-UUS C
IED application
148
Application manual
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