Siemens Siprotec 7UM611 Manual page 93

Loop Selection
• The corresponding phase-earth loop is used for a 1-pole pickup.
• With a 2-pole pickup, the phase-phase loop with the corresponding phase-to-phase
• With a 3-pole pickup, the phase-phase loop with the highest current value is used
Table 2-5
This loop selection type ensures that the fault impedance of system faults is measured
correctly via the unit transformer. A measuring error occurs with a 1-pole system short-
circuit, since the zero phase-sequence system is not transmitted via the machine
transformer (switching group e.g. Yd5). The following table describes the fault model-
ing and the measuring errors.
Table 2-6
7UM61 Manual
C53000-G1176-C127-3
voltage is used for impedance calculation.
and with equal current amplitudes, the procedure described in the last row of the
following of table is applied.
Measuring Loop Selection
Pickup
1-pole L1
L2
L3
2-pole L1, L2
L2, L3
L3, L1
3-pole, L1,2*L2,L3
with different ampli- L2.2*L3,L1
tudes L3.2*L2,L3
3-pole, L1, L2, L3
with equal ampli-
tudes
Fault Modeling and Measuring Errors on the Generator Side on System Faults
System Faults Fault Model on
the Generator
Side
3–pole short 3–pole short
circuit circuit
2–pole short 3–pole short
circuit circuit
1–pole short 2–pole short
circuit circuit
2.14 Impedance Protection (ANSI 21)
Measuring Loop
Phase-earth
Phase-phase,
Calculation of U
I I
and I
I I
Phase-ground, se-
lection of loop with
the highest current
U and I
I (Imax) I (Imax)
Phase-earth, any,
maximum current
amount
Loop Selection Measuring
Errors
Phase-earth always correct
measurement
Phase-earth loop always correct
with highest measurement
current
Phase-phase Impedance mea-
loop sured too high by
the zero imped-
ance
L1–E
L2–E
L3–E
L1– L2
L2– L3
L3– L1
L2–E
L3–E
L1–E
L1=IL2=IL3 then IL1
IL1=IL2 > IL3 then IL1
IL2=IL3 > IL1 then IL2
IL3=IL1 > IL2 then IL1
93