Functions
2.19 Impedance Protection (ANSI 21)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
153
Loop Selection
Table 2-9 Measuring Loop Selection
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 modeling and the measuring errors.
Table 2-10 Fault Modeling and Measuring Errors on the Generator Side on System Faults
- 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 voltage is used for
impedance calculation.
- With a 3-pole pickup, the phase-earth loop with the highest current value is used and with equal current
amplitudes, the procedure described in the last row of the following table is applied.
Pickup Measuring Loop
1-pole L1
L2
L3
Phase-earth L1-E
L2-E
L3-E
2-pole L1, L2
L2, L3
L3, L1
Phase-phase,
Calculation of U
LL
and I
LL
L1-L2
L2-L3
L3-L1
3-pole,
for dissimilar
amplitudes
L1,2*L2,L3
L2,2*L3,L1
L3,2*L1,L2
Phase-earth, selection of loop with the highest
current
U
L (Imax)
and I
L (Imax)
L2-E
L3-E
L1-E
3-pole,
for identical
amplitudes
L1, L2, L3 Phase-earth (any, maximum current amount) IL1=IL2=IL3 then IL1
IL1=IL2 > IL3 then IL1
IL2=IL3 > IL1 then IL2
IL3=IL1 > IL2 then IL1
System Faults Fault Model on the
Generator Side
Loop Selection Measuring Errors
3-pole short-
circuit
3-pole short-circuit Phase-earth Always correct measurement
2-pole short-
circuit
3-pole short-circuit Phase-earth, selection
of loop with the highest
current
Always correct measurement
1-pole short-
circuit
2-pole short-circuit Phase-phase loop Impedance measured too high by the zero
impedance
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