Table of Contents
Advertisement
2.17.1.2 Determination of the Short–Circuit Impedance
Loop Selection
7UM62 Manual
C53000-G1176-C149-3
For calculation of the fault impedance, the currents and voltages of the faulty loop are
decisive. The phase selective fault detector determines the faulted loop and releases
the corresponding measurement values for impedance calculation (see Table 2-7).
− 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-phase loop with the highest current value is used
and with equal current amplitudes, the procedure described in the lowest line of
Table 2-7 is applied.
Table 2-7
Measuring Loop Selection
Pickup
1-pole
2-pole
3-pole, with
L1.2*L2,L3
different amplitudes
L2,2*L3,L1
L3,2*L1,L2
3-pole, with
same amplitude
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, as the zero phase-sequence system is not transmitted via the machine
transformer (switching group e.g. Yd5). Table 2-8 describes the fault modeling and the
measuring errors.
L1
Phase-earth
L2
L3
L1...L2
Phase-phase,
L2, L3
Calculation of
L3, L1
Phase-earth, selection of
loop with the highest current
U
and
I (Imax)
L1, L2, L3
Phase-earth, any, maximum
current amount)
Impedance Protection (ANSI 21)
Measuring Loop
L1–E
L2–E
L3–E
L1– L2
U
I
and
L2– L3
I I
I I
L3– L1
L2–E
L3–E
I
L1–E
I (Imax)
IIL1=IL2=IL3 => IL1
IL1=IL2 > IL3 => IL1
IL2=IL3 > IL1 => IL2
IL3=IL1 > IL2 => IL1
123
Advertisement
Table of Contents
