Alstom MiCOM P846, Technical Manual

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P846/EN FD/D Firmware Design
(FD) 9-16
MiCOM P846
I
p
mZr
(1–m)Z
I
q
Zsq
Rf
I
f
Vp
Zsp
Eq Ep
P0124ENa
Figure 6: Two machine equivalent circuit
From this diagram:
Vp = m
Ι
pZr +
Ι
fRf …(equation 1)
The fault location, m, can be found if
Ι
f can be estimated allowing equation 1 to be solved.
1.5.2 Data acquisition and buffer processing
The fault locator stores the sampled data within a 12 cycle cyclic buffer at a resolution of 32
samples per cycle. When the fault recorder is triggered the data in the buffer is frozen such
that the buffer contains 6 cycles of pre-trigger data and 6 cycles of post-trigger data. Fault
calculation commences shortly after this trigger point.
The trigger for the fault locator is user selectable via the programmable scheme logic.
The fault locator can store data for up to four faults. This ensures that fault location can be
calculated for all shots on a typical multiple re-close sequence.
FD
1.5.3 Faulted phase selection
Selection of the faulted phase(s) is performed by comparing the magnitude of the pre fault
and post fault values of the three phase-to-phase currents. A single phase-to-ground fault
produces the same change on two of these signals and zero on the third. A phase-to-phase
or double phase-to-ground fault produces one signal that is larger than the other two. A
three-phase fault produces the same change on all 3 currents.
Current changes are considered to be the same if they are within 20% of each other. Phase
selection and fault location calculation can only be made if the current change exceeds
5%
Ι
n.
1.5.4 The fault location calculation
This works by:
a) First obtaining the vectors
b) Selecting the faulted phase(s)
c) Estimating the phase of the fault current,
Ι
f, for the faulted phase(s)
d) Solving equation 1 for the fault location m at the instant of time where
Ι
f = 0