Alstom MiCOM P546 - 3.8.1 Basic theory for ground faults; 3.8.2 Data acquisition and buffer processing; 3.8.3 Faulted phase selection; 3.8.4 The fault location calculation

Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546 (OP) 5-133
OP
3.8.1 Basic theory for ground faults
Figure 84 shows a two-machine equivalent circuit of a faulted po
wer system.
Ip
mZr
(1m)Z
Iq
Zsq
Rf
If
Vp
Zsp
Eq
Ep
P0124ENa
Figure 84 Two-machine equivalent circuit
From this diagram:
Vp = mpZr + fRf …(equation 1)
The fault location, m, can be found if f can be estimated allowing equation 1 to be solved.
3.8.2 Data acquisition and buffer processing
The fa
ult locator stores the sampled data within a 12 cycle cyclic buffer at a resolution of 48
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 recorder 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 reclose sequence.
3.8.3 Faulted phase selection
Phase
selection is derived from the current differential protection or the superimposed
current phase selector.
Phase selection and fault location calculations can only be made if the current change
exceeds 5% In.
3.8.4 The fault location calculation
The fault loca
tion calculation works by:
 First obtaining the vectors
 Selecting the faulted phase(s)
 Estimating the phase of the fault current f for the faulted phase(s)
 Solving equation 1 for the fault location m at the instant of time where f = 0