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Fault locator (FLOC)
The accuracy of the distance-to-fault calculation, using the non-compensated impe-
dance model, is influenced by the pre-fault load current. So, this method is only used if
the load compensated models do not function and the display indicates whether the non-
compensated model was used when calculating the distance to the fault.
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When calculating the distance to fault, pre-fault and fault phasors of currents and volt-
ages are filtered from disturbance data stored in digital sample buffers.
When the disturbance report function is triggered, the fault locator function starts to cal-
culate the frequency of the analog channel U1. If the calculation fails, a default frequen-
cy is read from the database to ensure further execution of the function.
Then the sample for the fault interception is looked for by checking the non-periodic
changes. The channel search order is U1, U2, U3, I1, I2, I3, I4, I5 and U5.
If no error sample is found, the trig sample is used as the start sample for the Fourier
estimation of the complex values of currents and voltages. The estimation uses samples
during one period before the trig sample. In this case the calculated values are used both
as pre-fault and fault values.
If an error sample is found the Fourier estimation of the pre-fault values starts 1.5 period
before the fault sample. The estimation uses samples during one period. The post-fault
values are calculated using the Recursive Least Squares (RLS) method. The calculation
starts a few samples after the fault sample and uses samples during 1/2 - 2 periods de-
pending on the shape of the signals.
The pre-fault time (tPre) should be at least 0.1 s to ensure enough samples for the esti-
mation of pre-fault trip values.
The phase selectors from the distance protection function provide the necessary infor-
mation for the selection of the loop to be used for the calculation. The following loops
are used for different types of faults:
•
for 2 phase faults: the loop between the faulted phases.
•
for 2 phase to earth faults: the loop between the faulted phases.
•
for phase to earth faults: the phase to earth loop.
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