ABB COM600 series User's & Technical Manual page 23

1MRS758734 COM600 series 5.1
Substation Analytics Technical Manual
Explanation of variations.png
Figure 4.3-20 Connection of a physical fault resistance is connected in different fault loops
The estimated fault point resistance (recorded data RF) for fault loops "AB Fault", "BC
Fault" or "CA Fault" is half of the total physical fault point resistance between the phases.
In case of earth faults, the estimated fault point resistance includes the arc and earthing
resistances. In case of a three-phase fault, the estimated fault point resistance equals the
arc resistance per phase.
Steady-state asymmetry and Load compensation
In reality, power systems are never totally symmetrical. The asymmetry produces steady-
state quantities in the form of zero and negative sequence voltages and currents. If not
compensated, these are the error sources for the fault distance calculation, especially in
the case of earth faults. In SCEFRFLO, all the fault distance calculation algorithms
utilize the delta (Δ) quantities which eliminate the steady-state asymmetry. The delta
quantities are also used for load compensation for short circuit faults (fault loops 12, 23,
31 and 123). The delta quantities describe the change in the measured quantities due to
the fault:
Δx = xfault - xpre-fault.
The Pre fault time setting is used for generating the delta quantities. The pre-fault values
are captured at least Pre fault time earlier than the actual fault moment occurs.
Load current is one of the main error sources for fault distance calculation. Its influence
increases with higher fault resistance values. SCEFRFLO employs independent load
compensation methods for each fault type to achieve optimal performance. For earth
faults (fault loops either "AG Fault", "BG Fault" or "CG Fault"), the load compensation
is done automatically inside the fault distance algorithm. For short circuit faults (phase-
to-phase fault impedance loops "AB Fault", "BC Fault", "CA Fault" and also loop "ABC
Fault"), current delta quantities are used for load compensation.
Triggering FaultLocator_sc_A
The fault distance estimate is obtained when FaultLocator_sc_A is triggered.
FaultLocator_sc_A requires a minimum of two signal fundamental cycles of measuring
time after a fault occurrence by omitting the signal initial transient and providing time
for filtering the fault distance estimate with the principle that the more there is time for
measuring, the better the fault distance estimate. Figure 4.3-21 illustrates the behavior
of fault distance estimate of FaultLocator_sc_A as a function of time.
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