ABB RELION 620 Series Technical Manual page 852

Section 5
Protection related functions
846
Estimation of fault resistance in different fault loops
The fault point resistance value provided by the impedance calculation is available in
recorded data Flt point resistance and it depends on the applied fault loop as shown in
Figure 438. In case of earth faults, the estimated fault point resistance includes the
total fault point resistance between the faulted phase and earth, for example, the arc
and earthing resistances. In case of phase-to-phase faults, the estimated fault point
resistance is half of the total fault point resistance between the phases. In case of a
three-phase fault, the estimated fault point resistance equals the total fault point
resistance as per phase value, for example, the arc resistance per phase.
GUID-8C1D00A2-1DFC-4904-B2D5-1CF7A88E9C4D V2 EN
Figure 438: Definition of a physical fault point resistance in different fault loops
Steady-state asymmetry and load compensation
In reality, power systems are never perfectly symmetrical. The asymmetry produces
steady-state quantities in the form of zero-sequence and negative-sequence voltages
and currents. If not compensated, these are error sources for fault distance calculation
especially in case of earth faults. All earth-fault distance calculation algorithms of
SCEFRFLO utilize the delta-quantities which mitigate the effects of the steady-state
asymmetry.
Load current is another error source 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. The
purpose of load compensation is to improve the accuracy of the fault distance
calculation models by estimating the actual fault current in the fault location. Delta-
quantities are used for this to mitigate the effect of load current on fault distance
estimation. For earth faults, the load compensation is done automatically inside the
fault distance calculation algorithm. For short circuit faults, load compensation is
enabled with setting Load Com PP loops. The default value is "Enabled". The
parameter should be set to "Disabled" only if the ratio between the expected fault
current and load current is large or when the fault distance estimate for short circuit
fault is required for each shot of an autoreclosing sequence.
The delta-quantity describes the change in measured signal due to the fault.
1MRS757644 F
620 series
Technical Manual