ABB RELION REX640, Техническое руководство, Страница: 1206 / 1960

Figure 688: Definition of a physical fault point resistance in different fault loops
Steady-state asymmetry and load compensation
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 use the delta quantities which mitigate the effects of the steady-state
asymmetry.
The 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 high or when the fault distance estimate for the 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.
∆ = +
x x x
fault pre fault
-
(Equation 305)
x
fault
Signal value during fault
x
pre-fault
Signal value during healthy state just before fault
Result quality indicator
The quality of the estimated fault distance is judged and reported in recorded
data as the Flt Dist quality together with the fault distance estimate. The Flt Dist
quality is a bit vector indicating detected sources of inaccuracy in the fault distance
estimate. If Flt Dist quality equals 1, the result is not affected by error sources. This
results in good quality for the fault distance estimate. If factors affecting negatively
Protection related functions 1MRS759142 F
1206 REX640
Technical Manual