ABB RELION REX640 Technical Manual page 1114

Section 5
Protection related functions
1108
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Figure 601: 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 large 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 -
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x Signal value during fault
fault
x Signal value during healthy state just before fault
pre-fault
1MRS759142 C
(Equation 271)
REX640
Technical Manual