ABB RELION Series Technical Manual page 497

1MRS758755 A
REC615 and RER615
Technical Manual
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Figure 254: 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.
x x x
∆ = +
fault pre fault -
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x Corresponds to the signal value during fault
fault
x Corresponds to the signal value during healthy state just before fault
pre-fault
Protection related functions
Section 5
(Equation 58)
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