Earth-fault distance calculation in case of compensated (resonant earthed)
networks where an earth-fault current raising resistor is used
In case of compensated (resonant earthed) networks where an earth-fault current
raising resistor is used, the setting
Flt Loc Det mode
is equal to "Comp, switched R".
The algorithm for calculation of earth-fault distance requires a measurable change
in the measured phase currents and phase-to-earth voltages during an earth fault.
Such a change can be accomplished by switching on the resistor of the PAW winding
(Power Auxiliary Winding) of the compensation coil during the fault.
The algorithm requires three independent time instances, which are assumed to be
known by the algorithm:
• t
0
= time instance before the fault
• t
1
= time instance before the change during the earth fault
• t
2
= time instance after the change during the earth fault
The effect of loads is considered by applying “Load modelling” algorithm.
The required settings of the algorithm are:
• R1 = Positive-sequence resistance of the protected feeder [ohm, prim]
• X1 = Positive-sequence reactance of the protected feeder [ohm, prim]
• R0 = Zero-sequence resistance of the protected feeder [ohm, prim]
• X0 = Zero-sequence reactance of the protected feeder [ohm, prim]
• s = Equivalent load distance [pu]
Determining timings of changes: determination of earth fault occurrence and time
instance t0
Timing t
0
is needed to store the current and voltage phasors right before the
occurrence of the earth fault. Timing t
0
is common to both triggering types
(
Calculation Trg mode
either “Internal” or “External”). Pre-fault (indicating healthy
network) phasors are continuously updated and stored at intervals of 300 ms until
the earth fault is indicated. When the earth fault is detected (internal detection), the
pre-fault phasor is stored at least 300 ms before the detection of the earth fault.
This time instance before the earth fault is noted as t
0
(see
Figure 685
).
Determining timings of changes: determination of changes during an earth fault
due to parallel resistor switching, time instances t1 and t2
• Time instance t
1
equals the time during earth fault, right before the switching of
the parallel resistor of the coil (see
Figure 685
)
• Time instance t
2
equals the time during earth fault, just after the switching of the
parallel resistor of the coil (see
Figure 685
)
In case of external triggering, time instances t
1
and t
2
are determined using input
TRIGG_T and delay setting
Time delay
. TRIGG_T input should be activated when the
parallel resistor is switched ON. Time instance t
1
is taken at least one fundamental
cycle before TRIGG_T rising edge. Time instance t
2
is taken according to the set
Time delay
after the rising edge of TRIGG_T signal. Setting
Time delay
should be
few hundreds of ms and it is needed due to oscillations in currents and voltages
after the switching of the parallel resistor.
TRIGG_T
activation must be given after the initial earth-fault ignition
transients. Only one activation of TRIGG_T is allowed during an earth
fault. However, further activations are internally disabled.
1MRS759142 F Protection related functions
REX640
Technical Manual
1201
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