Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-21
AP
2.3.12 Distance setup - filtering, load blinding and polarizing (Distance option only)
2.3.12.1 Digital filtering (Distance option only)
In most appli
cations, it is recommended that Standard filtering is used. This will ensure that
the relay offers fast, sub-cycle tripping. In certain rare cases, such as where lines are
immediately adjacent to High Voltage DC (HVDC) transmission, the current and voltage
inputs may be severely distorted under fault conditions. The resulting non-fundamental
harmonics could affect the reach point accuracy of the relay. To prevent the relay being
affected, a ‘Special’ set of filters are available.
Note: When using the long line filter the instantaneous operating time is
increased by about a quarter of a power frequency cycle.
2.3.12.2 CVTs with passive suppression of ferroresonance (Distance option only)
Set a Passive CVT
filter for any type 2 CVT (those with an anti-resonance design). An SIR
cutoff setting needs to be applied, above which the relay operation is deliberately slowed by
a quarter of a cycle. A typical setting is SIR = 30
, below which the relay will trip sub-cycle,
and if the infeed is weak the CVT filter adapts to slow the relay and prevent transient
overreach.
2.3.12.3 CVTs with active suppression of ferro
resonance (Distance option only)
Set an Active CVT filter for any type 1 CVT.
2.3.13 Load blinding (load avoidance) (Distance option only)
For
security, it is highly recommended that the blinder is Enabled, especially for lines above
150 km (90 miles), to prevent non harmonic low frequency transients causing load
encroachment problems, and for any networks where power swings might be experienced.
The impedance radius must be set lower than the worst-case loading, and this is often taken
as 120% overloading in one line, multiplied by two to account for increased loading during
outages or fault clearance in an adjacent parallel circuit. Then an additional allowance for
measuring tolerances results in a recommended setting typically 1/3
rd
(or even 1/4
th
in some
countries such as UK) of the rated full load current:
Z (Rated phase voltage Vn)/(I
FLC
x 3)
When the load is at the worst-case power factor, it should remain below the beta setting. So,
if we assume a typical worst-case 0.85 power factor, then:
Cos
-1
(0.85) plus 15
o
margin 47
o
And, to ensure that line faults are detected, (Line Angle -15
o
).
In practice, an angle half way between the worst-case leading load angle, and the protected
line impedance angle, is often used.
The MiCOM P54x has a facility to allow the load blinder to be bypassed any time the
measured voltage for the phase in question falls below an undervoltage V< setting. Under
such circumstances, the low voltage could not be explained by normal voltage excursion
tolerances on-load. A fault is definitely present on the phase in question, and it is acceptable
to override the blinder action and allow the distance zones to trip according to the entire
zone shape. The benefit is that the resistive coverage for faults near to the relay location
can be higher.
The undervoltage setting must be lower than the lowest phase-neutral voltage under heavy
load flow and depressed system voltage conditions. The typical maximum V< setting is 70%
Vn.
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