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2.3 PROTECTION SETTINGS
2
Figure 2–3: POWER SYSTEM ONE LINE DIAGRAM WITH SERIES CAPACITORS
For the fault at F3, the voltage at the relay is:
(
)
V
I Z
X
=
–
line
t
where
if gaps are used
X
X
=
t
C 3
= parallel combination of
X
t
If the Zone 1 function is set with a reach equal to Z
⋅
×
I Z
V
I Z
–
=
If gap flashing does not occur, or if there is not significant conduction in the MOV, then the operate signal,
×
equal to
I X
, the voltage drop across the capacitor. If the gaps flash, the operate signal will be very small, or if the
C
MOVs conduct, the voltage will be less than or equal to the MOV protective level. The level detector in the operate circuit of
the Zone 1 functions is set greater than the gap flashing level or the MOV protective level, so that the functions cannot
operate for faults beyond the zone of protection even in the presence of low frequency transients.
Now consider a fault at F2 directly in front of the Zone 1 functions. For this fault, V = 0, and the operate signal,
will therefore equal I · Z . If the source behind the function is very strong and the line is very long ( Z is big), the operate sig-
nal will be very large and fast operation will result.
To simplify the explanation, the above discussion was predicated on C3 being the only capacitor located between the relay
potential source and the fault. In determining the level detector setting, it is necessary to consider external faults beyond
any capacitor located at the remote bus in adjacent/parallel line sections. All of the capacitors that appear between the
source of the relay potential and those external faults must then be taken into consideration. Consider the forward fault at
location F4 in Figure 2–3: POWER SYSTEM ONE LINE DIAGRAM WITH SERIES CAPACITORS above.
1.
Capacitors C3 and C4 need only be considered if line side potential is used.
2.
If bus side potential is used then capacitors C2, C3, and C4 must be taken into consideration.
Capacitor C4 should be treated as follows when determining the level detector setting:
1.
With gaps: If the minimum fault current for a fault at F4 (including infeed) is above the gap flashing level, then exclude
C4 from the calculation. If the gaps do not flash then use the gap flashing level in making the calculations.
2.
With MOVs: If the MOVs are protected with gaps, and if the minimum fault current (including infeed) for a fault at F4 is
above the gap flashing level, then exclude C4 from the calculation. If there are no gaps around the MOVs or if the gaps
do not flash, then use the protective level of the MOVs when making calculations.
Note that only one capacitor is shown in location C4 where in actuality there may be more than one line containing capaci-
tors that emanates from the remote bus. Each capacitor must be evaluated individually in determining whether it should be
used in calculating the level detector setting. Use the highest protective/gap flashing level so obtained in calculating the
setting. After deciding which capacitors are involved, the protective levels of those capacitors are defined as follows:
n
1
∑
PL
=
----- -
PL
t
n
95
1
where: n = number of capacitors involved
= the gap flashing level in peak volts if gaps are used to protect the capacitors, or
PL
n
PL
= the protective level in peak volts if MOVs are used to protect the capacitors.
n
Once
has been determined, Z1PLEVDET can be calculated as follows:
PL
t
ZIPLEVDET =
0.15
+
2-10
F1
F2
Xc1
Xc2
C2
C1
Line
Bus
Side
Side
Infeed
ALPS
and the MOV impedance when MOVs are used
X
C 3
line
×
(
)
×
I
Z
X
I X
–
–
=
line
line
t
t
2
1.3 PL
t
ALPS Advanced Line Protection System
Xc3
Zline
C3
gap
MOV
, then the operating signal for the fault at F3 is:
2 CALCULATION OF SETTINGS
F3
F5
F4
Xc4
C4
Infeed
⋅
I Z
–
⋅
I Z
GE Power Management
, is
V
V
,
–
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