ABB Relion 670 Series Applications Manual page 278

Section 10
Unbalance protection
P is number of parallel units in the affected section = 9.
a
The per unit phase-to-neutral capacitance of the parallel group of capacitor units that include
blown fuses is calculated as,
=
C
s
C
g
IECEQUATION19347 V1 EN-US
Where, S
The per unit capacitance of the phase (both WYEs) that includes blown fuses is calculated as,
(
P C
=
a
C
p
IECEQUATION19348 V1 EN-US
Where,
P is the number of parallel units in a group = 9.
u
For a single WYE bank, P
Because of the element failure in the left section, per unit (line-to-ground voltage) unbalance
voltage in the neutral is calculated as,
=
V
NG
2
IECEQUATION19349 V1 EN-US
Internally fused capacitor units are subjected to the over-voltage across elements and fuses
within the unit as internal fuses blow and remove elements from the parallel group. The over-
voltage on the remaining elements and fuses are considered in addition to the over voltage on
units without the blown fuses. Therefore, the line-to-neutral voltage across the phase that
includes blown fuses is calculated as,
= +
V 1
LN
IECEQUATION19350 V1 EN-US
In an internally fused capacitor bank, the unbalance detection indicates the total number of
failed capacitor elements within a capacitor unit.
The trip level should be set so that:
• The number of operated fuses in the affected capacitor unit does not exceed the
maximum number recommended by the manufacturer.
• The voltage on the healthy capacitors does not exceed the contingency over-voltage
capability of the capacitor units (usually, 110% of the rated voltage). Note that the
element voltage in the unit with blown fuses may exceed 110% of the normal voltage.
Then, the per unit voltage on the capacitor units in the affected series group with blown fuses
is calculated as,
V
=
V
CU
IECEQUATION19351 V1 EN-US
If C = 0 p.u., then
g
=
V V
CU
IECEQUATION19352 V1 EN-US
272
´
S C
u g
( )
´ - +
S 1 1
u
number of series group of units in a phase = 1.
u
) ( )
´ + -
P P
s u a
P
u
= P and C
a u
3
-
1
+
C
p
V
NG
´
C
LN s
C
g
´
S
LN u
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= C .
p s
1MRK 511 401-UEN Rev. K
(Equation 161)
(Equation 162)
(Equation 163)
(Equation 164)
Bay control REC670
Application manual