ABB Relion 670 Series Applications Manual page 244

Section 10
Unbalance protection
L1
I
L1
L2
L3
Faulty section
C
L1l
Cx
Cx Cx
IEC19000200 V1 EN-US
V
Figure 113: Units arrangement in ungrounded double WYE connected capacitor bank
Assume that the voltage at the filter terminal is maintained at 1 p.u. The unbalance current is
calculated based on the element failure or operated fuses. The failure of the capacitor
elements are due to the short-circuit of an element inside the unit. The failure of the unit
occurs due to fuse operation (open-circuit). For further analysis, considering that the
capacitor failure has occurred only in the left section of L1 phase .
The capacitance values in the system are calculated by using the following equations:
P
= ´
e
C
u
S
e
IECEQUATION19024 V1 EN-US
Where, C
capacitance of the unit changes to:
=
C P
X
IECEQUATION19025 V1 EN-US
Where, f
If an element fails in the left section of phase L1 unit , the capacitance of the unit changes to:
238
I
L2
I
L3
Cu Cu Cu Cu
N
1
= ´
C 0.666 C
e e
is the capacitance of one unit. Whenever an element failure occurs in a unit, the
u
æ ö
C
´ ç
e ÷
-
e
è ø
S f
e e
is the numbers of elements failed
e
© Copyright 2017 Hitachi Power Grids. All rights reserved
Ce
Se
Cu Cu Cu Cu
I
U NB L1
1MRK 511 401-UEN Rev. K
Pe
C
Cu Cu Cu
Cu
N
2
Bay control REC670
Application manual
1 MΩ
Cu Cu Cu
IEC19000200-1-en.vsdx
(Equation 106)
(Equation 107)