ABB Relion 670 Series Applications Manual page 273

1MRK 511 401-UEN Rev. K
The following effects can be seen under this condition.
• The reactance of the faulty parallel group increases.
• The voltage across all the elements or units in the faulty parallel group increases.
• The total capacitor bank reactance increases.
• The total capacitor bank current decreases.
• The voltage across the healthy elements or units decreases.
• The decreased voltage is evenly distributed among the healthy elements or units.
The capacitor bank manufacturer determines the series or parallel arrangement based on the
trade-off between voltage rating, VAr requirements, and overall economics. The problem is to
quantify the voltage stress seen by the remaining capacitors in the capacitor bank. As the
remaining capacitors are exposed to different electrical stresses, the highest steady-state
capacitor voltage must be calculated following the fuse operation.
Two setting examples are provided in this section with the following assumptions:
• Voltage at the SCB connected bus terminal is maintained at 1 pu.
• Unbalance neutral voltage is calculated based on the element failure.
• Capacitor elements failure is considered as an short-circuit in case of an externally fused
configuration.
• Capacitor elements failure is considered as an open-circuit in case of an internally fused
configuration.
• Capacitor failure is considered only in phase L1 and left section of the double WYE
connection.
In case of an externally fused capacitor bank, the successive element failure in each unit is
considered in the series elements, while in an internally fused capacitor bank, the successive
element failure in each unit is considered in the parallel elements for calculation of the
unbalance neutral voltage. All the calculations with effect of capacitor element or unit failure
are done based on the IEEE standard C37.99-2000.
10.4.4.1 Example of an externally fused ungrounded double WYE configuration
Single line diagrams for the externally fused ungrounded double WYE capacitor bank with the
relevant application data are shown in
IEC19000928 V1 EN-US
Figure 128: Single line diagram for an ungrounded double WYE capacitor bank
Bay control REC670
Application manual
Figure 128
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and Figure 129.
Preprocessing
Function Block Unbalance Protection
SMAI
GRP1L x G1A I3P
SMAI
G2A I3P
GRP2N
Where x = 1 , 2, an d 3
Section 10
Unbalance protection
GUID-7BC160FC-BC6B-4CD9-870D-CFD3E8B42FCD v1
Neutral Voltage
SCUVPTOV
U3P
U3NEUT
IED
IEC19000928-1-en.vsdx
267