ABB Relion 670 Series Applications Manual page 269

1MRK 511 401-UEN Rev. K
10.4 Voltage unbalance protection of shunt capacitor bank,
SCUVPTOV
10.4.1 Identification
Function description
Voltage unbalance protection of the shunt
capacitor bank
10.4.2 Application
The unbalance protection function provides protection against faults in the shunt capacitor
bank (SCB). The decision of protection scheme depends on the following:
• Fusing method
• Size of the capacitor bank
• Method of grounding and the available installation of CT or VT
The SCUVPTOV function uses the neutral voltage measurement of an ungrounded single or
double WYE connected SCB. Generally, the ungrounded capacitor bank neutral is equipped
with a VT at full line voltage rating, because the capacitor bank neutral is momentarily at phase
potential when the capacitor bank is switched or when one capacitor unit fails in a bank
configured with a single group of units.
The unbalance present in the SCB is because of:
• The loss of individual capacitor units or elements
• The inherent system and the capacitor bank unbalances
The natural unbalance which exists on all the capacitor bank installations is because of the
system voltage unbalance and the capacitor manufacturing tolerances. In addition to these
natural unbalances, there are secondary unbalance errors, which are introduced by the
measurement device tolerances and variation, and by the relative changes in the capacitance
due to the difference in the capacitor unit temperatures in the capacitor bank.
The use of fuses for protecting the capacitor units and its location influences the design of the
capacitor bank unbalance protection. Removal of a failed element or unit by its fuse operation
increases the voltage across the remaining elements or units, causing an unbalance in the
capacitor bank. The unbalance protection detects the changes associated with the failure of a
capacitor element or unit.
The SCUVPTOV function considers both the bus voltage and the capacitor bank neutral
voltage, which is measured between the capacitor bank neutral and the ground of the system.
It uses the system bus voltages to remove the system unbalance from the measured neutral
voltage. If any internal fault occurs in the capacitor elements, then there is an unbalance in the
capacitor bank. This unbalance voltage appears across the neutral. The protection decision is
based on the magnitude of the neutral voltage.
Bay control REC670
Application manual
© Copyright 2017 Hitachi Power Grids. All rights reserved
IEC 61850 IEC 60617
identification identification
SCUVPTOV Uub>
Section 10
Unbalance protection
GUID-C41B59C8-9CA0-4B5F-9F90-354804BC1449 v1
GUID-62C0BF41-EB2D-41E1-A2C6-799C46C1BFD2 v1
ANSI/IEEE C37.2
device number
60 V
GUID-1768993C-62A5-4388-8092-67D3F01C2D01 v1
263