GE C70 Instruction Manual page 507

CHAPTER 8: APPLICATION OF SETTINGS
The following figure illustrates parallel capacitor banks, grounded, with taps and current unbalance protection. In this
case, three CT/VT modules are required.
Figure 8-11: Parallel banks, grounded, with taps and current unbalance protection
The normal functions of unbalance relaying are as follows:
• Trip the bank promptly if an unbalance indicates the possible presence of external arcing or a cascading fault within
the capacitor bank
• Provide early unbalance alarm signal(s) to indicate the operation of fuses (internally or externally fused capacitors) or
failure of capacitor elements (fuseless or unfused capacitors)
• Trip the bank for unbalances that are large enough to indicate that continuing operation can result in the following:
– Damage to remaining good capacitor units or elements from overvoltage
– Fuse malfunction
– Inappropriate filter operation (for capacitor banks that are a part of a harmonic filter)
– Other undesirable consequences
Note that there can be undetectable failure modes for each type of unbalance protection. These need to be backed up by
other types of protection.
8.3.2.2 Unbalance trip element considerations
Note the following points when using the unbalance trip element.
• Minimize the unbalance trip element time delay to reduce damage from an arcing fault within the bank structure and
prevent exposure of the remaining capacitor units to overvoltage conditions beyond their permissible limits. For a
single-phase or an open-phase condition, keep the time delay short enough to avoid damage to the current
transformer or voltage transformer and to the relay system.
• Have enough time delay on the unbalance trip element to avoid false operations due to inrush, system ground faults,
switching of nearby equipment, and non-simultaneous pole operation of the energizing switch. For most applications,
0.1 seconds is adequate. For unbalance relaying systems that operate on a system voltage unbalance (ground fault), a
delay slightly longer than the upstream protection fault clearing time is required to avoid tripping due to a system
fault. Longer delays increase the probability of catastrophic bank failure.
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
PROTECTION METHODS FOR CAPACITOR BANKS
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