GE C70 Instruction Manual page 447

9 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 9–11: PARALLEL BANKS, GROUNDED, WITH TAPS AND CURRENT UNBALANCE PROTECTION
The normal functions of unbalance relaying are listed below.
• 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 may 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 may be undetectable failure modes for each type of unbalance protection. These will need to be backed up
by other types of protection.
b) UNBALANCE TRIP ELEMENT CONSIDERATIONS
The following points should be noted when using the unbalance trip element.
• The unbalance trip element time delay should be minimized 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 lim-
its. For a single-phase or an open-phase condition, the time delay should also be short enough to avoid damage to the
current transformer or voltage transformer and to the relay system.
• The unbalance trip element should have enough time delay 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 applica-
tions, 0.1 seconds should be 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.
GE Multilin
9.3 PROTECTION METHODS FOR CAPACITOR BANKS
C70 Capacitor Bank Protection and Control System
C70 CAPACITOR BANK RELAY
50P 51P
60N
60P
(C)
59B
60P
87V (B)
60P
87V
(A)
51N
834763A1.CDR
9
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