Protection For Rack Faults; Overvoltage - GE C70 Instruction Manual

9.3 PROTECTION METHODS FOR CAPACITOR BANKS
• With grounded capacitor banks, the failure of one pole of the switching device or the single phasing from a blown bank
fuse will allow zero sequence currents to flow in system ground relays. Capacitor bank relaying, including the operating
time of the switching device, should be coordinated with the operation of the system ground relays to avoid tripping
system load.
• The unbalance trip element may need to be delayed to account for the settling time of the protection system on initial
energization and for the transient response of certain capacitor voltage transformers, etc., which may be a part of the
unbalance protection system.
• The unbalance trip scheme may include a lockout feature to prevent inadvertent closing of the capacitor bank switch-
ing device if an unbalance trip has occurred.
• To allow for the effects of inherent unbalance, the unbalance element trip should be set to operate at a signal level half-
way between the critical step and the next lower step. The critical step is the number of fuse operations or shorted ele-
ments that will cause an overvoltage on healthy capacitor units in excess of 110% of the capacitor unit rated voltage or
the capacitor unit manufacturer's recommended maximum continuous operating voltage. In addition, for internally
fused capacitor units, the critical step may be the number of internal fuse operations at which tripping should occur as
recommended by the capacitor manufacturer.
• If switch failure or single phasing due to a blown main fuse could result in the continuous voltage exceeding the relay
or VT rating, additional elements in the relay should be configured to trip the bank for this condition.
• The unbalance element detects only the unbalance in the capacitor bank and in supply voltage and will not respond to
capacitor overvoltage due to a balanced system voltage above nominal. The bank phase overvoltage should be config-
ured to trip the bank for this condition.
• The maximum system operating voltage, with capacitor bank energized, should be used for setting unbalance relays.
c) UNBALANCE ALARM ELEMENT CONSIDERATIONS
To allow for the effects of inherent unbalance within the bank, the unbalance relay alarm should be set to operate at about
one-half the level of the unbalance signal determined by the calculated alarm condition based on an idealized bank. The
alarm should have sufficient time delay to override external disturbances.
The most effective protection for an arc-over within the capacitor bank is provided by a fast unbalance element. A short
time delay for the unbalance elements minimizes the damage caused by rack faults. Intentional delays as short as 0.05
seconds have been used. This short unbalance time delay, however, should not be less than the maximum clearing time of
the capacitor-unit or element fuse. Although the unbalance trip element is the most effective protection for arc detection of
a series section, the neutral voltage type of unbalance element should not be relied upon for rack fault protection on capac-
itor banks where all three phases are not well separated.
Negative sequence elements can be used for inter-phase fault detection as they can be set to be more sensitive than
phase overcurrent relays, but tripping should be delayed to coordinate with the other relays in the system. A setting of 10%
of the rated capacitor current, taking into consideration the maximum system voltage unbalance and the maximum capaci-
tance variation together with a time delay setting of 15 to 25 cycles, may provide adequate coordination for faults external
to the bank. However, it may not prevent damage due to arcing faults within the bank structure.
The capacitor bank and other equipment in the vicinity may be subjected to overvoltages resulting from abnormal system
operating conditions. If the system voltage exceeds the capacitor or equipment capability with the capacitor bank on line,
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the bank should be removed with minimum time delay. Removing the capacitor bank from the system lowers the system
voltage in the vicinity of the capacitor, reducing the overvoltage on other system elements.
Especially for very large EHV capacitor banks, it is advisable to install three-phase overvoltage protection (ANSI 59B) to trip
the bank quickly for extreme overvoltage conditions. To avoid nuisance tripping during transient overvoltage conditions, in
some cases, tripping is delayed by a timer. The C70 relay provides inverse time overvoltage characteristics required to
decrease tripping time for large overvoltages.
Because this tripping is not due to a fault within the capacitor bank, the capacitor bank is not locked out.
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9.3.3 PROTECTION FOR RACK FAULTS (ARC-OVER WITHIN CAPACITOR BANK)
C70 Capacitor Bank Protection and Control System
9 APPLICATION OF SETTINGS

9.3.4 OVERVOLTAGE

GE Multilin