S&C Instruction Sheet 1011-540 7
Control Operation
This section explains how the BankGuard Plus Controls
the capacitor bank or reactor.
Overvoltage Protection for Capacitor Banks
Medium to large wye-connected shunt capacitor banks
commonly use twofold protection against short circuits:
• Individual capacitor units are protected by fuse links
that clear internal faults, reducing the probability of
case ruptures.
• The system is protected against major faults by the
bank protective control, such as power fuses or an S&C
Circuit-Switcher.
If a fuse link operates to isolate a failed capacitor unit,
the voltage across remaining units in the same series group
will increase. This increased voltage can overstress and
shorten the life of the other capacitor units in the group.
As subsequent units fail, their isolation leads to further
voltage increase on the remaining units. The result is an
accelerating cascade of overvoltage that destroys good
capacitor units.
This phenomenon is addressed in IEEE Standard
18-1992, “IEEE Standard for Shunt Power Capacitors.” The
standard specifies a curve that indicates the permissible
capacitor unit operating time at varying per-unit multiples
of the capacitor nameplate voltage rating. See Figure 2.
This curve applies for up to 300 applications of power-
frequency overvoltages of the magnitude and duration
shown. (The standard further requires that capacitors
be capable of continuous operation to at least 110% of the
rated voltage, including harmonics. Most capacitor manu-
facturers publish similar data, which may permit higher
working voltages.) When the voltage applied to the surviv-
ing capacitor units exceeds the manufacturer’s maximum
recommended working voltage (or in the absence of a
recommendation, the IEEE standard), the entire bank
should be removed from service.
Thus, large-sized capacitor banks require a third form
of protection: a control sensitive enough to detect the
isolation of the first failed unit in a capacitor bank but
sophisticated enough to disregard system and inherent
bank imbalances, spurious transients, and harmonics.
Also, the ability to alarm upon isolation of that capacitor
unit lets the user replace it before additional failures occur.
Protection for Reactors
When a developing turn-to-turn fault occurs in any phase
winding, the ungrounded, wye-connected shunt reactor
is protected from further damage by automatic switching
initiated by the BankGuard Plus Control. The control also
will isolate and lock out the entire shunt reactor when a
predetermined neutral-to-ground voltage is exceeded.
Figure 2. Capacitor unit power-frequency overvoltage versus time (IEEE standard 18-1992).
