Capacitor banks are used to correct the power factor of an AC system or to compensate for
reactive energy absorbed by electrical
system loads, and sometimes to make up
filters to reduce harmonic voltage.
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| Double star connected capacitor bank for reactive power compensation |
In terms of power system, the function of the capacitor is to improve the quality of the
electrical system.
They may be connected in star, delta and
double star arrangements, depending on the
level of voltage and the system load.
A capacitor comes in the form of a case with
insulating terminals on top. It comprises
individual capacitances which have limited
maximum permissible voltages
(e.g. 2250 V) and are series-mounted in
groups to obtain the required voltage
withstand and parallel-mounted to obtained
the desired power rating.
There are two types of capacitors as far as protection is concern:
- those with no internal protection
- those with internal protection a fuse is combined with each individual
capacitance.
The main faults which are liable to affect
capacitor banks are:
- overload,
short-circuit - frame fault
- capacitor component short-circuit
An overload is due to temporary or
continuous overcurrent:
- continuous overcurrent linked to raising of the power supply voltage,
the flow of harmonic current due to the
presence of non-linear loads such as static
converters (rectifiers, variable speed drives),
arc furnaces, etc. - temporary overcurrent linked to the
energizing of a capacitor bank step.
Overloads result in overheating which has
an adverse effect on dielectric withstand
and leads to premature capacitor aging.
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| Capacitor as Filter |
fault between live conductors, phase-to-phase or phase-to-neutral depending on
whether the capacitors are delta or star-connected. The appearance of gas in the
gas-tight chamber of the capacitor creates
overpressure which may lead to the opening
of the case and leakage of the dielectric.
A frame fault is an internal fault between a
live capacitor component and the frame
created by the metal chamber.
Similar to internal short-circuits, the appearance of gas in the gas-tight chamber
of the capacitor creates overpressure which
may lead to the opening of the case and
leakage of the dielectric.
A capacitor component short-circuit is
due to the flashover of an individual
capacitance.
- with no internal protection: the parallel wired individual capacitances are shunted
by the faulty unit:
the capacitor impedance is modified
the applied voltage is distributed to one
less group in the series
each group is submitted to greater stress,
which may result in further, cascading
flashovers, up to a full short-circuit. - with internal protection: the melting of the
related internal fuse eliminates the faulty
individual capacitance:
the capacitor remains fault-free,
its impedance is modified accordingly.
Capacitors should not be energized unless
they have been discharged. Re-energizing
must be time-delayed in order to avoid
transient overvoltage. A 10-minute time
delay allows for sufficient natural discharging.
Fast discharging reactors may be used to
reduce discharging time.
Overloads
- Overcurrent of long duration due to the
raising of the power supply voltage maybe
avoided by overvoltage protection that
monitors the electrical system voltage. This
type of protection may be assigned to the capacitor itself, but it is generally a type of
overall electrical system protection. Given
that the capacitor can generally
accommodate a voltage of 110% of its rated
voltage for 12 hours a day, this type of
protection is not always necessary. - Overcurrent of long duration due to the the flow of harmonic current is detected by an
overload protection of one the following
types:
thermal overload
time-delayed overcurrent,
provided it takes harmonic frequencies into
account. - The amplitude of overcurrent of short
duration due to the energizing of the capacitor
bank steps are limited by series-mounting
impulse reactors with each step.
Short circuits
- Short-circuits are detected by a time-delayed
overcurrent protection device. Current and
time delay settings make it possible to
operate with the maximum permissible load
current and to close and switch steps.
Frame faults
- Protection depends on the grounding
system. If the neutral is grounded, a time-delayed earth fault protection device is used.
Capacitor component short-circuits
Detection is based on the change in
impedance created
by the following:
- the short-circuiting of the component for
capacitors with no internal protection - by the elimination of the faulty individual
capacitance for capacitors with internal
fuses.
When the capacitor bank is double star-connected, the unbalance created by the
change in impedance in one of the stars
causes current to flow in the connection
between the neutral points. This unbalance
is detected by a sensitive overcurrent
protection device.
- Protection Guide and Control
- Publisher: Merlin Gerin | Download
