CAPACITOR BANK PROTECTION

BTech: ELP400S Electrical Protection 4

Department of Electrical, Electronic and Computer Engineering

Lecturer: Dr S Krishnamurthy

Total: 10 Marks

BAFUNDI CISHE

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Introduction:

In all industries that make use of machinery particularly motors, we find that the motors draw
inductive power from the grid. This results in loads having a lagging power factor. It becomes
necessary and of benefit to consumers to try by all means to correct this power factor and keep it
close to unity.

This is where the need for shunt capacitor banks arises as they supply the necessary reactive power
to correct these lagging power factors. There also comes a need to protect these capacitor banks as
they are of advantage to us.

In this assignment we will discuss the different shunt capacitor banks designs and the different
configurations. We will also show the protection schemes available and how to apply them.

Lastly we will discuss how to set up the system protection around these shunt capacitor banks.

Capacitor bank designs

a) Externally fused, with individual fuses for each capacitor unit.

Fig 1: Externally fused capacitor banks

Externally fused capacitor banks consist of a series of parallel-connected capacitor groups per phase.
Each capacitor unit is protected a fuse between the capacitor unit and the capacitor bank bus. This
configuration can be used for relatively high voltages because the external fuse is capable of
interrupting a high-voltage fault.
In externally fused capacitor banks, several capacitor element breakdowns may occur before the
fuse removes the entire unit. The external fuse will operate when a capacitor unit becomes
(essentially) short circuited, isolating the faulted unit.
Unbalance protection removes the bank from service when the resulting overvoltage becomes
excessive on the remaining healthy capacitor units.

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 b) Internally fused, with each element fused inside the capacitor unit.

Fig 2: Internally fused

An internal fuse is connected in series with each capacitor element. With internally fused capacitors,
when a capacitor element fails, the current through its individual fusible link will be considerably
higher than the normal current. This higher current will blow the fuse and isolate the failed element.
Internally fused capacitors have individual capacitor elements within a capacitor unit that are
disconnected when an element breakdown occurs. The risk of successive faults is minimized because
the fuse will isolate the faulty element within a few cycles. Unbalance protection removes the bank
from service when the resulting unbalanced voltage becomes excessive on the remaining healthy
capacitor elements or units.

c) Unfused with capacitor units connected in series strings between line and neutral

Fig 3: Unfused capacitor unit connected in series

This configuration is used for applications at or above 34.5 kV. The capacitor units are normally
designed with two bushings with the elements insulated from the case. The capacitor units are
connected in series strings between phase and neutral.
For unfused capacitor banks, a failed element is short-circuited by the weld that naturally occurs at
the point of failure. Unbalance protection removes the bank from service when the resulting voltage
becomes excessive on the remaining healthy capacitor elements or units.

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 d) Unfused, with the capacitor units connected in a variety of series and parallel
arrangements.

Fig 4: Unfused capacitor unit with series and parallel arrangement

The unfused shunt capacitor approach uses a series/parallel connection of the capacitor units. The
voltage across the remaining elements will increase more than in the fuse-less design.

Bank connections

Fig 5: Delta

Used at 2.4 KV
No over voltage occurs across capacitor units when isolation of the
failure unit

Fig 6: Grounded wye

34.5 KV and above

Low impedance path to ground for lighting surge currents and
surge voltages
Low impedance path for harmonic currents

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Fig 7: Ungrounded wye

Do not permit 3rd harmonic currents or capacitor discharge

current during ground faults

Fig 8: Ungrounded double wye

Used when capacitor bank too large

4650 kVAR per group for expulsion fuses

Fig 9: Grounded double wye

Also used when capacitor bank too

large 4650 KVAR per group.
Two neutrals are directly connected
together with single ground

Bank and system protection

The protection of shunt capacitor banks involves both bank and system protection schemes. Bank
protection schemes are provided for faults within the capacitor bank itself.

System protection schemes are provided to protect the capacitor bank from stresses that may be
caused by the system and to protect the substation and system from stresses that may be caused by
the operation of the capacitor bank.

Table 1: Bank and system protection

Bank protection
Condition Type of protection Remarks
Faulted capacitor element. External or internal fuse for fused Fuses should be fast to coordinate
banks with fast unbalance relay settings, but
should not operate during switching or
external faults.

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Fault from capacitor elements to case, Fuse for externally fused capacitor; For externally fused capacitor banks,
bushing failure, faulty connection in unbalance protection for internally fuses should be fast to coordinate with
capacitor unit. fused banks or banks without fuses fast unbalance relay settings, but
should not operate during switching or
external faults. For internally fused
banks or banks without fuses, the
unbalance protection should be
fast to avoid case rupture, but should
not operate during switching or
external faults

Fault in capacitor bank other than in Unbalance protection. Relay should Unbalance protection should be fast to
unit (arcing fault in bank). have a band-pass filter for the minimize damage to other units during
fundamental current or voltage for a major fault.
security.

Continuous overvoltage on capacitor Unbalance protection. Relay should Bank should be tripped for voltages >
elements or units due to faulted have a band-pass filter for the 110% of rated voltage or as
elements or fuse operations within the fundamental current or voltage for recommended by manufacturer on
bank. security. healthy capacitor units. An alarm may
be added for 5% unbalance or one unit
out. (In some critical applications an
alarm with delayed tripping above
110% of rated voltage is used

Rack-to-rack flashover in two Phase overcurrent or negative Fast operation is required to minimize
series group phase-over-phase sequence relay; unbalance current for dam-
single wye banks wye-wye capacitor banks. age

System protection
System surge overvoltage Surge arresters Selection of surge arrester may require
consideration of bank energy,
particularly for
larger capacitor banks

Power frequency system overvoltage. Phase voltage relays. For a distorted voltage waveform, the
capacitor dielectric is sensitive to the
peak voltage.

Harmonic current overloading Relay sensitive to harmonic current. Where excessive harmonic currents
are anticipated, harmonic relaying may
be required.

Bus fault in capacitor installation or a) Circuit breaker or circuit switcher Relays or power fuses should be as fast
major capacitor bank failure. with conventional relays, or as possible without nuisance
b) Power fuses. operations due to outrush currents
into nearby faults.

Fault in or near substation, but Inrush and outrush limiting reactors Reactors may be required to protect
outside capacitor installation circuit breakers, current transformer
circuits, and other components against
excessive currents or induced voltages.

Excessive inrush current. a) Insertion resistor or reactor in Energizing a capacitor bank in close
switch, breaker, or circuit switcher, or proximity to an energized capacitor
b) Inrush and outrush limiting reactors bank may result in excessive inrush
between capacitor banks, or currents, damaging circuit breakers or
c) Synchronous (zero voltage) closing switches, causing undesired fuse
of the switch or circuit breaker. operations, causing excessive voltages
in current transformers and relays,
causing arcing at gate latches, etc.

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System outage. Undervoltage relays. Capacitor banks (which may be)
energized through a transformer
without load on the transformer may
need to be switched off before re-
energizing the system.

Transmission line tripping (for a) Transfer tripping of the capacitor Capacitor banks directly connected to
capacitor banks connected to a bank switch, or a transmission line with no connected
transmission line segment). b) Undervoltage relays. load may need to be disconnected
from the line before reclosing the line

Breaker failure. Conventional breaker failure relays. Local or remote breakers should have
capacitor switching capability if they
trip the bank without parallel load due
to breaker failure considerations.

Capacitor unbalance protection

Unbalance protection is usually the main protection for arcing faults in a capacitor bank and other
abnormal conditions. Arcing faults can cause a lot of damage within a small space of time so the
unbalance protection should operate very quickly to minimise damage.

Protection for rack faults (arc-over within the capacitor rack)

In a capacitor bank the three phases are on separate structures. If an arc-over within the capacitor
rack occurs the instantaneous overcurrent relay will not be able to detect it because of its required
high setting. If an unbalance relay protection scheme fails to operate, more of the units on the same
phase can become involved until the bank overcurrent relays trip the bank or fuses clear. By this
time there will be extensive damage to the bank like blown fuses and ruptured capacitor units.
The best solution would be to use an unbalance relay with a very short delay. It must also not be less
than the maximum clearing time of the capacitor-unit or element fuse. The setting of the relay must
be sensitive enough to protect against sustained overvoltages that result from individual unit or
element failure and resultant fuse operation.

System protection

System protection may help minimise damage to capacitors if the following events occur:

External arcing:
Provide the capacitor bank with redundant unbalance protection and/or current unbalance or
negative sequence current protection to minimize damage. The overcurrent protection is not
normally sensitive to arcing within a capacitor bank and if it does sense a disturbance, it is normally
too slow.

Overvoltages:
If the system voltage exceeds the capacitor rating, the bank should be removed with minimum time
delay. Inverse time overvoltage relays can be used to protect the capacitor units from severe system
power frequency overvoltage conditions.
For very large EHV capacitor banks, it is best to install three-phase overvoltage relays (59B) to
monitor the bus voltage. The 59B relays must 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.

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Bank overcurrent protection
Protecting the capacitor bank against a major fault, such as a line-to-line fault or a line-to-ground
fault generally requires external protection, such as power fuses, circuit breakers, or relay circuits.
For best protection, the relays should be set as low and fast as possible.
Time-overcurrent relays can be applied with normal settings without encountering false operations
due to inrush currents. The desirable minimum pickup is 135% of nominal phase current for
grounded wye banks or 125% for ungrounded banks. Instantaneous relays, if used, should be set
high to override inrush or out-rush transients.

Loss of bus voltage

In some cases, it may be necessary to trip a capacitor bank if the supply bus voltage is lost.
Circuit breaker reclosing schemes and capacitor voltage discharging means should be considered to
avoid nuisance tripping or equipment damage upon loss of bus voltage with fast reclosing schemes.
The undervoltage relay should be set so that the relay will not operate for voltages that require the
capacitor bank to remain in service.

Capacitor bank breaker failure protection

If a circuit breaker is used for protection, it must have a breaker fail protection scheme as backup if
the breaker fails. So if the circuit breaker fails, the capacitor bank can be isolated by tripping the
adjacent breakers connected to the bank after a set time delay.

Surge arrester protection

Lightning and transient overvoltages may be controlled by using surge arresters.

Unbalance protection methods

a) Unbalance Protection for Ungrounded Single Wye Banks

Fig 10: single ungrounded wye

The simplest method to detect unbalance in single ungrounded wye banks is to measure the bank
neutral or zero sequence voltage. If the capacitor bank is balanced and the system voltage is balance
the neutral voltage will be zero. A change in any phase of the bank will result in a neutral or zero
sequence voltage. We can also use three phase-to-neutral voltage transformers with their

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secondaries connected in broken delta to an overvoltage relay. The latter scheme has the advantage
of not being sensitive to system voltage unbalance. Also, the unbalance voltage going to the
overvoltage relay is three times the neutral voltage for the same voltage transformer ratio it is more
sensitive

Unbalance Protection for Ungrounded Double Wye Banks

Fig 11: Ungrounded double wye

For an ungrounded double wye configuration we can connect a current transformer between the
two neutrals and an overcurrent relay. Another option uses a voltage transformer connected
between the two neutrals and an overvoltage relay. System voltage unbalances are avoided by both
schemes, and both are unaffected by third harmonic currents or voltages when balanced.

Unbalance Protection for Grounded Single Wye Banks

Fig 12: Grounded single wye

An unbalance in the capacitor bank will cause current to flow in the neutral. The figures show a
protection based on a current transformer or three single phase voltage transformers installed on
the connection between the capacitor bank neutral and ground.

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Unbalance Protection for Grounded Double Wye Banks

Fig 13: Grounded Double Wye Banks

The above figure shows a scheme where a current transformer is installed on each neutral of the
two sections of a double wye shunt capacitor bank. The neutrals are connected to a common
ground. The current transformer secondaries are cross-connected to an overcurrent relay so that the
relay is insensitive to any outside condition that affects both sections of the capacitor bank in the
same direction or manner.

Voltage differential protection method for grounded wye banks

Fig 14: Voltage differential for grounded single wye

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The voltage differential provides a very sensitive and efficient method. The scheme uses two voltage
transformers per phase: one connected to a tap on the capacitor bank; the other, at the bank bus for
single wye banks; or, for double wye banks, at a similar tap on the second bank.

Fig 15: Voltage differential for grounded double wye

Mid-Rack Phase to Phase Faults

Fig 16: Mid-Rack Phase to Phase Faults

The most efficient protection for mid-rack phase to phase faults is the negative sequence current.
Tripping shall be delayed to coordinate with other relays in the system.

Conclusion:

The protection of shunt capacitor banks is quite easy to implement and not necessarily expensive. It
makes use of the basic power system protective devices namely fuses, circuit breakers, current

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transformers and voltage transformers and relays. The devices used do not make use of complicated
techniques so it’s only up to the engineer to be familiar with the designs and connections.

References:

(1) IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitors Banks

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