Download shunt_capacitor_fundamentals

SHUNT CAPACITOR
FUNDAMENTALS AND
PROTECTION
Prepared By :
OVERVIEW
(1) Introduction
(2) Configurations
(3) Design
(4) Operation
(5) Protection
(6) Conclusions
INTRODUCTION
1.
2.
3.
4.
Shunt capacitor banks (SCB) are mainly installed to
provide capacitive reactive compensation/power
factor correction.
They are installed near the load terminals, in factory
substations, in the receiving substations to provide
leading volt-ampere-reactive.
By using shunt capacitors line drop is reduced and
the voltage regulation is improved.
They are switched in when kVA demand on the
distribution system rises and voltage of bus drops.
•
Advantages
1. Improvement of the voltage at the load.
2. voltage regulation.
3. reduction of losses
4. Maximize system capacity
5. reduction in Cu loss due to reduction in current
Disadvantages
1. proportional to the square of the voltage and
consequently when the voltage is low and the system
need them most, they are the least efficient.
Fig1. Single line diagram of SCB
Fig 2. High voltage shunt
capacitor
THE CAPACITOR UNIT
AND BANK
CONFIGURATION
The Capacitor Unit
1.
2.
The capacitor unit, Fig. 3, is the building
block of a shunt capacitor bank.
The capacitor unit is made up of
individual capacitor elements, arranged
in parallel/ series connected groups,
within a steel enclosure. The internal
discharge device is a resistor that
reduces the unit residual voltage to 50V
or less in 5 min.
Fig3. capacitor bank
SCB operation
• The SCB of the line abosrbz leadind var (i.e
generates lagging var). At the time of light
load the lagging vars produced by the lines
are much larger than that required by
loads. These surplus lagging vars must be
absorbeed by additional equipment to keep
voltage profile within limits.
• SCB are those reactive power
compensating equipment to generate
generate or absorb vars.
S=P+jQ
Where,
S=apparent power=VI(kva)
Q=reactive power=VIsinФ(kvar)
P=active power=VIcosФ(kw)
Capacitor unit capabilities
1.
2.
Relay protection of shunt capacitor banks
requires some knowledge of the capabilities and
limitations of the capacitor unit and associated
electrical equipment including: individual
capacitor unit, bank switching devices, fuses,
voltage and current sensing devices.
Capacitors are intended to be operated at or
below their rated voltage and frequency as they
are very sensitive to these values; the reactive
power generated by a capacitor is proportional to
both of them (kVar ≈ 2π f V2 ).
Bank Configurations
1. The use of fuses for protecting the capacitor units and it location is
an important subject in the design of SCBs.
2. They also affect the failure mode of the capacitor unit and influence
the design of the bank protection.
3.
Depending on the application any of the following configurations
are suitable for shunt capacitor banks:
(1) externally fused
(2) internally fused
(3) fuseless shunt capacitor bank
(4) unfused shunt capacitor bank
Externally Fused
1. An individual fuse, externally mounted between the capacitor unit and the
capacitor bank fuse bus, typically protects each capacitor unit.
2. The capacitor unit can be designed for a relatively high voltage because
the external fuse is capable of interrupting a high-voltage fault.
3. A failure of a capacitor element welds the foils together and short circuits
the other capacitor elements connected in parallel in the same group.
4. The remaining capacitor elements in the unit remain in service with a
higher voltage across them than before the failure and an increased in
capacitor unit current.
5. If a second element fails the process repeats itself resulting in an even
higher voltage for the remaining elements.
6. Successive failures within the same unit will make the fuse to operate,
disconnecting capacitor unit and indicating the failed one.
Fig4. Externally fused shunt capacitor bank
Internally Fused
1. Each capacitor element is fused inside the capacitor unit.
2. . Upon a capacitor element failure, the fuse removes the affected element
only. The other elements, connected in parallel in the same group, remain
in service but with a slightly higher voltage across them.
3.. Banks employing internally fused capacitor units are
configured with fewer capacitor units in parallel and
more series groups of units than are used in banks
employing externally fused capacitor units
Fig 5. Internally fused SCB
Fuseless Shunt Capacitor Banks
The capacitor units for fuseless capacitor banks are identical to those for
externally fused described above. To form a bank, capacitor units are
connected in series strings between phase
and neutral, shown in Fig. 6.
The protection is based on the capacitor elements
failing thus short- circuiting the group. When the
capacitor element fails it welds and the capacitor unit
remains in service. The voltage across the failed
capacitor element is then shared among all the
remaining capacitor element groups in the series.
The discharge energy is small because no capacitor units
are connected directly in parallel. Another advantage of
fuseless banks is that the unbalance protection does not
have to be delayed to coordinate with the fuses
Fig 6. Fuseless SCB
Unfused Shunt Capacitor Banks
Contrary to the fuseless configuration, where the units are
connected in series, the unfused shunt capacitor bank uses a
series/parallel connection of the capacitor units. The unfused
approach would normally be used on banks below 34.5 kV,
where series strings of capacitor units are not practical, or on
higher voltage banks with modest parallel energy. This
design does not require as many capacitor units in parallel
as an externally fused bank.
CAPACITOR BANK DESIGN
The protection of shunt capacitor banks requires understanding the
basics of capacitor bank design and capacitor unit connections. The
capacitors banks are arrangements of series/paralleled connected
units. Capacitor units connected in paralleled make up a group and
series connected groups form a single-phase capacitor bank.
When a capacitor bank unit fails, other capacitors in the same parallel
group contain some amount of charge. This charge will drain off as a high
frequency transient current that flows through the failed capacitor unit and
its fuse. The fuse holder and the failed capacitor unit should withstand this
discharge transient.
The optimum connection for a SCB depends on the best
utilization of the available voltage ratings of capacitor
units,fusing, and protective relaying.Virtually all substation
banks are connected wye.
various types of SCB designs are:
(1) Grounded wye-connection banks
(2) Underground wye-connection banks
(3) Delta connected banks
Grounded Wye-Connected Banks
Grounded wye capacitor banks are composed of series and
parallel-connected capacitor units per phase and provide a
low impedance path to ground. Fig. 7 shows typical bank
arrangements.
Fig. 7 - Grounded Wye Shunt Capacitor Banks
Advantages of the grounded capacitor banks include:
1. Its low-impedance path to ground provides inherent self-protection for
lightning surge currents and give some protection from surge voltages.
2. Offer a low impedance path for high frequency currents and so they
can be used as filters in systems with high harmonic content.
3. Reduced transient recovery voltages for circuit breakers and other
switching equipment
Some drawbacks for grounded wye SCB are:
1. Increased interference on telecom circuits due to harmonic circulation
2. Circulation of inrush currents and harmonics may cause
misoperation of protective relays and fuses
Ungrounded WyeConnected Banks
1. Ungrounded wye banks do not permit zero sequence
currents, third harmonic currents, or large capacitor
discharge currents during system ground faults to flow.
2. Overvoltage appearing at the CT secondaries are not as
high as in the case of grounded banks. However, the neutral
should be insulated for full line voltage because it is
momentarily at phase potential when the bank is switched or
when one capacitor unit fails in a bank configured with a
single group of units.
Fig. 8 - Ungrounded Wye Shunt Capacitor Banks
Delta-connected Banks
1.
Delta-connected banks are generally used only at
distributions voltages and are configured with a single
series group of capacitors rated at line-to-line voltage.
2. With only one series group of units no overvoltage
occurs across the remaining capacitor units from the
isolation of a faulted capacitor unit.
CAPACITOR BANK
PROTECTION
The protection of SCB’s involves:
a) protection of the bank against faults occurring within the bank including
those inside the capacitor unit
b) protection of the bank against system disturbances and faults.
Capacitor Unbalance
Protection
1. The protective elements found in a SCB for internal faults are: individual
fuses, unbalance protection to provide alarm/ trip and overcurrent
elements for bank fault protection.
2. Removal of a failed capacitor element or unit by its fuse results in an
increase in voltage across the remaining elements/ units causing an
unbalance within the bank.
3. Unbalance protection normally senses changes associated with the
failure of a capacitor element or unit and removes the bank from service
when the resulting overvoltage becomes excessive on the remaining healthy
capacitor units.
4.Unbalance protection normally provides primary protection for arcing faults
within a capacitor bank and other abnormalities that may damage
capacitor units
5. The unbalance protection should have minimum
intentional delay in order to minimize the amount of
damage to the bank in the event of external arcing.
6. The need for sensitive resulted in the development of unbalance
protection where certain voltages or currents parameters of the
capacitor bank are monitored and compared to the bank balance
conditions.
7. Capacitor unbalance protection is provided in many different
ways, depending on the capacitor bank arrangement and
grounding.
Protection of the SCB
Against System Disturbances
and Faults
System Overvoltage Protection
The capacitor bank may be subjected to overvoltages resulting from
abnormal system operating conditions. If the system voltage exceeds
the capacitor capability the bank should be removed from service. The
removal of the capacitor bank lowers the voltage in the vicinity of the
bank reducing the overvoltage on other system equipment. Time
delayed or inverse time delayed phase overvoltage relays are used.
Relays for Bank Closing Control
Once disconnected from the system a shunt capacitor
bank cannot be re-inserted immediately due to the
electrical charge trapped within the capacitor units,
otherwise catastrophic damage to the circuit breaker or
switch can occur. To accelerate the discharge of the
bank, each individual capacitor unit has a resistor to
discharge the trapped charges within 5min.
CONCLUSIONS
The protection of shunt capacitor banks uses simple, well known relaying
principles such as overvoltage, overcurrents.
Unbalance is the most important protection in a shunt capacitor
bank, as it provides fast and effective protection to assure a long and
reliable life for the bank. To accomplish its goal, unbalance protection
requires high degree of sensitivity that might be difficult to achieve.
THANK YOU