shunt power capacitor

7/27/2019 Shunt Power Capacitor

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SHUNT CAPACITOR

FUNDAMENTALS ANDPROTECTION

Prepared By :


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OVERVIEW

(1) Introduction

(2) Configurations

(3) Design

(4) Operation

(5) Protection

(6) Conclusions


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INTRODUCTION

1. Shunt capacitor banks (SCB) are mainly installed to

provide capacitive reactive compensation/power

factor correction.

2. They are installed near the load terminals, in factorysubstations, in the receiving substations to provide

leading volt-ampere-reactive.

3. By using shunt capacitors line drop is reduced and

the voltage regulation is improved.

4. They are switched in when kVA demand on the

distribution system rises and voltage of bus drops.


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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.

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


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Fig 2. High voltage shunt

capacitor

Fig1. Single line diagram of SCB


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THE CAPACITOR UNIT

AND BANK

CONFIGURATIONThe Capacitor Unit1. The capacitor unit, Fig. 3, is the building

block of a shunt capacitor bank.

2. The capacitor unit is made up of

individual capacitor elements, arranged

in parallel/ series connected groups,

within a steel enclosure. The internaldischarge device is a resistor that

reduces the unit residual voltage to 50V

or less in 5 min.


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Fig3. capacitor bank


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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 byloads. These surplus lagging vars must be

absorbeed by additional equipment to keep

voltage profile within limits.

SCB are those reactive powercompensating equipment to generate

generate or absorb vars.


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S=P+jQ

Where,S=apparent power=VI(kva)

Q=reactive power=VIsin(kvar)

P=active power=VIcos(kw)


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Capacitorunit capabilities1. Relay protection of shunt capacitor banks

requires some knowledge of the capabilities and

limitations of the capacitor unit and associated

electrical equipment including: individualcapacitor unit, bank switching devices, fuses,

voltage and current sensing devices.

2. Capacitors are intended to be operated at or

below their rated voltage and frequency as they

are very sensitive to these values; the reactivepower generated by a capacitor is proportional to

both of them (kVar 2 f V2 ).


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Bank Configurations

1. The use of fuses for protecting the capacitor units and it location isan 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


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External ly 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.


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Fig4. Externally fused shunt capacitor bank


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In ternal ly Fused

1. Each capacitor element is fused inside the capacitor unit.

2. . Upon a capacitor element failure, the fuse removes the affected elementonly. 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 areconfigured with fewer capacitor units in parallel and

more series groups of units than are used in banks

employing externally fused capacitor units


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Fig 5. Internally fused SCB


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Fuseless Shun t Capaci tor 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 elementsfailing thus short- circuiting the group. When thecapacitor element fails it welds and the capacitor unitremains in service. The voltage across the failedcapacitor element is then shared among all theremaining 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 nothave to be delayed to coordinate with the fuses


17/32Fig 6. Fuseless SCB


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Unfused Shunt Capaci tor Banks

Contrary to the fuseless configuration, where the units areconnected 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 onhigher voltage banks with modest parallel energy. This

design does not require as many capacitor units in parallel

as an externally fused bank.


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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.


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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 substationbanks are connected wye.

various types of SCB designs are:

(1) Grounded wye-connection banks

(2) Underground wye-connection banks

(3) Delta connected banks


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Grounded Wye-Connected BanksGrounded 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


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dvantages 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


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Ungrounded Wye-Connected Banks1. Ungrounded wye banks do not permit zero sequence

currents, third harmonic currents, or large capacitordischarge 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 ismomentarily at phase potential when the bank is switched or

when one capacitor unit fails in a bank configured with a

single group of units.


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Fig. 8 - Ungrounded Wye Shunt Capacitor Banks


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1. Delta-connected banks are generally used only atdistributions 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.

Delta-connected Banks


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CAPACITOR BANKPROTECTION

The protection of SCBs involves:

a) protection of the bank against faults occurring within the bank includingthose inside the capacitor unit

b) protection of the bank against system disturbances and faults.


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Capacitor UnbalanceProtection1. 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 anincrease 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 healthycapacitor units.

4.Unbalance protection normally provides primary protection for arcing faults

within a capacitor bank and other abnormalities that may damage

capacitor units


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5. The unbalance protection should have minimumintentional delay in order to minimize the amount ofdamage 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.


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Protection of the SCB

Against System Disturbances

and Faults

System Overvoltage ProtectionThe 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. Timedelayed or inverse time delayed phase overvoltage relays are used.


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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 todischarge the trapped charges within 5min.


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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 capacitorbank, 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.


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THANK YOU

shunt power capacitor

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