47563543 EPRI Inspection and Maintenance Guidelines for Circuit Breakers

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Inspection and Maintenance Guidelinesfor Circuit Breakers

1010618

Effective December 6, 2006, this report has been made publicly available in accordance

with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S. ExportAdministration Regulations. As a result of this publication, this report is subject to only

copyright protection and does not require any license agreement from EPRI. This notice

supersedes the export control restrictions and any proprietary licensed material notices

embedded in the document prior to publication.


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Inspection and Maintenance Guidelines

for Circuit Breakers

1010618

Technical Update, December 2005

EPRI Project Manager

L. van der Zel

ELECTRIC POWER RESEARCH INSTITUTE3420 Hillview Avenue, Palo Alto, California 94304-1395 PO Box 10412, Palo Alto, California 94303-0813 USA

800.313.3774 650.855.2121 [email protected] www.epri.com


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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OFWORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI).NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANYPERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITHRESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEMDISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULARPURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNEDRIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT ISSUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDINGANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISEDOF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THISDOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED INTHIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

International Switchgear Consulting Ltd

This is an EPRI Technical Update report. A Technical Update report is intended as an informal report ofcontinuing research, a meeting, or a topical study. It is not a final EPRI technical report.

NOTE

For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 ore-mail [email protected].

Electric Power Research Institute and EPRI are registered service marks of the Electric PowerResearch Institute, Inc.

Copyright 2005 Electric Power Research Institute, Inc. All rights reserved.

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CITATIONS

This document was prepared by

International Switchgear Consulting Ltd

14728 Upper Roper AveWhite Rock, BC, Canada, V4B-2C9

Principal InvestigatorB. Holm

Major Contributors

EPRI Life Extension Guidelines (Transmission Circuit Breaker Chapter)

This document describes research sponsored by the Electric Power Research Institute (EPRI)

This publication is a corporate document that should be cited in the literature in the following

manner:

Inspection and Maintenance Guidelines for Circuit Breakers, 1010618, EPRI, Palo Alto, CA.

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ABSTRACT

This document includes descriptions of the various types of circuit breakers, insulation media,

and operating mechanisms, as well as techniques for monitoring, testing and maintenance ofcircuit breakers.

Because of the high importance and cost of transmission substations, a significant amount ofwork has been done to address the condition assessments and maintenance practices for

transmission type circuit breakers. This information is contained in the EPRI Life Extension

Guidelines (LEG). Parts of this information have been included in this document.

Distribution substations have their own set of problems and issues that have not been addressedin the same level of detail as the higher voltage class of equipment. This document includes

additional information about distribution as well as transmission type circuit breakers, with

emphasis on monitoring, testing and condition assessment techniques that can be used to

diagnose incipient problems, as well as to avoid unnecessary dismantling work.

The purpose of these guidelines is to assist utilities in reviewing, altering or developing state ofthe art circuit breaker maintenance practices while maintaining or increasing system reliability.

There is a very large variety of circuit breakers in service. Maintenance programs must be

tailored to individual makes and models of breakers. This document is intended to provide

guidance that, together with information from manufacturers and individual utility experience,can be used to develop specific techniques, criteria and programs for circuit breaker inspections,

testing and maintenance.

Keywords: Circuit Breaker, Transmission, Distribution, Monitoring, Maintenance, testing,

replacement

.

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CONTENTS

1 DESCRIPTION OF CIRCUIT BREAKERS ______________________________________1

1.1 Introduction__________________________________________________________1

1.2 Basics of Arc Interruption ______________________________________________1

1.3 Operating Mechanisms ________________________________________________21.3.1 Solenoid Mechanisms _______________________________________________31.3.2 Motor Mechanisms__________________________________________________ 31.3.3 Spring Mechanisms _________________________________________________31.3.4 Pneumatic Mechanisms ______________________________________________41.3.5 Hydraulic Mechanisms _______________________________________________6

1.4 Insulating Media ______________________________________________________71.4.1 Oil _______________________________________________________________81.4.2 Air_______________________________________________________________9

1.4.3 Sulfur Hexafluoride (SF6)_____________________________________________91.4.3.1 SF6 characteristics ________________________________________________ 91.4.3.2 EPRI SF6 Handling Guide _________________________________________101.4.3.3 SF6 leak detection _______________________________________________101.4.3.4 Contamination___________________________________________________101.4.3.5 Moisture in SF6__________________________________________________ 111.4.3.6 Particles _______________________________________________________111.4.3.7 SF6 decomposition products________________________________________111.4.3.8 SF

6gas systems _________________________________________________12

1.4.4 Vacuum _________________________________________________________12

1.5 Bulk Oil Circuit Breakers ______________________________________________121.5.1 Introduction_______________________________________________________121.5.2 Interrupter Functions _______________________________________________131.5.3 Internal Tank Insulation _____________________________________________14

1.6 Minimum Oil Circuit Breakers __________________________________________151.6.1 Introduction_______________________________________________________151.6.2 Interrupter functions ________________________________________________ 16

1.7 Air Magnetic Circuit Breakers __________________________________________171.7.1 Introduction_______________________________________________________171.7.2 Interrupter functions ________________________________________________ 17

1.8 Air Blast Circuit Breakers _____________________________________________171.8.1 Introduction_______________________________________________________17

1.8.2 Interruption _______________________________________________________ 181.8.3 Interrupter Types __________________________________________________201.8.4 Auxiliary Interrupter Components______________________________________231.8.5 Air Compressor systems ____________________________________________251.8.6 Trouble and Failure Modes __________________________________________26

1.9 SF6 Two Pressure Circuit Breakers _____________________________________281.9.1 Introduction_______________________________________________________281.9.2 Interruption Process and Mechanisms__________________________________28


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1.9.3 Auxiliary Interrupter Components______________________________________32

1.10 SF6Single Pressure Circuit Breakers __________________________________32

1.10.1 Introduction_______________________________________________________321.10.2 Puffer Interrupters _________________________________________________ 361.10.3 Self-blast (also termed Auto-puffer) design-types._________________________38

1.10.4 Auxiliary Interrupter Components______________________________________401.11 Vacuum Circuit Breakers ____________________________________________40

1.11.1 Outdoor applications _______________________________________________401.11.2 Indoor applications _________________________________________________ 411.11.3 Mechanisms ______________________________________________________ 42

2 METHODS FOR MONITORING AND TESTING CIRCUIT BREAKERS ______________43

2.1 Causes of Circuit Breaker Failures ______________________________________43

2.2 Methods for Monitoring circuit breakers _________________________________432.2.1 Pressure monitoring ________________________________________________ 432.2.2 Temperature monitoring_____________________________________________44

2.2.3 New monitoring applications _________________________________________442.3 Methods for testing circuit breakers_____________________________________44

2.3.1 General visual inspections ___________________________________________442.3.2 Insulation ________________________________________________________45

2.3.2.1 Visual inspection of oil_____________________________________________ 452.3.2.2 Oil test on site ___________________________________________________ 472.3.2.3 Laboratory analysis of oil __________________________________________472.3.2.4 Power factor tests ________________________________________________ 472.3.2.5 High voltage withstand ____________________________________________472.3.2.6 Acoustic emissions _______________________________________________ 482.3.2.7 UHF emissions __________________________________________________482.3.2.8 SF

6analysis_____________________________________________________48

2.3.3 Mechanical _______________________________________________________ 482.3.3.1 Contact timing ___________________________________________________ 482.3.3.2 Circuit breaker speed _____________________________________________492.3.3.3 Vibration analysis ________________________________________________51

2.3.4 Current contacts___________________________________________________ 512.3.4.1 Resistance across contacts ________________________________________512.3.4.2 Thermo graphic testing ____________________________________________52

2.3.5 Controls _________________________________________________________52

3 INSTRUMENTATION _____________________________________________________ 53

3.1 Monitoring Systems __________________________________________________ 533.1.1 Mechanical _______________________________________________________ 53

3.1.2 Insulation ________________________________________________________54

3.2 Test Instruments_____________________________________________________553.2.1 Mechanical _______________________________________________________ 553.2.2 Insulation ________________________________________________________57

4 MAINTENANCE _________________________________________________________59

4.1 Instruction Books for Circuit Breakers______________Error! Bookmark not defined.

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4.2 External Inspections and Diagnostic Tests _______________________________59

4.3 Internal Inspections and Diagnostic Tests________________________________60

4.4 Bulk Oil Circuit Breaker Maintenance____________________________________624.4.1 Mechanical _______________________________________________________ 624.4.2 Dielectric_________________________________________________________63

4.4.3 Current contacts___________________________________________________ 634.4.4 Controls _________________________________________________________634.4.5 Test details _______________________________________________________ 64

4.5 Minimum Oil Circuit Breaker Maintenance________________________________654.5.1 Mechanical _______________________________________________________ 654.5.2 Dielectric_________________________________________________________664.5.3 Current contacts___________________________________________________ 664.5.4 Controls _________________________________________________________664.5.5 Test Details ______________________________________________________66

4.6 Air Magnetic Circuit Breaker Maintenance________________________________684.6.1 Mechanical _______________________________________________________ 68

4.6.2 Dielectric_________________________________________________________694.6.3 Current contacts___________________________________________________ 694.6.4 Controls _________________________________________________________694.6.5 Test Details ______________________________________________________69

4.7 Air Blast Circuit Breaker Maintenance ___________________________________704.7.1 Mechanical _______________________________________________________ 704.7.2 Dielectric_________________________________________________________714.7.3 Current contacts___________________________________________________ 714.7.4 Controls _________________________________________________________724.7.5 Test Details ______________________________________________________72

4.8 SF6

Two Pressure Circuit Breaker Maintenance ___________________________73

4.8.1 Mechanical _______________________________________________________ 734.8.2 Dielectric_________________________________________________________744.8.3 Current contacts___________________________________________________ 744.8.4 Controls _________________________________________________________744.8.5 Test Details ______________________________________________________74

4.9 SF6

Single Pressure Circuit Breaker Maintenance__________________________764.9.1 Mechanical _______________________________________________________ 764.9.2 Dielectric_________________________________________________________774.9.3 Current contacts___________________________________________________ 774.9.4 Controls _________________________________________________________774.9.5 Test Details ______________________________________________________78

4.10 Vacuum Circuit Breaker Maintenance __________________________________794.10.1 Mechanical _______________________________________________________ 794.10.2 Dielectric_________________________________________________________794.10.3 Current contacts___________________________________________________ 794.10.4 Controls _________________________________________________________804.10.5 Test Details ______________________________________________________80

4.11 Air Compressor Systems ____________________________________________814.11.1 Air system types___________________________________________________ 81

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4.11.2 Compressor system ________________________________________________ 824.11.3 In service inspections_______________________________________________ 824.11.4 Out of service inspections ___________________________________________83

4.12 Lubrication________________________________________________________ 854.12.1 Introduction_______________________________________________________85

4.12.2 Petroleum lubricants________________________________________________864.12.3 Synthetic lubricants ________________________________________________864.12.4 Solid lubricants____________________________________________________ 884.12.5 Lubricant types____________________________________________________ 884.12.6 Applications ______________________________________________________894.12.7 Present day practices_______________________________________________904.12.8 Recommendations _________________________________________________ 91

5 CONDITION ASSESSMENT________________________________________________92

5.1 Information Needed for Condition Assessment ___________________________92

5.2 Use of Condition Assessment Data _____________________________________93

6 REPLACEMENT/REFURBISHMENT_________________________________________956.1 Life limiting factors___________________________________________________96

6.2 Analysis of Accumulated Factors _______________________________________96

6.3 Review of the Available Options ________________________________________98

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TABLE OF FIGURESFigure 1 Diagram of a pneumatic system...................................................................................5Figure 2 Diagram of a hydraulic system .................................................................................7Figure 3 Relative dielectric strengths of oil, air, and SF

6for a 1-cm gap with optimum

electrodes at 0C ..........................................................................................................................8Figure 4 Typical bulk oil breakers.............................................................................................13Figure 5 Bulk oil circuit Breaker components and interrupter illustration ..............................14Figure 6 Typical live tank minimum oil breaker and interrupter arrangement .......................16Figure 7 Typical air blast breaker for distribution voltages....................................................19Figure 8 Typical air last breakers for transmission voltages, live, pressurized tanks............20Figure 9 Air blast interrupters, (A) mono blast (B) duo or partial duo blast ...........................21Figure 10 Typical SF

6two pressure dead tank breaker and SF

6blast valve arrangement .....29

Figure 11 Illustration of SF6

two pressure interrupter..............................................................30Figure 12 Typical SF

6single pressure dead tank and live tank configurations .......................34

Figure 13 Typical SF6single pressure outdoor distribution type configurations......................35

Figure 14 Illustration of puffer type interrupter principle..........................................................37Figure 15 Self blast self generated pressure (type 1) and self blast auto puffer (type 2)........39Figure 16 Typical outdoor vacuum breaker and vacuum interrupter illustration .....................41Figure 17 Typical indoor vacuum breaker and typical spring mechanism ..............................42Figure 18 Various stages of carbon contamination in minimum oil circuit breaker .................46Figure 19 Circuit breaker replacement versus refurbishing ....................................................95


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1DESCRIPTION OF CIRCUIT BREAKERS

1.1 IntroductionA power circuit breaker is a device for making, maintaining, and breaking (interrupting) an

electrical circuit between separable contacts under both load and fault conditions. Interruption of

electrical circuits has been a necessary part of electric utility systems since the first use ofelectricity. Initially, this interruption was achieved simply by separating the contacts in air. As

current levels became higher, arcing between the opening contacts presented greater problems,

which required the development of methods to deal with plasma arcs that occur during theopening process. This problem is more severe during faults or short-circuits, at which times

rapid, practically instantaneous, interruption of current is necessary as a protective measure forthe connected apparatus and system security. By the late 1920s, all principal methods of arc

interruption had been developed with the exception of the SF6 types, which came into being inthe late 1950s. Oil, air-magnetic, air-blast, and vacuum methods were all in use by 1930. Many

of the principles of these first modern breakers are still used in todays more highly developed

breaker designs.

1.2 Basics of Arc InterruptionWhen a switching-device conducting alternating current is in the act of opening, an arc is

formed. The arc commences as the last metal-to-metal electrical contacts separate. An arc is a

conductor. A number of factors must work together to extinguish an arc and interrupt a circuit.These factors include velocity, distance, cooling, current zero, and dielectric strength.

Velocity. The speed at which the circuit breaker contacts separate is an important part of circuit

interruption. The faster the contacts separate, the less time the arc has to heat the space and other

materials between the parting contacts, thereby reducing the conducting ability of the space. Theslower the movement of the moving contact, the greater the ability of the arc to maintain itself.

Distance. As the distance increases between the contacts as they open the arc is stretched. As the

arc stretches, the voltage, termed the arc-voltage, attempts to maintain current flow, but with theincreasing distance of the parting contacts, the arc becomes more vulnerable to the other factors

mentioned.Cooling. Interrupter cooling is a physical effect that removes heat created by an arc within a

circuit breaker interrupter. Increasing the temperature of gases causes them to become moreconductive. Therefore, cooling methods such as introducing forced air, gas, or insulating oil into

the area of the arc is important to arc extinction.

Current zero. Alternating current changes polarity, from positive to negative or negative to

positive, 120 times a second in a 60-Hz (cycle) sine wave (100 at 50 Hz). At the time the polarity


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changes, there is no flow of current. The instant an arc ceases is termed current zero. This

provides the opportunity for interrupting the arc.

Dielectric strength. Dielectric strength is the ability of an insulating medium to withstand agiven voltage over a given distance without conducting. As previously mentioned, circuit

breakers utilize different interrupting media of varying dielectric strengths. The dielectric

strength of insulating oil is many times greater than that of air at atmospheric pressure; however,the dielectric strength of air (as well as other gases) increases when pressurized. The dielectric

strength of a hard vacuum also exceeds the dielectric strength of air at atmospheric pressure.

When, at a current zero, a circuit breaker attempts to interrupt either load or fault current, a

voltage is generated across the open contacts of the circuit breaker to oppose this change incurrent. This voltage, the transient recovery voltage (TRV), is equal to the difference in the

voltages on the load side and source side of the circuit breaker after the breaker contacts have

parted and the current interrupted. The wave shape and magnitudes of these voltages depend onthe system configuration both before and immediately after the contacts open and the current

ceased.

The arc is stretched as the contacts continue to open. The stretching and cooling enables the arcto be quenched at current zero. As the contacts continue to move (open), the arc extinguishes at

each current zero crossing. The arc will remain extinguished if the improving dielectric strength

of the medium between the contacts is greater than the rising voltage across the open contacts. Ifthe dielectric strength across the contacts is not sufficient to withstand this voltage, a re-ignition

will occur and the arc will be re-established. At the next zero crossing, the arc will again

extinguish and, dependant on the type of circuit breaker, this process will continue until thedielectric strength necessary to withstand the voltage across the distance between the parted

contacts is re-established. Modern designs interrupt within two, sometimes three, zero crossings

following the contacts parting and they have no capability beyond this.

The heart of the circuit breaker is the interrupter. When the circuit breaker contacts open, theinterrupter interrupts the path of the resulting arc and directs the interrupting medium

(oil/air/gas) to cool and replace the arc column. In this way it extinguishes the arc at a current

zero after contact separation.

1.3 Operating MechanismsOperating mechanisms provide the energy to enable the interrupter to perform the mechanicalclosing and opening, and hence the electrical making and breaking, function of circuit breakers.

On some designs, energy from the closing operation is stored in the mechanism for the next

opening operation, such as charging opening springs during the closing operation. Other designsmake use of stored energy from a single source for opening as well as closing, such as

compressed air vessels, nitrogen accumulators or spring devices.

Method used for providing the stored energy to close or open circuit breakers includes electricmotors, solenoids, springs, pneumatic drives and hydraulics. A combination of these methods is

commonly used, with one method used for closing and another used for opening. The most

commonly used opening method is spring.

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Some mechanisms are characterized as being trip-free. The trip-free characteristic requires the

circuit breaker to open at any instant that a trip command is issued to the unit, even if the circuitbreaker is in the process of closing. To achieve this, the mechanism, interrupters and drive

system must be able to withstand the forces of the sudden change of direction. In other cases a

circuit breaker must close before it opens.

Circuit breaker control circuitry contains a feature termed antipumping. This antipumping

characteristic signifies that the circuit breaker will not repeatedly open and close if the electrical

open and close commands are applied and maintained to the circuit breaker simultaneously. Thisprevention is usually achieved within the control circuitry by requiring that the electrical close

command be removed before the unit can be closed a second time.

1.3.1 Solenoid MechanismsIn the solenoid type mechanism, a solenoid supplies the energy to close the circuit breaker. A

spring, which is charged during the closing operation, is used to open the unit. The closingsolenoid potential is supplied from either the station battery or by station ac rectified voltage.The closing and opening times of circuit breakers with this type of mechanism are quite slow,

with closing times as long as 40 cycles.

This type of mechanism is the oldest and simplest, but due to its relatively slow closing times it

has been largely replaced with one of the other types. It is a typical mechanism type for the

earlier designs of bulk oil and air magnetic circuit breakers, especially at lower system voltages.

1.3.2 Motor MechanismsSome bulk oil breaker types had motor closing mechanisms. These mechanisms used systems

with weights and clutches to control the closing action. Such mechanisms have not been

produced during the last few decades.

Improvements in electronics and electric motor controls, however, have now made motormechanisms attractive again. The newest motor mechanisms use motor drive for closing as well

as opening, with energy for the motor supplied by capacitors.

1.3.3 Spring MechanismsIn the spring type of mechanism, the energy to close the circuit breaker is stored in a largespring, which is usually compressed, but on some designs may be extended, by an electric motor

immediately following each close operation. A smaller spring, which is charged during the

closing operation, is used to open (trip) the breaker. This type of mechanism provides fasteroperating times than solenoid mechanisms, but has duty cycle limitations (one open-close-open

cycle) due to the lack of energy storage. The motor that provides the force to charge the closing

spring is usually a low power, single phase, ac motor, although dc motors are available. This typeof mechanism is typical of the earlier designs of bulk oil circuit breakers as well as in minimum

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oil, air magnetic, SF6 single pressure and vacuum circuit breakers. Spring mechanisms are now

widely used.

1.3.4 Pneumatic MechanismsA pneumatic mechanism uses compressed air for the energy source to close, and dependant onthe type, to open the circuit breaker as well. The mechanism is supplied with air from an air

storage tank. This tank is the energy storage reservoir and is charged by the compressed air

supplied from either a local air compressor or from a substation (switchyard) centrally locatedcompressed air system. The reservoir normally contains enough stored air to complete several

successful close open cycle operations. In one common design, to close the circuit breaker,

pressurized air is directed under the mechanisms main piston by means of a close control valve

(which is solenoid operated). Dependant on the design, the circuit breaker may be openedpneumatically (air-blast and some SF6 types) or by a spring that is charged during the closing

operation (bulk oil types). Circuit breakers equipped with a pneumatic mechanism have the

ability to open and close rapidly, resulting in interrupting times of 3 to 5 cycles for bulk oil types(depending on the circuit breaker type). Air blast and SF6 types are faster. Typical air system

operating pressures range from those for mechanisms for bulk oil and two-pressure SF6 circuit

breakers that are at 1.03 to 2.76 MPa (150 to 400 psig), up to 3 MPa to 9 MPa (450 to 1200 psig)range for air-blast circuit breakers. Where used for the early designs of single-pressure SF6, the

operating pressure is typically from 2.0 to 3.0 MPa (300 to 450 psig). See Figure 1 for a

simplified flow diagram. Air blast breaker operating mechanisms are an integral part of the

breaker. Each manufacturer uses a design unique to the specific circuit breaker type. There aretwo basic air blast concepts, and the mechanism arrangement is different for both.

The early designs are used up to 300 kV and were installed into the early 1960s, mainly fromEuropean manufacturers. These designs have series interrupters mounted on insulating supports.

These interrupters are forced open by a blast of high-pressure air from the air receiver via a blastvalve. The blast-valve is housed between the air-receiver tank and the base of each support

insulator. A pilot valve, itself operated by the opening coil, initiates the blast valve operation.

When these interrupters are open, a separate air motor is operated to open a switch arm that,when fully open, provides electrical isolation (although, because it cannot be locked open it is

not usually used as a disconnector {disconnect switch}). When this is complete the blast valve is

shut off and the interrupters are returned to the closed position by small springs incorporated intoeach set of contacts. The circuit breaker is closed by operation of the switch arm air motor only.

This drives the switch-arm closed and as the interrupters are already closed, this action makes the

electrical circuit. It is a rapid operation and the arm and contacts are capable of making the rated

short circuit current.

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Figure 1 Diagram of a pneumatic system

Later designs, those developed and installed from the early to mid 1960s until the late 1970swhen the single pressure SF6 designs became available, had a variety of interrupting techniques.

Generally these operated by a mechanism moving a control rod system to operate a control valve

mounted at each interrupter. On the larger 420 kV designs this could involve a single mechanism

and 36 interrupters.

The later designs of air-blast circuit breaker operation are initiated by the energization of theappropriate open/trip or close coil. Energizing the trip coil pushes the pilot valve, which allows

high-pressure air to activate the control valve. The control valve then lets the pressurized air

move the actuating piston. In turn, the piston pulls or pushes the insulated operating rod (via a

series of linkages, rods, and cranks) to operate the closing or opening valves in the interrupters.

A number of variations exist dependent on whether the air is used in the mechanism to move the

control rods to close and open, or just close with the opening action being derived from a springfollowing de-latching of the control rods by a trip coil and pilot valve system.

Multiple operations are usually possible without recharging the local air system receiver (tank).As can be seen, the ways in which the interrupters are operated and mechanism are used to close

and/or open them, are as varied as the circuit breakers themselves. It is interesting to note that

not all air-blast circuit breakers used pneumatic mechanisms. One US manufacturer produced alimited number of air blast breakers with a mechanism that utilized energy in a charged spring

for the closing operation.

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1.3.5 Hydraulic MechanismsHydraulic mechanisms act in a manner that is similar to the later air-blast and other pneumaticdesigns. The circuit breaker is closed by the hydraulic system. On bulk oil and double-pressure

SF6 circuit breaker types the interrupters are usually opened by a spring. Where used on single

pressure SF6, both closing and opening is by the hydraulic system. In all types, the hydraulicsystem utilizes an energy store within an accumulator. Here the pressure on the hydraulic oil is

maintained by compressing nitrogen to 20.7 to 34.5 MPa (3000 to 5000 psig) or by compressing

a spring mounted behind a piston. On some designs the nitrogen is contained within a bag held

within the accumulator (as illustrated in Figure 2), in others, the accumulator is divided by a free-piston that separates the oil from the nitrogen. This piston is free to move with the changing

pressure conditions within the accumulator. These mechanisms are capable of providing the

circuit breaker with very short interrupting times. As with pneumatic mechanisms, sufficientenergy can be stored to allow multiple open-close cycles without the pump running. See Figure 2

for a sample flow diagram.

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Figure 2 Diagram of a hydraulic system

1.4 Insulating MediaThe insulating medium in a circuit breaker may be oil, air, SF6 gas or vacuum. All media types

may perform the functions of insulation and arc interruption.

Figure 3 shows the relative dielectric strengths of oil, air, and SF6.

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Figure 3 Relative dielectric strengths of oil, air, and SF6 for a 1-cm gap with optimum

electrodes at 0C

1.4.1 OilOil in circuit breakers may serve two purposes. It may be used as insulation between phases and

between the phases and ground. It may also be used for arc extinguishing.

The principle behind oil circuit breakers relies on the fact that an electric arc developed acrosscontacts immersed in oil causes the oil to decompose and release hydrogen gas. Hydrogen is

known to be an excellent arc-extinguishing medium and has excellent dielectric properties. Inaddition, hydrogen rises rapidly, drawing in fresh, cool, oil from the main tank into the arcing

zone.

The main disadvantage of oil is its flammability, and the maintenance necessary to keep oil in

good condition.

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is practically free from contamination. However, in the factory some contamination may be

introduced to the circuit breaker enclosure during the preparation of gas-filled components forshipment. To minimize contamination, SF6 handling procedures, usually prepared by the

manufacturer, must be followed, not only in the factory but also on site whenever access is

necessary for installation and maintenance purposes. There are five principal contaminants that

must be identified and reduced or eliminated: Particles, moisture, oil contamination, gaseouscontamination, and arc-decomposition products.

1.4.3.5 Moisture in SF6

Commercially available SF6 gas has a very low moisture content, less than 40 parts per millionby volume (ppmv). But unless the circuit breaker is thoroughly evacuated before filling with SF6

gas, water molecules adhering to the solid surfaces inside the system will diffuse into the gas. A

low level of moisture does not degrade the dielectric strength of the gas. However, at about 50%

relative humidity, enough moisture is absorbed on the surface of the spacer insulators to decreaseflashover voltage slightly. At over 90% relative humidity, a flashover across the surface of the

insulators is almost certain to occur at operating voltage.

Normally, gas-insulated systems are evacuated to about 26.7 Pa (0.2 mm Hg) before filling with

SF6, and then checked for moisture content a few days thereafter. The likelihood of excessivemoisture in SF6 systems is very low. It should be recognized that the relative humidity will

change with variations in temperature and pressure. The moisture content of the gas is higher in

summer when ambient temperature is high, and lower in winter when more moisture adheres tosolid surfaces. The acceptable moisture level is normally such that this moisture will become a

frost rather than a liquid at the condensation temperature (frost-point).

1.4.3.6 Particles

Particles are a particular problem in metal-enclosed designs where the gas forms the insulatingmedium to earth in a highly stressed arrangement. Obviously any metallic and other clearlyconducting particles are likely to cause problems where they contaminate solid insulation. Where

such particles are in the form of light, long (25mm or 1 inch) slivers of materials such as

aluminum then they can be lifted into the highly stressed gas gap by the electric field and cause a

flash-over. This can also happen, more vigorously in fact, with similar sized plastic shavings,especially where they themselves have attracted some conducting dust.

1.4.3.7 SF6 decomposition products

SF6 is chemically inert up to 150C and will not attack metals, plastics, and other substances

commonly used in the construction of high-voltage circuit breaker components. However, at thehigh temperature caused by power arcs, it decomposes into various components, which are

principally SF4 and SF2, together with small amounts of S2, F2, S, F, etc., which are in part

corrosive to both glass and metals in the presence of moisture. The substances formed by thecombination of such elements with vaporized metals appear as a whitish powder that has good

insulating properties. The breaker contacts are designed with a wiping action to ensure self-

cleaning of the contacts current-carrying surfaces.

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1.4.3.8 SF6 gas systems

The SF6 gas system has two functions. First, it provides the dielectric strength that is necessary to

prevent flashovers between areas of differing voltage potential, whether during normal in-serviceuse, or during any system switching requirements or other system disturbance. Second, it is used

to interrupt the arc that occurs during opening operations. The SF6 within the circuit breaker

must contain only a minimal amount of moisture, because of the requirement of the breaker tomaintain proper dielectric strength. In most breakers the SF6 gas system consists of a filter

system that removes moisture, oil, gaseous and solid arc decomposition products. A compressor

is used to charge the high-pressure system. Various devices are used for adjusting and

maintaining proper gas pressure, pressure alarms and gauges, and heaters. Contamination by airwill reduce the dielectric and arc-quenching capabilities of the gas and will also introduce

oxygen, which may promote oxidization degradation.

1.4.4 VacuumVacuum was seen as ideal for arc extinguishing as far back as the 1920s. Reliable vacuumtechnology was developed in the 1970s and has become more generally accepted since the1980s. Vacuum breakers are now used extensively in distribution stations, up to 35 kV

applications.

Vacuum interrupter contacts are normally housed inside a ceramic container. The moving

contact requires only a very small stroke, and thus low energy input for mechanical operationcompared with other methods used for current interruption.

1.5 Bulk Oil Circuit Breakers1.5.1 Introduction

The use of oil as an interrupter medium has been common since the first application of circuit

breakers. During the early part of this century, oil breaker design was refined and, especially in

North America, quickly became the dominant circuit breaker for many years. In Europe andelsewhere it was in competition with air-blast and later minimum oil designs.

Bulk oil breakers up to 69 kV have mostly had all three phases housed in a single tank. At higher

voltages, each phase has normally had one tank per phase. See Figure 4. Bulk oil circuit breakers

have been commonly used at voltages up to 230 kV, and on some systems up to 345 kV.

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Bulk oil circuit breakers, both spring open. One tank per phase on the left. One tank

for all three phases on the right.

230 kV, one tank per phase, pneumatic close 25 kV, single tank, solenoid close

Figure 4 Typical bulk oil breakers

This figure includes a silica gel drier on the breaker tank vent. In some more damp and humid

locations this drier has been found to help minimize moisture accumulation; however, for most

applications this has not been necessary.

1.5.2 Interrupter FunctionsFigure 5is an illustration of typical bulk oil breaker components. As with all circuit breakers, theinterrupter is a critical part of the oil circuit breaker. The most common method of interruptionused in oil circuit breakers, shown in Figure 5, is called by several names, e.g., cross-blast or oilblast interrupter. In these designs the arc is drawn in front of a series of lateral vents often calledthe grid assembly. The heat of the arc vaporizes the oil in the assembly and the gases (mainlyhydrogen) form a bubble that increases the pressure against the arc, finally forcing it to be blowninto the grid vents.

When the pressure inside the interrupter becomes sufficiently high and the length of the arc is

adequately extended at current zero, the arc is extinguished.

The arc is always confined inside a bubble of gas formed from the oil, and this bubble extends

and expands through the grid vents and the surrounding shell vents to the outside of the two ormore interrupter assemblies in each pole (phase). The hot gases emerging from the vents are

initially still ionized. It is essential to ensure, by correct grid design that dielectric breakdowns do

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not occur between the outer vents of the shell system external to the interrupter assemblies.

Preventing dielectric breakdowns is particularly important for higher voltage interrupters wheremultiple series grid arrangements are used. It is equally important that the shell vents in the same

pole (phase) tank face away from each other.

At the time the arc is being extinguished, fresh oil is drawn into the interrupter grid assembly toreplace the arc-affected oil and thus cooling the arc zone and restoring the dielectric integrity of

the system.

One phase of typical bulk oil breaker(dead tank, outdoor)

Typical bulk oil breaker interrupter

Figure 5 Bulk oil circuit Breaker components and interrupter illustration

1.5.3 Internal Tank InsulationInternal insulation consists of the oil, lift rod, lift rod guides, support members, tank liner, and

inter-phase barriers. Some breaker types do not have tank liners. Contamination and/ordeterioration of these components reduce the circuit breakers reliability by reducing its overalldielectric and mechanical strength.

With the circuit breaker in the closed position, the lift rod provides insulation between the

energized crosshead (conductor) and the grounded mechanism. The oil and the tank liner provide

insulation between the energized conductors and the grounded tank. Inter-phase barriers providephase-to-phase insulation in some single-tank breakers.

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1.6 Minimum Oil Circuit Breakers1.6.1 Introduction

In bulk oil breakers, the oil serves as the main insulating medium as well as to extinguish the arc.

The minimum oil breakers were developed to reduce the oil volume primarily to the amount

needed to extinguish the arc. Arc extinction takes place inside a tube made of insulating material.

Minimum oil (also termed small oil volume) circuit breakers have been used at voltages up to230 kV. They were extensively developed during the 1960s as an alternative to the bulk oil type,

with its large oil volume, and the air-blast, with its need for expensive compressed air plant.

They also competed with the then new technology of the two-pressure SF6 types. Althoughwidely used at distribution as well as transmission voltages, some were found to be unreliable in

service, particularly when switching capacitive currents. Those designs in service that are sound

have given good service and refurbishment programs exist.

Minimum oil breakers at distribution voltages have been used for indoor as well as outdoor

applications, usually with spring mechanisms. For higher voltage applications, hydraulic as wellas spring mechanisms were commonly used. See illustrations of typical transmission type

minimum oil breaker in Figure 6.

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230 kV, Y-type configuration, four

interrupters per phase, spring mechanism

Typical Y-type interrupter

configuration

Figure 6 Typical live tank minimum oil breaker and interrupter arrangement

1.6.2 Interrupter functionsThe arc control for minimum oil breakers is based on the same principle as for bulk oil breakers.

To improve performance, oil may be injected or pumped into the interrupter to quench the arc.

The used oil is retained within the interrupter zone limiting the number of the short-circuitclearances possible before oil maintenance/overhaul is required. The arc is quenched in a similar

manner to that of the bulk oil design but is cooled by oil is forced into the arcing chamber,

usually insulation with slots, by a pumping action derived from the opening movement of the

interrupter contact drive shaft.

In transmission type live tank breakers, the interrupter is normally housed inside a porcelain

enclosure, as a single vertical arrangement per pole (phase) on top of the mechanism, or in a T orV type configuration with multiple interrupters (or breaks). In some cases the supporting

insulator column is replaced by a current transformer.

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1.7 Air Magnetic Circuit Breakers1.7.1 Introduction

Air magnetic breakers were used primarily as part of indoor metal clad switchgear, but also in

indoor free standing applications.

1.7.2 Interrupter functionsAir magnetic circuit breakers use atmospheric air to extinguish the arc by stretching it until the

dielectric strength of the gap is larger than the voltage across the gap. The longer arc has a largercooling surface, thus cooling increases and deionization of the gap between the contacts

improves. The increased length of the arc also increases its resistance and therefore decreases the

current flow and the amount of heat that is created. To increase the length of the arc, the arc isstretched by forcing it into an arc chute by either a natural convection of the hot gas, by blowing

air from below into the arc, or by using magnetic blowout coil. The magnetic blowout coil

creates magnetic force on the arc that pulls the arc into the arc chute.

One type of arc chute is made of insulating material. Its function is to stretch the arc. Another

type is made of metal. The metal barriers chop the arc into a series of many smaller arcs. The

voltages across these smaller arcs are much lower than the total voltage across the breakercontacts. This makes it easier to extinguish the arcs.

1.8 Air Blast Circuit Breakers1.8.1 Introduction

Air-blast breakers were being developed in parallel with oil breakers, mainly in Europe duringthe 1940s and 1950s when oil was scarce. During the 1950s they were installed in many parts of

the world in competition with the oil designs. There are two basic design types and for the

purpose of this document they will be termed the early and the later, or pressurized-head, types.It is the later design that formed the basis of many of the international grid systems of the mid

1960s when system voltages of 400 kV and higher and up to 4000A continuous current and 63kA short circuit were required. Due to these ever-increasing power-system voltages, the physical

size limitations of oil breakers, coupled with the large quantities of insulating oil that would be

required at the higher voltages, the use of oil circuit breakers became unrealistic. Development ofthe existing air-blast technology was necessary. Ultimately these pressurized-head later design-

types of air-blast circuit breakers ranged in voltage class from 115 to 800 kV and in interrupting

rating from 40,000 to 80,000 amperes. These air-blast circuit breakers have extremely rapid

interrupting times, typically opening the main contacts within 2 cycles (40ms at 50 Hz or 33.3ms at 60 Hz) from trip initiation.

During opening operations, the early designs direct a blast of high-pressure air from a ground-mounted receiver-tank to an interrupter assembly mounted on support columns. These circuit

breakers are of the live tank type. The term live-tank means that, with the breaker energized,

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the chambers containing the interrupting apparatus (the heads) will be at the potential of the

systems voltage. With the interrupters open, a separate switching arm is then rapidly opened andthe interrupters re-close as the blast is shut off. The interrupters are not permanently pressurized.

Even though various designs exist, the later, pressurized-head, air-blast circuit breakers are also

of the live-tank type. On these design-types the high-pressure air is used for electrical

insulation and arc extinguishing purposes, hence the term pressurized-head type.

Transmission type air-blast circuit breakers use compressed air for insulation as well as theinterruption and mechanical operation. For these designs a much drier air is required. This is

achieved in two stages, with a higher storage than usage pressure, but in addition, the high-

pressure air is dried before storage.

1.8.2 Interruption1.8.2.1 Early design-types

As described above, a blast of high-pressure dry air is directed onto the interrupters. This high-

pressure air is directed up a tube housed within the interrupter unit support column. It forces the

interrupters open and quenches the arc at the appropriate current zero, simultaneously an air-motor is used to open a separate switch-arm to provide an open atmospheric air gap and enable

the interrupters to be returned to the closed position when the air-blast is shut off. This switch-

arm provides the open condition. To close the circuit breaker the air-motor is driven back to

drive the switch-arm into the closed position and so make the circuit.

These distribution type air blast breakers were used primarily in indoor in metal clad enclosures

or in free standing applications with open bus work. A variety of ratings were available, usuallyup to 25 kV. High load current and interrupting ratings were available, making these breakers

well suited as generator breakers as well as distribution or industrial breakers. A typical

configuration for distribution bus breaker application is shown in Figure 7.

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25 kV bus breaker, indoor

Figure 7 Typical air blast breaker for distribution voltages

1.8.2.2 Later Design Types - pressurized head type

These air-blast circuit breakers utilize dry air under pressure to quench the arc that is formedduring an opening operation. This air is stored around the contacts within one of the numerous

series heads that make up a pole (phase) of the circuit breaker.( See Figure 8) As the contacts

separate and the current attempts to maintain its flow, an arc is formed. However, by design, thearc is directed in a designated course. With precision timing, a valve within the interrupting

chamber opens, allowing some of the air contained within the breaker to exhaust to atmosphere

directly through the path of the arc. At the opening of the blast valve, the interior of the breakerrapidly becomes somewhat depressurized. This depressurization results in a blast of air that cools

the arc, forcing it away from the parting contacts and out the arc chutes or arcing tubes. The arc

is eliminated by being elongated and cooled beyond its ability to maintain itself. With adequatedielectric strength between the open contacts, the exhaust valve is closed and the head re-

pressurized from the local receiver.

On all air-blast circuit breakers, the interrupter heads are modular. Normally a particularinterrupting head can be transferred from one position or breaker to another position or breaker if

the current-carrying capacity, interrupting capability, and the accessory equipment are the same.

The move could be successfully accomplished even if accessories are added, removed, orchanged to meet the requirements of the new position, as long as the ratings are the same at both

locations. Because of the modular design, all that is required of the manufacturer to increase the

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voltage of a particular type of breaker is to add the modular interrupter heads in series, within a

phase, to give the breaker the desired capability. Of course, the insulation level from phase toground has to be increased as well. Therefore, the height of the interrupter support columns and

drive rods must be increased. The added height is required due to the basic insulation level

necessitated by the increased voltage. Interrupter support columns are hollow ceramic insulators

of high mechanical (as well as electrical) strength. The interrupter operating rods pass throughthe opening of the center of the support column. In some cases this contains high-pressure air, in

others the high-pressure air is within a separate tube itself housed within the support column. The

zone between the tube and the column is kept dry by a low pressure conditioning air system. Inone manufacturers design, the space is filled with SF6 gas, as electrical insulation. As an

example of the difference in stack height, a typical 800-kV breaker reaches 12.5 meters (41 feet)

from ground level to the top of the interrupter (and depending upon the version of the breaker,uses either four or five interrupters per phase). In contrast, a 138-kV breaker of the same type is

less than 6 meters (20 feet) to the top (and has only one interrupter per phase, but of the same

basic type as the 800-kV breaker).

550 kV, T-configuration, internal

grading capacitors and closing resistors

230 kV, 63 kA, Y-configuration,

external grading capacitors

Figure 8 Typical air last breakers for transmission voltages, live, pressurized tanks

1.8.3 Interrupter TypesThere are two ways in which a blast of compressed air can be directed onto an arc: transversely

(at right angles) to act as a cross-blast, or longitudinally along the arcs length as an axial blast.

The cross-blast method has generally been found to be unsuitable for high-power, high-voltageapplications. Accordingly, all modern air-blast interrupters employ the axial blast principle by

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The exhaust passage(s) downstream are controlled by exhaust ports or valves. Each of these

arrangements must admit compressed air to the nozzles while the exhaust passages are open, andthen shut off the air supply to prevent the pressure reservoirs from being exhausted.

This minimum requirement can be met in all cases by blast valves placed in the duct leading to

the nozzles as shown, or across the electrodes just upstream from the nozzles. In the former case,one blast valve may be arranged to serve a number of interrupters at the same time and the

interrupter chambers are pressurized outside the interrupter. In the latter case, one blast valve can

serve only one interrupter and the interrupter chambers are permanently pressurized up to thevalve.

Where the interrupters are pressurized in the open position only, both the blast valves and theexhaust valves are used. The former admit the air to the nozzles, the latter stop the flow and keep

the interrupter pressurized. One blast valve may again supply more than one interrupter at a time

and one exhaust valve may be arranged to control two adjacent exhaust passages in a twininterrupter unit.

Where the interrupter chambers are permanently pressurized (i.e., in closed and open positions),

exhaust valves are used. The exhaust valves are used either on their own or in combination withblast valves placed across the electrodes. One exhaust valve may again serve two interrupters,

but separate blast valves must, in this case, be provided for each interrupter.

In so far as nozzle systems and pressurization are concerned, air-blast interrupters can be divided

into nine types, i.e., mono- or partial-duo- or duo-blast, each pressurized in one of the three

ways:

During interruption only

During interruption and in the open position

Permanently.

Two other terms, axial flow and radial flow, are sometimes used in descriptions or classification

of air-blast interrupters. Axial flow usually refers to interrupter constructions where, due to anarrangement of the upstream passages, the predominant direction of the air flow approaching the

nozzles is parallel to the axis of the interrupter as in Figure 9, Detail A. Radial flow usually

refers to constructions where most of the air tends to approach the nozzles centripetally asbetween the duo-blast electrodes in Figure 9, Detail B, or as would be the case if the mono-blast

electrodes in Figure 9, Detail A, were placed in the center of an air receiver. Neither of these

terms invalidates the axial-blast principle.

In circuit breakers that have both high-pressure air and SF6 gas separated by gaskets, there can be

a leakage of high-pressure air into the SF6 gas space. The high-pressure air contains far greater

amounts of moisture than the SF6 gas spaces are intended to contain. Therefore, leakage of airinto these spaces can set up the potential for a catastrophic failure. In some designs such leaks

are from seals that are difficult to replace under normal maintenance. In such cases this leaking

seal may be considered in the later section on Condition Assessment as a life-limiting-factor

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because it can be expected to involve major dismantling to correct the seal, the disruption and

cost may indicate that replacement is more sensible.

Further, with any air-blast circuit breaker, moist air entry into dry air chambers will degrade the

insulating quality and arc-quenching capability. Wet air can cause flashovers, re-strikes, or a

slow deterioration of insulated parts within the circuit breaker. If slow deterioration occurs andplastics are involved, there may be corrosive gases formed that will attack copper, aluminum, or

silver-plated surfaces. This corrosion can include contacts, valves, seating surfaces, and all

interior parts.

1.8.4 Auxiliary Interrupter ComponentsAccessories utilized for an interrupter also are dependent upon the voltage at which the breaker

is designed to operate, as well as the individual usage of the breaker. Following are descriptions

of interrupter accessories that may or may not be part of a breaker, depending upon itsapplication.

Grading Capacitors and Resistors

Grading capacitors can be used in parallel with the interrupter contacts to provide nearly uniform

voltage distribution of all contacts within a phase. This grading effect prevents the contacts

nearest the end or outside of a phase from being subjected to the majority of the duty while theinnermost contacts see a reduced amount of burden. Each manufacturers capacitor design is

different, some are inside the interrupter chamber and some are bolted on externally. Normally,

capacitors used internally have relatively low voltage withstand. These internal capacitors shouldnot be exposed to extended periods of energization with the breakers main contacts open and

with potential across the capacitor. When open, the circuit breakers associated disconnect

switches (disconnector) should be opened to protect the capacitors. Grading capacitors designedfor mounting external to the interrupter often have greater electrical strength; opening the

disconnects (disconnectors) may not be a requirement for capacitor protection.

On a breaker with several interrupters per phase, the capacitor values may differ between breaks,

dependant on the design. It is important that proper interrupter assembly includes an orderedplacement of capacitors. Normally, higher capacitance values are placed on the outside ends of

the interrupter breaks of a phase, decreasing in value toward the center of the phase. Improper

capacitor association may cause trouble and ultimately could cause failure of the circuit breaker.

Grading resistors are usually only found on the earlier designs of air-blast circuit breaker. Theyhave very high resistance and act in the same way as grading capacitors by sharing the voltage

across the series interrupters of the circuit breaker to make it as near equal across each gap aspossible. These resistors also cause a phase shift, easing the opening duty for the arc contacts.These resistors should not be confused with the opening (tripping) resistors used on the later

designs of air-blast (pressurized-head) circuit breakers.

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Opening Resistors

Opening resistors, or tripping resistors, are used to distribute the voltage during interruptions of

high short-circuit current and to dampen oscillations created by a breaking operation under the

specific short line fault condition. The resistor-switches that insert and remove the resistors aretimed to open their contacts a few milliseconds after the main contacts have parted. They are

usually re-closed with, or just before, the main contacts during a closing operation, although a

few designs close them early during the opening (tripping) operation. The value of these openingresistors is chosen to match the surge impedance of the line construction.

Closing Resistors

Closing resistors are normally utilized on breakers associated with long transmission lines. Theresistors are used to dampen the over voltage transients that occur when energizing long,

unloaded transmission lines. The closing resistors are inserted just prior to main contact closure

and the value is chosen to match the line length and construction. They may also be rated towithstand auto re-close operation.

Mufflers (Silencers)

During the 1970s it became necessary for manufacturers to develop sound mufflers (silencers) tolessen the audible impact of the air blast. Depending upon the circuit breaker configuration, one

or more mufflers (silencers) are added to an interrupter in order to reduce the operational noise to

a reasonable level.

Current Transformers

Current transformers (CTs) associated with air-blast circuit breakers are, like the breakers

themselves, quite varied in design. CTs are used for relaying purposes and some contain awinding with metering accuracy. Many CTs are free standing and are not integral to the circuit

breaker. Current transformers are located at the end of an interrupter head or string of interrupter

heads, and normally one is required for each phase. There are also circuit breakers that havemultiple current transformers per phase. The early air-blast designs integrated the CT into the

design as the outgoing post-support for the switch-arm of the design-type.

Freestanding current transformers can be utilized at any air-blast breaker location. Current

transformers of different manufacturers or types can be mixed on different phases of the samebreaker if properly applied. Many freestanding current transformers are filled with insulating oil.

Porcelain is normally used as chambers to contain the oil and to pass the primary leads through.

However, some are filled with SF6 gas and may utilize a composite application of plastics andsilicone for the material to house the SF6 gas insulation. Where these devices were integral with

the circuit breaker they used the porcelain SF6 chamber as an interrupter support column, as well

as the chamber through which the primary leads passed. Because these CTs are part of the circuitbreaker, they do not lend themselves well to relocation and they complicate the refurbish/replace

debate.

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1.8.5 Air Compressor systemsFor these circuit breakers the air is used for operation, interruption and insulation purposes. The

air used for interruption and insulation within the circuit breaker must contain only a relativelysmall amount of moisture, because of the requirement of the breaker to maintain proper dielectric

strength. The dielectric strength of the air is necessary to prevent flashover between areas of

differing voltage potential, whether during normal in-service use, or during any system switchingrequirements.

Compression of air from atmospheric pressure is a natural method of moisture removal, as air

under compression freely gives up its water molecules. The greater the amount of compression,the more moisture removed. The typical operating pressures of the early, non-pressurized designs

were of the order of 2.5 MPa (360psig) whilst the later pressurized-head designs operate at up to

8.0 MPa (1200 psig). The air for all air-blast circuit breakers is compressed to a pressure higherthan the operating pressure and stored either locally at the circuit breaker or in a central location

for the whole substation. The air storage pressure for both basic air-blast types ranges from 4

MPa (600 psig) to 21 MPa (3000 psig) and higher. Most of the earlier air-blast breakers

depended solely upon compression for moisture removal, but even under these compressionpressures, moisture remained and requiring draining from the storage where it condensed on

cooling. This air is suitable for the early designs where the air is used for interruption from ablast valve mounted in the base receiver/tank. Air for insulation is at a much reduced pressure of

the order of 0.1 MPa (15 psig) and hence much dryer.

Moist air could not be permitted to enter the interrupting portions of the later pressurized-headcircuit breakers, as in this case the compressed air is the insulating medium to earth (ground) and

between the open contacts. In order for more complete moisture removal, dryers were developed.

Earlier dryers passed the compressed air through a drying agent of silica gel that, after acceptingthe allotted amount of moisture, could be regenerated by heating the dryer and back flowing

previously dried air at a very low flow rate. Later dryers utilized a molecular sieve as a dryingagent, which did not require heat; only the backflow of dry air was required to complete the

regeneration process. These dryers were normally installed between the compressors and the air-

storage tanks (receivers). The storage air is then required to be reduced to the pressure at whichthe breaker operates. Each design has its particular requirements. Some operate at pressures in

excess of 3.5 MPa (500 psig), some as low as 2.5 MPa (360 psig). The pressure reduction takes

place in one or two stages, again depending on the manufacturers design.

For operating mechanisms the quality of the air is less important and need only be dry enough to

prevent internal corrosion of the various valve components and more importantly to prevent frostdamage during winter operations.

In the basic arrangement these air systems are often no more than a compressor mounted at or onthe individual circuit breakers with a local air receiver (tank) to store sufficient air for the

number of multiple operations specified and for the size and consumption of the circuit breaker

mechanism.

Other systems employ air compressors at a central substation location, usually two or three with

a greater delivery capability (depending upon the needs of the user), situated in a location

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accessible enough to supply all the breakers in a particular substation yard. These central systems

are normally contained within a house, either an existing house modified for that purpose orwithin a house specifically designed for the purpose.

There is also the hybrid system, which has one or more banks of two compressors, each of which

supplies a small number of circuit breakers. These compressors are not normally containedwithin a house but are often in weatherproof cabinets. The decision to use one system over the

others is governed by the philosophy of the power systems management.

Air compressor systems for air-blast circuit breakers offer an array of differing types and styles.

As stated above, some breakers were sold with an air compressor as part of the circuit breakerpackage, that is, one compressor for each circuit breaker. For these the maintenance activity is

closely linked to that of the circuit breaker. For the centrally located large compressors a pattern

of maintenance is linked to the needs of the individual machine and is generally not linked to the

restrictions of power-system access.

For the many types of circuit breaker using pneumatic mechanisms, and the air-blast types, the

compressor is one of the most important components to be considered for circuit breakermaintenance, but also for consideration during life estimation and the refurbish/replace debate

linked to life extension. Most compressors require a dedicated maintenance regime in order to

ensure a reliable air system and hence reliable circuit breaker operation. Generally, as with otherplant, compressors should be maintained as recommended by the compressor manufacturer,

keeping in mind that the duty requirements and the ambient environment will have an effect on

the frequency of that maintenance and system access may be a restriction.

Air leaks in the associated valves, piping, pressure switches, and gauges are an occasional

problem and should be corrected as soon as possible after detection. If not repaired, the leakscould cause excessive compressor run time, and hence additional maintenance, but more

importantly, an excessive leak on an air system on a circuit breaker could possibly lead to the

failure of the breaker to close or open correctly when required. At the very least leaks on theselocal systems may force the need for an additional maintenance power-system access to repair it,

depending on its location.

1.8.6 Trouble and Failure ModesThe main cause for concern in air-blast breakers is leakage of high-pressure air to atmosphere.

Problems caused by unrepaired leakage of air are as follows:

Moisture entry into the breaker. Moisture laden, atmospheric air is drawn in as thepressurized air leaks out. The leakage creates a venturi effect.

Wire drawing of metallic seating and sealing surfaces. Wire drawing is created bycontinuous passage of high-pressure air over a very small area, which cuts a groove into

the metal that can never be effectively sealed.

Excessive compressor run time. If large leaks, or a multitude of small leaks, are leftunrepaired then not only can they become larger and cause damage to the seal mating

surfaces but, just to keep pace with the leaks, a compressor can run enough hours to

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drastically reduce the time between maintenance periods and prematurely shorten the

expected life of the machine. In addition, compressor parts and accessories will wear outsooner than desired, potentially causing additional down time. Even the pressure

reduction fill valve will be adversely affected when called upon to operate excessively.

System integrity. Continued air leakage from an air-blast breaker will eventually affect

the availability of the breaker, requiring unplanned maintenance to be performedsometimes at the worst possible times, and often at great cost. These costs could includeloss of revenue at a time when the circuit is most needed.

Specific leakage problem. Some design variants of air-blast circuit breaker use SF6 gas inthe support insulation whilst others use a very low-pressure air system (0.02 MPa or

30psig). This air is very dry as it is obtained from a further reduction of pressure from the

main circuit breaker air system. The purpose of this air and the SF6 is to ensure that the

internal volumes of those zones of the circuit breaker not subject to high-pressure air aredry and able to provide an adequate dielectric capability. Leakage of the high-pressure air

into these zones can occur, causing disruption of their sealing arrangements with a

degrading of the dielectric capability.

General leakage problems. There are many causes of air leaks, but topping the list is thedeterioration of seals, seats, and gaskets of all types, including O-rings. Many seals aremade of synthetic (nitrile) rubber compound. Depending on the cross-sectional size of theseal and the operating pressure of the air, this material will take on a permanent set andbecome hard sooner than some other compounds. This process is termed oxygenembrittlement as the oxygen in the air modifies the molecular structure of the sealcompound. The deterioration is progressive as the oxygen permeates the material, hencethe size and pressure relationship. This natural aging process, coupled with a less thanperfect environment, causes the elastic seals to become less flexible and, in the worst case,completely brittle, thereby losing the ability to effectively seal the intended surface andallowing the pressurized air to escape to atmosphere. Original O-rings and seals, in some

breakers, have a life expectancy of approximately 15 years at best. In the compressed airsystems with the highest pressures, such as those on the central substation air storagesystems and the compressor supply and regulation valves, this may become less than fiveyears for some small section O-rings. Some manufacturers and users have used, orchanged to, materials of superior quality, extending the required replacement timeconsiderably but care is needed in choosing such materials. Leaks are also likely to befound at threaded joints, at fitting ferrules, around valves of all types, gauges, and pressureswitches. Even porous castings have been known to cause leakage problems. Wherever aleak is found to exist, prompt attention should be taken as the deterioration gets worse andconsequential damage is often caused to mating surfaces of seals and the seats of valves.

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1.9 SF6 Two Pressure Circuit Breakers

1.9.1

IntroductionThe first two pressure SF6 circuit breakers were developed in the United States and use the

structural concepts of bulk oil breakers.

Two-pressure SF6 circuit breakers generally have two compressors, one for the SF6 system and

one for the operating mechanisms compressed air system. The SF6 compressor works in aclosed loop. It is designed to take its input from the low pressure SF6 of the circuit breaker main

tank, re-compress it after filtration and deliver it to the high-pressure storage tank of the circuit

breaker.

1.9.2 Interruption Process and MechanismsThe following describes dead-tank and live-tank breakers and auxiliary interrupter components.

1.9.2.1 Dead-Tank Breakers

In this type of breaker the extinguishing chambers and the contacts of each phase are housed inan earthed (grounded) steel, or later aluminum, tank. These circuit breakers are called dead-

tank breakers because the tank is at earth (ground) potential. Figure 10 illustrates a typical SF6

two pressure dead tank breaker as well as one type of SF6 blast valve configuration.

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SF6 two pressure, 230 kV, four interrupters

per phase, pneumatic mechanism One of various SF6 blast valveconfigurations

Figure 10 Typical SF6 two pressure dead tank breaker and SF6 blast valve arrangement

Completely sealed and self-contained unit construction has been adopted for all SF6 circuit

breakers. The seal between the different sections uses ethylene-propylene-rubber (EPR) gaskets

and PTFE (Teflon) (the arc-resistant synthetic insulating material poly tetra fluor-ethylene) rings.Arcing does not significantly reduce the dielectric and arc-quenching properties of SF6. Contact

designs have been developed that can be subjected to repeated arc interruptions equivalent tomany years of service. Over 30 years of service experience has shown that hermetically sealed

SF6 circuit-breakers need not be opened for inspection and maintenance except at long intervals,

in the order of 10 to 15 years for these two-pressure designs. The contacts are immersed forinsulation purposes in an atmosphere of SF6 at a pressure of approximately 0.2 MPa

(30 psig). The bushing internal conductors are insulated from the steel tank enclosure by the

same insulating atmosphere. Contacts are constructed to minimize erosion due to arcing on theportions of the contacts that conduct current in the closed position of the breaker. The current is

directed through the sidewalls of the fixed contact and into a set of fingers, which are part of the

moving contact. An arcing horn, located within the finger cluster, projects a short distancebeyond the end of the fingers and into a cavity in the end of the moving contact. On opening, the

arc quickly transfers from the end of the finger cluster to the centrally located arcing horns and to

the end of the moving contact. Both contacts have surfaces that are faced with arc-resistant

material.

The interrupting function is performed by a high-velocity flow of SF6 through a PTFE (Teflon)

ring in an orifice or nozzle located inside the arc-extinguishing chamber. (SeeFigure 11) The

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gas is maintained at approximately 1.5 MPa (220 psig) within a high-pressure reservoir during

normal operation. At the start of contact movement in an opening operation, the blast valveopens under control of a pilot valve and allows high-pressure gas to flow through an insulating

tube to the interrupting orifice, thereby extinguishing the arc as the moving contact moves to the

open position. As the contact linkage reaches the open position, the pilot valve closes the main

blast valve and conserves gas pressure for the next operation.

After each interruption, a compressor system pumps the low-pressure gas from the circuit

breaker tanks to the high-pressure reservoir via a filter containing activated alumina. Because thegas liquefies at approximately 10C at 1.6 MPa (235 psig), a heating arrangement is provided

around the high-pressure reservoir to keep its temperature above this point.

Figure 11 Illustration of SF6 two pressure interrupter

Capacitor assemblies provide uniform distribution of voltage across each of the breaks.Electrostatic shields around the metal portions of the assembly maintain control of the electric

field between the interrupter and the tank.

In these breakers, the contacts are actuated mechanically by a pneumatic operating mechanism

that drives a mechanical linkage and bell crank drives mounted one each pole (phase) tank.These bell crank drives are the mechanical link with the contact mechanism, and they also

operate the SF6 gas blast valves. Compression-type accelerating springs, mounted close to the

contacts, drive the breaker to the open position and are latched by a roller trip system in theoperating mechanism.

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1.9.2.2 Live-Tank Breakers

A live-tank breaker is one in which the tank or interruption chamber is at line potential. The

same principle of interruption applies to live-tank circuit breakers. In these breakers, contacts areactuated mechanically by a pneumatic operating mechanism, fitted on each pole. This drives a

mechanical linkage and bell crank drives mounted at the top of the columns. These bell crankdrives are the mechanical link with the contact mechanism, and they also operate the gas blast

valves. Compression-type accelerating springs, mounted close to the contacts, drive the breakerto the open position and are latched by a roller trip system in the operating mechanism.

The interrupting units and the hollow porcelain columns are filled with SF6 at a pressure ofapproximately 200 kPa (30 psig), constituting the low-pressure system. A high-pressure reservoir

operating at approximately 1.5 MPa (220 psig) is accommodated in the breaker chassis at ground

potential and is connected via high-pressure pipe run through the hollow column to a receivertank located in the distribution head.

The blast valves, which are mounted in the upper receiver tanks, are opened when the breaker istripped. The high-pressure SF6 then flows at a high velocity through short pipes to the interrupternozzles and into the low-pressure chambers. A compressor located in the breaker chassis pumps

the gas back to the high-pressure reservoir through filters containing activated alumina. The

reservoir is heated to keep the gas temperature above 10C in ambient temperatures as low as -35C. The modular construction facilitates the use of the same interrupting unit with higher

voltage breakers. A good example of this principle is represented by the circuit breakers built in

the United States for 3- and 2-cycle interruption for 335 GVA at 500 kV with three double-breakunits supported by three porcelain columns. The tank forming the central section of the modular

unit is part of the low-pressure system. It is pressurized at approximately 0.3 MPa (45 psig), and

provides gas storage in close proximity to the interrupters. The cross-arm is hollow, forming the

gas passage to the two interrupting gaps, and surrounds a section of the blast valve containinghigh-pressure

47563543 EPRI Inspection and Maintenance Guidelines for Circuit Breakers

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