Module 1 Medium Voltage Switchgear Assemblies

SUBSTATION COMMISSIONING COURSE of 46

SUBSTATION COMMISSIONINGCOURSE

MODULE ONE

MEDIUM VOLTAGESWITCHGEAR ASSEMBLIES

Written by:Raymond Lee, Technical trainerCopyright ©2010

Electrical Industry Training Centre of Alberta4234 – 93 StreetEdmonton, Alberta, Canada

Phone: (780) 462-5729Fax: (780) 437-0248

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TABLE OF CONTENTS

Headings Page

Introduction:....................................................................................................31. Switchgear Standards..................................................................................4

1.1 Canadian Standards...........................................................................41.1.1 CSA Standards................................................................................41.1.2 EEMAC and CEMA Standards.......................................................5

1.2 American Switchgear Standards............................................................61.3 International Standards..........................................................................7

2. Switchgear and Switchgear Assemblies.....................................................92.1 What is a Medium Voltage Switchgear?...............................................92.2 Major Parts.............................................................................................9

2.2.1 Control devices:...............................................................................92.2.2 Switching and Interrupting Devices:.............................................102.2.3 Metering devices...........................................................................102.2.4 Protective Devices:........................................................................112.2.5 Regulating Devices:......................................................................112.2.6 Interconnections:...........................................................................122.2.7 Supporting Structures:...................................................................13

2.3 Characteristics......................................................................................152.3.1 Electrical........................................................................................152.3.3 Metal-Clad Vs Metal-Enclosed.....................................................16

2.4 Basic Parts...........................................................................................192.4.1 Vertical Structure..........................................................................192.4.2 Compartments...............................................................................20

2.4.2.1 Circuit Breaker Compartment.................................................212.4.2.2 Main Bus Compartment..........................................................262.4.2.3 Line Compartment..................................................................292.4.2.4 Control Compartment.............................................................312.4.2.5 Auxiliary compartment...........................................................31

2.5 Installations..........................................................................................332.5.1 Indoor Installation.........................................................................332.5.2 Outdoor Installation.......................................................................34

3 Testing........................................................................................................364. Glossary of Terms.....................................................................................395. Self Test Quiz............................................................................................436. Acknowledgement, References and Suggested Readings:........................47

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Introduction:

Module One explains the construction aspects of the Medium Voltage (MV) switchgear assemblies from its physical attributes to the various compartmentalized functional sections

By the end of this module, the participants should be knowledgeable in the construction aspects of the MV switchgear and be able to identify its major and basic parts. This knowledge will prepare the participants to work comfortably around the switchgear when performing commissioning tests.

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1. Switchgear Standards

Switchgear and control panels are found in power generating stations, distribution substations, commercial and institutional buildings, industrial plants, factories, paper mills and other facilities where electric energy is utilized or electric power is distributed.

The major North American standards that govern the design, manufacture and testing of the medium voltage switchgear assemblies are CSA, ANSI/IEEE, IEC, EEMAC and NEMA.

CSA - Canadian Standards Association (1919) ANSI - American National Standards Institute (1989) IEEE - Institute of Electrical and Electronics Engineers (1963) EEMAC - Electrical Equipment Manufacturers Association of Canada

(1976) NEMA - National Electrical Manufacturers Association (1926) CEMA – Canadian Electrical Manufacturers Association (1944),

became EEMAC with the merger of Electronic Association of Canada which developed out of the Radio Manufacturers Association of Canada (1929)

1.1Canadian StandardsCanada has two electrical standards, Canadian Standards Association (CSA) and Electrical and Electronic Manufacturers Association of Canada (EEMAC).

1.1.1 CSA Standards Canadian Standards Association (CSA) Standard C22.2 No 31- 10 covers deadfront indoor enclosed and outdoor enclosed assemblies of switchgear devices, such as:

Switches Interrupting devices Air circuit breakers Power circuit breakers Control, metering, protective and regulating equipment with

associated interconnections and supporting structures.

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CSA Standard C22.2 No 31-10 covers the construction and design features required for equipment with nominal voltage of 46kV or less. The components that are used to build the equipment must comply with the applicable requirements of the Canadian Electrical Code that governs such components.

CSA approvalMetal-clad switchgear approved for use in Canada will bear the CSA label. CSA labels are affixed by the manufacturers that have passed the CSA’s approval for their manufacturing process, and are subject to ongoing regular inspection to be in compliance with the requirements of the applicable CSA standards.

UL-C labels are affixed to equipment produced in the US that complies with the Underwriter Laboratories product safety certification program but tested to CSA standards. Switchgear with UL-C labels are accepted by the Canadian inspection authorities.

Equipment installed which does not bear the CSA or UL-C label are subjected to a special inspection by the Canadian inspection authorities for use in Canada and if approved, will be affixed by a special inspection sticker.

1.1.2 EEMAC and CEMA StandardsCSA Standards references EEMAC standards within its writings which is the Electrical and Electronic Manufacturers Association of Canada standards. In turn, EEMAC standards have referenced many of the applicable ANSI / IEEE standards for ratings, definitions and testing procedures.

EEMAC has not supported the development or updating of any EEMAC Industry Product Standards since 1994. While most EEMAC Standards are inactive and out of date, a number of them continue to be referenced in CSA standards.

CEMA sister organization in the US is NEMA and comparable standards in Canada are the CEMA standards.

Current EEMAC Standard that are referenced in CSA standards are: EEMAC G8-3.2; 1989; Standard for Metal-Clad and Station-Type

Cubicle Switchgear

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EEMAC G11-1, 1972; Guide for Measurement of Corona in Switchgear Assemblies.

EEMAC G14-1, 1987; Procedure for Testing The Resistance of Metalclad Switchgear Under Conditions of Arcing Due To Internal Fault

1.2 American Switchgear StandardsThe evolution of the standards originated with the first written standard by The Electric Power Club which today became NEMA.

NEMA was a merger between the Associated Manufacturers of Electric Supplies and The Electric Power Club, September 1926.

The American Institute of Electrical Engineers (AIEE) was in existence at the time and they also worked on their own switchgear standards. After World War II, the various standard organization worked together to consolidate the standards requirements. The resulting work culminated to form AIEE Standard 27. Year later AIEE merged with IRE to form IEEE and the AIEE Standard 27 became the IEEE 27, which formed the basis for ANSI/IEEE C37.20 of today.

Note: American National Standard Institute (ANSI), is a membership organization that coordinates the development of US voluntary standards in both the private and public sector. It is the US member body to ISO and IEC

C37.20 covered switchgear assemblies across the voltage spectrum. These include:

low voltage power circuit breaker switchgear to 600 Vac medium voltage metal-clad switchgear above 1000v up to 38k Vac metal enclosed load interrupter switchgear above 1000v up to 38k

Vac.

In 1987 C37.20 was revised into separate standards to produce 1 document for each major equipment classes.

C37.20.1 - IEEE Standard for Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear

C37.20.2 - IEEE Standard for Metal-Clad Switchgear C37.20.3 - IEEE Standard for Metal-Enclosed Interrupter

Switchgear

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C37.20.4 - IEEE Standard for Indoor AC Switches (1kV – 38 kV) for Use in Metal Enclosed Switchgear

Table 1 lists the principle standards that are applicable to Medium Voltage Metal-Clad Switchgear of today. The standards listed is not all inclusive. There are other standards that are applicable to relays, device numbers, wires and other equipment or devices within the switchgear assemblies.

The more significant standards for switchgear are the switchgear standard C37.20.2 and the breaker standards C37.04, C37.06, C37.09 and C37.010.

1.3 International StandardsThe International Electrotechnical Commission (IEC) is the international standards and conformity assessment body for all fields of electro-technology. IEC standards are synonymous with International standards. IEC was founded in 1906, based in Geneva and is made up of national committees from over 60 countries.

IEC Standard 62271-200 ‘AC metal-enclosed switchgear and controlgear for rated voltage above 1kV and up to and including 52kV’ is the comparable to the ANSI / IEEE Standard C37.20.2.

IEC Standard related to metalclad switchgear, are quite different from what is expected of switchgear designed and tested to the North American Standards. Ratings and test results are also different. It is beyond the scope of this module to discuss the difference and similarities of these two standards, but it is notable to say that the IEC designed switchgears can be in compliance to the requirements of the CSA standards. A special inspection sticker is still required for approval by the electrical inspection authority.

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TABLE 1: NORTH AMERICAN SWITCHGEAR STANDARDSSTANDARD REV SPONSOR TITLE WORKING

GROUPC37.04 1999 IEEE Standard Rating Structure for AC HV Circuit Breakers IEEEC37.06 2009 IEEE Standard for AC HV Circuit Breakers Rated on a Symmetrical

Current Basis – Preferred Ratings and Related Required Capabilities for voltages above 1000 V

NEMA

C37.09 1999 IEEE Test Procedure for AC HV Circuit Breakers IEEEC37.010 1999 ANSI /

IEEEApplication Guide for AC HV Circuit Breakers Rated on a Symmetrical Current Basis

IEEE

C37.20.2 1999 ANSI /IEEE

Metal-Clad and Station-Type Cubicle Switchgear IEEE

C37.20.6 2007 IEEE Standard for 4.76 TO 38 Kv Rated Grounding and Test Devices Used in Enclosures

IEEE

C37.54 2002 ANSI / NEMA

Conformance Test Procedures for Indoor Alternating-CurrentMV Circuit Breakers Applied as Removable Elements in Metal Enclosed Switchgear Assemblies

NEMA

C37.55 2002 ANSI / NEMA

Metal-Clad Switchgear Assemblies – Conformance Test Procedures

NEMA

C37.85 2002 ANSI / NEMA

Switchgear - Alternating-Current High Voltage Power Vacuum Interrupters – Safety Requirements for X-Radiation Limits

NEMA

C22.2 # 31 2010 CSA Switchgear Assemblies CSAC22.2 # 58 1989 CSA HV Isolating Switches CSAC22.2 # 193 1983 CSA HV Full-Load Interrupter Switches CSAC22.2 # 253 2009 CSA MV AC Contactors, Controllers and Control Centres CSAG8-3.2 1989 EEMAC Standard for Metal-Clad and Station-Type Cubicle SwitchgearG11-1 1972 EEMAC Guide for Measurement of Corona in Switchgear AssembliesG14-1 1987 EEMAC Procedure for Testing The Resistance of Metalclad Switchgear

Under Conditions of Arcing Due To Internal Fault

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2. Switchgear and Switchgear Assemblies

2.1 What is a Medium Voltage Switchgear?When we say “medium voltage,” we are referring to a range of voltage from 1000 volts to 38 kV (as per ANSI / IEEE 1585 – 2002, ac assumed).

CSA standard C22.2 No 31-10 applies to deadfront indoor enclosed and outdoor enclosed assemblies of switchgear devices such as switches and interrupting devices, air circuit breakers, power circuit breakers along with control, metering, protective and regulating equipment with associated interconnections and supporting structure.

MV Switchgear Definition:A MV switchgear are compartmentalized assemblies of switching and interrupting devices operating from 1000V to 38 kV supplied from an insulated main bus structure, along with control, metering, protective and regulating equipment with associated interconnections and supporting structure.

2.2 Major PartsThe major parts of the switchgear are provided within its definition. They are listed as:

Control devices Switching and Interrupting devices Metering devices Protective devices Regulating devices Interconnections Supporting structure

2.2.1 Control devices:Control devices provide local and / or remote control to operate the main switching or interrupting devices. Local breaker control can be performed via with door mounted push buttons, pistol grip breaker control switch or local centralized breaker control panel. Load-break disconnects are normally operated by a manual operating handle, they can be fitted with a electrical operator when required.

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Remote operations can be initiated via SCADA or other communication equipment using interface / interposing relays or through a remote terminal unit (RTU) that sends control signals in parallels with the local controls.

Control devices are: Local controls devices Remote controls devices

2.2.2 Switching and Interrupting Devices:Switching and Interrupting devices are installed in the primary circuit to control the flow electrical energy. The rating of the switching or interrupting devices will determine if the device is able to make or break load current, overload current or short-circuit current.

No-load / isolating disconnects are used for no-load operation. It main function is to provide a visible break and a lockable point for isolation purposes for maintenance / repair of downstream equipment.

Load break switches / disconnects are designed to interrupt normal load current. They are fitted with a fast acting make / break mechanism and arc quenching devices.

Load break switches / disconnects are able to make short circuit currents for inadvertent situations when they are closed onto a faulted condition, they are not designed to interrupt fault current. For those scenarios when load break switches are closed onto a fault, the upstream protective device shall operate to interrupts the short circuit currents and isolate the faulted part of the power system.

Switching and Interrupting devices are: Circuit breakers Load-break switches / disconnect; load interruptors / disconnectors No-load / isolating disconnects

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2.2.3 Metering devicesMetering devices monitors the system parameters such as voltage, current, frequency and phasing through the use of instrument transformers which convert and isolate the primary quantities into useable low voltage secondary quantities. Metering devices enables the operators to quantify the operating conditions of the system.

Instrument transformers: Current transformers (CT), Voltage transformer (VT), potential transformer (PT)

Switchboard instruments: Voltmeter, Ammeter, Current Demand,Watt, Var, Power Factor, Frequency, Synchroscope (120V, 5A)

Transducers: Volt, Amp, Watt, Var, Frequency, Temperature Revenue Meters: Watt-hour meter and Var-hour meter

2.2.4 Protective Devices:Protective devices monitor the system’s operating parameters for abnormal operating conditions. Depending on its function, protective devices may trip the supply breaker to isolate the faulted part of the power system or initiate an alarm to signal the operator to take corrective action.

Examples of abnormal operating conditions are: Under-voltage / over-voltage Under-frequency / over-frequency Under-speed / over-speed High temperature High vibration Reverse power Overload Short-circuit

Some protective devices are: Fuses Overload relays Temperature relays Protective relays

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2.2.5 Regulating Devices:Regulating devices are used to control the operating parameters of the power system such as voltage, current, power, speed, frequency, temperature and loading for machines, tie lines or other apparatus. These devices are commonly associated with power generating equipment and become an integral part of the control systems.

Regulating devices in switchgear are associated with forced air-cooled breakers and with metal enclosed capacitor banks.

Forced air-cooled circuit breakers employed a current sensing relay which initiate beaker cooling using fans when the load current approach 90-95% of the breaker continuous current rating. The fan will stop if the load current drops below the set-point value after a set time delay.The control scheme for capacitor banks can utilize the reactive energy sensing, under-voltage sensing or both to control the application of capacitors by vacuum contactors. Capacitor banks are used to reduce the current demand during heavy load periods and / or reduce the voltage drop.

2.2.6 Interconnections:Interconnection relates to the primary bus-bars connections between switchgear cubicles or between switchgear sections via enclosed bus structures / bus ways / ducts. The efficiency of all types of heavy current busbars depends upon careful design.

Temperature rise is the main design criteria used for establishing the continuous current rating for busbar design. The general requirements as per ANSI, IEEE and NEMA standards for switchgear is a temperature rise of no more the 65°C, although it may vary for different classes of equipment. The requirement is to prevent overheating of the insulators and insulation support systems enclosing the bus bars since high temperature will reduce the life of the insulation.

Factors affecting bus-bars temperature rise and bus bar design are: Size and type of materials used (copper or aluminum alloys) Insulated of un-insulated bars (insulated bars runs cooler from increased heat

radiation due to darker color) Maximizing the surface area for heat dissipation Size and material of the enclosure around the bus Air flow around the bus and enclosure Not impeding the natural air currents due to busbars placement

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Achieving uniform current densities in all parts of the conductor Minimizing skin effects and proximity effects for busbars

Due to the complex interactions of the above and other factors, continuous currenttests are required as part of the standard to determine the temperature rise of a bus system design.

Insulated Main Bus and Connections All main bus and connections are insulated in a medium voltage metal-clad

switchgear assembly. This is not necessarily the case with other types of assemblies such as a panelboards, where the main bus is not separated with

the branch breaker distribution system.

2.2.7 Supporting Structures:Supporting structure are the physical attributes of the switchgear enclosure. The construction of the enclosures must be strong and rigid to withstand the operating mechanical forces of the equipment it supports / houses and withstand the magnetic forces that are produced from the flow of short circuit currents.

Insulators are integral to the design of the supporting structures which supports the busbars.

Arc resistant switchgear are designed to resist the explosive forces generated during an arc flash through the use of pressure relief vents to direct the rapidly expanding air, hot metal and gases away from the operator’s working area.

Seimic rated gear is another design feature to consider for installations in earthquake prone regions. Such features are standard for nuclear installations.

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Switchgear examples:

Switchgear brings a number of very important functions together in one package.

Figure 1. Typical Medium Voltage Switchgear Assembly, Consisting of 5 Vertical Structures

Figure 2. Typical 3 Vertical Structure Assembly with Upper Compartment Vacuum Circuit Breaker Removed from its Compartment on Extension Rails

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2.3 Characteristics

2.3.1 Electrical

Voltage CategoryMedium voltage switchgear assemblies generally fall into one of four voltage category, they are:

5 kV 15 kV 27 kV 38 kV

Current ratingThe main bus continuous current rating is a function of the protective device protecting the main bus. Frame sizes for circuit breakers are 1200, 2000 or 3000 amperes, making the continuous current rating of the bus rated at 1200, 2000 or 3000 amp. Higher ratings are available when fans are used to force-cool the equipment.

Not all voltage levels offer the same continuous current ratings. The current ratings are determined by the system requirements and the circuit breakers selected to provide the protection.

Mechanical AspectsA typical medium voltage metal clad switchgear assembly has:

Removable / Draw-out circuit breakers Main bus compartment Incoming / outgoing compartments for line / cable connections Circuit breakers, control equipment compartments and other compartments

for auxiliary equipment (such as VTs) Insulated main bus and connections Metal barriers separating each vertical structure and each compartment

within each structure

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2.3.3 Metal-Clad Vs Metal-Enclosed Medium voltage metal-clad switchgear has the structures and compartments within each structure physically separated from each other by grounded metal barriers. Medium voltage switchgear assemblies are metal-clad.

Metal enclosed assembly often associated with low voltage equipment encloses the equipment in metal vertical structures. However, compartments are not separated from one another with metal barriers.

Metal Enclosed MetalcladApplicable Standards

ANSI C37.20.2 for Breakers ANSI C37.20.2 for BreakersANSI C37.20.3 for Switchgear CSA C22.2 # 31 includes Metalclad.CSA C22.2 # 31 for Switchgear Assy. EEMAC G8-3-2 for Metalclad

Structural AttributesFront and rear access (optional) Must have front and rear accessShutters not required Shutters requiredComparmentalization not required All live parts are compartmentalizedAdjacent compartments can be open Adjacent compartments are isolatedBus and primaries can be uninsulated Bus and primaries must be insulatedSwitching devices can be fixed mounted Switching devices must be draw-outInterrupting dev. Can be fixed mounted Interrupting dev. must be draw-out

Overcurrent Protection Device TypesSwitches with or without fuses (typ) Vacuum circuit breakers with O/C PN.

Overcurrent Protection Device FeaturesSwitches have low duty cycle Breakers have high duty cycle

Metalclad is metal enclosed, but metal enclosed is not metalclad

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Application Example

Figure 3. Application Example

The substation transformer steps the voltage down from 35 kV to 15 kV. It feeds a 15 kV medium voltage circuit breaker A. This is the main / incoming breaker of the medium voltage switchgear assembly

Three other medium voltage circuit breakers B, C and D are performing other functions. Breaker B and C are feeder breakers and breaker C is the transformer breaker.

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The rest of the system comprised of a 5 kV isolating switch, 5 kV fussed motor starter and a 5 kV motor load.

2.4 Basic PartsA switchgear assembly consists of as many vertical structures / sections required for the intended application. That could be one structure or many structures.

2.4.1 Vertical StructureWidth dimensions are usually 26, 36 or 42 inches, and height of 90 to 100 inches. Vertical structures are all bolted together to form one rigid, continuous assembly. The exact dimensions of the vertical structure are dictated by the voltage class and specification requirements.

The front structures are normally hinged metal doors. Access from the rear is provided through bolted, removable metal panels, or hinged doors.

Figure 4. Typical Medium Voltage Switchgear Assembly withOne Circuit Breaker Compartment Door Open

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Figure 5. Typical Medium Voltage Switchgear AssemblyRear View, with Several Rear Covers Removed

2.4.2 CompartmentsA medium voltage switchgear assembly is an integrated combination of a number of compartments.

Compartment types are: Circuit Breaker Compartment (A) Main Bus Compartment (B) Cable Connection / Line Compartment © Low Voltage / Control Compartment (D) Potential Transformer / Auxiliary Compartment (E)

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Figure 6. Sectional view of a Structure.

Figure 6 shows a lower breaker compartment, an upper VT compartment and a rear bus bar compartment. Compartments and vertical structures are physically separated from one another by metal barriers.

2.4.2.1 Circuit Breaker CompartmentThe removable draw-out circuit breaker is the heart of the switchgear. Each circuit breaker is provided with its own compartment, within which it performs its function.

The circuit breaker compartment provides: an enclosure for one circuit breaker a means to physically move the circuit breaker into or out of the

compartment between connected and disconnected position a means to make primary electrical connections, self aligning and self

coupling disconnectable secondary control wiring connections interlock to prevent insertion into the wrong compartment, prevents insertion

of a closed breaker and prevent breaker operation until fully inserted. extension rails to support the circuit breaker as it is pulled out of the

compartment

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Enclosure The circuit breaker compartment includes a top, bottom, right and left sides, and rear portions of the compartment. When the circuit breaker is racked into its connected position, the faceplate of the circuit breaker itself acts as the front part of the compartment.

Figure 7. Circuit Breaker Shown Being Manually Levered into Connected Position

In this position, the operating and current-carrying parts of the circuit breaker are compartmentalized from the rest of the assembly structure.

Primary Electrical Connections Circuit breakers have six primary electrical connecting devices. They are arranged two per phase; Phase-A, Phase-B and Phase-C line and load side connections.

Each primary connecting device has multiple contact fingers which will mate with the bus bar stationary primary connecting stabs. Breaker to primary bus connections is made via spring pressured contact finger clusters. Connections are automatically made as the circuit breaker is racked into position within the circuit breaker compartment and automatically disconnected as the breaker is racked out of the compartment.

Note: Do not move or handle the circuit breaker by the primary connecting device as damage may occur.

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Figure 8. Rear View of Circuit Breaker, Showing Six Primary Finger Clusters

When the circuit breaker is racked out of the connected position, the rear bus connection points are covered via a closing shutter to prevent contact with the live primary circuit. Conversely the shutters move out of the way during breaker insertion for connection to the primary circuit. This safety device is called the automatic shutter system.

Secondary Electrical Connections When the circuit breaker is racked into the connected position, secondary male and female contact plug engages. Control power is provided to the charging motor and other auxiliary devices through the plug connections. When the circuit breaker is racked out of the connected position to the disconnected position, the secondary contacts plug disengages. The secondary contact plugs can be manually engaged when the circuit breaker is in the TEST position within its compartment (Figure 10). Once the plugs are connected, the circuit breaker can be electrically operated while disconnected from the primary circuit.

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Figure 9. Secondary Contacts Being Manually Engaged with Circuit Breaker in Test Position

The secondary plug is often mounted on the underside of the circuit breaker, with the compartment’s secondary plug mounted on the compartment floor.

Figure 10. Secondary Control Plug Shown Mounted on Floor of Circuit Breaker Compartment

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Safety Interlocks Safety interlocks are required by governing standards, and must be supplied by all manufacturers. Safety interlocks in the compartment interact with matching circuit breaker interlocks. There are both electrical and mechanical interlocks.

The purpose of these interlocks is to ensure: The circuit breaker cannot be removed from the test/disconnected to the

connected position unless the main contacts are opened and secondary electrical connections are engaged

The circuit breaker cannot be removed from the connected to the test/disconnected position unless the main contacts are opened and secondary electrical connections are engaged

The secondary electrical connection cannot be broken when the circuit breaker is in the connected position

The circuit breaker cannot be removed from the compartment with the main contact closed or with the closing spring charged

Incorrect circuit breaker types are prevented from being inserted into the wrong circuit breaker compartment

Extension Rails Extension rails provide equipment support and permit circuit breakers and other auxiliary devices such as large removable transformers to be withdrawn from the compartment for inspection and maintenance.

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Figure 11. Vacuum Circuit Breaker Withdrawn from Compartment on Extension Rails

2.4.2.2 Main Bus Compartment The main bus comprised of a set of bus bars. They typically copper of aluminum bus bars mounted on bushing. The bushings are made of an insulating material. Two vacuum circuit breakers have been permitted to be installed in one vertical structure since the arcing current during contact part is contained within the vacuum bottles.

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Figure 12. Two Vacuum Breakers In One Vertical Structure (Side View)

Connections to the main bus within a vertical structure for circuit breakers and other devices are called Bus Joints. Bus connections between compartments must be insulated.

Figure 13. Three Main Bus Bars Extending Through Insulated Openings in the Side of Vertical Structure.

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Main BusThe main bus extends the length of the assembly from one vertical structure to another through insulated openings in the sides of the vertical structures. Depending upon the length of the assembly, main bus bars are not usually one continuous piece or standard lengths bolted together.

The bus system must be supported and braced to withstand the stresses created by short circuit currents. To minimize occurrence of faults, buses are insulated with fluidized bonded epoxy insulation and bolted joints are covered with insulating boots.

Isolating barriers are used within each vertical structure to compartmentalize the main bus from other compartments and equipment. Access to the main bus is provided by removing specific barriers.

Figure 14. Rear View of Partial Assembly with Barriers Removed to Show Insulated Main Bus Bars

Main bus conductors are sized to handle the current, while maintaining the rated temperature requirements established by standards. In medium voltage switchgear assemblies, the rated continuous current of the main bus is usually 1200, 2000 or 3000 amperes, although higher ratings are possible using forced air cooling consisting of fans controlled via thermostats.

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2.4.2.3 Line Compartment The rearmost compartment is the line compartment. It is accessible via removable bolted rear panels or rear doors. Space is provided in the compartment for incoming cable connections or other connecting means. Assembly designs permit entry and exit from the top or bottom. Surge arresters and large control power transformers, can also be mounted in the line compartment. The line compartment usually runs from top to bottom.

Figure 15. Two Vertical Structures of One-High Circuit Breaker Switchgear,Rear Panels Removed (Rear View)

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Figure 16. One Vertical Structure of Two-High Circuit Breakers, Showing How Cable for Top Compartment Gets Through Bottom Compartment (Rear View).

Ground BusAn un-insulated ground bus runs the length of the assembly, and is normally located in the lower part of the line compartment. The ground bus is capable of carrying the rated short circuit current of the system.

2.4.2.4 Control Compartment The control compartment is considered to be the space between the front of the circuit breaker face plate and the front door. Instruments, meters, relays and other control equipment can be mounted on the rear door panel. Relays, indications, control switches and other devices can be mounted in the door panel. The rear portion of door-mounted devices protrudes into the compartment space. If there is to be a large volume of control equipment, it can alternatively be mounted in a dedicated auxiliary compartment.

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Figure 17. Typical Circuit Breaker Compartment, with Door-Mounted Devices Protruding Through Door

2.4.2.5 Auxiliary compartmentAuxiliary compartment is usually located above or below a circuit breaker compartment. An auxiliary compartment could be an entire full-height vertical structure. Auxiliary compartments are frequently used to mount draw-out voltage transformers and large power control transformer.

Figure 18. Auxiliary Compartment Transformer Mounting

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Figure 19. Entire Vertical Structure Dedicated to Auxiliary Equipment

2.5 InstallationsThe two main installation types are:

Indoor Outdoor

Sub-classifications are based on enclosure types are: NEMA 1 non-gasketed NEMA 1A gasketed NEMA 2 drip-proof NEMA 12 dust-tight NEMA 3R non-walk-in NEMA 3R walk-in and others

Other sub-classification based of functionality are: Sprinkler proof Arc resistant Dust proof Seismic rated Weatherproof

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Switchgear assemblies are installed in a wide variety of locations under a wide range of climatic conditions.

2.5.1 Indoor Installation All medium voltage switchgear assemblies start out as indoor assemblies. Configurations are designed to use the smallest amount of floor space and materials taking into account such factors as temperature, humidity and other environmental factors and still meeting design requirement. Not all manufacture will come up with the same configuration.

Figure 20. Typical Two-High Configurations

2.5.2 Outdoor InstallationOutdoor switchgear incorporates and outdoor approved structure with an indoor switchgear components. Longer life can be achieve if the environment within the enclosure can be controlled to mimicked an indoor environment.

Three primary outdoor installations are: Aisle-Less switchgear Sheltered Aisle Switchgear Common Aisle Switchgear

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Aisle-Less SwitchgearAisle-Less Switchgear are house in a weatherproof enclosure around an indoor switchgear assembly. Weatherproof gasketed sealed doors are located on the front to allow for the drawout and removal of the circuit breakers. Removable weatherproof gasket panels or doors are provided for rear access. Additional provision of added ventilation with space heater aids in minimizing condensation of moisture.

Figure 26. Typical Outdoor Aisle-Less Construction (Side View)

Sheltered Aisle SwitchgearAn outdoor enclosure is constructed around an indoor switchgear assembly. An aisle is constructed as part of the enclosure at the front of the assembly. Weatherproof gasketed doors are normally at both end of the enclosure.

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Figure 27. Typical Outdoor Sheltered Aisle Construction (Side View)

Common Aisle SwitchgearThis is a large assembly and installed in two parts on both sides of the aisle.

Figure 28. Typical Outdoor Common Aisle Construction (Side View)

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3 Testing

The design and proof testing and the production testing of a medium voltage switchgear assembly varies, depending upon whether it is compliant with ANSI or IEC standards. A medium voltage switchgear assembly is built and tested in accordance with a specific set of engineering standards. Depending of the geographical location some of the applicable standards may be ANSI, IEEE, NEMA, CSA or EEMAC.

Unlike an individual component, which also must meet certain specific standards, the manufacturer must know and be able to prove by testing that each and every component functions properly when installed in the assembly. This is quite a task, considering the vast number of different devices and pieces that make up an assembly.

Figure 29. Dominant Regional Standards Throughout the World

In addition to regional standards, an assembly may frequently be required to meet specific local standards. Very specific tests could even be required for the equipment before it can be permitted for use in certain areas or applications. Two such applications are seismically-qualified and arc-resistant equipment. The design of arc-resistant equipment is dedicated to the safe control and release of arc-related overpressures.

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Design and Proof Testing The design and proof testing of the assembly to show compliance with applicable standards includes, but is not limited to:

Short circuit testing BIL testing Dielectric testing Continuous current testing Mechanical life testing Thermal testing Environmental testing

Production Testing Production testing of an assembly is performed in compliance with specific standards for the circuit breaker and the assembled housing individually. These production tests are also referred to as “routine tests.” Circuit breaker testing includes, but is not limited to:

Control wiring insulation test Charging motor insulation test Operation of switches, control devices, latches and interlocks Breaker operation at minimum, maximum, and rated control voltages Vacuum interrupter operation and withstand tests Breaker contact resistance Closing and opening timing tests Dielectric tests

The assembled housing’s testing includes, but is not limited to: Component nameplate verification for compatible application Dielectric tests Mechanical testing of all operational parts and devices Control wiring continuity verification Operation of all relays, instruments, meters and other devices Control wiring insulation tests Instrument transformer polarity verification tests Sequence of operation tests, if applicable

A medium voltage switchgear is a built-to-order product with many details to be considered and many selections to be made. Selecting each component and device to do the job, be acceptable to the customer, and comply with all regional and local standards is an undertaking. Every detail, from the electrical and mechanical

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requirements, down to the finish and composition of the structure must be considered.

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4. Glossary of TermsAisle Part of a sheltered aisle switchgear installation. Connected

accessway for multiple vertical enclosures in an outdoor installation. Weatherproof access doors are normally supplied at both ends of the aisle.

ANSI American National Standards Institute. It was organized to simplify and standardize production and construction.

Arc-Resistant A special certification that equipment must have for use in certain circumstances. The design of arc-resistant equipment is dedicated to the safe control and release of arc-related overpressures.

Automatic Shutter System

Circuit breaker compartment safety feature. When the circuit breaker is removed from its compartment, the compartment’s fixed primary conductors are automatically covered to prevent contact with live primary current-carrying parts.

Auxiliary Compartment

An optional compartment in a medium voltage switchgear assembly, usually located above or below a circuit breaker compartment. Used when there is a significant volume of auxiliary and/or control equipment required in the assembly.

Bus Joint The connection between the main bus and a circuit breaker (or other devices). Bus joints must be insulated.

Circuit Breaker A reusable overcurrent protection device. After tripping to break the circuit, it can be reset to protect the circuit again.

Circuit Breaker Compartment

A compartment in a medium voltage switchgear assembly that contains and electrically connects one or more circuit breakers.

Common Aisle Switchgear

An outdoor installation solution for very large assemblies. A variation on sheltered aisle switchgear, in which vertical structures are on both sides of the aisle.

Continuous Current

The amount of current the breaker can carry continuously at 60 cycles without exceeding the temperature rise limit, according to ANSI charts.

Control Compartment

The space between the front of the circuit breaker and the front doors of the assembly. This is where instruments, meters, relays and other control equipment are usually mounted.

Control Power Transformer

A transformer that provides a safe, reasonable low voltage source for relays, contactors and other devices.

Extension Rails A common feature of the circuit breaker itself. Allow the circuit

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breaker slide out of its compartment easily for inspection.Feeder Circuit Breaker

A reusable overcurrent protection device designed to protect a panel downstream from a medium voltage switchgear assembly.

Finger Cluster A configuration of spring-loaded conductive fingers mounted on the circuit breaker, often made of silver-plated copper. When the circuit breaker is levered into the CONNECTED position, its finger clusters engage the circuit breaker compartment’s stationary primary conductors to make electrical connection.

Ground Bus A grounded bus that runs the length of the assembly, and is normally located in the lower part of the line compartment. For safety reasons, the ground bus is capable of carrying the rated short circuit current of the installed circuit breakers for a certain amount of time.

IEC Abbreviation for International Electro-technical Commission. This organization is associated with equipment used internationally.

IEEE Institute of Electrical and Electronic Engineers. A professional organization of scientists and engineers whose purpose is the advancement of engineering.

Lever A term to describe the act of moving a circuit breaker from one position to another.

Line Compartment

Space is provided in this compartment for line terminations or other special devices. Assembly designs usually permit entry and/or exit of cable or bus from the top or bottom of the compartment.

Main Bus A set of electrical conductors, usually three per set. These individually insulated conductors provide for multiple connections into the electrical system. In medium voltage switchgear assemblies, the main bus usually takes the form of solid copper bars.

Main Bus Compartment

A compartment in a medium voltage switchgear assembly through which the main bus runs.

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Main Circuit Breaker

Also “Main Breaker.” A reusable overcurrent protection device designed to protect an entire medium voltage switchgear assembly.

Medium Voltage Switchgear Assembly

An integrated assembly of compartmentalized, removable circuit breakers with an insulated main bus, associated control devices, and auxiliary equipment designed to provide medium voltage circuit protection

Metal-Clad Equipment in the assembly is enclosed, and separated by metal barriers into individual compartments. Typically associated with medium voltage equipment.

Metal-Enclosed Equipment in the assembly is enclosed, but not necessarily separated by barriers. Typically associated with low voltage equipment.

NEMA Abbreviation for National Electrical Manufacturers Association. An organization of manufacturers of electrical products.

Outdoor Aisle-less Switchgear

An outdoor installation solution. Basically, the assembly of a weatherproof enclosure around an indoor switchgear assembly.

Outdoor Control Room

An outdoor installation solution. Encloses all the equipment in one weatherproof house. Normally, the construction takes place at the manufacturer’s location and then moved to the installation site

Safety Interlock A feature of both the circuit breaker compartment and the circuit breaker itself. Helps ensure safe and proper interfacing between the circuit breaker and its compartment. Interlocks are required by governing standards, and must be supplied by all manufacturers.

Seismically-Qualified

A special certification that equipment must have for use in certain earthquake-prone zones.

SF 6 An arc extinguishing technology involving the use of sulfur hexafluoride gas.

Sheltered Aisle Switchgear

An outdoor installation solution. An outdoor enclosure is constructed around an indoor switchgear assembly with an aisle constructed as part of the enclosure at the front of the assembly.

Surge Arrester A device that protects equipment from electrical surges.

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Switchgear An assembly of switching and interrupting devices, along with control, metering, protective and regulating equipment.

Vacuum Interrupter

An arc extinguishing technology. Features a pair of separable contacts enclosed in a vacuum-tight envelope. Because the environment inside the interrupter envelope is a vacuum, an arc cannot be sustained easily.

Vertical Structure

A metal enclosure for the other switchgear components. A switch gear assembly can consist of practically any number of adjacent vertical structures.

Voltage Transformer

Also “potential transformer.” A step-down transformer that steps down supplied voltage to a voltage usable by control components such as relays and meters.

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5. Self Test Quiz

Quiz 1:

Answer the following questions without referring to the material.

1. Medium voltage refers to a voltage range from __________ to __________.

2. In your own words, define the generic term “switchgear.”_____________________________________________________________ _____________________________________________________________ _____________________________________________________________

3. A medium voltage switchgear assembly can be composed of as many as five different types of compartments. The circuit breaker compartment is one of them. Name three others.____________________________________________________________________________________________________________________________________________________________________________________

4. The exact dimensions of the vertical structure are dictated by the __________ _______ and _____________ _______________ for the application.

5. The circuit breaker compartment accommodates the circuit breaker with a number of provisions. Name three of them.____________________________________________________________________________________________________________________________________________________________________________________

6. Medium voltage switchgear is defined by its metal _______ construction.

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Answer to Quiz 1:

1. Medium voltage refers to a voltage range from 1000 to 38 Kv.

2. Switchgear is assemblies of switching and interrupting devices, along with control, metering, protective and regulating equipment.

3. Types of compartments other than the breaker Main Bus Compartment, line Compartment, control Compartment Area,

auxiliary Compartment

4. The exact dimensions of the vertical structure are dictated by the voltage class and specification requirement for the application.

5. The circuit breaker compartment accommodates the circuit breaker with a number of provisions, Name 3 of them:

An enclosure for the breaker, means to move the breaker in and out of the enclosure, means to make primary and secondary connections, safety interlocks and extension rails.

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Quiz 2:

Answer the following questions without referring to the material just presented.

1. Main bus conductors are sized to handle the __________, while maintaining the rated ______________ _______________ established by standards.

2. In your own words, describe where controls are generally found in a switchgear assembly._____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________

3. There are two primary approaches to outdoor switchgear assembly installations. Name two of them.________________________________________________________________________________________________________________________

4. ANSI design and proof testing for switchgear assembly includes many types of tests. Seven types were mentioned. Name four.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

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Answer to Quiz 2:

1. Main bus conductors are sized to handle the current, while maintaining the rated temperature rise established by standards.

2. The space between the front of the circuit breaker and the front doors of the assembly is generally considered as the control compartment area. Door-mounted devices are viewable with the door closed. If there is a large volume of control equipment, it could be located in an auxiliary compartment.

3. Three primary approaches to outdoor switchgear assembly installations are:Outdoor aisle-less switchgear, Sheltered aisle switchgear (or common aisle switchgear).

4. Four of seven types of ANSI proof testing are:Short circuit, BIL, Dielectric, Continuous current, Mechanical life, Thermal and Environmental.

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6. Acknowledgement, References and Suggested Readings:

Acknowledgement:Special thanks to Clayton Engineering Company who allowed the use of their graphics and education materials for inclusion in this document.

References and Suggested Readings[1] Caylton Engineering Company

http://www.claytonengineering.com/cecoweb/pittsburgh.htm

[2] NEMA – National Elelctrical Manufacturing Associationhttp://www.nema.org/

[3] CSA – Canadian Standards Associationhttp://www.csa.ca

[4] ANSI – American National Standards Intitutehttp://www.ansi.org

[5] IEEE – Institute of Electrical and Electronic Engineershttp://www.ieee.org

[6] IEC – International Electrotechnical Commissionhttp://www.iec.ch/

[7] Electro Federation Canada http://www.electrofed.com/councils/EEMAC/

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http://www.electrofed.com/councils/EEMAC/

http://www.iec.ch/

http://www.ieee.org/

http://www.ansi.org/

http://www.csa.ca/

http://www.nema.org/

http://www.claytonengineering.com/cecoweb/pittsburgh.htm

Module 1 Medium Voltage Switchgear Assemblies

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Transcript of Module 1 Medium Voltage Switchgear Assemblies