fundamental concepts in substation design

8/2/2019 Fundamental Concepts in Substation Design

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Paner No

93D2

Conference Paper

Fundamental Concepts in Substation Design

James R. LusbyBlack and Veatch

0-7803-0940-5/93/$3.00 993 EE E

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FUNDAMENTAL CONCEPTS IN SUBSTATION DESIGNJames R. Lusby, P.E.

Project ManagerTransmission and Distribution Division

Black 8c Veatch

ABSTRACT: Electric utility substations vary widely indesign and appearance, depending on the purposes forthe substation and its location. A 69/13.8 kV distribu-tion substation near a rural community, for example,will be quite different in design and possibly appear-ance from a 500/230 kV substation at a nuclear powerplant. The fundamental design concepts, however, arethe same.

This paper describes some of the basic conceptswhich are common in the design of all substations, andprovides a general outline for substation design in anappendix. Substations and their component systemsare described, along with the various bus configurationsand types of construction in use today.

An overview of substation design and generalinformation for the planning and preparation requiredto design a substation are also presented.

What is a Substation?A substation is an installation that interconnects

elements of an electric utility's system. These elementscan include generators, transmission lines, distributionlines, and even neighboring utility systems. It iscommon to refer to the transmission and distributionelements as networks or again, as systems. Dependingon the size and complexity of a particular utility system,the transmission and/or distribution networks mayinclude more than one voltage level. For instance, autility's transmission network may include 115kV and230 kV transmission lines, while another utility's dis-tribution network may include both 13.8 kV and34.5 kV distribution lines.

The functions of a substation may include one orfollowing:To isolate a faulted element from the restof the utility system.To allow an element to be disconnectedfrom the rest of the utility system formaintenance or repair.To change or transform voltage levelsfrom one part of the utility system toanother.

(d) To control power flow in the utilitysystem by switching elements into or outof the utility system.To provide sources of reactive power forpower factor correction or voltage control.To provide data cOncerning system param-eters (voltage, current flow, power flow) foruse in operating the utility system.

(e)(f )

When the utility system is discussed, it is almostalways the "ac" or alternating current system that isreferred to. Electric energy in the United States andthroughout the world is generated and consumed as acelectric energy. Most utility system interconnectionsare synchronous ac connections at a transmission net-work voltage level. Some utility systems are intercon-nected asynchronously using direct current systems, or"dc." These dc interconnections are not common, andare much higher in capital cost than comparable acinterconnections.

There are many kinds of ac substations.Generating station substations transform generationvoltage (usually 15 kV through 23 kV) up to trans-mission network voltage (usually 69 kV through500 kV). Transmission switching substations inter-connect portions of the utility system transmissionnetwork, but do not include transformation betweenvoltage levels. Transmission step-down (or step-up,depending on your point of view) substations inter-connect portions of the utility system transmissionnetwork, and include transformation between trans-mission network voltage levels. Distribution step-downsubstations may or may not interconnect portions of theutility system transmission network, include transforma-tion between transmission network and distributionnetwork voltage levels, and interconnect portions of theutility system distribution network. Distribution sub-stations interconnect portions of the utility systemdistribution network, and may include transformationbetween distribution voltage levels.

DC interconnections are made with one of twospecialized types of substations. The first type, ac/dcor dc/ac conversion stations, interconnect the actransmission network of one utility system with a dc

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transmission line or network. The dc line or networkconnects with one (or more) other conversion stations.The second type, ac/dc/ac (back-to-back) conversionstations, interconnect the ac transmission network ofone utility system with the ac transmission network ofanother utility system. In effect, this is two conversionstations on the same site with the dc transmission lineinside the converter building.

Substation SvstemsExperienced substation designers break a sub-

station down into smaller pieces so that they candevelop design criteria, prepare calculations, writespecifications for purchasing and construction, andprepare descriptive drawings. Defining substationsystems and then dealing with each of them individuallymakes substation design manageable and understand-able, and these small pieces are the systems thattogether make up a substation.

A substation system is a set or arrangement ofcomponents so related or connected to perform acommon function. Each system has a definablefunction to which the system components contribute.A substation consists of many systems which may bebroadly classified into the following categories:

(a) Site Related Systems.(b) Switchyard Systems.(c) Control Building Systems.(d) Protection, Control, and Metering(e) Auxiliary Systems.

Systems.

A typical substation consists of a switchyard anda control building of some kind, as shown in Figure 1.The switchyard is usually the outdoor, open airinsulated, high voltage portion of the substation.However, the switchyard can still be air insulated, butenclosed within a building for aesthetic or environ-mental reasons. A switchyard can also be metal-enclosed and SF, gas insulated, thus requiring asmaller area than a comparable air insulatedswitchyard. Gas insulated switchyards (GIS) can beinstalled either outdoors or indoors.

The control building contains the equipment thatprotects, controls, and monitors the switchyard. Thecontrol building may be a small pre-engineered metalbuilding, or a large masonry and steel frame control/switchgear building that houses both protection,control, and metering equipment, and distributionswitchgear.

Both the switchyard and the control building areconstructed on a site that must be made secure,sometimes screened from the public, and has adequateaccess to public roads for the delivery and removal oflarge equipment and structures.

Following are descriptions of the various systemsof a typical substation, including the functions andcomponents of each system.Site-Related Svstems

Site-related systems are those that have to dowith substation security, appearance, and access. Thesesystems include the following.

Securitv Fence/Wall Svstem. The securityfences/wall system prevents entry of unauthorizedpersons to the substation, provides adequate electricalclearance from energized buses and equipment to areasaccessible to the public, and provides entry to thesubstation for equipment delivery, removal, andmaintenance. For some substations, another functionof this system is to screen the substation from thepublic. The components of the security fence/wallsystem are the fence or wall, the gates that allow entryto the substation, and the below-grade foundations thatsupport the fence or wall.

Site Access Svstem. This system allows accessto the substation from public roads, and access insidethe substation fence/wall for the installation, removal,and maintenance of the substation equipment, buswork,and structures. The site access system includes theaccess road from the designated public road to thesubstation, and the access ways and corridors estab-lished within the substation.

Site Grading, Drainage, and SurfacingSvstem. The site grading, drainage, and surfacingsystem provides a reasonably level switchyard for accessto equipment, positive drainage of storm water fromthe switchyard, a driveable surface within theswitchyard, and a layer of constant resistivity crushedrock above the ground grid for personnel safety. Thesite grading, drainage, and surfacing system includesthe drainage facilities and surfacing materials selected.Drainage facilities may simply consist of sloping theswitchyard from a higher elevation at the center to alower elevation at the fence lines, or may include aseries of collection basins piped together andcollectively drained to a retention facility such as apond or the city storm-water sewer system.

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FENCE ORWALL-

ACCESS

.....................,

......- I.... /. .- .........

. . . . . . . ....... ....-..... -,.

PUBLIC ROAD

Figure 1. Typical Substation

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Siie LandscaDinQ Svstem. The site landscap- Bus Svstem. The bus system interconnects theing system beautifies the site and complements the sub- high voltage portions of the various components of thestation fence/wall to screen the substation from public switchyard to form the required bus configuration forview. This system includes the various plantings and the substation.any sprinkler (irrigation) facilities selected for theparticular substation.Switchvard Svstemsfacilities installed in the substation high voltage conductors.switchyard. These systems include the following.

The components of the bus system include therigid and strain bus conductors, the fittings used toconnect the bus conductors to the switchyard equip-

Switchyard systems are those that describe the ment, and the insulators that support the bus

Measurinq and Relavinq CommunicationsSwitchinq EauiDment Svstem. The switching Equipment Svstem. The measuring and relayingequipment system connects and disconnects elements communications equipment system provides low voltageof the substation or utility system from the rest of the or low current inputs to the protective relaying andsubstation or utility system. Some components of this metering equipment which are proportional to thesystem, such as the circuit breaker, are capable of voltage or current which exists in the substation busesinterrupting (disconnecting) the very large quantities of and equipment, and transmits protective relaying equip-current associated with electrical short circuits, or ment communication signals on transmission line con-faults. Other components can switch (connect or dis- ductors. The measuring and relaying communicationsconnect) normal levels of load current, and some can equipment system includes potential transformersbe operated only if little or no current is flowing. (PTs), coupling capacitor voltage transformersSome components are normally operated electrically (CCVTs), current transformers (CTs), bushing currentfrom a remote location; others can only be operated transformers (BCTs), line traps, and line tuning units.manually at the component location. Other less widely used components include bushing

potential devices and linear couplers.The components of the switching equipment

system include circuit breakers, circuit switchers, circuit Reactive Power Compensation Equipmentreclosers, and disconnect switches. Power fuses can Svstem. The reactive power compensation equipmentalso be included in this system. The supporting system supplies large quantities of capacitive or reactivefoundation for circuit breakers and any other system power for power factor improvement or voltage con-equipment not supported by a switchyard structure are trol, limits fault current on buses or distribution lines,also included within this system. and supplies low impedance tuned paths to ground for

harmonic voltages, which are "nuisance" voltagesPower Transform ation Equipment Svstem. occurring at frequencies above 60 hertz.The power transformation equipment system transferspower between voltage levels in the utility system. In The components of the reactive power compen-effect, the components of this system create the trans- sation equipment system include capacitor banks andmission and distribution voltage levels of the utility reactors, installed individually or in combinations. Thesystem. Power is generated at the voltage level of the most common form of reactive power compensation isgenerator, and must be transformed up to transmission the installation of bulk capacitor banks within a sub-voltage levels, then back down to distribution voltage station which are switched on or off to supply capaci-levels by power transformers. The components of this tive power to the system. Current limiting reactors aresystem are the various types of power transformers sometimes installed on distribution feeders to addfound throughout the utility system, such as generator impedance to the feeder source impedance, thus limit-step-up transformers, transmission step-up trans- ing the current that can flow if the feeder is faulted.formers, and distribution step-down transformers. The Harmonic filters are installed in HVDC stations tosupporting foundation and oil spill containment short or shunt harmonic voltages to ground, thus pre-facilities for the power transformers are also included venting these harmonic voltages from entering thewithin this system. utility system. The supporting foundations for any

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system equipment not supported by a switchyard struc-ture are also included within this system.

Direct Stroke and Surge Protection Svstem.The direct stroke and surge protection system protectsthe switchyard and control building from being struckby lightning, and protects the insulation system of theswitchyard equipment from transient, high voltagesentering the substation from the transmission or distri-bution systems. These transient voltage waves can becaused by lightning strikes to the transmission or distri-bution lines, or from switching of the transmissionsystem. The components of this system include theshielding masts and wires and the surge arrestersinstalled within the switchyard. The supporting founda-tions for lightning masts are also included within thissystem.

Switchvard Support Structure Svstem. Theswitchyard support structure system provides supportfor the switchyard equipment and bus at the elevationsneeded for adequate electrical clearance from grade tothe bus or equipment live parts, and terminates out-going transmission or distribution line conductorswithin the switchyard. The components of this systeminclude the various stands for disconnect switches,measuring equipment, bus support insulators, surgearresters, and termination structures for overhead orunderground transmission and distribution lines. Thefoundations for the structures would be included withinthis system.

Grounding Svstem. The grounding systemprotects personnel within the substation from encoun-tering large potential differences during voltage orcurrent transients, provides a low impedance path totrue earth for proper protective relaying operation, anddissipates into the earth large current flows fromlightning strikes or faults. The components of thegrounding system include the buried ground rods andconductors that form the ground grid, and the con-ductors, called stingers, that attach equipment andsupport structures to the grid.

Racewav Svstem. The raceway system con-tains, supports, and protects from physical damagecontrol and power cables within the switchyard. Mostswitchyard raceway system components are installedbelow grade. The components of the raceway systemwithin the switchyard can include cable trench, ductbanks, conduit, manholes, junction boxes, and pullboxes.

Linhtina and Comm unication Svstem. Thelighting and communications system illuminates theswitchyard for security, illuminates switchyardequipment for emergency switching and repair, andprovides a means for personnel to be paged and to usetelephone communications. The lighting and communi-cations system includes the various light fwures,lighting control devices and interconnecting wiring, andthe telephone and paging devices and interconnectingwiriing installed throughout the switchyard.Control B uilding Svstems

Control building systems are those that describethe structure and supporting facilities that compose thecontrol building. These systems include the following.

Buildinn Architectural Svstem. The buildingarchitectural system provides a durable, weatherproof,and attractive enclosure for the substation equipmentrequiring indoor installation. The components of thebuilding architectural system include the roof, walls,interior partitions, doors, windows, penetrations, floorcoverings and paints, and plumbing. The selection ofbuilding appearance and colors would be made as partof the design of this system. Fireproofing of walls androof would also be designed as part of this system.

Building Structural Svstem. The buildingstructural system supports the roof and walls of thebuilding, and equipment mounted within the building,and provides level floors within the building. Thecomponents of the building structural system includethe structural steel frame, floors, and foundation.

Building Space Conditioning Svstem. Thebuilding space conditioning system provides a clean,uniform temperature and humidity environment withinthe building, and exhausts fumes and odors from insidethe building. This system is also known as the heating,ventilating, and air conditioning (HVAC) system. Thecomponents of the building space conditioning systemare the heating, air conditioning, and ventilating devicesinstalled within the building; the ductwork used to dis-tribute or collect air within the building; and thecontrols that operate the system equipment.

Building Grounding Svstem. The buildinggrounding system protects personnel from electricshock by connecting equipment enclosures to thesubstation grounding grid and provides low resistancepaths to ground for protective relaying equipment.Depending on the types of equipment installed within

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the building, the building grounding system may alsoprovide a low impedance path for fault current flow toground. The building grounding system is connected tothe switchyard grounding system and is actually anextension of the switchyard grounding system.

The components of the building groundingsystem are the ground rods and conductors buriedbelow the ground floor slab that extend the switchyardground grid below the building, the conductors thatground the steel frame of the building, and theconductors that ground the raceway and equipmentenclosures within the building.

Buildinq Racewav Svstem. The building race-way system contains, supports, and protects fromphysical damage control and power cables within thecontrol building. The components of the buildingraceway system can include cable trench, cable tray,conduit, electrical metallic tubing, wireway, junctionboxes, and pull boxes.

Buildinq Liqhtincl and CommunicationSvstem. The building lighting and communicationssystem illuminates the exterior entrances to the build-ing for security and the interior of the building foroperation and maintenance of the equipment installedwithin the building, emergency egress, emergencyoperation and repair of equipment within the building,and provides a means for personnel to be paged and touse telephone communication.

The components of this system include thevarious light fMures, lighting control devices and inter-connecting wiring, and the telephone and pagingdevices and interconnecting wiring installed throughoutthe control building. The switchyard lighting andcommunications system is an extension of this system.Protection, Control, and Meterina SvstemsProtection, control, and metering systems describe theprotective relaying, local and remote control, indication,monitoring, annunciation, and metering equipmentincluded in most substations. These systems includethe following.

Protective Relavinq Svstem. The protectiverelaying system protects substation equipment or theutility system from damage and limits the damage offaulted equipment by monitoring the operation of theutility system and by taking action automatically if theparameters being monitored exceed the limits presetfor the relaying system. The parameters beingmonitored can be voltage, current, or frequency, or

some combination of the three. The action taken canbe the tripping (opening) or closing of a switchingdevice such as a circuit breaker. The protectiverelaying system for a given substation is subdivided intorelaying schemes, each scheme designed to monitorand protect a portion (zone) of the substation or utilitysystem. Examples are the protection of powertransformers, substation buses, or transmission lines.The components of this system include the variousrelays and associated relaying communicationsequipment that make up the protective relayingschemes for the substation.

Control Svstem. The control system providesa means of manually operating electrically operateddevices, either from within the substation controlbuilding, or from a remote operating (dispatch) facility,and monitors selected system parameters and auto-matically operates equipment under normal circum-stances within preset limits. Devices that are manuallyoperated by this system can include circuit breakers,circuit switchers, power transformer load tap changers,and motor-operated disconnect switches. Examples ofnormal automatic operation a re the changing of powertransformer taps or the switchingon or off of capacitorbanks. The components of the control system includethe control switches mounted on the control panelswithin the control building, the automatic controlschemes installed within the control panels within thecontrol building, and the SCADA equipment whichallows remote operation of the substation equipment.

Meterinq Svstem. The metering system pro-vides a quantitative measurement of system parametersand displays those measurements for operator infor-mation or for record. Meters can show the amount ofvoltage to ground or between phases of substationbuses, the amount of current flowing in a substationbus, transmission line, or transformer, or the amountof instantaneous, average, or accumulated real orreactive power flowing through a substation bus, trans-mission line, or transformer. The metering system canalso input to the SCADA equipment, displaying read-ings of system parameters at the remote dispatchcenter. The components of the metering systeminclude the meters installed on the control panelswithin the control building. Also included are thetransducers that convert system parameters intoSCADA input signals.

Indication and Annunciation Svstem. Theindication and annunciation system informs operatingpersonnel of the status of switchyard equipment (openor closed) and draws attention to the misoperation or

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abnormal condition of substation equipment. Theindication of equipment status can be displayed withindicating lights installed on the control panels withinthe control building or displayed in the remote dispatchcenter via the SCADA equipment. The annunciationof equipment misoperation can be displayed by aflashing annunciator window and horn within thecontrol building or displayed in the remote dispatchcenter via the SCADA equipment.Auxiliarv Svstems

Auxiliary systems describe the auxiliary powerfacilities, interconnecting cabling, and fire protectionequipment that are included in most substations.These systems include the following.

AC Station Service Svstem. The ac stationservice system supplies electric power for the normaloperation of equipment auxiliaries, space conditioning,and lighting within the substation. Equipment a d -aries include power transformer cooling pumps andfans, power transformer load tap changers, circuitbreaker operating mechanism compressor motors, andequipment enclosure space heaters. The componentsof this system include the system source transformers,transfer switches, panelboards, ransformers required toreduce voltage levels within the system, and safetyswitches required for equipment power disconnection.

DC Station Service Svstem. The dc stationservice system supplies reliable electric power for theoperation of the protection and control systems, for theemergency egress of personnel, and operation of thesubstation. The dc station service system is normallysourced from the ac station service system through thebattery chargers which convert ac power to dc power.If the ac station service system loses its sources, the dcstation service system continues to supply power to itsconnected loads from the substation battery for a pre-determined period. The components of this systeminclude the substation battery, battery charger, transferswitches, and panelboards.

Power and Control Cable System. Thepower and control cable system interconnects the lowvoltage measurement and control components of theequipment within the switchyard and control buildingto implement the various protective relaying, control,metering, indication, and station service systems. Thissystem includes low voltage cables, usually rated600 volts or less.

The components of this system are the varioustypes of insulated cables used throughout thesubstation.

Fire Protection Svstem. The fire protectionsystem detects and a l a r m s fires within the controlbuilding or in the area of major switchyard equipmentsuch as power transformers and provides a means ofextinguishing fires.

This system includes the various detectors andalarm system devices that detect fires and the extin-guishers, hose cabinets, deluge systems, and otherdevices to extinguish fires.

Bus ConfiaurationsThe equipment and buses installed in the

substation switchyards are arranged and connected inspecific ways to form bus configurations. The industryhas developed several standard bus configurations thatvary in complexity, cost, and reliability.

The standard bus configurations are the radialbus, sectionalized radial bus, main and transfer bus,single breaker double bus, ring bus, one-half breaker,breaker and one-half, and double breaker double bus.The layout of a substation for any particular con-figuration may vary to accommodate differences inequipment type, size and arrangement, and site specificcriteria.Radial Bus

The radial bus configuration is shown inFigure 2, and consists of one main bus to which lines,transformers, and shunt capacitor banks are connectedthrough circuit breakers, circuit switchers, or motor-operated or manually-operated disconnect switches.

Radial bus substations are the simplest tooperate, but have the least system reliability andflexibility of operation. Bus faults and failure of abreaker to operate for a fault require an outage of thecomplete substation. In radial bus substations, it isnecessary to take an outage of a circuit to performperiodic or emergency maintenance on its associatedcircuit breaker.

As shown in Figure 3, breaker bypass switchescan be installed to allow removal of a circuit breakerfrom service for maintenance without an outage of theassociated circuit, but this leaves the circuit without

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Main BusIr AT

Disconnect4 k E l ~ E : r Switch,-j $Circuit Leading ToOther SubstationsI I I IFigure 2. Radial Bus

Main Bus1ircuitBreaker

Normally OpenBypass SwitchSwitchDisconnect

Circuit1.Figure 3. Breaker Bypass Switchrelay protection. When the breaker is isolated formaintenance, the bypass switch is closed, and thecircuit breaker and its associated disconnect switchesare opened. All protective relaying and control for thecircuit at the local substation are removed from servicewhen the circuit breaker is isolated. A fault on thecircuit with its associated circuit breaker bypassedrequires an outage of the complete substation.

Advantages of the radial bus substations overother configurations are lowest cost, small requiredland area, ease of expansion, simple operation, andsimple protective relaying. Disadvantages include lowreliability, low flexib&ty of operation for maintenance,and the removal of the substation from servicein casesof bus faults and failure of a breaker. The radial busconfiguration is generally applied in substations from


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distribution voltage through 161 kV and in locationswhere system reliability is not critical.Sectionalized Radial Bus

The sectionalized or split radial bus is shown inFigure 4, and is a modification of the radial bus. Thisconfiguration is two radial buses tied together througha sectionalizing or bus tie circuit breaker. The sec-tionalizing circuit breaker can be operated normallyopen or normally closed, depending on system require-ments. Bu s faults or the failure of a breaker (otherthan the tie breaker) to operate for a fault requires anoutage of only the affected bus section.

The grouping of circuits on bus sections isdetermined by examining system operating criteria.Circuits should be arranged to prevent outages onsimilar or redundant circuits. Circuits fed from thesame source or circuits feeding the same load shouldbe on different bus sections.

Breaker bypass switches can be applied insectionalized radial bus substations and operate thesame as in radial bus substations.

Sectionalized radial bus substations offer theadvantages of small land area, increased reliability overradial bus, increased flexibility of operation over radialbus, and ease of expansion. Disadvantages when com-pared to the radial bus include increased cost,increased complexity of operation, and increased com-plexity of protective relaying.

The sectionalized radial bus configuration isgenerally applied in substations from distributionvoltage through 161kV and in locations where systemreliability is not critical.Main and Transfer Bus

The main and transfer bus is shown in Figure 5,and is another modification of the radial bus. Thisconfiguration consists of a main bus and a transfer bus.All circuits are connected to the main bus throughcircuit breakers and to the transfer bus through trans-fer switches. The main and transfer buses are con-nected through a transfer bus circuit breaker.

The transfer bus circuit breaker protects a circuitduring maintenance of its associated circuit breaker.When a circuit breaker is removed from service formaintenance, the transfer circuit breaker and itsassociated disconnect switches are closed, the transferswitch for the circuit breaker to be serviced is closed,and the circuit breaker to be maintained and its asso-

ciated disconnect switches are opened. Reliability andprotection are not compromised during maintenance.Considerable attention must be given to the selectionof the protective relaying for the transfer circuitbreaker.

The advantages of main and transfer bus sub-stations when compared to other configurations ncludethe small land area required, ease of expansion,increased flexibility of operation over radial bus or splitradial bus, and the fact that any breaker can beremoved from service without an outage of the circuitserved. Disadvantages over the radial bus are theincreased cost, increased complexity of operation,increased complexity of protection, and no improve-ment in reliability.

The main and transfer bus configuration is gen-erally applied in substations from distribution voltagethrough 161kV and in locations where system relia-bility is not critical.Sinqle Breaker Double Bus

The single breaker double bus configuration(Figure 6) is a modification of the sectionalized radialbus. This configuration consists of two main busesconnected through a tie circuit breaker. Each circuithas one circuit breaker that can be connected to eithermain bus through disconnect switches. This configura-tion allows circuits to be connected to either main busto balance load, separate critical circuits, or placesources on each bus, and allows all circuits to beconnected to one bus in case of an outage on the otherbus. Switching of a circuit from one bus to the otheris not automatic, and requires manual switching.

Single breaker, double bus substations have thesame advantages and disadvantages as the split busradial, and additional disadvantages including ncreasedcost over split radial bus, increased complexity ofprotective relaying over split radial bus because of therequirement for switching of bus relaying current trans-former secondary circuits.

The single breaker double bus configuration isgenerally applied in substations from distributionvoltage through 161kV and in locations where systemreliability is not critical. It is also the least common ofthe radial bus configurations discussed.

Ring BusThe ring bus configuration (Figure 7) is in realitya series of sectionaliied radial buses connectedtogether to form a ring. Each bus is called a position.

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I

Bus-Tie Circuit BreakerfMain Bus No. 2

a

L B r e a k e rircuit x isconnectMain Bus No. 1

Switch

J Circuit

* f *Figure 4. Sectionalized Radial Bus

Main Bus

Figure 5. Main and Transfer BusSometimes a transmission line and a transformer arepaired on one ring position.

In this configuration, only one position isremoved from service for a circuit or bus fault. Thecircuit breakers which serve the faulted position areopened. The failure of a breaker to operate for a lineor bus fault will cause two positions to be removedfrom service.

This configuration allows for any circuit breakerto be removed from service for maintenance without anoutage on any circuit.

Line disconnect switches are often installed toallow a line to be removed from service and the ring toremain intact. The two circuit breakers sourcing theline are opened, the line disconnect switch is opened,and then the two circuit breakers are closed.

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- vMain Bus No. 2

Figure 6. Single Breaker Double Bus

Transfer-7reakerircuit

Ring bus substations are highly reliable and figuration, one of the breakers is usually at the otherflexible to operate. They are generally limited to a end of the transmission line. In Figure 8, Substa-maximum of eight positions to prevent splitting of the tions A, B, C, D, and E form an extended ring bus.ring. Sources of generation or redundant circuitsshould not be terminated on adjacent positions of the The advantages of this configuration are thering bus. This prevents a failed circuit breaker from same as for the ring bus, and on an individual sub-removingtwo sources of generation or two feeds to the station basis, the costs are even lower than for thesame load from service. radial bus.

T w

Ring bus substations have the advantages of highreliability, flexible operation, and low cost whencompared to the breaker and one-half configuration,removal of any breaker from service without circuitoutage, and the possibility of expansion to breaker andone half configuration. The disadvantages are thecomplex protective relaying and control and theeight-position limitation.

The ring bus configurationis generally applied insubstations from 115 kV through 345 kV, and inlimited application at 500 kV in locations where highsystem reliability is a requirement.One-Half Breaker

The one-half breaker configuration, shown inFigure 8, is a variation of the ring bus concept on amultiple substation basis. As with the ring bus, twobreakers must be tripped to isolate a faulted line ortransformer. In the case of the one-half breaker con-

The one-half breaker configuration is generallyapplied in substations from 69 kV hrough 161kV, ndin systems where several substations are located neareach other.Breaker and OneH alf

The breaker and one-half configuration is shownin Figure 9, and consists of two main buses.Connected between the main buses are bays whichconsist of three circuit breakers. A circuit isterminated between each two circuit breakers. In thisconfiguration, each circuit has a dedicated circuitbreaker and shares a circuit breaker with the adjacentcircuit, resulting in one and one-half breakers percircuit.

Frequently, a substation is designed to operateinitially as a ring bus up through expansion to sixpositions. Beyond six positions, the substation evolvesto a breaker and one-half configuration.

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Load

Source

tCircuit Disconnect+ - v - 4 witchftwBL+Source.tLoadFigure 7. Ring Bus (Four Position)

Sub Ar - - - - 1 Sub Dr - - - -1

Figure 8. One-Half Breaker Configuration

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-/

No. 1

x+

Main B u s

DisconnectSwi cliI$- ?t/ CircuitBreaker

Figure 9. Breaker and One-HalfTwo types of breaker and one-half contlgura- a main bus and a circuit to operate for a main bus faulttions, conventional and folded, are shown in Figures 9 requires that only the circuit adjacent to the circuit

and 10. In the conventional arrangement, transmission breaker be removed from service. The failure of alines must pass over one of the main buses, causing circuit breaker between two circuits to operate for aline termination structures to have higher pull-off fault requires the two adjacent circuits be removedpoints. Also, installation of line traps, current from service.transformers, and disconnect switches in the lines isdifficult. The folded arrangement locates line termina- This configuration allows any circuit breaker totion structures outside the main buses, allowing con- be removed from service for maintenance without anventional pull-off heights to be used. Installation of outage on any circuit.line traps, current transformers, and disconnectswitches in the lines is relatively easy. Also, the folded Line disconnect switches are sometimes installedarrangement can be "fitted to oddly shaped sites more to allow a circuit to be removed from service and alleasily than can the conventional arrangement. circuit breakers to remain closed.

In this configuration, only one circuit, the faulted Breaker and one-half substations are very reli-circuit, is removed from service for a fault. A main able and flexible in operation. Sources of generationbus fault does not require that circuits be removed or redundant circuits should not be connected in thefrom service. The failure of a circuit breaker between same bay. This prevents a failed breaker from

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LineDisconnectSwitch

Main MainBus BusNo. 1 No. 2

DisconnectSwitch

Figure 10. Folded Breaker and One-Halfremoving two sources of generation or two feeds to thesame load from service.

Advantages of breaker and one-half substationsover other configurations are very high reliability, veryflexible operation, removal of any breaker from servicewithout a circuit outage, and ease of expansion, with nolimit on the number of bays. Disadvantages are thelarge land area required, the high cost, and the com-plex protective relaying and control.

The breaker and one-half configuration is gen-erally applied in substations from230kV through ultra-high voltages, but can be applied at 69 kV, 115 kV,138kV, nd 161 kV. Because of its high reliability, itis often applied at major generation facilities and atlocations where system reliability is critical.Double Breaker Double Bus

The double breaker double bus configuration isshown in Figure 11and consists of two main buses.Connected between the main buses are bays consistingof two circuit breakers, and between the circuitbreakers, a circuit. In this configuration, each circuithas two dedicated circuit breakers. Only the faultedcircuit is removed from service for a fault. A bus fault

requires that no circuits be removed from service. Thefailure of a circuit breaker to operate for a bus faultrequires only that the circuit terminated in that bay beremoved from service.

This configuration allows any circuit breaker tobe removed from service for maintenance without anoutage on any circuit. Line disconnect switches areusually not required.

Double breaker double bus substations are themost reliable and are very flexible to operate. Theyrequire no separation of sources of generation orredundant circuits.

Double breaker double bus substations have theadvantages of very high reliability, very flexible opera-tion, the fact that breaker removal for maintenancewillnot cause an outage, and ease of expansion. Dis-advantages include high cost, the large land arearequired, and the complex protective relaying andcontrol.

A large substation may include both breaker andone-half bays and double breaker double bus bays totake advantage of the features of both.


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Bu s No . 24Figure 11. Double Breaker Double Bus

The double breaker double bus configuration isgenerally applied in substations from 230 kV throughultra-high voltages, nuclear generating facilities, majorgeneration facilities, and locations where system relia-bility is very critical.

Comparison of Bus ConfiQurationThe following tabulation compares the relative

constructed costs and levels of reliability of eachconfiguration for a substation serving six transmissionlines.

Approximatepe r Unit Cost Reliability

Radial 1.00 6Sectionalized Radial 1.17 5Main and Transfer 1.29 4Single Breaker 1.29 4Double BusOne-half Breaker 0.8-1.25 3.5Ring Bus 1.25 3Breaker and One-Half--Conventional 1.45 2--Folded 1.48 2Double Breaker 1.75 1Double Bus

Usually, some bus configurationscan be elimin-ated from consideration for a particular substation onthe basis of its function. A radial bus configurationwould not be considered for a nuclear generatingstation, nor would a double breaker, double bus con-figuration be considered for a distribution substation.Selection of a bus configuration for a particular sub-station should always take into account the ultimateanticipated development and function of that installa-tion. Figure 12 shows at what voltage levels eachconfiguration is typically applied.

Tvpes of ConstructionThe types of construction typically used for

switchyards in the US include the box structure, lowprofile rigid bus, low profile strain bus, and gasinsulated types.Box Structure

The box structure is generally applied at 138kVand below. It requires the least amount of land areaand uses layers of bus, disconnect switches and relatedequipment, one above the other, connected with ver-tical bus runs, and supported on a common structure.The support structure is generally structural steel con-struction. Rigid or strain bus can be used. The busconfigurations most easily applied with a box structureare radial, split bus radial, main and transfer, and

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69 k V 11 5 kV 138 kV 161 k V 230 k V 345 k V 500 kV- - _ _ _ _ _ _ _ - - -Radial/Sectionalized RadialMain and TransferSingle Breaker Double BusRing BusBreaker and One-HalfDouble Breaker Double BusOne-Half Breaker

Figure 12. Typical Bus Co nfiguration Voltage L evelssingle breaker, double bus. This construction isgenerally considered the least attractive, and ispredominantly installed in rural or industrial locations.Figures 13and 14 show a typical radial bus box struc-ture substation.Low Profile Riaid Bus

Low profile rigid bus construction has becomewidely used and is generally applied from 69 kVthrough EHV and UHV levels. The main and crossbuses run horizontally on two relatively low levels withthe substation equipment supported on individual struc-tures. This construction requires more land area thanthe box structure but provides a much less massiveappearance. Rigid bus is used to allow minimumphase-to-phase spacings. All of the bus configurationscan be applied using this construction. This construc-tion is generally installed in all locations, and has beensuccessfully installed in residential areas. Figures 15and 16 show a typical low profile, rigid bus substation.Low Profile Strain Bus

Low profile strain bus construction is similar inconcept to the low profile, rigid bus construction,except that overhead strain buses are used. It is gener-ally applied from 69 kV through EHV and UHV levels.Wider phase spacings must be used to compensate forstrain bus movement. Thus, strain bus substationsgenerally require larger land areas than do comparablerigid bus designs. Also, strain buses must containmultiple conductors to have equivalent ampacity ratingswith rigid bus. Strain bus designs are extensively usedat UHV levels because the very large phase spacingsrequired make the support of rigid bus more costly.Also, strain bus is used in high seismic areas to allowsome movement of equipment and bus during seismicevents. Allbus configurations can be applied using this

construction. Figures 17,18, and 19 show a typical lowprofile strain bus substation.Gas Insulated Construction

Gas insulated construction consists of completelyenclosed.buses and equipment insulated with SF6 gas.Because of the excellent insulating properties of thisgas, very compact phase spacings and therefore, sub-stations,can be constructed. Gas-insulated substationsare generally installed where land area for the sub-station is extremely limited and/or environmental con-tamination is severe. Since gas insulated substationsare shipped as factory-assembled units or modules,field erection time and cost are reduced. AU bus con-figurationscan be applied using this construction. It isgenerally applied from 115 kV through EHV levels.

Com parison of Con structionsAll of these constructions have applications for

which they are best suited. If a 115 kV substation isneeded to serve an industrial complex in a purely indus-trial area, appearance is not a principal criterion in thedesign. To provide guidance in the selection of thebest construction for a given situation, the tabulationbelow ranks each construction in terms of overall cost,land area required, and appearance. A similar busconfiguration with an identical number of circuitbreakers, switches, lines, etc., is considered for eachconstruction. A ranking of one is assigned the lowestin cost, least in land area required, and best inappearance.

Land Area-ost Reauired AmearanceOutdoor Box 1 2 4Structure

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OutdoorLow ProfileOutdoorStrain BusOutdoor GasInsulated

Figure 13. Radial Bus (Box Structure), Plan ViewLand Area a pleasing and acceptable appearance to the-ost ReQuired ADDearance neighborhood. These techniques may include one or

more of the following:3 3 2 a. Perimeter walls to enclose the substation.2 4 3 b. Special architectural materials in the con-

struction of major structures.Extra land for a buffer zone.1 1Earthen berms in the buffer zones to give

c. Extensive landscaping.d.e.As can be seen from these rankings, selection of

the type of construction to be used is a complexprocess which must be made in the context of the sitechosen. This selection directly impacts the cost of aparticular substation.

Aesthetic ConsiderationsWhen a major substation must be located in an

urban, residential area, utilities often incorporatespecial architectural techniques into the design to give

additional concealment.Completely enclosed gas insulated sub-station with underground transmissionline entrances.

f.

The use of special techniques to improve theappearance of substations increases their overall cost,but the good wi l l generated from their use often provesto be a prudent investment.

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Station PostInsulator

Figure 14. Radial Bus (Box Structure), Section A-AAn Outline for Substation Design

Attached as an appendix to this paper is anoutline of the steps in the planning, design, andconstruction of a substation. The first step is toidentify the need for the substation and what require-ments it must meet, both when first constructed and inthe future. The voltage levels, numbers of lines to beconnected, and level of reliability required aredetermined. The basic milestone dates for the projectare established. A preliminary one-line diagram andsite arrangement plan are prepared. Several alternatesites may be identified and a preferred site may beselected.

Once the need and basic requirements are deter-mined, the project is planned. This step includesestablishing a detailed schedule for the design andconstruction of the project, and preparing a preliminarycost estimate for the project. This step usually leads toobtaining authorization of the project by utilitymanagement.

Following project authorization, the conceptualdesign step establishes the basis for the detailed designof the substation. Basic information concerning thesubstation site is collected. Culminatingthis step is thepreparation of the design criteria report which docu-ments the design basis for the project.

Detailed design of the substation includes pre-paring the design calculations and drawings needed forboth the construction of the substation and aspermanent documentation of the substation. Designdrawings should always be arranged so that futureadditions can be incorporated into them as they aremade.

The equipment and materials needed for thesubstation are usualy purchased at the same time asthe detailed design is being prepared. Equipmentinformation must be included in the fmal design, andsome design must be performed before some of thematerials can be purchased. This step includes

I

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Electrical ConnectionHigh Bu s an d Bus Support1 11

A B C A B Cn i c A e c

Figure 15. Low Profile Rigid Bus, Plan ViewHigh

Rigid Bu s

Figure 16. Low Profile Rigid Bus, Section A-A

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Figure 17. Low Profile Strain BusHighStrain Bu s

Figure 18. Low Profile Strain Bus, Section A-A


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Suspension

Figure 19. Low Profile Strain B us, Section B-Bpreparing the specifications and bills of materialneeded for purchasing the equipment and materials.

The construction of the substation must bedescribed in some form of specification whether theconstruction is performed by construction contractors,or by the utilities own personnel. The design drawingsthat have been prepared for the substation alsodescribe the construction to be performed.

During construction, the design engineers shouldinspect the work to verify that it is being performed inaccordance with the specifications and drawings thathave been prepared for the substation. This step alsoincludes maintaining an accurate record of fieldchanges made to the design during construction.

After construction is complete, the designdrawings should be revised to include the field changesmade during construction, so that an accurate as-builtrecord is provided. This becomes very important whenfuture additions are made to the substation. Theproject design criteria report should also be updated toaccurately reflect the design basis for the substation.

Performing these steps will provide a wellplanned and designed substation, and a reliablepermanent record of the as-constructed substation.

ConclusionDesign of a reliable and cost efficient substation

begins with the establishing of need through systemplanning. It then progresses through an optimizinganalysis in conceptual design that balances reliability,aesthetic considerations, ultimate development, andcost; and culminates in detailed design in which cal-culations, drawings, and specifications are prepared.While this paper has discussed important concepts insubstation design, the extent to which each topic wasdiscussed has understandably been limited by time andspace. The author recommends that substationdesigners obtain and refer a t length to the publicationslisted below:1. Design Guide for Rural Substations, US

Department of Agriculture, Rural ElectrificationAdministration Bulletin 65-1, June 1978.

2. Guide for Design of Substation Rigid-BusStructures, IEEE Standard 605-1987.

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3. "Guide for Safety in Substation Grounding," 5. "The Rolling Sphere Method of Lightning Pro-tection for Substations: A Practical Application,"by Jeff C. Camden, Black & Veatch, 1990.IEEE Standad 80-1986."Guide for Determining the Maximum ElectricPower Station Ground Potential Rise andInduced Voltage from a Power Plant," IEEE4.

Standad 367-1987.

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APPENDIXAN OUTLINE FOR SUBSTATION DESIGNThis appendix lists activities that should be performedfor the complete design of almost any substation. Allof the steps listed are necessary, but the party perform-ing each step may vary depending on the capability ofthe utility for which the substation is being designed.Large utilities perform almost all of these activitiesthemselves, whereas small utilities may delegate mostof the activities to a consulting engineer. What isimportant is that it be clearly understood which activityis to be performed by which party.

I. System PlanningA. Identify the need for the substation.B. Identify the voltages to be included, and

the numbers of lines to be connected,both initially and ultimately, based on thebest planning studies available.

C. Perform system load flow and faultstudies to supply data needed for con-ceptual design.

D. Select a location for the substation basedon area needed and line routing.

11. Project PlanningA. Prepare a project CPM schedule. The

schedule evolves from a basic list ofmilestone dates determined during systemplanning to a complete design, procure-ment, and construction schedule.

B. Prepare a project cost estimate. Thisestimate is prepared initially duringsystem planning or conceptual design,updated during detailed design, and fina-lized as equipment and material procure-ment and construction are completed.

111. Conceptual DesignA. Based on the results of system planning,

develop a preliminary substation andsystem one-line diagram. This includesselecting the bus configurations to beused.

B. Based on the preliminary substation one-line diagram and the location selected,develop a preliminary substation site plan.

IV.

C .

D.

E.

F.

G.

H.

I.

J .

Develop the electrical and structuraldesign criteria to be used for the project.This includes determining the electricalclearances and spacings to be used.Determine the need for aesthetic treat-ments and environmental constraints thatw i l l affect the design.Select the equipment and bus ampacityratings based on load flow and faultstudies performed for the project.Obtain a site topographical survey for usein the grading and drainage design.Obtain soil borings at the site and per-form laboratory tests to determine thesoil parameters needed for foundationdesign.Perform soil resistivity tests at the siteto determine the values needed for thegrounding design.Develop the protection and controlschemes to be used for the substation andtransmission lines.Prepare a Project Design Criteria Reportwhich documents the design basis for thesubstation.

Detailed DesignA. Prepare the substation protection and

control one-line diagrams.B. Prepare the electrical and structural site

plan drawings. This includes any requiredlandscaping and architectural wall design.

C. Prepare the switchyard bus and equip-ment arrangement plan, section, anddetail drawings.

D. Prepare the switchyard direct strokelightning shielding analysis, and add theshielding masts and wires to the arrange-ment drawings.

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I

E.

F.

G.

H.

I.

J.

K.

L.

Prepare the switchyard support structuredrawings.

3. Control (dc) schematic diagrams.4. Panel internal wiring diagrams.

Prepare the switchyard foundation design,foundation plan, and d e t d drawings. 5. Interconnecting wiring diagrams.Prepare the switchyard grounding design,grounding plan, and detail drawings.

6. Circuit and raceway list.V. Equipment and Material Procurement

A. Prepare specifications and/or bills ofmaterial and procurement documentsneeded to purchase the equipment andmaterials for the substation.

Prepare the switchyard raceway design,raceway plan, and detail drawings.Prepare the switchyard lighting design,lighting plan, and detail drawings.

B. Bid, evaluate, and award contracts/purchase orders for equipment andmaterials.Prepare the control building equipmentarrangement plan and elevation drawings.

Prepare the control building architecturalplan and detail drawings.

C. Review manufacturers' shop drawings forequipment and materials. Revise thedesign drawings to reflect the actualequipment purchased.repare the control building structural

steel design and structural steel plan,and section and detail drawings, ifrequired.

VI. Construction Document PreparationA. Prepare specifications and procurement

documents needed to contract for con-struction of the substation. Generallytwocontracts are awarded, as follows.

Prepare the control building foundationdesign, foundation plan, and detaildrawings.

M.

1. General construction to preparethe site and construct foundationsand the control building.N. Prepare the control building groundingdesign, grounding plan, and detail

drawings. 2. Electrical construction to installgrounding, raceway, equipment,structures, bus, cables, etc.0.

P.

Q.

Prepare the control building racewaydesign, raceway plan, and detail drawings.

B. Bid, evaluate, and award contracts forconstruction.

Prepare the control building lightingdesign, lighting plan, and detail drawings.Prepare the control building space condi-tioning design, space conditioning plan,and detail drawings.

C. Review contractor shop drawings forcontractor-furnished equipment andmaterials.

D. Review contractor field tests of concretestrength, soil compaction, etc., forcompliance with the constructionspecification.

Prepare the protection and control draw-ings for the substation, then include thefollowing:

R.

1. Protection and control panel frontelevation drawings. VII. Construction Management

The activities for construction management varywidely depending on the utility's preferences.Usually, constructioncontract administrationand2. Three- l ine (ac schemat ic)diagrams.

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construction inspection are performed. As-builtfield records are kept.VIII. Engineering Completion

A. Revise the design drawings to include anyas-built field changes.

B. Update the Project Design CriteriaReport with any changes made during theproject to maintain an accurate finalrecord of the design basis.

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fundamental concepts in substation design

Documents