power dist. systems - eaton

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CA08104001E For more information visit: www.EatonElectrical.com

June 2006

Contents

Power Distribution Systems1

Sheet 0019

PowerD

istributio

n

Sy

stem

s

Power Distribution Systems

System Design

Basic Principles. . . . . . . . . . . 1.1-1

Modern Electric PowerTechnologies . . . . . . . . . . . 1.1-1

Goals of System Design . . . 1.1-2

Voltage Classifications; BILs Basic Impulse Levels . . . . . 1.1-4

3-Phase TransformerWinding Connections . . . . 1.1-5

Types of Systems Radial,Loop, Selective, Two-Source,Sparing Transformer, SpotNetwork, Distribution . . . . 1.1-6

Health Care FacilityDesign Considerations . . . 1.1-14

Generator Systems . . . . . . 1.1-17

Generator System Design

Types of Generators. . . . . . . 1.2-1

Generator Systems . . . . . . . 1.2-2

Generator Grounding. . . . . . 1.2-3

Generator Controls. . . . . . . . 1.2-4

Generator Short CircuitCharacteristics . . . . . . . . . . 1.2-4

Generator Protection . . . . . . 1.2-5

System Analysis

Systems Analysis . . . . . . . . . 1.3-1

Short Circuit Currents . . . . . 1.3-2

Fault Current WaveformRelationships . . . . . . . . . . . 1.3-3

Fault Current Calculationsand Methods Index . . . . . . 1.3-4

Determine X and R fromTransformer Loss Data . . . 1.3-19

Voltage DropConsiderations . . . . . . . . . . 1.3-23

System Application Considerations

Capacitors/Power Factor. . . 1.4-1

Overcurrent Protectionand Coordination

. . . . . . . . 1.4-3

Protection of Conductors. . . 1.4-5

Circuit Breaker CableTemperature Ratings

. . . . .

1.4-5

Zone Selective Interlocking . 1.4-5

Ground Fault Protection . . . 1.4-6

Suggested GroundFault Settings . . . . . . . . . . . . .

1.4-6

Grounding/Ground Fault Protection

Grounding Equipment,System, MV System,LV System . . . . . . . . . . . . . .

1.4-6

Ground Fault Protection . . . .

1.4-11

Lightning and SurgeProtection . . . . . . . . . . . . . .

1.4-14

Grounding Electrodes. . . . . .

1.4-14

Power Quality

Terms, Technical Overview . .

1.4-15

TVSS . . . . . . . . . . . . . . . . . . .

1.4-16

Harmonics andNonlinear Loads . . . . . . . . .

1.4-18

UPS . . . . . . . . . . . . . . . . . . . .

1.4-22

Other Application Considerations

Secondary Voltage . . . . . . . .

1.4-28

Energy Conservation . . . . . .

1.4-29

Building Control Systems . .

1.4-30

Cogeneration. . . . . . . . . . . . .

1.4-30

Emergency Power. . . . . . . . .

1.4-30

Peak Shaving. . . . . . . . . . . . .

1.4-31

Sound Levels. . . . . . . . . . . . .

1.4-32

Reference Data

IEEE Protective RelayNumbers . . . . . . . . . . . . . . .

1.5-1

Codes and Standards . . . . . .

1.5-6

Motor ProtectiveDevice Data. . . . . . . . . . . . .

1.5-7

Chart of Short CircuitCurrents for Transformers . .

1.5-9

Transformer FullLoad Amperesand Impedances . . . . . . . . .

1.5-10

Transformer Losses,TP-1 Losses . . . . . . . . . . . . .

1.5-12

Power Equipment Losses . . .

1.5-13

NEMA Enclosure Definitions

. .

1.5-13

Cable R, X, Z Data . . . . . . . . .

1.5-15

Conductor Ampacities . . . . .

1.5-16

Conductor TemperatureRatings . . . . . . . . . . . . . . . .

1.5-16

Formulas and Terms. . . . . . .

1.5-18

Seismic Requirements . . . . .

1.5-19

Designing a Distribution System

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Power Distribution SystemsSystem Design

Sheet 0021

Basic Principles

The best distribution system is onethat will, cost effectively and safely,supply adequate electric service toboth present and future probable

loads this section is included to aidin selecting, designing and installingsuch a system.

The function of the electric powerdistribution system in a building orinstallation site is to receive power atone or more supply points and deliverit to the individual lamps, motors, andall other electrically operated devices.The importance of the distributionsystem to the function of a buildingmakes it almost imperative that thebest system be designed and installed.

In order to design the best distributionsystem, the system design engineer

must have information concerning theloads and a knowledge of the varioustypes of distribution systems that areapplicable. The various categories ofbuildings have many specific designchallenges, but certain basic principlesare common to all. Such principles,if followed, will provide a soundlyexecuted design.

The basic principles or factors requir-ing consideration during design ofthe power distribution system include:

Functions of structure, presentand future.

Life and flexibility of structure.

Locations of service entrance anddistribution equipment, locationsand characteristics of loads,locations of unit substations.

Demand and diversity factorsof loads.

Sources of power; includingnormal, standby and emergency(see Section 42

).

Continuity and quality ofpower available and required(see Section 41

).

Energy efficiency and management.

Distribution and utilization voltages.

Bus and/or cable feeders.

Distribution equipment andmotor control.

Power and lighting panelboardsand motor control centers.

Types of lighting systems.

Installation methods.

Power monitoring systems.

Electric utility requirements.

Modern Electric PowerTechnologies

Several new factors to consider inmodern power distribution systemsresult from two relatively recent

changes. The first recent change isutility deregulation. The traditionaldependence on the utility for problemanalysis; energy conservation mea-surements and techniques; and asimplified cost structure for electricityhas changed. The second change is lessobvious to the designer yet will havean impact on the types of equipmentand systems being designed. It is thediminishing quantity of qualified build-ing electrical operators; maintenancedepartments; and facility engineers.

Modern electric power technologiesmay be of use to the designer and

building owner in addressing thesenew challenges. The advent of micro-processor devices (smart devices)into power distribution equipment hasexpanded facility owners options andcapabilities, allowing for automatedcommunication of vital power systeminformation (both energy data andsystem operation information) andelectrical equipment control.

These technologies may be grouped as:

Power monitoring and control.

Building management systemsinterfaces.

Lighting control.

Automated energy management.

Predictive diagnostics.

Various sections of this guide coverthe application and selection of suchsystems and components that may beincorporated into the power equip-ment being designed. See Sections 2,3, 4, 23

and 43

.

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System Design

Sheet 0023

5. Maximum Electrical Efficiency(Minimum Operating Costs)

:Electrical efficiency can generallybe maximized by designing sys-tems that minimize the losses inconductors, transformers and utili-zation equipment. Proper voltagelevel selection plays a key factorin this area and will be discussedlater. Selecting equipment,such as transformers, with loweroperating losses, generally meanshigher first cost and increasedfloor space requirements; thus,there is a balance to be consideredbetween the owners utility energychange for the losses in the trans-former or other equipment versusthe owners first cost budget andcost of money.

6. Minimum Maintenance Cost

:Usually the simpler the electrical

system design and the simplerthe electrical equipment, the lessthe associated maintenance costsand operator errors. As electricalsystems and equipment becomemore complicated to providegreater service continuity orflexibility, the maintenance costsand chance for operator errorincreases. The systems should bedesigned with an alternate powercircuit to take electrical equipment(requiring periodic maintenance)out of service without droppingessential loads. Use of drawouttype protective devices such as

breakers and combination starterscan also minimize maintenancecost and out-of-service time.

7. Maximum Power Quality

:The power input requirements ofall utilization equipment has to beconsidered including the accept-able operating range of the equip-ment and the electrical distributionsystem has to be designed to meetthese needs. For example, whatis the required input voltage, cur-rent, power factor requirement?Consideration to whether theloads are affected by harmonics(multiples of the basic 60 cycleper second sine wave) or generateharmonics must be taken intoaccount as well as transientvoltage phenomena.

The above goals are interrelatedand in some ways contradictory.As more redundancy is added tothe electrical system design alongwith the best quality equipment

to maximize service continuity,flexibility and expandability, andpower quality, the more initialinvestment and maintenanceare increased. Thus, the designermust weigh each factor basedon the type of facility, the loadsto be served, the owners pastexperience and criteria.

Summary

It is to be expected that the engineerwill never have complete load infor-mation available when the system isdesigned. The engineer will have toexpand the information made avail-

able to him on the basis of experiencewith similar problems. Of course, itis desirable that the engineer has asmuch definite information as possibleconcerning the function, requirements,and characteristics of the utilizationdevices. The engineer should knowwhether certain loads functionseparately or together as a unit, themagnitude of the demand of the loadsviewed separately and as units, the rated

voltage and frequency of the devices,their physical location with respectto each other and with respect to thesource and the probability and possi-bility of the relocation of load devices

and addition of loads in the future.

Coupled with this information, aknowledge of the major types of electricpower distribution systems equips theengineers to arrive at the best systemdesign for the particular building.

It is beyond the scope of this guide topresent a detailed discussion of loadsthat might be found in each of severaltypes of buildings. Assuming that thedesign engineer has assembled thenecessary load data, the followingpages discuss some of the varioustypes of electrical distribution systemsthat can be utilized. The description of

types of systems, and the diagramsused to explain the types of systemson the following pages omits thelocation of utility revenue meteringequipment for clarity. A discussion ofshort circuit calculations, coordination,voltage selection, voltage drop, groundfault protection, motor protection, andother specific equipment protectionis also presented.

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System Design

Sheet 0024

Voltage Classifications

ANSI and IEEE standards definevarious voltage classifications forsingle-phase and 3-phase systems.The terminology used divides voltage

classes into:

Low voltage.

Medium voltage.

High voltage.

Extra-high voltage.

Ultra-high voltage.

Table 1.1-1

presents the nominal sys-tem voltages for these classifications.

Table 1.1-1. Standard Nominal SystemVoltages and Voltage Ranges(From IEEE Standard 141-1993)

BIL Basic Impulse Levels

ANSI standards define recommendedand required BIL levels for:

Metal-Clad Switchgear(typically vacuum breakers).

Metal-Enclosed Switchgear (typi-cally load interrupters, switches).

Liquid Immersed Transformers.

Dry-Type Transformers.

Tables 1.1-2

through 1.1-6

containthose values.

VoltageClass

Nominal System Voltage

3-Wire 4-Wire

Low Voltage 240/120240480600

208Y/120240/120

480Y/277

Medium Voltage 2,4004,1604,8006,900

13,20013,80023,00034,50046,00069,000

4160Y/24008320Y/4800

12000Y/693012470Y/720013200Y/762013800Y/7970

20780Y/1200022860Y/1320024940Y/1440034500Y/19920

High Voltage 115,000138,000161,000230,000

Extra-High Voltage 345,000500,000765,000

Ultra-High Voltage 1,100,000

Table 1.1-2. Metal-Clad Switchgear Voltageand Insulation Levels (From ANSI/IEEEC37.20.2-1999)

Table 1.1-3. Metal-Enclosed SwitchgearVoltage and Insulation Levels(From ANSI C37.20.3-1987)

Table 1.1-4. Liquid-Immersed TransformersVoltage and Basic Lightning ImpulseInsulation Levels (BIL)(From ANSI/IEEE C57.12.00-2000)

BIL values in bold typeface

are listed asstandard. Others listed are in common use.

Voltage Recommendations byMotor Horsepower

Some factors affecting the selectionof motor operating voltage include:

Motor, motor starter and cable

first cost.

Motor, motor starter and cableinstallation cost.

Motor and cable losses.

Motor availability.

Voltage drop.

Qualifications of the buildingoperating staff; and many more.

The following table is based in parton the above factors and experience.Since all the factors affecting the selec-tion are rarely known, it is only anapproximate guideline.

Table 1.1-5. Selection of Motor HorsepowerRatings as a Function of System Voltage

Table 1.1-6. Dry-Type Transformers Voltageand Basic Lightning Impulse InsulationLevels (BIL) From ANSI/IEEE C57.12.01-1989)

BIL values in bold typeface

are listed asstandard. Others listed are in common use.Optional higher levels used where exposureto overvoltage occurs and higher protectionmargins are required.

Lower levels where surge arresterprotective devices can be applied withlower spark-over levels.

Rated MaximumVoltage (kV rms)

ImpulseWithstand (kV)

4.76

8.2515.0

60

95 95

27.038.0

125150

Rated MaximumVoltage (kV rms)

ImpulseWithstand (kV)

4.76 8.2515.0

60 75 95

15.525.838.0

110125150

Applica-tion

NominalSystemVoltage(kV rms)

BIL(kV Crest)

Distribu-tion

1.2 2.5 5.0

304560

8.7 15.0 25.0

7595

150

125

34.5 46.0

69.0

200250

350

150

200

250

125

Power 1.2 2.5 5.0

456075

304560

8.7 15.0 25.0

95110150

7595

34.5 46.0 69.0

200250350

200250

115.0138.0161.0

550650750

450550650

350450550

230.0345.0500.0765.0

9001,1751,6752,050

8251,0501,5501,925

750

9001,4251,800

650

1,300

Motor Voltage(Volts)

Motorhp Range

SystemVoltage

460 2,300 4,000

up to 500250 to 2000250 to 3000

480 2,400 4,160

4,60013,200

250 to 3000above 2000

4,80013,800

NominalSystemVoltage(kV rms)

BIL (kV Crest)

1.2 2.5 5.0 8.7

10 20 30 45

20 30 45 60

30 45 60 95

15.025.034.5

95

60110

125

95125

150

110150200

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Sheet 0025

Table 1.1-7. 3-Phase Transformer Winding Connections

PhasorDiagram

Notes

1. Suitable for both ungrounded and effectively grounded sources.

2. Suitable for a 3-wire service or a 4-wire service with a mid-tap ground.


2. Suitable for a 3-wire service or a 4-wire grounded service with XO grounded.

3. With XO grounded, the transformer acts as a ground source for thesecondary system.

4. Fundamental and harmonic frequency zero-sequence currents in the secondarylines supplied by the transformer do not flow in the primary lines. Instead thezero sequence currents circulate in the closed delta primary windings.

5. When supplied from an effectively grounded primary system does not see loadunbalances and ground faults in the secondary system.

1. Suitable for both ungrounded and effectively grounded sources.2. Suitable for a 3-wire service or a 4-wire delta service with a mid-tap ground.

3. Grounding the primary neutral of this connection would create a ground sourcefor the primary system. This could subject the transformer to severe overloadingduring a primary system disturbance or load unbalance.

4. Frequently installed with mid-tap ground on one leg when supplyingcombination 3-phase and single-phase load where the 3-phase load is muchlarger than single-phase load.

5. When used in 25 kV and 35 kV 3-phase 4-wire primary systems,ferroresonance can occur when energizing or de-energizing the transformerusing single-pole switches located at the primary terminals. With smaller kVAtransformers the probability of ferroresonance is higher.


2. Suitable for a 3-wire service only, even if XO is grounded.

3. This connection is incapable of furnishing a stabilized neutral and its use mayresult in phase-to-neutral overvoltage (neutral shift) as a result of unbalancedphase-to-neutral load.

4. If a 3-phase unit is built on a three-legged core, the neutral point of the primarywindings is practically locked at ground potential.

1. Suitable for 4-wire effectively grounded source only.

2. Suitable for a 3-wire service or for 4-wire grounded service with XO grounded.

3. 3-phase transformers with this connection may experience stray flux tankheating during certain external system unbalances unless the core configuration(four or five legged) utilized provides a return path for the flux.

4. Fundamental and harmonic frequency zero-sequence currents in the secondarylines supplied by the transformer also flow in the primary lines (and primaryneutral conductor).

5. Ground relay for the primary system may see load unbalances and groundfaults in the secondary system. This must be considered when coordinatingovercurrent protective devices.

6. 3-phase transformers with the neutral points of the high voltage and low

voltage windings connected together internally and brought out through anHOXO bushing should not be operated with the HOXO bushing ungrounded(floating). To do so can result in very high voltages in the secondary systems.


2. Suitable for a 3-wire service or a 4-wire service with a mid-tap ground.

3. When using the tap for single-phase circuits the single-phase load kVA shouldnot exceed 5% of the 3-phase kVA rating of the transformer. The 3-phase ratingof the transformer is also substantially reduced.

H2

H1 H3

X2

X1 X3

DELTA-DELTA Connection

Phasor

Diagram:

Angular Displacement (Degrees): 0

H2

H1 H3

X2

X1

X3

DELTA-WYE Connection

PhasorDiagram:


X0

H2

H1 H3

X2

X1

X3

WYE-DELTA Connection

PhasorDiagram:


H2

H1 H3

WYE-WYE Connection

PhasorDiagram:


X2

X1 X3

X0

H2

H1 H3

GROUNDED WYE-WYE Connection

PhasorDiagram:


X2

X1 X3

X0H0

H2

H1 H3

X2

X1 X3

DELTA-DELTA Connection with Tap

PhasorDiagram:


X4

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System DesignSheet 0028

Figure 1.1-4. Secondary Unit SubstationLoop Switching

Figure 1.1-5. Pad-Mounted TransformerLoop Switching

In addition, the two primary mainbreakers which are normally closedand primary tie breaker which is nor-mally open are either mechanicallyor electrically interlocked to preventparalleling the incoming source lines.For slightly added cost, an automaticthrow-over scheme can be addedbetween the two main breakers andtie breaker. During the more commonevent of a utility outage, the automatictransfer scheme provides significantlyreduced power outage time.

The system in Figure 1.1-3has highercosts than in Figure 1.1-2, but offersincreased reliability and quick restora-tion of service when 1) a utility outageoccurs, 2) a primary feeder conductorfault occurs, or 3) a transformer faultor overload occurs.

Should a utility outage occur on one ofthe incoming lines, the associated pri-mary main breaker is opened and thetie breaker closed either manually orthrough an automatic transfer scheme.

LoopFeeder

LoopFeeder

Load BreakLoop Switches

FusedDisconnectSwitch

Loop

Feeder

Loop

Feeder

Load BreakLoop Switches

Load BreakDrawout Fuses

When a primary feeder conductor faultoccurs, the associated loop feederbreaker opens and interrupts serviceto all loads up to the normally openprimary loop load break switch(typically half of the loads). Once it is

determined which section of primarycable has been faulted, the loop sec-tionalizing switches on each side ofthe faulted conductor can be opened,the loop sectionalizing switch whichhad been previously left open thenclosed and service restored to allsecondary unit substations whilethe faulted conductor is replaced.If the fault should occur in a conductordirectly on the load side of one of theloop feeder breakers, the loop feederbreaker is kept open after tripping andthe next load side loop sectionalizingswitch manually opened so that thefaulted conductor can be sectionalizedand replaced.

Note: Under this condition, all secondaryunit substations are supplied through theother loop feeder circuit breaker, and thusall conductors around the loop should besized to carry the entire load connected tothe loop. Increasing the number of primaryloops (two loops shown in Figure 1.1-6) willreduce the extent of the outage from a con-ductor fault, but will also increase the sys-tem investment.

When a transformer fault or overloadoccurs, the transformer primary fusesopen, and the transformer primaryswitch manually opened, disconnect-ing the transformer from the loop,

and leaving all other secondary unitsubstation loads unaffected.

Figure 1.1-6. Single Primary Feeder Loop System

A basic primary loop system whichutilizes a single primary feeder breakerconnected directly to two loop feederswitches which in turn then feed theloop is shown in Figure 1.1-6. In thisbasicsystem the loop may be normallyoperated with one of the loop section-alizing switches open as described

above or with all loop sectionalizingswitches closed. If a fault occurs in thebasic primary loop system, the singleloop feeder breaker trips, and second-ary loads are lost until the faulted con-ductor is found and eliminated fromthe loop by opening the appropriateloop sectionalizing switches and thenreclosing the breaker.

3. Primary Selective System Secondary Radial System

The primary selective secondaryradial system, as shown in Figure 1.1-7,differs from those previously described

in that it employs at least two primaryfeeder circuits in each load area. It is

Figure 1.1-7. Basic Primary Selective Radial Secondary System

Loop A Loop A

In cases where only one primary lineis available, the use of a single primarybreaker provides the loop connectionsto the loads as shown here.

52

Primary Metal-Clad

Switchgear Lineup

Bus A Bus B

Feeder A1 Feeder B1

Primary Feeder Breaker

Feeder B2

Feeder A2

Primary Main Breaker

To OtherSubstations

Typical Secondary UnitSubstation Duplex PrimarySwitch/FusesTransformer/600 V ClassSecondary Switchgear

52 52

52

5252

52 52

NO

NC

NO

NC

NO

NC

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Sheet 0029

designed so that when one primarycircuit is out of service, the remainingfeeder or feeders have sufficientcapacity to carry the total load. Halfof the transformers are normallyconnected to each of the two feeders.When a fault occurs on one of theprimary feeders, only half of theload in the building is dropped.

Duplex fused switches as shown inFigure 1.1-7and detailed in Figure 1.1-8are the normal choice for this typeof system. Each duplex fused switchconsists of two (2) load break 3-poleswitches each in their own separatestructure, connected together by busbars on the load side. Typically, theload break switch closest to thetransformer includes a fuse assemblywith fuses. Mechanical and/or keyinterlocking is furnished such thatboth switches cannot be closed at

the same time (to prevent paralleloperation) and interlocking such thataccess to either switch or fuse assem-bly cannot be obtained unless bothswitches are opened.

Figure 1.1-8. Duplex Fused Switch inTwo Structures

As an alternate to the duplex switcharrangement, a non-load break selectorswitch mechanically interlocked with aload break fused switch can be utilizedas shown in Figure 1.1-9. The non-loadbreak selector switch is physically

located in the rear of the load breakfused switch, thus only requiring onestructure and a lower cost and floorspace savings over the duplexarrangement. The non-load breakswitch is mechanically interlocked toprevent its operation unless the loadbreak switch is opened. The maindisadvantage of the selector switch isthat conductors from both circuits areterminated in the same structure.

Figure 1.1-9. Fused Selector Switch inOne Structure

This means limited cable space espe-cially if double lugs are furnished for

each line as shown in Figure 1.1-7andshould a faulted primary conductorhave to be changed, both lines wouldhave to be deenergized for safechanging of the faulted conductors.

In Figure 1.1-7when a primary feederfault occurs the associated feederbreaker opens, and the transformersnormally supplied from the faultedfeeder are out of service. Then manu-ally, each primary switch connected tothe faulted line must be opened andthen the alternate line primary switchcan be closed connecting the trans-former to the live feeder, thus restoringservice to all loads. Note that each of the

primary circuit conductors for FeederA1 and B1 must be sized to handle thesum of the loads normally connectedto both A1 and B1. Similar sizing ofFeeders A2 and B2, etc., is required.

If a fault occurs in one transformer,the associated primary fuses blowand interrupts the service to justthe load served by that transformer.Service cannot be restored to theloads normally served by the faultedtransformer until the transformeris repaired or replaced.

Cost of the primary selective secondary radial system is greaterthan that of the simple primary radialsystem of Figure 1.1-1because of theadditional primary main breakers, tiebreaker, two-sources, increased num-ber of feeder breakers, the use of pri-mary-duplex or selector switches, andthe greater amount of primary feedercable required. The benefits from thereduction in the amount of load lostwhen a primary feeder is faulted, plusthe quick restoration of service to all

or most of the loads, may more thanoffset the greater cost. Having two-sources allows for either manual orautomatic transfer of the two primarymain breakers and tie breaker shouldone of the sources become unavailable.

The primary selective-secondary radialsystem, however, may be less costly ormore costly than a primary loop secondary radial system of Figure 1.1-3depending on the physical locationof the transformers while offeringcomparable downtime and reliability.The cost of conductors for the twotypes of systems may vary greatlydepending on the location of thetransformers and loads within thefacility and greatly override primaryswitching equipment cost differencesbetween the two systems.

4. Two-Source Primary

Secondary Selective SystemThis system uses the same principleof duplicate sources from the powersupply point utilizing two primarymain breakers and a primary tie breaker.The two primary main breakers andprimary tie breaker being eithermanually or electrically interlockedto prevent closing all three at the sametime and paralleling the sources. Uponloss of voltage on one source, a manualor automatic transfer to the alternatesource line may be utilized to restorepower to all primary loads.

Each transformer secondary isarranged in a typical double-endedunit substation arrangement as shownin Figure 1.1-10. The two secondarymain breakers and secondary tiebreaker of each unit substation areagain either mechanically or electricallyinterlocked to prevent parallel opera-tion. Upon loss of secondary sourcevoltage on one side, manual or auto-matic transfer may be utilized to transferthe loads to the other side, thus restor-ing power to all secondary loads.

This arrangement permits quick resto-ration of service to all loads when aprimary feeder or transformer fault

occurs by opening the associated sec-ondary main and closing the second-ary tie breaker. If the loss of secondaryvoltage has occurred because of a pri-mary feeder fault with the associatedprimary feeder breaker opening, thenall secondary loads normally servedby the faulted feeder would have tobe transferred to the opposite primaryfeeder. This means each primaryfeeder conductor must be sized tocarry the load on both sides of all thesecondary buses it is serving under

PrimaryFeeders

Load BreakSwitches

Fuses

PrimaryFeeders

Non-Load BreakSelector Switches

Fuses

Load BreakDisconnect

Inter-

lock

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into the protector enclosure whichfunctions to automatically close theprotector only when the voltageconditions are such that its associatedtransformer will supply power to thesecondary network loads, and to auto-matically open the protector whenpower flows from the secondary to thenetwork transformer. The purpose ofthe network protector is to protect theintegrity of the network bus voltageand the loads served from it againsttransformer and primary feeder faultsby quickly disconnecting the defectivefeeder-transformer pair from thenetwork when backfeed occurs.

The simple spot network systemresembles the secondary-selectiveradial system in that each load areais supplied over two or more primaryfeeders through two or more trans-formers. In network systems, the

transformers are connected throughnetwork protectors to a common bus,as shown in Figure 1.1-12, from whichloads are served. Since the transform-ers are connected in parallel, a primaryfeeder or transformer fault does notcause any service interruption to theloads. The paralleled transformerssupplying each load bus will normallycarry equal load currents, whereasequal loading of the two separatetransformers supplying a substation inthe secondary-selective radial systemis difficult to obtain. The interruptingduty imposed on the outgoing feederbreakers in the network will be greater

with the spot network system.

The optimum size and number of pri-mary feeders can be used in the spotnetwork system because the loss ofany primary feeder and its associatedtransformers does not result in theloss of any load even for an instant.In spite of the spare capacity usuallysupplied in network systems, savingsin primary switchgear and secondaryswitchgear costs often result whencompared to a radial system designwith similar spare capacity. Thisoccurs in many radial systemsbecause more and smaller feeders

are often used in order to minimizethe extent of any outage when aprimary fault event occurs.

In spot networks, when a fault occurson a primary feeder or in a transformer,the fault is isolated from the systemthrough the automatic tripping of theprimary feeder circuit breaker and allof the network protectors associated

with that feeder circuit. This operationdoes not interrupt service to any loads.After the necessary repairs have beenmade, the system can be restored tonormal operating conditions by closingthe primary feeder breaker. All networkprotectors associated with that feederwill close automatically.

The chief purpose of the network busnormally closed ties is to provide forthe sharing of loads and a balancingof load currents for each primary ser-vice and transformer regardless ofthe condition of the primary services.

Also, the ties provide a means forisolating and sectionalizing groundfault events within the switchgearnetwork bus, thereby saving a portionof the loads from service interruptions,yet isolating the faulted portion forcorrective action.

The use of spot network systemsprovides users with several importantadvantages. First, they save trans-former capacity. Spot networks permitequal loading of transformers underall conditions. Also, networks yieldlower system losses and greatlyimprove voltage conditions. The volt-age regulation on a network systemis such that both lights and power canbe fed from the same load bus. Muchlarger motors can be started across-the-line than on a simple radialsystem. This can result in simplifiedmotor control and permits the useof relatively large low voltage motors

with their less expensive control.Finally, network systems provide agreater degree of flexibility in addingfuture loads; they can be connectedto the closest spot network bus.

Spot network systems are economicalfor buildings which have heavy con-centrations of loads covering smallareas, with considerable distancebetween areas, and light loads withinthe distances separating the concen-trated loads. They are commonly usedin hospitals, high rise office buildings,and institutional buildings where ahigh degree of service reliability is

required from the utility sources. Spotnetwork systems are especially eco-nomical where three or more primaryfeeders are available. Principally, thisis due to supplying each load busthrough three or more transformersand the reduction in spare cable andtransformer capacity required.

They are also economical whencompared to two transformer double-ended substations with normallyopened tie breakers.

Emergency power should be connectedto network loads downstream from

the network, or upstream at primaryvoltage, not at the network bus itself.

7. Medium Voltage DistributionSystem Design

A. Single Bus, Figure 1.1-13

The sources (utility and/or generator(s))are connected to a single bus. All feedersare connected to the same bus.

Figure 1.1-13. Single BusThis configuration is the simplestsystem, however, outage of the utilityresults in total outage.

Normally the generator does not haveadequate capacity for the entire load.A properly relayed system equippedwith load shedding, automatic voltage/frequency control may be able tomaintain partial system operation.

Any future addition of breaker sectionsto the bus will require a shutdown ofthe bus, since there is no tie breaker.

B. Single Bus with Two-Sources from the

Utility, Figure 1.1-14Same as the single bus, except thattwo utility sources are available.This system is operated normally withthe main breaker to one source open.Upon loss of the normal service thetransfer to the standby NormallyOpen (NO) breaker can be automaticor manual. Automatic transfer is pre-ferred for rapid service restorationespecially in unattended stations.

52

Utility

Main Bus

G

OneofSeveralFeeders

52

52

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Figure 1.1-14. Single Bus with Two-Sources

Retransfer to the Normal can beclosed transition subject to the approvalof the utility. Closed transition momen-tarily (5 10 cycles) parallels bothutility sources. Caution: when bothsources are paralleled, the fault currentavailable on the load side of the maindevice is the sum of the available faultcurrent from each source plus the motorfault contribution. It is recommendedthat the short circuit ratings of the bus,feeder breakers and all load sideequipment are rated for the increasedavailable fault current. If the utilityrequires open transfer, the disconnec-tion of motors from the bus must beensured by means of suitable time delayon reclosing as well as supervision ofthe bus voltage and its phase withrespect to the incoming source voltage.

This busing scheme does not precludethe use of cogeneration, but requiresthe use of sophisticated automatic syn-chronizing and synchronism checkingcontrols, in addition to the previouslymentioned load shedding, automaticfrequency and voltage controls.

This configuration is more expensivethan the scheme shown inFigure 1.1-13,but service restoration is quicker. Again,a utility outage results in total outage tothe load until transfer occurs. Extensionof the bus or adding breakers requiresa shutdown of the bus.

If paralleling sources, reverse current,

reverse power, and other appropriaterelaying protection should be addedas requested by the utility.

C. Multiple Sources with Tie Breaker,Figure 1.1-15 and Figure 1.1-16

This configuration is similar to configu-ration B. It differs significantly in thatboth utility sources normally carry theloads and also by the incorporationof a normally open tie breaker. Theoutage to the system load for a utilityoutage is limited to half of the system.

Utility #2Utility #1

Normal Standby

NC NO

Loads

52 52

Again, the closing of the tie breaker canbe manual or automatic. The statementsmade for the retransfer of scheme Bapply to this scheme also.

Figure 1.1-15. Two-Source Utility withTie Breaker

If looped or primary selective distribu-tion system for the loads is used, thebuses can be extended without a shut-down by closing the tie breaker andtransferring the loads to the other bus.

This configuration is more expensivethan B. The system is not limited to twobuses only. Another advantage is thatthe design may incorporate momentaryparalleling of buses on retransfer afterthe failed line has been restored to pre-

vent another outage. See the Cautionfor Figures 1.1-14, 1.1-15and 1.1-16.

In Figure 1.1-16, closing of the tiebreaker following the opening of amain breaker can be manual or auto-matic. However since a bus can be fedthrough two tie breakers the controlscheme should be designed to makethe selection.

The third tie breaker allows any busto be fed from any utility source.

Summary

The medium voltage system configura-tions shown are based on using metal-

clad drawout switchgear. The servicecontinuity required from electrical sys-tems makes the use of single sourcesystems impractical.

In the design of a modern mediumvoltage system the engineer should:

1. Design a system as simple aspossible.

2. Limit an outage to as small aportion of the system as possible.

3. Provide means for expanding thesystem without major shutdowns.

4. Relay the system so that only the

faulted part is removed fromservice, and damage to it is mini-mized consistent with selectivity.

5. Specify and apply all equipmentwithin its published ratings andnational standards pertaining tothe equipment and its installation.

Figure 1.1-16. Triple-Ended Arrangement

Utility #1

NC

Bus #1 Bus #2

Load Load

Utility #2

NC

NO

52 52

52

52 52

Caution forFigures 1.1-14, 1.1-15 and1.1-16:If continuousparalleling ofsources is planned, reverse current,

reverse power and other appropriaterelaying protection should be added.When both sources are paralleled forany amount of time, the fault currentavailable on the load side of the maindevice is the sum of the availablefault current from each source plusthe motor fault contribution. It isrequired that bus bracing, feederbreakers and all load side equipmentis rated for the increased availablefault current.

NO

NC

Bus #1 Bus #2

Utility #1 Utility #2

NC

NO NO

Utility #3

Bus #3

NC

Tie Busway

52 52 52

52

52

52

52 NOTypical Feeder52 5252

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Sources: Type 1 systems are requiredto have a minimum of two independentsources of electrical power. A normalsource that generally supplies theentire facility and one or more alter-nate sources that supply power whenthe normal source is interrupted. Thealternate source(s) must be an on-sitegenerator driven by a prime moverunless a generator(s) exists as thenormal power source. In the casewhere a generator(s) is utilized as thenormal source, it is permissible for thealternate source to be a utility feed.Alternate source generators must beclassified as Type 10, Class X, Level 1gensets per NFPA 110 2-2 capableof providing power to the load in amaximum of 10 seconds. Typically,the alternate sources of power aresupplied to the loads through a seriesof automatic and/or manual transferswitches. (SeeSection 25.) Thetransfer switches can be non-delayedautomatic, delayed automatic, ormanual transfer depending on therequirements of the specific branchof the EES that they are feeding. It ispermissible to feed multiple branchesor systems of the EES from a singleautomatic transfer switch providedthat the maximum demand on theEES does not exceed 150 kVA. Thisconfiguration is typically seen insmaller health care facilities thatmust meet Type 1 EES requirements.(SeeFigure 1.1-18.)

Figure 1.1-18. Small Hospital ElectricalSystem Single EES Transfer Switch

Table 1.1-9. Type 1 EES Applicable Codes

Systems and Branches of Service:TheType 1 EES consists of two separatepower systems capable of supplyingpower considered essential for lifesafety and effective facility operationduring an interruption of the normalpower source. They are the EmergencySystem and the Equipment System.

1. Emergency System consists ofcircuits essential to life safety and

critical patient care.

The Emergency System is an electricalsub-system that must be fed from anautomatic transfer switch or series ofautomatic transfer switches. ThisEmergency System consists of twomandatory branches that provide powerto systems and functions essential tolife safety and critical patient care.

A. Life Safety Branch suppliespower for lighting, receptacles,and equipment to perform thefollowing functions:

1. Illumination of means of egress.

2. Exit signs and exit direction signs.3. Alarms and alerting systems.

4. Emergency communicationssystems.

5. Task illumination, batterychargers for battery poweredlighting, and select receptaclesat the generator.

6. Elevator lighting control, com-munication and signal systems.

7. Automatic doors used for egress.

These are the onlyfunctionspermitted to be on the life safetybranch. Life Safety Branch equip-

ment and wiring must be entirelyindependent of all other loads andbranches of service. This includesseparation of raceways, boxes orcabinets. Power must be suppliedto the Life Safety Branch from anon-delayed automatic transferswitch.

B. Critical Branch supplies powerfor task illumination, fixed equip-ment, selected receptacles andselected power circuits for areasrelated to patient care. Thepurpose of the critical branchis to provide power to a limitednumber of receptacles and loca-tions to reduce load and minimizethe chances of fault conditions.The transfer switch(es) feeding thecritical branch must be automatictype. They are permitted to haveappropriate time delays that willfollow the restoration of the lifesafety branch but should havepower restored within 10 secondsof normal source power loss.The critical branch provides powerto circuits serving the followingareas and functions:

1. Critical care areas.

2. Isolated Power Systems inspecial environments.

3. Task illumination and selectedreceptacles in the followingpatient care areas: infantnurseries, medication prepareas, pharmacy, selectedacute nursing areas, psychiatricbed areas, ward treatmentrooms, nurses stations.

4. Specialized patient care taskillumination, where needed.

5. Nurse call systems.

6. Blood, bone and tissue banks.

7. Telephone equipment roomsand closets.

8. Task illumination, selectedreceptacles, and selectedpower circuits for the following:general care beds (at leastone duplex receptacle), angio-graphic labs, cardiac catheter-ization labs, coronary careunits, hemodialysis rooms,selected emergency roomtreatment areas, humanphysiology labs, intensive careunits, selected postoperativerecovery rooms.

9. Additional circuits and single-phase fraction motors as neededfor effective facility operation.

Normal Source

G

Non-EssentialLoads

AlternateSource

Entire Essential

Electric System(150 kVA or Less)

Description Standard Section

DesignSources

Uses

Emergency Power Supply Classification

NFPA 99NFPA 99

NFPA 99

NFPA 110

4.4.1.1.14.4.1.1.4 thru4.4.4.1.1.7.24.4.1.1.8 (1-3)

4

Distribution NFPA 99NEC

4.4.2517-30

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Table 1.1-10. Type 1 Emergency SystemApplicable Codes

2. Equipment System consists ofmajor electrical equipment necessaryfor patient care and Type 1 operation.

The Equipment System is a subsystemof the EES that consists of large electri-cal equipment loads needed for patientcare and basic hospital operation.Loads on the Equipment System thatare essential to generator operation are

required to be fed by a non-delayedautomatic transfer switch.

The following equipment must bearranged for delayed automatic transferto the emergency power supply:

1. Central suction systems for medicaland surgical functions.

2. Sump pumps and other equipmentrequired for the safe operation ofa major apparatus.

3. Compressed air systems formedical and surgical functions.

4. Smoke control and stairpressurization systems.

5. Kitchen hood supply and exhaustsystems, if required to operateduring a fire.

The following equipment must bearranged for delayed automatic ormanual transferto the emergencypower supply:

1. Select heating equipment.

2. Select elevators.

3. Supply, return and exhaustventilating systems for surgical,obstetrical, intensive care,coronary care, nurseries, andemergency treatment areas.

4. Supply, return and exhaustventilating systems for airborneinfectious/isolation rooms, labsand medical areas where hazard-ous materials are used.

5. Hyperbaric facilities.

6. Hypobaric facilities.

7. Autoclaving equipment.

8. Controls for equipment listed above.

9. Other selected equipment inkitchens, laundries, radiologyrooms, and central refrigerationas selected.

Table 1.1-11. Type 1 Equipment SystemApplicable Codes

Any loads served by the generator thatare not approved as outlined above as

part of the Essential Electrical Systemmust be connected through a separatetransfer switch. These transfer switches

must be configured such that the loadswill not cause the generator to over-load and must be shed in the event thegenerator enters an overload condition.

Ground Fault Protection per NFPA70 NEC article 230-95, ground fault

protection is required on any feeder orservice disconnect 1000 A or larger onsystems with line to ground voltages of150 volts or greater and phase-to-phasevoltages of 600 volts or less. For healthcare facilities (of any type), a secondlevel of ground fault protection isrequired to be on the next level offeeder downstream. This second levelof ground fault is only required forfeeders that serve patient care areasand equipment intended to support life.100% selective coordination of the twolevels of ground fault protection mustbe achieved with a minimum six-cycleseparation between the upstream and

downstream device.

NEC 517-17 prohibits the installationof ground fault protection between theon-site generator(s) and any EES transferswitch or on the load side of a transferswitch feeding EES circuits. (SeeFigure1.1-19 Additional Level of GroundFault). The intent of this code section isto prevent a ground fault that causes atrip on the normal system to also causea trip on the emergency system. Suchan event could result in complete powerloss of both normal and emergencypower sources and could not be recov-ered until the source of the ground

fault was located and isolated fromthe system. To prevent this condition,NEC 700-26 removes the

Figure 1.1-19. Additional Level of Ground Fault Protection Ground fault protection is required for service disconnects 1000 amperes and larger or systems with less than 600 volts phase-to-phase and greater

than 150 volts to ground per NEC 230-95.


General NFPA 99NEC

4.4.2.2.2517-31

Life Safety

Branch

NFPA 99

NEC

4.4.2.2.2.2

517-32

Critical Branch NFPA 99NEC

4.4.2.2.2.3517-33

Wiring NFPA 99NEC

4.4.2.2.4517-30(c)


General NFPA 99NEC

4.4.2.2.3517-34

Equipment NFPA 99NEC

4.4.2.2.3 (3-5)517-34(a)-(b)

Normal Source Normal Source(s)

G

480/277 V

ServiceEntrance

1000 Aor Larger

GFServiceEntrance

1000 Aor Larger

GF

480/277 V

GFGFGF GFGFGFGFGFGF GFGF

ServiceEntrance

1000 Aor Larger

GF

480/277 V

GF

Non-Essential Loads Non-Essential Loads

Essential Electrical System

Additional Levelof Ground Fault

Protection

Additional Levelof Ground Faultis not PermittedBetween Generatorand EES TransferSwitches.

(NEC 517-17a(1))

Additional Level of Ground Fault isnot Permitted on Load Side of EESTransfer Switches. (NEC 517-17a(2))

= Ground Fault Protection Required

Generator Breakers areTypically Supplied withGround Fault AlarmOnly. (NEC 700-26)

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ground fault protection requirementfor the emergency system source.Typically, the emergency systemgenerator(s) are equipped with groundfault alarms that do not automaticallydisconnect during a ground fault.

Table 1.1-12. Ground Fault ProtectionApplicable Codes

Wet Locations A wet location in ahealth care facility is any patient carearea that is normally subject to wetconditions while patients are present.Typical wet locations can include oper-

ating rooms, anesthetizing locations,dialysis locations, etc. (Patient beds,toilets, sinks, are not considered wetlocations.) These wet locations requirespecial protection to guard againstelectric shock. The ground fault currentin these areas must be limited not toexceed 6 mA.

In areas where the interruption of poweris permissible, ground fault circuitinterrupters (GFCI) can be employed.GFCIs will interrupt a circuit whenground fault current exceeds thepresent level (typically 2-5 mA).

In areas where the interruption of

power cannot be tolerated, protectionfrom ground fault currents is accom-plished through the use of an IsolatedPower System. Isolated Power Sys-tems provide power to an area that isisolated from ground (or ungrounded).This type of system limits the amountof current that flows to ground inthe event of a single line-to-groundfault and maintains circuit continuity.Electronic Line Isolation Monitors (LIM)are used to monitor and display leakagecurrents to ground. When leakagecurrent thresholds are exceeded, visibleand/or audible alarms are initiated toalert occupants of a possible hazardous

condition. This alarm occurs withoutinterrupting power to allow for thesafe conclusion of critical procedures.

Table 1.1-13. Wet Location Applicable Codes

Maintenance and Testing

Regular maintenance and testing ofthe electrical distribution system ina health care facility is necessary toensure proper operation in an emer-gency and, in some cases, to maintain

government accreditation. Any healthcare facility receiving Medicare orMedicaid reimbursement from thegovernment must be accredited by theJoint Commission on Accreditation ofHealth Care Organizations (JCAHO).JCAHO has established a group ofstandards called the Environment ofCare, which must be met for healthcare facility accreditation. Included inthese standards is the regular testingof the emergency (alternate) powersystem(s). The emergency power sys-tem must be tested in accordance withNFPA 110 Standard for Emergency andStandby Power Systemsguidelines at

intervals not less than 20 days and notexceeding 40 days. Generators mustbe tested for a minimum of 30 minutesunder the criteria defined in NFPA 110.

One method to automate the task ofmonthly generator tests is through theuse of PowerNetcommunications.With the PowerNet integrated metering,monitoring and control system, a facilitymaintenance director can initiate agenerator test, control/monitor loads,meter/monitor generator test pointsand create a JCAHO compliant reportautomatically from a central PC. Thereport contains all metered values,test results, date/time information, etc.

necessary to satisfy JCAHO require-ments. This automated generator testingprocedure reduces the labor, trainingand inaccuracies that occur duringmanual emergency power system tests.(SeePower Monitoring Section 2.)

Table 1.1-14. Maintenance and TestingApplicable Codes

Routine maintenance should be per-formed on circuit breakers, transferswitches, switchgear, generator equip-ment, etc. by trained professionalsto ensure the most reliable electricalsystem possible. See Section 43forEaton - Electrical Services & SystemsE-ESS), which provides engineers,

trained in development and executionof annual preventative maintenanceprocedures of health care facilityelectrical distribution systems.

Paralleling Emergency Generators

Without Utility Paralleling

In many health care facilities (andother large facilities with criticalloads), the demand for standbyemergency power is large enough torequire multiple generator sets topower all of the required EssentialElectrical System (EES) loads. In manycases, it becomes more flexible andeasier to operate the required multiplegenerators from a single locationutilizing Generator Paralleling Switch-gear. Figure 1.1-20on Page 1.1-18shows an example of a typical one-linefor a paralleling switchgear lineup

feeding the EES.A typical abbreviated sequence ofoperation for a multiple emergencygenerator and ATS system follows.Note that other modes of operationsuch as generator demand priority andautomated testing modes are availablebut are not included below. (ReferenceSection 43for complete detailedsequences of operation.)

1. Entering Emergency Mode

a. Upon loss of normal source,automatic transfer switchessend generator control systema run request.

b. All available generators arestarted. The first generator upto voltage and frequency isclosed to the bus.

c. Unsheddable loads and LoadShed Priority 1 loads are pow-ered in less than 10 seconds.

d. The remaining generators aresynchronized and paralleledto the bus as they come up tovoltage and frequency.

e. As additional generators areparalleled to the emergencybus, Load Shed Priority levels

are added, powering theirassociated loads.

f. The system is now inEmergency Mode.

2. Exit from Emergency Mode

a. Automatic transfer switchessense the utility source is withinacceptable operational toler-ances for a time duration set atthe automatic transfer switch.


ServicesFeeders

NECNEC

230-95215-10

Additional Level NECNFPA 99

517-174.3.2.5

Alternate Source NECNEC

700-26700-7 (d)


General NFPA 99NEC

4.3.2.2.9517-20

Isolated PowerSystems

NFPA 99NEC

4.3.2.6517-160


Grounding NFPA 99 4.3.3.1

Emergency PowerSystem

NFPA 99JCAHO

4.4.4.1.1EC.2.14(d)

Generator NFPA 110 8.4.2

Transfer Switches NFPA 110 8.4.5

Breakers NFPA 99NFPA 110

4.4.4.1.28.4.6

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b. As each automatic transferswitch transfers back to utilitypower, it removes its runrequest from the generatorplant.

c. When the last automatic trans-

fer switch has retransferred tothe utility and all run requestshave been removed from thegenerator plant, all generatorcircuit breakers are opened.

d. The generators are allowedto run for their programmedcool- down period.

e. The system is now back inAutomatic/Standby Mode.

With Utility Paralleling

Today, many utilities are offering theircustomers excellent financial incen-tives to utilize their onsite generationcapacity to remove load from the utilitygrid. These incentives are sometimes

referred to as Limited InterruptibleRates (LIP). Under these incentives,utilities will greatly reduce or eliminatekWhr or kW Demand charges to theircustomers with onsite generationcapabilities. In exchange, during timesof peak loading of the utility grid, theutility can ask their LIP rate customersto drop load from the grid by utilizingtheir onsite generation capabilities.

Health care facilities are ideally suitedto take advantage of these programsbecause they already have significantonsite generation capabilities due tothe Code requirements described.

Many health care facilities are takingadvantage of these utility incentives byadding generator capacity over andabove the NFPA requirements. Figure1.1-21on Page 1.1-19shows an exam-ple one-line of a health care facility withcomplete generator backup and utilityinterconnect.

NFPA 110 requirements state that thenormal and emergency sources mustbe separated by a fire rated wall.

The intent of this requirement is so thata fire in one location cannot takeoutboth sources of power. To meet thisrequirement, the Paralleling Switchgearmust be split into separate sectionswith a tie bus through a fire rated wall.For more information on GeneratorParalleling Switchgear, see Section 43.

Figure 1.1-20. Typical One-Line for a Paralleling Switchgear Line-up Feeding the Essential Electrical System (EES)

Utility

Metering

Utility

Transformer

ServiceMain

NormalBus

OptionalElectricallyOperatedStoredEnergyBreakers

Non-EssentialLoads

EP1 EP2 EPX

FxF2F1 EFxEF2EF1

Generators X=NumberofUnits

TypicalGeneratorBreaker

EmergencyBus

EquipmentATS #1

LifeSafetyATS #2

Critical

ATS #X

Typical

Panelboards

GxG2G1

OptionalEle

ctricallyOperatedStored

EnergyBreakers

LoadShed/LoadAddATS Units

OptionalClosedTransitionParallelingofGeneratorsandUtility

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Power Distribution SystemsGenerator System Design

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Generators andGenerator Systems

Typical Diesel Genset Caterpillar

Introduction

The selection and application of gener-ators into the electrical distributionsystem will depend on the particularapplication. There are many factorsto consider including code require-ments, environmental constraints,fuel sources, control complexity, utilityrequirements and load requirements.The Health Care Requirements forlegally required emergency standbygeneration systems are describedstarting on Page 1.1-14

. Systemsdescribed in this section are applicableto Health Care requirements, as wellas other facilities that may require ahigh degree of reliability. The electricalsupply for data centers, financialinstitutions, telecommunications,government and public utilitiesalso require high reliability. Threatsof disaster or terror attacks haveprompted many facilities to requirecomplete self-sufficiency forcontinuous operation.

Types of Engines

Many generator sets are relativelysmall in size, typically ranging fromseveral kilowatts to several mega-watts. These units are often requiredto come on line and operate quickly.

They need to have the capacity torun for an extended period of time.The internal combustion engine isan excellent choice as the primemover for the majority of theseapplications. Turbines may also beutilized. Diesel fueled engines are themost common, but other fuels usedinclude natural gas, digester gas,landfill gas, propane, biodiesel,crude oil, steam and others.

Some campuses and industrialfacilities use and produce steamfor heating and other processes.These facilities may find it economi-

cally feasible to produce electricity asa by-product of the steam production.These installations would typically beclassified as a cogeneration facilityproducing a fairly constant poweroutput and operating in parallel withthe electric utility system.

Types of Generators

Generators can be either synchronousor asynchronous. Asynchronousgenerators are also referred to asinduction generators. The constructionis essentially the same as an induction

motor. It has a squirrel-cage rotor andwound stator. An induction generatoris a motor driven above its designedsynchronous speed thus generatingpower. It will operate as a motor if itis running below synchronous speed.The induction generator does not havean exciter and must operate in parallelwith the utility or another source. Theinduction generator requires vars froman external source for it to generatepower. The induction generatoroperates at a slip frequency so itsoutput frequency is automaticallylocked in with the utility's frequency.

An induction generator is a popularchoice for use when designing cogen-eration systems, where it will operatein parallel with the utility. This type ofgenerator offers certain advantagesover a synchronous generator. Forexample, voltage and frequency arecontrolled by the utility; thus voltageand frequency regulators are notrequired. In addition, the generatorconstruction offers high reliability andlittle maintenance. Also, a minimumof protective relays and controls arerequired. Its major disadvantages arethat it requires vars from the systemand it normally cannot operate as a

standby/emergency generator.Synchronous generators, however,are the most common. Their output isdetermined by their field and governorcontrols. Varying the current in thedc field windings controls the voltageoutput. The frequency is controlledby the speed of rotation. The torqueapplied to the generator shaft bythe driving engine controls the poweroutput. In this manner, the synchro-nous generator offers precise controlover the power it can generate. Incogeneration applications, it can beused to improve the power factor ofthe system.

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Generator Fundamentals

A generator consists of two primarycomponents, a prime mover and analternator. The prime mover is theenergy source used to turn the rotorof the alternator. It is typically a diesel

combustion engine for most emer-gency or standby systems. In cogener-ation applications, the prime movermay come from a steam driven turbineor other source. On diesel units, a gov-ernor and voltage regulator are usedto control the speed and power output.

The alternator is typically a synchro-nous machine driven by the primemover. A voltage regulator controls itsvoltage output by adjusting the field.The output of a single generator ormultiple paralleled generator sets iscontrolled by these two inputs. Thealternator is designed to operate at aspecified speed for the required outputfrequency, typically 60 or 50 hertz. Thevoltage regulator and engine governoralong with other systems define thegenerators response to dynamicload changes and motor startingcharacteristics.

Generators are rated in power andvoltage output. Most generators aredesigned to operate at a 0.8 power fac-tor. For example, a 2000 kW generatorat 277/480 V would have a kVA ratingof 2500 kVA (2000 kW/ 08 pf) and acontinuous current rating of 3007 A

Typical synchronous generatorsfor industrial and commercial powersystems range in size from 100 to3,000 kVA and from 208 V to 13,800 V.There are other ratings available andthese discussions are applicable tothose ratings as well.

Generators must be considered in theshort circuit and coordination study asthey may greatly impact the rating ofthe electrical distribution system. Thisis especially common on large installa-tions with multiple generators andsystems that parallel with the utilitysource. Short circuit current contribu-tion from a generator typically ranges

from 8 to 12 times full load amperes.

2500 kVA 480 V 3( ).

The application of generators requiresspecial protection requirements.The size, voltage class, importanceand dollar investment will influencethe protection scheme associated withthe generator(s). Mode of operationwill influence the utility company'sinterface protection requirements.Paralleling with the electric utility isthe most complicated of the utilityinter-tie requirements. IEEE ANSI 1547provides recommended practices.

Generator Grounding

Generator grounding methods needto be considered and may affect thedistribution equipment and ratings.Generators may be connected in deltaor wye but wye is the most typicalconnection. A wye-connected genera-tor can be solidly grounded, lowimpedance grounded, high impedance

grounded or ungrounded. Section 1.4

discusses general grounding schemes,benefits of each and protectionconsiderations.

A solidly grounded generator may havea lower zero sequence impedance thanits positive sequence impedance. In thiscase, the equipment will need to berated for the larger available groundfault current. The generator ground canbe the same as the system ground or itcould be a separate ground. If they areseparate grounds, then the neutral will

need to be switched for 4-wire loads orground fault relays could misoperateand unbalanced neutral current maybe carried on ground conductors.

An IEEE working group has studied thepractice of low resistance grounding of

medium voltage generators within thegeneral industry. This workinggroup found that, for internal genera-tor ground faults, the vast majority ofthe damage is done after the generatorbreaker is tripped off-line and the fieldand the turbine is tripped. This is dueto the stored energy in the generatorflux that takes several seconds to dissi-pate after the generator is tripped off-line. It is during this time that the LowResistance Ground contributes hugeenergy over time into the generatorground fault. One of the solutions setforth by this working group is aHybrid High Resistance Grounding(HHRG) scheme as shown in Figure1.2-4

. In the HHRG scheme, the LowResistance Ground (LRG) is quicklytripped off-line when the generatorprotection senses the ground fault.The LRG is cleared at the same timethat the generator breaker clears leav-ing the High Resistance Ground por-tion connected to control the transientovervoltages during the coast-downphase of the generator thereby all buteliminating generator damage.

Figure 1.2-4. Hybrid High Resistance Grounding Scheme

Gen 59G

51G

87GN

86

Phase

Relays

HRG

LRGR

R

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Sheet 0045

Generator Protection

Generator protection will vary anddepend on the size of the generator,type of system and importance of thegenerator. Generator sizes are definedas: small 1000 kVA maximum up

to 600 V (500 kVA maximum whenabove 600 V); medium over 1000 kVAto 12,500 kVA maximum regardless ofvoltage; large from 12,500 50,000kVA. The simplest is a single generatorsystem used to feed emergencyand/or standby loads. In this case,the generator is the only sourceavailable when it is operating andit must keep operating until thenormal source returns.

Figure 1.2-5 Part (A)

shows minimumrecommended protection for a singlegenerator used as an emergency orstandby system. Phase and ground

time overcurrent protection (Device51 and 51G) will provide protection forexternal faults. For medium voltagegenerators, a voltage controlled timeovercurrent relay (Device 51V) isrecommended for the phase protec-tion as it can be set more sensitivethan standard overcurrent relays andis less likely to false operate on normaloverloads. This scheme may not pro-vide adequate protection for internalgenerator faults when no other powersource exists. Local generator control-lers may offer additional protectionfor voltage and frequency conditionsoutside the generators capabilities.

Figure 1.2-5 Part (B)

shows therecommended protection for multipleisolated medium voltage small gener-ators. Additional protection may bedesired and could include generatordifferential, reverse power and loss offield protection. Differential protection(Device 87) can be accomplished witheither a self-balancing set of CTs asin Figure 1.2-6 or with a percentagedifferential scheme as in Figure 1.2-7

on Page 1.2-6

. The percentage differ-ential scheme offers the advantageof reducing the possibility for falsetripping due to CT saturation. Theself-balancing scheme offers the

advantages increased sensitivity;needing three current transformersin lieu of six and the elimination ofcurrent transformer external wiringfrom the generator location to thegenerator switchgear location.

Figure 1.2-5. Typical Protective Relaying Scheme for Small Generators

Figure 1.2-6. Self-Balancing GeneratorDifferential Relay Scheme

51G

1

Preferred

Location

51

51G

1

1

Gen

51

1

Alternate

Location

1 1

51V 32 40

1

3

87

Gen

Generator Protection ANSI/IEEE

Std 242-1986

(A) (B)(A)Single Isolated Generator on LowVoltage System(B)Multiple Isolated Generator on MediumVoltage System

87-1

87-3

87-2

50/5A

50/5A

50/5A

R

Gen

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Sheet 0048

Generator Set Sizingand Ratings

Many factors must be consideredwhen determining the proper size orelectrical rating of an electrical power

generator set. The engine or primemover is sized to provide the actualor real power in kW

, as well as speed(frequency) control through the useof an engine governor. The generatoris sized to supply the kVA needed

atstartup and during normal runningoperation and it also provides voltagecontrol using a brushless exciter andvoltage regulator. Together the engineand generator provide the energynecessary to supply electrical loadsin many different applicationsencountered in todays society.

The generator set must be able to

supply the starting and runningelectrical load. It must be able to pickup and start all motor loads and lowpower factor loads, and recover with-out excessive voltage dip or extendedrecovery time. Non-linear loads likevariable frequency drives, uninterrupt-ible power supply (UPS) systems andswitching power supplies also requireattention because the SCR switchingcauses voltage and current waveformdistortion and harmonics. The har-monics generate additional heat in thegenerator windings and the generatormay need to be upsized to accommo-date this. The type of fuel (diesel,

natural gas, propane etc.) used isimportant as it is a factor in determin-ing generator set transient response.It is also necessary to determinethe load factor or average powerconsumption of the generator set.This is typically defined as the load(kW) x time (hrs. while under thatparticular load) / total running time.When this load factor or averagepower is taken into considerationwith peak demand requirementsand the other operating parametersmentioned above, the overall electricalrating of the genset can be deter-mined. Other items to consider includethe unique installation, ambient andsite requirements of the project. Thesewill help to determine the physicalconfiguration of the overall system.

Typical rating definitions for dieselgensets are: standby, prime plus 10,continuous and load management(paralleled with or isolated fromutility)

. Any diesel genset can haveseveral electrical ratings dependingon the number of hours of operationper year and the ratio of electricalload/genset rating when in operation.The same diesel genset can have astandby rating of 2000 kW at 0.8 powerfactor (pf) and a continuous rating of1825 kW at 0.8 pf. The lower continu-ous rating is due to the additionalhours of operation and higher loadthat the continuous genset mustcarry. These additional requirementsput more stress on the engine andgenerator and therefore the ratingis decreased to maintain longevityof the equipment.

Different generator set manufacturers

use basically the same diesel gensetelectrical rating definitions and theseare based on international dieselfuel stop power standards fromorganizations like ISO, DIN andothers. A standby diesel genset ratingis typically defined as supplying vary-ing electrical loads for the duration of apower outage with the load normallyconnected to utility, genset operating< 100 hours per year and no overloadcapability. A prime plus 10 rating istypically defined as supplying varyingelectrical loads for the duration of apower outage with the load normallyconnected to utility, genset operating

500 hours per year and overloadcapability of 10% above its ratingfor one hour out of twelve. A continu-ous rating is typically defined assupplying unvarying electrical loads(i.e., base loaded) for an unlimited time.The load management ratings apply togensets in parallel operation with theutility or isolated/islanded from utilityand these ratings vary in usabilityfrom < 200 hours per year to unlimitedusage. Refer to generator set manufac-turers for further definitions on loadmanagement ratings, load factor oraverage power consumption, peakdemand and how these ratings are

typically applied. Even though there issome standardization of these ratingsacross the manufacturers, there alsoexists some uniqueness with regardsto how each manufacturer appliestheir generator sets.

Electrical rating definitions for naturalgas powered gensets are typicallydefined as standby or continuous withdefinitions similar to those mentionedabove for diesels. Natural gas gensetsrecover more slowly than dieselgensets when subjected to blockloads

. Diesel engines have a muchmore direct path from the engine gov-ernor and fuel delivery system to thecombustion chamber and this resultsin a very responsive engine-generator.A natural gas engine is challengedwith air-fuel flow dynamics and amuch more indirect path from theengine governor (throttle actuator)and fuel delivery system (naturalgas pressure regulator, fuel valve andactuator, carburetor mixer, aftercooler,intake manifold) to the combustionchamber and this results in aless responsive engine-generator.Diesel gensets recover about twiceas fast as natural gas gensets.

For the actual calculations involvedfor sizing a genset, there are readilyaccessible computer software pro-grams which are available on thegenset manufacturer's Internet sitesor from the manufacturer's dealersor distributors. These programs areused to quickly and accurately sizegenerator sets for their application.The programs take into considerationthe many different parameters dis-cussed above including the size andtype of the electrical loads (resistive,inductive, SCR etc.), reduced voltage

soft starting devices (RVSS), motortypes, voltage, fuel type, site condi-tions, ambient conditions and othervariables. The software will optimizethe starting sequences of the motorsfor the least amount of voltage dipand determine the starting kVA neededfrom the genset. It also providestransient response data includingvoltage dip magnitude and recoveryduration. If the transient response isunacceptable, then design changescan be considered including oversizingthe generator to handle the additionalkvar load, adding RVSS devices toreduce the inrush current, improving

system power factor and othermethods. The computer softwareprograms are quite flexible in that theyallow changes to the many differentvariables and parameters to achievean optimum design. The softwareallows, for example, minimizingvoltage dips or using paralleledgensets vs. a single genset.

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Sheet 0049

Genset Sizing Guidelines

Some conservative rules of thumbfor genset sizing include:

1. Oversize genset 20 25% forreserve capacity and for motor

starting.2. Oversize gensets for unbalanced

loading or low power factorrunning loads.

3. Use 1/2 hp per kW for motor loads.

4. For variable frequency drives,oversize the genset by atleast 40%.

5. For UPS systems, oversize thegenset by 40% for 6 pulse and15% for 6 pulse with input filtersor 12 pulse.

6. Always start the largest motor

first when stepping loads.

For basic sizing of a generator system,the following example could be used:

Step 1: Calculate Running Amperes

Motor loads:

200 hp motor . . . . . . . . . . . . 156 A

100 hp motor . . . . . . . . . . . . . 78 A

60 hp motor . . . . . . . . . . . . . . 48 A

Lighting Load . . . . . . . . . . . . . . 68 A

Miscellaneous Loads . . . . . . . . 95 A

Running Amperes

. . . . . . . . . . 445 A

Step 2: Calculating Starting Amperes

Using 1.25 Multiplier

Motor loads:

. . . . . . . . . . . . . . . . . . . . . . . 195 A

. . . . . . . . . . . . . . . . . . . . . . . . 98 A

. . . . . . . . . . . . . . . . . . . . . . . . 60 A

Lighting Load . . . . . . . . . . . . . . 68 A

Miscellaneous Loads . . . . . . . . 95 A

Starting Amperes

. . . . . . . . . . 516 A

Step 3: Selecting kVA of Generator

Running kVA =(445 A x 480 V x 1.732)/1000 = 370 kVA

Starting kVA =(516 A x 480 V x 1.732)/1000 = 428 kVA

Solution

Generator must have a minimumstarting capability of 428 kVA and min-imum running capability of 370 kVA.

Also, please see Section Factors Gov-erning Voltage Drop on Page 1.3-21

for further discussion on generatorloading and reduced voltage startingtechniques for motors.

Generator Set Installationand Site Considerations

There are many different installationparameters and site conditionsthat must be considered to have a

successful generator set installation.The following is a partial list of areasto consider when conducting thisdesign. Some of these installationparameters include:

Foundation type (crushed rock,concrete, dirt, wood, separateconcrete inertia pad etc.).

Foundation to genset vibrationdampening (spring type, corkand rubber etc.).

Noise attenuation (radiator fanmechanical noise, exhaust noise,air intake noise).

Combustion and cooling air

requirements.

Exhaust backpressure requirements.

Emissions permitting.

Delivery and rigging requirements.

Genset derating due to highaltitudes or excessive ambienttemperatures.

Hazardous waste considerationsfor fuel, antifreeze, engine oil.

Meeting local building andelectrical codes.

Genset exposure (coastalconditions, dust, chemicals etc.).

Properly sized starting systems(compressed air, batteries andcharger).

Allowing adequate space forinstallation of the genset and formaintenance (i.e., air filter removal,oil changing, general gensetinspection etc).

Flex connections on all systems thatare attached to the genset and arigid structure (fuel piping, founda-tion vibration isolators, exhaust, airintake, control wiring, power cables,radiator flanges/duct work etc.).

Diesel fuel day tank systems

(pumps, return piping). Fuel storage tank (double walled,

fire codes) and other parameters.

Please see the generator set manufac-turers Application and InstallationGuidelines for proper applicationand operation of their equipment.

Figure 1.2-9. Typical Genset InstallationNote: Courtesy of Caterpillar, Inc.

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1June 2006

Power Distribution SystemsSystem Analysis

Sheet 0051

Systems Analysis

A major consideration in thedesign of a distribution system is toensure that it provides the requiredquality of service to the various

loads. This includes serving eachload under normal conditions and,under abnormal conditions, providingthe desired protection to serviceand system apparatus so thatinterruptions of service are minimizedconsistent with good economic andmechanical design.

Under normal conditions, the impor-tant technical factors include voltageprofile, losses, load flow, effects ofmotor starting, service continuity andreliability. The prime considerationsunder faulted conditions are apparatusprotection, fault isolation and servicecontinuity. During the system prelimi-

nary planning stage, before selectionof the distribution apparatus, severaldistribution systems should be analyzedand evaluated including both economicand technical factors. During this stageif system size or complexity warrant,it may be appropriate to provide athorough review of each system underboth normal and abnormal conditions.

The principal types of computerprograms utilized to provide systemstudies include:

Short circuit identify 3-phase andline-to-ground fault currents andsystem impedances.

Arc flash calculates arc flashenergy levels which lead to theselection of personal protectiveequipment (PPE).

Circuit breaker duty identifyasymmetrical fault curren

power dist. systems - eaton

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