108.pdf

Broadwater, R., Sargent, A., Lee, R. E. Power Distribution The Engineering Handbook. Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000

1998 by CRC PRESS LLC

108Power Distribution

108.1 Equipment108.2 System Divisions and Types108.3 Electrical Analysis, Planning, and Design108.4 System Control108.5 Operations

Robert BroadwaterVirginia Polytechnic Institute &State University

Albert SargentArkansas Power & Light

Robert E. LeePennsylvania Power & Light

The function of power distribution is to deliver to consumers economic, reliable, and safe electricalenergy in a manner that conforms to regulatory standards. Power distribution systems receiveelectric energy from high-voltage transmission systems and deliver energy to consumerservice-entrance equipment. Systems typically supply alternating current at voltage levels rangingfrom 120 V to 46 kV.

Figure 108.1 illustrates aspects of a distribution system. Energy is delivered to the distributionsubstation (shown inside the dashed line) by three-phase transmission lines. A transformer in thesubstation steps the voltage down to the distribution primary system voltagein this case, 12.47kV. Primary distribution lines leave the substation carrying energy to the consumers. Thesubstation contains a breaker that may be opened to disconnect the substation from the primarydistribution lines. If the breaker is opened, outside the substation there is a normally opensupervisory switch that may be closed in order to provide an alternate source of power for thecustomers normally served by the substation. The substation also contains a capacitor bank usedfor either voltage or power factor control.


Figure 108.1 Distribution system schematic.


Four types of customers, along with representative distribution equipment, are shown in Fig.108.1. A set of loads requiring high reliability of service is shown being fed from an undergroundthree-phase secondary network cable grid. A single fault does not result in an interruption to thisset of loads. A residential customer is shown being supplied from a two-wire, one-phase overheadlateral. Commercial and industrial customers are shown being supplied from the three-phase,four-wire, overhead primary feeder. At the industrial site a capacitor bank is used to control powerfactor. Except for the industrial customer, all customers shown have 240/120 V service. Theindustrial customer has 480Y/277 V service.

For typical electric utilities in the U.S., investment in distribution ranges from 35 to 60% of totalcapital investment.

108.1 EquipmentFigure 108.1 illustrates a typical arrangement of some of the most common equipment. Equipmentmay be placed into the general categories of transmission, protection, and control.

Arresters protect distribution system equipment from transient overvoltages due to lightning orswitching operations. In overvoltage situations the arrester provides a low-resistance path toground for currents to follow.

Capacitor banks are energy storage devices primarily used to control voltage and power factor.System losses are reduced by the application of capacitors.

Conductors are used to transmit energy and may be either bare or insulated. Bare conductorshave better thermal properties and are generally used in overhead construction where contact isunlikely. Insulated cables are used in underground/conduit construction. Concentric neutral andtape-shielded cables provide both a phase conductor and a return path conductor in one unit.

Distribution lines are made up of conductors and are classified according to primary voltage, thenumber of phases, number of conductors, and return path. The three-phase, four-wire,multigrounded system is the most common primary system, where one conductor is installed foreach of the three phases and the fourth conductor is a neutral that provides a return current path.Multigrounded means that the neutral is grounded at many points, so that the earth provides aparallel path to the neutral for return current. Three-phase, three-wire primary systems, ordelta-connected systems, are rarely used because faults therein are more difficult to detect. Alateral is a branch of the system that is shorter in length, more lightly loaded, or has a smallerconductor size than the primary feeder.

Distribution transformers step the voltage down from the primary circuit value to the customerutilization level, thus controlling voltage magnitude. Sizes range from 10 to 2500 kVA.Distribution transformers are installed on poles, ground-level pads, or in underground vaults. Aspecification of 12470Y/7200 V for the high-voltage winding of a single-phase transformer meansthe transformer may be connected in a line-to-neutral "wye" connection for a system with aline-to-line voltage of 12470 V. A specification of 240/120 V for the low-voltage winding meansthe transformer may be used for a three-wire connection with 120 V midtap voltage and 240 Vfull-winding voltage. A specification of 480Y/277 V for the low voltage winding means thewinding may be wye-connected for a three-phase, four-wire service to deliver 480 V line-to-line


and 277 V line-to-neutral.Distribution substations consist of one or more step-down power transformers configured with

switch gear, protective devices, and voltage regulation equipment for the purpose of supplying,controlling, switching, and protecting the primary feeder circuits. The voltage is stepped down forsafety and flexibility of handling in congested consumer areas. Overcurrent protective devicesopen and interrupt current flow in order to protect people and equipment from fault current.Switches are used for control to interrupt or redirect power flow. Switches may be operated eithermanually or remotely with supervisory control. Switches are usually rated to interrupt load currentand may be either pad or pole mounted.

Power transformers are used to control and change voltage level. Power transformers equippedwith tap-changing mechanisms can control secondary voltage over a typical range of plus orminus 10%.

Voltage regulators are autotransformers with tap-changing mechanisms that may be usedthroughout the system for voltage control. If the voltage at a remote point is to be controlled, thenthe regulator can be equipped with a line drop compensator that may be set to regulate the voltageat the remote point based upon local voltage and current measurements.

108.2 System Divisions and TypesDistribution transformers separate the primary system from the secondary. Primary circuitstransmit energy from the distribution substation to customer distribution transformers. Three-phasedistribution lines that originate at the substation are referred to as primary feeders or primarycircuits. Primary feeders are illustrated in Fig. 108.1. Secondary circuits transmit energy from thedistribution transformer to the customer's service entrance. Line-to-line voltages range from 208 to600 V.

Radial distribution systems provide a single path of power flow from the substation to eachindividual customer. This is the least costly system to build and operate, and thus the most widelyused.

Primary networks contain at least one loop that generally may receive power from two distinctsources. This design results in better continuity of service. A primary network is more expensivethan the radial system design because more protective devices, switches, and conductors arerequired.

Secondary networks are normally underground cable grids providing multiple paths of powerflow to each customer. A secondary network generally covers a number of blocks in a downtownarea. Power is supplied to the network at a number of points via network units, consisting of anetwork transformer in series with a network protector. A network protector is a circuit breakerconnected between the secondary winding of the network transformer and the secondary networkitself. When the network is operating properly, energy flows into the network. The networkprotector opens when reverse energy flow is detected, such as may be caused by a fault in theprimary system.


The distribution system is planned, designed, constructed, and operated based on the results ofelectrical analysis. Generally, computer-aided analysis is used.

Line impedances are needed by most analysis applications. Distribution lines are electricallyunbalanced due to loads, unequal distances between phases, dissimilar phase conductors, andsingle-phase or two-phase laterals. Currents flow in return paths due to the imbalance in thesystem. Three-phase, four-wire, multigrounded lines have two return pathsthe neutral conductorand earth. Three-phase, multigrounded concentric neutral cable systems have four return paths.The most accurate modeling of distribution system impedance is based upon Carson's equations.With this approach a 5 5 impedance matrix is derived for a system with two return paths, and a7 7

impedance matrix is derived for a system with four return paths. For analysis, these matricesare reduced to 3 3 matrices that relate phase voltage drops (i.e., VA;VB ;VC ) to phasecurrents (i.e., IA; IB ; IC ), as indicated by2

4 VAVBVC

35 =

24 ZAA ZAB ZACZBA ZBB ZBCZCA ZCB ZCC

3524 IAIBIC

35

Load analysis forms the foundation of system analysis. Load characteristics are time varying anddepend on many parameters, including connected consumer types and weather conditions. Theload demand for a given customer or group of customers is the load averaged over an interval oftime, say 15 minutes. The peak demand is the largest of all demands. The peak demand is ofparticular interest since it represents the load that the system must be designed to serve. Diversityrelates to multiple loads having different time patterns of energy use. Due to diversity, the peakdemand of a group of loads is less than the sum of the peak demands of the individual loads. For agroup of loads,

Diversity factor =Sum of individual load peaks

Group peak

Loads may be modeled as either lumped parameter or distributed. Lumped parameter load modelsinclude constant power, constant impedance, constant current, voltage-dependent, andcombinations thereof. Generally, equivalent lumped parameter load models are used to modeldistributed loads. Consider the line section of length L shown in Fig. 108.2(a), with a uniformlydistributed load current that varies along the length of the line as given by

i(x) =I2 I1

Lx + I1

The total load current drawn by the line section is thus

IL = I2 I1

An equivalent lumped parameter model for the uniformly distributed current load is shown in Fig.

108.3 Electrical Analysis, Planning, and Design


108.2(b).

Figure 108.2 (a) Line section model with distributed load current; (b) lumped parameter equivalentmodel.

Load forecasting is concerned with determining load magnitudes during future years fromcustomer growth projections. Short-range forecasts generally have time horizons of approximatelyfive years, whereas long-range forecasts project to around twenty years.

Power flow analysis determines system voltages, currents, and power flows. Power flow resultsare checked to ensure that voltages fall within allowable limits, that equipment overloads do notexist, and that phase imbalances are within acceptable limits. For primary and secondary networks,power flow methods used in transmission system analysis are applied. For radially operatedsystems, the ladder method is used. The actual implementation of the ladder method may vary withthe type of load models used. All ladder load flow methods assume the substation bus voltage isknown. An algorithm for the ladder method is as follows:

Step 1. Assume a value for all node voltages throughout the circuit. Generally, assumedvoltages are set equal to the substation voltage.


Step 2. At each load in the circuit, calculate the current from the known load value andassumed voltage.Step 3. Starting at the ending nodes, sum load currents to obtain line section currentestimates, performing summation until the substation is reached.Step 4. Having estimates of all line section currents, start at the substation and calculate linesection voltage drops and new estimates of node voltages.Step 5. Compare new node voltages with estimates of previous iteration values. Thealgorithm has converged if the change in voltage is sufficiently small. If the algorithm hasnot converged, return to step 2.

Dynamic load analysis includes such studies as motor-starting studies. Rapid changes in largeloads can result in large currents, with a resultant drop in system voltage. If the dip in voltage is toolarge or too frequent, then other loads are adversely affected, such as in an annoying flicker oflights. This study generally employs a power flow calculation that is run at a number of pointsalong the dynamic characteristic of the load.

Fault analysis provides the basis for protection system design. Generally, superposition is used toadd load currents obtained from power flow analysis to fault currents. Thus, in the model used tocalculate fault currents, load currents are neglected. Sources of fault current are the substation bus,cogenerators, and large synchronous motors on the feeder or neighboring feeders. A variety offault conditions are considered at each line section, including three-phase-to-ground,single-phase-to-ground, and separate phases contacting one another. In performing thecalculations, both bolted (i.e., zero-impedance) faults and faults with an impedance in the fault pathare considered. Of interest are the maximum and minimum phase and return path fault currents, aswell as the fault types that result in these currents.

Reliability analysis involves determining indices that relate to continuity of service to thecustomer. Reliability is a function of tree conditions, lightning incidence, equipment failure rates,equipment repair times, and circuit design. The reliability of a circuit generally varies from point topoint due to protection system design, placement of switches, and availability of alternative feeds.There are many indices used in evaluating system reliability. Common ones include systemaverage interruption frequency index (SAIFI), system average interruption duration index (SAIDI),and customer average interruption frequency index (CAIFI), as defined by

SAIFI =Total number of customer interruptions

Total number of customers served

SAIDI =Sum of customer interruption durations

Total number of customers

CAIFI =Total number of customer interruptions

Total number of customers aected

Phase balancing is used to balance the current or power flows on the different phases of a linesection. This results in improved efficiency and primary voltage level balance. The average currentin the three phases is defined as


Iavg =IA + IB + IC

3

The maximum deviation from Iavg is given by

Idev = maximum of fjIavg IA j; jIavg IB j; jIavg IC jg

Phase imbalance is defined as

Phase imbalance =Idev

Iavg

Planning involves using load forecasting and other analysis calculations to evaluate voltagelevel, substation locations, feeder routes, transformer/conductor sizes, voltage/power factor control,and restoration operations. Decisions are based upon considerations of efficiency, reliability, peakdemand, and life cycle cost.

Overcurrent protection is the most common protection applied to the distribution system. Withovercurrent protection, the protective device trips when a large current is detected. The time to tripis a function of the magnitude of the fault current. The larger the fault current is, the quicker theoperation. Various types of equipment are used. A circuit breaker is a switch designed to interruptfault current, the operation of which is controlled by relays. An overcurrent relay, upon detectingfault current, sends a signal to the breaker to open. A recloser is a switch that opens and thenrecloses a number of times before finally locking open. A fuse is a device with a fusible member,referred to as a fuse link, which in the presence of an overcurrent melts, thus opening up the circuit.

Breakers may be connected to reclosing relays, which may be programmed for a number ofopening and reclosing cycles. With a recloser or a reclosing breaker, if the fault is momentary, thenthe power interruption is also momentary. If the fault is permanent, then after a specified numberof attempts at reclosing the device locks open. Breakers are generally more expensive thancomparable reclosers. Breakers are used to provide more sophisticated protection, which isavailable via choice of relays. Fuses are generally used in the protection of laterals.

Protective equipment sizing and other characteristics are determined from the results of faultanalysis. Moving away from the substation in a radial circuit, both load current and available faultcurrent decrease. Protective devices are selected based on this current grading. Protective devicesare also selected to have different trip delay times for the same fault current. With this timegrading, protective devices are coordinated to work together such that the device closest to apermanent fault clears the fault. Thus reclosers can be coordinated to protect load-side fuses fromdamage due to momentary faults.

108.4 System ControlVoltage control is required for proper operation of customer equipment. For instance, in the U.S.,"voltage range A" for single-phase residential users specifies that the voltage may vary at theservice entrance from 114/228 V to 126/252 V. Regulators, tap-changing under load transformers,


and switched capacitor banks are used in voltage control.Power factor control is used to improve system efficiency. Due to the typical load being

inductive, power factor control is generally achieved with fixed and/or switched capacitor banks.Power flow control is achieved with switching operations. Such switching operations are

referred to as system reconfiguration. Reconfiguration may be used to balance a load amonginterconnected distribution substations. Such switching operations reduce losses while maintainingproper system voltage.

Load control may be achieved with voltage control and also by remotely operated switches thatdisconnect load from the system. Generally, load characteristics are such that if the voltagemagnitude is reduced, then the power drawn by the load will decrease for some period of time.Load control with remotely operated switches is also referred to as load management.

108.5 OperationsThe operations function includes system maintenance, construction, and service restoration.Maintenance, such as trimming trees to prevent contact with overhead lines, is important to ensurea safe and reliable system. Interruptions may be classified as momentary or permanent. Amomentary interruption is one that disappears very quicklyfor instance, a recloser operation dueto a fault from a tree limb briefly touching an overhead conductor. Power restoration operations arerequired to repair damage caused by permanent interruptions.

While damaged equipment is being repaired, power restoration operations often involvereconfiguration in order to restore power to interrupted areas. With reconfiguration, power flowcalculations may be required to ensure that equipment overloads are not created from the switchingoperations.


SYSTEM OF ELECTRIC LIGHTINGWilliam Stanley, Jr.Patented January 5, 1886#333,564This invention described a system for wiring lamps in series/parallel combinations to maintain aconstant load in the cross circuits even when individual lights were switched in and out.

An excerpt:My invention consists in arranging and connecting the wires constituting each cross-circuit in

the manner hereinafter shown, and connecting with the wires and lamps switches and resistancesin such a way that the resistance in each cross-circuit shall remain constant so long as any lamptherein is lighted, and so that each cross-circuit shall be interrupted and no current pass through itwhen no lamp in it is lighted.


Stanley obtained 130 patents in his lifetime and was instrumental in the development of ACelectric power distribution for Westinghouse and others. He developed practical transformers, aself-starting motor, a two-phase AC generator and an electric meter with magnetic suspensioninstead of jeweled bearings. (1994, DewRay Products, Inc. Used withpermission.)

Defining TermsCurrent return path: The path that current follows from the load back to the distribution

substation. This path may consist of either a conductor (referred to as the neutral) or earth, orthe parallel combination of a neutral conductor and the earth.

Fault: A conductor or equipment failure or unintended contact between conductors or betweenconductors and grounded objects. If not interrupted quickly, fault current can severelydamage conductors and equipment.

Phase: Relates to the relative angular displacement of the three sinusoidally varying voltagesproduced by the three windings of a generator. For instance, if phase A voltage is 120 6 0 V,phase B voltage 120 6 120 V, and phase C voltage 120 6 120 V, the phase rotation isreferred to as ABC. Sections of the system corresponding to the phase rotation of the voltagecarried are commonly referred to as phase A, B, or C.

Tap-changing mechanism: A control device that varies the voltage transformation ratio betweenthe primary and secondary sides of a transformer. The taps may only be changed by discreteamounts, say 0.625%.

ReferencesBroadwater, R. P., Shaalan, H. E., Oka, A., and Lee, R. E. 1993. Distribution system reliability

and restoration analysis. Electric Power Sys. Res. J. 29(2):203-211.Carson, J. R. 1926. Wave propagation in overhead wires with ground return. Bell System Tech.

J. 5: 40-47.Engel, M. V., Greene, E. R., and Willis, H. L. (Eds.) IEEE Tutorial Course: Power Distribution

Planning. 1992. Course Text 92 EHO 361-6-PWR IEEE Service Center, Piscataway, NJ.Kersting, W. H. and Mendive, D. L. 1976. An Application of Ladder Network Theory to the

Solution of Three-Phase Radial Load Flow Problems. IEEE Winter Meeting, New York.

Further InformationBurke, J. J. 1994. Power Distribution Engineering. Marcel Dekker, New York.Redmon, J. R. 1988. IEEE Tutorial Course on Distribution Automation. Course Text 88 EH0

280-8-PWR IEEE Service Center, Piscataway, NJ.Electric Utility Engineers, Westinghouse Electric Corporation. 1950. Electrical Transmission and

Distribution Reference Book. Westinghouse Electric Corporation, Pittsburgh, PA.Gnen, T. 1986. Electric Power Distribution System Engineering. John Wiley & Sons, New York.Lakervi, E. and Holmes, E. J. 1989. Electricity Distribution Network Design. Peter Peregrinus,

London.Pansini, A. J. 1992. Electrical Distribution Engineering. Fairmont Press, Liburn, GA.


The Engineering HandbookContentsPower Distribution108.1 Equipment108.2 System Divisions and Types108.3 Electrical Analysis, Planning, and Design108.4 System Control108.5 OperationsDefining TermsReferencesFurther Information

108.pdf

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