Transformers

Transformers play a central part in the design of distribution systems;they reduce the high voltage of the primary to the low utilizationvoltage of the secondary. As with other elements of the distribution circuit,the energy losses and the drop in voltage due to the current flowingthrough...

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Reactors

PrimaryWhere relatively high-voltage primary feeders (23-kV and above)operate in metallic sheathed cables and are rather long, the capacitanceeffect of the cable may cause undesirable voltage rises along the feeder.Reactors connected between the primary conductors and the neutral...

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Capacitors

Voltage regulation can also be improved by the application ofshunt capacitors at the substation, out on the primary feeder, or both.The current drawn by a capacitor has a leading power factor characteristicand will cause a voltage rise from the location of the capacitor back tothe...

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Boosters

An increase or decrease in the primary voltage can also be obtainedby the installation of a transformer in the line to provide a fixed voltagedrop. A distribution transformer, connected as an autotransformer, maybe used to boost or buck the feeder voltage at the point of its installation.The...

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Taps

Where voltage improvement can be obtained by some fixedamount which will not cause voltages to exist outside permissible limitsduring both light and heavy load conditions, taps can be changed on thedistribution transformers on certain portions of the feeder.For example, assuming...

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Voltage Regulators

Where the most economical size of conductor results in voltagedrops or regulation greater than permissible, alternatives may be considered.These may include the installation of larger-size conductors,or a voltage regulator, or both, economics indicating the selection. Herethe economic...

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The Primary System

The primary system comprises the facilities that deliver powerfrom the distribution substation to the distribution transformers. Thesetake the form of one or more distribution feeders or circuits emanatingfrom the substation, each supplying a portion of the entire load servedfrom...

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The Secondary System

Transformer-secondary CombinationThe combination of transformers, secondary circuit or main, andthe consumers’ services makes up the secondary system. Secondarysystems are predominantly single-phase, except for larger commercialand industrial consumers, who are supplied from three-phase...

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Services

The electrical design of distribution facilities is based on the loadsthey are to carry safely and the permissible voltage variations; the finaldesign, however, cannot be divorced from mechanical, economic,and other considerations. Several different designs may serve the sameelectrical...

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Service Reliability

Reliability of service generally is interpreted to mean the continuityof service or the lack of interruption to service. For a distribution system,or any of its parts, absolute reliability or continuity of service 100percent of the time for 100 percent of its consumers is an impossibility,although...

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Voltage Requirements

Electric devices utilizing secondary or low voltage in the UnitedStates have been standardized by almost all manufacturers at 120/240 V.While many utilities are following these standards for their systems, thereare a significant number operating at 115/230 V and some at 125/250 V(and...

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Future Requirements

Good engineering requires that probable future growth of loads beconsidered in planning. This is usually provided for by spare capacityin the present design of the several elements, or by provisions for possiblefuture additions or alterations, or both of these. Load growth israrely...

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Fluctuation in Demand

There are three main factors that greatly influence the magnitudeof maximum demand and the time of its occurrence. The most frequentis the weather as it affects light intensity during daylight hours andtemperatures throughout the day and year.The sharpest factor and perhapsthat of...

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Consumer Classification

As aids in planning, consumers may be conveniently classifiedinto certain categories and certain ranges of load densities expressed inkVA per square mile (where this unit is too broad to be useful, watts persquare foot for specific occupancies may be used).Further classifications...

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Consumer Factors

It is obvious that an individual consumer is not apt to be using allof the electrical devices that constitute his or her “connected load” at thesame time, or to their full capacity. It would evidently be unnecessaryto provide facilities to serve such a total possible load, and much...

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Connected Loads

A good place to start is the tabulation of all electric devices (lamps,appliances, equipment, etc.) that consumers can connect to their supplysystem. The ratings of the devices at specified voltages (and sometimesfrequency and temperature) limits are usually contained in the nameplateor...

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Load Characteristics

In the planning of an electrical distribution system, as in any otherenterprise, it is necessary to know three basic things:1. The quantity of the product or service desired (per unit of time)2. The quality of the product or service desired3. The location of the market and the individual...

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Overhead versus Underground

Although the original distribution system pioneered by ThomasEdison was a direct current low-voltage system installed underground,the widespread expansion of electric systems was based principally onthe adoption of alternating current (through the application of transformers)and...

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Distribution System Considerations

In determining the design of distribution systems, three broad classificationsof choices need to be considered:1. The type of electric system: dc or ac, and if ac, single-phase orpolyphase.2. The type of delivery system: radial, loop, or network. Radial systemsinclude duplicate and...

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