Transformers

Transformers play a central part in the design of distribution systems;
they reduce the high voltage of the primary to the low utilization
voltage of the secondary. As with other elements of the distribution circuit,
the energy losses and the drop in voltage due to the current flowing
through them to supply loads are factors in the selection of the size and
location of transformers.

Losses
Energy losses in a transformer are generally of two kinds:
1. No-load loss (also known as iron or core loss) results from the mag
netizing or exciting current flowing in the primary coil regardless
of the load carried. Its value of about 0.5 percent at rated full load
may vary substantially at voltages above or below rated values.
Although small as a power loss, it goes on constantly, accumulating
into significant annual energy losses (in kilo-watt-hours).

2. Full-load losses (earlier known as copper losses) result from the
load current passing through the resistance of both primary and
secondary coils. This I^R loss varies with the square of the current
carried and therefore depends on the shape of the load curve. Since
the current flowing in a circuit is inversely proportional to the
voltage, the copper loss is inversely proportional to the square of
the voltage; hence, for the same-size transformer, the losses in the
primary coil are substantially less as the voltage ratings increase.
No-load and full-load losses for the various sizes of transformers vary
with different manufacturers and are usually specified by them in some
percentage of normal voltage and full-load ratings. No-load losses may be
expressed in watts or as a percentage of the full rated load in watts.

Impedance—Resistance and Reactance
Copper losses, as well as voltage regulation, require that resistance
and reactance values (and their vector sum, impedance) of the
transformer be known. These three values represent both primary and
secondary coils of the transformer. They are usually specified by the
manufacturer as a percentage related to the percentage voltage drop.
That percentage gives a value in volts when applied to either primary
or secondary voltage; from that voltage and the full rated current the
values in ohms may be derived.

The percentage impedance given for a transformer represents (and
is equivalent to) the percentage drop from normal rated primary voltage
that would occur when full rated load current flows in the secondary;
thus the percentage impedance can be used to determine the impedances
(in ohms) of the primary and secondary as follows.


Voltage Drop
The determination of voltage drop through the transformer employs
values of impedance, resistance, and reactance, as indicated in the
previous discussion of primary and secondary systems. The drop must
be referred to either the primary or the secondary side:
Voltage drop (primary) = Ip(Rp cos θ + X sin θ)
where Ip is the load current and cos θ the power factor.
Voltage drop (secondary) = Is(Rs cos θ + X sin θ)
% voltage drop voltage drop (primary) voltage drop (secondary)
——————— = —————————— = ———————————
100 rated primary voltage rated secondary voltage

These same phase-to-neutral values of Z, R, and X can also be employed
in polyphase circuits. Since phase to phase voltage (for a threephase
circuit) is √3 times the phase-to-neutral value and the voltage
value in the equation is squared, the ratio between phase-to-phase and
phase-to-neutral characteristics is 3 to 1. If the transformer kVA value is
the three-phase total kVA and Ep is the phase-to-phase voltage,

Single-phase
The standard single-phase distribution transformer is generally designed
with the secondary coil in two parts, which may be connected in
parallel for two-wire 120-V operation, or in series for three-wire 120/240
V operation. The latter is the most commonly used connection for singlephase
distribution systems. The load is balanced between two 120-V
circuits; with perfect balance, no current flows in the center or neutral
wire. Refer to Figure 4-6a.

Three-phase
For three-phase systems, the wye-connected secondary can serve
single-phase loads at 120 V for each phase; when the load is balanced,
the neutral will carry no current. This connection can also supply threephase
power loads at 208 V between phases, and it is best adapted for
use on secondary networks. It does have the disadvantage of a lowered
three-phase (208 V) voltage supply to three-phase motors with standard
ratings of 240 V; the 32-V difference, or 13.3 percent, below the rating
may affect the operation of the motors. To remedy the situation, the secondary
voltage is often raised to 125 V, yielding about 217 V between
phases or about only 10 percent less than the standard 240-V rating,
more likely to be within the design tolerances for satisfactory operation.
See Figure 4-6b and c.

The primary supply to this four-wire wye secondary connection
can be either delta- or wye-connected; in the latter case, the wye is usually
grounded to prevent voltage unbalances from unbalanced secondary
loads from distorting phase relationships. Often, further economy is
achieved if both the primary and secondary circuits employ a common
neutral conductor.

Small amounts of three-phase power loads may be supplied on a
chiefly single-phase system by a small-diameter-conductor extension of
another phase and the installation of a small-capacity single-phase transformer
in an open-wye or open-delta bank (on the primary side) with
the principal single-phase transformer.

The secondary of this second
single-phase transformer is connected in an open-delta configuration
with the secondary of the principal transformer, providing a small threephase
delta power supply to the small three-phase requirement. Because
of the phase relationship of the voltage and current, however, only 86
percent of the capacity of this second, small single-phase transformer
can be utilized. This is an economical method of supplying a small,
isolated three-phase load in the midst of an area supplied from singlephase
facilities.

Two-phase
Although two-phase systems are virtually extinct, there are still
some two-phase power loads in existence. These may be supplied from
three-phase delta or wye systems through proper connections of two
single-phase transformers.

Boost-Buck
Earlier, reference was made to the use of single-phase transformers
to boost or buck the line voltage of a primary feeder. Here, the primary
and secondary of the transformer are connected in series, essentially operating
as an autotransformer. The incoming primary coil is connected
across the primary circuit, while the outgoing primary is connected
between a common terminal of the primary of the transformer and the
terminal of the secondary coil; the voltage of the secondary coil is either
added to boost the primary voltage or subtracted to buck it. The capacity
of the secondary coil limits the primary current that may flow through
it.
Three-phase Units
Connections for three-phase transformers and lead markings are
shown in the IEEE classification of polarities.

Autotransformers
Under certain conditions, when the ratio of transformation desired
is low, usually not greater than about 5 to 1, and electrical isolation
between primary and secondary circuits is not essential, the autotransformer
has some advantages.

The autotransformer consists of one winding, a part of which may
serve as both primary and secondary. In a two-winding transformer, all
of the energy is transformed by magnetic action. In the autotransformer,
a portion only is transformed magnetically and the remainder flows
conductively through a part of its windings. Since only a portion of the
energy is transferred, the autotransformer can be smaller than a twowinding
unit; comparable costs of the unit and its installation are less.

Read More - Transformers

Reactors

Primary
Where relatively high-voltage primary feeders (23-kV and above)
operate in metallic sheathed cables and are rather long, the capacitance
effect of the cable may cause undesirable voltage rises along the feeder.
Reactors connected between the primary conductors and the neutral or
ground are inserted in the feeder at appropriate points to hold customer
voltages within permissible limits; shunt reactors act in a similar fashion
as shunt capacitors.

Secondary
Where two or more transformers supply a common load, the transformers
may not share the load equitably. This may be due to differing
secondary voltages at the transformers’ terminals either because the
primary feeder voltages are different, or because the transformers have
different impedances. Reactors inserted in the secondary leads of one or
more of the transformers are installed in an effort to equalize the voltages
and make the transformers share the load equitably.

This phenomenon is often evident in low-voltage secondary networks, and especially
in “spot networks.” In this latter case, the terminals of the reactor coil of
one transformer are interconnected to those of another transformer with
the leads reversed. Hence, the voltage drop in each of the reactances is
added to or subtracted from the several secondary voltages, tending to
balance the load among the several supply transformers.

Read More - Reactors

Capacitors

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

The voltage rise will be equal to the reactance of the
circuit (back to the source) multiplied by the capacitor current (taking
into account their vector relationship). The rise in voltage is independent
of the load on the circuit and is greatest at the location of the capacitors
and decreases to the source.

Capacitors provide a constant increase in the level of voltage at the
location of the capacitor that is the same under any load condition of
the feeder, from light to heavy loading. If capacitors are installed so that
they may be switched on during heavy load periods and off at light load
periods, voltage regulation can be improved. If a bank of capacitors is
so arranged that some of its units can be switched on and off separately,
voltage regulation can be improved even further.

Primary Feeder
When they are installed out on a primary feeder, the capacity of
the capacitors (in kVA) and the location on the feeder where they are
to be installed depends on the manner in which the loads are distributed
on the feeder, the power factor of the loads, the feeder conductor
size and spacing between conductors, and the voltage conditions
along the feeder.

Like the line voltage regulator, capacitors should be
installed approximately at the point where the voltage at heavy load
is at the minimum permissible level (with some consideration given
to load growth). The conditions under light load will determine what
portion of the capacitance installed may be fixed and what may be
switched.

Substations
Capacitors may also be installed at substations on the bus supplying
the outgoing distribution feeders. They are usually installed in
relatively large-capacity banks, and it is usually necessary to switch off
portions of them at periods of light load to prevent excessively high
outgoing voltage


The voltage drops along the feeders supplied from
this substation bus remain the same as do their power factors, since the
relationship between the voltage and current flowing through each of
the feeders supplying their loads is unaffected by the capacitors added
to the substation bus. The voltage level of each of the entire feeders is
raised depending on the capacitance added at the substation, but the
voltage spread on each feeder remains the same.

In many instances, the principal reason for the capacitors at the substation bus is not necessarily
to control the bus voltage, but, by counteracting the effect of induction
(or reactance), to reduce the current to that necessary to supply the load
at approximately unity power factor, thereby permitting larger loads to
be supplied by the same transmission and substation facilities.

Series Capacitors
Capacitors can also be installed in series with primary feeders to
reduce voltage drop, but they are rarely employed in this fashion. Where
shunt capacitors, connected in parallel with the load, correct the component
of the current due to the inductive reactance of the circuit, series
capacitors compensate for the reactive voltage drop in the feeder.

A capacitor in series in a primary feeder serving a lagging-power
factor load will cause a rise in voltage as the load increases. The power
factor of the load through the series capacitor and feeder must be lagging
if the voltage drop is to decrease appreciably. The voltage on the
load side of the series capacitor is raised above the source side, acting
to improve the voltage regulation of the feeder. Since the voltage rise
or drop is produced instantaneously with the variations in the load, the
series capacitor response as a voltage regulator is faster and smoother
than the induction or TCUL-type regulator; moreover, no contact-making
voltmeter and load compensator are required for its operation.

During fault conditions, however, the large fault current passing
through the series capacitor can develop excessive voltage across the
capacitor, sufficient to cause its destruction. It is essential, therefore,
that it be taken out of service as quickly as possible. A resistor and
air gap are connected between the terminals of the series capacitor.
When the voltage becomes sufficiently high, the gap breaks down and
permits the capacitor to be short-circuited through the resistor; the
resistor dampens out any oscillatory discharge current so the gap can
break down and restrike repetitively without damaging the capacitor.
Auxiliary relays operate to short-circuit and bypass the capacitor if the
fault persists.

Because of the potential hazard, series capacitors as voltage regulators
are usually restricted to supplying single large consumers where
flicker may result from frequent motor starts or from electric welders,
furnaces, and similar devices that may cause rapid and repetitive load
fluctuations.

Read More - Capacitors

Boosters

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

The percentage of boost or buck will depend on the ratio of the primary
and secondary coils, including the tap used, of the transformer selected.
The capacity of the unit is determined by the current-carrying capacity
of the secondary coil, through which the entire line current will flow.
(Refer to Figure 4-6k.)

The use of a distribution of normal design in this way is usually
done in an emergency. It is an unsafe method, as the secondary is
connected directly to the primary. For safety reasons, special attention
should be given when connecting or disconnecting such units.

Read More - Boosters

Taps

Where voltage improvement can be obtained by some fixed
amount which will not cause voltages to exist outside permissible limits
during both light and heavy load conditions, taps can be changed on the
distribution transformers on certain portions of the feeder.

For example, assuming an evenly distributed load on a feeder, the
taps on the transformers in the first third of the feeder from the substation
can be changed to lower the secondary voltage a fixed amount; the
taps on the second or center third of the feeder may be left on their normal
setting; and those on the farthest third of the feeder may be changed
to raise the secondary voltage a fixed amount.

The taps on the transformers merely change their ratios of transformation.
If the normal ratio is (say) 20 to 1 to give a secondary voltage of
120 V, tap changes on those nearest the substation would result in a 21
to 1 ratio and a voltage drop of approximately 6 V, which, if subtracted
from a high permissible voltage of 126 V, will still leave a voltage of 120
V; or put another way, the tap change allows the highest permissible
voltage at the substation to be raised 6 V without exceeding the permissible
high-voltage limit at the first consumer. Similarly, on those farthest
from the substation, tap changes can result in a 19 to 1 ratio and a voltage
increase of 6 V, allowing additional voltage drops in the feeder up to
6 V before the permissible low voltage at the last consumer is not met.

Read More - Taps

Voltage Regulators

Where the most economical size of conductor results in voltage
drops 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. Here
the economic comparison is based on the annual carrying charges of the
conductor installed together with those for the regulator—the energy
losses in the regulator and its operating and maintenance costs.

Sizes
Regulator sizes specify the percentage of regulation in definite
steps—e.g., 5 percent, 7-1/2 percent, 10 percent, etc.—and hence the size
of conductor that will give satisfactory regulation with each size of regulator
is determined and the total annual costs for each alternative are
compared. These are also compared with the annual costs for conductors
that would prove satisfactory without a regulator. The alternative with
the least total annual cost is the one preferred.

Controls
The regulator does not reduce the voltage variation along the feeder
with which it is associated. It does reduce the voltage spread at the point
of supply to that feeder, or a portion of the feeder. Refer to Figure 4-32.
The regulator can be applied at the substation to reduce the supplyvoltage
spread on individual feeders or on the bus supplying a number
of feeders. Unless the feeders are of about the same length and have the
same kinds and magnitudes of loads, individual feeder regulators are
generally preferred.

Where feeder voltages drop below permissible limits, voltage
regulators may be inserted in the primary circuit to correct the condition.
They should be located at the point on the feeder where, under
full load, the voltage falls below the permissible limit; they are usually
located some distance before this point in order to provide for some
future increase in the loading of the feeder. Voltage regulators may be
of either the induction type or of the tap-changing-under-load (TCUL)
type; these are described in Chapter 12. They may be either single-phase
or three-phase units.

Voltage-regulating Relays
Regulators are usually controlled automatically, though they may
be manually operated in association with a voltmeter. In older units
(many of which still exist), the element for automatic control is essentially
a contact-making voltmeter, which makes a contact to cause the
regulator to raise the voltage when the voltmeter reads the minimum
permissible outgoing voltage, and another contact to lower the voltage
when the voltmeter reads the maximum permissible outgoing voltage.
In newer units, electronic (solid-state) relays accomplish this function
without any moving parts.

Line-drop Compensators
Where it is desired to regulate or maintain the voltage band at some
distance from the source of the distribution feeder (e.g., at the first consumer
or at some other point farther out on the feeder), a line-drop compensator
is used with the contact-making voltmeter. The line-drop compensator
is an electrical miniature of the line to the point where the regulation
is desired. See Figure 4-4. Resistance and reactance values of the line are
calculated and a resistance and reactance proportional to these values are
set on the compensator; the line current, through a current transformer,
flows through the compensator, producing a voltage drop proportional to that current.

This drop is subtracted from the line voltage at the regulator terminals, thus
applying at the contact-making voltmeter a voltage (varying with the load) repre-
senting the voltage at the point of compensation on the feeder. Refer to
Figure 4-32.

The point of compensation should be selected so that the consumer
farthest from the regulator will have at least the lowest permissible voltage
under the heaviest load while the consumer nearest the regulator
will have the highest permissible voltage under light-load conditions.

Networks
Where the regulators (at the substation) control the voltage on feeders
supplying a secondary network, steps must be taken to prevent the
regulators from becoming “unstable,” i.e., some moving to their maximum
increase position while others on adjacent feeders move to their
minimum positions; this condition can reverse itself and be continuous,
creating periodic voltage variations that might be annoying, and creating
troublesome circulating currents. This is especially true for three-phase
regulators that cause a phase displacement. Mechanical interconnections,
in-phase regulators, and phase shifters are sometimes used to prevent this
instability. Where two feeders only are involved, stability can be maintained
by using current from one line in the compensator for the other.

On some feeders, a lowering in voltage may be necessary under
periods of light load or where other means of raising voltage are employed,
such as taps, boosters, and capacitors.

Read More - Voltage Regulators

The Primary System

The primary system comprises the facilities that deliver power
from the distribution substation to the distribution transformers. These
take the form of one or more distribution feeders or circuits emanating
from the substation, each supplying a portion of the entire load served
from that substation. The feeders are made up of mains (or trunks) from
which branches (or laterals, or spurs) are provided to supply the several
transformers serving loads within the feeder’s designated area.

Feeder Mains
The feeder mains are usually three-phase three- or four-wire circuits,
and the branches are predominantly single-phase, although they
may consist of two or three phases of the three-phase circuit if the loads
carried on them are large or require polyphase supply.

Like the secondary circuit, the design of the primary feeder is based
on the maximum voltage variation permissible at the farthest consumer.
This depends on the size, type, and location of the loads to be supplied,
the size of the conductors, and the operating voltage, which may also be
limited by local codes and regulations.

Conductor Size
The size of the conductors for the “main” portion of the feeder is
usually larger than that of the branches. While a conductor’s size may be
reduced as it proceeds farther from the substation because of the smaller
load it is normally required to carry, this is seldom done. The size of the
conductor of the main near the substation is often carried all the way
to its extremities; indeed, it is sometimes made even larger than normal
operating conditions would dictate. This not only provides for rapid
growth, in which it may be found desirable to divide the load so that the
direction of supply may be reversed, but also provides spare capacity to
carry all or part of the load of adjacent feeders under contingency conditions.

Moreover, the larger conductor size may substantially reduce the
voltage variation on this portion of the circuit, permitting greater freedom
in the design of the branches. The sizes of wire for both main and
branches, as in the secondary system previously discussed, will depend
on the voltage variation or regulation desired and on economy, which
includes evaluation of losses in the conductors.

Sectionalizing
Provision for moderating the effects of faults on the circuit usually
takes the form of fuses and switches. Each of the single-phase branches
is connected to the main through a fuse; a fault on the branch will blow
the fuse and isolate the fault, leaving the remainder of the circuit intact.

A fault on the three-phase main will affect the entire circuit; the size of
the conductors may be such that the fault current will be beyond the
capability of being safely interrupted by a fuse, and the circuit breaker at
the substation is called upon to handle the fault current and disconnect
the faulted circuit from the substation bus (which may also supply other
circuits).

Switches are installed in the main of the feeder, enabling the
main to be sectionalized, isolating the fault between two switches or other
sectionalizing devices. The unfaulted portion back to the substation is
reenergized by closing the circuit breaker at the substation; the unfaulted
portion beyond the fault is energized from adjacent sources; the portion
containing the fault will remain de-energized until the fault condition is
repaired and the circuit restored to normal operation. Where the feeder
main may consist of two or more parts, circuit breakers in the form of
“reclosers” may be installed on each of the parts. Refer to Figure 4-3a.

In Figure 4-3b, the primary circuit is formed into a loop; for example,
the trunks of two circuits are connected through a circuit breaker
to form such a loop, both circuits emanating from the same source. The
circuit so formed may then be sectionalized through a number of circuit
breakers into a number of sections determined by the distribution engineer.

A fault on such a circuit deenergizes only that portion between the
two circuit breakers on either side of the fault. The circuit may be further
sectionalized by disconnects (operated only on the deenergized section)
manually or remotely operated. This allows other now deenergized parts
of the circuit to be energized, leaving only a small section between disconnects
deenergized until the fault is cleared and repairs made.

This is a different form of thinking of how faults should be handled, eliminating
the possibility of blackout that exists in the secondary network described
above. During this period when sabotage may be of major concern, this
mode of supply should be given consideration. (This mode is also applicable
to transmission lines for essentially the same reasons.)

Another mode of primary supply for greater reliability is to tie
the ends of spurs of the circuit into a network as shown in Figure 4-3c.
Here each transformer installation is fed from two directions in a sort
of “radial mesh” mode. Besides the additional service reliability feature,
IR losses may be reduced justifying in part some of the extra expense
involved. The reduction in the flow of fault currents because of the now
multiple paths may affect the operation of fuses, if any, in the circuit for
protective purposes and should be taken into consideration.

Both methods described are less costly than the network described
earlier and should be taken into account when comparing plans for a
particular installation.

Reclosers
Reclosers are designed to open when a fault occurs on that part of
the main in which they are connected; a timing device, however, enables
them to reclose a predetermined number of times for short durations. If
the fault is of a temporary nature, such as wires swaying together or a
tree limb falling on them, the recloser will remain closed and service will
be restored; should the fault persist, the recloser will remain open and
disconnect that part of the main from the circuit.

Transformer Fuses
Similarly, a distribution transformer may be connected to the primary
main or branch through a fuse. A fault or overload on the transformer
or its associated secondary circuit will cause the fuse to blow and disconnect
the faulted section from the remainder of the primary circuit.

Load Balancing
On polyphase portions of the feeder, on both main and branches,
loads are balanced between phases as closely as practical by connecting
transformers and single-phase branches to alternate phases of the circuit;
this provides a more uniform balancing of loads along the line (contributing
to better load and voltage conditions) than would balancing in
large blocks of loads. An approximate method multiplies each load by
its distance from the substation; the sum of these, uniformly distributed,
should be about the same for each phase.

Operating Voltage
The selection of the primary operating voltage is probably the factor
having the greatest influence on the design of the primary system.
It has a direct effect on the length of the feeder and its loading, the
substation supplying the feeders and the number of feeders, the number
of consumers affected by an outage, and on maintenance and operating
practices (which, in turn, affect annual carrying charges). Several voltage
levels have evolved into “standard” nominal values of primary voltages:
2400, 4160, 7620, 13,200, 23,000, 34,500, 46,000, and 69.000 V.

Delta and Wye Circuits
Many of the older systems employed delta circuits with phase-tophase
voltages approximating 2400 V; as loads increased, it was found
economical to convert these into wye circuits with phase-to-phase voltages
approximating 4160 V, but with phase-to-neutral voltages remain
ing at the 2400-V level, permitting the use of the same transformers,
insulators, and other single-phase equipment. The wye circuit necessitated
a fourth, neutral conductor grounded in many places; later, a
single conductor common to both the primary and secondary systems
was employed safely, effecting greater economies.

Delta to Wye Conversion
As loads grew and load densities increased, resort was had to
higher voltages, making use of subtransmission circuits, a great many
of which were delta circuits operating at phase-to-phase voltages of
approximately 13,200 V; the wye voltage or phase-to-ground voltage of
this level is 7620 V. For the same economy reasons the 13,200-V phaseto-
phase delta subtransmission supply circuits to distribution substations
were converted to 13,200-V phase-to-neutral wye circuits having
phase-to-phase voltages of 23,000 V.

Distribution circuits at these higher
voltages required fewer substations, whose acquisition in the more
developed areas became increasingly difficult. Circuits at these higher
voltages also found employment in rural areas where distances between
consumers were greater and load diversities lower.

This process continued with the development of distribution
circuits operating at 34,500 and 46,000 V from subtransmission lines
operating at these voltages. Advantage is taken of taps on transformers
supplying these circuits, sometimes as much as 10 percent, in adapting
these feeders to distribution requirements. Other voltages, outside the
ranges mentioned above, may be found, e.g., 3000, 6600, 8800. 11,000,
and 27,000 V.

Advantages
The principal advantages to such conversions from delta to wye
systems are:
1. The wye system affords greater feeder capacity and usually improved
voltage regulation.
2. Existing transformers, insulators, and other material can be used;
in most cases, spacing between conductors is left unchanged.
3. Single-phase branches need not have any work done on them.
4. Existing secondary neutral conductors can be used as the fourth
and neutral conductor in establishing the wye circuit. Where a new
neutral conductor is required, it can be installed safely and readily
in the secondary position on the pole with no conflict with the
higher-voltage energized conductors.
5. The entire circuit need not be converted to the higher voltage at
one time, but can be converted piecemeal over a period of time; a
portion of the three-phase delta circuit can be maintained from a
relatively small step-down transformer (pole-mounted) connected
to the new supply three-phase wye circuit.
6. Transformers and other equipment at the substation can be rearranged
and reutilized, like those on distribution lines.
7. Where the neutral is grounded at the substation and at many
points along the feeder, the voltage stresses on the insulation of
the lines, transformers, and other devices are limited to the lowest
possible value; should an accidental ground occur on any phase, it
will be cleared as the circuit breaker opens.
8. Important savings can be realized in the equipment installed on
wye systems: transformers need only one high-voltage bushing;
only one cutout and lightning arrester are required (if a completely
self-protected transformer is used the cutout can be eliminated);
and the single high-voltage line conductor may be mounted on one
pin at the top of the pole, eliminating the need for a cross arm (and
contributing to a neater appearance of the line).

Disadvantages
There are some disadvantages to the conversions from delta to wye:
1. The load and voltage advantages of the higher voltage apply only
on the three-phase main and not on the single-phase branches, as
they continue to operate at the existing voltage.
2. A ground on a phase conductor constitutes a short circuit, which
will de-energize at least that portion of the circuit. On delta
circuits, normally operated ungrounded, one or more accidental
grounds on the same phase of the circuit will not cause any
interruption to service. (The occurrence of a ground on another
phase, however, will create a short circuit between phases, possibly
connected together through long lengths of conductors; if
the impedance of the intervening conductors between grounds is
large, the fault current flowing to ground may not be sufficient to
open the circuit breaker at the substation, and much damage can
ensue until its magnitude either causes the circuit breaker to open
or the conductors burn themselves clear at some point. A delta
circuit may be hazardous, as a worker, unaware of a ground that
may exist at a point farther away, may come in contact with an
ungrounded phase wire.)
3. Because of the grounded nature of the wye system, greater care,
reflected in greater maintenance costs (e.g., greater and more frequent
tree trimming), may be required to achieve the same degree
of reliability as in a delta circuit.
4. The higher voltage and the many grounds in a wye circuit may
cause greater interference to communications circuits that parallel
the power circuits.
5. Some local regulations and codes may require greater safety factors
in the construction of facilities operating at the higher voltages.

Higher-voltage Circuits
When the need is indicated for a still higher-voltage distribution
circuit, major reconstruction and a complete replacement of transformers
and other devices is usually necessary. The new higher-voltage circuit
is generally designed for immediate wye operation, omitting the intermediate
delta operation. In addition to the greater construction costs,
additional maintenance and operating costs must be considered in determining
the economics of going to higher voltages. Beyond about 15 kV,
handling such lines and equipment requires either “live line” tools and
methods or the de-energizing of lines and equipment. This latter condition
may require additional sectionalizing facilities, including a greater
number of extensions between feeders to enable loads to be transferred
from the circuit to be de-energized.

The greater load-carrying ability of the higher-voltage primary
circuits tends to have them serve larger areas and a greater number of
consumers, so that an interruption to an entire circuit will have a greater
effect on the area served. Rapid sectionalizing and reenergizing means
are therefore more necessary and must be considered in evaluating the
service reliability factor in economic studies.

Voltage Drop and Losses
Sizes of conductors of primary circuits are also based on acceptable
voltage drop and losses in the conductor and the cost of the facilities; the
mechanical requirement may be the decisive factor. The principles and
methods given for secondary circuits also apply here.

Branches of the primary circuit may supply from one to a great
many transformers. Where only one transformer is involved, voltage
drops and losses may be calculated as a concentrated load at the end of
the line. Where the branch is relatively long and serves a few transformers
widely spaced, these values may be derived from a circuit considered
to have a distributed load.

Where the length is short, or where a larger,
more closely situated number of transformers exist, the circuit may be
considered as supplying a uniformly distributed load; the total loads of
these transformers can be assumed to be concentrated at a point half
the length of the branch (from the tap-off at the main to the last transformer)
in calculating the maximum voltage drop, and at one-third the
distance (from the tap-off at the main) for calculating losses in the entire
length of the branch. For single-phase circuits, the characteristics of the
neutral conductor should also be considered. For polyphase branches,
each phase and the transformers connected to it may be considered separately;
the loads on the separate phases may be considered balanced and
the neutral ignored.

Voltage and loss calculations for the three-phase main portion of
the feeder may be considered to be concentrated at the tap-off point of
the main; these, together with the transformers connected to the main,
can be considered as a uniformly distributed load on the main. In some
instances, the main may proceed from the substation for a certain length
before serving any branches or transformers. In this case, the main can
be considered in two parts. The portion to which branches and transformers
are connected may be considered to have uniformly distributed
load, with voltage and losses calculated accordingly.

The untapped portion of the main (from the substation to the first load connected to
it) may be considered to be a line with the entire load (the uniformly
distributed load mentioned earlier) concentrated at its end (where the
first load is connected). The loads may be assumed to be balanced and
the neutral neglected. The total voltage drop is the sum of the drops in
the two portions of the main; the total losses in the feeder main are also
the sum of those in the two portions.

In considering the total annual cost of the primary line for compari
son with the annual cost of the losses in it, in addition to the cost of the
conductors in place, the cost of poles, insulators, switches, etc., must also
be included as well as the annual costs of operation and maintenance.
Voltage drops and energy losses are reduced substantially as the
applied voltage values increase. For primary circuits, particularly those
operating at higher voltages, these values are considerably less than for
comparable secondary quantities.

As indicated earlier for secondary systems, the most economical
size of conductor for a proposed load (present, future, and contingency)
may be determined by an analysis of the annual carrying charges for the
system considered and the annual cost of energy losses in the conductor.

Conductor Size
A conductor size, though as near as possible to that indicated by
the economic analysis, may still be subject to other considerations. The
permissible voltage drop in the several parts of the circuit will determine
the minimum size of conductor; if this size is greater than the indicated
economical size, economy is disregarded; if smaller, the economical size
should be chosen.

The choice of conductor size, however, will not only be limited to
those which will carry the load with satisfactory voltage variations, but
the size chosen must also be mechanically able to support itself even
under unfavorable weather conditions, if overhead, and to withstand installation
cable stresses if underground.

As a rule, for overhead systems,
conductors smaller than no. 6 AWG medium- to hard-drawn copper are
not recommended, because of strength limitations, nor are those larger
than no. 4/0, because of the difficulty in handling. For underground
systems, soft-drawn copper may be used because of its ease in handling;
no. 6 or no. 8 is the minimum for reasons of strength as well as load and
voltage limitations, and no. 4/0 and 350,000-cmil are the largest sizes
that may burn themselves clear under short-circuit conditions; where a
conductor larger than these sizes is required, two smaller-size conductors
in parallel may be substituted.

As indicated earlier for secondary systems, the sizes of conductors
employed for primary (and secondary) circuits should be standardized
for any one system, and limited to relatively few in number. Such
standardization simplifies, and adds to economy in, their manufacture,
purchasing, stocking, and handling in the field.

While the discussion applies principally to radial-type systems, it is
also applicable to primary network systems, the network being divided
into a number of adjacent radial-type circuits; the analysis will be very
similar to that indicated for secondary networks.

Read More - The Primary System

The Secondary System

Transformer-secondary Combination
The combination of transformers, secondary circuit or main, and
the consumers’ services makes up the secondary system. Secondary
systems are predominantly single-phase, except for larger commercial
and industrial consumers, who are supplied from three-phase systems.
While the discussion will be limited to single-phase systems, the principles
and methods employed in their design will serve for other types
of secondary systems.

The number of consumers’ services and their loads, the voltage
drop, the size of conductors, and the spacing and size of transformers
are all variables that are interdependent. They are factors which must
be considered in combination to arrive at a satisfactory design. There
are many theoretical combinations of these factors that will achieve economical
solutions to the problems of design.

For practical purposes, however, these combinations can be reduced
to fewer and more manageable numbers. Certain assumptions
can be made safely:
1. The load can be considered uniformly distributed along a secondary
whose length can be considered fixed. Although not strictly
true, this assumption does represent a majority of conditions, but
concentrated or scattered loads must be considered separately.
2. The length of secondary circuit is fixed either by geography or by
the type of design; e.g., each city block could be fed by one or more
secondary circuits. Refer to Figure 4-1.

3. In practice, the number and sizes of conductors and transformers
are limited, usually to two or three in number, and to certain standard
sizes because of manufacturing, purchasing, stocking, and
construction economies.

The problem then is to determine the proper combination of conductor
or wire, transformer, and transformer spacing for the least annual
cost, using the materials available while providing for satisfactory voltage
variations, including flicker. Also, the design should consider not
only present loads, but the economics of supplying future loads as well.
Computers permit study of a greater number of combinations.

Conductor Size
It may be well to begin with a determination of the size of conductor.
The maximum demand for each of the consumers is known and a
coincidence or diversity factor, determined by analyses or from previous
experience, applied. Assuming uniformly distributed loads, the loading
on each half of the circuit (each direction from the transformer) can
be expressed as load density in kilowatts per thousand feet or similar
units.

Voltage Drop
For determining voltage drop, the load can be assumed to be concentrated
at the midpoint of the secondary main between the transformer
and the last consumer, i.e., one-quarter of the length of the conductor
from the transformer. The total load connected to one-half the circuit is
converted into a coincident maximum demand in amperes.

A maximum tolerable voltage drop (to the last consumer) is assumed
and divided by the coincident demand in half the circuit,
expressed in amperes. The result will give the maximum permissible
resistance of the conductor. On the basis of its length (one-quarter of the
circuit), the resistance per unit (1000 ft) can be determined. The standard-
size conductor whose unit resistance is equal to or less than that
calculated can be selected.

This assumes the loads are at or near unity power factor; where
this is not so, impedance values based on the spacing between conductors
must be used. Also, the drop in one conductor is calculated,
which assumes no current in the neutral conductor and a load balanced
equally between the two energized or line conductors; where this is not
so, voltage drop in the neutral conductor must also be calculated and
the greater of the drops in the two line conductors used in selecting the
standard-size conductor.

Losses
The next step is to determine the loss in the secondary mains. The
value of current and the unit resistance of the conductor are known; for
the purposes of determining losses, the full load can be considered to be
at one-third the distance from the transformer. This approximate value
in watts or kilowatts is multiplied by an estimate of the “equivalent
hours” duration to obtain the energy losses in watt-hours or kilowatthours.

This should be multiplied by 4 for the entire length of the two
conductors (neglecting the neutral).
It should be noted that while load curves for a particular period
(day, month, year) vary with the value of current, corresponding curves
for losses vary with the square of the current, even though the curves
may have a similar configuration.

Like the load factor, a loss factor is
the ratio of average power loss for a certain period of time (day, month,
year) to the maximum loss or loss at peak load (for a stipulated time:
15, 30, or 60 min) during the same period. This value can be determined
with sufficient accuracy by analysis of a few typical daily load curves
for the period involved. This loss factor always lies between the load
factor (for long, sustained peak loads) and the square of the load factor
(for short, sharp peaks). The loss factor multiplied by 24 equals the daily
equivalent hours.

Returning to the energy losses, in watt-hours or kilowatt-hours,
these are evaluated at the system cost per kilowatt-hour (which includes
not only fuel costs, but carrying charges on equipment, operating costs,
and other overheads). This value is compared with the carrying charges
(including maintenance costs and appropriate overheads) on the installed
cost of the conductors. If the two values are reasonably close, the conductor
selected is economically satisfactory, according to Kelvin’s law.

Kelvin’s Law
Kelvin’s law is generally expressed as follows: The most economical
size of conductor is that for which the annual charge on the investment
is equal to the annual cost of energy loss.

If these two values are not reasonably close, another size of conductor
may be chosen, or the length of the secondary main (and its
connected loads and its coincident maximum demand) may be changed;
and either process may be repeated until the values of annual charges
and annual cost of energy losses are reasonably close. 

Transformer Size
Having determined the tentative size of conductor, the next step is
to determine the size of the transformer to be installed. The value of the
diversified coincident demand for the loads connected to the secondary
main having been determined, the nearest standard-size transformer (in
kVA) to the demand (in kW) is tentatively selected. To allow for future
growth and not to prejudice the life of the transformer, the size chosen
is usually larger than the demand.

The most economical load of a transformer is that for which the annual
cost of its copper loss is equal to the annual carrying charges of the
transformer installed plus the annual cost of the core loss. The core loss
can be considered constant regardless of the load carried by the transformer.

Values of core loss and transformer resistance, both expressed as
percentages at full load, vary with the manufacturer, vintage, size, and
other characteristics, and are found in the transformer specifications;
core loss is usually a fraction of one percent, while resistance is usually
less than 2 percent (reflecting the high efficiency of transformers). Here,
too, if the two values are not close, another size of transformer may be
chosen, or the secondary circuit may be changed so that two or more
transformers supply the load. It may be necessary to review the conductor
size and loads for the new resulting circuits.

It is obvious that any secondary-transformer configuration represents
a compromise. Much depends on the relative costs of material
and labor, which may vary widely from time to time and from place to
place. Further, other considerations may play a great part in the final
determination; e.g., conductor sizes may change to meet mechanical
requirements.

Future Growth
To provide for future growth, loads are adjusted upward by a percentage
estimated to represent probable increase over a specified period
of time. Facilities to serve these increased loads are designed in the same
manner described. The difference in investment costs for each design is
evaluated in terms of the future worth of the present increment of cost
of the additional facilities provided for growth. This is compared to the
cost of installing the facilities at the future time. If it is less, it is desirable
to provide for the future load at the time of initial installation. If not,
provision for future load growth should be dropped, or scaled down to
values and timing that will justify some value of additional cost.

To accommodate the load growth, the transformer and conductors
can be replaced with larger ones, or more popularly, the secondary circuit
can be divided into two or more parts without changing conductors;
a suitably sized transformer is then added to the newly formed secondary
circuits. Comparison of costs and annual carrying charges dictate the
method selected.

Networks
The analysis described pertains to radial secondary circuits. Where
networks are involved, the same principles and methods can be applied
by assuming the network to be divided into a number of adjacent radialtype
circuits, as shown in Figure 4-2; no appreciable error is introduced.

The general principles and methods applied to overhead singlephase
radial-type secondary circuits may be applied to underground
circuits and three-phase three- or four-wire circuits by proper adjustment
of terms to fit the cases. With underground circuits, the lesser current-
carrying capacity of a size of conductor, without overheating, must
be taken into account. In network design, the ability to burn clear the
conductors in the cable under fault or short-circuit conditions should
also be ascertained. These additional considerations may be taken into
account after the economic studies are made.

Rural Systems
Where consumers are scattered, such as in rural areas on in the
case of three-phase consumers in an area supplied essentially at single
phase, the load may be served either by extending the secondary from
one transformer or bank of transformers, or by installing a separate
transformer or transformers to serve those consumers. Annual carrying
charges, including costs of losses, should be compared in selecting the
method of supply.

There are many other problems in the design of secondary systems,
but they lend themselves to the application of the same basic principles
and methods, with proper consideration given to their particular requirements.

Read More - The Secondary System

Services

The electrical design of distribution facilities is based on the loads
they are to carry safely and the permissible voltage variations; the final
design, however, cannot be divorced from mechanical, economic,
and other considerations. Several different designs may serve the same
electrical requirements adequately; each, in turn, may be modified by
mechanical considerations.

The design ultimately selected must reflect
economic considerations: specifically, the design that results in the least
annual expense in supplying the load or loads in question. This necessarily
involves the evaluation of losses, as well as capital, maintenance,
and operation expenses. Often, other considerations must also be taken
into account including government regulations (at all levels), national
and local industry construction and safety codes, taxes, public relations,
and some other, intangible requirements.

SERVICES
Rather than design a separate service for each consumer, it is more
practical and economical to determine the capacity and construction
requirements on a group basis for different types of consumers. The
maximum demand for a consumer group is determined by the connected
load, to which a demand factor may be applied. The factor can
be an estimate based on observation or a logical analysis of the operations
of the several devices comprising the load, or it may be obtained
from previous experience. Further, service entrance equipment specified
by national and local codes (and often installed by the consumer) has
minimum ratings based on the number and kinds of circuits installed
within the consumer’s premises.

Each group represents consumers whose maximum demands fall
within certain ranges, expressed in volt-amperes or kVA, and for which
certain conductor sizes are specified, listed in Table 4-1. These values
are based on a single-phase three-wire 120/240 V supply via three-conductor
self-supporting cable for a 100-ft length, carrying the maximum
amperes listed and producing a 1 percent voltage drop. Losses at the
maximum loads are less than 2 percent. For longer service drops (sometimes
in rural areas), these values may be exceeded.

Three-conductor self-supporting service cables are almost always
specified because of their appearance and ease of installation as compared
to older (now almost obsolete) open-wire-type services; voltage
drop for the same load and length is slightly less than for the open-wire
type. The sizes of conductors specified are more than ample to support
the mechanical stresses imposed on them, even in severe weather conditions.

Local conditions, including varying costs of both labor and materials,
rates of growth, and other factors, may substantially change the
values shown in Table 4-1. Services for the relatively fewer larger commercial
and industrial consumers served at secondary voltages are usually
determined individually.

Read More - Services

Service Reliability

Reliability of service generally is interpreted to mean the continuity
of service or the lack of interruption to service. For a distribution system,
or any of its parts, absolute reliability or continuity of service 100
percent of the time for 100 percent of its consumers is an impossibility,
although this goal can be approached. The costs to achieve such goals,
even partially, are usually not warranted.

Degree of Service Reliability
As a practical matter, all consumers may not require a uniformly
high degree of service reliability. For some consumers, an extremely
high degree of service is essential; these may include hospitals, military
establishments, some larger theaters, department stores, apartment
buildings, hotels, etc., where the safety of the public is concerned; often
auxiliary sources of supply are provided to supplement the utility
company supply.

For some other types of loads, a high degree of reliability is desirable
but not so essential from the public safety viewpoint; smaller
apartments and theaters are examples of these, as well as some
manufacturing or service processes where interruption may result in
substantial monetary losses. To the average residential or commercial
consumer, however, a short interruption (and in some cases even an occasional
long one) is more of an inconvenience than a hazard or cause
for monetary loss.

As a rule, provisions for higher degrees of service reliability involve
higher expenditures, for both additional facilities and increased
maintenance. The expenditure to provide reliability should bear some
proportion to the degree of reliability needed. Various system designs,
outlined in Chapter 2, provide for varying degrees of service reliability,
from a simple, unsectionalized radial feeder to a low-voltage secondary
network supplied from a multiplicity of primary feeders isolated from
each other. Each type of service should produce revenues to justify the
additional expenditures for achieving the service reliability desired or
required; exception may be made for such public services as hospitals
and military establishments.

Overhead versus Underground
In this regard, comparisons between overhead and underground
systems should be borne in mind. Overhead systems are generally much
less costly but are more vulnerable to the hazards of nature (wind, ice,
lightning, flood, etc.) and to the actions of people (vehicles hitting poles,
kites, etc.); they are, however, easier to maintain, especially as faults can
be more easily located and repaired. Underground systems, generally
more expensive and less vulnerable to the vagaries of nature and people,
nevertheless require longer times for the location and repair of faults
that may occur.

Reliability Indices
Service reliability indices are maintained to measure and obtain
trends in the performance of a distribution system and its components.
Some of these include number of interruptions per consumer served;
number of consumers affected per consumer served; number of consumer
hours of interruption per consumer served; average duration of
interruption (hours) per consumer affected; average number of consumers
affected per consumer served; and average duration (hours) per consumer
served. Further indices are maintained as to causes and duration
of interruptions in the several parts of the system, e.g., on the basis of
miles of conductor installed, on the miles of circuit, by voltage classification,
or by geographic divisions. Compilation and analyses of these data
lend themselves to computer application.

Trends
The trends, over a period of time, not only measure the effectiveness
of system designs (and operating procedures), but also point out
areas of need for further improvement of service continuity.

Read More - Service Reliability

Voltage Requirements

Electric devices utilizing secondary or low voltage in the United
States have been standardized by almost all manufacturers at 120/240 V.

While many utilities are following these standards for their systems, there
are a significant number operating at 115/230 V and some at 125/250 V
(and a small and diminishing number at 110/220 V). For polyphase or
three-phase loads, the established ratings are 208 and 416V for wye-connected
systems and 240 and 480 V for delta-connected systems. In a few
cities, and some downtown and heavy load centers, network systems
supply a single-phase voltage of 277 Volts and a three-phase voltage of
480 V. (A few two-phase 115/230 V systems still exist.)

With distribution systems designed for practical voltage tolerances
expressed in volts plus or minus in relation to their normal, “standard,”
base single-phase voltage of 115, 120, and 125 V, voltages of 110 to 130
V can exist at the terminals of the loads (lamps, appliances, etc.). This is
a spread of ± 10 V, or ± 8.3 percent on a 120-V base, to which a flicker
voltage drop of 3 V, or 2.5 percent, should be added to allow for motor
starts. This would then give a total spread of 23 V, or 19.1 percent, from
+ 8.3 to – 10.8 percent—approximately within most manufacturers’ rated
maximum tolerances of ± 10 percent for motors and heating devices.

Closer coordination between manufacturers and utilities could do much
to improve this situation. Electronic devices are more sensitive to voltage
variations, and difficulty may be experienced with their operation at
variations of this magnitude.

Some utilities provide for an estimated voltage drop of up to 2 V in
the consumer’s wiring by specifying normal voltage at the service point
2 V higher than mentioned earlier, i.e., 122 instead of 120 V (117 and 127
V on other bases). Their designs, however, provide for the same high
and low voltage limits, but the variations above and below the usual
base are unequal, e.g., 128 V high, 122 normal, 114 low. The 23-V spread
and—10.8 percent variation mentioned above will be the same.

Not only does satisfactory operation of lights, appliances, and
other devices make for good consumer relations, but the effect of high
and low voltages (principally because of unity power factor lighting
loads) on both revenue and fuel conservation measures should also be
considered.

Higher voltages—660 V, 2400 V, and others—are also employed
for larger motors, rectifiers, and some other purposes. Consumers using
such large voltages are usually served at primary voltages and meet
their own utilization voltage standards.

Read More - Voltage Requirements

Future Requirements

Good engineering requires that probable future growth of loads be
considered in planning. This is usually provided for by spare capacity
in the present design of the several elements, or by provisions for possible
future additions or alterations, or both of these. Load growth is
rarely uniform throughout an area, so that growths in various parts of a
system will be different from each other and from that of the system as
a whole.

Economics
How far present capacity should provide for future load is largely
a question of economics: the cost of carrying excess capacity until it is
needed versus the cost of replacing smaller units with larger when it
becomes necessary. This is a problem of the future worth of present
expenditure, which is affected by fluctuations in rates of interest and
inflation. Standard sizes of the materials and equipment involved automatically
provide for a limited amount of spare capacity for growth,
so that any economic analysis can only be approximate. The relatively
large proportion of labor to material in the construction of a distribution
system or its parts lends itself to the installation of capacity greater than
its immediate need. Such spare capacity incidentally provides a cushion
for accommodating some of the unforeseen fluctuations in demands
described above.

Past Performance
Data from past performances, such as total system loads, substation
loads, and feeder loads, can be used as a basis for estimating such
growth. The variations from year to year, or from month to month, can
furnish a trend for such growth; separate trends can be developed for
different parts or areas. Where such data are nonexistent or patently
unreliable, estimates can include a fixed percentage growth above the
values on which planning is made.

Future Performance
To obtain some idea of what may occur in the future, it may be well
to look back a generation or two. Earlier, consumers’ appliances could
be contained in a relatively short table. To attempt to list all the electrically
operated devices, appliances, and gadgets presently to be found
in homes and commercial establishments would be an almost endless
task. To attempt to foretell what may develop in the future would be an
exercise in futility.

The advent of widespread air conditioning and space heating, together
with the almost universal use of television, not only substantially
changed consumers’ maximum demands and consumption, but also materially
affected loads, diversity, coincidence and (for larger units) power
factors, and utilization factors as well.

While the demand factor may indicate how the connected loads
are being used, the utilization factor indicates how the capacity of the
supply system is being used. Since the capacity of the supply system is
determined by its thermal capability, the increased sustained demand on
these facilities will lower their thermal capability, and hence the system
capability.

The greater use of electronically operated computers will tend to
call for narrower limits of voltage control (regulation and flicker) and a
greater degree of service reliability by stiffening the supply distribution
system, or through the installation of auxiliary equipment owned and
maintained by the consumer or rented as another service by the utility;
the choice will be determined by future developments.

Read More - Future Requirements

Fluctuation in Demand

There are three main factors that greatly influence the magnitude
of maximum demand and the time of its occurrence. The most frequent
is the weather as it affects light intensity during daylight hours and
temperatures throughout the day and year.

The sharpest factor and perhaps
that of least duration is special events which result in a temporary
slowdown of activities or a greatly increased usage of lighting, radio,
and TV and associated increases in water pumping, cooking, and other
loads. The largest factor is changes in business conditions accompanied
by significant changes in industrial demands and consumption; while
much less significant, fluctuations in both residential and commercial
consumer demands also follow such changes in business conditions.

The nature, magnitude, and time of these fluctuations are generally
unpredictable. Some estimate of them can be gleaned, however, from
past experiences, which may vary widely in different areas of the country.
Provision for these fluctuations should be taken into account in the
planning of distribution systems.

Read More - Fluctuation in Demand

Consumer Classification

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

Further classifications may be based on such items as the dependence
on electric service because of the critical nature of the consumer’s
operations, under either normal or emergency conditions; the resultant
cost if critical processes are interrupted; or the sensitivity of loads to
small voltage deviations.

Read More - Consumer Classification

Consumer Factors

It is obvious that an individual consumer is not apt to be using all
of the electrical devices that constitute his or her “connected load” at the
same time, or to their full capacity. It would evidently be unnecessary
to provide facilities to serve such a total possible load, and much more
economical to provide only for a probable load, the load creating the
demand on the distribution facilities.

Maximum Demand
The actual load in use by a consumer creates a demand for electric
energy that varies from hour to hour over a period of time but reaches
its greatest value at some point. This may be called the consumer’s
instantaneous maximum demand; in practice, however, the maximum
demand is taken as that which is sustained over a more definite period
of time, usually 15, 30, or 60 min. These are referred to as 15-, 30-, or 60-
min integrated demands, respectively.

Demand Factor
The ratio of the maximum demand to the total connected load is
called the demand factor. It is a convenient form for expressing the relationship
between connected load and demand. For example, a consumer
may have ten 10-hp motors installed; at any one time, some will not be
in use and others will not be fully loaded, so that the actual demand may
be only 50 hp; the demand factor is 50 divided by 100, or 50 percent.

The demand factor differs for different types of loads, and by averaging
a large number of loads of each type, typical demand factors can
be obtained. These values are important in determining the size of facilities
to be installed for a particular service; they are extremely useful in
making estimates in planning new distribution systems or in expanding
existing ones.

Load Factor
The load factor is a characteristic related to the demand factor,
expressing the ratio of the average load or demand for a period of time
(say a day) to the maximum demand (say 60 min) during that period.
For example, a consumer household may have a maximum demand of 2
kW during the evening when many of its lights, the TV, the dishwasher,
and other appliances are in use. During the 24-h period, the energy
consumed may be 12 kWh; thus the average demand or load is 12 kWh
divided by 24 h, or 0.5 kW, and the load factor in this case is 0.5 kW
divided by 2 kW, or 25 percent. This provides a means of estimating
particular consumers’ maximum demand if both their consumption and
a typical load factor for their kind of load are known.


Diversity
Consumer load diversity describes the variation in the time of use,
or of maximum use, of two or more connected loads. Load diversity
is the difference between the sum of the maximum demands of two
or more individual consumers’ loads and the maximum demand of
the combined loads (also called the maximum diversified demand or
maximum coincident demand). For example, one consumer’s maximum
demand may occur in the morning, while another’s may occur in the
afternoon, and still another’s in the early morning hours.

Diversity Factor
The diversity factor is the ratio of the sum of maximum demands
of each of the component loads to the maximum demand of the load
as a whole (or the coincident maximum demand). For example, each of
the loads mentioned above may have a maximum demand of 100 kW,
while the coincident maximum demand on the system supplying the
three may be only 150 kW. The diversity factor is then 300 (100 + 100 +
100) divided by 150, or 2, or 200 percent. Such diversity exists between
consumers, between transformers, and between feeders, substations, etc.

Note that the demand factor is denned so that it is always less than 1 or
100 percent, while the diversity factor is the reciprocal of the demand factor
and is always greater than 1 or 100 percent. This is a most important
factor in the economical planning and design of distribution facilities.

Coincidence Factor
The coincidence factor is the ratio of the maximum coincident total
demand of a group of consumers to the sum of the maximum demands
of each of the consumers.

Utilization Factor
The ratio of the maximum demand of a system to the rated capacity
of the system is known as the utilization factor. Both the maximum demand
and the rated capacity are expressed in the same units. The factor
indicates the degree to which a system is being loaded during the load
peak with respect to its capacity. The rated capacity of a system is usually
determined by its thermal capacity, but may also be determined by voltage
drop limitations, the smaller of the two determining the capacity.

Power Factor
The ratio of power (in watts) to the product of the voltage and
current (in volt-amperes) is called the power factor. It is a measure of
the relation between current and voltage out of phase with each other
brought about by reactance in the circuit (including the device served).
Since facilities must be designed to carry the current and provide for
losses which vary as the square of the current, and for voltage drops
which are approximately proportional to the current, it is necessary that
current values be known. The power factor enables loads and losses designated
in watts to be converted to amperes. Transformer sizes, wire and
cable sizes, fuses, switch ratings, etc., are all based on values of current
they must carry safely and economically.

Read More - Consumer Factors

Connected Loads

A good place to start is the tabulation of all electric devices (lamps,
appliances, equipment, etc.) that consumers can connect to their supply
system. The ratings of the devices at specified voltages (and sometimes
frequency and temperature) limits are usually contained in the nameplate
or other published data accompanying the devices. The devices can
be classified into four broad general categories: lighting, power, heating,
and electronic. Each of these has different characteristics and requirements.

Lighting Loads
Included under lighting are incandescent and fluorescent lamps,
neon lights, and mercury vapor, sodium vapor, and metal halide lights.
Nominal voltages specified for lighting are usually 120, 240, and 277
Volts (variations may exist from the base 120-V value, e.g., 115 and 125
V). All operate with dc or single-phase ac; the discussion will be in terms
of ac, with comments concerning dc operation where applicable.

Incandescent Lighting
Incandescent lamps operate at essentially unity power factor.
Their light output drops considerably at reduced voltage, being some
16 percent less with a 5 percent lowered voltage, and decreasing at a
geometrically faster rate from then on. They are also sensitive to sudden
rapid voltage variations, producing a noticeable (and annoying) flicker
at variations of as little as 3 Volts (on a 120-V base). Street lighting of the
incandescent type can be operated in a multiple or a series fashion. The
former operates as other lighting in a multiple or parallel circuit, while
the light output for the series type depends on the amount of deviation
from the standard value of current flowing through it (usually 6.6, 15,
or 20 A); it is sensitive to variations of as little as 1 percent in the value
of the current. The life of incandescent lamps is considerably reduced at
voltages appreciably above normal.

Fluorescent and Neon Lighting
Fluorescent lamps and neon lights operate at power factors of
about 50 percent, but usually have corrective capacitors included so
that, for planning purposes, they may also be considered to operate at
100 percent or unity power factor. Their light output, per unit input of
electrical energy, is considerably greater (25 percent or more) than that
of a similarly rated incandescent lamp. The life of fluorescent lamps
and neon lights is affected by the number of switching operations they
undergo. If fluorescent lamps are used on dc circuits, special auxiliaries
and series resistance must be employed; operation is inferior to that on
ac, with much less light produced per unit of energy and rated life reduced
20 percent. Neon lights are not usually employed on dc circuits.

Fluorescent lamps, neon lights, mercury and sodium vapor, and metal
halide lights may, if improperly installed or when deteriorating, cause
radio and TV interference.

High-intensity Vapor Lighting
Mercury vapor (high pressure) and sodium vapor (high and low
pressure) and metal halide lights operate at power factors of 70 to 80 percent,
but also are associated with capacitors to raise the effective value
to 100 percent. They are not as susceptible to voltage variations as are
incandescent lamps.

Their light output and life expectancy are greater
than those for fluorescent lamps. They may be employed on dc circuits,
but require additional starting auxiliaries. They are generally restricted
to applications where large amounts of lighting are desirable, such as
on expressways, in large manufacturing areas, or in photographic work;
they are somewhat more expensive than other types and have the disadvantage
of taking some time after being energized before maximum
light output occurs.

Power Loads
Generally included in power loads are motors of all sizes: direct
current shunt, compound and series types; alternating current singlephase
and polyphase, induction and synchronous types; and universal
(series) for both dc and ac operation. Table 3-1 summarizes the characteristics
and general application of these various types of motors.

Single-phase Fractional-horsepower Motors
The majority of fractional horsepower motors, generally used in
appliances of various kinds, are single-phase and operate at power factor
values of 50 to 70 percent, but many have corrective capacitors associated
with them. When they operate without speed controls or starters,
their starting currents may cause lights on the same circuit to flicker;
where starts are relatively frequent, as with refrigerators and oil burners,
the flicker may be annoying.

Induction Motors
Most commercial and industrial ac motors are of the induction
type; limited speed control may be obtained in some types by varying
the applied voltage. Where accurate speed control is desirable, such as
for elevators and printing presses, dc motors are employed, sometimes
served from ac sources through motor-generator sets. Induction motors
may operate at power factors of 50 to 95 percent but generally operate
on the order of 80 to 90 percent; at less than full load, the power factors
may drop to 50 to 60 percent. Most large motors for industrial loads
(from about 2 hp and larger) are usually three-phase (although many
older two-phase motors still exist). Voltage variations of about—10 percent
can be accommodated with little lowering of motor efficiency and
power factor values.

Synchronous Motors
Synchronous motors, usually of large sizes, can operate at power
factors leading or lagging 100 percent by adjusting their excitation: overexcitement
draws leading current, under-excitement lagging current.

Often this type of motor is used for power factor correction for the entire
installation.
Since larger motors are apt to cause voltages to dip when starting,
circuits separate from lighting circuits are provided to eliminate flicker
problems; sometimes separate supply transformers are also provided.
Also causing similar flicker problems are chemical and electrolytic devices
and mechanical devices operated by coils or solenoids.

Heating Loads
The heating category may be conveniently divided into residential
(small) and industrial (large) applications.
Residential Heating
Residential heating includes ranges for cooking; hot water heaters;
toasters, irons, clothes dryers, and other such appliances; and house heating.
These are all resistance loads, varying from a relatively few watts
to several kilowatts, most of which operate at 120 V, while the larger
ones are served at 240 V; all are single-phase. The power factor of such
devices is essentially unity. The resistance of the elements involved is
practically constant; hence current will vary directly as the applied voltage.

The effect of reduced voltage and accompanying reduced current
is merely to cause a corresponding reduction in the heat produced or a
slowing down of the operation of the appliance or device. While voltage
variation, therefore, is not critical, it is usually kept to small values since
very often the smaller devices are connected to the same circuits as are
lighting loads, although hot water heaters, ranges, and other larger loads
are usually supplied from separate circuits. (Microwave ovens employ
high-frequency induction heating and are described below.)

Industrial Heating
Industrial heating may include large space heaters, ovens (baking,
heat-treating, enameling, etc.), furnaces (steel, brass, etc.), welders, and
high-frequency heating devices. The first two are resistance-type loads
and operate much as the smaller residential devices, with operation at
120 or 240 V, single-phase, and at unity power factor. Ovens, however,
may be operated almost continuously for reasons of economy, and some
may be three-phase units.

Electric Furnaces
Furnaces may draw heavy currents more or less intermittently
during part of the heat process and a fairly steady lesser current for the
rest; on the whole, the power factor will be fairly high since continuous
operation is indicated for economy reasons. The power factor of a furnace
load varies with the type of furnace from as low as 60 percent to
as high as 95 percent, with the greater number about 75 or 80 percent.
Sizes of furnaces vary widely; smaller units with a rating of several
hundred kilowatts are single-phase, while the larger, of several thousand
kilowatts, are usually three-phase. Voltage regulation, while not critical,
should be fairly close because of its possible effect on the material in the
furnace.

Welders
Welders draw very large currents for very short intermittent periods
of time. They operate at a comparatively low voltage of 30 to 50
V, served from a separate transformer having a high current capacity.
Larger welders may employ a motor-generator set between the welder
and the power system to prevent annoying voltage dips. The power fac
tor of welder loads is relatively low, varying with the load. The timing
of the weld is of great importance and may be regulated by electronic
timing devices.

High-Frequency Heating
High-frequency heating generates heat in materials by high-frequency
sources of electric power derived from the normal (60-Hz) power
supply. High-frequency heating is of two types: induction and dielectric.

Induction heating. 
In induction heating, the material is conducting
(metals, etc.) and is placed inside a coil connected to a high-frequency
source of power; the high-frequency magnetic field induces in the material
high-frequency eddy currents which heat it. Because of the skin effect, the
induced currents will tend to crowd near the surface; as the frequency is
increased, the depth of the currents induced will decrease, thus providing
a method of controlling the depth to which an object is heating.

Dielectric heating. In dielectric heating, a poor conducting material
(plastic, plywood, etc.) is placed between two electrodes connected to a
high-frequency source; the arrangement constitutes a capacitor, and an
alternating electrostatic field will be set up in the material. (Some slight
heating will also be set up from the induction effect described above, depending
on the conducting ability of the material.) The alternating field
passing uniformly through the material displaces or stresses the molecules,
first in one direction and then in the other as the field reverses
its polarity. Friction between the molecules occurs and generates heat
uniformly throughout the material. Such friction and heat are proportional
to the rate of field reversals; hence, the higher the frequency, the
faster the heating. Because of heat radiation from the surface, however,
the center may be hotter than the outside layers. Residential-type microwave
ovens are an application of dielectric heating.

Oscillators
Oscillators are used as the source of high-frequency power required
for both induction and dielectric heating. This is an electronic
application, and its characteristics and requirements are described in the
following section.

Electronic Loads
The electronic load category includes radio, television, x-rays,
laser equipment, computers, digital time and timing devices, rectifiers,
oscillators for high-frequency current production, and many other elec
tronically operated devices. In general, these employ electron tubes or
solid-state devices such as transistors, semiconductors, etc. Practically
all of these devices operate at voltages lower than the commercial power
sources and employ transformers or other devices to obtain their specific
voltages of operation.

They are all affected by voltage variations.
Voltage variations may have a marked effect on electron tubes, affecting
their current-carrying abilities or emissions as well as their life
expectancy. Because of the reduced life of the heater element and higher
rate of evaporation of active materials from the cathode surface, the cathode
life of electron tubes may be reduced as much as one-half by only a
5 percent rise in cathode voltage. Industrial-type tubes are normally designed
to operate with a voltage tolerance of ± 5 percent, though closer
tolerances are often specified.

While voltage variations also affect the operation of solid-state devices,
the effect on their life expectancy is not as serious as in the case of
electron tubes. On the other hand, variations in frequency of the power
supply have little effect on electron tubes but may have a pronounced
effect on solid-state devices.

Both types of devices are very sensitive to voltage dips, and, from
the power supply viewpoint, operate at essentially unity power factor.
Some applications, such as computers, may require an uninterrupted
source of supply, and various schemes are employed to achieve this, including
the use of motor-generator sets capable of running on batteries
for a limited time; the motor-generator set also eliminates the problems
of voltage dips on the commercial power supply.

Except for some rectifier applications, most of these devices operate
from single-phase ac supply circuits; large rectifiers may be supplied
from three-phase sources.

Oscillators for commercial purposes employ industrial-type electron
tubes in conjunction with capacitors and inductances that may be
varied to produce the desired high-frequency sources. The regular tolerances
in voltage supply from commercial power sources are suitable for
this application.

Read More - Connected Loads

Load Characteristics

In the planning of an electrical distribution system, as in any other
enterprise, 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 desired
3. The location of the market and the individual consumers
Logically, then, it would be well to begin with the basic building
blocks, the individual consumers, and then determine efficient means of
supplying their wants, individually and collectively.

Read More - Load Characteristics

Overhead versus Underground

Although the original distribution system pioneered by Thomas
Edison was a direct current low-voltage system installed underground,
the widespread expansion of electric systems was based principally on
the adoption of alternating current (through the application of transformers)
and the very economic overhead type of construction.

While the chief limitation to the adoption of underground systems
is economic, there are other reasons that argue against its selection. The
necessity for ducts, for manholes, and for cables that require expensive
insulation and lead sheaths, short pulls, and a relatively large number
of splices, and the special requirements to make equipment waterproof
and safe for installation underground all tend to make investment costs
several times as great as for overhead systems of comparable characteristics.

Where loads become so great, however, that the number of pole
lines and the congestion of conductors on such lines become impractical
from safety, operational, and appearance viewpoints, there is no alternative
but to place the lines underground. In such areas, traffic conditions
are usually so severe that difficulty is experienced in building and
maintaining overhead systems; moreover, the heavy traffic itself presents
additional hazards from vehicles striking the poles.

While an underground system is not exposed to damage and interruptions
from storms, traffic, etc., on the other hand, when trouble does
occur, it is very much more difficult and time-consuming to locate and
repair than in the overhead system. For this reason, additional provisions
and expenditures are made for maintaining service reliability;
these include duplicate facilities, throwover schemes, networks, etc.
Also, the lesser ability for heat radiation in an underground system does
not permit the loading and overloading of conductors and equipment
possible with overhead systems.

With plastics taking over the functions of insulation and sheathing
in underground cables, and the ability of these materials to be buried
directly in the ground, the economic advantage of overhead systems,
though still favorable, is markedly reduced.

The recent greater emphasis on environment (appearance) also has contributed to a greater pressure
for underground installations. Overhead systems will, however, prevail
to a very great extent for some time, and will be in almost exclusive use
in rural areas.

Read More - Overhead versus Underground

Distribution System Considerations

In determining the design of distribution systems, three broad classifications
of choices need to be considered:
1. The type of electric system: dc or ac, and if ac, single-phase or
polyphase.
2. The type of delivery system: radial, loop, or network. Radial systems
include duplicate and throwover systems.
3. The type of construction: overhead or underground.

DESIRED FEATURES
Electrical energy may be distributed over two or more wires. The
principal features desired are safety; smooth and even flow of power, as
far as is practical; and economy.

Safety
The safety factor usually requires a voltage low enough to be safe
when the electric energy is utilized by the ordinary consumer.

Smooth and Even Flow of Power
A steady, uniform, nonfluctuating flow of power is highly desirable,
both for lighting and for the operation of motors for power purposes.
Although a direct current system fills these requirements admirably, it
is limited in the distance over which it can economically supply power
at utilization voltage.

Alternating current systems deliver power in a fluctuating manner
following the cyclic variations of the voltage generated. Such fluctuations
of power are not objectionable for heating, lighting, and small motors,
but are not entirely satisfactory for the operation of some devices
such as large motors, which must deliver mechanical power steadily and
therefore require a steady input of electric power.

This may be done by supplying electricity to the motors by two or three circuits, each supplying
a portion of the power, whose fluctuations are purposely made not
to occur at the same time, thereby decreasing or damping out the effect
of the fluctuations. These two or three separate alternating current circuits
(each often referred to as a single-phase circuit) are combined into
one polyphase (two- or three-phase) circuit. The voltages for polyphase
circuits or systems are supplied from polyphase generators.

Economy
The third factor requires the minimum use of conductors for delivery
of electric energy. This usually calls for the use of higher voltages
where conditions permit and the elimination of some conductors by
providing a common return path for two or more circuits.

Read More - Distribution System Considerations