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.
