consumer may consist of several more or less distinct parts that are nevertheless
somewhat interrelated, described generally in Chapter 1. The
part considered “distribution,” i.e., from the bulk supply substation to
the meter at the consumer’s premises, can be conveniently divided into
two subdivisions:
1. Primary distribution, which carries the load at higher than utilization
voltages from the substation (or other source) to the point
where the voltage is stepped down to the value at which the energy
is utilized by the consumer.
2. Secondary distribution, which includes that part of the system
operating at utilization voltages, up to the meter at the consumer’s
premises.
Primary Distribution
Primary distribution systems include three basic types:
1. Radial systems, including duplicate and throwover systems
2. Loop systems, including both open and closed loops
3. Primary network systems
Radial Systems
The radial-type system is the simplest and the one most commonly
used. It comprises separate feeders or circuits “radiating” out of the substation
or source, each feeder usually serving a given area. The feeder
may be considered as consisting of a main or trunk portion from which
there radiate spurs or laterals to which distribution transformers are connected,
as illustrated in Figure 2-11.
The spurs or laterals are usually connected to the primary main
through fuses, so that a fault on the lateral will not cause an interruption
to the entire feeder. Should the fuse fail to clear the line, or should a fault
develop on the feeder main, the circuit breaker back at the substation or
source will open and the entire feeder will be de-energized.
To hold down the extent and duration of interruptions, provisions
are made to sectionalize the feeder so that unfaulted portions may be
reenergized as quickly as practical. To maximize such re-energization,
emergency ties to adjacent feeders are incorporated in the design and
construction; thus each part of a feeder not in trouble can be tied to an
adjacent feeder.
Often spare capacity is provided for in the feeders to
prevent overload when parts of an adjacent feeder in trouble are connected
to them. In many cases, there may be enough diversity between
loads on adjacent feeders to require no extra capacity to be installed for
these emergencies.
Supply to hospitals, military establishments, and other sensitive
consumers may not be capable of tolerating any long interruption. In
such cases, a second feeder (or additional feeders) may be provided,
sometimes located along a separate route, to provide another, separate
alternative source of supply.
Switching from the normal to the alternative
feeder may be accomplished by a throwover switching arrangement
(which may be a circuit breaker) that may be operated manually or
automatically. In many cases, two separate circuit breakers, one on each
feeder, with electrical interlocks (to prevent connecting a good feeder to
the one in trouble), are employed with automatic throwover control by
relays.
Loop Systems
Another means of restricting the duration of interruption employs
feeders designed as loops, which essentially provide a two-way primary
feed for critical consumers. Here, should the supply from one direction
fail, the entire load of the feeder may be carried from the other end, but
sufficient spare capacity must be provided in the feeder. This type of
system may be operated with the loop normally open or with the loop
normally closed.
Open Loop
In the open-loop system, the several sections of the feeder are connected
together through disconnecting devices, with the loads connected
to the several sections, and both ends of the feeder connected to the
supply. At a predetermined point in the feeder, the disconnecting device
is intentionally left open. Essentially, this constitutes two feeders whose
ends are separated by a disconnecting device, which may be a fuse,
switch, or circuit breaker.
In the event of a fault, the section of the primary on which the fault
occurs can be disconnected at both its ends and service reestablished to
the unfaulted portions by closing the loop at the point where it is normally
left open, and reclosing the breaker at the substation (or supply
source) on the other, unfaulted portion of the feeder.
Such loops are not normally closed, since a fault would cause
the breakers (or fuses) at both ends to open, leaving the entire feeder
de-energized and no knowledge of where the fault has occurred. The
disconnecting devices between sections are manually operated and may
be relatively inexpensive fuses, cutouts, or switches.
Closed Loop
Where a greater degree of reliability is desired, the feeder may be
operated as a closed loop. Here, the disconnecting devices are usually
the more expensive circuit breakers. The breakers are actuated by relays,
which operate to open only the circuit breakers on each end of the
faulted section, leaving the remaining portion of the entire feeder energized.
In many instances, proper relay operation can only be achieved
by means of pilot wires which run from circuit breaker to circuit breaker
and are costly to install and maintain; in some instances these pilot wires
may be rented telephone circuits.
To hold down costs, circuit breakers may be installed only between
certain sections of the feeder loop, and ordinary, less expensive disconnecting
devices installed between the intermediate sections. A fault will
then de-energize several sections of the loop; when the fault is located,
the disconnecting devices on both ends of the faulted section may be
opened and the unfaulted sections reenergized by closing the proper
circuit breakers.
Primary Network Systems
Although economic studies indicated that under some conditions
the primary network may be less expensive and more reliable than some
variations of the radial system, relatively few primary network systems
have been put into actual operation and only a few still remain in service.
This system is formed by tying together primary mains ordinarily
found in radial systems to form a mesh or grid. The grid is supplied by
a number of power transformers supplied in turn from subtransmission
and transmission lines at higher voltages.
A circuit breaker between the transformer and grid, controlled by reverse-current and automatic reclosing
relays, protects the primary network from feeding fault current
through the transformer when faults occur on the supply subtransmission
or transmission lines. Faults on sections of the primaries constituting
the grid are isolated by circuit breakers and fuses.
This type of system eliminates the conventional substation and
long primary trunk feeders, replacing them with a greater number of
“unit” substations strategically placed throughout the network. The additional
sites necessary are often difficult to obtain. Moreover, difficulty
is experienced in maintaining proper operation of the voltage regulators
(where they exist) on the primary feeders when interconnected.
Secondary Distribution
Secondary distribution systems operate at relatively low utilization
voltages and, like primary systems, involve considerations of service
reliability and voltage regulation. The secondary system may be of four
general types:
1. An individual transformer for each consumer; i.e., a single service
from each transformer.
2. A common secondary main associated with one transformer from
which a group of consumers is supplied.
3. A continuous secondary main associated with two or more transformers,
connected to the same primary feeder, from which a group
of consumers is supplied. This is sometimes known as banking of
transformer secondaries.
4. A continuous secondary main or grid fed by a number of transformers,
connected to two or more primary feeders, from which a
large group of consumers is supplied. This is known as a low-voltage
or secondary network.
Each of these types has its application to which it is particularly
suited.
Individual Transformer—Single Service
Individual-transformer service is applicable to certain loads that
are more or less isolated, such as in rural areas where consumers are far
apart and long secondary mains are impractical, or where a particular
consumer has an extraordinarily large or unusual load even though situated
among a number of ordinary consumers.
In this type of system, the cost of the several transformers and the
sum of power losses in the units may be greater (for comparative purposes)
than those for one transformer supplying a group of consumers
from its associated secondary main.
The diversity among consumers’
loads and demands permits a transformer of smaller capacity than the
capacity of the sum of the individual transformers to be installed. On
the other hand, the cost and losses in the secondary main are obviated,
as is also the voltage drop in the main. Where low voltage may be undesirable
for a particular consumer, it may be well to apply this type of
service to the one consumer.
Common Secondary Main
Perhaps the most common type of secondary system in use employs
a common secondary main. It takes advantage of diversity between
consumers’ loads and demands, as indicated above. Moreover,
the larger transformer can accommodate starting currents of motors with
less resulting voltage dip than would be the case with small individual
transformers.
In many instances, the secondary mains installed are more or less
continuous, but cut into sections insulated from each other as conditions
require. As loads change or increase, the position of these division
points may be readily changed, sometimes holding off the need to install
additional transformer capacity. Also, additional separate sections can
be created and a new transformer installed to serve as load or voltage
conditions require.
Banked Secondaries
The secondary system employing banked secondaries is not very
commonly used, although such installations exist and are usually limited
to overhead systems.
This type of system may be viewed as a single-feeder low-voltage
network, and the secondary may be a long section or grid to which the
transformers are connected. Fuses or automatic circuit breakers located
between the transformer and secondary main serve to clear the transformer
from the bank in case of failure of the transformer. Fuses may
also be placed in the secondary main between transformer banks.
Some advantages claimed for this type of system include uninterrupted
service, though perhaps with a reduction in voltage, should a
transformer fail; better distribution of load among transformers; better
normal voltage conditions resulting from such load distribution; an ability
to accommodate load increases by changing only one or some of the
transformers, or by installing a new transformer at some intermediate
location without disturbing the existing arrangement; the possibility that
diversity between demands on adjacent transformers will reduce the
total transformer load; more capacity available for inrush currents that
may cause flicker; and more capacity as well to burn secondary faults
clear.
Some disadvantages associated with this type of system are as follows:
should one transformer fail, the additional loads imposed on adjacent
units may cause them to fail, and in turn their loads would cause
still other transformers to fail (this is known as cascading); the transformers banked must
have very nearly the same impedance and
other characteristics, or the loads will not be distributed equitably
among them; and sufficient reserve capacity must be provided to carry
emergency loads safely, obviating the savings possible from the diversity
of the demands on the several transformers.
Banked secondaries, while providing for failure of transformers,
do not provide against faults on the primary main or feeder. Further, a
hazard on any transformer disconnected for any reason may result from
a back feed if the secondary energizes the primary (which may have
been considered safe).
Secondary Networks
Secondary networks at present provide the highest degree of service
reliability and serve areas of high load density, where revenues
justify their cost and where this kind of reliability is imperative. In some
instances, a single consumer may be supplied from this type of system
by what are known as spot networks.
In general, the secondary network is created by connecting together
the secondary mains fed from transformers supplied by two
or more primary feeders. Automatically operated circuit breakers in
the secondary connection between the transformer and the secondary
mains, known as network protectors, serve to disconnect the transformer
from the network when its primary feeder is de-energized; this prevents
a back feed from the secondary into the primary feeder.
This is especially important for safety when the primary feeder is de-energized from fault
or other cause. The circuit breaker or protector is backed up by a fuse so
that, should the protector fail to operate, the fuse will blow and disconnect
the transformer from the secondary mains.
The number of primary feeders supplying a network is very important.
With only two feeders, only one feeder may be out of service at
a time, and there must be sufficient spare transformer capacity available
so as not to overload the units remaining in service; therefore this type
of network is sometimes referred to as a single-contingency network.
Most networks are supplied from three or more primary feeders,
where the network can operate with the loss of two feeders and the
spare transformer capacity can be proportionately less. These are referred
to as second-contingency networks.
Secondary mains not only should be so designed that they provide
for an equitable division of load between transformers and for good
voltage regulation with all transformers in service, but they also must
do so when some of the transformers are no longer in service when their
primary feeders are de-energized.
They must also be able to divide fault
current properly among the transformers, and must provide for burning
faults clear at any point while interrupting service to a minimum
number of consumers; this often limits the size of secondary mains,
usually to less than 500 cmil × 103, so that when additional secondary
main capacity is required, two or more smaller size conductors have to
be paralleled. In some networks, where insufficient fault current might
cause long sections of secondary mains to be destroyed before the fault
is burned clear, sections of secondary mains are fused at each end.
Because these networks may represent very large loads, their size
and capacity may have to be limited to such values as can be successfully
handled by the generating or other power sources should they become
entirely de-energized for any reason. When they are de-energized
for any length of time, the inrush currents are very large, as diversity
among consumers may be lost, and this may be the limiting factor in
restricting the size and capacity of such networks.
Voltages
For all types of service, primary voltages are becoming higher.
Original feeder primary voltages of about 1000 V have climbed to nominal
2400, 4160, 7620, 13,800, 23,000, and 46,000 V. Moreover, primary
feeders that originally operated as single-phase and two-phase circuits
are all now essentially three-phase circuits; even those originally operated
as delta ungrounded circuits are now converted to wye systems,
with their neutral common to the secondary neutral conductor and
grounded.
Secondary voltages have changed from nominal 110/220 V singlephase
values to those now operating at 120/240 V single-phase and
120/208 or 120/240 V for three-phase circuits, the 120-V utilization being
applied to lighting and small-motor loads while the 208- and 240-V
three-phase values are applied to larger-motor loads. More recently, secondary
systems have employed utilization voltage values of 277 and 480
V, with fluorescent lighting operating single-phase at 277 V and larger
motors operating at a three-phase 480 V. To supply some lighting and
small motors single-phase at 120 V, autotransformers of small capacity
are employed to step down the 277 V to 120 V.
Secondary voltages and connections will be explored further in
discussing transformers and transformer connections.
