Although there is no “typical” electric power system, a diagram
including the several components that are usually to be found in the
makeup of such a system is shown in Figure 1-1; particular attention
should be paid to those elements which will make up the component
under discussion, the distribution system.
including the several components that are usually to be found in the
makeup of such a system is shown in Figure 1-1; particular attention
should be paid to those elements which will make up the component
under discussion, the distribution system.
While the energy flow is obviously from the power generating
plant to the consumer, it may be more informative for our purposes to
reverse the direction of observation and consider events from the consumer
back to the generating source.
Energy is consumed by users at a nominal utilization voltage that
may range generally (in the United States) from 110 to 125 V, and from
220 to 250 V (for some large commercial and industrial users, the nominal
figures are 277 and 480 V). It flows through a metering device that
determines the billing for the consumer, but which may also serve to
obtain data useful later for planning, design, and operating purposes.
The metering equipment usually includes a means of disconnecting the
consumer from the incoming supply should this become necessary for
any reason.
The energy flows through conductors to the meter from the secondary
mains (if any); these conductors are referred to as the consumer’s
service, or sometimes also as the service drop.
Several services are connected to the secondary mains; the secondary
mains now serve as a path to the several services from the distribution
transformers which supply them.
At the transformer, the voltage of the energy being delivered is
reduced to the utilization voltage values mentioned earlier from higher
primary line voltages that may range from 2200 V to as high as 46,000 V.
The transformer is protected from overloads and faults by fuses
or so-called weak links on the high-voltage side; the latter also usually
include circuit-breaking devices on the low-voltage side. These operate to
disconnect the transformer in the event of overloads or faults. The circuit
breakers (where they exist) on the secondary, or low-voltage, side operate
only if the condition is caused by faults or overloads in the secondary
mains, services, or consumers’ premises; the primary fuse or weak link in
addition operates in the event of a failure within the transformer itself.
only if the condition is caused by faults or overloads in the secondary
mains, services, or consumers’ premises; the primary fuse or weak link in
addition operates in the event of a failure within the transformer itself.
from lightning or line voltage surges by a surge arrester, which
drains the voltage surge to ground before it can do damage to the transformer.
The transformer is connected to the primary circuit, which may
be a lateral or spur consisting of one phase of the usual three-phase
primary main. This is done usually through a line or sectionalizing fuse,
whose function is to disconnect the lateral from the main in the event of
fault or overload in the lateral. The lateral conductors carry the sum of
the energy components flowing through each of the transformers, which
represent not only the energy used by the consumers connected thereto,
but also the energy lost in the lines and transformers to that point.
The three-phase main may consist of several three-phase branches
connected together, sometimes through other line or sectionalizing fuses,
but sometimes also through switches. Each of the branches may have
several single-phase laterals connected to it through line or sectionalizing
fuses.
Where single-phase or three-phase overhead lines run for any
considerable distance without distribution transformer installations connected
to them, surge arresters may be installed on the lines for protection,
as described earlier.
Some three-phase laterals may sometimes also be connected to the
three-phase main through circuit reclosers. The recloser acts to disconnect
the lateral from the main should a fault occur on the lateral, much as a
line or sectionalizing fuse. However, it acts to reconnect the lateral to the
main, reenergizing it one or more times after a time delay in a predetermined
sequence before remaining open permanently. This is done so
that a fault which may be only of a temporary nature, such as a tree limb
falling on the line, will not cause a prolonged interruption of service to
the consumers connected to the lateral.
The three-phase mains emanate from a distribution substation, supplied
from a bus in that station. The three-phase mains, usually referred
to as a circuit or feeder, are connected to the bus through a protective circuit
breaker and sometimes a voltage regulator. The voltage regulator is
usually a modified form of transformer and serves to maintain outgoing
voltage within a predetermined band or range on the circuit or feeder
as its load varies.
It is sometimes placed electrically in the substation
circuit so that it regulates the voltage of the entire bus rather than a
single outgoing circuit or feeder, and sometimes along the route of a
feeder for partial feeder regulation. The circuit breaker in the feeder acts
to disconnect that feeder from the bus in the event of overload or fault
on the outgoing or distribution feeder.
The substation bus usually supplies several distribution feeders
and carries the sum of the energy supplied to each of the distribution
feeders connected to it. In turn, the bus is supplied through one or more
transformers and associated circuit breaker protection. These substation
transformers step down the voltage of their supply circuit, usually called
the subtransmission system, which operates at voltages usually from
23,000 to 138,000 V.
The subtransmission systems may supply several distribution
substations and may act as tie feeders between two or more substations
that are either of the bulk power or transmission type or of the distribution
type. They may also be tapped to supply some distribution load, usually
through a circuit breaker, for a single consumer, generally an industrial
plant or a commercial consumer having a substantially large load.
The transmission or bulk power substation serves much the same
purposes as a distribution substation, except that, as the name implies,
it handles much greater amounts of energy: the sum of the energy individually
supplied to the subtransmission lines and associated distribution
substations and losses. Voltages at the transmission substations are
reduced to outgoing subtransmission line voltages from transmission
voltages that may range from 69,000 to upwards of 750,000 V.
The transmission lines usually emanate from another substation
associated with a power generating plant. This last substation operates
in much the same manner as other substations, but serves to step up to
transmission line voltage values the voltages produced by the generators.
Because of material and insulation limitations, generator voltages
may range from a few thousand volts for older and smaller units to some
20,000 volts for more recent, larger ones. Both buses and transformers in
these substations are protected by circuit breakers, surge arresters, and
other protective devices.
In all the systems described, conductors should be large enough
that the energy loss in them will not be excessive, nor the loss in voltage
so great that normal nominal voltage ranges at the consumers’ services
cannot be maintained.
In some instances, voltage regulators and capacitors are installed at
strategic points on overhead primary circuits as a means of compensating
for voltage drops or losses, and incidentally help in holding down
energy losses in the conductors.
In many of the distribution system arrangements, some of the
several elements between the generating plant and the consumer may
not be necessary. In a relatively small area, such as a small town, that
is served by a power plant situated in or very near the service area, the
distribution feeder may emanate directly from the power plant bus, and
all other elements may be eliminated, as indicated in Figure 1-2. This is
perhaps one extreme; in many other instances only some of the other elements
may not be necessary; e.g., a similar small area somewhat distant
from the generating plant may find it necessary to install a distribution
substation supplied by a transmission line of appropriate voltage only.
In the case of areas of high load density and rather severe service
reliability requirements, the distribution system becomes more complex
and more expensive. The several secondary mains to which the
consumers’ services are connected may all be connected into a mesh
or network. The transformers supplying these secondary mains or network
are supplied from several different primary feeders, so that if one
or more of these feeders is out of service for any reason, the secondary
network is supplied from the remaining ones and service to the consumers
is not interrupted.
To prevent a feeding-back from the energized
secondary network through the transformers connected to feeders out
of service (thereby energizing the primary and creating unsafe conditions),
automatically operated circuit breakers, called network protectors,
are connected between the secondary network and the secondary of the
transformers; these open when the direction of energy flow is reversed.
The two examples cited here are perhaps the two extremes in the
design of distribution systems, the first the simplest, the latter the most
complex. There are many variations in between these, and the basic ones
will be described in their appropriate places.
Only distribution systems, however, will be the subject of further
description and discussion in this book. In general, these include the
distribution substation, primary feeders, transformers, secondary mains,
services, and other elements between the substation and the consumers’
points of service.
