Location versus Distribution Voltage
Perhaps the first consideration regarding a distribution substation
is its location. In general, it should be situated as close to the load center
to be served as practical. This implies that all loads can be served
without undue voltage regulation, including future loads that can be
expected in a reasonable period of time. The difficulty in obtaining substation
sites is an important factor in selecting the distribution voltage,
both in original designs and in later conversions.
The higher the distribution voltage, the farther apart substations
may be located, but they also become larger in capacity and in the number
of customers served. Thus, the problem of the number and location
of distribution substations involves not only the study of transmission
and subtransmission designs, but more emphasis on service reliability
and consideration of additional costs that may be justified. The subjects
of sectionalizing, field-installed voltage regulators and reclosers, capacitors,
and ties to adjacent sources are discussed elsewhere, but are pertinent
to the problem.
Supply Feeders and Circuit Breaker Requirements
The number and sources of supply subtransmission feeders to the
distribution substation will depend not only on the load to be served,
but also on the degree of service reliability sought. Some rural substations
may be supplied from only one subtransmission feeder, while
substations serving urban and suburban areas have a minimum of two
supply feeders and may have several more. Each additional incoming
feeder, however, adds to the bus and switching requirements, including
auxiliary devices for their protection, all of which add to costs.
Circuit Breaker Arrangements
Some basic arrangements of incoming high-voltage circuit breakers
and transformers are shown in Figure 4-8. Each scheme progressively
adds to the reliability of service to the substation and the loads it supplies.
For example, in scheme a, a failure on the transmission line or
substation transformer or bus will trip the breaker back at the transmission
source, and service may not be restored until the fault is found and
repaired; in scheme b, such failures will trip the circuit breaker but service
can be restored as soon as the fault is isolated; in schemes c, d e, f, and
g (the last incorporating a ring bus), failures on the incoming transmission
lines, transformers, or high-voltage circuit breaker will not interrupt
(except for a short time or momentarily) the supply to the bus serving
distribution feeders.
Since the cost of high-voltage circuit breakers, together
with their accessories, is often as great as or greater than the cost
of the transformers with which they may be associated, it is essential
that the cost of additional circuit breakers not outweigh the protective
advantages gained. It may prove desirable that a minimum number of
circuit breakers be installed initially and others added as deemed necessary
for any improvement in service reliability that time, increments of
load, and customers’ requirements may indicate.
Interrupting Duty
The circuit breakers must not only interrupt the normal load current,
but must be mechanically able to withstand the forces resulting
from the large magnetic fields created by the fault current flowing
through them. Since the field will depend on the magnitude of the fault
current, which in turn also depends on the voltage of the circuit, the
stresses that must be accommodated depend on both of these values.
A circuit breaker, therefore, is rated not only on its applied voltage and
normal current-carrying capacity, but on its interrupting ability, expressed
in volt-amperes (or kVA or MVA); for example, 100-A, 35-kV, or
50,000-kVA interrupting “duty” or capability.
Insulation Coordination—BIL
Circuit breakers and other equipment are subject to high-voltage
surges resulting from lightning or switching operations, and the insulation
of their energized parts must be capable of withstanding them.
Lightning or surge arresters are installed on the conductors and buses
of each phase as close to the circuit breakers as practical, with the intent
of draining off the voltage surge to ground before it reaches the
breaker.
To provide adequate insulation economically and to restrict and
localize possible damage to the circuit breaker, the insulation provided
for the several parts is coordinated. Internal parts are insulated as
equally as practical, but their insulation is generally stronger than that
of the bushings, which in turn is stronger than that of the “discharge”
point of the associated arrester. Thus, a surge not drained to ground
by the arrester will next tend to flash over at the bushings, outside
the tank, where damage would be confined, comparatively light, and
easier to repair. In general, the insulation of the weakest point in the
circuit breaker should be weaker by such a margin as to ensure it will
break down before the insulation of the principal equipment it is protecting.
The coordination of insulation requires the establishment of a basic
insulation level (BIL) above which the insulation of the component parts
of the system should be maintained, and below which lightning or surge
arresters and other protective devices operate. This is discussed further
in connection with protective devices.
Substation transformers also have their insulation coordinated with
that of associated circuit breakers, buses, and other devices.
Capacitors
As mentioned earlier, banks of capacitors may be connected to the
high-voltage incoming bus in connection with voltage regulation and
increasing the capacity or capability of the substation to supply load.
All or portions of these banks may be switched on and off to provide
flexibility in maintaining voltage regulation and power factors. This is
done with one or more circuit breakers, and arresters or other protective
devices as indicated.
Transformers
Substation transformers may consist of three-phase units or banks
of three single-phase units. The size of these individual installations may
range from 150 kVA (three-phase) in small rural stations to upwards of
25,000 kVA at larger urban and suburban substations. Their impedances
are generally low, restricting unregulated voltage variations at the bus
to a few percent, except where fault current levels are high. In this case,
transformer impedances are increased to limit fault current duty to design
limits.
The impedances of the transformer banks in a station should match
each other as closely as practical to have the banks share the load as
equally as practical.
The transformers may be connected in a delta or wye pattern, on
both the incoming high-voltage (subtransmission) side and the outgoing
low-voltage (primary circuit) side. The transformers are ordinarily
of the two-winding standard type, operating much as the distribution
transformers.
For many reasons, including the random and nonuniform movement
of the molecules in the core of the transformer, the alternating
magnetic field that is set up may be distorted, producing serrated sine
waves on both sides of the transformer. These serrations can be broken
down into a series of harmonics or waves with frequencies of 3, 5, 7,
etc., times the basic frequency (usually 60 cycles per second). If the transformers
have a ground on either side, the harmonics or fluctuations flow
to ground and the original sine wave essentially remains undistorted. If
the windings are connected in delta fashion, these fluctuations circulate
around the delta, filtering out the harmonics and eliminating them from
the sine wave formed in the windings; however, they do cause some
unnecessary heating.
Where the transformer windings are connected in a wye arrangement
without a ground or neutral back to the source, the harmonics may
be particularly bothersome. To overcome these, each of the single-phase
transformations (singly or within a three-phase unit) is provided with
a third, small-capacity winding; the three such windings are connected
in delta (even though the main primary and secondary windings are
connected in wye). The delta thus formed allows the harmonics to circulate
within it, producing a little heat but essentially filtering them out,
so that the sine wave produced on both the high and low sides of the
transformer will be a more pure sine wave.
Low-side Bus Arrangements
The low sides of the transformers are connected to their buses
usually through circuit breakers. Several configurations are shown
in Figure 4-9. Some provision is usually made for permitting circuit
breakers, switches, regulators, and other devices to be taken out of
service for maintenance or for other reasons without causing an interruption
to the outgoing distribution feeders. Each of the outgoing
distribution feeders is usually equipped with its own circuit breaker.
The relays operating these, as well as the transformer high-side circuit
breakers, and the capacitors (if any) are coordinated so that only the
proper circuit breaker will operate to clear a fault that may occur on
some portion of the system.
Voltage Regulators
Each distribution feeder may have its voltage individually regulated,
employing three single-phase regulators or one three-phase regulator.
If all of the distribution feeders have approximately the same load
cycles and voltage regulation (even if corrected by capacitors, field regu-
lators, or other means out on the feeder) the bus to which they are connected
may be regulated in place of individual feeder regulators. While
this calls for a certain amount of compromise, it may prove economical
in many instances.
Mobile Substations
Substations are often designed for three single-phase transformers
so that, where they are connected in delta on the incoming side, they can
operate in open delta in the event of failure of one of the units. In some
instances, a spare single-phase transformer is installed at the substation
so that, in the event of failure of one of the transformers, a replacement
can be made readily.
With the advent of lighter transformers and improved transportation
equipment, it has proven practical to mount a three-phase
transformer and associated switching and surge arresters on a trailer
especially designed for that purpose. Such a mobile substation can be
readily transported to a substation where a failure has occurred. The
terminal arrangements of both the mobile substation and the fixed
substation are so designed that often service can be restored more
quickly than by reconnecting the spare unit (which no longer need be
provided).
The mobile substation not only can be effective where the failure
may involve more than one transformer, but can service a number of
substations in a more economical fashion than the installation of spare
transformers at many, if not all, substations. Further, it may also be installed
as a separate, temporary substation, picking up portions of the
load of one or more substations whose facilities may be overloaded.
