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.
