Good quality electric service requires that the voltage at the consumers’ premises be kept within an acceptable voltage range for satisfactory operation of consumer equipment. At the 120-volt level, this is 110–126 volts at the utilization point.

It is customary for utilities to hold voltage at the customer meter location between 114 and 126 volts, which allows for a 4-volt drop to the utilization point in the residence. The location of the voltage extremes are usually at the first and last customer locations on the primary feeder.

During peak load conditions the first customer usually receives the highest voltage and the last customer the lowest. The variations from light to heavy load at these locations will establish the voltage range for the circuit.

As a first step in the control of voltage on such a circuit, most utilities will regulate the primary voltage at the substation. This takes care of variations in the voltage supplied to the substation and the variation on the feeder up to the first customers. The equipment usually used for regulation is tap changers on the substation transformers or separate feeder voltage regulators.

For most urban feeders, no other regulating equipment is needed, although shunt capacitor banks are often installed to supply part of the kilovar portion of the load. On larger or longer feeders, both voltage regulators and shunt-capacitor banks may be needed out on the feeders to provide supplementary voltage control and reactive supply.

In general, the control of voltage is more economical if both voltage regulators and shunt capacitors are used and if distribution voltage control is coordinated with voltage control of the transmission system and of generation.

Capacitors are applied as an economic tool to reduce system losses by supplying kilovars locally. Shunt capacitor banks including fixed and switched banks are used on primary feeders to reduce voltage drop, reduce power loss, and improve power factor.

The closer to the load they can be installed, the greater the economic benefit. Capacitors are not only an economic tool for the distribution system, but they can eliminate the need for adding reactive sources in the bulk power system. Kilovars supplied directly to load areas reduce the current in all portions of the system.

This releases transmission capacity and reduces system losses. At light load, the capacitors installed for full load operation may cause too high a voltage on the distribution system. Therefore, many capacitors will have to be switched off during these periods. Various means are used to perform the switching.

Voltage Regulators
Voltage regulators usually are an autotransformer with automatic tap-changing under load. Automatic measuring and tap-changing equipment holds the output voltage within a predetermined bandwidth.

By using the smallest practical bandwidth, more voltage drop can be allowed along the feeder, still keeping the consumer voltage within acceptable limits. The means for achieving this are an integral part of the regulator controls called the line drop compensator.

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