How does a distribution voltage regulator work?

Power distribution voltage regulators maintain power distribution system voltages within a defined range. Regulated voltages ensure that electrical products and equipment will operate optimally. Heavy electrical loads and long electrical distribution lines drag down system voltages. Voltage regulators enable utilities to maintain voltage levels within acceptable ranges giving utility customers the assurance that electrical equipment will operate properly. 

How conventional transformers work

To explain how a distribution voltage regulator works, we will start by discussing a conventional transformer. The conventional transformer has two windings and a steel core. The voltage input across the primary winding is transformed to a different voltage at the output across the secondary winding. The voltage transformation is dictated by the ratio of the turns of the windings. Also, in a conventional transformer, the primary and secondary windings are electrically isolated from each other; the transformation of voltage is accomplished magnetically through the steel core. 

What is an auto transformer? 

One kind of transformer, an autotransformer, can be used to change voltages in a different way. In the autotransformer, the voltage is also transformed magnetically through the steel core, but there is also a direct electrical connection between the primary and secondary windings. This electrical connection provides for the secondary winding to start or end at the same voltage as the primary winding. The voltage across the secondary winding is added to or subtracted from the voltage across the primary winding. The auto transformer is then said to have either an additive or subtractive property when the voltage output is either the sum or difference of the voltages across the two windings.

Because of the position of the windings in the circuit in the autotransformer, the primary is called the shunt winding and the secondary is called the series winding. The series winding is named as such because the winding is in series with the flow of power through the autotransformer. 

How voltage regulating autotransformers work

The autotransformer becomes a voltage regulator with taps added into the coil of the series winding and a load tap-changer connected to the taps. The load tap-changer makes changes to the number of turns used in the secondary winding and thus adjusts the amount of voltage either added to or subtracted from the input voltage. The tap changer is also able to change the polarity of the series winding connections resulting in an additive or subtractive property in the voltage regulator. This results in the capability of the voltage regulator to raise (boost) or lower (buck) the input voltage. 

An electronic control is added to the voltage regulator to operate the tap changer. The control monitors the output voltage of the voltage regulator using voltage information from a potential transformer. The control then sends commands for operation of the tap changer to boost or buck the voltage and maintain the distribution system voltage within a specified band.

Learn more about the fundamentals of voltage regulators here.

What voltage regulating devices are used in a power system? 

On a distribution system, there are two primary types of voltage regulating devices. The first of these is called a Single-Phase Step voltage regulator (SVR). In order to regulate a three-phase power distribution system, three SVRs are required. Each SVR will be equipped with a potential transformer to measure the voltage individually in each phase. Traditionally, three individual controls were required to monitor the voltage and operate the tap changer. More recently, a multi-phase control has been introduced to operate the three SVRs, but typically, the three phases will still be regulated independently.

The second type of device is called an On-Load Tap Changer (OLTC). An OLTC is a voltage regulating device associated with a substation transformer. The OLTC is a three-phase device in that it regulates all three phases of a power distribution system in unison. The OLTC will have a single control and the voltage on one phase will be monitored. The voltage on the monitored phase will be regulated directly when the control operates the tap changer. The two other phases will be regulated based upon the voltage regulation of the one monitored phase.

What are the typical voltages for the distribution system? 

The information below contains typical voltages for power distribution systems.

Wye connected Systems 60 Hz systems (Phase to Ground/Phase to Phase Voltages)

2400/4160

4160/7200

7200/12470

7620/13200

7970/13800

13800/23900

14400/24940

19920/34500

Delta connected systems 60 Hz systems

2400

4160

4800

7200

7620

7970

12470

13200

13800

Delta Connected system 50 Hz systems

6600

11000

22000

33000