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Distribution Transformers

Voltage Regulators

Siemens invented the voltage regulator in 1932 and pioneered its use in the United States. Voltage Regulators are tapped step autotransformers used to ensure that a desired level of voltage is maintained at all times.

Complete Power Transformers Guide
Complete Power Transformers Guide (photo credit:

A voltage regulator comprises a tapped autotransformer and a tap changer. The standard voltage regulator provides ± 10 % adjustment in thirty-two 0.625 % steps. Voltage Regulators with ± 15 % and ± 20 % regulation are available for some designs.

Voltage regulators are liquid-immersed and can be 1-phase or 3-phase. They may be self-cooled or forced air-cooled. Available at 50 or 60 Hz and with 55 or 65 °C temperature rise, they can be used in any electrical system to improve voltage quality.

Voltage regulator ratings are based on the percent of regulation (i.e., 10 %).

For example, a set of three 1-phase 333 kVA regulators would be used with a 10 MVA transformer (e.g., 10 MVA 0.10/3 = 333 kVA). 1-phase voltage regulators are available in ratings ranging from 2.5 kV to 19.9 kV and from 38.1 kVA to 889 kVA (fig. 5.9-3).

Fig. 5.9-3: 1-phase voltage regulator, JFR
Fig. 5.9-3: 1-phase voltage regulator, JFR

3-phase voltage regulators are available at 13.2 kV or 34.5 kV and from 500 kVA to 4,000 kVA.

Voltage regulators can be partially or completely untanked for inspection and maintenance without disconnecting any internal electrical or mechanical connections. After the unit is untanked, it is possible to operate the voltage regulator mechanism and test the control panel from an external voltage source without any reconnections between the control and the regulator.

Standard external accessories

The standard accessories are as follows:

  • External metal-oxide varistor (MOV) bypass arrester
  • Cover-mounted terminal block with a removable gasketed cover. It allows easy potential transformer reconnections for operation at different voltages
  • Oil sampling valve
  • Two laser-etched nameplates
  • External oil sight gauge that indicates oil level at 25 °C ambient air temperature and oil color
  • External position indicator that shows the tap changer position Mounting bosses for the addition of lightning arresters to the source (S), load (L) and source-load (SL) bushings. They shall be fully welded around their circumference.

Accessories and options

  • Remote mounting kit: Extra-long control cable shall be provided for remote mounting of the control cabinet at the base of the pole.
  • Sub-bases: To raise the voltage regulator to meet safe operating clearances from the ground to the lowest live part.
  • Auxiliary PT: Operation at different voltages.


All voltage regulators shall be tested in accordance with the latest ANSI C57.15 standards.

Standard tests include:

  • Resistance measurements of all windings
  • Ratio tests on all tap locations
  • Polarity test
  • No-load loss at rated voltage and rated frequency Excitation current at rated voltage and rated frequency Impedance and load loss at rated current and rated frequency Applied potential
  • Induced potential
  • Insulation power factor test
  • Impulse test
  • Insulation resistance
Title:Power Transformers – SIEMENS
Size:2.72 MB
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Power Transformers Guide
Power Transformers Guide

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One Comment

  1. Haydar
    Mar 23, 2014

    I am protection engineer

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