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Voltage regulators: fundamentals of voltage regulators

What are voltage regulators?

Voltage regulators maintain power distribution system voltages within a defined range. Regulated voltages ensure that electrical products and equipment will operate optimally. 

Electrical products and equipment are typically designed to operate within a limited voltage band. Poorly regulated voltage often results in adverse and unacceptable equipment performance. Problems that can occur include damage electronics, inefficient operation of motors, dim light bulbs and a myriad of other issues.

Heavy electrical loads and long 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. Voltage regulators provide 32 steps, 5/8ths percent per step, for a total regulation of +/- 10% of system voltage. 

The latest voltage regulator technology provides important advantages over traditional designs:

  • Electronic controls with advance features supporting smart-grid technology and SCADA communications
  • Multi-phase control operating two or three regulators with a single controller
  • Quik-Drive tap changers provide operation through 32 steps in under 10 seconds improving power quality by providing faster recovery from large voltage swings
  • Tap changers with vacuum technology, higher load ratings, and reduce maintenance requirements 

Why use voltage regulators?


Over time, increased load density, feeder length and line losses can result in an unacceptable voltage drop. Load current variations typically result from new loads as well as daily and seasonal changes in load profiles.

Yet, electrical equipment has power quality criteria that requires a constant voltage despite variations in load current. Supplying unregulated voltage can cause lighting brownouts, overheat and shorten motor life and cause premature failure of electronics.

Voltage regulators solve the voltage drop challenge and hold line voltage within predetermined limits to support the proper operation of lights, appliances and motors.

While the primary purpose of a voltage regulator is to provide regulated voltage that meets power quality criteria, voltage regulator electronic controls can also enable conservation voltage reduction, metering and integrated volt/var control (IVVC). 


How do voltage regulators work?

Voltage regulators maintain power distribution system voltages within a defined range. Regulated voltages ensure that electrical products and equipment will operate optimally. 

A voltage regulator is an auto transformer that is able to add or subtract voltage to provide consistent system voltage levels.  A voltage regulator control senses system voltage and commands the tap changer to operate when voltage changes are needed. The tap changer operation changes the configuration of the auto transformer coil resulting in a change in the voltage.  


Types of voltage regulators

Single-phase step-voltage regulators can be applied in wye and delta connected systems.

  • In delta connected systems the open and closed delta configurations are available
  • In an open delta configuration, two regulators are connected to regulate all three phases
  • In closed delta configuration, three regulators are used to regulate between the phases

Multi-phase pad-mounted voltage regulators  

Multi-phase pad-mount voltage regulators are designed for use in public and commercial areas with underground distribution lines and provide a smaller footprint than traditional overhead installations. Pad-mounted regulators are available with single-phase, 2-in-1 and 3-in-1 configurations and can be configured for wye or delta systems. The multi-unit designs offer 2 or 3 single-phase regulators in one tank for substation and underground applications.

Pad-mounted voltage regulators provide the functionality of traditional, round-tank, pole-mounted and substation voltage regulators. Like traditional designs, pad-mounted designs regulate distribution line voltages from +/- 10% in 32 steps when used in wye-connected banks.

Without overhead lines or substation fences, less land is needed for voltage regulator installation. These low-profile voltage regulators can help reduce onsite installation costs and preserve site aesthetics.

Voltage ratings for pad-mounted regulators are available from 2500 volts to 34,500 volts for 60 Hz and 50 Hz systems.

Single-phase, 32-step voltage regulators

Single-phase voltage regulators are tap-changing autotransformers that regulate distribution line voltages from +/- 10% in 32 steps. Voltage regulator control senses system voltage and is able to operate the tap changer to correct voltage within the specified range.

Voltage ratings for single-phase regulators are available from 2400 volts to 34,500 volts for 60 Hz and 50 Hz systems. Smaller kVA sizes can be supplied with support lugs for pole mounting and with substation or platform tie down options. Larger sizes are provided with substation bases with pad-mounting provisions. 

Single-phase, pad-mounted voltage regulators

Single-phase pad-mounted voltage regulators provide the functionality of traditional round-tank pole-mounted and substation voltage regulators in a pad-mounted design.

The regulators are outdoor, oil-immersed, step type regulators that provide +/-10% regulation in 32 steps when used singly or in wye-connected banks. Voltage ratings are available from 2500 volts to 34,500 volts for 60 Hz and 50 Hz systems. 


Voltage regulators: Construction

Voltage regulators are constructed from three basic parts:

  • Autotransformer: a transformer with part of one winding common to both the primary and secondary windings
  • Load tap changer: the switch is designed to work under load to change the configuration of a transformer coil, providing greater regulator versatility
  • Voltage regulator control: the control senses the system and automatically commands the tap changer 

Voltage regulators: Load tap changers

Load tap changers are sized for a specific range of current and voltage applications.

New vacuum interrupting tap changers provide reliable, efficient and flexible voltage regulation for substations. This next generation tap changer provides for years of virtually maintenance-free operation and reduced maintenance costs.

The Quik-Drive tap changer improves power quality by tapping all 33 positions in less than 10 seconds, which is 5 to 10 times faster than traditional spring devices, and results in better power quality and faster recovery from large voltage variations, helping protect customer equipment.

Compared to traditional spring-loaded devices, these tap changers incorporate:

  • Fewer parts
  • Take less to maintain
  • Last longer
  • Cost less over the life of the equipment 

Voltage regulators: Control

New generation voltage regulator controls provide enhanced power quality. These controls can be mounted in a box on the regulator tank or remotely from the unit. While designed for current applications, they provide forward compatibility and the ability to be applied on nearly any voltage regulator in service today.  

The latest generation technology can provide multi-phase voltage regulation, so that two or three regulators can be operated with the use of single control. This new capability provides a single point of contact for communications, true multi-phase metering and fewer controls to program and maintain.

With integrated intelligence and communications, voltage regulators provide new functionality and intelligence that can be used in a variety of operational strategies using site metrics. Yet, they typically use the same function codes and interface of earlier controls. Field technicians familiar with earlier regulator control models should be able to use new technology with minimal training.

Designed with communications in mind, controls can support integrated communications, multiple protocol availability and digital metering for Class 1 accuracy. With Instantaneous metering, time- and date-stamped demand metering and profile records, controls provide voltage limiting capability, voltage reduction, reverse power flow operation and tap position tracking – all to improve power quality. 

Voltage regulation integrated volt/var control

Voltage regulators: Troubleshooting / basic operation

Bypassing voltage regulators

Bypassing refers to installing or removing a regulator from service. For either operation, the regulator must be in neutral and a minimum of four indicators are recommended to confirm neutral setting.

It is important to note that both installation and removal of an energized voltage regulator with the tap changer off of neutral will short circuit part of the series winding. If the voltage regulator cannot be placed in neutral, then an outage should be taken, and downstream and upstream power sources should be disconnected.

Refer to the appropriate installation, operation and maintenance instructions.

Setting up voltage regulator controls

While voltage regulators provide new functionality and intelligence, they typically use the same function codes and interface of traditional technology. Field technicians familiar with earlier regulator control models should be able to use new technology.

Read the regulator control reference guide for more information on basic set up. 


Voltage regulators: Size and type

There are two predominant types of voltage regulators defined by the American National Standards Institute (ANSI): ANSI Type A and ANSI Type B designs. For applications that are rated 875 amperes and up, there are two additional types of regulators: Type TX and Type AX.

The external construction, connection to the utility electrical system and use is the same. The difference between ANSI Type A and Type B regulators is in the internal construction. 

ANSI Type A regulators are also known as “straight” designs with series winding located on the load side of the shunt winding. Type A regulators have separate control power transformers (PT) to measure the voltage between the L and SL bushings for sensing, and to supply control and motor power. 

ANSI Type B regulators are “inverted” with series winding located on the source side of the shunt windings. ANSI Type B regulators do not have a separate PT, instead the control windings in the main coil measure the voltage between the L and SL bushings for sensing and to supply control and motor power. 

Applications that are rated 875 A and above, can use Type TX and Type AX designs. Type TX is a series transformer design used on 2.5 kV applications, while Type AX incorporates a series autotransformer design and is typically used on 5.0 and 7.62 kV applications.  


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