Choosing voltage regulators to handle reverse power flow

January 23, 2017

Simply put, reverse power flow occurs when the normal current direction in a distribution system reverses and flows the other way. Usually this happens when a need arises to feed a line from another substation from the opposite end of the system. Reasons may be to perform maintenance, handle seasonal load shifts, or because of an emergency. It generally applies to voltage regulators when they are installed as line regulators, located in the middle of the system and not in a substation.

Because reversing power flow is usually a planned action, it presents little hazard. “The main issues that must be addressed when running RPF are having proper voltage settings to account for the change in load center distances and the ability of the regulator to detect the new load side voltage,” said Andrew Gruber, Solomon Corporation’s Decatur Plant Regulator and OCR Shop Supervisor. For example, a bank of regulators at the end of a 10-mile run may regulate a load center (town, business, etc.) directly after their placement in normal conditions. In RPF, their new load center would be 10 miles away and that difference must be accommodated.

The difference between regulator designs

The majority of voltage regulators come in two different designs: Straight (Type A) or Inverted (Type B). The major difference is the location of the exciting (shunt) winding.

Straight design regulators have their exciting windings located between the Source and the Source-Load connections. This winding can be thought of as the primary winding of the regulator in the same way that a transformer has a primary. The tapped series winding of the regulator is connected to the exciting winding in an autotransformer configuration and is the winding that sees the outgoing transformed voltage.

Straight design regulators have a separately wound Potential Transformer (PT) connected across the Load and Source-Load leads to be able to sense outgoing voltage. Certain models also provided a core wound source side PT that can supply motor voltage and meter incoming voltage.

Inverted design regulators have their exciting windings located between the Load and the Source-Load connections. Because the exciting winding will reflect the outgoing voltage, there isn’t a need to separately wind a PT for this design. The load side PT is wound in the main core and uses the exciting winding as its primary. Some regulator manufacturers give the option to install a separately wound PT on inverted design regulators for source side sensing purposes.

Clearing up some misinformation

Reverse power flow is a common issue across the majority of our customer’s systems. It is also an issue with much misinformation out there about it.

Many of our customers in the past have requested inverted regulators because they have been told only ANSI type B could run RPF. Customers have also requested regulators with two PT’s to accommodate for RPF. In the past, there were technological limitations on how to run RPF that required two PT’s. Since power flow essentially swaps the roles of the Source and Load bushings on the regulator, there has to be a way to sense the new load side voltage to properly run a regulator in reverse power flow. Straight design regulators with the internally wound source side potential already had the ability to accommodate reverse power flow. Inverted regulators needed the optional source side PT installed.

Now, both straight and inverted regulators can run RPF with the right controller installed. New microprocessor controls have eliminated the need for two potential transformers to accommodate for the voltage metering requirements during RPF. “Because both common ANSI type regulators can handle RPF needs, it helps customers to have more cost effective options in choosing their equipment for RPF,” Andrew said.

How new technology affects reverse power flow capabilities

Today, reverse power flow capabilities depend more on the control capabilities than anything else. Algorithms calculate source side voltage based upon the sensed load side voltage and the tap position of the regulator, which allows for reverse power flow capabilities on regulators that only have one PT. These algorithms are accurate and reliable. This has allowed companies to repurpose single PT regulators in areas that might see reverse power flow operation, with just an upgrade of the control panel.

Choosing the correct regulators is essential to keeping your system running safely and at peak efficiency. Solomon Corporation offers technical support and can help you choose the correct voltage regulators for your system. For more information or to request a quote, visit us at or call 800-234-2867.