What Is a Phase-Shifting Transformer (PST)?

Published 27 Aug 2026Updated 27 Aug 20267 min readAdvancedGrid

Quick answer — A phase-shifting transformer (PST), also called a quadrature booster, is a series unit that controls power flow by injecting a phase-shifted voltage. It changes the phase angle across the line, thereby redirecting real-power flow from overloaded lines to under-loaded parallel paths in a meshed network.

Contents

What it controls
Construction
Why it matters

What is a phase-shifting transformer (PST) and what does it do?

What it controls

In an AC network, real power flows largely according to the phase-angle difference between two points. The PST adds a controllable phase shift, letting the operator adjust how much power flows on a given line.

Construction

A PST uses a series winding (in the line) and an excitation winding (fed from the bus), with an on-load tap-changer controlling the injected voltage and hence the phase angle.

Why it matters

PSTs relieve congestion, balance loaded and lightly loaded parallel lines, increase transfer capacity and prevent overloads — increasingly important in interconnected networks and markets.

References & standards

IEEE C57.135 (PST guide)
IEC 60076-1

Book series Academy

Related manufacturers

Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Virginia Transformer Corp · Pennsylvania Transformer Technology

Related components

On-load tap changers Conductors & core steel

Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.

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