Phase-shifting transformer

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Overview
Phase-shifting transformers (PSTs) enable precise control over active power flow in AC transmission networks. They are used to steer direction and magnitude of power flows on specific transmission lines to stabilize grids, prevent overloads, relieve congestion, and make better use of existing infrastructure. Built on decades of transformer design expertise, PST solutions support secure, reliable, and flexible power system operation.

How PSTs work
A PST introduces a controllable phase-angle shift between sending and receiving ends of a transmission line. Commonly implemented as an indirect PST design, it combines a shunt (tap) unit with a series unit that injects a variable voltage component into the line. PSTs can be realized as two-core solutions for high power ratings or as single-core solutions (typically up to 230 kV) where space or transport constraints apply.

Key capabilities and uses

• Active steering of active power flows to relieve overloaded corridors and balance parallel circuits.
• Restoring controllability in meshed and cross-border networks by limiting unscheduled loop flows.
• Increasing usable transfer capacity of existing transmission corridors to defer or avoid new line build-out.
• Supporting renewable integration by redirecting variable generation away from congested nodes.
• Contributing to grid stability, N-1 compliance, post-fault recovery, and controlled system restoration.

Design and technical features
Siemens Energy PSTs are available in three-phase, single-core and two-core designs with multiple configuration options to suit grid topologies and site constraints. Typical attributes include high phase-shift capability, fine tap resolution, broad operating range, robust mechanical design, options for low-noise execution, ester insulation, advanced cooling concepts, and provisions for digital monitoring and online diagnostics to support condition-based maintenance and integration with substation automation.

Application configurations
PSTs can be installed in in-line or by-pass arrangements and are typically applied to interconnectors, congested internal corridors, renewable integration hubs, and other steady-state power-flow management locations in high- and extra-high-voltage transmission systems.

Decision guide (table)
Grid situation | Typical challenge | How a PST helps
Persistent congestion on specific corridors | Certain transmission lines are regularly overloaded while parallel paths remain underutilized | Actively redistributes power flows to relieve congestion and balance line loading
Unplanned power flows in meshed or cross border grids | Power follows impedance rather than commercial schedules, causing unscheduled cross border exchanges | Restores controllability of power flows and limits unintended impacts on neighboring systems
Limited options for grid expansion | New lines face permitting, cost, or time constraints | Maximizes the efficiency of the existing grid and defers or avoids costly reinforcements
High redispatch or curtailment volumes | Frequent operational interventions increase system and market costs | Reduces the need for redispatch and curtailment through targeted flow control
Increasing renewable integration | Variable generation leads to rapidly changing flow patterns | Supports secure integration of renewables by (re)balancing active power flows
Growing cross border power exchange and market coupling | Higher transfer volumes stress interconnected networks | Enables controlled cross border exchanges while maintaining system security
Strict N-1 security requirements | Line outages or contingencies challenge system stability | Supports N-1 compliance by enabling controlled power flow

Typical operational and business benefits

• Better utilization of existing transmission infrastructure and deferral of costly reinforcements.
• Reduced redispatch/curtailment costs and lower system losses through targeted flow control.
• Increased operational flexibility, improved system resilience, and enhanced ability to integrate renewables and support market-based exchanges.

Characteristics / technical specifications

• Function: Active control of active power flow via controllable phase-angle shift.
• Design variants: single-core and two-core designs; three-phase execution; inline or by-pass configurations.
• Voltage & power: Engineered for high-voltage transmission applications up to the highest transmission classes; single-core solutions typically up to 230 kV where applicable; very high power ratings supported.
• Topology: Indirect PST designs combining shunt (tap) unit and series injection unit; two-core for large ratings where redundancy/flexibility required.
• Control: Fine tap resolution and broad operating range for precise phase-shift control.
• Mechanical & site options: Compact mechanical design, low-noise options, ester insulation availability, advanced cooling concepts, and site-specific configurations.
• Monitoring: Options for digital monitoring, online diagnostics, and integration into substation automation and asset management systems.
• Application: Cross-border interconnectors, congested internal corridors, renewable integration hubs, and corridors requiring N-1 compliance and system restoration support.