Power flow control solutions have evolved significantly since series reactors and series capacitors were first deployed in the 1920s. In this article, I focus on comparing a leading advanced power flow control (APFC) solution, SmartValve™, to a conventional form of power flow control, the phase-shifting transformer (PST).
What are APFC solutions and PSTs?
Advanced power flow control (APFC) refers to modular power electronics-based devices that are part of the FACTS (Flexible Alternating Current Transmission Systems) family of devices. APFC solutions can dynamically control power flows on transmission and distribution grids by changing the reactance of the line on which they are deployed in real time. SmartValve, a patented, single-phase, modular Static Synchronous Series Compensator (m-SSSC), is the most well-known and widely deployed APFC solution. SmartValves are deployed as part of a SmartValve System, with at least one SmartValve installed per phase. Given their modular, voltage-agnostic design, SmartValves can be easily and quickly added to existing deployments over time to scale the voltage injection output, and devices can be relocated if network needs change.
In contrast, a phase-shifting transformer (PST) is an older form of power flow control equipment that is transformer-based and controls power flows by changing the phase angle of the voltage at the output terminals compared to the input terminals; PSTs use an electro-mechanical tap changer to change the phase angle in discrete steps. PSTs have been used on transmission grids for decades to manage the large and consistent needs for power flow control, such as on high voltage transmission lines between countries. However, PSTs typically have long lead and delivery times because they require a bespoke design and significant civil works before they can be installed. In some regions, PSTs are more commonly known as quadrature boosters (QBs) or phase angle regulators (PARs).
With rapid increases in electricity demand and use of inverter-based resources such as wind and solar, the need for more dynamic, faster-switching power flow control solutions has become increasingly pressing. Three important factors may advantage APFC over traditional PST solutions: the flexibility to deliver solutions quickly, the potential to incrementally expand solutions and the ability to defer investment decisions until an asset owner has more certainty on network needs.
SmartValve advantages to PST
Five dimensions where the two approaches diverge most.
APFC · Deployment flexibility
Modular, flexible solution
SmartValves are modular, voltage-agnostic devices that can be scaled to incrementally increase power flow control capabilities.
PST · Deployment flexibility
Fixed solution
PSTs are fixed solutions and must be sized far in advance, which increases up-front capital expenditure and overinvestment risk.
APFC · Controllability
Real-time, dynamic control
SmartValve uses proven voltage source converter (VSC) technology capable of injecting a voltage in quadrature with the line current, synthesizing a capacitive or inductive reactance across a continuous range. This lets operators incrementally and continuously change power flows in real time.
PST · Controllability
Mechanical tap changes
PSTs control power flow in discrete steps with mechanical tap changes, which are much slower than APFC voltage injection and limit contingency response and renewable variability handling.
APFC · Siting considerations
Flexible deployment options
SmartValve can often be installed within the existing substation footprint, removing the need for land acquisition and permitting. Devices can be double-stacked or distributed along a circuit so large solutions need not sit in one location.
PST · Siting considerations
Significant space required at a single location
PSTs are bulky devices that often require expanding an existing substation or building a new one, plus special transport measures and oil containment.
APFC · Lead and delivery time
12–18 months
SmartValve can be deployed within 12 to 18 months of ordering, using standard transport equipment with no special permits or road closures required.
PST · Lead and delivery time
Minimum 5 years
PSTs can take upwards of five years to implement due to their bespoke design, and are often challenging to transport, requiring specialized equipment and permits.
APFC · Redundancy
Modular system
SmartValve Systems typically run more than one unit per phase, each with an integrated fast-acting bypass, so the loss of one unit does not mean loss of total system capability.
PST · Redundancy
Single point of failure
The loss of one PST plant item is the loss of the entire PST. Without a spare, the system stays out of service until repairs are complete, and recent U.S. cases have taken over six months to fix.
To illustrate the differences further, two real-world examples of large-scale transmission projects follow: a UK utility that used SmartValves instead of a PST because it needed a modular, quick-to-deploy solution, and a US utility that will use SmartValves to extend the asset life of a PST at a critical grid seam otherwise at risk of future component failure.
Case example
The National Grid case: using SmartValves as an alternative to PSTs
National Grid Electricity Transmission (NGET) is using SmartValves at 275 kV and 400 kV at three existing substations to provide power flow control across five circuits, adding 2 GW of extra grid capacity. Before deciding on this solution, NGET submitted several network reinforcement options, including PSTs, or quadrature boosters as they are more commonly known in the UK, to the Network Options Assessment, which evaluates multiple solutions for the same network need to determine the option with the least worst regret across several scenarios.
As the least worst regret analysis values flexibility across multiple future scenarios, the SmartValve solution performed well. It could be delivered quickly to meet the near-term network need and scaled up over time, letting NGET defer investment decisions until there was sufficient certainty on need. The deployments at two of the substations were expanded in 2022 and 2023 with additional SmartValves, installed on top of the original devices without requiring any additional substation space, which would not have been possible with a PST.
Case example
The VELCO case: extending the asset life of a PST
VELCO, a utility in Vermont, experienced issues with a PST at its Sandbar substation connecting New York and Vermont state. The PST failed in 2021, requiring a replacement part to be moved over 60 miles. After six months of repair and maintenance, the substation was back in service, at a cost of 3.5 million dollars in total. The failure was likely caused by excessive tap changes used to manage shifting power flows on the VELCO network.
To prevent a future failure, VELCO is installing 12 SmartValves at 115 kV in series with the Sandbar PST, providing 180 MVAr of controllable capability, reducing PST tap changes by nearly 60 percent and adding 50 percent more power flow control capability along with 50 percent redundancy should the PST become unavailable. The chosen alternative is 16 percent less expensive than a series PST option and can be deployed in half the time. Construction begins in spring 2026, with commissioning anticipated summer 2027, and the project is funded in part by the United States Department of Energy’s Grid Resilience and Innovation Partnership.
In summary, while both APFC and PSTs provide power flow control capabilities, they differ significantly in flexibility, control, space required and redundancy. There is already notable APFC adoption in the US, and it is expected to continue to grow due to new regulatory rulings such as FERC Order 2023 and Order 1920, which require consideration of APFC in interconnection and transmission planning studies respectively.
Federally funded programs such as the Grid Resilience and Innovation Partnership are further supporting APFC deployment. To learn more, contact Smart Wires at info@smartwires.com.
Download the full APFC versus PST comparison
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Sarah Rozon
Sarah Rozon is Senior Analyst at Smart Wires, collaborating with North American utilities and government organizations to implement grid-enhancing technologies. Before joining Smart Wires, she was an Engineer in Training at Hydro Ottawa, where she developed business cases for system renewal upgrades, devised five-year business improvement strategies and built system-wide reliability reports to prioritize asset replacement and renewal initiatives.