Power flow control solutions have evolved significantly since series reactors and series capacitors were first deployed in the 1920s. In this article, I’ll 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. See EPRI’s definition of APFC here. 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 where there is 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 the 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. There are three important factors that 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

To illustrate the differences further, I’ll share two real-world examples of large-scale transmission projects where 1) a UK utility used SmartValves instead of a PST because they needed a modular, quick-to-deploy solution, and 2) a US utility will use SmartValves to extend the asset life of a PST at a critical grid seam that would otherwise be at significant risk of future component failure.

• The National Grid case: using SmartValves as an alternative to PSTs

National Grid Electricity Transmission (NGET) are using SmartValves at 275 kV and 400 kV at three existing substations to provide power flow control across five circuits, unlocking 2 GW of extra grid capacity. Before deciding on this solution, NGET submitted several network reinforcement options to resolve the identified network need to the Network Options Assessment (NOA) which evaluates multiple solutions for the same network need to determine the option with the ‘least worst regret’ (LWR) across several scenarios. Alternate options evaluated included PSTs (or Quadrature Boosters as they are more commonly known in the UK) and other network reinforcements such as reconductoring overhead lines and transformer replacements.

As the LWR analysis values flexibility across multiple future scenarios, the SmartValve solutions performed well.  They can be quickly delivered to meet the near-term network need and scaled up over time. The shorter lead time enabled NGET to defer making investment decisions until there was sufficient certainty on need, and resolve near-term grid issues while alternate options would still have been in planning or construction. This also avoided the risk of deploying an oversized solution that could have become a stranded asset in some scenarios.

The deployments at two of the substations were expanded in 2022/23 with additional SmartValves to meet an increased need for power flow control following the closure of a nearby power station. The original deployment had been designed to enable the easy scaling up of the projects so this expansion did not require any additional substation space, instead the additional SmartValves were safely installed ‘on top’ of the original devices. This would not have been possible if a PST solution was used instead.

• The VELCO case: extending the asset life of a PST

VELCO, a utility in Vermont, have experienced issues with a PST at their Sandbar substation connecting New York and Vermont state. This PST failed in January 2021, requiring a replacement PST to be moved over 60 miles and installed to provide power flow control later that year[1]. After two years of repair and maintenance, the original PST from Sandbar substation was returned to service at a different location, costing $3.5 M in total. The failure of the original PST was likely caused by excessive tap changes used to manage shifting power flows on the VELCO network. The shifting power flows were largely caused by the increased integration of intermittent generation (wind and solar) onto the grid.

VELCO predicts that tap changing requirements will continue to increase due to growing wind generation in New York State, which increases the risk of future PST failures. To prevent a future PST failure, VELCO chose to install SmartValves in series with the new Sandbar PST to reduce the number of tap changes on the PST by 99.8% (nearly zero tap changes anticipated) and provide 50% more power flow control capability. This solution also provides 50% redundancy so that the line can remain in-service after a PST failure.

The chosen alternative, adding SmartValves in series with the existing PST, is 16% less expensive than a series PST option and can be deployed in half the time. The SmartValve solution can also double its future output within the same footprint by stacking new SmartValves on top of the existing ones to fully replace the PST once it reaches end of life in 2032. Since SmartValve is a voltage-agnostic device, it will remain a suitable solution if/when VELCO decides to uprate the circuit to 230 kV in the future.

This project will be deployed in 2026 and is funded by the United States Department of Energy’s (DOE) Grid Resilience and Innovation Partnership (GRIP). GRIP funds projects that increase the flexibility, efficiency, and reliability of the US electric power system, with particular focus on increasing transmission capacity. For this innovative project, GRIP is funding 50% of the capital costs. Access the project’s factsheet here. 

In summary, it’s clear that while both APFC and PSTs provide power flow control capabilities, they are quite different in terms of their 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 like FERC Order 2023 and Order 1920, which require consideration of APFC in interconnection and transmission planning studies, respectively. 

Federally funded programs like GRIP are further catalyzing APFC deployment. If you would like to learn more about any of the above, please contact us at info@smartwires.com.

[1] https://www.vermontspc.com/sites/default/files/2024-04/Sand_Bar_PST_Asset_Condition_VSPC_V1.pdf