Steering Electricity: How Grid Control Devices Unlock Transmission Capacity

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I’m preparing to speak to engineers at GE Vernova during Engineering Week at the request of Cornelis Plet, CTO of GE Vernova Grid Systems Integration. It is a useful moment to step back and look at a class of technologies that rarely make headlines but quietly shape how modern power systems operate, and to deepen my own understanding. The conversation around electrification often focuses on generation capacity or new transmission lines, but a growing part of the engineering challenge is extracting more performance from the infrastructure already in place.

Grid enhancing technologies fit squarely into that category. They are tools that allow operators to move more electricity through existing transmission networks without building entirely new corridors. Among them, Flexible AC Transmission Systems, usually called FACTS, and newer Advanced Power Flow Control technologies are important because they address a specific limitation of alternating current networks. They help operators do more with the wires already strung across the landscape, and they often do it with much less social resistance than constructing new lines.

The core constraint behind these technologies is straightforward. Electricity demand is rising in many regions as transportation, buildings, and industry electrify. At the same time the geography of generation is shifting. Wind and solar resources are often located far from cities. Hydropower plants sit in remote regions. Large nuclear plants and fossil plants are often distant from the loads they serve. Transmission networks were built over decades to connect those resources to demand centers, but expansion of those networks has slowed. Building new high voltage lines now often takes ten to fifteen years in North America or Europe once environmental review, permitting, and legal challenges are included. In many cases the bottleneck in an electricity system is no longer generation capacity but transmission capacity. Wind turbines may be curtailed in Texas when west Texas lines fill up. Renewable generation in northern England may be limited when flows toward southern demand exceed corridor limits. Hydropower transfers in Brazil or Canada may run into stability constraints on long lines. In each of these cases the wires exist, but the grid cannot always use them efficiently.

The reason lies in the physics of alternating current networks. Electricity does not follow instructions from grid operators. It follows the path determined by impedance in the network. Impedance is how much a power line resists the flow of electricity, similar to how the width and roughness of a pipe affect how easily water flows through it. When power enters a transmission system it spreads across every available path according to the electrical characteristics of each line. The process resembles water flowing through a network of channels. If two parallel rivers connect the same lakes, water distributes itself according to the width and slope of each channel. Operators cannot tell the water to take only one route.

Electricity behaves similarly. When a generator sends 1,000 MW into a network with several parallel lines, the flows divide automatically. One line may end up carrying 600 MW while another carries 300 MW and a third carries 100 MW depending on impedance. If the first line has a thermal limit of 500 MW the operator must reduce total transfers even though the other lines still have capacity. The constraint is not the physical absence of wires. The constraint is the inability to steer the flows precisely.

FACTS technologies were developed to address related stability and voltage problems that arise in large AC grids. The term Flexible AC Transmission Systems refers to a group of power electronics based devices that control voltage, reactive power, or impedance on transmission lines. They appeared in commercial deployments in the late twentieth century as semiconductor switching devices became capable of handling very large electrical currents. Examples include Static Var Compensators, Static Synchronous Compensators called STATCOMs, series compensation systems, and phase shifting transformers.

These are power electronics or transformer based devices installed at substations or directly on transmission lines to control how electricity behaves on the grid. A Static Var Compensator or STATCOM can rapidly inject or absorb reactive power, which helps stabilize voltage and prevents parts of the network from becoming unstable when large amounts of electricity move across long distances. Reactive power is the portion of electricity that moves back and forth in the grid to maintain voltage, similar to the pressure in a water pipe that keeps water ready to flow even when it is not yet moving to a tap.

Series compensation systems place capacitors in series with a transmission line, effectively lowering the electrical resistance the line presents to alternating current so that more power naturally flows through that corridor. Phase shifting transformers change the timing of the electrical wave between two parts of the grid, which nudges electricity to choose one transmission line over another. A useful way to think about it is traffic lights at a highway merge. By adjusting the timing of the lights, you can encourage more cars to take one lane and fewer to take another. The transformer does something similar for electricity, subtly shifting the timing of the electrical signal so power spreads more evenly across parallel transmission lines.

These devices do not generate electricity and they do not replace transmission lines. Instead they slightly adjust the electrical characteristics of the network so that power flows distribute themselves more evenly across available lines, allowing the grid to move more electricity safely through infrastructure that already exists. The purpose of these devices is not to increase the thermal capacity of conductors. Instead they stabilize the system so operators can safely operate lines closer to their real limits. Voltage stability and oscillations can constrain a corridor long before the wires reach their thermal rating. When that occurs a FACTS device that injects or absorbs reactive power can stabilize the voltage profile and allow more megawatts to move across the same corridor.

Real deployments illustrate the scale of the effect. A well known case is the Manitoba Minnesota transmission corridor connecting Canada and the United States. Installation of a Static Var Compensator on the 500 kV interconnection increased transfer capability by roughly 200 MW from a roughly 1,000–1,500 MW base according to engineering documents filed with regulators in Ontario and Minnesota. The device provides dynamic reactive power support that stabilizes voltage during disturbances, allowing operators to run the line closer to its thermal limit without risking collapse.

A similar case occurred on the transmission network serving Mexico City. Engineers installed a large Static Var Compensator rated at about 600 Mvar near the Temascal hydroelectric complex, which feeds power toward the capital along a 400 kV transmission corridor. Before the installation, operators limited transfers along that corridor to roughly 1,300 MW to avoid voltage instability during disturbances. By stabilizing voltage and supporting reactive power on the line, the SVC allowed the system to operate closer to its physical limits, increasing the safe transfer level to about 1,500 MW. In practical terms, the device enabled roughly 200 MW of additional power to flow toward Mexico City without building new transmission lines. In both cases the additional power flow came from improved stability rather than stronger conductors.

Another widely cited installation is the STATCOM at the Marcy substation in New York State. The project deployed roughly 200 MVA of dynamic reactive power capability on the 345 kV network. The New York Independent System Operator and project documentation describe improvements in voltage stability that allow higher transfers across key paths linking western generation to downstate demand. In stability constrained systems the value of reactive power support is easy to quantify. If the system could previously move 1,400 MW safely before reaching a voltage stability threshold and a STATCOM allows the same corridor to operate at 1,600 MW, that 200 MW difference represents a meaningful expansion of transmission capacity without installing new lines.

The impact of FACTS is also visible in grids with large renewable penetration. The Texas grid provides a clear example. During the expansion of wind generation in west Texas, several Static Var Compensators were installed to support voltage stability on transmission paths carrying wind power toward the Dallas and Houston regions. Documents referenced in energy studies describe four SVC installations that stabilized voltage profiles and enabled greater transfer of wind generation along existing lines. In this case the devices allowed the grid to carry more renewable electricity without violating stability constraints during disturbances.

Brazil provides another illustration because its grid spans thousands of kilometers and connects remote hydroelectric plants to coastal cities. Long transmission corridors introduce oscillation and voltage stability risks. Brazil’s operator has installed a combination of series compensation and STATCOM devices along these corridors. The result is improved controllability of long distance power flows and higher usable transfer limits. Engineering studies published by universities in Brazil and Europe describe how these systems damp oscillations that occur when large hydro plants and distant load centers interact through long AC lines.

The second generation of grid control technology shifts focus from stability toward steering power flows directly. Advanced Power Flow Control devices adjust the electrical characteristics of a line so that electricity distributes itself differently across the network. In simple terms they change the effective impedance of a transmission path. Because AC power divides according to impedance, a small change in one line’s impedance can shift hundreds of megawatts from one route to another. If two parallel lines connect the same substations and one is overloaded, increasing its impedance slightly causes electricity to move onto the other line. Nothing physical about the wires changes. The redistribution occurs because the network equations governing power flow change.

Modern modular APFC devices make this process easier to deploy than traditional FACTS installations. Some designs attach directly to transmission lines rather than requiring large substation installations. Companies developing these systems place series devices on the line that adjust reactance electronically. Grid operators can then influence how electricity spreads across parallel circuits. This capability has been deployed in several systems including the United Kingdom. National Grid has installed modular series controllers on 275 kV lines in northern England where wind generation and demand centers are connected by multiple parallel circuits. The goal of the program is unlocking as much as 1.5 GW of additional transfer capability across parts of the network according to statements from National Grid describing the project.

The logic behind that figure becomes clearer with a simplified example. Imagine three parallel transmission lines connecting two regions. Each line has a thermal limit of 1,000 MW. Because of impedance differences the flows may divide unevenly so that the first line carries 1,000 MW while the second carries 700 MW and the third carries 500 MW. Operators must restrict total transfer to avoid overloading the first line even though the other two have unused capacity. If a power flow controller increases the impedance of the overloaded line slightly, the flows might redistribute to 900 MW, 800 MW, and 700 MW. Total transfer increases from 2,200 MW to 2,400 MW without touching the conductors or towers.

Studies of these technologies provide additional context, though it is important to distinguish modeling results from operational outcomes. Academic studies of SVC and STATCOM installations report increases in transmission line loadability of roughly 10% to 20% in many systems. In cases where voltage stability was the dominant constraint, some studies report improvements approaching 40% to 50%. These figures come from system simulations and specific corridor analyses rather than universal rules. In real systems the improvement depends on the nature of the constraint. If the binding limit is thermal heating of the conductor, power flow control offers little benefit. If the limit arises from voltage instability or uneven flow distribution, the impact can be large.

Understanding where these technologies work best requires looking closely at the topology of the grid. FACTS and APFC devices deliver the most value when multiple transmission paths connect the same regions. Renewable energy corridors provide a common example. Wind farms in a remote region may connect to the rest of the grid through several parallel lines that converge near a major substation. If one path becomes overloaded while others have spare capacity, power flow control can rebalance the network. The same applies to urban bottlenecks where several circuits feed a metropolitan area. In those cases the technology helps distribute flows more evenly across the infrastructure that already exists.

There are also clear limits. FACTS and APFC devices cannot increase the thermal rating of a conductor. If every line in a corridor is already carrying its maximum current, steering flows will not create additional capacity. The only solutions in that situation are reconductoring, building new lines, or raising voltage. While these approaches work best when there are multiple transmission corridors joining regions, they can still provide benefits on parallel lines on the same transmission pylons.

Reliability standards also shape the usable benefit. Transmission planning follows an N minus one rule that requires the grid to remain stable even if a major component fails. If a power flow controller itself trips offline during a fault, operators must ensure the system still operates safely. Planning studies sometimes discount a portion of the theoretical capacity increase for this reason. Operational benefits may still appear in daily dispatch, but long term planning must account for the possibility that a control device is unavailable.

Costs are another consideration. FACTS installations range widely in price depending on size and complexity. Large STATCOM installations can cost tens of millions of dollars or more. Modular APFC devices often cost less but still represent significant investments. Grid operators compare these costs against alternatives such as reconductoring or building new transmission lines. In many cases the economics favor grid enhancing technologies because permitting delays for new lines can extend projects by a decade.

One advantage that rarely appears in engineering equations is social acceptance. Transmission projects in the developed world usually face strong opposition from communities along proposed routes. Environmental reviews, visual impact concerns, and legal challenges can delay projects for years. Grid enhancing technologies avoid many of these barriers because they operate within existing corridors. Installing a STATCOM at a substation or placing modular controllers on an existing line rarely triggers the same level of public resistance as constructing a new 400 kV corridor across rural landscapes.

For this reason utilities increasingly view these technologies as part of a broader toolkit. Reconductoring with advanced conductors can increase thermal capacity by replacing aluminum steel reinforced wires with higher performance materials. Dynamic line rating systems adjust thermal limits based on real time weather conditions. Topology optimization software evaluates switching configurations to route flows more efficiently. FACTS and APFC devices provide stability and steering capabilities. Together these approaches can increase transmission capacity by meaningful amounts without building entirely new infrastructure.

Looking at the electricity system through this lens reveals a shift in how engineers approach grid expansion. The first century of power system development focused on building the physical network. Transmission towers marched across continents as demand grew. The next phase increasingly involves extracting more performance from that network using electronics, software, and control systems. Instead of adding more wires everywhere, engineers are learning how to guide electricity through existing wires more effectively. This isn’t the smart grid, this is the smarter grid, as we’ve been making the grid smarter for decades.

A useful metaphor is the evolution of transportation systems. Early highway networks expanded by building additional lanes and new roads. Modern traffic systems still build new roads when necessary, but they also rely heavily on traffic management centers that coordinate signals, adjust speed limits, and direct vehicles toward less congested routes. These tools do not eliminate the need for new infrastructure, but they allow the existing network to carry far more traffic than its designers originally expected. FACTS and APFC technologies play a similar role in electricity networks. They help operators guide the invisible flow of electrons so that the grid we already built can carry more power than we once believed possible.