Those of us who like to monitor air traffic with ADS-B aggregators such as FlightAware and ADS-B Exchange tend to see some interesting flight paths. I’m not talking about the truly ambitious pictures drawn by pilots, or even the more ribald ones, but rather flights that follow paths that seem to make little sense from either a commercial or leisure standpoint.

Most of these mystery flights have long straight stretches interrupted by occasional tight loops, and often cover great distances across rural and urban landscapes alike. A glance at the ADS-B data indicates that these flights are usually pretty close to the ground, and are often completed by helicopters. Occasionally, the registration of the aircraft will even indicate ownership by some “three-letter” federal agency.

Although mystery helicopters flying odd patterns in the sky seems like a good excuse to don a tinfoil hat and head to one’s bunker, chances are pretty good that these aircraft are engaged in a far less nefarious and far more useful endeavour: aerial transmission line patrols. These flights are key to keeping the transmission lines that form the backbone of the grid in tip-top shape, especially at a time of unprecedented growth in load and a shift in the generation profile away from fossil fuels towards renewables.

Federal Alphabet Soup

Although the grid as we know it today in North America appears to be a monolithic machine, it’s actually a far-flung collection of interconnected sub-grids, operating more or less in concert to provide uninterrupted service to 400 million people. While part of that cooperation can be explained by market forces doing what they do best, a lot of the interoperability that makes the grid work and gives it the reliability we’ve come to expect can be traced to government regulations.

In the United States, the regulations that bulk power system (BPS) operators must follow come from the Federal Energy Regulatory Commission (FERC), a federal agency of the Executive Branch that ultimately answers to the President through the Secretary of Energy. FERC is somewhat analogous to the Federal Communications Commission in that regard, but while the FCC creates standards and enforces them directly, FERC delegates its standards-setting and enforcement authority to a separate body, the National Electric Reliability Corporation, or NERC.

For as critical to modern life as the grid is, the existence of a body dedicated solely to ensuring its reliability is a shockingly recent development. In its current form, the NERC has only existed since 2005, created in response to the 2003 blackout in the Northeast United States. Before that, NERC was the National Electric Reliability Council, which itself only came into being in 1968 in response to a prior Northeast blackout in 1965. Both versions of NERC sound a little like closing the barn doors after the horses have gotten out, but engineering something as large and complex as the grid is largely a learn-by-doing exercise, and NERC’s regulations are what BPS operators use to ensure that their systems are in line with current best practices.

On Patrol

Patrolling transmission lines is one of the main ways that BPS operators make sure they’re up to snuff with NERC rules. These patrols give an up-close and personal look at the transmission lines and the structures that support them, along with the rights-of-way (ROWs) along which they’re built, and any defects noted during these inspections can be scheduled for repair before they cascade into widespread system failures.

Transmission line patrols can take many forms, but the simplest to perform in some regions is probably a ground patrol. Ground patrols are often as simple as a single engineer driving a truck along a transmission line right-of-way, visually inspecting each tower along the way. Ground patrols such as these are limited by what can be seen with the linesman’s Mark I eyeballs or perhaps a pair of binoculars, but they’re still a valuable part of the patrolling process. The “boots-on-the-ground” approach also has the advantage of potentially coming across broken equipment that has fallen from structures, like the nuts and bolts that hold together towers, or even fragments of failed insulators. Occasionally, ground patrols will come across the carcasses of unfortunate animals that have completed a circuit,

But given the huge geographic footprint of transmission lines, some of which span hundreds of miles and often pass over remote and rugged landscapes, ground patrols can be limiting. They tend to be very time-consuming; transmission lines often cross privately owned property, and while the rights-of-way usually allow BPS operators to legally access the property, in practice, coordinating with owners to unlock gates can complicate matters. Add to that factors such as the potential need to cross streams or wetlands, potential for property damage from truck tires, and the fact that inspection is limited to what’s visible from the ground, and ground patrols can be difficult.

The obvious solution to these problems is to get above it all and inspect transmission lines from the air. Airborne inspection offers significant advantages over ground patrols, but the chief benefit is speed. Airborne inspections can inspect long stretches of a transmission line far faster than a ground patrol, and without worrying about access issues. Airborne patrols can also make inspections over rough terrain a relative snap, although such inspections often call for more experienced pilots.

It would seem that aerial power line patrols are an ideal use case for UAVs, and indeed, many of the 300 to 400 aerial inspection companies operating in the United States today offer drone-based inspection services. But even with the vastly less expensive per-hour cost of operating a drone, helicopter inspections dominate the industry today. There are a couple of reasons for this, but the most important are speed and payload capacity. A typically equipped Bell 407 helicopter, for example, carries enough primary and reserve fuel to inspect 170 miles (273 km) of transmission line with a single takeoff and landing. A UAV patrol, on the other hand, usually has to operate within line-of-sight of the operator, and has to land frequently for battery changes. This leads to frequent relocations of the base of operations, resulting in some of the same access problems as ground patrols. It’s also significantly slower than helicopter patrols, taking up to five times longer to complete an equivalent length of line as a helicopter patrol.

Helicopters also have UAVs beat when it comes to payload capacity. Even large UAVs are limited in how many instruments they can carry, whereas a helicopter has effectively no limit. This makes helicopters a multispectral imaging platform, with HD visible-light video to capture images of potential structural problems, forward-looking infrared (FLIR) scanners that look for overheating due to corrosion in a splice or an internal defect in the conductors, and LiDAR scanners that can image the entire ROW and the structures within it. But perhaps most significantly, UAVs can’t carry aloft an experienced linesman, whose training can be key to quickly locating something that needs a closer look from the sensor platforms onboard.

My Corona

The breakdown voltage of air is approximately 30 kV, and while this figure varies slightly with atmospheric conditions such as temperature and humidity, it is generally well below the voltage on most transmission lines in the BPS. That makes flashover a possibility anywhere in the system, and the potential damage caused by an intense high-current discharge to both transmission system components and the surrounding environment makes it critical to detect defects that could lead to it.

Luckily, physics provides an early warning system in the form of corona discharge. Corona discharge occurs when the air surrounding a conductor becomes ionized, turning into a conductive plasma. It can happen anywhere along the transmission system, but it’s particularly likely to happen at places where the electric field is concentrated, such as sharp points. These are generally avoided when designing the system, but faults can occur that lead to their formation, such as broken strands in conductors. Sometimes these defects are visible to the naked eye, but more often, they reveal themselves with characteristic emissions in the ultraviolet part of the EM spectrum.

Corona discharge starts when a strong electric field accelerates free electrons in the air surrounding a defect. If the field is sufficiently strong, the kinetic energy of these electrons causes other air molecules to be ionized, starting an electron avalanche. These excited electrons propagate outward to a distance where the electric field is no longer strong enough to accelerate them, at which point the excited electrons return to their ground state and emit a photon of light. Since air is 78% nitrogen, the photons are mostly in the UV range, with just 5% being in the just barely visible end of the spectrum. This gives corona discharge its characteristic purplish-blue glow.

The other principal component of air, oxygen, comes into play as well. The free electrons in the corona discharge can split diatomic oxygen, leaving behind two negative oxygen ions. Each of these can then combine with a diatomic oxygen molecule to form ozone (O₃), a powerfully reactive oxidizer that can quickly corrode aluminum in conductors and steel in the support structure. The ozone can also combine with atmospheric nitrogen to form nitrogen oxides that, in the presence of water and oxygen, eventually create nitric acid. This strong acid can quickly strip the zinc coating from galvanized steel and attack passivated coatings on parts. Without these coatings, metal parts are unprotected from the elements and can quickly corrode and lose mechanical strength.

Corona discharge can be extremely costly to BPS operators. Specialized corona discharge cameras are used to detect corona faults. These cameras filter out the abundant UV-A and UV-B light in sunlight using a “solar blind” filter. This leaves only shortwave UV-C light below 280 nm in wavelength, which the ozone layer completely blocks out. Any light in this band has to come from nitrogen fluorescence, which makes it an effective way to detect corona discharge.

Corona cameras usually have a UV beam splitter to send light to a pair of detectors, one to capture the visible light coming from the scene and one that captures only the light remaining after passing through a solar-blind filter. The few photons of UV light that make it through the filter are amplified by a UV image intensifier, which uses a photocathode to release multiple electrons for each UV photon. These are accelerated in a strong electric field toward a phosphor screen, which converts them to visible light, which is picked up by a CCD camera and combined with the visible light scene. This shows the corona discharge as an overlay that allows operators to see where the discharge is originating from.

In the Weeds

One of the more stringent sets of NERC regulations is FAC-003-5, Transmission Vegetation Management. It might seem a little incongruous for an organization that sets standards for nuclear power plants and cybersecurity of critical infrastructure to worry about tree trimming, but studies show that vegetation contacts account for 16% to 23% of all outages in the US and Canada. Most of those outages occur in the distribution system, which is bad enough, but if vegetation were to contact lines in the transmission system, the failure cascade could be devastating. For an example of how bad vegetation contacts in the transmission system can be, look no further than the 2003 blackout in the northeast US, which started when overloaded 345 kV transmission lines in Ohio sagged into foliage. A software issue then compounded the problem, causing safety systems to trip and plunging customers from Ontario to the Mid-Atlantic states into darkness.

FAC-003-5 isn’t exactly light reading, going into great detail as it must to define terms and set actionable standards. The gist of the document, though, is contained in just a few tables that list the Minimum Vegetation Clearance Distances (MVCD) for both AC and DC systems. In general, the MVCDs increase with the nominal line voltage, which makes sense; the higher the voltage, the greater the potential flashover distance. More surprisingly, though, is that MVCDs increase dramatically with elevation. This has to do with the dielectric strength of air, which depends on its density. That means the thinner air at higher altitudes has a greater flashover distance, so more clearance is required.

For all the havoc vegetation contacts can wreak, the MVCDs are surprisingly narrow. For a nominal 800-kV line, the MVCD at sea level is a mere 11.6 feet (3.6 m), and only increases to 14.3 ft (4.4 m) over 14,000 ft (4268 m) elevation. These are minimum distances, of course, calculated using equations that take into account the breakdown voltage of air and the potential for flashover to vegetation. In practice, though, BPS operators keep ROWs well-groomed, aiming for to keep trees far beyond the MVCD requirements. Operators are especially watchful for trees at the edges of ROW that might be more than the MVCD away from the lines while standing, but could fall during a storm and make contact.

Assessing vegetation encroachments into the ROW is another job that can be tackled quickly by aerial patrols. The sensor platform in this case is often as simple as a spotter with a pair of binoculars or a camera, but in many cases, LiDAR sensors are used to scan the entire right of way. The LiDAR sensor is tied into the aircraft’s GPS system, resulting in a geotagged point cloud that can be analyzed after the flight. Three-dimensional visualizations of the transmission lines, their supporting structures, the ground below, and everything within and adjacent to the ROW can be viewed interactively, making it easy to spot trees with the potential to cause problems. These visualizations allow users to virtually “fly the line,” giving BPS operators a view that would be impossible to achieve even by flying a drone dangerously close to the lines.