[Note that this article is a transcript of the video embedded above.]

On June 4, 2022, a small piece of equipment (called a lightning arrestor) at a power plant in Odessa, Texas failed, causing part of the plant to trip offline. It was a fairly typical fault that happens from time to time on the grid. There’s a lot of equipment involved in producing and delivering electricity over vast distances, and every once in a while, things break. Breakers isolate the problem, and we have reserves that can pick up the slack. But this fault was a little bit different.

Within seconds of that one little short circuit at a power plant in Odessa, the entire Texas grid unexpectedly lost 2,500 megawatts of generation capacity (roughly 5% of the total demand), mainly from solar plants spread throughout the state. For some reason, a single 300-megawatt fault at a single power plant magnified into a loss of two-and-a-half gigawatts, dropping the system frequency to 59.7 hertz. The event nearly exceeded Texas’s “Resource Loss Protection Criteria,” which is minimum loss of power that requires having redundancy measures in place. Another fault in the system could have required disconnecting customers to reduce demand. In other words, it was almost an emergency.

If you lived in Texas at the time, you probably didn’t notice anything unusual, but this relatively innocuous event sent alarm bells ringing through the power industry. Solar plants, large-scale batteries, and wind turbines don’t produce power like conventional thermal power plants that make up such a big part of the grid. The investigation into the 2022 Odessa disturbance found that it wasn’t equipment failures that caused all the solar plants to drop so much production all at once, at least not in the traditional sense. Instead, a wide variety of algorithms and configuration settings in the power conversion equipment reacted in unexpected ways when they sensed that initial disturbance.

The failure happened just before noon on a sunny summer day, so solar plants around the state were at peak output, representing about 16% of the total power generation on the grid. That might seem high, but there have already been times when solar was powering more than a third of Texas’s grid, and that number is only going up. The portion of the grid comprised of solar power is climbing rapidly every year, and not just in Texas, but worldwide. So the engineering challenges in getting these new sources of power to play nicely with the grid that wasn’t really built for them are only going to become more important. And, of course, I have some demos set up in the garage to help explain. I’m Grady and this is Practical Engineering. In today’s episode, we’re talking about inverter-based resources on the grid.

Solar panels and batteries work on direct current, DC. If you measure the voltage coming out, it’s a relatively constant number. This is actually kind of true for wind turbines as well. Of course, they are large spinning machines, similar to the generators in coal or natural gas plants. But unlike in thermal power plants that can provide a smooth and consistent source of power through a steam boiler, winds vary a lot. So, it’s usually more efficient to let the turbine speed vary to optimize the transfer of energy from the wind into the blades. There are quite a few ways to do this, but in most cases, you get a variable-speed alternating current from the turbine. Since this AC doesn’t match the grid, it’s easier to first convert it to DC. So you have this class of energy sources, mostly renewables, that output DC, but the grid doesn’t work on DC (at least not most of it).

Nearly all bulk power infrastructure, including the power that makes it into your house, uses an alternating current. I won’t go into the Tesla versus Edison debate here, but the biggest benefit of an AC grid is that we can use relatively simple and inexpensive equipment (transformers) to change the voltage along the way. That provides flexibility between insulation requirements and the efficiency of long-distance transmission. So we have to convert, or more specifically invert, the DC power from renewable sources onto the AC grid. In fact, batteries, solar panels, and most wind turbines are collectively known to power professionals as “inverter-based resources” because they are so different from their counterparts. Here’s why.

The oldest inverters were mechanical devices: a motor connected to a generator. This is pretty simple to show. I have a battery-powered drill coupled to a synchronous motor. When I pull the trigger, the drill motor spins the synchronous motor, generating a nice sine wave we can see on the oscilloscope. Maybe you can see the disadvantages here. For one, this is not very efficient. There are losses in each step of converting electricity to mechanical energy and then back into electrical energy on the other side. Also, the frequency depends on the speed of the motor, which is not always a simple matter to control. So these days, most inverters use solid-state electronic circuits, and look what I found in my garage.

These are practically ubiquitous these days, partly because cars use a DC system, and it’s convenient to power AC devices from them. I just hook it up to the battery, and get nice clean power from the other end… haha just kidding. These cheap inverters definitely output alternating current, but often in a way that barely resembles a sine wave. Connecting a load to the device smooths it out a bit, but not much. That’s because of what’s happening under the hood. In essence, switches in the inverter turn on and off, creating pulses of power. By controlling the timing of the pulses, you can adjust the average current flowing out of the inverter to swing up and down into an approximate sine wave. Cheaper inverters just use a few switches to create a roughly wave-like signal. More sophisticated inverters can flip the switches much more quickly, smoothing the curve into something closer to a sine wave. This is called pulse width modulation. Boost the voltage on the way in or the way out, add some filters to smooth out the choppiness of the pulses, and that’s how you get a battery to run an AC device… but it’s not quite how you get a solar panel to send power into the grid. There is a lot more to this equipment.

For one, look at the waveform of my inverter and the one from the grid. They’re similar enough, but they’re definitely not a match. Even the frequency is a little bit off. I will not be making an interconnection here, since I don’t have a permit from the utility, but even if I did, this inverter would let out the magic smoke. A grid-tie inverter has to be able to both synchronize with the phase and frequency of the grid and be able to vary the voltage of the waveform to control how much current is flowing into or out of the device. The synchronization part often involves a circuit called a phase-locked loop. The inverter senses the voltage of the grid and sets the timing of all those little switches accordingly to match what it sees. So, these are often called grid-following inverters. They synchronize to the grid frequency and phase and only vary the voltage to control the flow of power. And that hints at one of their challenges: they only work when the grid is up.

I’ve done a video all about black starts, so check that out after this if you want to learn more, but (in general), inverter-based resources like solar, wind, and batteries can only follow what’s already on the grid. When the system’s down, they are too, regardless of whether the sun’s shining or the wind’s blowing. That’s why most grid-tied solar systems on houses can’t give you power during an outage.

There’s another interesting thing that inverters do for solar panels, and I can show you how it works in my driveway. I have a solar panel hooked up to a variable resistor, and I’m measuring the voltage and current produced by the panel. You can see as I lower the resistance, the output voltage of the panel goes down and the current it supplies goes up. But this isn’t a linear effect. I recorded the voltage and current over the full range, and multiplied them together to get the power output. If you graph the power as a function of voltage, you get this shape. And you can see there’s an optimum resistance that gets you the most power out of the panel. It’s called the maximum power point. If you deviate on either side of it, you get less power out. In other words, you’re leaving power on the table. You’re not taking full advantage of the panel’s capacity.

What’s even more challenging is that point changes depending on the temperature of the panel and the amount of sun hitting it. I ran this test again with a few more clouds, and you can see how the graph changes. So nearly all large solar photovoltaic installations use what’s called a Maximum Power Point Tracker (or MPPT) that essentially adjusts the resistance to follow that point as it changes with sunniness and temperature. It’s really a separate device from the inverter, but often they’re located right next to each other or inside the same housing. Even this panel came with a charge controller that has this MPPT function, and you can see it adjusting the flow of current to constantly try and stay at the peak of the curve while it charges this battery. These can be used for an entire installation, but in many cases, each panel or group of panels gets its own MPPT because that curve is just a little bit different for each one. Tracking the peak power output individually can often squeeze a little more capacity out of the system.

Squeezing out capacity is essential to address another challenge associated with inverter-based resources on the grid: frequency. The rate at which the voltage and current on the grid swing back and forth is an important measure of how well generation and demand are balanced. If demand outstrips the generation capacity, the frequency of the grid slows down. Lots of equipment, both on the generation side and the stuff we plug in, is designed to rely on a stable grid frequency, so if it deviates too far, stuff goes wrong: Devices malfunction, motors can overheat, generators get out of sync, and more. It’s so important that rather than let the frequency get too far out of whack, grid operators will disconnect customers to get electrical demands back in balance with the available supply of power, called an under-frequency load shed. Things go wrong on the grid all the time, so generators have to be able to make up for contingencies to keep the frequency stable. Here’s the quintessential example: an unexpected loss of generation.

Say a generator trips offline, maybe because of a failed lighting arrestor like the Odessa example. The system frequency immediately starts dropping, since power demand now exceeds the generation. And the frequency will keep dropping unless we inject more power into the system. The first part of that, called Primary Frequency Response, usually comes from automatic governors in power plants. If we do it fast enough, the frequency will reach a low point, called the nadir (NAY-dur), and then recover to the nominal value. The nadir is a critical point, because if it gets too low, the grid will have to shed load in order to recover. The other important value is called the rate-of-change-of-frequency, basically the slope of this line. It determines how much time is available to get more power into the system before the frequency gets too low, and there are several factors that play into it: How much generation was lost in the first place, how quickly we can respond, and how much inertia there is on the grid. Thermal power plants that traditionally make up the bulk of generating capacity are gigantic spinning machines. They’re basically a bunch of synchronized flywheels. That kinetic energy helps keep them spinning during a disturbance, reducing the slope of the frequency during an unexpected loss.

Maybe you can see the problem with a simple grid-following inverter. It’s locked in phase with the frequency, even if that frequency is wrong. And it has no physical inertia to help arrest a deviation in frequency. If we keep everything the same and just increase the share of inverter-based resources, any loss of generation will mean a steeper slope, reducing the time available to get backup supplies onto the grid before it’s forced to shed load. Larger renewable plants, like solar and wind farms, are increasingly required to participate in primary frequency response, injecting power into the grid immediately when the frequency drops. And some inverters can even create synthetic inertia that mimics a turbine’s physical response to changes in frequency. But there’s another challenge to this.

Dealing with an over-frequency event is relatively straightforward: just reduce the amount of energy you’re sending into the grid. But, response to an under-frequency event requires you to have more energy to inject. In other words, you have to run the plant below its maximum capacity, just in case it gets called on during an unexpected loss somewhere else in the system. For a power company, that means leaving money on the table, so in most places, the energy markets are set up to pay power plants to maintain a certain level of reserve capacity, either through operating below maximum output or including battery storage in the plant.

The last big thing that inverter-based resources have to manage is faults. Of course, you need protective systems that can de-energize solar or wind resources when conditions on the grid could lead to damage. These are expensive projects, and there’s almost no limit to the things that can go wrong, requiring costly repairs or replacement. But, for the stability of the grid, you can’t have those protective systems being so sensitive that they trip at the hint of something unusual, like what happened in Odessa. This concept is usually referred to as “ride-through.” Especially for under-frequency events, you need inverters to continue supplying power to the grid to provide support. If they trip offline, or even reduce power, in response to a disturbance, it can lead to a cascading outage. This is kind of a tug of war between owners trying to protect their equipment and grid operators saying, “Hey, faults happen, and we need you not to shut the whole system down when they do.” And reliability requirements are getting more specific as the equipment evolves, because every manufacturer has their own flavor of protective settings and algorithms.

As inverter-based resources continue to grow rapidly in proportion to the overall generation portfolio, their engineering challenges are only becoming more important. We talked about a few of the big ones: lack of black start ability, low inertia, and performance during disturbances. And there are a lot more. But inverters also provide a lot of opportunities. They’re really powerful devices, and the technology is improving quickly. They can chop up power basically however you want, and they aren’t constrained by the physical limitations of large generating plants. So they can respond more quickly, and, unlike physical inertia that will eventually peter out, inverters can provide a sustained response. There are even grid-forming inverters that, unlike their grid-following brethren, can black start or support an isolated island without the need for a functioning grid to rely on. We’re in the growing pains stage right now, working out the bugs that these new types of energy generation create, but if you pay attention to what’s happening in the industry, it’s mostly good news. A lot of people from all sides of the industry are working really hard on these engineering challenges so that we’ll soon come out with a more reliable, sustainable, and resilient grid on the other end.