IBRs with Planning

Problems can be solved, if we acknowledge and address them

Without Planning: Rinse and Repeat

In a recent post, Inverters on the Grid, I described some of the problems with Inverter Based Resources. My example of IBR failure was the May 2021 “incident” in Texas which led to 1100 MW of wind and solar tripping off the grid. In that case, a surge arrester failed at a gas-fired power plant in Odessa, Texas. This failure (at a 190 MW plant) led to 1100 MW of wind and solar tripping off the grid because their inverters couldn’t ride through the brief fault.

I ended my post with a quote from the NERC report on the 2021 Odessa incident. NERC wrote: “In many cases, industry is not proactively identifying abnormal performance of inverter-based resources…the recommendations outlined in NERC reliability guidelines are not being adequately adopted….Plants stated that no mitigating actions are being done (or planned) to improve the performance of the resources involved in the event.”

NERC (North American Reliability Corporation has many prescriptive industry standards that are approved by FERC. Failure to comply with these standards is a finable offense. However, in terms of ride-through events, the performance requirements are NERC guidelines which are recommendations for the industry and are not enforceable.

When I wrote my previous post, I was concerned that incidents like this could be repeated. I should also have been concerned that I was not reading Doomberg closely enough! As Doomberg reported (Inverted Priorities) in January of this year, the 2021 Texas incident was repeated in June 2022. The second time, it had even greater consequences. In 2022, Texas lost 2,555 MW of generation, compared to 1,300 in 2021.

Why did the situation get worse the second time? Part of the reason was the unintended consequences of the changes that were implemented after the 2021 incident.

About Inverters

There are two kinds of inverters:

• Grid-following inverters (IBRs)

• Grid forming inverters. (GFMs)

While the difference between the two types seems complex, it isn’t too bad. An AC current is basically a sine wave. As with any wave, it has a frequency and (if you look in comparison to other waves) it has a phase. That is, two waves can have the same frequency, but if they don’t have sine peaks at the same time, they are said to be “out of phase.” The phases are important because of leading and lagging and cancellation and reactive power. Don’t worry about that stuff. (Maybe we’ll look at it another time.) The important part is that an IBR must follow everything that is going on with the circuit that it joins. It will have the same frequency, phase, etc. The IBR follows the existing situation.

On the other hand, a GFM (grid-forming inverter) can make its own frequency, phase etc. Why would you want an inverter that can do that? Well, if the grid goes down, the IBR goes down too. It has nothing to follow. In contrast, a GFM can make 60-cycle AC for your home.

Ups and Downs with GFMs

But what about that phase stuff? If your home is islanded with its own GFM, no worries. (Well, very few worries. The line feeding your home may well remain energized, endangering linemen who are coming to fix the grid.) But what about GFMs and the grid? It’s time to get back to Texas and the NERC report on the 2022 Odessa disturbances.

In 2021, inverters in Odessa tripped due to Phase Loop Locking (PLL). PLL is a system built into inverters that is designed to protect them from loss of synchronism. However, in analyzing the 2021 incident, it was determined that the PLL was causing inverters to trip during a disturbance. (Let me recommend Tech Target on What is a Phase Lock Loop? It is the clearest explanation that I have found. )

Since they had caused a problem, all PLLs on the Texas Bulk Power System were disabled after the first Odessa incident. This did not solve the problem completely. Another level of the problem became visible.

(Emphasis added in the sections below):

In 2022, to quote the NERC report (page 8) “In the 2022 Odessa Disturbance, inverters did not trip on PLL loss of synchronism since many of those protections had been disabled; however, the inverters from this same manufacturer tripped on passive anti-islanding function, which misinterpreted the grid phase angle shift upon fault recovery as an islanding signature.”

Guidelines already exist for this issue. As the NERC report states (pages 8 and 9)

“NERC Reliability Guideline: BPS-Connected Inverter-Based Resource Performance, published in 2018, specifically stated that passive anti-islanding protection should be disabled for all BPS-connected inverter-based resources since this is predominantly a distribution-centric form of protection that is not appropriate for the BPS.”

To rephrase that last paragraph: NERC is stating that passive islanding protection is good for the distribution grid (the local grid that your house is connected to) but inappropriate for the Bulk Power System (BPS), which includes transmission lines and power plants.

Synchronous Condensers

At the end of Doomberg’s post on Inverted Priorities, he describes how ERCOT recommended that installing synchronous condensers would effectively boost the reliability of the West Texas System. They would indeed. Synchronous condensers are used in many ways to bolster a grid.

Synchronous condensers are basically big spinning machines. They adjust the invisible part of the grid (VARs) which is how system voltage is maintained, and they increase resilience to frequency changes due to their inertia. The West Texas grid already has two synchronous condensers. ERCOT has suggested that six new condensers would be needed. Synchronous condensers help an inverter-dominated grid behave more like a grid with traditional generators.

In Shorting the Grid, (page 31), I described how ISO-NE often curtailed Kingdom Community Wind because the wind farm had not installed the synchronous condensers that they had promised to install. Angry words went back and forth between ISO-NE and the governor of Vermont. Governor Shumlin claimed that ISO-NE was getting in the way of Vermont’s renewable mandates.

Kingdom Community Wind (63MW) later installed the synchronous condensers. The wind turbines were then allowed on the grid more often. However, with no new transmission being built, the wind farm was still curtailed more than they would have liked. Synchronous condensers are only part of the solution.

The condensers are also an expensive part of the solution. KCW has 21 Vesta turbines, 3 MW each. Looking up Vestas, I found estimates of $2 million to $4 million for turbines this size. If we assume 21 turbines at $3 million each, that would be $63 million. The synchronous condensers were widely reported to be $10 million for this wind installation. That is a significant addition to the capital costs.

Inverter problems can be solved

Using inverters for big solar installations is a relatively new thing, but inverters have been used on the grid for years. Inverters are often used for connections between grids.

For example, New England is connected to the hydropower-dominated grid in Quebec. We import bulk power from Canada.

The generating plants in Quebec are approximately 1700 kilometers from the intertie points in the New England system. These are very long distances to move electrical energy. If the two systems were connected via traditional AC transmission, there would be more loss of power due to the distance. There might also be transient stability issues. Small disturbances could cause the AC tie line to trip offline often. By using HVDC transmission with inverter-based equipment, the system is more resilient and reliable.

In other words, we use inverters to import a lot of power from Canada. The Canadian power is converted to DC, and then that DC power is converted to AC in the United States. Problems can be solved.

And DC lines are not just something that happens between New England and Quebec. Grids are connected by DC lines all over the world. But it takes planning. Connecting two grids with a DC line is a big deal. It is overly simplistic to think that we can just add big batches of inverter-based power because we are adding renewables.

Of course, the grid operator will try to set things up so the new renewable power will fit in. But as we can see from Texas, this can be far more complicated than it looks at first. We need more than a bunch of DC lines and inverters. We need planning. And we need expensive equipment such as synchronous condensers.

Musings in Conclusion

One of the main problems with inverters is a sort of three-body-problem that is a hallmark of all electrical circuits. Change one thing on a circuit, and you change other things. The consequences of a change are not completely predictable, especially if a fault arises on the circuit. This is often stated more mildly as “circuits are not linear systems.”

Modeling the circuit is an important aspect of power engineering. You can’t always predict it, but you can model it. At least, you hope so. Extensive modeling is undertaken before a DC intertie is built. We need similar modeling when we have extensive use of IBRs on the Bulk Power System. Of course, the grid operators try to model the effect of IBRs, but DC tie lines are much rarer than solar installations. Modeling renewables on the bulk power system is a big job.

We must respect our power circuits. We can’t take them for granted. We can’t take the work of the engineers for granted.

Thank an electrical engineer if you meet one! They tend to be shy, but everyone likes to be thanked.

And yes, it is time for me to thank two electrical engineers who reviewed an earlier version of this post. Russell Schussler (“planning engineer” on social media) and John Simonelli, recently retired from ISO-NE operations. Any errors in the post are mine alone.

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Some of my recent appearances:

Sprott Radio Podcast: Wisdom from the Electric Grandma Great fun with electric kettles! Interview by Ed Coyne. 26 minutes

Path to Zero Podcast: Increasing Electrification Demands on the Electric Grid Interview by Tucker Perkins of the Propane Education and Research Council. I was the first to be interviewed twice on this podcast. 53 minutes. I was also honored to write the forward for Perkins’s recent book.

Vermont Viewpoint: Electric Grid in Vermont and elsewhere Interview by long-time friend Pat McDonald. Pat is my go-to authority on everything Vermont. (Yes, I live here. But I don’t know Vermont the way she knows Vermont.) 43 minutes.

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Graphics and a note

Wave form graphic: https://en.wikipedia.org/wiki/Phase_(waves)#/media/File:Phase_shift.svg Peppergrower own work

Synchronous Condenser from Siemans press release: Siemens Energy awarded contracts to enable Uniper to provide grid stability services in Great Britain

Miserable note. I was trying to check the NERC report and the link gave me this statement: NERC.com is down for routine maintenance from June 22nd, 2024 at 12AM (Eastern) until June 23rd, 2024 at 6PM (Eastern). Sorry for any inconvenience.

Luckily, I have extensive quotes from the NERC report within this post.