The Need for Grid-Forming Inverters in the Future Power System

The Need for Grid-Forming Inverters in the Future Power System

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Okay. My name is Ben Kroposki,  and I am serving as the   organizational director for the Universal  Interoperability for Grid-Forming Inverters,   or as we like to say, UNIFI Consortium.  And I'm going to be talking today about   the need for grid-forming inverters in  the future power system, and specifically   why this is the most critical topic we need to  address to achieve 100 percent clean energy goals. So 2020, which was last year, was a seminal  year. A lot of important things happened.  

And let me just touch on some  of these critical things. First, the entire supply in the United States of  energy is shifting, and since – you know, over the   last decade, we've seen a dramatic decrease  in the use of coal, whereas natural gas and   renewables have increased, and nuclear and hydro  have remained steady. But 2020 was the first year,   at least on this graph, and maybe since the use of  coal, that renewables surpassed either nuclear or   coal in energy generation in the US. So  with the 20 percent energy share, that's  

a pretty significant achievement, where it finally  passed coal and nuclear in energy generation. So let's dig in a little  bit more to this renewables.   So if we take a look at the renewable portion of  that, the 20 percent that was annually produced   from renewable sources, you see 8.5 from wind,  7.3 percent from hydro, 2.3 solar, and a little  

bit from biomass and geothermal. Again, 2020  was another watershed year in that wind actually   produced more energy than hydropower. And you  can see over the last several years that wind   and solar are the two forms of energy that are  the most rapidly increasing on the grid today.

So if we take a look at that and we  say why is this really happening,   that renewables are starting to take off, we've  been working on renewables for 30 plus years,   trying to make them cost competitive, but really,  finally, if you look down in the 2019, 2020 area,   this is when the utility scale wind and utility  scale solar have become the most cost competitive   forms of new energy. Now this is across the  entire United States, and depending on what   region you're in, it's going to vary, but in  general, we've seen these tremendous decreases,   70 percent in wind, 90 percent  in solar, over the last decade. And although this is for the price –  levelized cost of electricity for new plants,   and so that's why a lot of new plants that are  going in are either wind, solar, or natural gas,   these prices are so low that they are actually  rivaling the cost of electricity from existing   nuclear and gas plants, and in a lot of cases  around the country are below actual operating   costs for existing coal plants. And this is just  a huge change in the industry that is happening,  

and we are going to see this continue for the  next decade plus, where wind and solar are going   to be increasingly used, because they are the  lowest form cost of new electricity going in. So what's really driving this besides  the economics? The economics is always   going to be the main driver of  practically everything that we do.   But we're also seeing states and utilities  across the United States put together clean   energy targets. So you can see from this graph  here there's quite a number of states that have   state clean energy mandates over 50 percent,  several that are 100 percent clean energy.  

And these green-shaded areas you can see in here,  now those are utility service territories. So you   have places like Xcel Energy here in the Midwest.  You also have places like Duke Energy here in the   Carolinas, where the individual utilities are  setting these 100 percent decarbonization goals.   And so a combination of these has really  led to a push in clean energy technologies. Now in addition to that, we're – we have finally  seen the new administration come up with these   clean energy targets. And if you take a look  at what we're talking about, 100 percent clean   electricity by 2035, and 100 percent clean energy  economy by 2050. And the engineer at least in  

me is sitting there thinking, hmm, are – what do  these goals really mean? How aggressive are they? If you take kind of a little thought experiment  and you say, hmm, 100 percent clean electricity   by 2035, and we have 20 percent nuclear, 7  percent hydro, maybe you've got 10 percent   clean hydrogen fuels of some sort in the  future, but that still leaves 70 percent   that is going to have to be made up  from wind and solar technologies. And this is a really big number, so  let's remember that 50 to 70 percent   of the entire system has to be made of wind and  solar, so what do we need to do to get there? So let's start taking a look at what we  know about running grids with high levels   of renewable systems. First off, we'll start  on the very small end of the scale here. If   you look at the graph here, along the X axis you  can see system size in gigawatts, and it ranges   from maybe 100 kilowatt type system to megawatts,  all the way up to 1 gigawatt here in the middle,   and out to 1,000 gigawatts, which is the installed  capacity of the continental United States. And on the Y axis we're looking at  percent wind and solar. So first off,   we're going to look at small island power systems,   and two very important numbers to set some  context. First, this red square, that is the   amount of wind and solar at any particular time.  So we call that the instantaneous level. Whereas  

these gray circles, those are annual energy,  and so that's going to be how much energy is   produced from wind and solar over an entire year.  But if you take a look at these very small island   systems that range from around 150 kilowatts  in size up to around 10 megawatts in size,   you can see, right, we know how to make systems  that run at really high levels of renewables. So how are we able to do this? So these systems,   number one, they have very good wind and solar  resources. Number two, they also typically have   a lot of energy storage connected to them.  As an example, you can see the El Hierro  

system in the Canary Islands has a pumped hydro  system that's associated with the wind plant.   But in general, these systems really oversize  their wind and solar capacity. For example,   this is – on Ta'u Island, this is over a megawatt  of solar PV for around 150 kilowatt size load. But we do know how to make these types of systems  work. Typically, they only have one device inside   of them, being the grid-forming device, and  everything else just synchronizes to that.

As we go to larger-scale systems, you  can see here on large islands, and here,   some good examples would be Kaua'i in the  Hawaiian islands, Maui in Hawaii, Crete   in Greece, and the country of Ireland,  and the entire Irish power system. As you can see here, we get fairly high annual  levels, somewhere in the 30 to 40 percent range,   and we're incrementally increasing the  instantaneous level of these grid-forming sources.   You can see 90 percent to 60 percent. In fact,  this data is from 2019, and even since then we've   see increases in these levels. As you can see  here, Maui was just as of this year able to get   up to closer to 90 percent, and Ireland also got  up around 70 percent on an instantaneous level.

Bus you can see we're still, even  as we get larger and larger systems,   we're not getting really to 100  percent, and there's a variety of   technical reasons for this that we'll  be going through in this presentation. Now as we go to larger interconnects, I picked  two examples here, ERCOT, which is the Texas   power system, and the Western Interconnect of the  Untied States. As you can see here, these levels   range from 10 to 26 percent annually and then 24  to 58 percent on an instantaneous level, although   this is older data, too. And since even this year,  ERCOT was up around 66 percent instantaneously. But at the end of the day, what we need to be  able to do is look at these large-scale systems,   figure out the technical constraints, and get  them to operate as if they were some of these   smaller-scale island power systems. And  this is a non-trivial problem to solve,  

but what we want to be able to do is come  up with technical solutions to get us to 100   percent inverter based on an instantaneous level  with really large power systems. And to do this,   we need to really unify the operations of  inverter based systems and synchronous generators. So let's do a quick analysis, just kind of  a thought study here, back of the envelope   calculation, just to see why is getting to  100 percent so important. So let's do a little   quick case study. This is using some data from  EirGrid, which is the utility operator on the  

Irish power system. Again, if you look at  their 2018 data, which is what we're going   to look at, they were running around 65 percent  instantaneously, which, again, this is actually   limited by the system operator, which got them  to around 30 percent on the annual energy basis. But by 2030, they want to be getting up  to 70 percent on an annual energy basis,   and 100 percent instantaneously. And why  70 percent? Well, that's on your way to  

100 percent clean energy system, and  if you remember from an earlier slide,   for the United States, probably up where  we need to be, in the 2035 time period. So let's just take a quick look at this example  to understand why getting to 100 percent is   important. What you see here is the wind  data from 2018 in the Irish power system,   and you can see they're limiting this level of  wind on the particular system to 65 percent.   This is actually limited by the system operator  themselves. But you can see that for the most part   the wind is going up and down, crazy, throughout  the year, going from January to December.

Now what happens when we start adding in  enough wind to get us to 70 percent on an   annual energy basis? Remember, this was  around 30 percent in the red line here.   The blue line gives you around 70 percent on an  annual energy basis, and back of the envelope,   we just quickly multiplied these numbers by around  2.5 – or 250 percent, or 2.5 times the amount,   and that would give you enough energy to provide  70 percent of the annual energy needs by wind.   But you can see all of a sudden we are getting  way over 100 percent at any particular time. Now this graph is pretty crazy to look at,  so let's reorganize this data a little bit   to make it a little simpler. What we're going  to do is create something similar to what   especially people in the power system know as a  load duration curve, but we're going to create a   variable renewable duration curve. So we're  going to resort this data from high to low.  

As such, we're going to slowly move all the data  around. And then we create this curve, right?   We've resorted all these blue data points into  this curve, which is called a variable renewable   duration curve. But it really quickly helps you  understand some important parts of the system. So now what I've done is I've transposed that  70 percent renewable duration curve on here,   and then the 30 percent or 29 percent duration  curve. And if you take a look at this,   some important things come across. First off,  

okay, so this amount of energy is provided by  wind, which is 30 percent of the area under   this flat line. And the rest of this has got  to be provided by other sources, whether it's   coal or natural gas or whatever resources you have  available. But at the same time, you really – you   can't go over 100 percent, so you're having to  basically curtail all of this electricity here.

Now if all of a sudden we were able to install up  to 70 percent wind, we wouldn't magically get that   much energy, because if, for example, we're still  limiting the instantaneous level to 65 percent,   we would only pick up this amount of the curve  here. And so now instead of 30 percent, we   actually get up to around 45 percent on  an annual basis. And still, you need other   sources to provide the rest of that energy.  And we had to curtail all of this wind here.

Now if we think about what we really want to  be able to do, what we want to be able to do is   come up with some type of technical solution  to get us to 100 percent, because all of a   sudden that frees up a really big chunk of this  annual energy that we're currently curtailing.   So if all of a sudden we were able to go to 100  percent, instead of 45 percent, we're now up to   58 percent, and that's getting us a lot closer to  this 50 to 70 percent overall need on the system. Now of course, I'm just doing it for Ireland here,  but if you do this for any particular system,   you will get something similar to this, where  going to 100 percent instantaneously will allow   you to pick up the most cost effective portion  of electricity. Now you can see we're still   curtailing a lot of electricity in this scenario,  but at the end of the day, that has to be solved   with things like energy storage, increased  transmission, or demand response or moving load. But the biggest and cheapest and quickest way to  access the most renewable energy will be through   getting this to 100 percent instantaneous levels  of inverter based resources. And that's really why   I'm saying that this is the most critical  problem to work on in the short term.  

Longer term, we're going to have to solve  all these additional problems to get   and move this excess energy over into this portion  here and reduce our dependency on other sources. Okay, so that kind of explains why we want  to try to get to 100 percent and why that's   so critically important. If we take a  look at inverters and machines together,   let's understand why this is a  challenge and what we need to address. So if we take a look at currently operating  large-scale power systems, for the most part,   you have – they're based on synchronous  generator technology, so whether it's hydropower,   coal, nuclear, natural gas. All of these  have a synchronous generator inside of them. And when we electrically connect them together,  so you can imagine this being a transmission line   connecting two generators together, we  create a large virtual power machine   that is all synchronized  together and outputting an AC   waveform, whether that's at 50 Hz like  in Europe, or 60 Hz in the United States.

When you connect induction machines or inverter  based technology, they don't necessarily have   a physical connection that is created by  these synchronous generators. So you have a   somewhat looser connection, if you will. But what  we need to do in the future is as we develop PV   and wind technologies and inverters, that  they sort of replicate what's going on in   synchronous generators, although they don't  have all the physical characteristics.   But if we can come up with sort of rules of the  road on how to unify the operations of synchronous   generators and other synchronous machines  and inverter based resources at any scale,   that will enable us to either run at 100 percent  inverters, 100 percent synchronous generators,   or everywhere in between, which is what  we're going to need in future power systems,   with wind and solar resources  varying throughout the day.

There are a variety of technical challenges  with inverter based technologies, and this   particular consortium is going to focus  on trying to address a lot of these.   I'll highlight them here, even though I  won't go into massive amounts of detail.   We'll be doing those in future  parts of this seminar series. But if you think about the system, we also –  we always need to be able to maintain frequency   stability and voltage stability. So those will  be critical aspects. Now although we can do that   actually currently with grid following inverter  technology by adjusting the control algorithms   in there to actually what we call respond fast, or  have fast frequency response characteristics that   would emulate some of the inertial characteristics  you would see in synchronous generators. But there's additional considerations that  need to be taken into account, things like   system protection. So I often use this graph  as a way to just quickly understand why system  

protection is so important, and what inverters  bring into a system and why it's a challenge. If you take a look at this graph here, what  we're assuming here is that a fault happens   on this system here at basically time zero, and  when a fault happens on a grid with a synchronous   generator, you get some output characteristic  of the current that looks like this.   So it's maybe six times rated output  current for several cycles as it decays. But this huge spike in current very quickly allows  protective relaying devices or fuses to recognize   a fault has occurred and operate properly  in order to isolate that part of the system.   When you deal with inverter technologies, as  you can see down here, you can have several   response characteristics. So one response  characteristic may be this red line,   where the inverter sees that spike happen,  and actually reacts extremely quickly,   and turns itself off in a quarter cycle, and  then just goes flat in terms of current output.

Another thing, it could see that short circuit and  actually keep providing current into that fault,   but roughly at around its rated power  level, for seven cycles, and then drop off.   But the challenge here is that neither of  those curves looks like this blue curve. And   so it's much more difficult to detect if you  have a system with high levels of inverters   what the actual fault currents are. So we need to  look at a variety of different ways to protect the   grid in the future when it's running with  high levels of inverter based resources. Another important thing that we need to  work on is this grid forming capability,   which is the whole reason for this consortium  that we're talking about. But in addition to   those grid forming capabilities, the ability  to black start a system, so bringing it up   from scratch, from a blackout condition.  Historically, we use either hydropower  

plants or natural gas plants or something that  we're starting to help restore the grid power,   and then we bring it all back online. But  with inverter based grids, we need to look at   having that capability of black starting the  entire system, including providing things   like transformer inrush currents when you're  restarting those, or line-charging currents. There's additional issues around the various  control systems that are implemented in inverters,   and potentially interactions with  those, as well as cyber security   concerns when you have so many  distributed technology devices. Let me just real quickly go back to this black  start capability, because I wanted to highlight an   example of grid forming technologies. This is  actually an example of not even a project. This   was actually a necessity that we had an NREL,  where we were able to run our building loads on a   large PV plant plus a wind turbine plus a battery.  For over 72 hours we provided these building loads  

in this 100 percent wind-PV-battery grid during a  site outage that we had at our Flatirons campus.   But we were able to actually  connect all of these together   and operate this type of system, and were  able to demonstrate black start capabilities. One of the unique things in this particular   system was the way that we were developing  the controls so that they would actually   maintain stable operations, even though you had  pretty wildly fluctuating wind and solar output.   You can see down here this orange line is the  solar output, and then the green is the wind   output from the turbine, and then the battery  system was either absorbing power or producing   power to maintain this nominal building load that  you see as this steady state condition in gray.

But even though we were able to demonstrate  this, one of the things that we would like to   eventually do is have all of the inverters that  were associated with these different components   actually all be independently grid forming and  compatible with each other. In this particular   case, we were able to run this system because we  had one device, the battery system, that acted   as the grid forming and grid  reference signal in the system. So I've explained a little bit about what's  going on in the power system industry currently   right now with renewables. I've explained  what we know about how to operate grids.  

And I've explained why getting to 100 percent  inverter based grids is critically important. But   let's talk about what we really are starting here,  which is the UNIFI Consortium, a way of bringing   the industry together to solve all of these  challenges and enable these types of systems. So our vision for the future is  future power systems that have any   mix of machines and inverter  based resources at any scale   that enable this affordable, secure, reliable,  clean, and resilient power grid. If you take   a look over here, you can see a recreation  of those graphs I was showing you earlier,   but what we need to be able to do is address  these fundamental challenges that are limiting   these types of large scale grids for getting  to a near 100 percent system, and basically   create a way to seamlessly integrate grid forming  technologies with the power system of the future,   and get these systems operating at as close to  100 percent instantaneously, which will enable the   annual averages in terms of renewable  technologies to get above 50 percent.

So inside the UNIFI Consortium, we plan  to conduct research and development,   demonstrate these grid forming concepts at  scale, develop best practices and standards,   and train the next generation workforce  for how the future power grid will operate. So let's just think back in history  a little bit, and we're really   not doing something entirely new here. If you  think and go back all the way to the early   1900s, when the power system was just  forming, we're really standing on the   shoulders of giants. You take a look here  at the precursor of what became the IEEE,   and people got together and really started putting  together presentation material about how to start   to parallel all of these synchronous generations  that were popping up all over the country. This interoperability between all the synchronous  generators really led to the interconnections that   we know today, and that operate and provide power  across the United States and around the world.  

And it's really the sustained engagement  of researchers and practitioners – it was   super critical for how we can interconnect  generators to realize this future power grid.   And UNIFI is going to provide this platform for  engagement to realize the grid of the future,   where we're integrating inverter based resources  and synchronous generators completely together. So what do we plan to do inside this consortium?  If we think about it, we're going to unify the old   technology, the synchronous machines, and the new  technologies, with grid forming and grid following   inverters. We're going to unify local controls  and global controls across the system. We need  

to unify how these things operate at both slow and  fast time scales to be compatible with each other.   We also need to unify how large scale inverters  and aggregations of these technologies integrate   with much smaller, because inverters are  unique. They can be all the way from 200 watts   up to several megawatts. And finally, we're going  to unify how solar, wind, and storage technologies   integrate with the electric power system  to create this clean grid of the future. I want to highlight the partners that we have.  A lot of you are on the phone here today. but  

as you can see, we have a huge collection of  people spanning the national laboratories,   the universities there in the middle, and  a huge number of our industrial partners,   both on the inverter manufacturing sector as  well as utilities and system operators, as well   as the industry labs and real time simulation  and software vendors. But we need this huge   cross-section of the industry to actually make  this a reality, because what we're trying to do in   this entire consortium is change the fundamental  way that the entire power system operates. So I'll talk at a high level about what's going  on in this particular consortium. We have it   divided into three main thrusts: research and  development, demonstration and commercialization,   and outreach and training. And inside each  of these thrusts there are key research  

areas – let's just look at the research and  development one. Modeling and simulation,   controls, hardware, integration and validation.  And we have several leads across this space.   These are not the only people working in here.  We're actually tying in all of the partners   that you saw on the previous page to contribute  into each of these things as they get developed. In addition to the project partners, which are  partners that are either receiving DOE funds or   contributing cost share, we have an additional set  of industry partners that we're reaching out to   across the entire scope. So additional sets of  utilities, additional vendors, additional software  

providers and system operators, as well as other  consortia that are in this space, in power systems   or in power electronics. But we are looking at a  kind of connection between the power system world   and the power electronic world in a way that  allows for this entire grid to operate smoothly. The other thing I want to mention is the  geographic diversity of the partners that we have   on this project. All across the United States, you  can see little dots here that represent different  

locations, but it goes all the way from Alaska to  Puerto Rico, from the Hawaiian Islands to New York   and the New England States. But in addition  to that, we have strong ties to international   organizations around the world, including the  Global Power System Transformation Initiative,   as well as several of the research institutes  around the world, because everybody is kind of   going through this transformation simultaneously,  and we want to make sure that we are making best   use of those interconnections, both internally in  the United States and externally around the world,   to make sure that we as an entire world  can make this transition together. I will highlight a couple of the  things that we plan on doing in this   consortium, and a lot of this will  be, again, further talked about   in future presentations in  this particular seminar series.   But we're establishing the US as the leader in  this space across the PV, wind, and storage, for   cutting edge research in grid forming systems. We  want to foster this ecosystem that ties together  

the R&D, the commercialization and demonstration,  the outreach and training in this space. The other thing that's most important and a piece  here that I'll talk about on the next slide,   also, is around the idea of developing these  interoperability guidelines at a system level,   along with the functional requirements at  the individual inverter level. And these   will be critical for how we actually maintain  this operability across the entire system. In addition, we want to cultivate this inclusive  culture. As we've opened up this particular   seminar series here, we want to invite more and  more people into this sort of family, to listen   in around grid forming technologies, to create  this cooperation around sustained innovation.  

And we want to convene this continuous  collaboration between the inverter   manufacturers on one end and systems operators  and utilities on the other, to bridge the gap   between power system and  power electronic industries. So let me just talk a little bit about these  interoperability guidelines and functional   requirements, because this is kind of the key  piece of this consortium that will be developed   over the next several years. Even this year,  we'll start at this, and then through research,   demonstration, and collaboration, we'll  be able to eventually translate a lot of   these type of breakthroughs into more  standardized products and standards. So if we take a look at interoperability  guidelines, what we really are focused there   on is how do we promote the coordination and  seamless operation of many different grid   forming technologies from multiple vendors  while ensuring stability and reliability?   So this is the key thing here, right? From  a systems level perspective, they just want   all of these different technologies to connect,  make sure the system is stable and reliable,   but from many, many different vendors that are  going to be providing inverter based technologies. So we'll be looking at how do you develop  scalable secondary controls, how do you   maintain system level stability, how do you  look at voltage and frequency regulation,   black start capabilities, and ensure cyber  secure communications across these areas? When you go down to the functional requirements  from individual grid forming plants or individual   inverters themselves, how do we make sure  that we're specifying how these requirements   operate so that they are developed in a vendor  agnostic fashion and satisfy all the system level   requirements coming from the top level system?  And inside there, you're talking about real   time control with dynamic protection, autonomous  primary control signals, how do you manage the   input and output from communications signals, how  do you look at aggregations to maintain stability,   and what type of power quality and  protection requirements are needed? So this is really the heart of the consortium  itself, and really, this is going to be – to   develop this, we're going to need the entire  industry to get on board and move this forward. So just to sort of start  wrapping this up a little bit,   as you saw here, our UNIFI Consortium, starting  off with 4 national labs, 12 universities,   6 inverter manufacturers, 8  utilities, 2 system operators,   3 real time simulation and software vendors, but  the reality is we are trying to get the entire   industry connected to this project. So we have  over 80 letters of support on this particular  

project, and we'll be reaching out to many more  parts of the industry as we move this forward. This particular group of people was the initial  development around droop control and virtual   oscillator control that you may have heard so  much about, if you're in this grid forming space.   We've got lots of experience in the past  doing large demonstrations of inverter based   resources, providing grid services. The team  has extensive experience in the standards   organizations and leadership roles in IEEE  1547 and the new P2800 standards. And we've  

really tried to put together a broad coalition  of the world-renowned experts in this space. The key goal here is to unify the integration  of machine and inverter technologies,   and as we've seen in the past, interoperability  drove the interconnections that we have today,   and interoperability will drive the  innovation of the grid of the future.   And we think that UNIFI can be a key  driving force in the US power sector and   renewable energy industries to achieve  this 100 percent clean power by 2035. So our first step in this entire journey has  been with you here today. We've just kicked off  

our seminar series. As you can see here, more  information will be coming out about all of the   follow-on talks in this particular series. And  then we plan on continuing this past the fall   into next year and beyond as a way  to get people to talk about grid   forming technologies and learn  from each other in this space. So I want to thank all of the additional  speakers that have signed up for this fall,   and look forward to hearing all of these  future presentations. And with that,   I'll say thank you, and we'll  take any questions. Thank you.

2021-09-14 16:51

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