Libraries for HVDC & Modular Multilevel Converters (MMC/SM)

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Libraries for HVDC & Modular Multilevel Converters (MMC/SM)

“The Modular Multilevel Converter (MMC) system offers many advantages over conventional voltage source converters and in DC power transmission, microgrid or renewable energy applications. MMC’s distinctive topology provides a wide variety of new features, necessitating the use of a sophisticated controller for extra control requirements.”

For this article, we spoke with Wei Li, currently working in OPAL-RT’s AXES (Application eXpertise and Electrical Simulation) department, with the team focusing on HVDC tools. He is working on developing MMC (modular multi-level converter) modelling tools for those simulating this converter type and mixed topology.

Interviewer (IV): “Hello and thanks for speaking with us. Would it be right to say, Wei Li, that HVDC and MMC are something that the market wants right now?”

Wei Li (WL): “Yes, MMC (Modular Multilevel Converter) is a technical term for a type of converters, whether they’re from AC to DC systems or vice versa. Normally we say they’re a subset of HVDC—which itself is kind of a link to connect two AC systems. MMC is looking like the future of HVDC technology. There are currently several huge engineering projects commissioned using this MMC technology for HVDC.”

IV: “So why is the market necessarily ready for this now?”

WL: “Because it has several technical advantages–people have been doing their research. And now, people are using these technologies to build. Compared to previous technologies, it’s a bit difficult, and there are several challenges for the controller. For our systems, we provide real-time Hardware-in-the-Loop (HiL) simulation and test benches. The main function of the HiL real-time test bench is so that these manufacturers can validate their controllers before they commission them for real in the field. Especially when they test high voltage, high power–it’s hard, and not necessarily what you want to start with, to test in the field.”

“Five years ago it was difficult, but now when they view the success of the current HVDC projects, they will choose to test their controllers in our systems. This has several advantages for them: the first advantage is that they test at an earlier stage—for prototyping and before there’s even hardware involved. Second, even if you’ve already built your controller, it’s expensive, potentially dangerous, and you can’t test all the scenarios—so being able to prototype-test is another advantage. You can’t test fault scenarios on purpose, because you could interrupt service or damage equipment, among other things. And you have to know your controller or protection system, for example, is going to work well in the worst case scenarios—this is another prototype-stage testing advantage.”

“We already have several HVDC MMC projects under our belts, and they’ve already used our models and simulators as hardware test benches. So we’ve been doing this for several years, and it’s been very successful. But as the technology evolves, for real systems, users have tried other, different technologies. Because of this, our real-time simulator has to follow the trends going on in the markets. So in China, for example, the bid for a contract had already been made by several manufacturers, and now it’s being built next year. With MMC, there are hundreds of modules stacked up for high voltage usage. Before, in one system or one project, all modules would have been the same: either all half-bridge or all full-bridge. And now they’re mixed for some technical advantages. People might have had difficulty simulating these before. Maybe they had a workaround, but it wasn’t straightforward.”

“Now what we do in our models, we can have mixed submodule types: 80% one, 20% the other, etc. So to replicate these scenarios—because in the real world it’ll be mixed types–they have to be able to simulate the mix and to change the ratio of the mix at will. The behaviour of the system during the fault is new and different, and we need to be able to simulate that. So when they design, they test various scenarios to see which is optimal.”

“We already have this new feature, we’re going to announce it, to tell people we can do this. For these new features, people can test mixed modules in one project, and during the simulation, you can change the ratio of modules in a simulation and see what changes, and validate the design based on those changes. Eventually, it ends up with the controller at a hardware-in-the-loop (HiL) test bench.”

IV: “It sounds like we already have this product or test bench?”

WL: “We have a library called MMC in both eFPGASIM and HYPERSIM. Each box can have hundreds of modules. We support both platforms, and we use hardware and software, and now we have full MMC libraries. We knew how we had to do it, but we now have a product ready–a test bench–that is MMC/mixed SM capable that deals with the advances the market has made.”

“So it’s more like a new feature announcement. I’ve been working here for more than 10 years, and I have an electrical engineering background. My specialty is power systems, and when I joined, we had the EMS (electromagnetic systems) team: we dealt with special needs for customers with power system modelling, but at that time it was not specific to MMC modelling. As the team grew, each team member started to be responsible for various specific areas. This is maybe 6-7 years ago. Now my team has joined AXES. Now we’re the FACTS & HVDC team.”

IV: “How do we hear about these new developments in the market, as the market starts to make technological advances?”

WL: “First, we closely watch what the markets need. We try to understand what are the research topics that are current in academia, then we write and read papers on these topics; we go to conferences; we keep our ear to the ground. For this particular topic, it’s a hot topic. There are several MMC projects going on around the world, and many conventional HVDC projects. We have close contact with those utilities and manufacturers, so we’re pretty easily able to stay current.”

“With the industry people, we try to understand what is their planned project. For academic people, we want to see what they’re researching. And understand it into the future by 5-10 years, or however far as we’re able to. Now most MMC is for HVDC use, and STATCOM is another device that provides reactive power. So we also provide the solutions, the tools, etc., for academia, because besides the obvious benefits, it lets us see into the future a little bit with the research topics.”

“When we provide real-time simulation solutions, the first try is always to use the general method: to build the models using the various basic components in the power systems. These theories have existed for many years. When people have to validate their controllers of MMC in real time, this becomes complex and difficult. Thousands of modules and we have to simulate it in real time. It’s almost impossible using conventional simulation methods. We have to look at this specific circuit, do optimization in the model, and do research ahead of time. This is another way to stay ahead of the market. Then we can go beyond conventional methods because we’re already optimized for the mixed topology. Then people will say, ‘I want to build different topologies’ in the field. So when they do this, we have to make it possible for them to simulate all their changes in real time.”

IV: “It also sounds like when we get huge projects, everything scales so much. Is that what you mean?”

WL: “Yes, because there are so many components and they all work together, and they have to do real-time simulation, which is fast. This is why we came up with an optimized way to do it, and we can do it in real time, and with much better resolution and detail. You have to optimize and look at specific operations when you get scaled all the way up. When people come to you and tell you there are mixed modules, you know you have work to do. Luckily we have great R&D people and customers who’ve identified their needs. When they start to take the bids and make the systems, we’ve already done the background work, so we’re caught up and ahead of the curve.”

“Different customers have different needs. So some customers will build HVDC, and they have to validate their controller with HiL. So when they design their systems, the topology, etc., they normally do it by themselves. And for commercial reasons, the design phase is usually confidential. Once this system is ready, they need some HiL: test benches that can precisely simulate their system. We know what they want, we prepare the model, and we have to meet their requirements to simulate their system in real time. Rarely, if we can’t do it right away with available tools, we have to figure out a solution, optimize a model, or develop something. So our main goal or point of success is to simulate their system in real time. We need a solution that is not only fast enough but with enough detail and accuracy–these are the three key points.”

“reaIn academia, it’s a little bit different, they try to figure out what are the possibilities, sometimes with very creative designs. They’ll work with us, it’s usually more iterative, and they may want us to provide some models, or to try something new. But a complete test cycle for them, as I understand it, is they design the system and controller using offline simulation; then they will try to build scaled-down equivalent systems, with the controller that is directly downloaded to a real-time simulator. This is rapid control prototyping (RCP)–it’s only a prototype controller. So during this cycle, they try to optimize their parameters and validate that the controller works. And then before the controller goes to the field, they’ll hook it up to a simulator. We call this HiL. And then finally they bring the controller to connect to the actual device.”

IV: “And we have products in all those categories, right?”

WL: “Exactly. From the beginning, our products provide offline simulation and then fully digital real-time simulation. Then our real-time simulator can be used either as a prototype controller or as a plant. As a prototype controller, we have all this control logic downloaded to a real-time simulator and connected to some hardware device to control. As a plant, it’s used not only for steady-state systems but also if not mostly to simulate the behavior during faults. We have a specific type of testing mode that is really good for customers that use two independent simulators: one acts as the controller, one as the plant. There is no closer way to simulate the real-life behavior, with noise in I/Os and delayed latency.”

“Considering this very important thing that are I/Os is essential as they can cause trouble in many cases. So testing all this together in the same simulation adds more certainty, it saves money, it solves problems ahead of time, and with HiL you can easily do many tests in one day—when you finish one test you just reset the model to steady-state and test again. This can be done within only a few seconds. In the field, you need to de-energize all the equipment and start again. For the first multi-terminal HVDC in the world, they had to do 650-1,000 tests before they commissioned it into the field in order for it to be viable.”

“You can imagine how, if you do so many tests, and if each day you can save some time, you can save an awful lot of time. And time is money in this field for sure. In China, they do these tests within one year. And if they do it in the field, 1,000 tests, at a rate of one a day, it would be three years. But they do all this in a few months. And they do each test several times.”

IV: “The scale of all that is just staggering.”

WL: ”Yeah, it’s literally awesome. And we’re pleased to be able to help people out.”

IV: “Wei Li, thanks for speaking with us today.”

About the Interviewee

Wei Li
received his B.Eng. degree from Zhejiang University, China, and an M. Eng. degree from the National University of Singapore, as well as a Ph.D. degree from McGill University, Canada. He is a Senior Power System Simulation Specialist with OPAL-RT Technologies, Montréal. His fields of interest are in power electronics, renewable energy, and distributed generation. His current research focuses mainly on real-time simulation and controls of modular multilevel converter HVDC systems and FACTS devices.