Measurements in Modern Power Electronics

00:00:00: Sound on power on your power electronics podcast, powered by PCIM Europe.

00:00:09: Hello and welcome to Sound On Power On. I'm your host, David Hegarty, and in this episode I am talking to Jörg Latzel who is the sales manager for Yokogawa Test and Measurement in Germany.

00:00:21: Jörg started out as a telecommunications engineer with a focus first on antenna measurements and measurement instruments, then later product management for fiber optic test. He later moved into sales and support at Ando Electric for communication measurement solutions, for the German market.

00:00:37: In 1998 he moved to Yokogawa, initially in fiber optic measurements, then for more than the last 10 years, in his current role which includes our topic for today, power measurements. Welcome Jörg.

00:00:50: Let's start with a short introduction to Yokogawa.

00:00:53: We were founded in Tokyo in 1915, and our focus - beside the measurement technology - is also on control. So a big part of Yokogawa is looking after, for example, chemical control, or also power plants.

00:01:13: We have data acquisition over months or years. And another part of our operation is also telecommunications. And of course all measurement technologies - including power measurement - And we still progress on optical spectral measurement.

00:01:40: And our focus the last 40 years has been to improve measurement accuracy, resolution, and also focus on measurement data capture.

00:01:54: Our main goal is to equip engineers with tools that make it possible to measure even small steps of improvement. So, if you look at measurement technologies, then we want users to be able to measure an efficiency improvement of just half a percent. Because it's important to understand if you got half a percent better, or if your latest development steps maybe made things worse. In the end, we can say that improvement can only be measured if your measurement instrument is good enough to realize this is an improvement. Yeah, that's Yokogawa.

00:02:41: And the power electronics test and measurement at Yokogawa, what's the history there, where does that come from?

00:02:48: In principle, the root of Yokogawa is based in energy measurements. Our founder, Mr. Yokogawa, developed the first energy measuring system, in principle an energy counter. So our roots go back to exactly this kind of measurement. We've been in the power analyzer world already for over 40 years. So even before the boom in renewable energies and power electronics, we have been developing accurate, integrative power meters.

00:03:30: Today if you put "Yokogawa power analyzers" into Google, you will be able to find reports going back to the early 70s, and white papers, or technical reviews back to the late 70s.

00:03:48: What has driven us since then is a close relationship with our customers, to the users. So, for example, at the time solar energy had its real big boom, at that time, we held seminars, we discussed onsite, directly with users, how best to use, how to wire a power analyzer, and how to get it connected. And that gave us the chance to get direct feedback from users, what else they needed, how the user interface should develop.

00:04:25: And of course, the increased capacity, so, the increased presence of power devices, also to control electric motor inverters, that's something that drove us since the early 90s.

00:04:42: During the early 90s, I was studying electronics with a focus on telecommunications. A professor was very keen on making us believe that switching power supplies would be the future. And yeah, today we can say he was right.

00:05:04: But switching power supplies - and that's what was not visible at that time - changed a lot of things, apart from having an efficient power conversion.

00:05:18: Switching power supplies use short pulses - what today we call pulse width modulated signals. And if you use short pulses - and that's the challenge everyone is confronted with today - this challenges is, that even if we are talking about the ground wave at 50 Hertz, or 100 Hertz, if you make it a pulse, it will have high frequency content. So we cannot rely just on the power measurement at 50 Hertz. We must be more broadband. And this is something Yokogawa was already considering from that period.

00:06:04: But it's not just a frequency challenge. It's a power challenge too. I mean, if you're using a 10 milliOhm shunt resistor to make a measurement, even if you're testing a device rated at 22 kilowatts - an instantaneous water heater, for example - that's around 30 Amps through that shunt resistor. So it has to handle energy of 30 Watt hours.

00:06:23: Absolutely correct. And exactly that is what the developer of measurement solutions must consider. They must be aware: you will have to handle energy. And if you know that 30 Watts through a small device will definitely heat it up. That means there will also be an influence on the resistor's value, it means it may fluctuate.

00:06:45: And that is why our solutions, our shunt resistors, have a special design, to treat this temperature. Our shunt resistors have a design to dissipate heat. So the challenge is not to change the temperature too much, but to take it away, to remove the energy. And therefore a special design like our larger shunt resistors - a coaxial design - the shunt resistors and power analyser's design is necessary. And this is, in principle, the challenge we continue to work on, and develop further on our power analyers.

00:07:24: Would one way out of this be to use oscilloscopes rather than power analyzers?

00:07:30: Yes. So there are applications, that a scope is a good fit to. We need to consider, if you're using an oscilloscope, we normally have to use a probe for the voltage, as well as a probe for the current. And this is where the big difference is between a power analyzer and an oscilloscope.

00:07:55: If you use a current probe, there is some inductivity. And if you use a differential probe, you need to have this for one phase, because if you want to measure power, you need voltage on one side, and the current on the other side. And these two probes are not in phase, as these are delivered and connected to your oscilloscope, these are not in phase.

00:08:25: Such a measurement solution has some kind of adjustment. Some do an auto adjustment to put the current and the voltage in phase. But this needs to be done in advance of the measurement. If you just go and say, "I will use the oscilloscope to measure power" , then if you don't consider this, there could be a skew between these two, then it will be very difficult to measure any power accurately.

00:08:55: Again, a power analyzer is ideal to measure full periods: a full period, because power is always defined as measured over complete periods.

00:09:12: Okay, so what applications are you recommending that people use scopes for?

00:09:16: Yeah, so if people want to develop and improve the efficiency of switching circuits, they need to touch also the switching cycles, for example, of the SiC and GaN transistors. And to check these, an xy plot is very helpful of the voltage and current, to help determine the safe operating area, and to improve the current and the voltage driving the SiC and GaN transistors.

00:09:50: If you do these measurements with an oscilloscope - for example, the Yokogawa oscilloscopes deliver a long history, if you have one with a long history memory available - you can make a statistical evaluation, for example, to find transients: How often transients happen during the on/off switching cycle? Or are there transients at all?

00:10:15: And all these raw data are covered in the in the history memory. Even after the measurement, you can still use this data to evaluate whether something happened, have there been any transients? And that's something an oscilloscope is good for. Though, of course, the power analyzer is also recording these data. But it's not made for this kind of switching cycle. So for this kind of highly dynamic, very short - in milliseconds or microseconds - changing power, there an oscilloscope is a really good fit.

00:10:55: Okay. When you're measuring inrush currents, because that's the other issue with switching transients, how would you approach that?

00:11:03: So if you look at this, we sometimes get this question, "How do we measure this inrush current?", And why is it that important?

00:11:13: In principle, years ago, to illuminate a room, we had to use 60 Watt or 100 Watt light bulbs. Today, we are talking about, in the smart home area, about light bulbs that have a continuous power of 5 Watts, or something like that. But - and this is the point - the switching circuits, also of these light bulbs, are mainly equipped with a rectifier, with a flattening capacitor. And when you switch them on, there is a really high current, because of the capacitor charging, and this creates - especially if you connect many such light bulbs or switching circuits together - this creates a very high pulse.

00:12:04: This inrush current suddenly appears with switching on this light. And depending on the switch you use, there can be some additional transients that influence back to the grid. And that's why it is important not only for the developers, but also for the people at the end, selling such kind of tools, they must be aware, what can happen when I switch this light on.

00:12:36: So, an inrush current can vary from switch to switch: just imagine once you switch such a light on, and the capacitor already charges. You turn it off, and then you switch it on again immediately, then the current is not as high. And, and all these processes, under different conditions, need to be evaluated.

00:12:58: There we have a special, let me call it, power scope available to measure this kind of quick change and quick spike.

00:13:11: So, this information we need, and it is probably more imaginable if we continue with this example of the light bulbs.

00:13:22: These light bulbs may have a continuous current of 50 milliamps. And connect 100 of these together, then that means these 50 milliamps will cause a current of 5 Amps, under continuous driving, under continuous operation.

00:13:47: But this 50 milliamps continuous current when using the light bulb, it may end up if it's switched on for the first time after, let's say one day, with a spike of 5 Amps. And if you take the 100 light bulbs connected together, for example, in a football stadium, or wherever, and switch it on now, then from these continuous 5 Amps, you have a spike of around 500 Amps. That may influence the circuit breaker, it switches off. So that's why it is so important to measure these transients and inrush currents, but that cannot be done with an integrative solution.

00:14:34: And the power scope you mentioned here, this was developed in response to the need for this, or is the power scope something that was in the portfolio anyway?

00:14:46: Yes. We had that in the portfolio in the past, and we continued developing it. So, today this model - and it will be presented also at the PCIM this year - is the power scope called PX 8000. And yes, in fact, it is made for inrush currents, but also for highly dynamic processes.

00:15:11: So, it could be if you have an entrance control with a barrier that is raising and closing again, this is a very short period that the power is present, or the power needs to be measured. For such kind of tools, it's more the instantaneous measurement that needs to be done.

00:15:34: But if we are talking about inverters, for fridges, for electric engines, and such, there is a lower dynamic. Accelerating an electric vehicle or reducing the speed, this is what we call medium or low dynamic. That requires an integrative measurement.

00:15:55: But the ultimate thing is getting a precision measurement, an accurate measurement, that's reliable.

00:16:01: Exactly. And that's what the majority of people are focusing on. The majority of processes, like generating electric energy from wind, or driving an electric vehicle, there the efficiency is very high. So the inverters, for example, in solar energy, they have an efficiency of 98%. And there it is about the long-term efficiency measurement: How efficient is such inverter? And that requires a really high accuracy.

00:16:38: And what's your approach to ensuring the precision of these measurements?

00:16:43: In fact, it really goes back to the special design of the shunts, because the current will be measured by the voltage that is created over the shunt resistor, and especially this design is important - and it needs to be - and that's a fact we focus on with our special design. In fact, the shunt resistor should be low resistance, for such kinds of current measurement. But a low resistance, in principle, also means it has a high inductance. And that's why we developed a special design for that. So that's our key.

00:17:28: Especially there is another thing that our users want to rely on, is that the stability over years must be good. And we already discussed the temperature influence on shunt resistors. And this temperature: if a shunt resistor is getting warm, for example, it will age. And this could mean that there is a change in resistance over the years, or over the months of usage, especially at high currents. And that leads me again, back to this high energy consumption with instantaneous heat.

00:18:17: If such a shunt resistor, is not well taken care of, of a heat sink, then your result will be unstable. So we have been confronted with the request to deliver a solution that can measure 30 Amps over four hours. And there are solutions on the market that cannot do that very well: It appears that not everyone is looking after this heat development in the shunt, if it's connected to a high current.

00:19:00: So, if you have a 30 Amps current signal, then of course your internal shunt resistor is warming up. And if you don't care, the resistance is changed, and then also the power measurement value will be changed. This is why the special design of the shunt is the key technology we have to deliver stable results. And if an even higher accuracy - because we are proud of having a power analyzer with a basic accuracy of 0.03% - and by a special aging process of the shunt resistors, we are able to deliver a customized solution that has an even higher basic accuracy than this 0.03%. And that's created by a special aging process of such shunt resistors.

00:20:04: And a lot of this comes from the fact that you've been in this market for a long time.

00:20:08: Right, absolutely. That has definitely driven us since the first electronic devices were put on the market. This was driving us.

00:20:23: But apart from this long history, you need to be looking to new applications, sometimes before they're obvious. How can you be sure you understand the needs of the people using your equipment?

00:20:34: Having engineers from the development team going with us to customers, presenting solutions, discussing solutions, and also returning back to the development team, and trying to resolve the demands that we heard from the customer side. And discussing this with the experienced engineers and developing further.

00:21:01: And what additionally helps us is that, at the beginning we talked about workshops and seminars at customer sites: we continue running these workshops. Often our customers bring their test devices. It can be a vacuum cleaner, it can be a tool, it can be a chainsaw, whatever you can imagine, ... battery driven tools. And based on this, we discuss demands.

00:21:30: And just taking one example of a tool, a battery driven tool, and this battery driven tool had very, very short wires to connect the battery with the engine. There was, in principle, no chance to connect a sensor, because the wire was so short. And it was not connected by a connector, it was welded.

00:22:03: So, this made us listen: what could be the solution? Yes, a solution can be a split core sensor. But that's something - that kind of information - you get if you are in touch with customers. That helps. Together with experienced people, with young people that have learned about the latest technologies, and the connection to the customer. That's something that helps us develop tools in the right direction.

00:22:34: Seeing the complexity of the measurement situations that are arising nowadays is also, I mean, it's not just one signal anymore, it's not...

00:22:43: Absolutely. Absolutely correct. And especially the development of new engines and controllers for new electric engines, that is changing. There are three-phase solutions, there are six-phase electric engines under development. So, a lot of things are going on, depending on who is taking care. And for such kind of solutions, you need to have the right answer on the power analyzing tool.

00:23:12: With this development of the circuit technology, the motor control is really getting a bit more complex. Let's take an example, the WT 5000: you can equip it with up to seven channel inputs. And for example, that fits perfectly for an inverter measurement, to manage three-phase input, then the DC intermediate channel, and the three-phase output.

00:23:40: Of course, you can configure a WT 5000 to fit also only for seven channels, or you can synch more together to measure even more channels. So, that means with one instrument, you can measure up to seven electric channels. So, up to seven phases.

00:24:02: But as we are often used to with e-drive technology, you additionally have the chance to have the input for direct torque, for motor evaluation. So, then simultaneously with one instrument, with one standalone instrument - of course, it can be controlled remotely - but in principle, you can use the instrument as standalone, and measure and gather all the data for a full motor evaluation, including speed, torque,... . And all synchronized with the power measurement.

00:24:40: The challenge is today, you're measuring hundreds or even thousands of Amps. So, what are your thoughts on the different types of sensor technologies necessary to cover the full span?

00:24:50: So, until now, we discussed - with the examples we had, with the instantaneous water heater, we were talking about direct measurements. And if we are talking, for example, about charging progresses, high power charging, or about the powertrain, then we have to measure at least 1,000 Amps, or even more; 2,000; it can go up to 5,000 Amps. And that cannot be measured directly.

00:25:22: Our recommendation is to use zero flux sensors. We are working with Signaltec, that are LEM zero flux sensors. And our recommendation, in that case, is to use zero flux sensors that are current driven. So, that means you have a wire put through the zero flux sensor, and the zero flux sensor is creating a lower current, that is connected to the power analyzer.

00:25:55: There are solutions available that transform the current from a zero flux sensor to a voltage. And why do we experience issues with that? It's pretty easy: The inverters, as discussed, are driven by pulse width modulated signals, that are mainly short pulses. And these short pulses, consisting of various frequencies, even high frequencies of a couple of kilohertz, because of the short pulse. And these frequencies can be received by a cable, for example the cable that is connecting a zero flux sensor to the power meter. And this influence is a voltage. So, it's inducing a voltage into that cable. And as long as you want to measure this voltage, you will have a disturbing influence in that power measurement.

00:26:58: And that's why we know that current measurements are much safer versus the interference of the fast switching electromagnetic fields that are around in the environment of inverters; because on the current, the influence is insignificant.

00:27:23: Looking to the future, do you think the market is now as developed as it's ever going to be, or are there more innovations coming down the pipeline, that you can see?

00:27:33: The hardware development is the key for the future. And we will continue improving resolution on one side, and accuracy and stability on the other side. But if you take the example of the world's leading power meter that we launched in 2004, this instrument was a kind of reference until 2016. And that was a period of more than 11 years before we launched our latest flagship, the WT 5000.

00:28:22: So, what we of course do is to improve the user interface, to listen to the users and to make it better. But our key is also the hardware. And we really are proud to have developed shunt resistors, we have developed measurement principles that are leading. And we will continue this, but we do not expect every year a new product. It's a longer life cycle in this area.

00:28:55: If you have the highest accuracy in class, the next step shall be to be even more accurate. But that will take slightly longer than electronic development in the mass market.

00:29:10: Because you were talking about the hardware, what about the software side of things?

00:29:16: A colleague of mine referred to "software eats hardware". And it's an interesting saying. And on the other side, it's "garbage in, garbage out": you can only be as good as your hardware.

00:29:35: It reminds me a bit of the period when I was starting as a sales engineer. I had the task of demonstrating an optical spectrum analyzer, including, of course, an optic power measurement, to a group of students. And these students - as they are today - were honest and frank. And I put an instrument on the table with a floppy disk drive, and that was around 2013. And they started laughing, "What shall I do with a floppy disk drive in 2013?" (That time, floppy disk were still available.)

00:30:19: We had an instrument on the table, best in class optical resolution. But the hardware is the part that makes this resolution. Yes, the software is important. Many people are using a different kind of tools, including vibration measurement - which we can do as well, by the way - but people at the end need to decide based on what the physical factors are, and what the physical accuracy is. Because you will not be able to make a physical measurement more accurate by software.

00:31:01: You can, of course, smooth, you can calculate, you can integrate by mathematical calculations. But at the end, the hardware is what counts, and that's what we take very seriously.

00:31:17: And taking this seriously, because most of your product development is done in Tokyo, how are you managing to keep the connection open then to the applications that are arising around the world?

00:31:31: Since I'm with the company, it has always been, and it continues to have a close connection. So we can get in touch with the development immediately if there is a high demand. In all the other cases, we have frequent visitors from the development team in Japan joining with us, visiting customers, and understanding the application and feeding back to the development team.

00:32:03: One thing that is very important in international business: Yokogawa is doing the same for Europe, for the US, for all continents, for all countries, and collecting the information and the demands.

00:32:20: The thing is that also our customers are working on an international base. Even if you buy a Yokogawa solution, for example, in Germany, and you want to use it for your test bench (as an international customer) in China; and you experience some difficulties, there is no need to send the unit back to Germany where you purchased it, even if it's a warranty repair. We have local service centers worldwide. And that makes the phrase true "think global and act local".

00:33:05: And of course, the immediate chance to interact with customers is going to be at the exhibition in June. So what are you planning to do there?

00:33:16: We will have all that we discussed - which means solutions for semiconductor devices, for inverters, for power trains, for lighting, renewable energies, energy measurement - we will present solutions there. All the instruments we will have there in an environment where we can also show some measurements. For example, we will be able to see the inrush current measurement.

00:33:51: The PCIM is a good exhibition, including all the specialists from our side, from customer side. And we have been presenting at the PCIM for more than 15 years. And it's always great to have this kind of information exchange between development people, between customers. At the PCIM we concentrate on all these kind of power measurements.

00:34:27: Thank you very much, Jörg, for this fascinating look into some of the challenges of measuring power electronics. And I wish you a successful time at PCIM Europe 2024.

00:34:37: If anything in this episode has sparked your interest, there are links to Yokogawa in the transcript.

00:34:42: And if you found this episode useful, please remember to subscribe for future episodes. And if you think this episode, or the podcast itself, would be of interest to anybody you know, please do share it. For sound on power on and the PCIM Europe, I am David Hegarty, and I look forward to our next episode.

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