Mr. Grasel, you are the CEO of Neo Messtechnik. Could please introduce your company to us?
The products of NEO Messtechnik focus on applications all over the electrical grid and renewable energy technologies. We are a “Power” company. Our key products include Power Quality Analysers, Grid Impedance Analysers and Photovoltaic Testing Instruments. We focus on high-end products and aim to lead by technology. The Grid Impedance Analyser was nominated for the Innovation Award of the Austrian electrotechnical association. The new Photovoltaic Analyser PM-10 was sold to all continents just one week after market release and the Power Quality Analyser PQA8000H is the reference instrument on the market today, addressing all anticipated requirements for the next ten years.
Do you have some news for us regarding grid problems?
The electrical power grid is experiencing significant transformations. Advanced semiconductor technologies like IGBTs or MOSFETs are facilitating numerous novel applications for renewable energy technologies and the grid itself. With the widespread adoption of photovoltaic systems, electric vehicle charging stations, and the emergence of DC grids, the electrical grid is facing new challenges such as load flow management and power quality assurance. Additionally, these developments are altering the fundamental characteristics of the grid, notably in terms of High Frequency Grid Impedance. Power quality issues are no longer confined to industrial settings; they are becoming prevalent across the entire electricity grid. Despite these advancements, standardization efforts are still ongoing, and for many new phenomena, including DC currents and supraharmonics, there are often no emission limits yet.
Power quality professionals work a lot with Rogowski coils and current clamps. Are these still appropriate measuring devices today?
Each technology indeed presents its own set of advantages and limitations. Rogowski coils are very convenient to use due to their easy mounting process, but they are unable to measure DC currents. Iron-core clamps are only used for secondary measurements at current transformers (1A / 5A). These clamps don’t allow measuring DC currents and in addition, are limited in bandwidth (~20 kHz). Additionally, hall-based DC clamps suffer from imprecise measurement of small DC components due to the significant error induced by the earth’s magnetic field. .
Active power electronics (EV charger, PV) and its high switching frequency (GaN, SiC) cause emissions in the frequency range up to 150 kHz (and above). SiC based frequency inverters already achieve switching frequencies between 50 and 100 kHz.
Danisense offer the premium option with their AC/DC zero flux transducers. The high bandwidth allows to cover the full range from DC to 500 kHz. The flux-compensation technology ensures high accuracy in the ppm range and the error due to earth magnetic field or other magnetic is extremely low. With the PQA8000H device, µA can be measured precisely. The voltage output of the Danisense transducer makes it easy because a lot of our devices can work with voltage signals directly.
Some users occasionally notice large inaccuracies in current sensors with regard to the specified accuracy in the higher frequency range. Have you also already experienced this issue?
This is actually quite often the case. As a manufacturer of power analysers and system provider we meticulously assess each sensor acquired in our laboratory. Frequently, the specifications outlined in the datasheets are not fulfilled. Even upon ordering, we anticipate that certain specifications may not align with reality, such as with hall sensors for example. However, Danisense products consistently demonstrate remarkable reliability in this regard. The specifications provided in their datasheets are precise, and the German national metrology institute, PTB, utilizes Danisense current transducers as a benchmark for frequency measurements.
Can you explain in which cases you need AC/DC current sensors?
We usually require AC/DC sensors for the following applications:
- Power & Efficiency Analysis
For high-precision power analysis measuring AC and DC parts of the signal with high bandwidth, high resolution and high accuracy is a must. - Power Quality Analysis
For power quality analysis not only the AC parts are important but also the DC and subharmonic part. Many national technical guidelines define maximum values for a parasitic DC component in the low-voltage range. We have research projects and master and bachelor thesis working on DC emissions of active power electronics such as of photovoltaic and electric vehicle chargers. First results show that emission limits are violated in most cases with regards to the existing national directives. Beside that we are researching the propagation of DC currents along the grid. - DC Grids
DC grids are a big topic in the future at all voltage levels. Wherever an AC to DC conversion or DC/DC conversion takes place, you need to measure both AC and DC. The IEC TR 63282 already emphasizes the need of measuring DC and AC components in low voltage DC grids (LVDC). Of course, power quality monitoring in DC grids looks different to AC grids, but there are still high switching frequencies (PWM, DC/DC) causing high frequency emissions (Keywords: Supraharmonics, EMC, Common Mode).
Can I also use AC current sensors for the power analysis if there is an AC application?
In my previous roles at various companies, we specialized in developing power analyzers. For the efficiency evaluation of frequency inverters, for example, current measurements with AC current sensors are not possible. Nowadays, 99% of all devices use power electronics and the conversation from AC to DC generates currents in a wide frequency range, starting from DC up to several MHz. Consequently, you need to use high precision zero-flux transducers to measure power and efficiency precisely. AC/DC converters today have efficiency level between 90 and 98%.
Danisense transducers are the perfect fit for such applications. The DC accuracy is brilliant and the high bandwidth, high dynamic and very low phase error over the full frequency range are ideal for the measurements with the highest accuracy requirements. Even small phase errors can lead to significant errors in the power reading, making Danisense’s technology particularly well suited for these applications.
I expect quite small DC values in the power quality analysis. Is this technically possible when the 50 Hz amplitude is very high?
As the DC values are in the two-digit mA range with a simultaneously high 50 Hz amplitude, we need very precise and stable current sensors that provide us with reliable values. Danisense current transducers are very suitable here. We also use Danisense high-precision current transducers for grid impedance measurements which become increasingly important.
Why is a DC component in AC networks so important to measure?
Inductive components such as motors and transformers are pre-magnetized by a DC component. AS a result, the magnetic operating point specified by the manufacturer is shifted. This in turn leads to higher losses in the equipment and their faster ageing. If transformers are pushed into the magnetic saturation range by a DC offset, the humming and thus the noise pollution increases significantly. In addition, the transformer becomes a non-linear device, which then also generates harmonics.
Energy meters equipped with passive current transformers can also be affected in their accuracy by a DC component.
With regards to the effects of DC components, we have initiated further research into how the DC components behave in real grids. A paper will be published at the
7th International Conference on Smart Energy Systems and Technologies, SEST 2024
https://sest2024.polito.it/
However, there are unit certificates for inverters through a defined national approval process. This should ensure that the devices meet the latest standards?
When the DC component is measured, the question arises: “How is this done? Hall current clamps are often too inaccurate. In these cases, we recommend fluxgate Danisense transducers as they can detect very small DC currents in the range of µA or mA precisely. Further, Danisense laboratory is accredited to the IEC 17025.
In addition, only one inverter is tested for unit certification, so the manufacturer is free to decide whether or not to carry out this test.
What do you suggest how inverter manufacturers can improve their devices regarding the DC component?
Often the DC component on the inverter output is not monitored. If this component is fed into the control circuit of the inverter, the control unit could initiate a corresponding compensation. At least at the type test stage, the DC component should be tested.
Particularly in the case of larger systems, where a transformer directly behind the inverter spans up to medium voltage, a measurement of the DC component is not performed at all. The transformer does not transfer a possible DC component.
As I understand it now, there are only DC currents in the low-voltage grid. Does this mean that higher voltage levels are not affected by these problems?
At the moment, the low-voltage grid is more affected. Almost every electrical device that we connect to the power grid features integrated power electronics. Photovoltaic systems, charging stations for electric vehicles or heat pumps use active power electronics based on IGBTs or MOSFETs, which operate at high switching frequencies and also emit low direct currents. DC currents in AC networks are completely ignored, as the current sensors used for monitoring applications cannot detect DC currents.
Nevertheless, DC currents on higher voltage levels are getting attention for two reasons. On the one hand they can couple through earthed transformer neutral points into network sections that are earthed on both sides. One example are geomagnetically induced currents (GICs). It is also possible to detect stray currents from neighbouring supply networks. For example, 16.7 Hz currents from railroad systems can also enter the earthed transformer neutral point. On the other hand, the use of power electronics in the high voltage grid will increase significantly over the next years – keyword: Smart Transformer.
At higher voltage levels, power quality measurements are probably much more complex?
Yes, that’s right. We definitely need high-voltage dividers for the voltage measurement. For current measurement, bushings on transformers can be used. The current sensors used here do not require high-voltage insulation.
Through our cooperations in the high-voltage sector, we ensure that our devices work perfectly with the voltage dividers currently available. We have also received the first inquiries regarding fluxgate current transducers in the high-voltage application.
Do you have any final statements for our readers?
Yes, we see more and more often how laboratory equipment of the test and measurement business becomes necessary at a later stage in large-scale network applications. For example, our power quality analyser is also a high-precision power analyser in terms of hardware.
Thank you very much for this very informative interview Mr. Grasel.
Dipl.-Ing. Bernhard Grasel
CEO
NEO Messtechnik GmbH
Hauptstrasse 7 (Büro)
Sonnweg 4 (Lager)
2871 Zöbern, Austria (AT)
bernhard.grasel@neo-messtechnik.com
M:+43 660 38 155 86
T: +43 2642 20301
www.neo-messtechnik.com