Accurate and certified DC current meters for electric vehicle charging and precision network applications


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Deloitte’s 2020 report expects total electric vehicle sales to grow from 2.5 million in 2020 to 11.2 million in 2025, and then reach 31.1 million by 2030, representing a compound annual growth rate of 29% over the decade. This means that electric vehicles will provide nearly a third of the total market share for new vehicle sales. This will require a huge investment in charging stations, and consumers will demand that the accuracy and reliability of the DC measurement – which will regulate how much they will have to pay – be tightly regulated and monitored. Zes Zimmer, a leading German manufacturer of power devices, is partnering with Danish current-sense power transformer company Danisense to provide accurate DC measurement solutions. Electric vehicle charging stations are proliferating as the uptake of electric vehicles progresses. Germany has many charging stations in cities and towns as well as increasingly in workplaces. There are already charging stations available that can provide up to 350 kW. Customers must be able to rely on the accuracy of measuring transmitted DC power because there is a direct correlation between power consumed and billed. At the beginning of 2020, Zes Zimmer, one of the technical leaders in the field of energy analysis, was contacted by the German standards organization VDE, which was working to ensure proper verification, calibration and certification of DC meters used in electric vehicle charging stations. Zes Zimmer already provides devices that accurately measure up to 32 amperes, but for larger installations ranging from hundreds of kilowatts the requirement is to measure much larger currents, in the region of a few hundred amperes. This requires the use of external current sensors. Figure 1: Measurement setup using the Zes Zimmer LMG641 power analyzer Figure 1 shows the measurement setup using a Zes Zimmer LMG641 power analyzer and an external power converter in the current sense DS600 from Danisense. To calibrate the DC meter, the power supply is generated by a precision PSU and fed into both the power meter and the reference power analyzer, in parallel. Additionally, the power meter’s pulse output is connected to the LMG600’s external cycle input, allowing the analyzer to synchronize its measurement cycle for voltage, current, power and energy with the counter pulses. Since the input of the external cycle is sampled at more than 5MHz, the pulses will be captured reliably and accurately. Once the pulse periods are synchronized with the measurement cycle of the power analyzer, their length can be recorded. Combined with the use of a highly stable and accurate DC source, this allows for precise time intervals and thus the power is calculated to be checked simply. The LMG600 power analyzers from Zes Zimmer allow the GUI to be customized to mimic an application. This GUI can be fed with values ​​from the built-in script editor and only shows parameters relevant to the application. An example is shown in Figure 2. Figure 2: Example of a GUI Figure 2: Example of a GUI On its own, the LMG600 series provides accurate DC measurement across the input current range from 500 μA to 32 arms. To extend the current input range, Zes Zimmer is partnering with Danisense. Based on the zero-flow principle, Danisense current transducers provide measurement accuracy up to 1 ppm, and the combination of the Zes Zimmer LMG600 and Danisense DS600 power analyzers results in a highly accurate and user-friendly calibration test system. There are many different types of current measurement technologies, from basic transformers and the Hall effect to more complex systems. The limiting factor is usually the required accuracy, and simple devices cannot deliver high accuracy. Danisense’s Fluxgate Gateway is a compensated closed-loop technology with a constant excitation frequency and zero second harmonic flow detection. [A1] (Fig. 3). It combines complex magnetic performance with advanced signal processing, and by using second harmonics, signals can be extracted to provide measurement of current in conductors and their DC current value to extremely high levels, very frequently. Furthermore, Danisense uses a double-balance fluxgate structure, which propagates two opposing magnetic cores, similar in concept to the Wheatstone bridge. This provides a natural compensation, eliminating the effect of any drift due to environmental conditions such as temperature. This is important if a DC current of a few hundred amperes is pushed to some while the sensor is overheating. Therefore, temperature stability is essential. Figure 3: Fixed excitation frequency closed-loop compensation technique and second harmonic zero flux detection [A2]Zes Zimmer provides a cable with built-in intelligence to facilitate simple setup of existing Danisense transducers, such as the DS600 shown in Figure 4 and marketed by Zes Zimmer as a Plug’n’Measure PCT600, which enables automatic identification of the sensor type connected and the LMG600’s current input configuration. Every important parameter, such as exact scaling factor, delay compensation variable, last calibration date, and sensor type will be automatically read by the power analyzer. Furthermore, the sensors are actively powered via the LMG600, so a separate power supply is not required. Danisense offers a range of products that allow current metering to be extended up to 2,000 arms (3000 peak or dc) and even higher. Written by Loic Moreau, Vice President of Marketing at Danisense and Patrick Fox, Director of Business Development at Zes Zimmer Please visit the eBook for the full article.


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