Ensuring high accuracy DC current metering for EV charging, microgrid applications

Article By : Loic Moreau and Patrick Fuchs

As EV charging stations proliferate, customers need to be able to rely on the accuracy of the DC measurement due to the relationship between DC energy consumed and billing.

A 2020 report by Deloitte forecasts total EV sales growing from 2.5 million in 2020 to 11.2 million in 2025, then reaching 31.1 million by 2030, representing a compound annual growth rate of 29% over the decade. This means that EVs would secure nearly one-third of the total market share for new car 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 the amount they have to pay — is tightly regulated and controlled. Zes Zimmer, a leading German power instrumentation company, is working in partnership with Danish current-sense transducer company Danisense to deliver precision DC metering solutions.

EV charging stations are proliferating as the uptake in EVs progresses. In Germany, there are many charging stations in cities and towns and increasingly also at workplaces. There are already charging stations available that can provide up to 350 kW. Customers need to be able to rely on the accuracy of the measurement of the DC energy transferred because there is a direct link between the energy consumed and the billing.

At the beginning of 2020, Zes Zimmer, one of the technical leaders in the field of power analysis, was approached by German standards organization VDE, which was working to ensure the proper verification, calibration, and certification of DC meters used in EV charging stations. Zes Zimmer already provides instrumentation that accurately measures up to 32 A, but for larger installations ranging into hundreds of kilowatts, the requirement is to measure much greater currents, in the region of a few hundred amps. This requires the use of external current sensors.

Measurement setup with a Zes Zimmer LMG641 power analyzer
Figure 1: Measurement setup with a Zes Zimmer LMG641 power analyzer

Figure 1 shows the measurement setup with a Zes Zimmer LMG641 power analyzer and an external DS600 current-sense transducer from Danisense. To calibrate the DC meter, the energy supply is generated by a precision PSU and fed both into the energy meter and the reference power analyzer, in parallel. In addition, the energy meter’s pulse output is connected to the LMG600’s external cycle input, allowing the analyzer to synchronize its measuring cycle for voltage, current, power, and energy to the meter’s pulses. Because the external cycle input is sampled with more than 5 MHz, the pulses will be captured reliably and precisely.

Once the pulse intervals are synchronized to the measuring cycle of the power analyzer, their length can be recorded. In combination with the use of a highly stable and accurate DC source, this permits the accuracy of the intervals and thus the energy counted to be simply verified. Zes Zimmer’s LMG600 power analyzers allow the GUI to be customized to mimic a specific application. This GUI can be fed with values from the built-in Script Editor and displays only those parameters that are relevant to the application. An example is shown in Figure 2.

GUI example
Figure 2: GUI example

On its own, the LMG600 series provides accurate DC measurement across a current input range of 500 μA to 32 Arms. To extend this current input range, Zes Zimmer is partnering with Danisense. Based on the zero-flux principle, Danisense current transducers deliver a measurement accuracy down to 1 ppm, and the combination of Zes Zimmer LMG600 power analyzer and Danisense DS600 results in a highly accurate yet simple-to-use calibration test system.

There are many different types of current-measurement technologies, from basic shunt and Hall-effect devices to more complex systems. The determining factor is usually the accuracy required, and simple devices cannot deliver at high accuracies. Danisense proprietary fluxgate is a closed-loop–compensated technology with fixed excitation frequency and second-harmonic zero-flux detection[A1]  (Figure 3). It combines complex magnetic performance with advanced signal processing, and by using second harmonics, signals can be extracted to provide a measurement of the current in the conductors and its DC current value to extremely high levels, very repeatably. Furthermore, Danisense employs a dual-balanced fluxgate structure, which deploys two magnetic cores in opposition, similar in concept to a Wheatstone bridge. This provides 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 amps is driven for some while the sensor heats up. Therefore, temperature stability is essential.

Closed-loop compensated technology with fixed excitation frequency and second-harmonic zero-flux detection
Figure 3: Closed-loop compensated technology with fixed excitation frequency and second-harmonic zero-flux detection [A2] 

Zes Zimmer supplies a cable with in-built intelligence to facilitate simple setup of Danisense current transducers, such as the DS600 shown in Figure 4 and marketed by Zes Zimmer as Plug’n’Measure PCT600, which enables automatic identification of the sensor type connected and configuration of the LMG600 current input. Every important parameter, such as the precise scaling factor, delay compensation variable, last calibration date, and sensor type, will be read and used automatically by the power analyzer. Moreover, the sensors are actively powered via the LMG600, so separate power supplies are not required. Danisense offers a range of products that enable the current measurement to be extended up to 2,000 Arms (3,000 A peak or DC) and even higher.

Zes Zimmer power analyzers in combination with the Danisense sensors are being used in the field by institutions like the VDE to test the accuracy of the DC meters installed at EV charging stations to standards such as Eichrecht conformity resp. E-VDE-AR-E 2418-3-100. They are also being used by manufacturers of DC meters to do pre-compliance tests before they send them off for independent certification to ensure that they will get a positive result. So the fact that no additional equipment is required and all the setup is simplified and matched through the partnership between Zes Zimmer and Danisense is very useful.

DC microgrid

Another application that calls for a similar approach is 1,500-V DC microgrids. In applications in which power is being generated by wind or solar power, for example, why convert to AC? Why not use the DC power directly? It is predicted that the house of the future will run on DC, resulting in significant efficiency benefits. This could be especially valuable if a vehicle is being charged in the garage overnight. This leads to a system called a DC microgrid. The high-voltage AC grid will still exist, but alongside it, a DC box connects to the DC generators, mostly renewable energy generators, and consumer products. Again, testing, calibration, and certification of such systems will call for highly accurate, stable DC current measurement.

It is vital to build customer trust and confidence that DC meters used in EV charging stations and other energy systems are highly accurate and regularly calibrated. The combination of Zes Zimmer power analyzers and Danisense current-sense transducers provides a precise, stable measurement that is repeatable and simple to facilitate. This is why renowned institutions such as VDE are using this approach.

This article was originally published on Electronic Products.

Loic Moreau is vice president of marketing at Danisense.

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