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Table 1. Maximum Percentage Current Error per EN 50463-2 Current Range


Class 0.2R 1% to 5% IN 5% to 10% IN 10% to 120% IN 1% 0.40% 0.20% Class 0.5R 2.50% 1% 0.50%


Table 2. Maximum Percentage Voltage Error per EN 50463-2 Voltage Range <66% VN


66% to 130% VN


Class 0.2R 0.40% 0.20%


DC EnErgy MEtEring: rEquirEMEnts anD stanDarDisation While the standardisation of dc energy metering may not seem too difficult to achieve compared to the existing ac metering standards ecosystem, industry stakeholders are still debating the requirements for different applications, asking for more time to iron out the exact details of dc metering. IEC is working on IEC 62053-41 in order to


define requirements specific for dc static meters for active energy with accuracy classes of 0.5 per cent and one per cent. The standard proposes a range of nominal


voltages and currents, and sets limits on the maximum power consumption of the voltage and current channels of the meter. Moreover, like the ac metering requirement, specific accuracy is defined across the dynamic range, as well as the current threshold for no-load condition. In the draft, there is no specific requirement


for the bandwidth of the system, but a fast load variation test is required to be successfully accomplished, defining implicit requirement on the minimum bandwidth of the system. DC metering in EV charging applications is sometimes compliant with the German standard


Nominal


Class 0.5R 1%


0.50% Class 1R 5% 1.50% 1%


Class 1R 2% 1%


ConClusion: a Proof of ConCEPt stanDarD CoMPliant DC MEtEr Analog Devices is an industry leader in precision sensing technology, offering a complete signal chain for precision current and voltage measurements to meet the restrictive standards requirements. The next section will show a proof of concept for a dc energy meter compliant with the upcoming application-specific standard IEC 62053-41. Considering the space of billing-grade dc


energy metering in microgrids and data centres, we can hypothesise the requirements shown in Table 3. Cheap and accurate current sensing can be achieved by using a small value and low EMF


shunt (<1 μVEMF/°C). Keeping the shunt resistance small is fundamental to reduce the self-heating effect and keep the power level below the limits required by the standard. A commercial 75 μΩ shunt will keep the


power dissipated below 0.5 W. However, one per cent of the 80 A nominal


current will generate a small signal of 60 μV on a 75 μΩ shunt, requiring a signal chain in the range of sub-microvolt offset drift performance. The ADA4528, with a max offset voltage


of 2.5 μV and a max offset voltage drift of 0.015 μV/°C, is well suited to provide ultralow drift, 100 V/V amplification for the small shunt signal. Therefore, the simultaneous sampling, 24-bit ADC AD7779 can be directly connected to the amplification stage, with a 5 nV/°C input referred offset drift contribution. High dc voltage can be accurately measured with a resistive potential divider of 1,000:1 ratio directly connected to the AD7779 ADC input. Finally, a microcontroller implements a


Figure 10. Proof of concept - prototype.


Table 3. DC Energy Meter Specifications - Proof of Concept Rating


Dynamic Range


Voltage Current


Accuracy


1% to 5% INOM


5% to 120% INOM


Temperature Meter Constant Voltage and Current Bandwidth 12


±400 VDC ±80 A


1% 0.50% –25°C to +55°C


1000 imp/ kWh 2.5 kHz


–40°C to +70°C storage


100:01:00 100:01:00


Measurement (Max Range)


±600 V ±240 A


simple sample-by-sample, interrupt driven metrology functionality, where for each ADC sample the interrupt routine:


Reads voltage and current samples Calculates instantaneous power (P = I × V)


Accumulates the instantaneous power in an energy accumulator


Checks if the energy accumulator exceeds the energy threshold to generate an energy pulse and clears the energy accumulation register


Moreover, in addition to the metrology


functionality, the microcontroller enables system-level interfaces such as RS-485, LCD display, and push buttons.


Analog Devices www.analog.com August 2021 Instrumentation Monthly


VDE-AR-E 2418 or old railway standard EN 50463-2. According to EN 50463-2, accuracies are specified per transducer, and the combined energy error is then a quadrature sum of voltage, current, and calculation error.


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