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Modern ac meters are significantly more complex, accurate, and protected from tamperings. Now, a state-of-the-art smart meter can even monitor its absolute accuracy and detect signs of tampering 24/7 while installed in the field. This is the case for the Analog Devices ADE9153B metering IC, enabled with mSure technology. Energy meters - either modern, traditional, ac, or dc - are all classified by their
impulses per kWh constant and percentage class accuracy. The number of impulses per kWh denotes the energy update rate, or resolution. The class accuracy certifies the maximum measure error of the energy. Similar to the old mechanical meter, energy in a
given time interval is calculated by counting these impulses; the higher the pulse frequency, the higher the instantaneous power, and vice versa.
DC Meter Architecture The basic architecture of a dc meter is represented in Figure 5. In order to measure the power consumed by the load (P = V × I), at least one current sensor and one voltage sensor are required. When the low side is at earth potential, current flowing through the meter is commonly measured on the high side to minimise the risk of unmetered leakages, but current can also be measured on the low side, or both sides if required by the design architecture. The technique of measuring and comparing currents on both sides of the load is often used to enable the meter with fault and tamper detection capability. However, when the current is measured on both sides, at least one current sensor needs to be isolated in order to deal with the high potential across the conductors.
Figure 3. Fewer components are required in a dc supply for data centres, and there are lower losses than with traditional ac distribution.
Voltage Measurement Voltage is typically measured with a resistive potential divider, where a ladder of resistors is used to proportionally reduce the potential to a level compatible with the system ADC input. Due to the large amplitude of the input signal,
an accurate voltage measurement can be easily achieved with standard components. However, attention must be paid to temperature coefficients and voltage coefficients of the chosen component, in order to guarantee the required accuracy across the entire temperature range. As previously discussed, dc energy meters for
Figure 4. Renewable energy integration in a dc data centre.
applications such as EV charging stations are sometimes required to bill exclusively for the energy transferred to the vehicle. In order to fulfill the measurement requirement, dc energy meters for EV chargers may be required to have multiple voltage channels, enabling the meter to sense the voltage also at the entry point of the vehicle (4-wire measurement). DC energy metering in a 4-wire configuration ensures that all the resistive losses of the charging pile and the cable are discounted from the total energy bill.
Current MeasureMent for DC energy Metering
Electric current can be measured either by direct connection or indirectly, by sensing the magnetic field generated by the flow of the charge carrier. The next section will discuss the most popular sensors for dc current measurement.
Shunt Resistor Direct connection current sensing is a tried and tested method of measuring ac and dc current. The flow of current is routed through a shunt resistor of known value. The voltage drop across the shunt resistor is directly proportional to the current flowing as described by the well-known Ohm’s law (V = R × I), and it can be amplified and digitised, providing an accurate representation of the current flowing in the circuit. Shunt resistor sensing is a cheap, accurate,
Figure 5. DC energy meter system architecture. 10
and powerful method for measuring current from mA to kA, with theoretically unlimited bandwidth. However, the method suffers from some disadvantages.
August 2021 Instrumentation Monthly
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