Summary:In the energy meter industry, many meters use shunts as current sensors. This is mostly because they...
In the energy meter industry, many meters use shunts as current sensors. This is mostly because they cost less than other types of current sensors and provide magnetic immunity that makes them a good choice for one-phase meters. However, this shunt-based approach also presents some challenges, including the need to isolate each shunt from the meter's core and heating issues caused by shunts.
Shunt-based metering requires two important features: complete isolation between the shunt and a meter's core and coherent ADC outputs for detecting the same phase currents on all phases. Fortunately, these requirements are addressed in a new family of analog-to-digital converters (ADCs) from Analog Devices.
The new ADS131M04 devices are multichannel, simultaneously-sampling, 24-bit, 2nd order delta-sigma ADCs that have internal calibration features and wide dynamic range. These ADCs are ideal for energy metering applications that need high accuracy and low power consumption.
Isolation between shunt and meter is achieved through a simple resistive voltage divider (R32-R48). The divider is arranged in a parallel-strip configuration with a shunt on one side, the ADCs on the other side, and the ground between them. The ADCs must be isolated from the circuit to avoid a common-mode voltage that can damage the meter's analog-to-digital converter (ADC).
To address this problem, an isolation amplifier is added between the ADCs and the divider. This amp can reduce the common-mode voltage to an acceptable level.
Another way to ensure isolation is to insert the shunt in the grounded conductor, instead of the ungrounded conductor. This eliminates the common-mode voltage from the shunt and prevents it from damaging the ADC or the instrument's internal circuitry.
Shunt-based metering also requires an accurate ADC to sense the low output voltages produced by shunts at low currents. This is especially true of shunt-based meters used in residential applications that are referenced to the Mains. If a shunt-based meter is not properly calibrated, it may read a high-current, low-voltage circuit as a high-current, low-voltage one and result in inaccurate billing for utility customers.
In addition, a shunt-based metering system could be tampered with by placing a magnetic device on the shunt to paralyze its operation. This magnetic tampering can affect the CT that is used to sense the line current, resulting in false meter readings and electricity theft.
A tampered meter could also have its RMS voltage and active power removed. This tampering would be difficult to detect because the RMS voltage and active power are not measured. Nevertheless, the presence of the shunt and CT still gives a clue that a tampering event has occurred.
The design has an AC/DC power supply that can be triggered to enter a current-detection mode when the RMS voltage or active power are missing. The meter can then be triggered to exit this current-detection mode when the AC power is restored to normal.
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