In this product how-to article Radomir Kozub describes how to use Freescale’s 8-bit MC9S08LH64 to build an intelligent and accurate single-phase power metering design, while keeping a low cost bill of materials.
Electro-mechanical power meters have now been replaced by electronic versions. Electronic meters have a number of advantages over their electro-mechanical predecessors. Namely, the mechanical construction is more cost-effective and is simpler due to the fact that there are no moving parts.
Electronic PMs also have better accuracy coupled with wider dynamic range. Electro-mechanical PMs have a standard dynamic range of 1:80 at two percent accuracy; whereas today’s static PMs have a dynamic range of power measurement of approximately 1:1000 with one percent accuracy.
This article focuses on the construction and features of a static single phase power meter with direct measurement. Static means that Power Meters (PMs) do not contain any mechanical parts; rather, static meters are microcontroller based.
Direct measurement is the current flowing to the load - it is sensed on the shunt resistor. Voltage and current is measured using a high-precision AD converter; power is then calculated using the measured values and finally the power is summed in time so that total active energy [Wh] can be determined. Considering that voltage is stable in the mains, dynamic range of the PM is given by the dynamic range of the current measurement.
In addition, electro-mechanical PMs register and display only active energy value; while electronic PMs can measure and show additional information like active power, voltage, current RMS, peak values, line frequency, power factor or temperature. These values may then be read via an electronic interface, such as RS232, RS485, MBUS or IRDA.
A static PM provides enhanced security as well. As today’s electricity rates continue to climb, tampering has become an increasingly common phenomenon. An electronic-based power meter has the capability to sense direction change in current flow, partial earth, missing neutral condition or lid removal.
Power consumption is an important component in meter design as well. Millions of the PMs currently installed consume a significant amount of power. Thus, the power budget for the meters themselves must be strictly limited. Total losses in current sensor and meter electronics are limited to 2W and 10VA.
Static single phase power meters should be compliant with EN62056 and EN50470 regulation. Particular requirements for static electricity meters for active energy (class indexes A, B and C) are stated in EN50470-3.
This regulation describes the requirements for voltage, current circuits, self power consumption and various tests for accuracy, the influence of noise, temperature, over and under voltage, over current or harmonics.