Low-cost electric energy meter designed with AD7755

Overview

This article introduces a low-cost, high-precision electric energy meter designed using the AD7755 produced by Analog Devices Inc. (ADI) in the United States. This energy meter is single phase. It is designed as a two-wire system, but it can also easily adapt to the design requirements of special areas. For example, the power distribution system in the United States usually requires residential electricity meters to adopt a single-phase, three-wire system.

Using AD7755 to design an electronic energy meter is a low-cost single-chip solution for energy measurement. AD7755 contains two analog-to-digital converters (ADCs), a reference voltage source and all signal processing circuits used to calculate active power. AD7755 also contains digital frequency converters (F1, F2) that directly drive electromechanical counters (i.e., power recorders) and high-frequency pulse output circuits (CF) for checksum communication.

For the function of AD7755, please refer to the information on pulse output energy measurement integrated circuit.

Table 1 Accuracy requirements

Note: 1. The current ranges with specified accuracy shown in Table 1 are expressed in basic current (Ib). According to section 3.5.1.1 of the IEC1036 (1996-09) standard, Ib refers to the current value consistent with the relevant performance of a directly connected electric meter. The maximum current (Imax) refers to the maximum current that maintains the specified accuracy.
    2. The power factor (PF) listed in Table 1 characterizes the phase relationship between the fundamental (45-65Hz) voltage and current waveforms. In this case PF is simply defined as PF=cos(φ)φ, where represents the phase angle between pure sinusoidal current and voltage.
    3. According to Section 3.5.5 of the IEC1036 (1996-09) standard, the meter rating is defined as the range of percentage error. The percentage error is determined as: Percent error = (measured value - true value) / true value 100%

design purpose

In accordance with the international standard IFC1036 (1996-09) for level 1 and level 2 AC active power meters, it is the main basis for this design. For readers who are more familiar with the ANSIC12.6 standard, please see the last section of this article "Comparison of ANSI C12.16 and ICE1036 Standards". This section only provides some explanations of the main contents of the IEC10236 standard corresponding to ANSI.

In order to meet the accuracy requirements in various situations, such as the accuracy requirements when the power factor (PF) is equal to 1 and the accuracy requirements when the PF is low (PF=±0.5), the design technical indicators should be much higher than the basic technical indicators. In addition, the dynamic range of the electric energy meter also exceeds 500. The IEC1036 standard stipulates that the specified accuracy requirements are achieved within the 5%Ib-Imax current range, and the specified accuracy requirements are achieved within the meter current range, see Table 1. The typical value of Imax is 400% to 600% of Ib. Table 1 lists the accuracy requirements for static energy meters. The current range (dynamic range) corresponding to a company's accuracy is expressed by the basic current (Ib)

Figure 1 shows the working principle diagram of a simple, low-cost energy meter designed for postal use AD7755. In order to provide the necessary current-to-voltage conversion for the AD7755, this design uses a shunt and a simple voltage divider network to attenuate the line voltage. The electric energy recorder (kWh) is a simple electromechanical meter composed of a two-phase stepper motor. The F1 and F2 pins of AD7755 can directly drive this kind of counter. The CF pin can also provide a high-frequency output with an instrument constant of 3200imp/kWh (number of pulses/kilowatt hours). Therefore, this high-frequency output can be used with LED and photoelectric coupler output. This high-frequency output can also speed up the calibration process of electric meters, thereby providing this fast calibration method for producing electric meters. This design uses the resistor network R5-R4 to change the attenuation of the line voltage to calibrate the meter.

Design principles

The AD7755 produces an output frequency that is proportional to the time average of the product of the two input voltage signals. The input voltage values ​​added to AD7755 are V1 and V2. The detailed functions of AD7755 have been introduced in the AD7755 loose-leaf information, see the "Principle of Operation" section. This leaflet provides formulas for calculating the output frequencies F1 and F2 (driving counters), which are proportional to the product of the rms values ​​of the input voltage signal, V1 and V2. For convenience, formula (1) is re-given here. In order to calibrate the electric energy meter to a fixed constant, the V2 value is calibrated using formula (1).

F=(8.06×V1×V2×G×F 1-4 )/V REF 2 (1)

Among them, F is the pulse frequency output by pins F1 and F2, F1-4 is divided by the main clock, G is the gain, and VREF is the reference voltage.

Figure 1 shows the working conditions of the electric meter. The line voltage is 220V and the maximum current (Imax) is 40A. However, as long as the input signals of Channel 1 and Channel 2 are properly calibrated, the energy meter can be designed to operate at any line voltage and maximum current.

AD7755 has 4 frequency selection methods suitable for this type of electricity meter (that is, directly driving the counter), and Imax is as high as 120A. When Ib of this type of meter is selected as 5A, the current range of the specified accuracy is 2%Ib~Imax, or the dynamic range is 400 (i.e. 100mA~40A). The electric meter constant of the electromechanical meter is 100imp/kWh, which means it needs to be recorded 1kWh of electrical energy requires the AD7755 to send out 100 pulses. Section 4.2.11 of the IEC1036 standard stipulates that electromechanical recorders count in decimal form starting from the lowest digit, and each bit is counted in decimals. Therefore, the counter uses a 5-digit integer and 1 decimal display mode, that is, the highest digit corresponds to 10,000, the lower digit corresponds to 1000, 100, 10, 1, and the lowest digit corresponds to 1/10. The instrument constant used for calibration and testing is selected as 3200imp/kWh.