71M6545 Datasheet, PDF (66/134 Pages) Maxim Integrated Products – Four-Quadrant Metering, Phase Metrology Processors Flash/RAM Size

Data Sheet 71M6545/H
PDS_6545_009
3 FUNCTIONAL DESCRIPTION
3.1 Theory of Operation
The energy delivered by a power source into a load can be expressed as:
t
E = ∫V (t)I (t)dt
0
Assuming phase angles are constant, the following formulae apply:
 P = Real Energy [Wh] = V * A * cos φ* t
 Q = Reactive Energy [VARh] = V * A * sin φ * t
 S = Apparent Energy [VAh] = _P² _{+ Q}²
For a practical meter, not only voltage and current amplitudes, but also phase angles and harmonic
content may change constantly. Thus, simple RMS measurements are inherently inaccurate. A modern
solid-state electricity meter IC such as the Teridian 71M6545/H functions by emulating the integral
operation above, i.e. it processes current and voltage samples through an ADC at a constant frequency.
As long as the ADC resolution is high enough and the sample frequency is beyond the harmonic range of
interest, the current and voltage samples, multiplied with the time period of sampling yields an accurate
quantity for the momentary energy. Summing-up the momentary energy quantities over time results in
accumulated energy.
500
400
300
200
100
0
0
-100
5
10
15
20
-200
-300
-400
Current [A]
Voltage [V]
Energy per Interval [Ws]
Accumulated Energy [Ws]
-500
Figure 21: Voltage, Current, Momentary and Accumulated Energy
Figure 21 shows the shapes of V(t), I(t), the momentary power and the accumulated power, resulting from
50 samples of the voltage and current signals over a period of 20 ms. The application of 240 VAC and
100 A results in an accumulation of 480 Ws (= 0.133 Wh) over the 20 ms period, as indicated by the
accumulated power curve. The described sampling method works reliably, even in the presence of dynamic
phase shift and harmonic distortion.
66
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