71M6521DE Datasheet, PDF (48/107 Pages) Teridian Semiconductor Corporation – Energy Meter IC

71M6521DE/DH/FE Data Sheet
FUNCTIONAL DESCRIPTION
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] = P ² + Q²
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 71M6521DE/DH/FE 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 will yield an accurate quantity for the momentary energy. Summing up the momentary
energy quantities over time will result 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 15: Voltage. Current, Momentary and Accumulated Energy
Figure 15 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 20ms. The application of 240VAC and 100A results in an
accumulation of 480Ws (= 0.133Wh) over the 20ms 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.
Page: 48 of 107
Rev 2