STPM10 datasheet(39/47 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

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The reactive power, then, presents a ripple at twice the line frequency. Since the average

value of a sinusoid is 0, this ripple does not contribute to the reactive energy calculation

over time. Moreover, in the STPM10 the reactive power is not used for meter calibration or

to generate the stepper pulses, so this ripple does not affect the overall system

performance.

7.23.3

Apparent power and RMS values

The RMS values are calculated starting from the following formulas:

Equation 18:

√

�� ∫

��2

��

(��)�� =

��

ω⋅√2

Multiplying the equation abov

e by ω, the IRMS value is obtained:

Equation 19:

�� =

��

√2

The RMS voltage value is obtained by:

Equation 20:

�� = √

�� ∫

��2

��

(��)�� =

��

√2

For the apparent power, another value is produced:

Equation 21:

√

��

∫ ��2

��

(��)�� =

�� ⋅ ω

√2

Multiplying equation 18 and equation 21, the apparent power is produced:

Equation 22:

�� =

��

��∙√2

∙

��⋅ω

√2

��

The DSP performs the integration of the computed powers into energies. These integrators

are implemented as up/down counters and they can roll over. 20-bit output buses of the

counters are assigned as the most significant part of the energy data records. An

application reads the counters at least every second, to avoid missing any rollover.

7.24

STPM10 calibration

Energy meters based on the STPM10 device can be calibrated in a fast and simple way.

The calibration is essentially based on the single calibration of the voltage and current

channel considering their RMS values rather than on the frequency of the output pulse

signal. When two channels are calibrated, all other measurements are calibrated too. This

allows the calibration to be performed in only one point, thereby shortening the production

time of the meter. This procedure is possible due to the following key factors:



The device comprises two independent meter channels for line voltage and current,

respectively. Each channel includes its own digital calibrator, to adjust the RMS in the

range of ±12.5% in 256 steps, and a digital filter, to remove any signal DC component.

None of the final results are subject to the calibration procedure because they are

achieved from such corrected signals by mathematical modules implemented by

hardwired DSP

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