STPM01FTR datasheet(46/56 Pages) STMICROELECTRONICS

46 / 56 page

Theory of operation

STPM01

46/56

8.24.3

Apparent power and RMS values

The RMS values are calculated starting from the following formulas.

Shunt or Current Transformer

[Eq. 20]

multiplying Eq 20 by

ω, the I

RMS value is obtained:

[Eq. 21]

The RMS voltage value is obtained as:

[Eq. 22]

For the Apparent Power another value is produced:

[Eq. 23]

Multiplying Eq.20 and Eq. 23, the Apparent power is produced:

[Eq. 24]

Rogowsky Coil

In this case we have:

[Eq. 25]

while VRMS is calculated as in Eq. 22.

The Apparent Power is simple calculated multiplying Eq.25 and Eq. 22.

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

integrators are implemented as Up/Down counters and they can rollover. 20-bit output buses

of the counters are assigned as most significant part of energy data records. It is a

responsibility of an application to read the counters at least every second not to miss any

rollover.

)

(

⋅

∫

RMS =

)

(

RMS

∫

)

(

⋅

′

∫

⋅

)

(

RMS

′′

∫

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