STPM10 datasheet(39/51 Pages) STMICROELECTRONICS

39 / 51 page

STPM10

Theory of operation

Doc ID 17728 Rev 3

39/51

Equation 13

[see Figure 26 - 11]

In this way, the AC part V•I•cos(2

ωt + ϕ)/2 has been removed from the instantaneous

power.

The absence of any AC component allows for a very fast calibration procedure. It requires

only the setting of (using the internal device programming registers) the voltage and current

sensor conversion constants, using the effective voltage and current (Vrms, Irms) readings

provided by the device’s built-in communication port, avoiding the time-averaged readings of

the active power or the need for line synchronization.

7.23.2

Reactive power

The reactive power is produced using the previously-computed signals. In case of shunt

sensor the voltage signal is derived while the current signal is not. A first computation is to

multiply the DS value of the integrated voltage channel with the value of the integrated

current channel, which yields:

Equation 14

The second is to multiply the filtered DS value of the voltage channel with the value of the

filtered current channel:

Equation 15

From the above results, Q1(t) is proportional to 1/

ω, while Q2(t) is proportional to ω. The

correct reactive power would result from the following formula:

Equation 16

Since the above computation would need significant additional circuitry, the reactive power

in the STPM10 is calculated using only the Q1(t) multiplied by

ω, which means:

Equation 17

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