AN2299

Phase calibration

Rev2

19/26

4

Phase calibration

4.1

Phase error definition

STPM01 does not introduce any phase shift between voltage and current channel.

However, voltage and current signals come from transducers, which could have inherent

phase errors. Phase shift is usually introduced by the current sensor, since the voltage

sensor, normally a resistor divider, does not introduce any delay. For example, a phase error

of 0.1° to 0.3° is not uncommon for a current transformer (CT). These phase errors can vary

from part to part, and they must be corrected in order to perform accurate power

calculations.

At phase angle current to voltage

φ= 0° (power factor PF = 1) the amplitude errors of

voltages and currents determine the actual accuracy of the instrument.

In this case there is no phase shift between voltage and current and Eq. 1 becomes:

This is the target measured power to reach with calibration.

At phase angles other than zero, e.g. ±60° (PF = 0.5) the phase angle errors between

voltages and currents of each phase also affect the actual accuracy in addition to the

amplitude errors described above.

Let us assume that voltage and current RMS values are already calibrated, so they do not

introduce any error in measured power.

Figure 5.

Influence of phase angle errors in power measurement

Formula for the relative error of power can be expressed as:

PU

I

•

=

Eq. 31

PU

I

φ

cos

•

=

real active power;

P'

UI

φδ

+

()

cos

•

=

measured active power;

φ[°]

‘=cos(φ−δ)

=cosφ

‘=cos(

φ+δ)

P

P

P

e

P'

P

-----

1

–

100

Φδ

+

()

cos

φ

cos

-----------------------------

1

–

=

•

100

•

=

[%]

Eq. 32