We have question regarding implementation of 3-phase power metering based on FFT metering libraries. We are deciding whether to use qt timer based U-I phase shift compensation or software based. 1. Looking at AN5007 section 6.2 there is a FFT sample proce

[davidmalovrh]

Question regarding implementation of 3-phase power metering based on FFT metering libraries, deciding whether to use qt timer based U-I phase shift compensation or software based.

1. Looking at AN5007 section 6.2 there is a FFT sample process

    - how to trigger SD conversion (312us interval)?

    - is there additional delay needed between SD triggering for each phase as in Figure 5.?

2. In case we use software based U-I phase shift compensation

    - how to fill phase-shift correction buffer for METERLIBFFT3PH_InitMainBuffPhX?

    - is it ok to calculate phase shift correction based on power factor during calibration
      process and use same value for each harmonic in buffer?

MarMi‌

ludek

[egoodii]

But you might also note that if you take, for instance, 32 samples per phase, that's 96 samples total, or one every 3.75 degrees.  120 degrees is indeed exactly a 32-sample delay.  So with these regularly-placed interleaved-samples of all three phases you get exact in-phase alignment of a binary-count of samples on each phase (convenient for FFTs of course!) --- if sample '0' is 0-degrees on phase A, sample 32 will be 0-degrees on phase B, and sample 64 will be 0 degrees on phase C.

[Ludek]

All phases (ADC results) are absolutely independent in this case. You may imagine it as three 1-ph meters, in fact. You will get three independent binary buffers. For your 32-samples example it means, that you sample input periodic signal every 11.25 degree (360/32) on each phase:

• sample '0' on phase 'A' = 0degree, sample '1' on phase 'A'= 11.25degree ...... sample '31' on phase 'A' = 348.75degree
• sample '0' on phase 'B' = 0degree, sample '1' on phase 'B'= 11.25degree ...... sample '31' on phase 'B' = 348.75degree
• sample '0' on phase 'C' = 0degree, sample '1' on phase 'C'= 11.25degree ..... sample '31' on phase 'C' = 348.75degree

sample '32' = sample '0' of the next period, etc., etc.

[Ludek]

• fs=fadc/OSR, for example: 6.144MHz/1024=6kHz => sampling interval is 166usec => for 50Hz main you get 120 samples
• There should be an additional delay (FDL) for the 3-ph topology, according the FFT_KM3x.pdf document, page 20. This delay simulates the ADC-SAR conversion time (~9usec) due to not having all ADC results at one time - we have only one SAR and need three results.

   2.

• METERLIBFFT3PH_InitMainBuffPhX function sets the pointer to the phase shift correction buffer [0.001°] and U/I pointers to the real and imaginary buffers - see the Chapter 5.11 in AN4255. See also simple application examples (meterlibfft3ph_test.c) in AN4255SW associated file how to use and fill these buffers.
• Phase shift correction values can be calculated using power factor (or P, Q, S values) during the calibration process - the 1st harmonic only, other harmonics are not used (0) during the calibration. See also the 1-ph application example - DRM163 - there is applied this calibration process. You should only adopt it for your 3-ph application.