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Computational issues and compensation algorithms

AN3322

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Computational issues and compensation algorithms

This section deals with issues that can influence measurements and computation and were

not mentioned in Section 1.4: Error sources on the STM32 meter. The topics covered in this

section deal with coherent sampling, precise frequency measurement, proper zero crossing

detection and phase shift correction. The correct approach used when dealing with this topic

leads to the precision improvement and makes it possible to keep precision in the required

range.

Coherent sampling is the sampling of the periodical signal where the number of the samples

per period of the signal is still the same. The sampling time of the first sample is still in the

same time position in relation to the phase of the sampled signal. The timing of the following

samples is still the same and constant. In this case, 44 samples per period of the mains

(50 Hz, period 20 ms) are always kept.

Non-coherent sampling is, on the contrary, the sampling of the periodical signal where the

number of the sample per period of the signal is not the integer number. The sampling does

not need to start in the same position in relation to the phase of the sampled signal. The

timing of the subsequent samples can vary. For example, 44.76 samples per period of

mains are acquired. It means that the sample rate is a bit higher than required in the case of

coherent sampling.

2.1

Non-coherent sampling issues

If non-coherent sampling is present (it means no integer number of samples per period of

mains is taken), it is possible to observe an additional disrupting signal in the graph of

computed instant power from such sampled current and voltage. The additional disrupting

signal consists of two frequencies: a high frequency FH and a low frequency FL.

Figure 11. Instant power(1)

1. Y-axis: voltage, X-axis: cycles of mains/100 (50 Hz).

When using the non-coherent sampling, the measured period of the signal is not completely

covered and every period and this coverage differs from period to period. The amount of the

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