User manual

3 Design and measurement methods

Fig. 28. Determining interharmonics subgroups (50 Hz system)

Each interharmonic subgroup is the sum of RMS for seven (for 50 Hz mains) or nine (for 60 Hz

mains) spectral lines obtained by Fourier transform. The exception is zero subgroup, i.e. sub-har-

monic subgroup that contains one line more - 5Hz. It is presented in Fig. 28 with an example of

50 Hz network. Interharmonic subgroup of 0 order, i.e. subharmonic, consists of eight lines with

frequencies from 5Hz to 40Hz. Each following interharmonic subgroup consists of seven lines lo-

cated between harmonic subgroups, e.g. subgroup of 1st order includes spectral lines of frequen-

cies from 60 Hz to 90 Hz. In case of subharmonic subgroup, the range of spectral lines was ex-

tended by 5 Hz line, otherwise the energy in this lowest frequency band would not be included and

would be lost. All subsequent spectral lines are included either in the harmonic subgroup or inter-

harmonic subgroup.

Similarly as in case of the harmonics, interharmonics are calculated at least to 50th order; for

mains frequency of 50 Hz it gives a range of slightly above 2.5 kHz, and for 60 Hz, a range of slightly

above 3 kHz.

3.9 Measurement of ripple control signals.

The option for measuring ripple control signals is available only for PQM-703.

The analyzer allows user to monitor two user-defined frequencies in the range up to 3000 Hz.

After exceeding the threshold limit defined by the user, the analyzer records the signal level for a

specified period of time (up to 120 seconds). As a standard, the analyzer measures the average

values of signals for the time interval selected in settings (the main averaging period). When record-

ing acc. to EN 50160 is selected, then additionally all 3-second average values are recorded for

both frequencies - they are compared with limits specified in the standard (when the report is pre-

pared).

3.10 Measurement of transients

The option for measuring transients is available only for PQM-703.

Analog-to-digital converters, typically used in power quality analyzers, have relatively low sam-

pling frequency and are insufficient to provide required accuracy of transient recording due to the

short-term nature of these disturbances and their wide frequency spectrum. For this reason, PQM-

703 uses a separate 4-channel converter with a maximum sampling frequency of 10 MHz. This

corresponds to the time between individual samples of 100 ns. In this mode it is possible to record

the fastest transients, and the recording time reaches 2 ms.