User manual

PQM-702, PQM-703 Operating Manual

measurement window and is approximately 5 Hz.

The analyzer collects 2048 samples per measurement window (for 50 Hz and 60 Hz), thus it

fulfills the requirement for FFT stating that the number of samples subject to transformation equals

a power of 2.

It is essential to maintain a constant synchronization of the sampling frequency with the mains.

FFT may be performed only on the data containing an integer multiple of the network period. This

condition must be met in order to minimize the so-called spectral leakage, which leads to falsifying

information about the actual levels of spectral lines. The analyzer meets these requirements, as the

sampling frequency is stabilized by the phase locked loop (PLL).

Because the sampling frequency may fluctuate over time, the standard provides for grouping

the main spectral lines of harmonics with the spectral lines located in their direct vicinity. The reason

is that the energy of components may partially pass into adjacent interharmonics components.

Two methods of grouping are provided:

 harmonic group (includes the main line and five or six adjacent interharmonic components),

 harmonic subgroup (includes the main line and one of each adjacent lines).

Fig. 27. Determining harmonics subgroups (50 Hz system)

3.8 The method for measuring interharmonics

Interharmonics measurement is carried out according to IEC 61000-4-7 and IEC 61000-4-30.

They provide a method of calculating the individual components of interharmonics in power

quality analyzers.

The whole process, similarly as in case of harmonics, consists of several steps:

 synchronous sampling (10/12 periods),

 FFT (Fast Fourier Transform),

 grouping.

FFT analysis for the test window of 10/12 period (approx. 200 ms). As a result, we obtain a set

of spectral lines from 0 Hz (DC). The distance between successive lines results from the duration

of the measurement window and is approximately 5 Hz.

Example

In order to calculate the 3rd harmonic component in 50 Hz system, use

150 Hz main spectral line and adjacent lines 145 Hz and 155 Hz. The re-

sulting amplitude is calculated using RMS method.