How to overcome the obstacles in harmonic measurement?

　　As we all know, harmonics are a common interference signal generated by nonlinear loads, which will affect the normal operation of equipment and is a parameter that is of great concern when designing electrical equipment. Especially for grid-connected equipment, the harmonics generated by it will flow into the power grid and cause pollution, affecting the power quality of the nearby power supply environment.

　　IEC61000-4-7 Harmonic Measurement Standard

　　To accurately measure the fluctuating voltage and current in grid-connected equipment, it is necessary to perform FFT analysis on the signal with a time window of 10 signal cycles and a spectrum resolution of 5Hz according to the formula specified in the IEC61000-4-7 standard (corresponding to the national standard GB/T-17626.7-2008). However, to ensure that the FFT results are true and valid, the measuring instrument needs to deal with the common spectrum aliasing and spectrum leakage problems of FFT. Ordinary power meters do not have targeted measures for these two problems, so there are deviations in the harmonic measurement results, and their harmonic analysis function has become useless.

　　Figure 1 IEC61000-4-7 harmonic measurement standard

　　How to solve the two "obstacles" of harmonic measurement, frequency aliasing and spectrum leakage? To this end, ZHIYUAN Electronics PA300 series power meter is equipped with the following two "unique weapons" to solve the two "obstacles" of harmonic measurement, spectrum aliasing and spectrum leakage.

　　Figure 2 PA300 series power meter

　　Suppress spectrum aliasing and spectrum leakage from the source

　　Anti-aliasing filter: According to the sampling theorem fs≥2f, if the maximum frequency f contained in the input signal is greater than half of the sampling frequency fs, the sampled signal cannot be restored correctly. The power meter has a built-in pure hardware anti-aliasing filter, which can remove the high-frequency clutter of the measured signal to prevent the occurrence of spectrum aliasing.

　　Figure 3: High-frequency noise causes spectrum aliasing

　　Phase-locked loop technology: When the sampling period is not an integer multiple of the signal period, the intercepted sampled data is not a complete periodic signal, and spectrum leakage will occur during FFT, affecting the accuracy of amplitude measurement. In order to minimize spectrum leakage, PA300 uses phase-locked loop frequency multiplication technology to synchronize the sampling frequency with the fundamental frequency of the input signal, eliminating the occurrence of spectrum leakage from the root.

　　Figure 4 Synchronous sampling effectively avoids spectrum leakage

　　Power meter function expansion--PAM software

　　While ensuring accurate harmonic measurement, solving harmonic problems usually requires viewing harmonic data, harmonic bar graphs, trend graphs, and waveform graphs. These functions can be measured and displayed through PAM software. Data can be uploaded to the PC via USB, GP-IB, RS-232 or Ethernet communication interfaces, and synchronized display and data report export can be achieved on the PC through PAM software, truly solving harmonic measurement problems for users.

　　Figure 5 PAM software