It is well known that harmonics are a common interference signal generated by a non-linear load, which affects the normal operation of the device and is a parameter of great concern in the design of electrical equipment. Especially for grid-connected equipment, the harmonics generated by it will flow into the grid and cause pollution, which will affect the power quality of the nearby power supply environment.
IEC61000-4-7 harmonic measurement standard
To accurately measure the fluctuating voltage and current in the grid-connected equipment, it is necessary to use the time window of 10 signal periods and the spectrum of 5 Hz according to the IEC61000-4-7 standard (corresponding to the national standard GB/T-17626.7-2008). Resolution performs FFT analysis on the signal. However, if you want to ensure that the FFT results are true and effective, the measuring instrument needs to deal with the common spectral aliasing and spectrum leakage problems of the FFT. Ordinary power meters do not have specific measures for these two problems, so the harmonic measurement results are biased, and the harmonic analysis function is also a chicken rib.
Figure 1 IEC61000-4-7 harmonic measurement standard
How to solve the "blocking tiger" of the two harmonic measurements of frequency aliasing and spectrum leakage? In addition to the harmonological requirements of the IEC61000-4-7 harmonic measurement standard, the Zhiyuan electronic PA300 series power meter is equipped with the following two “single weapons†to solve the two harmonic measurements of spectrum aliasing and spectrum leakage. Stop the road."
Figure 2 PA300 Series Power Meter
Suppress spectral aliasing and spectrum leakage from the source
Anti-aliasing filter: According to the sampling theorem fs≥2f, if the input signal contains the maximum frequency f > half of the sampling frequency fs, the sampled signal will not be restored correctly. The power meter has a built-in pure hardware anti-aliasing filter that removes the high-frequency clutter of the signal under test to prevent spectral aliasing.
Figure 3: High frequency noise causes spectral 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 occurs during FFT, which affects the accuracy of the amplitude measurement. In order to minimize the spectrum leakage, the PA300 uses the phase-locked loop frequency multiplication technology to synchronize the sampling frequency to the fundamental frequency of the input signal, and eliminates the spectrum leakage phenomenon 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 histograms, trend graphs, and waveform graphs. These functions can be measured and displayed through PAM software. The data can be selected to be uploaded to the PC by USB, GP-IB, RS-232 or Ethernet communication interface, and the synchronous display and data report export can be realized on the PC side through the PAM software, which truly solves the problem of harmonic measurement for the user.
Figure 5 PAM software
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