Python demonstration of power quality harmonics

Описание: We all know that the voltage and current of alternating current (AC) theoretically follow clean sine waves. However, in reality, the observed current waveforms often exhibit periodic non-sinusoidal shapes.

Periodic non-sinusoidal waveforms can be represented using Fourier series. Each term in the Fourier series is called a harmonic component of the distorted waveform. The frequencies of these harmonics are integer multiples of the fundamental frequency.

According to the definition by IEEE 519: "Harmonics are defined as periodic fluctuations in voltage or current whose frequency is an integer multiple of the fundamental frequency of the power system (50/60Hz)."

The most common index for assessing the degree of harmonic impact on power quality is the Total Harmonic Distortion (THD). It is expressed by calculating the ratio of the sum of the effective values of all harmonic currents to the effective value of the fundamental current.

Using the Python programming language, we can demonstrate the generation and impact of harmonics, as well as explain the working principles of active harmonic filters (AHF).

Firstly, harmonic numbers and current amplitudes are defined in a text file. Python then reads these parameters from the file, generates the fundamental and total current waveforms, and calculates the THDi. Then, we will display the obtained current waveforms respectively.

In the first subplot, the total current and fundamental waveforms are displayed, with the THDi value annotated.
The second subplot shows the sum of harmonic currents excluding the fundamental.
The third subplot plots the frequency spectrum of the harmonics.
The fourth subplot displays the output current of the active harmonic filter.

Two frequently asked questions are addressed:

Is it possible for THDi to be greater than 100%?

The answer is Yes, THDi can exceed 100%, especially when the fundamental current decreases. For example, if we reduce the amplitude of the harmonic fundamental current from 100A to 10A, the THDi value becomes 664%, indicating that THDi can indeed be greater than 1.

what's active harmonic filter working principle

AHF collects current information from the load using current transformers (CT). The AHF's DSP rapidly calculates the amplitude and spectrum of harmonics, controlling the AHF to output an opposing harmonic current, thereby achieving harmonic compensation. It is evident that the response speed and compensation accuracy are key parameters for evaluating AHF performance.

For details on the standards required for harmonic compensation in practical projects, stay tuned. In the next video, I will introduce the three versions of IEEE 519 and analyze their requirements for harmonic compensation.

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