Frequently asked questions about harmonics

Harmonics are not a new concept. However, not everyone knows it. In this article, we will answer harmonic questions to better understand this type of wave.

Frequently asked questions about harmonics

What are harmonics?

Harmonic is an unwanted form of noise. This type of wave directly affects grid quality. It should be paid attention to when the total harmonic currents are higher than the allowed level. Harmonic current is a current whose frequency is a multiple of the fundamental frequency. More detail…

Harmonics are characterized by complete oscillation over the fundamental industrial frequency spectrum. The harmonic component in the AC source is defined as the sine component of a wave cycle whose frequency is equal to the integer times the fundamental frequency of the system.

fh = h.fb

In which: h is a positive integer.

High-level harmonics measuring machine

What are the sources of harmonics?

• Industrial loads: Power electronics, arc furnaces, welding machines, electronic starters, circuit breakers of large power transformers, etc.
• Civil loads: Gas discharge lamps, televisions, photocopiers, computers, microwaves, etc.

With many different measures, one can reduce some harmonics to a negligibly small value. Completely suppressing harmonics is certainly not possible.

What causes harmonics?

The cause of the harmonic generation is due to nonlinear loads in the electrical system. The input voltage of the nonlinear load is sinusoidal but the current through it is non-sinusoidal.

Any waveform is the sum of sinusoidal waveforms. When homogenized from one cycle to another it can be described as the basic sine wave and a multiple of the fundamental frequency. That means including the basic sine wave and the series of a high-harmonic sine waveform, called the Fourier series.

The calculation process can be independent of each individual harmonic. The calculated result of each frequency will be combined into a form of Fourier series to get the general output waveform if needed. Usually, only the amplitude of the harmonics is needed.

When both positive and negative half of a waveform has a homogeneous form, the Fourier series contains only odd harmonics. Because most devices that produce harmonics usually have a uniform waveform. So in reality, we only need to pay attention to odd harmonics 3,5,7, …

Are there any methods that help reduce harmonics?

Replacing damaged equipment which is caused by harmonics is an expensive solution, increasing investment and operating costs but still unable to reduce the influence of harmonics. Therefore, it is advisable to choose less expensive but highly effective solutions to control harmonics such as:

Use an AC reactor or DC reactor for inverter

This is the best solution for applications that need filtering for heavily disturbed grid sources. At the same time asking harmonic reduction is not a top priority.

Reactor for inverter

12-pulse rectifier solution

This solution offers the highest efficiency in reducing harmonics but the process is the most complex.

Use inverter with low harmonics

The inverter uses harmonic reduction technology without using an external filter or multi-pulse transformer. Total harmonic current distortion (THDi) is less than 5%.

Use filters

This method is applicable to multiple inverters mounted in parallel on the same distribution line. The main task is power compensation, voltage harmonic compensation and current harmonic compensation.

Types of harmonic filters

• Zero-harmonic filter (ZSF): reduces or eliminates the zero-harmonic with zigzag winding transformers. Filter efficiency is about 60% – 95%.
• Active Harmonic Filter (active harmonic filter): minimizes harmonics by producing opposite harmonics. Filter efficiency of 80% – 95%.

How harmful are harmonics on the grid?

5th harmonic harms the grid distortion

This is a question about harmonics that are most concerned by business owners, factories, and factories.

Harmonics are unwanted forms of noise that directly affect power quality. They occur when using non-linear loads (inverters, voltage converters, UPS, etc.). Harmonic impacts very badly on the equipment and machinery used in the factory such as:

• Reduce engine life.
• CB overload, overheating and an explosion of the transformer (while the amount of electricity used is still smaller than the norm).
• The circuit breaker, the fuse may be affected for unknown reasons.
• Reduce capacitor life, even cause abnormal capacitor banks.
• Interfering with affecting telecommunications equipment, automation systems such as PLC, Role, …
• The measuring device is not working properly.
• Waste of energy …

Capacitor explosion due to high harmonic

-> Learn more: Solutions to filter harmonics