Harmonics in Modern Power Systems: Definition, Causes, and Solutions

Understanding Harmonics: A Rising Concern in Power Systems

As modern power systems evolve with rapid advancements in power electronics, one issue has become increasingly critical yet often overlooked: harmonics. The integration of high-power semiconductor devices such as thyristors, MOSFETs, and IGBTs has led to a significant rise in non-linear loads across power grids. These loads distort the sinusoidal nature of current and voltage waveforms, resulting in harmonics — a major power quality challenge that can no longer be ignored.

In earlier systems dominated by linear loads, power quality was largely the responsibility of the utility. However, as today's energy users adopt more diverse and sophisticated equipment, power quality indicators such as harmonics, voltage flicker, and imbalance are now heavily influenced by the user side. A clear understanding of harmonics is essential for anyone involved in system design, operation, or troubleshooting.

1. What Are Harmonics?

According to China's national standard GB/T 24337 (Power Quality - Interharmonics in Public Power Grids), harmonics are defined as the components of an electrical signal whose frequencies are integer multiples of the system's fundamental frequency.

In China, the standard grid frequency is 50Hz. Harmonics at 100Hz, 150Hz, 200Hz, etc., are thus referred to as the 2nd, 3rd, 4th harmonics, and so on. Harmonics can be classified in two major ways:

By sequence: Positive sequence, negative sequence, and zero sequence.

By frequency multiple:

• Odd-order harmonics (3rd, 5th, 7th, etc.)
• Even-order harmonics (2nd, 4th, 6th, etc.)

Odd harmonics are more commonly encountered in practice and can be monitored via system back-end data or waveform analysis tools.

2. What Causes Harmonics?

Harmonics typically originate when sinusoidal voltages are applied to non-linear loads. Common sources include:

• Power electronic equipment: Variable frequency drives (VFDs), rectifiers, and inverters, which generate dominant 3rd, 5th, 7th and higher-order harmonics.
• Industrial machinery: Arc furnaces, fluorescent lighting, and induction heaters.

These devices disrupt the linear relationship between voltage and current, causing waveform distortion and the injection of harmonic currents into the system. Over time, this can lead to overheating, insulation failure, equipment malfunction, false tripping, and even widespread system instability.

In recent years, the growth of renewable energy systems (e.g., PV and wind) has introduced more inverters and converters into the grid, increasing the harmonic footprint and placing higher demands on modern power system control.

3. How to Mitigate Harmonics: A Multi-Pronged Strategy

Effective harmonic mitigation requires a combination of proactive design, strategic equipment selection, and corrective technologies. Key approaches include:

(1) Source-side control: The most cost-effective and thorough method. By optimizing load design and selecting low-harmonic devices, the generation of harmonics can be minimized at the source.

(2) Equipment protection: For sensitive devices, apply isolation or electromagnetic interference (EMI) mitigation strategies to reduce the impact of harmonics.

(3) Filtering solutions:

• Passive filters: Cost-effective and suitable for filtering specific fixed-frequency harmonics.
• Active filters: More versatile and effective in dynamic environments but come at a higher cost.

Note: Filter deployment must be carefully evaluated to avoid potential resonance with the grid.

Conclusion

Harmonic distortion is an increasingly urgent issue in modern power systems. As energy infrastructures grow more complex, identifying, analyzing, and mitigating harmonics must become a standard practice for engineers and energy professionals. With the right knowledge and tools, power quality can be safeguarded, ensuring both reliability and efficiency in the next generation of electrical networks.

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