What interharmonics are & do and where they come from?

Introduction

The use of advanced power electronics and communication systems is improving power system efficiency, flexibility, and reliability, but it is also increasing interharmonic distortion. Knowledge of interharmonics, their sources, effects, measurement, limits, and mitigation will help the industry prevent interharmonics from adversely affecting the power system.

Interharmonic Definitions

The IEEE defines interharmonics as:

A frequency that isn’t a whole number multiple of the power system’s base frequency (like 50 Hz or 60 Hz).

The IEC defines interharmonics as:

Between the regular harmonics of the power frequency, there are additional frequencies that aren't whole-number multiples of the base frequency. They can appear as discrete frequencies or as a wide-band spectrum.

Table 1 – Harmonic and Interharmonic

Definitions

Interharmonic Sources

Power system interharmonics are most often created by two general phenomena. The first cause is sudden, irregular changes in current and voltage. This occurs when loads are in an unstable state or when adjustments to voltage or current control are applied.

The second source of interharmonics is static converter switching not synchronized to the power system frequency (asynchronous switching).

Oscillations between series or parallel capacitors or when transformers or induction motors saturate can also produce interharmonics.

· Arcing Loads

· Induction Motors

· Electronic Frequency Converters

· Variable Load Drives

· Voltage Sourced Converters (VSCs)

One consequence of the increased control provided by VSCs is the production of interharmonics.

· Power Line Communications

· Smart meter communication

Interharmonic Effects

Interharmonics, like harmonics, add additional signals to the power system. These additional signals can cause various effects, especially when amplified by resonance.

 Two of the most common and significant effects of interharmonics are light flicker and power line communication interference. Both effects arise from non-periodic signals.

· Light Flicker

· Power Line Communication Interference

Interharmonic Measurement and Standards

In the United States, specific interharmonic limits have not been widely adopted. The IEEE has issued several standards for interharmonics: IEEE 519.1 in 1992, IEEE 519.2 in 2001, and IEEE 1453.1 in 2010. Some countries have adopted IEC Standard 61000-4-7 for interharmonic measurement.

Measuring Interharmonics

Measuring interharmonics is challenging due to their non-periodic nature. Traditional Fourier series methods often fail to capture these irregular signals. Instead, experts have proposed several alternative methods, such as the Generalized Cross Correlation Method, Hilbert-Huang Transform, and Windowed FFT. Each method offers varying degrees of effectiveness.

Interharmonic Limits

There is a general agreement that setting specific interharmonic limits is not practical. The impact of interharmonics can differ greatly based on their frequency, magnitude, and location.

Instead of imposing specific limits, apply existing harmonic distortion limits to the total harmonic and interharmonic distortion (THID). Establish specific interharmonic limits only when they cause particular issues.

Approaches to Setting Interharmonic Limits

Several approaches exist for determining interharmonic limits:

• Problem-Based Limits: Set limits according to the specific issues caused by interharmonics.
• Cumulative Effect Limits: Use limits based on the overall impact of interharmonics on the power system, similar to harmonic limits but covering a broader frequency range.
• Source-Based Limits: Establish limits based on the source of interharmonics, such as voltage source converters or arc furnaces. Each approach has its pros and cons, and the best method depends on the specific situation.
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Mitigating Interharmonics

Dealing with interharmonics is complex due to their non-periodic nature and diverse frequency range. Various methods can be used for mitigation:

• Passive Filters
• Active Filters: These filters adjust to changes in frequency and size, making them effective against a wide range of interharmonics.
• Voltage Source Converters
• Load Management
• Communication Systems

Summary

Interharmonics pose a growing concern for the power industry due to the increasing use of power electronics and communication systems. Understanding their sources, effects, measurement, limits, and mitigation is crucial for preventing adverse impacts on power systems. Despite the challenges in measuring and mitigating interharmonics, several effective approaches exist, tailored to specific circumstances.

One of the most effective solutions for interharmonic mitigation is the use of Active Power Filters (APFs). APFs are advanced devices designed to flexibly filter out unwanted frequency components, including interharmonics, from the power system. They provide significant advantages over passive filters, which can only target specific frequencies and might not effectively address a wide range of interharmonics.

Principle and Function of APF in Interharmonic Mitigation

Active Power Filters (APFs) work by injecting corrective currents into the power system to eliminate unwanted harmonics and interharmonics. They use advanced power electronics and real-time digital controls. The main parts of an APF are power converters (often IGBTs) and a control system that detects distortions in the power system.

Key Functions of APFs:

• Real-time Detection and Compensation: APFs use sensors and real-time digital processing to monitor the power system. When they detect interharmonics, they generate a compensating current to counteract these disturbances.
• Adaptive Filtering: Unlike passive filters, APFs adjust their filtering to match changing conditions in the power system. This flexibility allows them to address a wide range of interharmonic frequencies effectively.
• Reducing Resonance Risks: APFs help prevent resonance, which happens when interharmonic frequencies align with the natural frequencies of the system's components. By canceling these frequencies, APFs stop the increase in interharmonic levels.
• Enhancing Power Quality: APFs improve overall power quality by reducing interharmonics. This results in fewer equipment issues, less interference with communication systems, and a more stable and reliable power grid.

Enhanced System Efficiency: APFs contribute to reducing energy losses associated with interharmonics. This results in lower operational costs and improved efficiency of the power system.

Applications of APFs

APFs are widely used in various applications, including:

Industrial Environments: To mitigate interharmonics produced by variable frequency drives, arc furnaces, and etc.

Commercial Buildings: To ensure stable and clean power supply for sensitive electronic equipment and communication systems.

Renewable Energy Systems: To handle the interharmonics generated by renewable energy sources like wind turbines and solar inverters.

HVDC and STATCOM Systems: To manage the interharmonics in high-voltage direct current (HVDC) and STATCOMs.

Active Power Filters (APFs) are essential for managing interharmonics in today's power systems. They efficiently detect and correct a wide range of interharmonic frequencies and adapt to changes in the power system. APFs enhance power quality, making them vital for reliable and efficient power systems. APFs use advanced technology and real-time control to address interharmonic issues, ensuring stable and high-quality power for various uses.

For a tailored solution to your power quality needs, we invite you to consult with our experts. Contact us today to learn how our advanced APFs can optimize your power system’s performance and reliability.

Please contact us: sales@yt-electric.com