What is a harmonic filter?

introduce

In today’s world, with the wide application of power electronic equipment, harmonic problems have become one of the major challenges for electrical systems. Harmonic filters are an effective solution for reducing these distortions. This article provides a comprehensive overview of harmonic filters, including their types, applications, and their respective advantages and disadvantages.

What are harmonics? Why do harmonics cause problems?

Harmonics are essentially sinusoidal signals with frequencies that are integer multiples of the fundamental frequency of the electrical system (typically 50 or 60 Hz). These harmonics are mainly generated by nonlinear loads, such as:

• Motor controller (frequency converter)

• rectifier

• Switching power supply

• Welding Equipment

• Computers and electronic devices

They occur.

Problems caused by harmonics include:

1. Increased heat loss in equipment

2. Reduced service life of insulating materials and transformers

3. Noise generation in communication systems

4. Failure of the protective relay

5. In a three-phase system, the neutral current increases.

6. Power Factor Reduction

What is a harmonic filter?

Harmonic filters are devices designed to reduce or eliminate certain harmonics in an electrical system. These filters are usually composed of reactive components (inductors and capacitors) and are installed in the system in parallel or in series.

Types of harmonic filters

1. Passive Filter

Passive filters are the most common type of harmonic filters, and consist of a combination of inductors, capacitors, and resistors.

Types of passive filters:

A) Single Tuning Filter

• The simplest and most widely used passive filter

• With the aim of eliminating specific harmonics

• It is typically used for lower harmonics, such as 5th, 7th, 11th, and 13th.

• Simple structure and relatively low cost

Equivalent circuit:  LC circuit in series tuned to the desired harmonic frequency.

B) Two-tuning filter

• Capable of removing two different harmonics simultaneously.

• It takes up less space than two individual filters.

• Lower maintenance costs

• Increased design complexity

C) High-pass filter

• Eliminate Various High-Frequency Harmonics

• It is usually used for harmonics above 17 degrees.

• Different types: Level 1, Level 2 and Level 3

d) Type C filter

• A special type of high-pass filter

• Fundamental Low Frequency Loss

• Suitable for systems with fundamental frequency oscillation.

2. Active Filter

Active filters use power electronics technology to inject opposing harmonics.

Advantages of active filters:

• Capable of removing multiple harmonics simultaneously.

• Dynamic and fast response to load changes

• Independent of system impedance

• Reactive power compensation capability

defect:

• High upfront cost

• The control system is more complex.

• Special maintenance required

Types of active filters:

• Active bypass filter

• Active Filter in Series

• Hybrid filters (combination of passive and active filters)

3. Filter mixer

By combining passive and active filters, you can take advantage of both types of filters.

Harmonic Filter Design Guidelines

1. Harmonic Spectrum Measurement:  Harmonic Measurement and Analysis in the System

2. Calculation of current and voltage harmonics

3. Filter type selection:  based on harmonic spectrum and system specifications.

4. Calculate the filter parameters:  values of inductors, capacitors and resistors.

5. Check the effect of the filter on the system:  Make sure that no resonance is generated in parallel or in series.

6. Filter Loss Calculation

7. Review costs and benefits

Basic Passive Filter Calculations

For a single filter tuned to the harmonic frequency h:

Resonance frequency:

  =  fr 12πLC fr  = 2π  L  C1​ 

This value must be equal to the required harmonic frequency:

fr = h × f1  fr h f   1  ​ =    ×  

( $is\; the$fundamental frequency  of the system )

Filter impedance at resonant frequency:

Zr=R  Z  r  ​=  R

Quality Factor (Q):

Q=XLrR=hXLR  Q  =  R  X  L  r  ​​=  R  h  X  L

Harmonic Filter Location

1. Source Side:  Used for multi-load protection.

2. Load Side: Eliminate harmonics generated by specific loads .   

3. On the main bus:  Protect the entire system

Practical considerations when installing filters

1. Effect of filters on power factor:  Passive filters also typically generate reactive power.

2. Parallel resonance:  There is a risk that some harmonics will be amplified rather than removed.

3. Loss of power in the filter

4. Effects on transient waves

5. Additional protection system required

Comparison of passive and active filters

Harmonics and Filters Related Standards

1. IEEE 519-2014:  Standard for Harmonic Control in Power Systems

2. IEC 61000-3-6:  Evaluation of harmonic emissions from power systems

3. EN 50160:  Power Quality Specification for Distribution Systems

Case Studies and Practical Examples

Example 1: A manufacturing plant with high nonlinear loads

Problem:  The transformer temperature rises, the automatic controller fails.

Solution:  Install individually tuned filters for the 5th and 7th harmonics, as well as a high-pass filter.

Results:  Total harmonic distortion decreased from 25% to 5%, and transformer temperature decreased by 15°C.

Example 2: Data Centers with Severe Harmonic Issues

Problem:  Increased neutral current causes frequent UPS failures.

Solution:  Install an active filter in parallel that can compensate for the 3rd, 5th, and 7th harmonics.

Results:  The neutral current was reduced by 70%, the useful life of the UPS was extended.

Future Development Trends of Harmonic Filter Technology

1. Smart filters:  They use artificial intelligence algorithms to detect and eliminate harmonics.

2. Modular filters:  capacity can be increased in stages.

3. Integration with energy storage systems

4. Multifunctional filter: Integrates  reactive power compensation, phase balance, and harmonic filtering.

5. Using superconducting materials to reduce losses

in conclusion

Harmonic filtering is an effective solution to harmonic distortion problems in today’s electrical systems. The choice between passive, active, or hybrid filters depends on a variety of factors, including harmonic spectrum, budget, available space, and specific system requirements. Thanks to technological advances, smarter and more efficient filters are emerging, capable of meeting the increasingly complex challenges of the electrical systems of the future.

To design and select the right filters, it is essential to perform a detailed harmonic analysis and consult with experts in the field. Investing in appropriate harmonic filters not only improves power quality, but also extends equipment life, reduces energy losses, and increases system reliability.