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Passive Filter Design

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • Passive and active filters are used to protect sensitive, expensive equipment from harmonics.

  • Shunt passive filters block harmonic currents by diverting their path to the ground instead of the power system.

  • Shunt passive filter components are rated for only a fraction of the power system-rated voltage. This rating decreases the size of the filter components and their cost.

Passive Filter Design for Harmonic Reduction

In electrical engineering, any frequency other than the fundamental frequency can be regarded as harmonics. Harmonics are not desirable, and in most cases, they must be stopped from reaching the system. Active and passive filter designs for harmonic reduction are used to accomplish this. 

What Are Harmonics?

Harmonics are noises or disturbances in voltage or current. When a waveform consists of harmonics, apart from the fundamental frequency component, there will be components with frequencies equal to multiples of the fundamental frequency. For example, if the fundamental frequency of a voltage or current waveform is 60 Hz, then 2x 60=120 Hz and 3x60=180 Hz frequency components present in the waveform correspond to the second and third voltage or current harmonics, respectively.

Current harmonics are induced by non-linear loads such as variable speed drives, magnetic circuits, motors, etc. Voltage distortions result from the effect of harmonics using source impedance.

Effects of Harmonics

Some of the ill effects caused by harmonics in power systems are:

  1. Increased heating of transformers, motors, conductors, etc.
  2. Increased maintenance time
  3. Intermittent temporary shutdowns
  4. Decreased life span of the equipment
  5. Poor equipment reliability
  6. Increase in electricity cost
  7. Increase in electricity waste
  8. Higher power consumption

It is essential to prevent harmonics from disturbing healthy electrical and electronic systems. Filters are one of the proven methods to minimize disruptive harmonics.

Filters for Harmonic Reduction

Devices that eliminate or minimize harmonics from reaching the system downstream are called harmonics filters. Harmonic filters can be classified based on the rating of voltage, power, and the number of phases. The major division of harmonic filters is active and passive filters.

Active and Passive Filter Design for Harmonic Reduction

Passive and active filters are used to protect sensitive equipment from harmonics. Active and passive filter designs for harmonic reduction vary.

Active Filters

Passive Filters

Use components such as 1GBTs, MOSFETs, BJTs, ICs, etc. for harmonic elimination

Use components such as resistors, capacitors, and inductors

No reactive components

Reactive components are present

Generates signals equal and opposite to the original harmonics; this cancels the harmonics

Designed to operate at resonance frequency compared to harmonics

Doesn’t affect the power factor

Causes poor power factor

Depending on the control, the active filter can eliminate different harmonics

Frequency specific; for eliminating harmonics, the passive filter components vary

Passive Filter Design Types

Passive filters can be classified into the following types:

Passive Filter Type

Purpose

Classification

Band-pass filter

Filters lower order harmonics

Single-tuned filters or double-tuned filters

High-pass filter

Filters higher order harmonics

C-type high pass filter

The figure below gives the construction of various passive filters.

Passive Filter Designs

Passive Filter Designs

Choosing Between Active and Passive Filters

Active filters and passive filters are both effective in eliminating harmonics. Depending on the load configuration and system configuration, either passive or active filters are selected. In certain cases, a combination of both can be used.

Series and Shunt Passive Filters

Passive filters are combinations of capacitors, inductors, and resistors. They are tuned to resonate at a frequency called resonant frequency. Passive filters exhibit different impedance values to different frequencies applied.

A passive filter connected in series to a system forms a series passive filter. Series passive filters offer high impedance to the harmonics and prevent them from reaching the system. Passive filters are usually installed in series with busbars, switch gears, main switchboards, motor control systems, etc.

The most common way of connecting passive filters is in parallel; this is called a shunt passive filter. These filters block the harmonic currents by diverting their path to the ground instead of the power system. They can improve the power factor by providing reactive power.

In most power system applications, shunt passive filters are designed due to the following reasons:

  1. Series passive filter components are rated for full load current, which includes the fundamental frequency component. The size of passive components is large and the cost of the filter is high.

  2. The shunt passive filter components are rated for only a fraction of the power system-rated voltage. This rating decreases the size of the filter components and their cost.

For applications involving the removal of specific harmonics, single or double-tuned passive filter designs are a great choice. Cadence's full suite of analysis tools can help you design passive filters for harmonic reduction.

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