What is Harmonic Filter : Circuit, Working & Its Applications

In electrical power systems, harmonics is a voltage otherwise current occurred at different basic frequencies. It is frequently considered as noise within the power line. The harmonics within the power system can be categorized into two types like voltage harmonics and current harmonics. But it is very essential to eliminate harmonics within power systems by using a harmonic filter, otherwise, the power connection needs to redesign to control nonlinear loads.

The current harmonics can be induced through variable speed drives (VSD) which are called non-linear load. Similarly, the voltage harmonic generally occurs due to the generation of distorted voltage through the effect of the current harmonics using the source impedance. This article discusses an overview of a harmonic filter definition, it’s working with applications.

What is Harmonic Filter?

A device that is used to mitigate, reduce harmonics to acceptable ranges in a power system is known as a harmonic filter. This harmonic filter is used at strategic operating circuit points for eliminating or reducing harmonic distortion and resonance. Generally, harmonic distortion can be caused due to excess currents in & out of appliances.

Harmonic filter avoids huge amounts of harmonics from equipment damage, reduces operation & reduces operating costs. So these are used in different industries like HVAC, oil, gas, mining, wastewater to avoid components damage or utility providers fines.

Harmonic Filter
Harmonic Filter

The harmonic filter can be designed with a set of different electronic components like resistors, capacitors, inductors for preventing unnecessary harmonic currents, transmitting them to earth. These filters are mainly designed for preventing specific frequency harmonics.

Working Principle

The harmonic filter working principle is to decrease distortion through deflecting harmonic currents within less-impedance lanes. These filters are capacitive at the basic frequency, so used to generate the reactive power necessary through converters & also for correction of power factor.

Harmonic Filter Types

Generally, harmonics filters are classified into different types based on the voltage applied, rated power, 1-phase or 3-phases & other different parameters which depend on the load. These are very effective filters that protect expensive electrical equipment from excess power because of harmonics.


There are two types of harmonic filters like active and passive. The major difference between these two is the components used while designing the filter. The active harmonic filter uses active components like ICs, BJTs, IGBTs & MOSFETs whereas the passive harmonic filter uses different passive components like resistors, capacitors, and inductors.

Active Harmonic Filters

Active filters are very easy to switch the harmonics of the power line without depending on the reactive power of the basic frequency. These filters use an outstanding technique wherever the filter utilizes self-generated harmonic components & infuses this into the power line to cancel the unnecessary harmonics.

As compared to active filters, passive harmonic filters eliminate harmonics very easily within the power line but, its design is very difficult. So the designer should design these filters based on the requirements of reactive power for the load. So in that condition, passive filter design is not easy and it causes poor power actor operation for specific load conditions.

Active Harmonic Filter
Active Harmonic Filter

Active filters are available using various topologies to remove the harmonics within the power line. The following steps need to consider while designing an active harmonic filter.

  • This filter uses different types of semiconductor switches that need the power to function.
  • Voltage source inverter with different power switches
  • Sampling & control reference using the power line
  • PWM system as harmonics injects PWM firing signal into the power system.

Passive Harmonic Filters

Passive harmonic filters are easily available and most frequently used. These filters utilize typical passive components like capacitors, resistors, etc to make a tank circuit. This circuit is mainly designed in such a way to operate at a similar resonance frequency as compared to the unnecessary harmonics.

Passive Harmonic Filter
Passive Harmonic Filter

These filters block the unnecessary harmonics to supply and it changes the harmonic current supply into the temperature & defends the end load or device. There are four types of passive harmonic filters used like high pass filter, series filter, c type filter, and bandpass filter.

Harmonic Filter in Capacitor Bank

Power factor correction needs a design to compensate reactive power demand & inject harmonic currents through the equipment into the power supply system. Generally, power supply systems in industries include a capacitor bank & an arrangement of tuned filters. Analysis of communication between harmonic filters & capacitor banks in the power supply system of the industry is carried out.

Actually, capacitor banks are components for the harmonic filters, so when someone is using capacitor banks to enhance the power factor, it’s a good thing to provide in some coils & now he has a Harmonic Filter particularly for the neutral and it is extremely good.

The harmonic filter with capacitor bank is widely called a high voltage power capacitor bank. So, this equipment mainly includes 3-phase all film power & capacitors with surge protection. These equipment are enclosed or open, low or high power, fixed power, incorporated or for utilize with separate motors. With the compensation of reactive power system, power factor (PF) can be enhanced & harmonics are covered up to reduce electricity bill through convincing payback stage.

Harmonic Filter Circuit Diagram

The circuit diagram of the bandpass harmonic filter circuit is shown below. This filter is one of the passive harmonic filters types. This is a double-tuned type harmonic filter that can be designed with a single resistor, two capacitors & two inductors. This filter is mainly used for harmonic filtration purposes with high order.

This working of this filter can be done by merging the typical bandpass filter’s parallel resonance through the series resonance of the RL circuit.

Bandpass Harmonic Filter Circuit with Characteristics
Bandpass Harmonic Filter Circuit with Characteristics

In the above circuit diagram, a fundamental bandpass filter circuit is given. This circuit includes two parts. In the primary part of the circuit, a “C2’ capacitor & ‘L2’ inductor are connected in a series connection whereas in the next part, an inductor, a resistor & a capacitor are allied in parallel connection. Both the parts are also connected within the series connection. In the above diagram, the impedance characteristics including the frequency are illustrated.


The advantages of harmonic filters include the following.

  • These filters are used to reduce harmonics within a power system
  • It enhances the power supply flow
  • These filters provide the best results to maintain the system safe
  • These filters provide energy efficiency to keep the life of the power system safe
  • It decreases power bills through harmonics controlling within the power supply.
  • Active harmonic filters assist you to make the highest power supply consumption.
  • These filters offer many advantages in a single system. As compared to active filters, hybrid filters are not expensive and it provides a combined advantage of both the filters like active & passive.
  • Maintenance costs can be reduced while connecting active filters
  • Hybrid harmonic type filters are used to offer power factor development & harmonic correction.


The disadvantages of passive harmonic filter & active harmonic filter include the following.

  • Filtering characteristics are affected strongly through the source impedance
  • At the source side, amplification of currents at fixed frequencies can come into view because of the parallel resonance among the passive filter & the source.
  • Extreme harmonic flow of currents into the passive filter because of the voltage distortion occurred through the possible series resonance by the source.
  • The active filter is very difficult to design a current source with large-rated through quick current response.
  • Maximum initial & running prices
  • The passive filter size is large because of the huge ac capacitors.
  • The control system is extremely complex & the protection within the operation is decreased. Once the active filter is working is stopped, then their specific conditions while the passive filter is damaged.

Harmonic Filter Applications

The applications of harmonic filters include the following.

  • VFDs
  • Servo Drives
  • Water & Wastewater
  • Gas & Oil
  • HVAC
  • Automated Machinery
  • Industrial devices
  • Printing & Packaging Machinery
  • Mining industries
  • Process of Automation
  • Why Harmonic Filters are used?
  • These filters are used to decrease the harmonic currents supplying within the power system from the source to decrease the distortion of harmonic voltage within the system.

What does an Active Harmonic Filter do?

Active harmonic filters are parallel devices that work like a noise cancellation system. They inject equivalent & quite opposite frequencies to decrease harmonics. These filters also offer added current to rectify the power factor.

How do Filters Reduce Harmonics?

Harmonic filters decrease distortion through redirecting harmonic currents within low-impedance lanes.

What is Harmonic Filter for VFD?

Harmonic Filters are mainly used to decrease VFD distortion.

Which Filter is used to Reduce Higher Order Harmonics?

The high order harmonics can be reduced through High pass passive harmonic filters for flexible control on the broad range of frequencies

How do you Mitigate Harmonics?

The filtering method is used to mitigate harmonics in industries once the harmonic distortion has been increased gradually.

Do Harmonics Pass-Through Transformers?

Connecting a transformer within a series connection will reduce harmonics significantly & provide transient safety benefits. So, transformer connections can be used to decrease harmonic currents within a 3-phase system.

Thus, this is all about an overview of Harmonic Filter and its working with applications. Harmonics in power systems are a positive integer that is multiplied with basic frequency. It is a current or voltage that occurs at various fundamental frequencies and it is frequently called noise within the power line. Here is a question for you, what are the main causes of harmonics?

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