Overview: Harmonic Distortions (Passive Harmonic Filters)

What are Harmonics?

Harmonics are current and voltage waveform distortions in power supply. Harmonics occur in integer multiples of the fundamental (supply) frequency - 50 Hz in India/Europe, 60 Hz in North America.

How do Harmonic Filters work?

A distorted waveform creates heat in the power delivery equipment that can cause severe damage to both domestic and industrial equipment that use the utility power supply. Harmonics are created due to the presence of non-linear loads which draw non-sinusoidal currents from an essentially sinusoidal voltage source. As a consequence, this leads to corruption of the supply voltage in the main power lines.

Some of the non-linear equipment that trigger harmonics include:

  • Rectifiers, Inverters (Diodes / Thyristors)
  • Choppers
  • Welding machines
  • UPS / SMPS
  • HVAC systems
  • Variable Frequency motor Drives (VFDs)
  • Electronic starters & Battery Chargers
  • CFL lights / Electronic Regulators
  • TV and computer devices
  • Refrigerators & Microwave Ovens
  • Printers / Copiers /Fax Machines

Harmonics are an integral aspect of today's Power Systems and every effort has to be made through proper design and architecture to mitigate the problem of Harmonics.

What are the Impacts of Harmonics on the Power System?

  • Large load currents in the neutral wires of a 3-phase system can cause overheating, resulting in a potential fire hazard.
  • Tripping of circuit breakers and other protective devices due to the Harmonic current peak values can be many times higher than sinusoidal waveforms.
  • Increased iron and copper losses or eddy currents due to stray flux losses, increases heat which causes transformers to fail.
  • Harmonic distortion has direct impact on the Power Factor. Increased Harmonics increases the Phase angle difference between Voltage and Current.
  • Added heat from harmonic currents can cause insulation breakdown on cables due to Skin Effect which reduces the ampere capacity of the Cable.
  • Harmonics resonance causes overheating, frequent circuit breaker tripping, irregular fuse operation, capacitor failure, electronic equipment malfunction, flicking lights and telephone interference.
  • Voltage Harmonics with high peak values can weaken insulation in cables, windings and capacitors.
  • Current Harmonics in motor windings can create Electromagnetic Interference (EMI).

What is the Function of Harmonic Filters?

One of the most effective ways to reduce harmonics in power systems is using Harmonic Filters. Harmonic Filters are capable of eliminating excess currents in 2 ways:

  • Prevent excess current generated by a load (appliance) from being propagated to the supply line 
  • Remove excess current in the line before it reaches the load.

Harmonic Filters contain a combination of inductors, capacitors and resistors (LCR) circuits that are capable of removing unwanted harmonic currents. The LCR circuitry is connected either in series or parallel, depending on the specific application and configuration.

What are the different types of Harmonic Filters?

Passive Harmonic Filters

Passive Harmonic Filters are designed to provide an alternate path for the harmonic currents and thus keep the main supply lines close to the fundamental frequency. For every harmonic current, separate Harmonic Filter stage has to be added.

In high impedance applications, filter is connected in series with the load and blocks the harmonic currents. In low impedance applications, filter is connected in parallel with the load. All the harmonic current specific to a specific Harmonic will flow through this shunt Filter that can then be grounded.

Advantages of Passive Harmonic Filters:

  • Completely prevents a specific harmonic current from entering a system, as all the Harmonic Current will pass through this filter which can then be grounded.
  • Rugged, inexpensive, low maintenance

Disadvantages of Passive Harmonic Filters

  • Risk of over-heating of the Filter as the entire current for a specific harmonic will flow through this filter.
  • Not ideal for AC drives as passive filters generate significant amount of reactive power

Active Harmonic Filters:

These filters consist of combinations of LCR circuits along with power electronic circuitry. They operate by injecting in the opposite direction, the harmonics generated by the load. This reduces the harmonics in the line. These filters are capable of reducing a broad range of harmonic frequencies. Typically used in commercial installations with load less than 200 kVA. Capable of eliminating current distortion that could lead to equipment heating and current overloads. Ideal for systems that have multiple loads like UPS, VFD and DC drives.

Characteristics of Active Harmonic Filters include:

  • Digital filters, current controlled.
  • Have high power density and low space requirements.
  • Reactive power compensation is possible
  • Selective filtering of individual frequencies.
  • Active solutions are much more flexible in their use but expensive.
  • Active filters cannot be overloaded regarding harmonic filtering.

Advantages of Active Harmonic Filters include:

  • Reactive power compensation
  • Dynamic compensation
  • Load balancing
  • Flicker compensation
  • Tuned filter circuits
  • Energy savings
  • Voltage stabilization

Active Harmonic Filters are ideal in the following applications:

  • Large variation of loads, broad range of harmonics that need to be compensated, non-linear loads are widely distributed into smaller network loads.
  • Lighting and computer lines
  • Multi-utility Office buildings, such as Hospitals

Hybrid Filters: Combines the features of both passive and active filters. Typically used Industrial applications with load in excess of 500 kVA. Capable of catering to different power and performance levels.

What are the Harmonic Filters Product Ranges that EMISINDIA Offers?

EMIS Passive harmonic filters are designed and manufactured to provide an economical solution in harmonic mitigation in Power Systems by maintaining THID 5% in compliance with current and voltage requirements of IEEE 519.


  • Increases system efficiency and equipment life
  • Reduces losses
  • Eliminates nuisance tripping
  • High attenuation of individual harmonics
  • Improves reliability of the system
  • Reduce Operation and maintenance cost

Industry Applications of EMIS Harmonic Filters