Nonlinear loads such as power electronics based equipment and electric furnaces are sources of harmonic currents leading to harmonic distortion, which is one of the most important parameters for defining the power quality. Most of the commercial loads such as personal computers, photocopy machines, power supplies, and compact fluorescent lamps; and industrial loads such as AC and DC motor drives inject harmonic currents into the network that they are connected to.

The harmonic distortion in a network causes:

- Equipment heating
- Insulation failure due to overheating and higher voltage peaks than rated fundamental voltage (50Hz or 60Hz) sinusoidal signal
- Equipment malfunction (false zero cross detection on power electronics devices)
- Communication interference
- Fuse and breaker mis-operation

Passive harmonic filters are the most commonly used devices for reducing the harmonic distortion in a network. These filters are built up from passive RLC components, i.e. resistors, inductors and capacitors.

At low voltage level, usually iron core reactors are used as harmonic filter inductors. They may be also used at medium voltage level in some applications.

The reactors in these filters serve to provide a resonance path together with the capacitors existing in the harmonic filter. By appropriately tuning the resonance frequency of a harmonic filter, the unwanted harmonic currents injected by the nonlinear loads can be prevented from going into the electrical grid.

In the design stage, the calculations of both the series resonance (the frequency at which the filter impedance becomes minimum) and the parallel resonance (the frequency at which the equivalent network and filter impedance becomes maximum) are important. In practice, the most commonly used filter type is single-tuned filter which consists of the series connection of a capacitor and a reactor. For this configuration, series resonance frequency is calculated as follows:

where, L is the inductance of reactor in Henries, and C is the per phase equivalent capacitance of the capacitor bank in Farads. On the other hand, the ratio of reactor's reactance to capacitor's reactance at the fundamental frequency is called p factor.

f_{r} = 134 Hz for p = 14%

f_{r} = 189 Hz for p = 7%

f_{r} = 210 Hz for p = 5.64%

are the industrial standard values in 50Hz networks for single-tuned filters, where the series resonance frequency is not tuned to an integer multiple of the fundamental frequency, i.e. any harmonic component, but it is tuned to a non-integer multiple of the fundamental frequency, i.e. an interharmonic. Then, this configuration is called a de-tuned filter. De-tuning is done in order to avoid any harmonic filtering, but to provide reactor power compensation only, while eliminating the risk of parallel resonance with any harmonic or interharmonic component existing in the grid, and also reducing the inrush currents of the harmonic filter. However, in order to provide a series resonance frequency tuned to 5th harmonic for a 50Hz network,

It is evident here that the series connection of the reactor and capacitor increases the amount of voltage on the capacitor above network voltage. This increase is in relation with the value of p as follows:

where U_{rt} is the network voltage and U_{c} is the capacitor voltage. It is important to consider this rise of voltage while choosing the voltage ratings of the capacitor bank.

Apart from single-tuned filters, harmonic filter reactors may also be used in second order and C-type filters according to the type of load and purpose (Different types of filters are given in figure below). Moreover, when they are used in medium voltage level, they may be used in series with Flexible AC Transmission Systems (FACTS) devices such as Static VAr Compensator (SVC) and Static Synchronous Compensator (STATCOM) in order to reduce the amount of harmonics that would be injected by these systems into the electrical grid.

Industrial customers are forced to obey harmonic current and harmonic voltage limits defined with respect to voltage level and with respect to the ratio of short circuit power to load power, in standards such as IEEE 519.92. Therefore, careful design of the tuning frequency and rating of the reactors considering a wide frequency band including both the harmonics and interharmonics are important.

All Hilkar iron core harmonic filter reactors are custom designed for different applications by considering the voltage, current, inductance, type of application (or filter type), harmonics, interharmonics, size, transient events such as switchings, and loss characteristics that are required to provide the most efficient design at the most economical prices. All the routine tests are performed in accordance with EN 60289 or other standards depending on customer request. Type test reports are available on request. All the test reports are submitted to customer. Basic testing program includes some or all of the following tests:

- Routine Tests (Inductance, Resistance, One Minute AC Insulation Voltage Withstand Test and Impulse Voltage Withstand Test)
- Short Circuit Withstand Test
- Temperature Rise Test
- Sound Level Test
- Seismic Test