Though all orders of harmonics are harmful to an electrical power system, the 5thand 7thorder harmonics are harmful to the electrical networks. One of the solutions to mitigate these lower-order harmonics is to install harmonic filters. We will discuss in detail the criteria of harmonic filter selection.

The harmonic filters can be categorized into three categories i.e. passive filters, active filters, hybrid filters. The passive filter has passive components like a resistor, inductor, capacitor. The active filter is designed using semiconductor devices like IGBTs.

The hybrid filters are basically the combination of active and passive filters. For the cost-effectiveness of the installation, the hybrid filters are used to mitigate the lower orders harmonics using the passive filters and the higher orders harmonics using the active filters. A combination of the active and passive filters is known as a hybrid filter.

**The passive filters can be further categorized into tuned and de-tuned filters.** The frequency of the 5^{th} and 7^{th} order harmonics is 250 & 350 Hz respectively. The passive filters are generally used for mitigation of 5^{th} and 7^{th} order harmonics because the magnitude of the current of lower order harmonics is more than the magnitude of the current of higher-order harmonics.

**Tuned Filter** **Selection**

A tuned filter works on the principle of providing the least impedance path for one or two harmonic frequencies and has a tuning frequency, which is within +/- 10 % of the harmonic frequency to be filtered. Tuned filters of particular harmonics order carry more harmonic current as they offer low impedance to the dominant harmonics.

Therefore, the tuned filter design has to be done carefully. The tuned filter is formed with a combination of inductance and capacitance so they are bulkier as compared to the de-tuned filter and active filter.

The tuned filters can be installed only after carrying out a detailed harmonics study. Inherently, tuned filters do not adapt easily to change in power network configuration and their efficiency is usually altered when network configuration changes.

Whenever new non-linear loads get added to the network after the installation of the filters, the harmonics studies are again required to be carried out. Several tuned filters are to be used in parallel if more than one harmonic frequency is to be filtered. The self-resonance frequency is related to the tuning factor, which is defined as the ratio of the inductive to capacitive reactance of the filter expressed as a percentage.

The major problem with tuned filters is that if the inductive reactance of the electrical network is equal to the capacitive reactance of the system the resonance phenomenon occurs. The resonance phenomenon can cause the failure of the power factor correcting capacitors.

**De-tuned Filter Selection**

To solve this issue of resonance the filter is deliberately detuned for particular harmonics order. For example, the filter for 5^{th} order harmonics current is designed for the frequency lower than 250 Hz. With detuning, some percentage of the harmonics current remains unfiltered, but the resonance can be avoided.

The tuning factor can be given to the tuned filters to avoid the resonance problem. The tuning factor and tuning frequency of the harmonic filter are given by the following expressions:

**Tuning factor, p (%)**

=[(Inductive reactance) / ((Capacitive reactance)] x 100

= (XL/XC )*100

**Tuning frequency of the filter,**

Fr=Fs/√0.01p Hz (P is in percentage)

For 5 % detuning Fr=50/√ 0.05** = 224 Hz**

Where f_{s} = Fundamental system frequency

**Filter frequency with de-tuning**

De-tuning Factor (%) | Filter Frequency (Hz) |

5 | 224 |

7 | 189 |

8 | 177 |

14 | 134 |

**Active Filter**

The PWM INVERTER technology is the basic building block of an Active Harmonic Filter. Active filters are new generation harmonic filters that use modern technologies and devices to provide revolutionary filtering solutions.

The active filter produces the harmonic currents spectrum in phase opposition to that produced by the non-linear loads, and these generated harmonics current of the active filters are injected into the system in real-time to ensure effective cancellation of all harmonics present in the network.

In addition to canceling the harmonic current, an active filter is also capable of improving the power factor by providing both capacitive and inductive reactive power in a step-less manner. Presently, active filters are the most technologically advanced solutions available for harmonic filtering.

The Active filter uses a CPU for detecting the order and magnitude of the harmonics present in the network and generates a harmonic current spectrum in phase opposition to the measured spectrum.

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