Harmonic filtering

In cases where the preventive action presented above is insufficient, it is necessary to equip the installation with filtering systems.

There are three types of filters:

• Passive
• Active
• Hybrid

Passive filters

Typical applications

• Industrial installations with a set of non-linear loads representing more than 500kVA (variable-speed drives, UPSs, rectifiers, etc.)
• Installations requiring power-factor correction
• Installations where voltage distortion must be reduced to avoid disturbing sensitive loads
• Installations where current distortion must be reduced to avoid overloads

Operating principle

An LC circuit, tuned to each harmonic order to be filtered, is installed in parallel with the non-linear load (see Fig. M28). This bypass circuit absorbs the harmonics, thus avoiding their flow in the distribution network.

Generally speaking, the passive filter is tuned to a harmonic order close to the order to be eliminated. Several parallel-connected branches of filters can be used if a significant reduction in the distortion of a number of harmonic orders is required.

Fig. M28 – Operating principle of a passive filter

Active filters (active harmonic conditioner)

Typical applications

• Commercial installations with a set of non-linear loads representing less than 500kVA (variable-speed drives, UPSs, office equipment, etc.)
• Installations where current distortion must be reduced to avoid overloads.

Operating principle

These systems, comprising power electronics and installed in series or parallel with the non-linear load, compensate the harmonic current or voltage drawn by the load.

Figure M29 shows a parallel-connected active harmonic conditioner (AHC) compensating the harmonic current (Ihar = -Iact).

The AHC injects in opposite phase the harmonics drawn by the non-linear load, such that the line current Is remains sinusoidal.

Fig. M29 – Operating principle of an active filter

Hybrid filters

Typical applications

Industrial installations with a set of non-linear loads representing more than 500kVA (variable-speed drives, UPSs, rectifiers, etc.)

• Installations requiring power-factor correction
• Installations where voltage distortion must be reduced to avoid disturbing sensitive loads
• Installations where current distortion must be reduced to avoid overloads
• Installations where strict limits on harmonic emissions must be met

Operating principle

Passive and active filters are combined in a single system to constitute a hybrid filter (see Fig. M30). This new filtering solution offers the advantages of both types of filters and covers a wide range of power and performance levels.

Fig. M30 – Operating principle of a hybrid filter

Selection criteria

Passive filter

It offers both power-factor correction and high current-filtering capacity.Passive filters also reduce the harmonic voltages in installations where the supply voltage is disturbed. If the level of reactive power supplied is high, it is advised to turn off the passive filter at times when the percent load is low.

Preliminary studies for a filter must take into account the possible presence of a power factor correction capacitor bank which may have to be eliminated.

Fig. M31 – Example of MV passive filter equipment

Active harmonic conditioners

They filter harmonics over a wide range of frequencies and can adapt to any type of load. On the other hand, power ratings are limited.

Fig. M32 – Active Harmonic Conditionner (AccuSine range)

Hybrid filters

They combine the performance of both active and passive filters.