Loads operated by electronic devices are increasingly adopted in more and more industrial and commercial applications such as: Variable Speed Drives, rectifiers, UPS systems, DC power supplies, welding machines, computers, TV, energy efficient lamps, photocopiers. These loads generate waveform distortions that become threat to network components.

In particular, voltage and current harmonics generate a series of problems, like:

• Cable overheating
• Undue tripping of circuit breakers
• Blowing of fuses
• Capacitor overloading and network resonance
• Neutral cable overload
• Transformer premature ageing
• Electronic appliances disturbance

The active filter is an electronic device that measures the line current harmonic content, calculates the individual harmonic components in the network and for each inject an equal current (per module and harmonic order) but in phase opposition. In this way, it eliminates the present harmonics and leaves unchanged the current at network frequency. The active filters are preferred when the network harmonic content is on a wide spectrum (for example, the 3^rd, the 5^th, the 7^th, the 11^th, the 13^th) and/or when there is a resonance risk.

The active filter is dimensioned for current, considering the total rms value of the harmonic currents that are to be deleted from the network.

The active filters are made with digital technology and are able to guarantee high performance in terms of :

• Speed of response
• Robustness and reliability for use in heavy industrial environments
• Speed of maintenance/repair
• Adaptation to changes in the network harmonic coontent due to changes in the network topology and/or new non-linear loads presence