Criteria for choosing Automatic Equipment according to network conditions:
Once the maximum necessary power has been determined, the choice of which type of equipment to adopt must be based on the conditions of the electrical network and the types of loads present.
The selection table below, drawn up on the basis of general plant characteristics (and thus not usable for planning purposes), aims to provide an indication of the power factor correction system generally suited to the most frequently encountered conditions; electrical systems with mains voltage of 440V-50Hz, characterized by the presence of distorting loads with a spectrum composed of 5th, 7th, 11th and 13th harmonics.

Where:- 

THD: Total Harmonics Distortion of the current in the network. If no measurements of this parameter is available, it can be estimated by multiplying the ratio between the apparent power of the distorting loads and the total apparent power of the system by the coefficient 30 (NB: this method will provide only an approximate value and is based on an assumption of averagely distorting loads with a spectrum composed of 5th and 7th harmonics).

- THDmaxc: Total Harmonics Distortion of the max current accepted on the capacitors.
In cases where the distorting loads represent an overall power exceeding 25% of the available apparent power, it's always recommended to use power factor correction equipment with Harmonic Block Reactors to avoid amplifying the harmonic currents present in the network and to limit low-frequency electromagnetic pollution.
Moreover, it must always be verified that there are no significant harmonics in proximity to the frequency of parallel resonance between the equivalent capacitance of the capacitors and equivalent inductance of the plant (usually estimated as the equivalent inductance of the transformer), which may be calculated.
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