Technical Papers: Power factor correction and harmonic filtering in electrical plants

In electrical plants the loads draw from the network electric power (active) as power supply source (e.g. personal computers, printers, diagnostic equipment, etc.) or convert it into another form of energy (e.g. electrical lamps or stoves) or into mechanical output (e.g. electrical motors).

To get this, it is often necessary that the load exchanges with the network (with net null consumption) the reactive energy, mainly of inductive type. This energy, even if not immediately converted into other forms, contributes to increase the total power flowing through in the electrical network, from the generators, all along the conductors, to the users. To smooth such negative effect, the power factor correction of the electrical plants is carried out.

The power factor correction obtained by using capacitor banks to generate locally the reactive energy necessary for the transfer of electrical useful power, allows a better and more rational technical-economical management of the plants.

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Moreover, the present spreading of direct current users, such as electronic circuits and electric drives, involve the generation of current harmonics which are injected into the network, with the consequent pollution and distortion of the waveforms on other connected loads. Therefore, the use of harmonic filters, both of passive as well as of active type, contributes to improve the overall power-quality of the network, carrying out also power factor correction at the network frequency, when such filters are properly sized.

This technical paper has the purpose of analyzing these problems without going into technical details, but, starting from the definition of power factor correction, from an analysis of the technical-economical advantages and describing the forms and modalities to achieve power factor correction, it wishes to guide to the convenient choice of the devices for the switching of the capacitor banks and the filtering of the harmonics. In fact, after a first descriptive part, the ABB offer is illustrated in terms of power factor correction devices, intended not only as suitable capacitors, but also as those devices able to carry out switching and protection of the capacitor banks. Besides, some solutions are given for both the passive and active filtering of the current harmonics generated by distorting non-linear loads.

To integrate this technical paper there are also six annexes providing:

·        Tables for the quick choice and coordination of       circuit breakers and contactors for switching and protection of capacitor banks of a determined power;

·        indications on how the reactive power generated at the variations of supply voltages changes and necessary considerations to prevent reactive power from being injected into the network;

·        considerations        on power  factor correction and           filtering under distorted steady-state conditions to point out how canonical power factor correction implies a reduction of the value of the harmonics present in the network;

·        descriptions of the voltage and  current          characteristics during the switching on and discharging of capacitor banks;

·        considerations        on power factor     correction  in photovoltaic plants;

·        remarks about        the  contribution of harmonics to  the evaluation of the current in the neutral conductor of three-phase systems.