electrical-engineering-portal.comhttp://electrical-engineering-portal.com/why-its-important-to-know-which-type-of-power-factor-correction-to-use

Google+

Why it’s important to know which type of power factor correctionto use

Why it’s important to know which type of power factor correction to use (on photo: 300 KVAR automatic power factor correction (APFC) panel; credit to:eamfco.com)

Practical and economical feasibility

It’s important to understand where the capacitors are to be installed for a better exploitation of suchadvantages. There are no general rules applicable to every type of installation and, in theory, capacitors can beinstalled at any point, but it is necessary to evaluate the relevant practical and economical feasibility.

According to the location modalities of the capacitors, the main methods of power factor correction are:

1. Distributed power factor correction

2. Group power factor correction

3. Centralized power factor correction

4. Combined power factor correction

5. Automatic power factor correction

Now, let’s say a word about each of above listed methods…

1. Distributed power factor correction

Distributed power factor correction is achieved by connecting a capacitor bank properly sized directly to the terminalsof the load which demands reactive power.

The installation is simple and inexpensive. Capacitor and load can use the same protective devices againstovercurrents and are connected and disconnected simultaneously.

This type of power factor correction is advisable in the case of large electrical equipment with constant load and powerand long connection times and it is generally used for motors and fluorescent lamps.

Figure 1 – Common connection diagrams for the power factor correction of motors

Figure 1 above shows the common connection diagrams for the power factor correction of motors. Let’s explain eachof the cases from connection diagrams.

In case of direct connection (diagrams 1 and 2), the following risk may be run: after the disconnection from thesupply, the motor will continue to rotate (residual kinetic energy) and self-excite with the reactive energy drawn fromthe capacitor bank, and may turn into an asynchronous generator. In this case, the voltage on the load side of theswitching and control device is maintained, with the risk of dangerous overvoltages (up to twice the rated voltagevalue).

When using diagram 3, the compensation bank is connected only after the motor has been started and disconnectedin advance with respect to the switching off of the motor supply. With this type of power factor correction the networkon the supply side of the load works with a high power factor; on the other hand, this solution results economicallyonerous.

Go back to Index ↑

2. Group power factor correction

It consists in improving locally the power factor of groups of loads having similar functioning characteristics by

Figure 2 – Connection diagram of group power factor correction

Figure 3 – Centralized power factor correction

installing a dedicated capacitor bank.

This is the method reaching a compromise between the inexpensive solution and the proper management of theinstallation since the benefits deriving from power factor correction shall be felt only by the line upstream the pointwhere the capacitor bank is located.

Go back to Index ↑

3. Centralized power factor correction

The profile of loads connected during the day has a primaryimportance for the choice of the most convenient type of powerfactor correction.

For installations with many loads, where not all the loadsfunction simultaneously and/or some loads are connected forjust a few hours a day, it is evident that the solution ofdistributed power factor correction becomes too oneroussince many of the installed capacitors stay idle for a long time.

Therefore the use of one compensation system only located atthe origin of the installation allows a remarkable reduction ofthe total power of the installed capacitors.

In centralized power factor correction automatic assemblies arenormally used (see below automatic power factor correction)with banks divided into steps, installed directly in the maindistribution boards. The use of a permanently connectedbank is possible only if the absorption of reactive energy isquite constant all day long.

The centralized solution allows an optimization of the costs of the capacitor bank, but presents thedisadvantage that the distribution lines on the load side of the power factor correction device shall besized keeping into account the full reactive power absorbed by the loads.

Go back to Index ↑

4. Combined power factor correction

This solution derives from a compromise between the two solutions of distributed and centralized power factorcorrection and it exploits the advantages they offer.

In such way, the distributed compensation is used for high power electrical equipment and the centralizedmodality for the remaining part.

Combined power factor correction is prevailingly used in installations where large equipment only are frequently used;in such circumstances their power factor is corrected individually, whereas the power factor of small equipment iscorrected by the centralized modality.

Go back to Index ↑

5. Automatic power factor correction

In most installations there is not a constant absorption of reactive power for example due to working cycles for whichmachines with different electrical characteristics are used.

ABB’s power factor monitoring, type ‘Dynacomp’

In such installations there are systems for automatic power factor correction which, thanks to a monitoringvarmetric device and a power factor regulator, allow the automatic switching of different capacitor banks, thusfollowing the variations of the absorbed reactive power and keeping constant the power factor of the installationconstant.

An automatic compensation system is formed by:

Some sensors detecting current and voltage signals;

An intelligent unit which compares the measured power factor with the desired one and operates the connectionand disconnection of the capacitor banks with the necessary reactive power (power factor regulator);

An electric power board comprising switching and protection devices;

Some capacitor banks.

Automatic power-factor correction panel (photo credit: eamfco.com)

To supply a power as near as possible to the demanded one, the connection of the capacitors is implemented step bystep with a control accuracy which will be the greater the more steps are foreseen and the smaller the difference isbetween them.

Go back to Index ↑

Reference // Power factor correction and harmonic filtering in electrical plants – ABB