Practical and economic feasibility
It is important to understand where the capacitors should be
installed for better exploitation of these advantages. There
are no general rules applicable to each type of installation and, in theory,
capacitors can be installed at any time, but it is necessary to assess the
relevant practical and economic feasibility.
Depending on how the capacitors are located, the main power factor
correction methods are as follows:
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 the methods listed above ...
1. Correction of the
distributed power factor
Distributed power factor correction is achieved by connecting a
properly sized capacitor bank directly across the load that requires reactive
power.
Installation is simple and inexpensive. The
capacitor and the load can use the same overcurrent protection devices and are
connected and disconnected simultaneously.
This type of power factor correction is recommended in the case of
large electrical equipment with constant load and power and long connection
times, it is generally used for motors and fluorescent lamps.
Figure 1 above shows common connection diagrams for motor power
factor correction. Let's explain each of the cases from the connection
diagrams.
In case of direct connection (diagrams 1 and 2),
the following risk can be run: after disconnection of the power supply, the motor will continue to run (residual kinetic energy) and to self-excited with
the reactive energy extracted of the capacitor bank and could turn into an
asynchronous generator. In this case, the voltage on the load side of the
switching and control device is maintained, with the risk of dangerous
overvoltages (up to twice the value of the nominal voltage).
When using diagram 3, the compensation bank is
connected only after the motor has been started and disconnected beforehand
with regard to cutting off the power supply to the motor. With this type
of power factor correction, the network on the supply side of the load operates
with a high power factor; on the other hand, this solution is economically
expensive.
2. Correcting the generator
power factor
It consists of locally improving the power factor of
groups of loads with similar operating characteristics by installing a
dedicated capacitor bank.
This is the method that makes it possible to find a compromise
between the inexpensive solution and the good management of the installation
since the advantages resulting from the correction of the power factor must
only be felt by the line upstream of the point where the capacitor bank.
3. Centralized power factor
correction
The profile of the loads connected during the day is of paramount
importance in choosing the most appropriate type of power factor correction.
For installations with many loads, where all the loads do not
operate simultaneously and/or some loads are only connected a few hours per
day, it is obvious that the solution of the distributed power factor the correction becomes too expensive since many installed capacitors
remain inactive for a long time.
Consequently, the use of a compensation system located only at the
origin of the installation allows a remarkable reduction in the total power of
the installed capacitors.
In centralized power factor correction, automatic assemblies are
normally used (see automatic power factor correction below) with banks
divided into steps, installed directly in the main
distribution boards. The use of a permanently connected bank is
only possible if the absorption of reactive energy is fairly constant throughout
the day.
The centralized solution allows an optimization of the costs of
the capacitor bank, but has the disadvantage that the distribution lines on the
load side of the power factor correction device must be dimensioned taking into
account for the total reactive power absorbed by the loads.
This solution stems from a compromise between the two
solutions of distributed and centralized power factor correction and
it exploits the advantages that they offer.
In this way, the distributed compensation is used for high
power electrical equipment and the centralized model for the rest.
Combined power factor correction is predominantly used in
installations where only large equipment is frequently used; in such circumstances,
their power factor is corrected individually, while that of small equipment is
corrected by the centralized model.
5. Automatic power factor
correction
In most installations, the absorption of reactive power is not
constant, for example, due to work cycles for which machines with different
electrical characteristics are used.
In these installations, there are automatic power factor
correction systems which, thanks to a power factor monitoring
device and a power factor regulator, allow the automatic switching of different
capacitor banks, thus following the variations of the power. reactive
absorbed and keeps the power factor of the installation constant.
An automatic compensation system is formed by:
·
Some sensors detect current and voltage signals;
·
An intelligent unit that compares the power
meter with the desired one and operates the connection and disconnection of the
capacitor banks with the necessary reactive power (power factor regulator);
·
An electrical supply panel comprising
switching and protection devices;
·
Some capacitor banks.
Supplying a power as close as possible to the demand, the
connection of the capacitors is implemented step by step with a control
precision all the greater as the number of steps provided is large and the difference
between them is small.
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