Automatic Correction Systems Of The Power Factor
Correct The Power Factor: Why
In electric circuits the current is: - in phase with the voltage before an ohmic load (eg resistors); - delayed if the load is inductive (eg motors, vacuum transformers); - Phased in advance if the load is capacitive (eg capacitors).
For example, the total current (I) absorbed by a motor is determined by the vector sum of: - IR, ohmic current due to the resistive component of the load; - IL, reactive current due to the inductive component of the load.
The following powers are associated with these currents: - P, active power associated with the resistive part of the load; - Q, reactive power associated with the inductive part of the load; - A, apparent power.
The inductive reactive power having a zero average value in the period is not useful for the production of mechanical work and constitutes an additional burden for the energy supplier, which will have to oversize its generators and the transmission and distribution networks.
The parameter that defines the absorption of reactive inductive power is the power factor.
The power factor is defined as the ratio between the active power and the apparent power.
If there are no harmonics, the power factor equals the cosine of the angle between the current vector and the voltage vector (cosφ).
The Power Factor Decreases When The Reactive Power Absorbed Increases.
An equipment that works with a low cosφ has the following disadvantages: - High power losses in the transmission in the electric lines; - High voltage drops; - Greater dimensions of generation, transport and transformation equipment.
For all the above is understood the importance of eliminating or at least diminishing the effects of a low power factor. The capacitors serve to achieve this result.
Correct The Power Factor: How?
Installing a capacitor bank can reduce the reactive power absorbed by the inductive loads present in the installation and consequently increase the value of the power factor.
The modalities for correcting the power factor are numerous and their choice is based on the nature and daily evolution of the loads, their distribution in the installation and the type of service. The main choice must be made between correction of the distributed power factor and correction of the centralized power factor.
In the case of distributed correction, the correction units are located very close to the loads to be corrected. In the case of centralized correction, a single automatic battery is installed before all the loads to be corrected and immediately after the cos punto measurement point (for example in the MV / LV transformer cabinet or in the main distribution board) .
Technically, the distributed correction is the best solution: the capacitors and the user equipment behave the same during the daily exercise, so the regulation of the power factor becomes systematic and strongly linked to the corrected load.
In addition, with the distributed correction of the power factor, the reduction of reactive energy affects both the distribution company and the user. In industrial facilities, for example, the savings that can be achieved with the correction of the distributed power factor is shown both in the form of tariffs and in the form of a better sizing of all the lines of the factory that connect the MT cab. / BT with the equipment.
Another important advantage of this type of correction of the power factor is the easy and cheap installation, since the power factor and load correctors are activated and deactivated at the same time and can have the same protections against overloads and short circuits .
The daily evolution of loads is of fundamental importance when choosing the most convenient type of correction.
In many facilities, not all computers work at once and some even work only for a few hours a day. It is evident that the solution of the correction of the distributed power factor is too expensive for the high number of capacitors that should be placed and many of them would be unused for a long time.
The correction of the distributed power factor is appropriate when most of the required reactive power is concentrated in few high power loads that work many hours a day.
The correction of the centralized power factor is convenient, however, in the case of installations with many heterogeneous loads that work sporadically. In this case the power of the battery is much lower than the total power that would have to be supplied with the distributed correction.
It is advisable to connect the battery permanently only if the absorption of reactive energy during the day has a certain regularity, otherwise it will have to be maneuvered to obtain the power factor in advance.
If the absorption of reactive power is very variable during the operation of the installation, it is advisable to carry out an automatic regulation by fractioning the battery in several levels. You can perform the manual maneuver when you have to operate the battery a few times a day.
Correct The Power Factor: How Much?
The choice of the power of the capacitor bank to install it in a device (QC) depends directly on: - the value of the cosφ2 that one wishes to obtain; - the value of the start cosφ1; - the active power installed.
The relationship is: QC = P x (tanφ1 - tanφ2) QC = capacitive reactive power to be installed (kvar) P = installed active power (kW) QL, QL '= inductive reactive power before and after the installation of the capacitor bank A, A '= apparent power before and after correction of the power factor to formula can also be written as follows : QC = kx P Where the parameter k can be easily calculated using the table on the next page. Example: suppose we have installed a load that absorbs an active power equivalent to 300kW with an initial power factor of 0.70 and we want to increase it to 0.97, according to the following table we obtain: k = 0.770. And therefore: QC = 0,770 x 300 = 231kvar
Correcting the power factor: harmonics in electrical networks
In many industrial electrical installations or in the tertiary sector, the presence of non-linear equipment (inverters, welding machines, grinders, computers, drives, etc.) determines a distortion of the current (and both harmonics), which is synthesized by the numerical parameter THDI%: if the current is sinusoidal its THDI% is zero, therefore the current will be more deformed the higher its THDI%. Its presence in the network leads to numerous problems in the elements of an electrical installation: - In the rotating machines parasitic pairs arise (with the consequent vibrations) that damage the mechanical duration. The increase in losses also causes unwanted overheating with the consequent damage of the isolations; - In the transformers cause increased losses in copper and iron with possible damage to the windings. The possible presence of continuous voltage or current components can lead to saturation of the nucleus with a consequent increase in the magnetizing current; - The capacitors suffer the effects from the point of view of heating and increasing the voltage with a reduction in the half-life.
The shape of the wave of the current generated by a non-linear load, if it is periodic, can be represented as the sum of several sinusoidal waves (one at 50Hz called fundamental and others with multiple frequency of the fundamental called harmonics). In general, it is not advisable to correct the power factor of a line with high harmonic content without taking the appropriate measures. This is because, although capacitors capable of withstanding heavy overloads can be built, the correction of the power factor made only with the capacitors translates into an increase in the harmonic content, with the negative effects that we have just indicated.
The most convenient solution to avoid this kind of problems is the blocking filter (Detuned Filter) that is obtained by putting in series some reactances to the capacitors that, by moving the resonance frequency below the lowest existing harmonic, can protect the capacitors and at the same time they avoid dangerous resonances.
Correct The Power Factor: Conclusions
In an installation with low power factor, in most cases the cost related to the installation of the power factor correction equipment is amortized in a few years. In addition to eliminating the fines in the receipt, the technical-economic advantages due to the installation of a capacitor bank are the following: - reduction of the losses in line and in the transformers due to the lower current absorbed; - decrease in voltage drops in the lines; - Optimization of the dimensioning of the installation.
Capacitors used
In our automatic power factor correction systems, we exclusively use three-phase high-gradient metallized polypropylene capacitors impregnated with resin (without PCB).
The fundamental difference with respect to the standard polypropylene capacitors is the way in which the dielectric film is metallized: if in the standard capacitors the thickness of the metallic layer deposited on the surface of the polypropylene is constant, for the capacitors «with high gradient» the metal layer has a thickness that is suitably modulated. The modulation of the thickness of the metallization allows to improve notably the performance of the capacitors (and therefore of the power factor correction systems of which they are the fundamental component) in terms of: The modulation of the thickness of the metallization allows to improve notably the performance of the capacitors (and therefore of the power factor correction systems of which they are the fundamental component) in terms of:
- increase of the specific power (kvar / dm3) with the consequent reduction of the dimensions of the power factor correction systems; - Improvement of the robustness against permanent and transient surges, for greater reliability also in installations with voltage surges due to the network or to the maneuvers in the installation; - better behavior to the internal short circuit.
Reactive Power Regulators
The reactive power regulator is, together with the capacitors and the reactors (in the blocking filter panels), the fundamental component of the automatic power factor correction system.
In fact it is the "intelligent" element, which is in charge of controlling the phase shift of the current absorbed by the load, depending on which commands the activation and deactivation of the capacitor banks in order to maintain the power factor of the capacitor. installation above the minimum threshold foreseen by the Energy Authorities.
The RPC reactive power regulators used in the automatic ORTEA power factor correction systems have been designed to guarantee the desired power factor while minimizing the effort of the capacitor banks; precise and reliable in the functions of measurement and regulation, are simple and intuitive in their installation and consultation.
The flexibility of the regulators allows modifying all the parameters of the logic to customize its operation, adapting it to the effective characteristics of the installation that needs to be corrected (threshold of the power factor, speed of activation of the batteries, waiting time for re connection of a battery, presence of photo-voltaic, etc.).
The regulators used by ORTEA also offer important functions for the maintenance and management of the power factor correction equipment, whose purpose is the identification and solution of problems in the installation that could cause damage with the consequent reduction of the useful life Of the same.
Custom Equipment
In case of high powers, in addition to the possibility of connecting two or more units of the same size in parallel taking advantage of the potential of the master / slave controller, ORTEA, thanks to its extremely flexible organization, can develop automatic correction systems in a short time of the power factor made according to the technical specifications of the Client.
Said equipment is assembled in a single industrial cabinet and carries a single input dis connector instead of the traditional dis connectors installed in all units connected in parallel.
Among other things, it is also possible to realize systems with dimensions of particular intervals, automatic switch in the entrance, painting with colors to choose and degree of protection IP superior (up to IP55).
