Power Factor Correction: why?
In an electric circuit, the current is:
– in phase with the voltage when the load is resistive (e.g., resistors)
– lagging when the load is inductive (e.g., motors, off-load transformers)
– leading when the load is capacitive (e.g., capacitors).
For example, the total current (I) absorbed by a motor is given by the vector sum of:
– IR, active current due to the load resistive component
– IL, reactive current due to the load inductive component.
The following powers are directly associated with the above currents:
– P, active power linked to the load resistive component
– Q, reactive power linked to the load inductive component
– A, apparent power.
The mean value of the reactive power in a wave period is nil. Therefore, the reactive power does not contribute towards the generation of mechanical work and constitutes an additional burden for the energy supplier, forcing it to oversize its generators and transmission/distribution lines.
The parameter that defines the absorption of inductive reactive power is called power factor (φ) and is represented by the ratio between active and apparent power.
Assuming there are no harmonics in the system, the power factor is equal to the cosine of the angle between the voltage vector and the current vector (cosφ).
The power factor decreases when the reactive power increases.
A system working with low power factor shows the following disadvantages:
– High power loss in transmission/distribution energy lines.
– High voltage drop.
– Oversized design of generating, transporting and transforming plants.
Hence the importance of solving or at least reduce the effects generated by low power factor. Capacitors are used for this purpose.
Power Factor Correction: how?
By installing a capacitor battery, it is possible to reduce the reactive power absorbed by the inductive loads connected thus increasing the power factor.
There are several ways to perform the power factor correction and the choice depends on daily load duty-cycle, load distribution and type of service.
The main choice is between distributed or centralized power factor correction.
If the correction system is distributed, the units are located in the vicinity of each load for which the power factor needs to be corrected.
If the correction system is centralized, a single automatic capacitor bank is installed upstream all the loads and immediately downstream the point where the power factor is measured (for example, inside the MV/LV distribution transformer cabin on in the main distribution switchboard).
Technically speaking, the distributed system is the solution to prefer: capacitors and load follow the same profile during daily service, which makes the power factor correction systematic and strongly linked to the affected load.
Moreover, in case of distributed configuration, both the user and the Distributing Body benefit from the reactive power reduction. In industrial plants, for example, savings are achieved in terms of tariffs but also in terms of better design of all the electric lines in the facility connecting the MV/LV cabin to the loads.
Another noticeable advantage prided by this type of correction, is that the installation is simple and not expensive.
Power factor correction systems and loads are energised and de-energised at the same time, thus exploiting the same protections against overload and short-circuit.
The daily load duty-cycle is of critical importance when choosing the most suitable power factor correction system.
Very often, not all the loads work at the same time and some are operative only for a few hours during the day. In this case, it is clear that the distributed configuration would be too expensive due to the high number of correction systems that would need to be installed and the idle time of several units.
The distributed configuration is most efficient when the majority of the required reactive power is concentrated on few high power loads working for many hours during the day.
The centralized configuration, on the other hand, is suitable for situations where there are many diverse loads working sporadically. In this case, the bank power is much lower that the overall power that would be necessary with a distributed configuration.
It is recommended to permanently connect the correction unit only if the daily reactive power absorption is sufficiently regular, otherwise the unit must be handled in order to avoid the power factor to swap to a leading value.
Should the reactive power absorption be very changeable during the plant operating time, it is recommendable to choose an automatic correction system splitting the bank into several steps. The manual operation can be foreseen only when the correction unit needs to be operated only a few times during the day.
Power Factor Correction: how much?
The choice of the capacitor bank power to install (QC) depends directly on:
– desired cosφ2 value
– starting cosφ1 value
– installed active power.
The formula 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 the power factor correction.
Said formula can also be written as: QC = k x P
where k can be easily calculated with the table in the following page.
Example: assuming that the installed load absorbs an active power equal to 300kW with a starting power factor equal to 0,70 and that an increase to 0,97 is desired, the coefficient k can be obtained from table 1: k = 0,770. QC is therefore equal to: QC = 0,770 x 300 = 231kvar
Power Factor Correction: harmonics in electric lines
Current distortion (i.e., harmonics) in industrial or tertiary electric plants is generated by non-linear loads such as inverters, welders, rectifiers, computers, drives and so on. The distortion is represented by the number THDI%: if the current is sinusoidal, the THDI% is nil. The more the current is distorted, the higher is its THDI% value.
Their connection to the mains causes several problems in an electric system:
– Rotating machinery: generation of eddy torques (and consequent vibrations) that undermine the mechanical structure. The loss increase causes undesired overheating and isolation damage.
– Transformers: increase of core and winding losses, with potential winding damage. The potential presence of DC voltage or current components may saturate the magnetic core, thus increasing the magnetizing current.
– Capacitors: overheating and voltage increase, both causing a reduction of the expected life.
If periodic, the waveform of the current generated by a non-linear load can be represented as the sum of several sinusoidal waves at different frequency (the wave at 50Hz is called fundamental, whilst the ones at frequency multiple of the fundamental are called harmonics).
It is generally not recommended to correct the power factor in a system with high harmonic content without any device dealing with the harmonics.Even though capacitors able to withstand high overloading could be provided, power factor correction performed only via capacitors actually increases the harmonic disturbance and the related negative effects.
The best solution for this type of issue is the detuned filter obtained by connecting reactances in series to the capacitors. The reactances shift the system resonance frequency below the lowest existing harmonic thus protecting the capacitors and avoiding dangerous resonance phenomena.
Power Factor Correction: conclusions
Usually, in a plant with low power factor, the payback of the installation costs is most likely achieved within a few years.
Beyond the elimination of potential penalties from the energy bills, the technical and economical benefits deriving from the installation of a power factor correction system are listed below:
– decrease of the losses in lines and transformers due to the lower absorbed current
– decrease of line voltage drop
– optimization of the plant sizing.
Inside ORTEA automatic power factor correction systems there are only three-phase polypropylene metalized high gradient capacitors resin filled (PCB free).
The fundamental difference with respect to the standard polypropylene capacitors is how the dielectric film is metallized: if in the standard capacitors the metal layer thickness deposited on the polypropylene film surface is constant, for the «high gradient» ones the metal layer has a suitably modulated thickness.
The metallization thickness modulation greatly improves the capacitors (and therefore of the power factor correction systems, of which they are the a critical component) performance in terms of:
– Increase in power density (kvar/dm3) with a consequent power size reduction of the power factor correction systems;
– Robustness against voltage surges, for g eater reliability even in systems with the presence of voltage fluctuations due to the network or maneuvers on the system;
– Improved behaviour in relation to internal short circuit withstand.
Reactive power regulators
The reactive power regulator, together with capacitors and reactors (in detuned filter cabinets) is, the key component of the automatic power factor correction system.
It is actually the 'intelligent' element responsible for the verification of the power factor of the load, depending on which it controls the ON/OFF switching of the capacitors batteries. By doing so, the regulator maintains the system power factor above the minimum threshold set by the Energy Authority.
The reactive power regulators RPC used in automatic ORTEA power factor correction systems are designed to provide for the desired power factor while minimizing the wear & tear of the banks of capacitors.
Accurate and reliable in their measuring and control functions, the regulators are simple and intuitive to install and use.
The flexibility of ORTEA regulators enables the modification of all the parameters, so that they can be customize to suit the actual characteristics of the system that needs correction (threshold power factor, step switching sensitivity, steps reconnecting time, presence of photovoltaic systems, etc.).
ORTEA regulators also offer important features for maintaining and managing the power factor correction bank in order to identify and solve problems which could otherwise lead to damage and life expectancy reduction.
For high rating units, beyond the 'master - slave' configuration which exploits the controller ability to connect in parallel several units, ORTEA can develop very quickly automatic power factor correction systems personalized on the Customer's specification. All this thanks to ORTEA's flexible and responsive organization.
These units are assembled inside a single industrial cabinet and are fitted with a single switch disconnect instead of the traditional switches mounted on each parallel module.
Among other characteristics, it is possible to design the unit with special step-sizes, input automatic circuit breakers, different paint colour and higher IP protection (up to IP55).
Designed to suit the most common switchboard sizes, ORTEA's rack system is the ideal solution for OEM and switchboard manufacturers. The rack is:
– Available with or without blocking reactor
– Available in a single rack from 9.4kvar to 150kvar
– Fitted with busbars sized to withstand up to 400kvar
– Easy to assemble (busbars and NH fuses incorporated in the rack support)
ORTEA racks are fitted with three-phase self-regenerating high energy-density metallised polypropylene capacitors ensuring enhanced performance, low losses and contained dimensions.
The lateral adjustable slides allow for quick and easy assembling operations inside any cabinets.
Thanks to its extensible brackets, the 480mm rack can be mounted in a 800mm cabinet thus enabling a flexible combination of sizes and total reactive power.
The busbar system can withstand a maximum reactive power equal to 400kvar (at 415V, 50Hz).
Rack can be added to the system at any time.
Each auxiliary and control component is supplied already wired to the terminal block assembled on the rack.
Each rack is supplied complete with:
– Contactors for capacitors
– Self-extinguishing cables (EN50267-2-1)
– Three-pole NH00 fuse holder
– NH00 gG power fuses
– Three-phase self-regenerating high energy-density metallised polypropylene capacitors
– Three-phase connecting system via tinned copper bars
– Discharging resistors
– Only for the H203 type, blocking reactor with 180Hz tuning frequency
All the components conform with the safety legislative provisions.
Standard accessories (in every rack)
– Telescopic side brackets, suitable for 600-400mm deep cabinets
– Tinned copper connection bars, complete with bolts
– IP20 Plexiglass protection
– Adapting brackets for installation inside cabinet with different width (800-1000mm)