PFC - General Overview
The increasing demand of electrical power and the awareness of the necessity of energy saving is very up to date these days. Also the awareness of power quality is increasing, and power factor correction (PFC) and harmonic filtering will be implemented on a growing scale. Enhancing power quality improvement of power factor saves costs and ensures a fast return on investment. In power distribution, in low and medium-voltage networks, PFC focuses on the power flow (cos ϕ) and the optimization of voltage stability by generating reactive power to improve voltage quality and reliability at distribution level.
What is power factor?
Power factor is a way of describing how efficiently electrical power is consumed. It refers to power in an
alternating current (AC) electrical circuit, either for a single piece of equipment or all of the electrical equipment at a site. The power that is drawn from the network can be described as consisting of two parts – useful power and reactive power. Useful power is the power that equipment needs to achieve the task at hand and it is measured in kW. Reactive power is drawn in addition to useful power by a reactive load and is measured in kVAR. The consumption of reactive power does not contribute to achieving the task. The useful power and the reactive power together determine the power drawn from the network, that is, the total power (also known as apparent power), measured in kVA. Total power is not the linear sum of useful power and reactive power. However, lessening the effects of reactive power will reduce the current needed from the network to complete the same tasks. Power factor is the ratio of delivered useful power to the total power taken from the supply. An ideal ratio is 1.0, that is, a perfect match between power drawn from the network and useful power for the task. This is also known as unity power factor. In reality, many loads do not easily achieve unity power factor due to their inherently reactive nature. However, it is possible to compensate for the reactive power. By doing so, less power will be needed from the network to achieve the same tasks, resulting in energy and cost savings. If the ratio is under 0.85, the power factor is generally considered to be poor. Correction would typically achieve 0.95-0.98.
How reactive power is generated
Every electric load that works with magnetic fields (motors, chokes, transformers, inductive heating, arc welding, generators) produces a varying degree of electrical lag, which is called inductance. This lag of inductive loads maintains the current sense (e.g. positive) for a time even though the negative-going voltage tries to reverse it. This phase shift between current and voltage is maintained, current and voltage having opposite signs. During this time, negative power or energy is produced and fed back into the network. When current and voltage have the same sign again, the same amount of energy is again needed to build up the magnetic fields in inductive loads. This magnetic reversal energy is called reactive power. In AC networks (50 / 60 Hz) such a process is repeated 50 or 60 times a second. So an obvious solution is to briefly store the magnetic reversal energy in capacitors and relieve the network (supply line) of this reactive energy. For this reason, automatic reactive power compensation systems (detuned / conventional) are installed for larger loads like industrial machin- ery. Such systems consist of a group of capacitor units that can be cut in and cut out and which are driven and switched by a power factor controller.
Apparent power S = √P² + Q²
Active power P = S * cos ϕ
Reactive power Q = S * sin ϕ
With power factor correction the apparent power S can be decreased by reducing the reactive power Q
Implementing power factor correction techniques
Understanding the system
The specification of a PFC system requires knowledge of several factors including:
The voltage level and typical usage of the reactive loads on-site.
This will affect the type of system and the amount of correction needed. Adding capacitors is a basic PFC technique, but other systems might be more appropriate in some circumstances. For instance, active PFC is an electronic PFC system that controls the amount of power drawn by a load in order to obtain a power factor as close as possible to unity. In most applications, the active PFC system controls the input current to the load using power electronics. However, active PFC systems can also be tailored to provide varying PFC characteristics for sites with unique profiles and power constraints.
The usage profile across the site
This is particularly important if PFC equipment has been installed at some time in the past and the electrical
requirements have changed since, for example, by the substitution or addition of some machinery. If so, have the
power factor resurveyed.
The power quality required by the on-site loads
Equipment generally needs a high quality supply with stable characteristics – that is a ‘pure’ supply. Improperly
specified, PFC can introduce harmonics, where unwanted voltages at various frequencies can be superimposed onto
the mains voltage. Adverse effects caused by harmonics include heating of equipment and electromagnetic
interference, such as that to communication systems, telephones, radio, TV and computers. In extreme cases, it can lead to failure of equipment. Additional equipment, such as line filters can smooth the mains voltage to reduce any prominence of harmonics.
The simplest form of PFC involves fitting capacitors. Many PFC devices exist to accommodate each type of situation. It is worth shopping around specialist companies and taking expert advice on the system that will suit you. If a single machine has a poor power factor, capacitors can be connected in parallel with the device, that is, connected to the live and the neutral terminals of the reactive device, so that they compensate for the poor power factor whenever the machine is switched on. This is a form of ‘fixed’ PFC. If the power factor at a site is permanently poor and no single item of equipment is solely responsible, fixed PFC can be employed also. In this case, the PFC capacitors will be connected across the main power supply to the premises, that is, the capacitor banks’ terminals are connected to each of the three phase cables as they enter the site. In this case, PFC can be linked with the switchgear. However, there are other circumstances where PFC is not so straightforward. Where many machines are switching on/off at various times, the power factor may be subject to frequent change. In these cases the amount of PFC needs to be controlled automatically – that is, the banks of capacitors need to be selectively switched in and out of the power circuit appropriately. There are various
solutions on the market for automatically controlled PFC - See PFC Controllers and active PFC.