Power factor correction is a technology designed to restore a power factor as close as possible to unity. Power factor correction is normally achieved by the addition of capacitors to the electrical network. This will reduce losses in the power grid, and ultimately lower your electric bill.
What is Power Factor?
Power Factor is the ratio between the useful power (kW) and total (apparent) power of an electrical item or an installation. It is a measure of the efficiency of electrical power conversion into output.
The power factor can be expressed in two ways:
Power factor (pf) = Useful power (kW) divided by the total power (kVA),
Power factor (pf) = The cosine of the angle between useful power and total power = cos ø.
Value of Power Factor and Losses
The ideal power factor is unity, or one. If the power factor is less than unity, extra energy will be required in order to complete the actual task at hand.
Almost all power flow in the power distribution and supply systems causes losses.
- A load with a power factor of 1 is the most efficient load for the supply system.
- When the load has a power factor of 0.8, the loss in the supply system becomes high, and the bill for the consumer gets higher.
Thus, a relatively small improvement in power factor can significantly reduce losses since losses are proportional to the square of the current.
If the power factor drops to less than one, there will be ‘missing’ power called reactive power, which unfortunately is needed to provide a magnetising field for motors and other inductive loads to function properly.
Reactive power is also known as wattless, magnetizing or wasted power, and it adds to the burden placed on the power grid and the consumer’s bill.
Reason of Poor Power Factor
A poor power factor is usually due to the following two reasons.
- A significant phase difference between the voltage and current at the load terminals
- the current waveform is distorted or has high harmonic content.
Poor power factor usually occurs with inductive loads such as an induction motor or a power transformer, a ballast in a luminaire, a welding set, or a furnace. The distorted current waveform can be produced by a rectifier, an inverter, a variable speed drive, a switched-mode power supply, discharge lighting, or other electronic loads.
Improving Poor Power Factor
Poor power factor due to inductive loads can be improved by adding power factor correction equipment, but a poor power factor resulting from distorted current waveforms obviously needs remodeling of the equipment or the addition of harmonic filters.
Inverters that claim to have a power factor of 0.95 or more are actually operating at a power factor between 0.5 and 0.75. It should be noted that the 0.95 figure is based on the cosine of the angle between voltage and current, which does not take into account that currents that have irregular waveforms contribute to increased losses.
Power Factor Correction
An inductive load requires a magnetic field to operate. The magnetic field causes the current to be out of phase with the voltage (the current lags the voltage).
Power factor correction is the process of compensating for the lagging current by creating a leading current by connecting capacitors to the supply. A sufficiently high capacitance is connected so that the power factor is set to as close as possible to unity.
Power Factor Correction
Power factor correction is the term given to a technology that has been used since the turn of the 20th century. It is used to restore the power factor of an electrical system to as close to unity.
Power factor correction is normally achieved by the addition of capacitors to the electrical network which compensates for the reactive power demand of the inductive load and thus reduces the burden on the supply. There should be no effect on the operation of the equipment.
The power factor correction, usually in the form of a capacitor, is added to the distribution system to counteract the magnetic fields as much as possible in order to reduce losses in the distribution system and to further reduce the electricity bill.
Capacitors are used in almost all power factor correction equipment to draw current, which leads the voltage, thereby generating a leading power factor. If capacitors are connected to a circuit that operates at a nominally lagging power factor, the extent that the circuit lags is reduced proportionately.
Typically the corrected power factor will be 0.92 to 0.95. Some utilities offer incentives for their customers to maintain a power factor of over 0.9, for example, while others penalize those with a low power factor.
A number of ways exist to meter this, but in the end, in order to reduce wasted energy in the distribution system, the consumer is required to apply power factor correction. Today, most Network Operating companies penalize for power factors below 0.95 or 0.9.
Benefits of Power Factor Correction
When the power factor correction is applied correctly, the following benefits can be achieved:
- Environmental benefit: Enhanced energy efficiency results in lower power consumption so fewer greenhouse gas emissions are produced and fossil fuels are not depleted by power stations.
- Reduction of electricity bills.
- Extra kVA is available from the existing supply.
- Reduction of I2R losses in transformers and distribution equipment.
- Reduction of voltage drop in long cables.
- Extended equipment life – Reduced electrical burden on cables and electrical components
How is Power Factor Correction is Achieved?
Power factor correction is achieved by adding capacitors in parallel to motor or lighting circuits. This can be done at the equipment or distribution board or at the origin of the installation.
Static Power Factor Correction
By connecting the correction capacitors to the motor starter, you can apply static power factor correction to each individual motor. A disadvantage can occur when the load on the motor changes and can result in under or overcorrection.
Static power factor correction must not be applied at the output of a variable speed drive, solid-state soft starter or inverter as the capacitors can cause serious damage to the electronic components.
Power factor corrections that are correctly sized should not cause overcorrection. A motor’s power factor correction is typically based on its non-load (magnetizing) power. Since the reactive load of a motor is greater than the kW load, overcompensation should be avoided.
Care should be taken when applying power factor correction star/delta type control so that the capacitors are not subjected to rapid on-off-on conditions. Typically the correction would be placed on either the Main or Delta contactor circuits.
Power factor correction applied at the origin of the installation consists of a controller monitoring the VAr’s and this controller switches capacitors in or out to maintain the power factor better than a preset limit (typically 0.95)
Where ‘bulk’ power factor correction is installed, other loads can in theory be connected anywhere on the network.