Hello Keith
Traditionally, we have always described power factor as the cosine of the angle between the voltage and current in a network.
Provided that the current and voltage are both sinusoidal, then this is still correct.
On our electrical networks today, we have an increasingly significant harmonic current due to the high level of electronic devices connected to the supply.
In some regions, this high level of harmonic current has proven to be a major problem, resulting in severe regulations being put in place to limit the harmonic currents and more particularly the resultant harmonic voltages.
In other regions, there has not yet been any significant move to penalize harmonics as they have not yet been seen to present major identified problems.
Harmonic currents can significantly increase the transmission and distribution losses in the network in the same way that reactive currents can, but unlike reactive currents, harmonic currents in a finite network impedance, can cause harmonic voltages that can interfere and damage other connected equipment.
In the region where I am based, we have had major harmonic problems with MV harmonic voltages as high as 16%. This has resulted in major damage to connected equipment. - failed induction motor rotors due to excessive slip losses, capacitor failures, transformer and switchgear failures, and electronic equipment operation and reliability compromised.
There has been a very strict regime put in place here to limit harmonic currents and the problems are now reducing.
In some regions, there has been no recognician of harmonic problems and so there are no enforced regulations and/or penalties.
If we look at electronic inverters with no reactors in the rectifier circuit, they can exhibit current distortions of 85% to 115%. The addition of reactors, either AC line reactors or DC Bus chokes can reduce the full load current distortion down to 35% - 40%.
By definition, Power factor is the ration between KW and KVA. A high current distortion results in a low distortion power factor.
VFDs with not reactors can exhibit distortion power factors below 0.7 and VFDs with reactors typically exhibit a distortion power factor of around 0.91
If you are in a region where there are no enforced regulations covering harmonics, and the power factor metering is based on displacement power factor, then there is no advantage in distortion power factor correction unless you are experiencing problems.
Many modern installations are using modern smart metering and if this is set to measure True Power Factor rather than displacement power factor, you can be heavily penalized for a bad power factor even though your displacement power factor is close to unity.
True Power Factor correction is best achieved by the use of active filters as these can correct both distortion currents and reactive currents.
Capacitors, detuned or not detuned, should not be used on circuits with significant harmonic currents present, or with active filters or active front end drives present. They can amplify frequencies other than line frequency due to circuit resonances.
If your client is penalised for poor True Power Factor, then you must apply active or passive filters, capacitors will not help much and could cause resonance problems.
Mark Empson
Advanced Motor Control Ltd
