Detuned Power Factor Correction Case Study – Plastics Factory
We were asked to attend site to carry out a power factor correction survey on a plastics factory who, due to their impending increase in load, had a requirement for more power factor correction. As a normal part of our power factor correction survey our survey engineer measured the voltage harmonics to determine whether any power factor correction recommend would have to incorporate detuning reactors to avoid harmonic overloading of the capacitors. During this survey the voltage harmonics were recorded as being in excess of 8%. Such a level of harmonic voltage distortion is beyond even the IEC compatibility limits, meaning that any equipment connected to such a distorted supply cannot be guaranteed to function correctly.
BY INSTALLING OUR DETUNED POWER FACTOR CORRECTION SYSTEM WE MANAGED TO NOT ONLY PREVENT A DAMAGING HARMONIC RESONANCE, BUT ACTUALLY REDUCE THE LOAD HARMONICS, POTENTIALLY INCREASING THE LIFE OF THE CLIENTS PLANT.
The installation at the time of the survey included a 400 kVAr power factor correction bank arranged in four steps of 100kVAr, all of which was in circuit at the time of the test. The immediate suspicion was that the power factor correction capacitors were forming a resonant or at least partial resonant circuit with the supply transformer leading to a magnification of the loads natural harmonic levels. In order to test this theory the capacitor banks were all switched out of circuit and the harmonic voltage measured once again. Upon doing this the voltage distortion was found to be in the region of 3.5% to 4.0%, clearly supporting the theory that the capacitors were exacerbating a previously moderate harmonic issue. Upon closer analysis of the specific harmonics contributing to the overall harmonic distortion it was discovered that the 5th harmonic was the dominant harmonic.
In order to determine the true extent of the problem a period of harmonic logging was commissioned with the original power factor correction operating. Figure 1 shows the profile of the voltage total harmonic voltage distortion captured with the original power factor correction running. The first couple of days of the analysis were during the weekend and consequently no plant was operating. Once the plant started operating on the Sunday night there was an immediate jump in the voltage THD, with the distortion level peaking at more than 8%. Figure 2 shows the profile of the 5th harmonic voltage taken during the same period. It can be seen that this profile is almost identical to the THD profile, indicating that the 5th harmonic is by far the dominant harmonic.
From the predicted increase in loading it was calculated that the total future power factor correction requirement was 600 kVAr. Clearly this amount of power factor could not be connected without detuning reactors as it would result in harmonics levels even higher than the elevated values existing at the time. In cases such as this the most common solution would be to install capacitors with 189 Hz detuning reactors, which prevent the magnification of harmonics for all of the harmonics from the 5th order upwards. Normally the total harmonic distortion of a supply consists of contributions from a number of harmonics whose magnitude decreases as the harmonic order increases. As has already been mentioned, the harmonic distortion at this site is overwhelmingly dominated by the 5th harmonic and as such any reduction in this harmonic would have a profound impact on the total harmonic distortion. With this in mind it was recommended that a 210 Hz detuned power factor correction system was installed at this site. The effect of tuning the power factor correction to 210 Hz is that as well as preventing magnification of all of the major harmonics it actually provides filtration of the 5th harmonic.
The larger the capacitor bank, the greater the filtration effect is and bearing in mind the recommended capacitor bank size for this site was 600 kVAr the potential filtration was significant.
Based upon the commercial and technical arguments presented for this project, the installation of a 600 kVAr, 210 Hz detuned power factor correction system was commissioned. Figure 3 and Figure 4 show, respectively, a profile of the voltage total harmonic distortion and 5th harmonic distortion with all of the replacement detuned power factor correction in circuit. It can be seen that as opposed to the original level of 8%, the voltage THD with the detuned capacitors in circuit was barely 2%. In addition to this the 5th harmonic voltage no longer dominates the voltage THD to the same degree as before, this time only constituting approximately 1.6% of the 2.2% distortion.
Conclusion
From the preceding results it is apparent that the old power factor correction was producing a serious harmonic resonant condition. The additional harmonics imposed upon the supply transformer would have produced significant additional stress upon the supply transformer, potentially reducing the lifespan of the transformer and compromising the integrity of the supply.
By installing 210 Hz detuned power factor correction, a solution to the poor load power factor has been implemented that provides both an economical and technical advantage, resulting in a supply that operates efficiently with little potential for mal-operation of connected equipment due to excessive voltage harmonic distortion.
