# Effect of Harmonics on Ferroresonance in Low Voltage Power Factor Correction System--A Case Study
## 4 Experimental Results and Discussion
### Discussion  
Experimental results have shown that ferroresonance can occur in a low voltage detuned power factor correction system when high voltage distortion is present on the low voltage busbar. The high voltage distortion results from nonlinear loads connected to the low-voltage busbar injecting current harmonics into the system, but the voltage distortion on the medium-voltage busbar can also affect the voltage distortion on the low-voltage busbar. Since most of the nonlinear loads in the industrial system under study are variable speed drives with diode bridge rectifiers, the fifth current harmonic has the highest magnitude. As shown, when the ferroresonance in C2 is not triggered, these current harmonics flow into the medium voltage grid and generate harmonic voltages, with the highest value of harmonic voltage on the low voltage busbar. When the ferroresonance occurs, the harmonic spectrum of the detuned reactor current and voltage becomes very rich, with the 5 th harmonic having the highest value. With a detuning factor of 5.67%, the resonant frequency of the LC circuit is 210 Hz, which is very close to the 5 th harmonic,  
Figure 21: C2—Vector diagram of 5 th harmonic of the medium voltage feeder voltage and current (**a**) before and (**b**) after switching on the capacitor stage.  
Figure 22: C2—Secondary voltage and current of transformer T1 and stage current without ferroresonance.  
and under normal operating conditions, the detuned reactor with the PFC capacitor filters significant values of the 5 th harmonic. However, when the 5 th voltage harmonic on the low-voltage busbar is high, the detuned reactor can be overloaded with high current harmonics, as shown by the experimental results. This indicates the well-known problem of dimensioning passive devices (power factor correction, filters) in the presence of harmonics. If passive compensation or filter is present in the industrial installation, any change in the load type (e.g. adding a nonlinear load) or in the system configuration must be verified by simulating its effect on the harmonic distortion in the system and on the installed passive devices. If this is not the case, various phenomena can occur, such as the one analyzed in this paper. Moreover, new installations are made up of highly efficient components, so the resistance required to damp resonances is insufficient, making them susceptible to this type of event.  
Ferroresonance first appeared in the industrial plant studied after two conventional (fixed speed) compressor drives were replaced by variable speed drives. This resulted in increased voltage harmonic distortion on the low voltage busbar at some operating points where the nonlinear/linear load ratio is high. As a result, switching on the capacitor stage at these operating points triggered the ferroresonance, and for the stages that were already switched on, the high harmonic distortion caused the detuned reactors to be overloaded. This situation could have been avoided if the power factor correction had been implemented with active filters, reducing the higher harmonics but are the most expensive solution. The risk of ferroresonance would have been lower if the detuned reactors had been installed with the detuning factor of 7% (the resonant frequency of an LC circuit is 189 Hz). However, this solution would have reduced, but not eliminated, the risk of ferroresonance in the industrial plant under study. Finally, the total voltage harmonic distortion level of almost 7% in C2 is very high and should be reduced. Practical experience has shown that the total voltage harmonic distortion level of 5% is the practical limit above which various harmonic-related problems can occur.