Mitigation Of Harmonics Caused by Frequency Converters with Passive Filters

Some specific applications are made between the frequency converter and the mains and between the frequency converter and the induction motor in order to eliminate the negative effects on the mains and the motor. By eliminating the negative effects, like harmonics, caused by the inherent of frequency converters, the mains side (especially with the filters to be used on the input side) enables the use of a cleaner and more efficient energy for both the primary consumer and other consumers. On the other hand, the operating life of the induction motor and the frequency converter and thus the whole system to which these two components are connected is extended by eliminating the negative effects of the frequency converters on induction motors.

A compressor with a 7,5 kW induction motor fed by frequency converter was taken as a reference to measure the performance of the filters. At the first stage, the harmonic levels and power values of the current situation were measured. Then, the same measurements were repeated by adding the selected filters as per the power values and the results were compared with each other. 2% and 4% line reactor and LCL type passive filter were used on the inlet side.

1.     Harmonics in case the use of Frequency Converter

2.     Harmonics in case the use of 2% Line Reactor

The same tests were repeated by connecting a line reactor with a 2% impedance between the mains and the frequency converter. By keeping the compressor pressure constant, 100% loading of induction motor was ensured. The current-voltage waveform obtained under these conditions is given below.

When Figure 2.1 is examined, it is observed that the current waveform distortion has slightly decreased. The line reactor added before the fequency converter increases the impedance of the system and makes the current waveform smoother. The inductor, which is contained in its structure, does not allow sudden changes of the current and thus leads to improvement in the current waveform. The obtained voltage and current harmonic spectrum are given below.

 As seen from Figure 2.2, it is observed that 2% line reactor reduced the THD value from 46,1% to 34,6%. The harmonic values of current and voltage are given in the table below.

When Table 2.2 is examined, it is observed that the 2% line reactor reduces the THD value of the current and therefore decreases the distortion power. The total appearent power is reduced as well as the distortion power. Since reactors have an inductive reactive characteristic, they cause the reactive power increase due to the capacitors in DC busbar unit of frequency converters. In addition, this situation causes cos? value to drop a bit more than before. However, since the harmonics were decreased, the power factor value increased from 0,85 to 0,88.

3.     Harmonics in case the use of 4% Line Reactor

The same tests were repeated by connecting a line reactor with 4% impedance between the mains and the frequency converter. The obtained voltage-current waveforms are given below.

When Figure 3.1 is examined, it is observed that the current waveform gets slightly better than the 2% line reactor. The obtained voltage and current harmonic spectrums are given below.

As expected, the 4% line reactor reduced the THD? value from 34.6% to 31.6% by reducing the harmonic distortion slightly more than the 2% line reactor. As the impedance value increases, the harmonic mitigation rate increases as well. Yet, high impedances cause extra voltage drops. This reduces the voltage at the input of the frequency converter, causing it to draw more current and to operate with low performance. The harmonic values of current and voltage are given in the table below.

When Table 3.2 is examined, it was observed that the cos? value decreased to 0.87 due to the inductive characteristic of the line reactors as mentioned previously. However, with increasing inductive reactance, it is seen that harmonic distortion value and distortion power decrease.

4.     Harmonics in case the use of LCL Type Passive Filter

 Passive filters are also connected in series as well as the other line filters. However, they have capacitors besides inductors. These kind of filters have a specific cut-off frequency. This value is generally adjusted according to the 5th and 7th harmonic orders that appear predominantly in such applications. The passive filter used in this application has a cut-off frequency of 250 Hz. Passive filters filter the harmonics of the 5th harmonic with the LC arm and broadband harmonics with the series connected inductor. When the passive filter is added to the system, the improvement in the voltage-current waveform is clearly seen from the figure below.

Figure 4.1 shows that the current waveform approaches the sinusoidal form. The obtained voltage and current waveforms are given below.

The current harmonics are largely mitigated and decreased from 46,3% to 8,7%. The harmonic values of current and voltage are given in the table below.

The 5th harmonic current in the system where only the frequency converter exists was in the 5 A level was reduced to 0.6 A levels with the addition of the passive filter to the system. A similar reduction occured in 7th harmonics from other dominant harmonics. Thus, a low harmonic distortion is achieved and losses due to harmonics are reduced. The power and THD values obtained in the case of the passive filter are given in the table below.

Since the power lost due to the harmonics is reduced, the total power consumed in proportoion to this is reduced as well. Passive filters make reactive compensation to the system due to the capacitor they contain. Therefore cos? value is obtained as 1. As the system meets the need for reactive power through the capacitor, the reactive power that spent is obtained as zero. The power factor, which is obtained due to both the cos? value being 1 and the low harmonic distortion value, is the highest value among the tests.

 5. Evaluation

  When the harmonic filter is added to the system where the inductor motor fed by frequency converter is measured as 46.3%, the obtained THD? value is given in the table below. In the same table, the cost ratios of these filters were added to determine the optimum filter in terms of both performance and cost. The cost of 2% line reactor is taken as the base cost and the base value of current harmonic is taken as 46,3%.

References

[1]. Gül?en, G., Frekans Konvert?rlü ?ndüksiyon Motorlar?n Harmonik Analizi ve Optimum Filtre Tasar?m?n?n Belirlenmesi, Yüksek Lisans Tezi, Mersin üniversitesi Fen Bilimleri Enstitüsü, Mersin, 2018.