In this study a new method is proposed in order to improve the harmonic suppression efficiency of Active Power Filter for the problems caused by the distorted and unbalanced voltages with unbalanced load currents in control strategy. The proposed control method gives an adequate compensating current reference even for non ideal voltage and unbalanced current conditions. The results of simulation study are presented to verify the effectiveness of the proposed control technique in this study.
1. Active Power Filter
2. Park Transformation
3. Clark Transformation
4. Self Tuning Filter
5. Unbalanced Load Currents and Voltages
Fig. 1. Block diagram of the APF
EXPECTED SIMULATION RESULTS:
Fig. 2. Three phase unbalanced and distorted (non-ideal) grid voltage
Fig. 3. Unbalanced varying load current under non-ideal voltage condition.
Fig. 4. Voltage waveforms, a-) Three phase unbalanced and distorted grid voltages, b-) Grid voltages after transformation to α-β ( at the input of STF), c-) The obtained balanced and undistorted two phase voltage waveforms (at the output of STF), d-) The obtained balanced and undistorted three phase voltage waveforms (at the input of PLL).
Fig. 5. DC-link terminal voltage with the proposed control method
Fig. 6. Three phase converter currents
Fig. 7. Grid current after filtering by proposed control method under distorted and un-balanced grid voltage with unbalanced load conditions
In this paper, we have considered to design a control method in order to generate correct reference current signal to satisfy the requirements of harmonic suppressing and reactive power compensation for the unbalanced nonlinear loads combination under case of non-ideal grid voltage conditions. In the propose method, the distorted and unbalanced voltages first processed by using Self Tuning Filter to determine the correct angular positions. Then second STF is used to extract balanced load current waveforms after obtaining the fundamental and harmonic components of instantaneous currents by using park transformation. In this study, additional low-pass or high-pass filter is not used to extract harmonic components from the fundamental. The simulation studies shows that the proposed control technique gives an adequate compensating current references.
 W. M. Grady, S. Santoso, "Understanding power system harmonics", IEEE Power Eng., Rev. 21, pp. 8-11.
 B. Singh, K. Al-Haddad, K, A. Chandra, "A review of active filters for power quality improvement", IEEE Transaction on Industrial Electronics., vol. 46, no. 5, pp. 960–971, 1999.
 N. Mariun, A. Alam, S. Mahmod, H. Hizam, "Review of control strategies for power quality conditioners", PECon 2004, Power and Energy Conference, vol., no., pp. 109- 115, 29-30 Nov. 2004.
 H. S. Song, “Control scheme for PWM converter and phase angle estimation algorithm under voltage unbalance and/or sag condition”, Ph.D. thesis in Electronic and Electrical Engineering. South Korea, 2000.
 M. Abdusalam, P. Poure, S. Saadate, “A New Control Scheme of Hybrid Active Filter Using Self-Tuning-Filter,” POWERENG 2007 International Conference on Power Engineering, Energy and Electrical Drives, 2007, vol., no., pp.35-40, 12-14 April 2007.