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Saturday, 17 March 2018

Real-time control of shunt active power filter under distorted grid voltage and unbalanced load condition using self-tuning filter

In this paper, an alternative control method is proposed to improve the harmonic suppression efficiency of the activepower filter in a distorted and an unbalanced power system to compensate for the perturbations caused by the unbalanced nonlinear loads. The proposed method uses a self-tuning filter (STF) to process the grid voltage in order to provide a uniform reference voltage to obtain the correct angular position of the phase locked loop. Moreover, the required compensation currents are obtained by implementing another STF in the transformed set of currents in order to separate the fundamental and the harmonic currents. This allows the calculation of a precise reference current for the unbalanced, the non-linear and the variable load conditions. The proposed control method gives an adequate compensating current reference even for a nonidealvoltage and unbalanced current conditions. The real-time control of the filter under the distorted and the unbalanced power system is developed in an RT-LAB real-time platform. The results obtained in the software-in-the-loop configuration are presented to verify the effectiveness of the proposed control technique.




Fig. 1 Block diagram of the APF and the proposed control method


Fig. 2 Voltage and the current waveforms
a Ideal grid voltage
b Non-ideal (distorted and unbalanced) grid voltage
c Load currents under the ideal grid voltage
d Load currents under the non-ideal grid voltage

Fig. 3 Compensation of the grid current harmonics by using the conventional dq method
a Under the ideal voltage
b Under the non-ideal voltage, and by using the conventional pq method
c Under the ideal voltage
d Under the non-ideal voltage

Fig. 4 Compensation of the grid current harmonics
a By using the STF-based pq theory under the ideal voltage
b By using the STF-based pq theory under the ideal voltage
c By using the proposed method under the ideal voltage
d By using the proposed method under the non-ideal voltage

In this paper, the design of a control method that generates thecorrect reference current signal in order to satisfy the requirements of a harmonic suppression and a reactive power compensation, for the unbalanced non-linear load combinations under the case of the non-ideal grid voltageconditions have been discussed. An alternate method is proposed where two STF are applied to manage the distorted and the unbalanced voltage and current. In the proposed method, the distorted and the unbalanced voltages are first processed by using the STF to determine the correct angular positions. Then, a second STF is used toextract the balanced load current waveforms. This methodeliminates the need for additional low-pass or high-pass filtering when extracting the harmonic components from the fundamental. A step-by-step performance study, in a real-time environment, shows that the proposed control technique is able to generate the proper compensating reference current during the steady state and the dynamic load change conditions under the distorted and the unbalanced grid voltage conditions.
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