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Energy Procedia 74 (2015) 1015 - 1025

International Conference on Technologies and Materials for Renewable Energy, Environment and

Sustainability, TMREES15

Fuzzy logic controller of five levels active power filter

L. Zelloumaa, B. Rabhib S. Saadc, A. Benaissad, M. F. Benkhorise

aLEVRES Laboratory, University of El-Oued, Algeria bLMSE Laboratory, University of Biskra, Algeria cLSE Laboratory, University of Badji Mokhtar-Annaba-Algeria dLAADI Laboratory, University of Djelfa, Algeria eIREENA-CRTT Laboratory, Department of Electrical Engineering, Ecole Polytech Nantes, University of Nantes,Saint Nazaire, 44600, France

Abstract

This paper presents five-level inverter as a shunt active power filter (SAPF) to compensate reactive power and suppresses harmonics drawn from a diode rectifier supplying RL load under distorted voltage conditions. The harmonic current extraction is based on the use of self tuning filter (STF) and fuzzy logic controller employed to control harmonic current and inverter DC voltage. The aim of the present work is to obtain a perfect compensation by extracting accurate harmonic currents to improve the performances of the five-levels active power filters.

The proposed scheme is validated by computer simulation using MATLAB Fuzzy Logic Toolbox in order to show the effectiveness and ability of this method. The results have demonstrated that the proposed shunt active power filter with STF and Fuzzy Logic Controller (FLC) have produced a sinusoidal supply current with low harmonic distortion and in phase with the line voltage.

Keywords: Shunt active power filter, Five-level inverter, Fuzzy logic control, self tuning filter, Harmonics, Reactive power compensation, PWM control.

1. Introduction

The intensive use of static converters in the industry has lead to an increase in harmonic injection in the power system and a lower power factor. The harmonics causes problems in power systems and in consumer products such as equipment overheating, capacitor blowing, motor vibration, excessive neutral currents and low power factor.

Conventional active power filter was deeply explored and used to eliminate harmonics and compensate reactive power [1-3]. These active power filters are limited in medium power applications. Multilevel configuration were introduced and employed to achieve high power filters.

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativec0mm0ns.0rg/licenses/by-nc-nd/4 .0/).

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) doi: 10.1016/j.egypro.2015.07.738

Actually, the use of multilevel inverters for high power drives and harmonic compensation [2-5] become a great issue. Multilevel pulse width modulation inverters can be used as active power filter for high power applications solving the problem of power semiconductor limitation.

The aim of the present work is to obtain a perfect compensation by extracting accurate harmonic currents to improve the performances of the five-levels active power filters.

The performances of different reference harmonic current extraction methods under balanced sinusoidal alternating current (AC) voltages conditions [6-11], such as the so-calledp-q theory, Synchronous Reference Frame theory (SRF) and recently ADALINE neural network [6] provide similar performances. Differences appear when working under distorted and unbalanced AC sources. In real conditions, the voltage sources are distorted affecting greatly the filter performances [10].

In this paper, the reference current identification under distorted voltage conditions is based on using self tuning filter (STF). This filter is used to extract the fundamental component directly from the distorted voltage or/and current in a-P reference frame.

The controller is the main device of the active power filter operation and has been a subject of many works in recent years. In this paper, fuzzy logic controllers are used for harmonic current and inverter DC voltage control to improve the performances of the five levels shunt APF.

The advantages of fuzzy controller over conventional controllers, robustness, no need to accurate mathematical model, can work with imprecise inputs, and can handle non-linearity [12-22]. The performance of fuzzy controller and reference current identification using STF are evaluated through computer simulations under distorted voltage conditions. The obtained results showed that, the proposed active power filter scheme with STF and Fuzzy Logic Controller (FLC) has provided a sinusoidal supply current with low harmonic distortion and in phase with the line voltage.

2. Shunt APF configuration

The proposed shunt APF is employed to reduce harmonic currents and compensate reactive power. The topology presented in Fig.1 shows a three-phase five-level VSI used as shunt APF. In order to obtain an inverter (active filter) of N levels, N—1 capacitors are needed [23-24].

The voltage across each capacitor is equal to Vdc/(N—1), Vdc is the total voltage of the dc source. Each couple of switches (S11, S15) form a cell of commutation, the two switches are ordered in a complementary way. The inverter produces five voltages according to the expression presented below:

V10 = k1Vd- (1)

Where, Vio is the phase-to-middle fictive point voltage, Ki is the switching state variable (Ki =1, 1/2, 0, -1/2, -1), Vdc is the dc source voltage, and i is the phase index (i = a, b and c). The voltage values are presented in table 1 (Vdc/2, Vdc/4, 0, -VdC/2, -VdC/4) [23-24].

Table 1: Obtaining of five -level inverters.

Ki Tii Ti2 Ti3 Ti4 Ti5 Ti6 Ti7 Ti8 Vio

1 1 1 1 1 0 0 0 0 Vdc/2

1/2 0 1 1 1 1 0 0 0 Vdc/4

0 0 0 1 1 1 1 0 0 0

-1/2 0 0 0 1 1 1 1 0 -Vdc/4

1 0 0 0 0 1 1 1 1 -Vdc/2

2.1. Reference current calculation 2.1. 1.Self tuning filter

M. Abdusalam et al [11] have presented in their work the principle, frequency and dynamic response of the STF under distorted conditions. Current or voltage signal, before and after filtering can be expressed by the following equation:

= - k(*) " M*)]" — XP(S) (2)

= - Ms) - Xft(s)] + — xa(s) (3)

Where, xa(s) ,Xp(s), xa(s) and Xp(s)are a current or a voltage signal, respectively and wc is the pulsation of STF.

The block diagram of the STF tuned at the pulsation wc and the frequency response of the STF versus different values of the parameter - are given in [11]. It is observed that no displacement is introduced by this filter at the system pulsation. It has been also noticed that small value of - increases filter selectivity. Fig.2, shows the self-tuning filter tuned to the pulsation wc.

2.1.2 Harmonic identification

Several methods [1-6] were proposed for harmonic currents identification. Mainly, methods based on the FFT (Fast Fourier Transformation) in the frequency domain and methods based on instantaneous power calculation in the time domain and recently artificial neural network method. The block diagram of the STF used with p-q theory to extract harmonic currents is illustrated in Fig. 3.

Fig. 3. Block diagram of the new STF-based harmonic isolator.

In this study, the p-q theory method is used enabling the compensation of harmonic currents, reactive power and unbalanced currents. The reference currents (harmonic currents) extraction is based on a-ft transformation to obtain real and imaginary powers. The voltages (VSa, VSb and VSc) and currents (ILa, ILb and ILc) are transformed to bi-phase system according to the following equation:

Vs 2 /

After that, the STF extracts the fundamental component at the pulsation wc directly from the currents in the a-P axis. Then, the a-P harmonic components of the load currents are computed by subtracting the STF input signals from the corresponding outputs. The obtained signals are the AC components, ia and ip, which correspond to the harmonic components of the load currents ILi, IL2 and IL3 in the stationary reference frame.

Then, self-tuning filter is applied to these a-P voltage components. This filter enables suppressing any harmonic component of the distorted mains voltages and as a result the harmonic extraction is improved. After computation of the fundamental component Vap and the harmonic currents iap, thep and q powers are calculated as follows:

After computation of the fundamental component V ap and the harmonic currents powers as follows:

we calculate the p and q

p ( v a V ia

_q _ I" ^ V a y

Instantaneous powers are composed from a constant part and a variable part corresponding to fundamental and harmonic currents respectively.

- 2 , - 2

V« + Vp

After adding the active power required for regulating DC bus voltage, pc, to the alternative component of the instantaneous real power, p (Fig. 3), the current references in the a-P reference frame are i*ap . Then, the filter reference currents in the a-b-c coordinates are defined by:

2.2. Inverter control strategy

This control uses a fuzzy logic controller which starts from the difference between the injected current (active filter current) and reference current (identified current) [2] that determines the reference voltage of the inverter (modulating wave). This standard reference voltage is compared with four carrier triangular identical waves.

These carrier waves have the same frequency and are arranged on top of each other, with no phase shift, so that they together vary from maximum output voltage to minimum output voltage . PWM synoptic block diagram of currents control is presented in Fig. 4.

2.3. Fuzzy logic control application

Fuzzy logic serves to represent uncertain and imprecise knowledge of the system, whereas fuzzy control enables taking a decision even if we can't estimate inputs/outputs only from uncertain predicates. The synoptic scheme of Fig.5 shows a fuzzy controller, which possesses two inputs and one output. The inputs are namely the error (e), which is the difference between the reference current (harmonic current) and the active filter current (injected current) (e = iref -if) and its derivative (de) while the output is the command (cde).

Fuzzy Controller

Fig.5. Fuzzy controller synoptic diagram

2.4. Active power filter current control

The purpose of current control is to obtain sinusoidal source currents in phase with the supply voltages. The conventional PI controllers are replaced by fuzzy logic controllers [12-21] and their characteristics and fuzzy rules are given in [2].

2.5. DC capacitor voltage control

Several power filter controllers were studied and used such as IP, RST, hysteresis, adaptive control and fuzzy logic controller.

In the present work fuzzy logic control algorithm is implemented to control the DC capacitor inverter voltage based on DC voltage error e(t) and its variation Ae(t) in order to improve the dynamic performance of APF and reduce the total harmonic source current distortion. The fuzzy logic rules are summarized in [2].

3. Computer simulation

The described fuzzy logic controller and STF have been tested on a distorted wave produced by a diode bridge rectifier with RL load and five levels PWM inverter used as shunt APF. The simulation is conducted using parameters illustrated in Table 3. The obtained switching signals of the three-phase five-level inverter are presented in Fig. 6. The load current before connecting shunt active power filter is illustrated in Fig. 7. This current is highly distorted and its THD is equal to 15.66% and its spectrum is shown in Fig. 8.

Table.3. Simulation parameters.

Supply: Vs, Rs, Ls 220 V, 0.010, 0.1 mH

System frequency 60 Hz

DC Load: Rd, Ld 0.50, 0.3 mH.

Active Filter: Vdc, Lf, Rf , 800 V, 0.2mH,

Cj=C2 =C3=C4 0.0050,

Switching frequency 10 K Hz

>i i m i

11 □

□ .16

Time (s)

Fig. 6. Switching pulses of APF arms (S11, S12, S13, S14).

Time (s)

Fig. 7. Supply current Is waveform without filtering

= so--!—1----1 — I----1—!— +—!—I—I —

E I I I I I I I I I

1= 60--!—1----1 — I----1—!— +—!—I—I —

I I I I I I I I I

^ 40--I---I----1---I----1---1---4----1---J----1----

s 20--I---I----1---1----1---1---A---1---J----1----

0_i__i_i_I_i_I_i_i_I_i_I_i_i_i_i_i_

0 2 4 6 8 10 12 14 16 18 20

Harmonic order

Fig. 8. Source current spectrum without filter.

4. Results and discussions

The obtained results are given to show the active power filter performances using fuzzy control schemes and self tuning filter (STF). Five-level shunt active power filter performances depend on current references quality, p-q theory and STF are used for harmonic currents extraction, the obtained current is shown in Fig. 9. This control method enables harmonic currents and reactive power compensation simultaneously; the obtained current and voltage waveforms are in phase as illustrated in Fig. 10.

It can be deduced that five level shunt APF contains less harmonic as presented in Fig. 11 with a THD of source current equal to 0.85% after compensation which is below the limit of 5%. The five-level voltages are (Vao): 400V, 200 V, 0 V, -400 V and -200 V, corresponding respectively to Vdc/2, Vdc/4, 0, -Vdc/4 and -Vdc/2, the DC voltage source (Vjc) is 800 V.

The output voltages Vao and Van are shown in Figs. 12 and 13. The five level active filter with fuzzy logic controllers and STF has imposed a sinusoidal source current waveform as illustrated in Fig. 10 and a constant and ripple free DC voltage as illustrated in Fig. 14.

Time (s)

Fig. 9. Active filter current I.

Fig. 10. Power factor correction (Vs, Is).

6 8 10 12 14 16 18 20

Harmonic order

Fig. 11. Source current spectrum with filter.

Fig. 12. APF output voltage Vao

Fig. 13. APF output voltage Var

Tirine (s)

Fig. 14. DC voltage of the condenser

5. Conclusion

In the present paper theoretical study with simulation of a fuzzy logic controller with SFT and five level shunt active power filter based on p—q theory to extract reference currents and PWM to generate switching signals have shown high shunt APF performances in reducing harmonics, and power factor correction. The results have proved that DC capacitor voltage and the harmonic currents control using FLC and STF are very important. This type of controller and filter are adapted easily to severe conditions such as distorted voltage conditions.

The three phase five level shunt active power filter is simulated and the THD measured verifies the reduction of harmonics to a low level when the fuzzy logic control is employed. The five level APF with FLC and STF provide numerous advantages such as perfect and accurate harmonic currents extraction, improvement of the supply current waveform, less harmonic distortion and its use in high power/medium voltage with a lower maximum device rating.

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