LinkFuzzy Control for an Active Power Filter, Sensing the Line Current Only

7/28/2019 LinkFuzzy Control for an Active Power Filter, Sensing the Line Current Only

1/6

DC LinkFuzzy Control for an Active Power Filter, Sensing the LineCurrent Only.Juan Dixon, Jose Contardo.

Pont ifkia Universidad Catdlica de Chile.Casilla 306, Correo 22, Santiago, CHILEfa x 56-2-552-2563e-mail: [email protected]. cl

Abstract. A different approach for controlling a shuntactive power filter is presented. The dc link is controlledwith fuzzy logic, and the current generated for the filterto compensate harmonics and power factor is not sensed.The sensors acts directly on the mains line currents,forcing them to be sinusoidal, and in phase with themains voltage supply. The amplitude of the current iscontrolled by the fuzzy system, which operates throughthe error between the dc link voltage of the PW Mmodulator (active power filter), and a pre-establishedreference voltage, Vref. The main advantages of thisapproach are the following: a) the control block becomessimpler because there is no need to evaluate the currenttemplate for the filter currents, b) the dc link fuzzycontrol has better dynamic behavior than conventionalPI control, and c) the filter can op erate simu ltaneously asa power factor compensator, and as a four quadrantrectifier-inverter system.

I. INTRODUCTIONConven tionally, passive LC filters have been usedto eliminate line current harmonics, and to improvethe load power factor. However, in practicalapplications these passive second order filters presentmany disadvantages such as aging and tuningproblems, series and parallel reson ance, and others. Inorder to o vercome these problems, active power filtershave been researched and developed [1,2]. In recentyears, shunt active power filters, based on current-

controlled PWM converters have been widelyinvestigated. They have grad ually been recognized as aviable solution to the problems created by high-powernonlinear loads [3,4]. These filters work as currentsources, connecte d in parallel with the non -linear load,generating the harmonic currents the load requires. Inthis form the mains only needs to supply thefundamental, avoiding contamination problems alongthe transmission lines. However, shunt active filters

Luis MoranUniversidad de Co neepcidnCasilla 53-C, (Conception, CHILE

e-mail: lmoran@manet. die. udec.clfa 6-41 246-999

have some drawbacks;, because until now they aredSicult to impllement in large scale, the control iscomplicated, and the cost is high. For example, to ge tthe current reference template in shunt active powerfilters, the load currenit, and the filter current have tobe sensed. From the load current, a complicatedhardware has to be implemented to extract the signalof the harmonic content [5, 61. With this signal, thefilter current is controlled to inject the harmonics backto the system, bur with 180 degrees phase-shift. In thisway, the harmonics from the load are canceled.However, if the signal template of the harmoniccontent is not properly evaluated, the compensationbecomes inadequate.One solution proposed in this paper is to use aconventional, four-quadrant PWM rectifier as a shuntactive power filter. This is accomplished by simplyconnecting the non-linear loads between this rectifierand their line current sensors. This solution reducescost and allows an important simplification in control.Besides, the system can be controlled by Suzzy logic[7]. This approach presents the following particularcharacteristics: a) the Sour quadrant rectifier-invertersystem can operate as a n active filter, and as a powerfactor compensa tor simultaneou sly, b) the dc link fuzzycontrol has better dynanuc behavior tha n conventionalPI control, and c) the calntrol block is simpller becausethere is no need to evalu ate the current templa te for thefiltering currents, because it follows a sinusoidaltemplate in phase with the voltage supply.

11. TH EPROPOSEDYSTEMTh e figure 1 shows the schematic of the shuntactive power filter propclsed. It only measures the dclink voltage, and the m ains line current. As it can beobserved, almost any current-controlled PWMconverter can be used as; a shunt active power filter,without additional electronic circuitry.

0-7803-3840-5/97/$10.001997 EEE 1109


2/6

CONVENTIONALWVM RECTIFIER .

3-PHASEE 3 7Fig. 1. Active power filter proposed

In the control strategy presented here, the activepower filter has been implemented with a four-quadrant, current controlled PWM rectifier, in which anon-linear load has been connected between it andtheir current sensors. Doing that, th e rectifier begins tobehave as a shunt active power filter, but withoutloosing its characteristics as a four-quadrant rectifier.The current-controlled rectifier does not detect thepresence of the non-linear load. It simply try to keepthe mains current sinusoidal. In this way, there is noneed to sense, neither the non-linear load current northe filter current.In this way, the PW M converter can work as a fourquadrant rectifier, as a power factor compensator, andas a filter simultaneously. In other words, is aMultiple-Function System. The control strategy isconventional in the sense that the dc link voltage of theconverter is controlled by adjusting the amplitude ofthe input ac currents. Due to the multiple capability ofthis ap proac h, a good dyna mic behavior is desirable.For this reason, a b z z y control in the dc link has beenimplemented, which allows more flexibility and betterdynamic response.

111. HARDWAREMPLEMENTATIONThe h ardware was implemen ted with a 2-kVA,four-quadrant, current hysteresis controlled, IGBTconverter. As a non-linear load, a diode rectifier withinput inductance, and RL dc load was used. The blockcalled FUZZY CONTROL in figure 1, generates the

sinusoidal templates to control the magnitude of themains current, Imax, They are controlled through theerror between the dc link voltage Vdc of the PWMrectifier, and a pre-established reference voltage Vref.The phase of the sinusoidal template is adjustedthrough the m ains voltage supply.The fuzzy controller has been implemented with a"Freedoml6" v2.1 board from Intec Inoventures Inc.[SI. This board is built around Motorola's 16 bi t68HC16 microcontroler wich not only has morecomputer power than a PC-AT, but, also featuresdigital signal processing (DSP) capabilities and a hostof speclfic features such as pulse counting, high speedinputs and outputs and 8 channnels of 10 A/Dconversion. The board has the advantage that ca n beprogrammed in C language through a PC serialinterface, and can be debuged through the parallelport. Once the programe is finished, the Freedom16can be programed i n ROM for a stand alone operation.

Despite the F16 has 8 A/D converters, theimplemented control was built with an external 8 bi tconverter. In this form, was possible to increase theconversion time and produce a faster response in thecontrol loop. As the F16 does not suport D/Aconvertion, the output was also implemented with anexternal 8 bit DA converter. The figure 2 shows thehardware implementation for th e fuzzy control loop,an d figure 3 shows the PC nterface for programingand debuging in F16.

Line Shunt Filter

f lULoad

VoltageMesurement

U- u-Po&. Input dataFig. 2. Fuzzy control loop

1110


3/6

I - IPort L.4Digital IO

256kRAM256kFlehEEPRCM

Fig. 3. PC nterface for programing and debuging

IV. SOFTWAREMPLEMENTATIONTo imp lement the fuz zy control strategy, a PI fuzzycontrol with 49 rules has been selected [7, 91. Th einputs are the error voltage e(k), and its incrementalvariation de(k). The output is the amplitude of themains current, Imax(k). To make the system work atunity power factor (power factor compensator), themains current is kept in phase with the m ains voltage.Th e Fig. 4shows the fuzzy control block implemented.

Fuzzyfca tion Decision Rules Deh zzy fcat ionFig. 4. Fuzzy co ntrol block

The fuzzy controller is characterized for thefollowing:- Seven fuzzy sets for each input- Seven fuzzy sets for the output- Triangular membership functions- Fuzzyfkation using continuous universe of- Implication using the "min"operator- Inference mechanism based on fuzzy- Defuzzyfication using the "cefltroid'method

discourse

implication

All fuzzy variables have the same partition andmembership functions. The fuzzy control has 7membership functions called from N egative Big (NB)

to Positive Big (PB). 'The figure 5 shows a unitarydiscusion universe wich can be modified by simplegain on each variable. The idea of this partition isto simpllfl the numlber of calibration variables,reducing them to one gain for each variable: GE forerror, GD for derror and finaly GU for DImax.1

Degree ofMembershipP

-1 -213 -113 0 113 213 1Uniwme of Dscusion

Fig. 5 . Unitary discusion universeThe final output to the system is calculated as

Imax(k) =GU*DImax(k:)+Imax(k-1). Figure 6showsthe flow ch art for the sofitware implem entation.

forERROR 4

pS-1RRORDERROR

CheckRules

Defuzzyfy

I m k)=I (k-I)DI MAX (k)

Fig. 6. Flow chart of the fuzzy control.


4/6

v. SIMULATION RESULTS It is clear that the transient response in the mainscurrent, and in the dc link voltage, is faster in thefuzzy alternative. With fuzzy, the dc voltage in thefilter (Vdc) is recovered in less than 40 [ms] (twocycles), but with conventional PI the same situationtakes more than 60 [ms]. The load is a rectifier withfiring angle a =30'.

Some simulations were performed using M athlab.The Figures 7 an d 8 shows a comparison between PIContro l and Fuzzy Control. The load is a thyristorrectifier, working with a =30'. The pa ramete rs of thefilter are: Vdc=250 [v, =2.5 [mH], C=10,000 [ S I .In a), a step from 0 to 20 [A] change in the loadcurrent (thyristor bridge rectifier) is displayed. In b)the opposite situation has been simulated.-

I I I I

I I i I ,o a o 20 3 0 0 0 5 0 6 0 -to e o

X-," , ( 2 O A f d L V >---o 10 2 0 30 d o so s l j 7 0 eo.__ - _ _ -Fig. 7. Trans ient response with PI Controla) from 0 to 20 [A] load currentb) from 20 to O[A] load current

0 1 0 2 0 30 'L O S O 60 7 0 B O__ .. .~ .. ........... .........

VI. EXPERIMENTALESULTSFor the experiments, a 2-kVA, four-quadrant,current-controlled PWM rectifier was used. Therectfie r was implemented with IGBTs, and their maincomponents were: C=4 ,400 pF, an d L=2.5 mH. The dcvoltage was adjusted to three different values: 100, 150an d 200 volts.Th e figure 9 shows the waveforms obtained for

steady-state operation. The first oscillogra m (a) showsthe source current. The second (b), shows the currentthrough the non-linear load (diode rectifier), and thethird (c) shows the current through the active powerfilter (fourquadra nt PW M rectifier). It can be observedthat the waveform of the line current is quitesinusoidal, showing the good performance of theproposed control strategy.

Fig. 9. Steady-state operation(a) line current (4 A/div)(b)load current (4A/div)(c) filter current (4 Nd iv )

Th e figure 10shows the in-phase operation of theinput current respect to the main s voltage supply (unitypower factor operation). The first oscillogram (a)shows the source voltage. The second (b), shows thesource current. The th ird (c) shows the current throughFig. 8. Transient response with Fuzzy Controla) from 0 to 20 [A] load currentb) from 20 to O[A] load currentthe non-linear load (diode rectifier), and the fourth (d)shows the current throu gh the active power filter (four-quadrant PWM rectifier).

1112


5/6

The figure 11 shows a comparison between PIcontrol (upper signal;), and fuzzy control. The firstoscillogram of each experiment (a) shows the dcvoltage drop when Vref=lOO volts. The letters (b), (c)and (d) show the source current, the non-linearcurrent, and the filter current respectively. In thisexperiment, two step chang es have been generated. Inthe first step, a dc load at the diode rectifier ou tm t

(a)

@I(c)a4

IOR Y T R I G n M R N U 0- OB- l a - 1 9 terminals is connected. In the second step a dc load 1sdirectly connected at the dc link of the active powerfilter (four-quadrant PWM rectifier). The oscillogramsshow that the fuzzy control recovers the steady-state ina shorter time. The scales are: V=25 V/div, and 1=4Nd iv. The source frequency is 50 Hz.

m s , . o ~ V CH;.B.Q.~.CI a s : 5 s ~2 C ~ I O K I8813Fig. 10. In-phase operation of the system(a) phase-to-neutral source voltage [50V/div](b) source current [4 Aldiv](c) current through the non-linear load(d) current thro ugh the active power filter

Fig. 11. Comparison between PI Control (upper signals), and Fuzzy Contirol (lower signals)(a) dc voltage drop [25 V/div]; (b) source current (4Aldiv](c) non-linear load current [4 Mdiv ] (d) filter current [4A/div]

-------.-- a)PI CONTROLTh e figure 12 shows the transient responseobtained for a sudden disconnection of the load at theactive power filter dc link. The upper oscillograms arefor PI Control, and the lower correspond to FuzzyControl. Again it is possible to realize that fuzzy hasbetter dynamic behavior than PI control. It can befast recovery of the dc link capacitor voltage. Theoscillogramsshow: (a) the dc voltage error (25 V/div),an d (b) the input source current (4 Aldiv). The dioderectifier was not connected, but the transient responsein that case is very similar

a)noticed a power reversal in the load current, to allow FUZZY CONTROL

Fig. 12 Transient response for a suddlendisconnectionof the load at t he active filter dc link

1113


6/6

CONCLUSIONSA different approach for controlling a shunt activepower filter has been presented. The dc link iscontrolled with fuzzy logic, and the current generated

for the filter to compensate harmonics and powerfactor is not sensed. The amplitude of the mainscurrent is controlled by the fuzzy system, through theerror between the dc link voltage of the PW Mmodulator (active power filter), and a referencevoltage, V ref. T he m ain advantages of ~s approachare the follow ing: a) the con trol block becomes simplerbecause there is no need to evaluate the currenttemplate for the filter currents, b) the dc link fuzzycontrol has better dynamic behavior than conventionalPI control, and c) the filter can operate simultaneouslyas a power compensator, and as a four quadrantrectifier-inverter system.

ACKNOWLEDGMENTSThe authors want to thank Dr. Aldo Cipriano forhis advise in the design of the fuzzy system. Theauthors also want to than Conicyt for his financialsupp ort throug h Project Fondecyt number 1940997.

REFERENCES

[l ] Akagi, H. Kanazawa, Y. y Nabae, A. "Instantaneous Reactiv ePower Compensators Comprising Switching Devices withoutEnergy Storage Components", IEEE Trans. on Ind. Appl., VolIA-20 N"3, MayiJune 1984, pp. 625-630.

[2] Akagi, H., Nabae, A y Atoh, S., "Control Strategy ofA ctivePower Filters Using Multiple-Voltage Source P WConverters", IEEE Trans. on Ind. Appl., Vol. IA-20 N"3,MayIJune 1 986, pp. 460-465.

[3] W. . Grady, M. J. Samotyj, and A H. Noyola, "Survey ofActve Power Line Conditioning Methodologies", IEEE Trans.on Power Delivery, Vol. 5 , No 3. July 1990, pp. 1536-1542.

[4] L. Malesani, L. Rosseto, and P. Tenti, "Active Filters forReactive Power and Harmonic Compensation", Proc. of Th eIEEE PESC'86, June 1986, pp. 321-330.

[6] Tepper, S., Dixon, J., Venegas, G., y Moran, L.,'!4 SimpleFrequency Independent Method for Calculating the ReactiveAnd Harmonic Current in a Nonlinear Load" InternationalPower Electronic Conference, IPEC'95, Yokohama, Japan, April3-7 1995.[7] &se, B. K., "Expert System, Fuzzy Logic, andNeura l NetworkApplicafions in Power Electronics and Motion Control",

Proceedings ofthe IEEE, Vol. 82, No 8, August 1994 , pp. 1303 -1323.

[8] Freedom 16 Advanced 16 bit Controller, User's Manual. IntecInoventures Inc., 27 51 Arbutus Road, Victoria, B. C., Canada,V8N-5X7.

[9] Dorf, R., "Modern Contro l Systems", Addison-Wesley 1989.

[ 5 ] Dixon, J., Garcia J., y Moran, L., "A control System for ThreePhase Active Power Filter, which SimultaneouslyCompensates Power Factor and Unbalanced Loads",Proceedings ofth e IECO N93, Nov. 15-19 1993, Hawaii, USA,pp.1083-1087.

1114

LinkFuzzy Control for an Active Power Filter, Sensing the Line Current Only

Documents

Transcript of LinkFuzzy Control for an Active Power Filter, Sensing the Line Current Only