an integrated high power factor three-phase ac-dc-ac converter for ac-machines implemented in one...

8/8/2019 An Integrated High Power Factor Three-phase AC-DC-AC Converter for AC-Machines Implemented in One Micro Con

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An Integrated High Power Factor Three-phase AC-DC-ACConverter for AC-machines Implemented in one MicrocontrollerFrede Blaabjerg & John K. PedersenAalborg UniversityInstitute of Energy Technology

Pontoppidanstraede 101DK-9220 Aalborg East, DenmarkAbstiact- A high power facto r three-phase AC-DC-AC conv erterfor AC-machines has beendesigned and tested. The AC-DC-ACconverter is controlled by one single l6-bit microeontroller. Theconverterhas twoPWM-VSIbridgeswith common snubber- anddrivecircuit topologies.The high power factor rectifier is spacevector controlled and the inverter controls an AC-inductionmachine by an energy optimized strategy. Common softwaretasks are utilized in order to m i n i memory demands in themicrocontroller. Measurements show a high power factor of theconverter, aswell as the conv erter very rapidlycanchange frommotor to generator operation during reversing of the AC-machine. It is concluded that the AC-DC-AC converter workswell, can be designed very compact and it represents some of thesta hf- the ar t integration and performance in three-phase AC-DC-AC converters.

I. INTRODUCTIONIn the last decades more and more speed controlled

electrical machines are based on an squirrel cage inductionmachine controlled by a PWM-VSI inverter. Much efforts aredone in improving the performance of the inverter-machinebut less efforts are done in investigating the disturbancesonthe line side. For the present new standards appear for theelectrical equipment which demand the benefit of acting as ahigh power factor load. These standards can easily be alimiting factor for the power electronic equipment in thefuture. In a number of applications it is also beneficial tohave regenerating capabilities in order to achieve a minimumof energy consumption. This can also be solved by using arectifier with a high power factor function.

The development in power electronic devices and micro-controllers has been very rapid [l] which in PWM-VSIinverter driven AC-machines have caused higher switchingfrequencies and the possibility to achieve a more complexcontrol strategy. -New inverters are highly integrated(Smartpower) and this gives a reliable design. Some micro-controllers which are special designed for AC-machinecontrol gives also a high dynamical performance.

Different topologies for AC-DC-AC converters have beendemonstrated. [2] and [3] demonstrate a complete AC-DC-AC converter system for AC-machines with a single phasesupply. [2]has a low input power factor and [3] has a high

input power factor but with no regenerating capability. [4]shows a three-phase GTO-based AC-DC-AC converter witha high performance but with the use of a lot of signalprocessors and it operates with a low switching frequency.The goal of this paper is to demonstrate a highly integratedthree-phase high power factor AC-DC-AC converter control-led by only one microcontroller. The converter has especiallythe benefit to conduct power in two directions which can bespecial useful in some applications. It has also the possibilityto generate fully voltage on the machine output terminals.Finally, a single microcontroller solution gives a possibilityto use some common softwaretasks like the modulationtaskand controller task and a lot of a priori information exist inthe system. The information can be used for special controlstrategies of AC-machines like energy optimized strategies.

This paper will first explain the overall system includingsignal flow and transducers. Secondly, the hardware isdescribed like the used snubbers and drive circuits. A controlstrategy for the rectifier and the AC-machine is developedaswell as the implemented software structure is described.Finally, tests are performed to show some of the benefits inthe developed system like line current, FFT-analysis of theline current, power factor measurements and dynamicalmeasurements when the AC-machine is reversing.

II. SYSTEM DESCRIPTIONThe AC-DC-AC converter systemhasa rectifier-module as

AC-DC converter which is connected to the AC-line and aninverter-module as DC-AC converter which is co ~ec t edtothe AC-machine. It has also transducers, drivers and a 16-bitmicrocontroller. The system is shown in Fig. 1.

A three-phase inductanceL, is placed between the rectifierand the AC-line to "ize the current ripple. An inrushcircuit is built-in for control of the inrush current duringstart-up. On the line side three-phase voltages and two phasecurrents are measured. The DC-link voltage and current arealso measured. A tach0 generator is used for speed measure-ment which is necessary for the energy optimized controlstrategy of AC-machine. Twelve drive circuits which aredesigned with decentralized overcurrent protection are theonly interface between the power-modules and themicrocontroller.0-7803-1243-0/!33$03.000 1993 IEEE

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Rectlfler- I nver ter -m o d u l e il.r.h. module

ACL i n

I ne

1

SAEBOC 1 6 6/

Software T a s k s'Control of rect i f ier*Con t ro l of A C-mach ine'Monitor ing- c u r r e n t- v o l t a g e

-power'Modulat ion'Communicat ion

Fig. 1 . System description of the AC-DC-AC converter.TheAC-DC-AC converter is controlled by a 16-bit microcon-troller (SAB80C166) which serves many tasks. It serves asmodulator and controller for the rectifier and inverter/AC-machine, it monitors the currents, the voltages, the power,the speed and finally, it communicates to a terminal. Fromhere it is possible on-line to change different parameters likecontroller constants, DC-link voltage and speed. Some tasksare common for the inverter and rectifier which reduce theneed for ROM in the system. The power rating of the AC-DC-AC converter is 2.5 k W input power. The rectifier israted for 3 x 400 V, 50 Hz and the inverter has an outputcapability of 3 x 0-400 V, 0 - 100 Hz. The switchingfrequency is 4.8 kHz.

111. HARDWARE DESCRIPTIONThe hardware in the AC-DC-AC converter is kept to a

minimum and a high reliability is obtained. The rectifier andinverter bridge are two integrated SIEMENSIGBT six-packmodules (lo00 V/15 A). Each transistor has decentralizedovercurrent protection in the drive circuit with a collector-emitter voltage measurement [9]and with reduced gateemit-ter voltage VR during short circuit. The reduced gateemittervoltage is kept constant in 10 p s and if the transistor is stilloverloaded the transistor turns off and gives an error signalto the controller, otherwise the transistor tums fullyon again.Fig. 2 shows the principle of the drive circuit for one IGBTtransistor.

The drive circuit with reduced gate voltage during shortcircuit is realized by discrete devices but can easily beimplemented in an ASIC.

"SH. IFEEDBACK

E

V -Fig. 2. Drive circuit with reduced gate voltage during short circuitV + = 15 V, VR = 8 V, V = -15 V, VSH,, - 20 A.

The power devices are protected with two Undeland-snub-bers 181 to minimize dv/dt , di/dt and the overvoltage. Fig.3 shows one phase-bridge for the rectifier and the inverterincluding the snubbers.

R e c t i f i e r I n v e r t e r,-------------------------~I I I

II

I L*li I Ls 2 I

I I I I,-------------------------~ , - - - - - - - - - - - - - - - - - - - - - - - - - - - - - IFig. 3 . Snubbers in the main circuit for both rectifier and inverter.b2= L,, = 15a.C, = 3 .3 nF. CO= 440nF. R, = 1 1 n.= 30pH.

A split-inductance snubber (Ls2, L,) is used on theinverter side in order to minimize disturbances from the loadside [8]. It is possible to minimize the number of snubbercomponents by omitting Ls3. Hereby can & and CObe com-mon for the two bridges and recovering of both snubberenergy from inverter and rectifier is possible but the benefitof noise reduction is lost.

Current measurements are performed by hall-sensors. AC-line voltage transducers are done by voltage dividers andthere after transformed to a 2-axis voltage vector. DC-linkvoltage transducer is realized by a voltage divider and anoptocoupler. The speed is measured by a tacho generator.

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IV. CONTROL STRATEGYThe control of the AC-DC-AC converter can be divided in

two parts, the rectifier controland the AC-machine control.The rectifier control is based on a space vector control andthe AC-machine control is based upon an energy-optimizer.R e c t i j k control

The basic control method of the rectifier is equivalent to asynchronous machine operating together with a line which isshown in Fig. 4a. Fig. 4b shows a phasor diagram for theline voltage U,, the line inductance and the rectifiervoltage U,.

Fig. 4. Basic control strategy for the rectifiera) Single-phase diagram of line and rectifierb) Phasor diagram

The stationary control algorithm is given by-U , = - U r+ j w L , I r .

where3 = Line-zero voltage in the line-rU = Line-zero voltage in the rectifierw = Natural frequency in the linelL = Linecurrent

(1)neglects the resistance in the system which can be done inmost systems.The active power flow P and the reactive power Q into therectifier are given by

Controlling the current iL by so it is in phase with theline voltage& the AC-DC-AC converter operates as a re-sistive load and if the current ld a small total harmonicdistortion a high power factor is obtained. However, astationary model cannormally not be used for achieving highdynamic performance in the system. The rectifier is control-led by space-vectors in a rotating d q axis system. Transfor-mation of the system (a-0)shown in Fig. 4into a d q axisrotating system gives simplified expressions for controlpurposes. Fig. 5shows the coordinate transformation.

at

e = atFig. 5 . Coordinate transformation of line and rectifier voltage and cumntfrom fixed a-0coordinates to rotat ing d q c oor di t es .

By placing the line voltage vector on the d-axis of the rota-ting coordinate-frame a simplified dynamical model can beobtained in the rotating frame.

d i ,dt0 = Ri, + L , - + u ~ , ~+ j w L , i d

1id - 'DC iD C(4)

whereR = Resistancein the AC-line inductanceLsand the

switchesuDC = DC-link voltageiDc = DC-linkcurrent

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(6) is stronglyunlinear and it is necessary to make a lineari-zation [7]. A first order Taylor expansion has been used.Following linearization is done.

U 1U d j e fP- PI-DC,ref

(9)2 2'D C = "DC,O 'DC - 'DC,OwhereU D c , o = Linearization point of DC-link voltage

I DCUd

Remark the linearizationhas a variable gaindependent on theworking point. (4) - (9) give a block diagram as shown inFig. 6.

X-F+

I I I I I

R + p L SController Controtler

Fig. 6 . Block diagram for rectifier in the AC-DC-AC converter.Different methods exist for controlling such system [4] -

%ref , 1PI-Controller ' R + P L s-

[7].-The easiest and most effective way is a direct decouplingmethod by mearmring idand i,. The decouplingis obtained by

Q = 0q

whereUd,& =uq,rc/ =Two PI-controllers serve as current controllers and a

simple P-controller controls the DC-link voltage. Thereference current iq,=f is 0 because the rectifier should havea high power factor (Q= 0). A better dynamical perfor-mance in the DC-link voltage is achieved by a feedforwardcompensation by measuring the power in the DC-link. Thefeedforward compensation current id,f can be obtained from

Reference voltage of udReference voltage of U,

(7)

id$ = zKI3 ' D C i D C'd,l

whereK, , K2 :Proportional gains in the feedforward compn-sation

Kz is chosen so the dynamic still is stable in the current loopsand it is also dependent on the line voltage.

Fig. 7. Final implementation of the rectifier control assuming complete decoupling. R=0.5 0. &= 18 mH . C= 1.65 mF.

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AC-machine controlA standarcl operation method for AC-machine control is

with a constant voltage/frequency ratio. The basic operationprinciple is

U f = K , + U , (13)where

U , = Output voltage at stand stilluf = Output phase voltagewS = Impressed stator frequencyK, = Voltagelfrequency ratio

AW = Wrcf - < (15)The ratio K, is controlled to a maximum value K, in the

transient situation given by a change in speed reference urefor a load change in order to enable fullytorque developmentin the machine. The o p t k t i o n starts again when (15) isfulfilled. The machine speed is controlled by a PI-controller.The speed controller and the adaptive controller are designedso speed dynamic is much faster than the dynamic of theadaptive controller.

The adaptive controller is based on an iterative calculationof the voltage/fiequency ratio K, as illustrated in Fig. 9.

This control strategy c a ~ ~be improved by an energyoptimized control strategy for the AC-machine which is basedupon measurement of input power [lo]. In this system i t isobvious to use the power in the DC-link because the DC-linkcurrent and voltage are already known in the microcontroller.The strategy uses the power measurement to an on-lineadjustment of the voltage/frequency ratio so the powerconsumption is kept to a minimum due to optimized mag-nitisation of the induction machine.

The control strategy is shown in Fig. 8.The calculatedDC-link power Pdc,m is a mean value given as

K,

f K s t e pKmKm1nPdc,mwherefm = Integration time of DC-link powert

Based on Pdc,m the adaptive controller calculates a newsary K, is only performed in a stationary situation identifiedf resuenCY ratio K, which OptimizationOf the neces- Fig. 9 . n l u m t i o n of the iterative calculation in the ad aptive controller.by

MIC R O C O N T R O L L E R

LI IIIt IInvertermoduleII 1

Fig. 8. Energy optimized control strategy for AC-induction machine with o n-line adaption of machine magnitization level based on Dc-link powermearunment.

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Fig. 11 shows a high stationary performance both in motorand generator operation.

The power factor PF is higher than0.99 at 90% load. Theharmonic content of the current is also measured at full loadand shown in Fig. 12.

I E C - 5 5 5..................... ,..........................l ;0 .

. o..00 :

0 i o 0 :?A n - I A A B

4

I

Froqurncy (Hz)Fig. 12. Harmonic analysis of the line current. P = 2.2 kW.

Low harmonic currents exist in the line current. Themaximum current specified in IEC-555 standard is alsomarked and the AC-DC converter fulfils these demands.

The power factor PF and the angle of the first harmonicbetween the voltage and current (cos(+)) as a function of theload is shown in Fig. 13.

1 .ohvQ

Bg 0.8b*0-LUL8 0.6

...;+..: ..... 1 .....:.....i.....i...... Po+ tactor ;

..../. . ._;_....I . ..._i _....;. . . ._;_. . . . j . . .._;_........,.. ......... l ........... I . . . . . . . . . . . . . . . . . . . . . . .

....,. . . .

0.0 0.5 1 .o I .5 2. 0Input power (kW)

Fig. 13 . Power factor PF and the angle (cos(+)) for the first harmonicbetween the voltage and the cu m nt a8 a hnction o f the load.

Fig. 13shows a very high cos(+) in the whole operatingarea which means the control strategy works well (Q = 0).The AC-DC-AC converter acts like a resistive load. Thepower factor decreases at very low load which is explainedby some unlinearities in the voltage transducers and in therectifier.

Dyn-mica1 testAC-machine. Fig. 14shows the line voltage and current.The dynamical performance is also tested by reversing the

Rsrardq of AC-machins 104wI i M T 0 l t . l . :

1-100.w 0.05 0.10 0.15 0.20 0.25 0.30Time (I)

Fig. 14. Line voltage and line current when the AC-machine is reversing.

Fig. 14shows that the AC-DC-AC converter very rapidchange from motor operation to generator operation. It alsoshows the current instantaneously changes and the currentcontrollers works well. However, some of the poorer powerfactor at low load (seeFig. 13)can be seen in the currentwaveform. Aliasing phenomenon from the oscilloscope canalso explain the waveform.

During the reversal of the AC-machine the speed and theDC-link voltage is also measured. The waveforms are showninFig. 15.

1 I 1

I 1 -1m0 1 320

h e (4Fig. IS. DC-link voltage and AC-machine speed during reversal.Fig. 15 shows that the DC-link voltage is almost constantdu-ring the reversal of the AC-machine. This indicates areasonable chosen feedforward technique. A faster speedchange can be allowed because the line current is not maxi-mum. In the case of the maximum allowable line current isobtained during generator operation to the line it is beneficialto allow a higher voltage increase in the DC-link to obtain afaster AC-machine response.

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Finally, measurements are performed on the d- and q-axiscurrent in the rectifier during reversal of the AC-machine.The AC-machine is loaded.The measurements are performedby the microcontroller. Fig. 16 shows these currents.

-2 -20.0 0.2 0.4 0.6 0.8 I .o 1.2Time (1)

Fig. 16. Measured d- and q-axis cu m n t in the rectifier at reversal of theAC-machine.

Fig. 16 shows that the controller very fast changes the d-axiscurrent at reversal of the AC-machine. The q-axis current iskept approximately zero which indicates an acceptabledecoupling and very fast current loops.

VII. CONCLUSIONA high performance and integrated AC-DC-AC converter

controlled by one microcontroller has been designed andtested. A 16-bit microcontroller controls the rectifier by spacevector control and the AC-machine is controlled by a speedloop with a proposed energy-optimal control scheme which isbased upon an adjustment of the voltage-frequency ratio. Theconverter has a power rating of 2.5 kW and is rated for 3 xDecentralized overcurrent protection is achieved on eachtransistor by measuring the collector- emitter voltage. TwoUndeland snubbers protect and control the IGBT- transistorsin the rectifier and the inverter.A direct decoupling method is used in the space vectorcontrol for the rectifier and two PIcontrollers control theactive and the reactive power in the converter. The reactivepower is keptto zero by the controller and a high power factor is obtained.However, the control strategy opens an opportunity also tocompensate for reactive power. The DC-link voltage iscontrolled by a simple Pcontroller and a power feed-forwardcompensation technique is integrated in order to achieve ahigh dynamic. The AC-machine has a simple control strategywith an energy optimizing performance. The powerfulmicrocontroller will also be able to include field vectorcontrol of the AC-machine if necessary in some specialapplications. Both the rectifier and the inverter utilize double

400V, 50 HZ input and 3 x 0-400V, 0 -100 HZ output.

sidqi space vector modulation because it gives a mini"distortion and torque ripple.Test results show stationary a high power factor both

motor and generator operation. Harmonic analysis shows asexpected that new standards are fulfilled.The testsshow alsoa high dyn am id performance in the rectifier. The presentedconverter represents the state-of-the-art of integration andperformance in three-phase AC-DC-AC converters.

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VIII. ReferencesB.K. Bow, "Recent Advances in Pow er Electronics". IEEE Trans.on PE, Vol. 7, No. 1, 1992. pp. 2-16.K. Thiyagarajah,V.T. Ranganathan,B.S. R amakrishnaIyengar, "AHigh Switching Frequency IGBT P W M Rectifierhverter Systemfor AC Motor D rives Operating from Single Phase Supply". IEEETrans. on PE, Vol. 6, No. 4, 1991, pp. 576-584.S. Murk-Nielsen, J.W. Jensen, J.K. Pedersen, F. Blaabjerg, "ACompact High Performance 16-Bit Microprocessor Controlled AC-servodrive". Qmposium onIndustrial Elecnvnics/ IEEE, 1992, pp .146-150.H. Kohlmeier, D. Sch me r , "Control of a Double V oltage InverterSystem Coupling a Three-phase Mains with anACdrive" .Proceed.of IASAEEE Ann. Meet., 1987, pp. 59 3-599.T.G. Habetler, "A Space Vector-based Rectifier Regulator forAClDClAC converters". Proceed. of EPE '91, 1991, pp. 2.101-2.106.M.Weinhold, "Appropriate Pulse Width Modulation for a"e-Phase PWM AC-to-DC Converter". EPE Journal, Vol. l , No. 2.1991, pp. 139-148.H. Sugimoto, S. Morimoto, M. Yano, "A high PerformanceControl Methods of a Voltage-Type PWM Converter". Proceed. ofPESC '88AEEE, 1988, pp. 360-368.F. Blaabjerg. "Snubben in PWM-VSI-Inverter". Proceed. ofPESC'9lAEEE, 1991, pp. 104-111.J.K. Pedersen, F. Blaabjerg. "An Optimum Drive and C h qCircuit Design with C ontrolled Switching for a Snubbedem PWM-VSI-IGBT Inverterleg". Proceed. of PESC '92/ZEE?, 1992, pp.289-297.J.J. Cathey, P. Famouri. "Loss Minimization Control of anInduction Motor Drive". Proceed. of USAEEE,Annual Meeting,1989 , pp. 226-23 1.

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