A New Predictive Current Control Technique for Multilevel Converters

Gerard LedwichSchool of Electrical Engineering

Queensland University of TechnologyGPO Box 2434, Brisbane,QLD, 4001, Australia

Abstract

In this paper, a novel predictive current controltechnique is proposed for a three-phase multilevelinverter, which controls the capacitors' voltages andload currents with low switching losses. The advantageof this contribution is that the technique can be appliedto more voltage levels without significantly changingthe control circuit. The three-phase three-level inverterwith a pure inductive load has been implemented totrack reference currents using analogue circuits andprogrammable logic device.

1. Introduction

Multilevel voltage source inverters are a newgeneration of inverters suitable for high power andhigh voltage applications because of reduced harmoniccontents and low voltage stress across load. A three-level inverter known as a neutral point inverter isproposed by Nabae [1] and is applied to higher levelinverters as diode-clamp multilevel inverters. Anotherpopular voltage source multilevel inverter is a flying-capacitor topology [2], which both real and reactivepower flow can be controlled. This topology has alladvantages of the multilevel inverters; while largenumbers of capacitors, balancing the capacitors'voltages and precharging the capacitors at the initialtime are the disadvantages of this topology. The nextproposed topology is a multilevel inverter usingcascaded traditional inverters. This type of inverter canavoid extra clamping diodes or flying-capacitors butrequires separate DC sources for active powerconversions [3]. The main advantages of this topologyare that each cell is identical and the manufacturingand the maintenance costs are reduced and pulse widthmodulation techniques can easily be applied toinverters. In most high performance applications of avoltage source Pulse Width Modulation (PWM)inverters, the current control is the essential part of theoverall control systems. Therefore, the performance ofthe inverter system largely depends on the quality ofthe applied current control strategy. Current controltechniques for traditional PWM inverters have beenconsidered in different methods by several authors [4].

Current controlled PWM inverters have the severaladvantages compare to conventional open-loop voltagesource PWM inverters such as: control ofinstantaneous current waveform with high accuracy,peak current protection and overload rejection [5].In this paper, a novel predictive current controltechnique for a three-phase multilevel inverter isproposed. This method considers how to improveunbalanced voltages of capacitors using voltagevectors in order to minimise switching losses.

2. Current control properties

Fig. 1 shows a three-phase three-level voltage sourceinverter with a three-phase R-L and back emf voltage.For each phase of the load, the output voltage of theinverter can be expressed in terms of the loadcomponents as follows:

dippVn =Rip+L -+epdt

(1)

Where,p =phasea, borcVp, = line-to-neutral voltage (phase voltage)ip= line currentL load inductanceR load resistanceep= back emf

I s, -)isXEdT ica ,rI 2T b , S4

Ca IcVd -F

Fig.1: A three phase three-level inverter with flyingcapacitor topology

1-4244-0549-1/06/$20.00 (2006 IEEE.

Firuz Zare

By assuming that the capacitors' voltages are keptaround Vd , the output voltage of the inverter can be

2determined as regards of the inverter switching statesas follows:

VdVp)( 2 (Si + S)| i=1,3,5;p=a,b,c (2)Where, Si(,= ..6) has 1 or 0 values when the switch is"on"l or "off', respectively.In this paper, the main idea is to develop a currenttracking algorithm for multilevel converters with lowswitching losses and balancing the capacitors voltages;thus a prediction of back emf voltage is not taken intothe account in the following equations. The back emfvoltages for the period from (k-1) to (k) may bepredicted by an increment of the back emf voltageposition. A purely inductive load (R=O, e,=O) andusing the above transformations, the load currentderivates in d-q coordinate can be expressed in termsof the switching states and input parameters as follows,

iq =d4 [S3 +S4 S5 S6] (3)q 4LVd[d[S +S 3 -S4 S5 S6] (4)

d 2L 2 2 2 2Multilevel inverters generate different voltage levelsusing capacitors, which work as voltage sources. Thesecapacitors are charged or discharged when loadcurrents pass through them during operation and tooperate properly, the current controller has to achievecorrect switching states in order to balance thecapacitors' voltages. As shown in Fig. 1, the currents offlying-capacitors can be determined in terms of theswitching states and the load currents as follows:

icp = (Si -Si+1 )ip i=1,3,5;p=a,b,c (5)The voltage across each capacitor can be expressed asfollows:

cp a dt p=a,b,b (6)Making an average of each variable over a oneswitching cycle, Ts, the duty cycles of the switches canbe determined by assuming that the voltage and thecurrent do not change during Ts. Thus, the followingequations define the capacitors' voltages and the loadcurrents in terms of the load and input parameters:

Aq= 4s d[d3+d4-d5-d6]Vd _d d d5 dAid = -[di +d2 - 2 2 22L 2 2 2 2

AVcp =(di C-di+l)lpTs =1,3,5;p=a,b,cCP

(8)

(9)

Where, di (i=1..6) is the duty cycle of ith switch;Aid & Aiq are the current errors; AVp ( =ab,c) is theerror between the reference DC voltage and thecapacitor voltage. Using Eq.(7-9), the duty cycles of allswitches, di (i=,..6) can be found in order to generate thedesired currents and to balance the capacitors' voltages

at the end of each switching cycle. In this case, twoswitches per leg are required to operate during eachswitching cycle in order to control capacitors' voltageswhich increase the number of switching to fourtransitions per leg as shown in Fig.2.(a). Anotheralternative to control the capacitors' voltages is basedon a decision at a transition time which minimisesswitching to two transitions per leg as shown inFig.2.(b). For example, if the voltage across thecapacitor Ca is less than reference value at thebeginning of nth switching cycle, the controller chargesthe capacitor based on Eq.(7). As shown in Fig.2.(b) &Fig.3.(a) and assuming that the load current is positive,the voltage across the capacitor can be increased up todlaIT when S2 is off over the switching cycle. Then,Ca

if the capacitor voltage is more than reference value atthe beginning of (n+l)th switching cycle, the currentcontroller can discharge the capacitor down to-d2I1Ts when SI is off over the switching cycle as

Cashown in Fig.2.(b) and Fig.3.(b). Achieving bigcapacitors, the voltage ripple across each capacitor canbe decreased and the controller can minimise theswitching numbers.

thnth (n+1)

Four swiirhings

I~ ~ ~ 77l/S 1Is.'S (a) 15

nth (n+1)*{4woTithTgs4

L r. I zr| | 2 |Ts (b) Ts

Fig.2: Pulse pattern using (a) four switching percycle (b) two switching per cycle

Fig.3: Circuit diagram when the capacitor in leg"6a" is (a) charged (b) discharged

Therefore, the following pulse patterns have beenproposed for three-phase three-level inverter:For 0 < (dj/>= 1]35)+dj 9 < 0.5, one of the switches, Sj orSj+ is off over one switching cycle, Ts and the otherone is on based on the duty ratio value as shown inFig.4.(a).For 0.5 < (dj ]=,3,s)+dj 9) < 1, one of the switches, Sj orSj+ is on over one switching cycle, Ts and the otherone is on based on the duty ratio value as shown inFig.4.(b).

SIS12 - SISSI2SI SI S2 S S S LTS Ts Ts

(b)Fig.4: Pulse patterns of leg "a" of three-levelinverter for different values of duty cycle(a) O< (di + d2)

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3 13 23 xi to S31 S3

Time (ms)(a)

-21O 10 20 30 40 50 fiO

° 2

O 10 20 30 40 50 60O(D)

Time (ms)(b)

Fig.7: Simulation results (a) load current (b)capacitors' voltages with different initial values

The noise signal strongly affects the analoguemeasurement section of the current controller. Thisproblem can affect the performance of the controllerand a PCB circuit with a good EMI protection candecrease the influence of noise signal in the controlsection. To simplify the control section, the resistancevalue of the load was neglected and iR terms wasassumed to be zero while to improve the quality of thecontroller this term can be considered in thecalculations.

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60

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0 Time (ms) 10 20

Fig.8: Experimental results of output voltages ofinverter with predictive current control technique

S-4W3

2u1

O

C

* -1

= -2a -30-J -4

Time (ms)Fig.9: Experimental results of load current of

inverter with predictive current control technique

4. Conclusions

Multilevel voltage source inverters are a newgeneration of power inverters suitable for high powerand high voltage applications because of reducedharmonic contents and low voltage stress across load.In this paper, a novel predictive current controltechnique is proposed for a three-phase multilevelinverter, which controls the capacitors' voltage andload current with low switching losses. This methodalso controls the capacitor voltage using adjacentvoltage vectors in order to minimise switching losses.The advantage of this contribution is that the techniquecan be applied to more voltage levels withoutsignificantly changing the control circuit. The three-phase three-level inverter with a pure inductive loadhas been implemented to track three-phase referencecurrents using analogue circuits and programmablelogic device to verify the validity of the currentcontroller.

5. Reference

[1] Nabae, I.Takahashi, H.Akagi, "A new neutral-point-clamped PWM inverter", IEEE Transactions-on-Industry-Applications, 1981, Vol. IA, No.5, pp.518-523.[2] Meynard-TA; Fadel-M; Aouda-N, "Modeling ofmultilevel converters", IEEE-Transactions-on-Industrial-Electronics. vol.44, no.3; June 1997; p.356-64.[3] Fang-Zheng-Peng; Jih-Sheng-Lai; McKeever-JW;VanCoevering-J, "A multilevel voltage-source inverterwith separate DC sources for static VAr generation",IEEE-Transactions-on-Industry-Applications. vol.32,no.5; Sept.-Oct. 1996; p.1130-8.[4] Victor Anunciada, "A new current mode controlprocess and applications", IEEE Transactions-on-Power-Electronics, Oct 1991, Vol. 6, No. 4.[5] Marian P.Kazmierkowski, Luigi Malesani,"Current control techniques for three-phase voltage-source PWM converters: A Survey", IEEETransactions-on-Power-Electronics, Oct 1998, Vol. 45,No. 4.

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