New Control Strategy of Three-Phase Five-Level NPC ...core.ac.uk/download/pdf/82678266.pdf2. Analysis of Five-level NPC Rectifier/Inverter System 2.1. Topology Fig.1 shows the main

Energy Procedia 18 ( 2012 ) 1382 – 1391

1876-6102 © 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of The TerraGreen Society.

doi: 10.1016/j.egypro.2012.05.154

New Control Strategy of Three-Phase Five-Level NPC

Rectifier - Inverter System for Induction Machine Drive

Rabia GUEDOUANIa,*,Bachir FIALAa,E.M BERKOUKb,M.S. BOUCHERITb

a Laboratoire des Systèmes Electriques et Industriel, University of Sciences an Technology ‘Houari Boumediene’B.O. Box 32 El Alia Bab-Ezzouar, Algiers, Algeria.

b Laboratoire de commande des Processus Ecole Nationale Polytechnique BP182, 10 avenue Hassen badi. El harrach, Algiers, Algeria.

Abstract

This paper proposes a control strategy of a three –phase five-level double converter for induction motor drives. The converter consists of the five-level NPC rectifier, DC link, and the five-level NPC inverter. In this control strategy, the DC link voltages are controlled by using a closed loop with an optimized stabilization system called clamping bridge. It provides a fast and flexible control of the converter capacitor voltage. This method will redress the imbalance of DC link voltage. This control strategy is completely independent from the load control, leading to a simpler implementation. The three-phase five-level NPC rectifier-inverter system is an ideal interface between a utility and renewable energy sources such as photovoltaic or wind generator. Keywords: Five-level NPC converter, PWM strategy, DC link voltage, regulation voltage, unity power factor, stabilization system, induction machine, renewable energy.

1. Introduction

In recent years, multilevel power converters for high power applications have been actively investigated [1-3]. In particular, three-level drive systems have been put to practical uses [1-2-3]. __________ * Corresponding author. Tel.: +213 559 29 47 52. E-mail address: [email protected] (R.GUEDOUANI)

Available online at www.sciencedirect.com

Open access under CC BY-NC-ND license.

http://creativecommons.org/licenses/by-nc-nd/3.0/

Rabia Guedouani et al. / Energy Procedia 18 ( 2012 ) 1382 – 1391 1383

The general function of the multilevel inverter is to synthesize a desired AC voltage from several levels of DC voltages. For this reason, multilevel inverters are ideal for connecting either in series or in parallel an AC grid with renewable energy sources such as photovoltaic or fuel cells or wind generator. Additional applications of multilevel converters include such uses as medium voltage adjustable speed motor drives, static var compensation, dynamic voltage restoration [3]…..

Several topologies under serious consideration from industry are developed [3], however the Neutral Point Clamped (NPC) structure, proposed by Nabea and al [2], remains the popular circuit due to its similarity to a conventional two-level voltage source inverter, and its ease of control [1-2-3].

The five-level inverter appears to offer interesting options for higher power drives without need for simultaneous device switching [2-3-4]. The output voltage waveform of the five-level NPC inverter is composed of intermediary voltage levels, which are typically obtained from capacitor voltage sources. However, the major problems with this configuration are in achieving four balanced voltage within the DC voltage source.

Different solutions have been proposed to overcome this problem [1-2-4-5-8]. A first approach [3] is to provide a single DC link voltage (from a three phase rectifier), and subdivide it into four equal voltage levels by using a split bank of capacitors [7]. The problem with the split capacitor arrangement is that during normal operation a net mean current is drawn from nodes 2 and 4 of the DC input voltage source by the load, and this will cause the link capacitors to charge or discharge, causing an imbalance in the DC input voltage source levels. During transient state, or if the PWM scheme and device switching characteristics are slightly unbalanced between the output phases, a net mean current may be drawn from the neutral point, node 3, again leading to a capacitor voltage imbalance[3-5-6].

The easiest approach is to simply supply the DC link voltage with three-phase five-level NPC rectifier. We have proposed in [8] three-phase PWM five-level NPC rectifier as input stage of the three-phase five-level NPC voltage source inverter. We have shown in [8] that this first solution is not enough to maintain the equal voltage division mainly when a large load torque is applied.

In this paper, we propose a feed-back control, in dq rotating frame, of DC link voltage for five-level NPC rectifier-inverter system. The proposed control strategy uses closed loop with an optimized stabilization system called clamping bridge. It permits to have the capacitor voltage balancing with network unity power factor. In the first section, we elaborate the knowledge and control models of three-phase five-level NPC rectifier-inverter system. The second section presents the triangular-sinusoidal control strategy using four carriers. In the third section, we propose a control strategy of a three –phase five-level double converter for induction motor drives by using a closed loop with an optimized stabilization system called clamping bridge. The last section shows some important results of the proposed control strategy. As an application, we study the speed performances drive of the three-phase high power induction machine fed by this system.

2. Analysis of Five-level NPC Rectifier/Inverter System

2.1. Topology

Fig.1 shows the main circuit configuration of the three-phase five-level double converter. It consists of the five-level rectifier, the DC link, and the five-level inverter feeding the induction motor. The rectifier and inverter employ the neutral point clamping (NPC) structure. Each converter composed of three arms [6]. Each leg of this structure has six pairs of switching sis devices in series and two in parallel. The diodes let to have zero voltage for ViM (ViM is the voltage of the phase relatively to the middle point M).

1384 Rabia Guedouani et al. / Energy Procedia 18 ( 2012 ) 1382 – 1391

Rectifier

sa3

sa2

sa1

sa4

sa5

sa6

Phase a Phase b Phase A Phase B Phase C

Inverter

VS1

VS2

VS3

Uc1

Uc2

Uc3

Uc4

IM

iA

iB iC

ia

ib

ic

M

sa7

sa8

a b c A B C

n

Phase c

i’d2

i’d1

i’d0

i’d3

i’d4

id2

id1

id0

id3

id4 DC Link

N

p

4

1

2

3

The DC link consists of four capacitors in series. For a DC link voltage of Vdc, if the voltage across each capacitor is Vdc/4, the voltage stress across each switching device will be limited to Vdc/4. Thus this topology would be suited to high voltage and high power applications.

Fig 1. Schematic of a three-phase five-level rectifier/inverter system

The topology analysis of such structure shows that seven different configurations En (n is the number of the configuration) possible [6] and are illustrated in Table 1.

Table .1 Different configurations of Five-Level NPC converter

Configurations Electrical quantity

E0 = {ø} Ii = 0 A

E1={si1 si2 si3} ViM = UC1+UC2

E2={si1 si2 si7} ViM = UC1

E3={si1 Di1} ViM = 0

E4={si4 si5 si8} ViM = -UC3

E5={si4 si5 si6} ViM = -UC3- UC4

E6={si4 Di0} ViM = 0

Five complementary laws are possible for five-level NPC converter. The optimal one is [6-7-8]:


24 ii SS

15 ii SS (1)

36 iSiS

Sis : control signal of the semi-conductor sis; i = (a,b,c) for the rectifier and =( A,B & C) number the phase of the inverter; s: number of switching. Uck : capacitor voltage of the DC link (k=1, 2, 3 & 4).

2.1. Knowledge model

In controllable mode, we define for each semi-conductor sis of the converter the connection function Fis as follows [6-8]:

offturnedissif0onturnedissif1

iS

iSSiF (2)

By using the proposed complementary law, the output voltage of the rectifier/inverter VkM relatively to the middle point M, is given by the following system [8-11]:

436543654213211321 CCiiiCiiiCCiiiCiiiiM UUFFFUFFFUUFFFUFFFV (3)

The system (3) shows that the five-level NPC converter is equivalent to four two-level or two three-level NPC converter in series.

The DC input currents (id1, id2, id3, id4, id0) of the three-phase five-level NPC inverter expressions, using the load currents (iA, iB, iC), are given as:

CBAd iFiFiFi 3727171

Cb

Bb

Ab

d iFiFiFi 3121112

CBAd iFiFiFi 3828183 (4)

Cb

Bb

Ab

d iFiFiFi 3020104

43210 ddddCBAd iiiiiiii

The rectified currents (i’d1, i'

d2, i'd3, i'

d4, i'd0) of the three-phase five-level NPC rectifier expressions,

using the input network currents (ia, ib, ic), are given as follow:

cbad iFiFiFi 37'

27'

17'

1'

cb

bb

ab

d iFiFiFi '31

'21

'112

'

cbad iFiFiFi 38'

28'

18'

3' (5)

cb

bb

ab

d iFiFiFi '30

'20

'104

'

4'

3'

2'

1'

0'

ddddcbad iiiiiiii

Where F’is is switching control of the switches rectifier.


The systems (3), (4) and (5) are used to establish the model of the five-level NCP rectifier/inverter system in matlab-simulink environment.

3. Multilevel Control Strategy

Different triangulo-sinusoidal strategies are developed for multilevel converter [1-7-8]. These strategies are extended from two-level carrier-based PWM techniques to multilevel inverters by making the use of several triangular carriers and one reference signal by phase. For N-level inverter, (N-1) carriers with the same frequency fc and same peak to peak amplitude Ac are shifted by 1NTc with Tc is the period of the carrier. The reference wave form has peak to peak amplitude Am and frequency fm, it is centred in the middle of the carriers signal. This reference is continuously compared with each carrier. In this paper, we develop triangulo-sinusoidal strategy using four carriers. This strategy uses the property that the three-phase five-level NPC rectifier/inverter is equivalent to four two –level converters in series[1-7-8]. This strategy is characterised by the amplitude modulation index ma and the frequency ratio mf, which are defined as:

cma A

Am ,m

cf ffm (6)

The algorithm of this strategy can be summarized as follow: Step 1: Compute of the intermediate voltages

CUiMprefi VthenUVif 11

011 iMprefi VthenUVif


CUiMprefi VthenUVif 22 (7)

033 iMprefi VthenUVif




Step 2: Compute of the out put voltage

4321 iMiMiMiMrefi VVVVV

4. Control Process

4.1. DC voltage control

The control flow of five-level rectifier/inverter system is as follows. Fig. 2 shows the control circuit structure of DC voltage of five-level converter. First, the DC link voltage Vdc, which is the potential difference between the P and N level, is detected. The error between Vdc and its command value V*

dc passes the PI controller, after which this value is multiplied to the gain K (K=Vdc/Vds). This later is determined by using the instantaneous power conservation principle, with neglecting the rectifier losses, in dq frame. This value then become the direct input line current command I*

d.


4.2 Input current control

The errors between the line current references (i*d, i*q), in rotating dq frame, and detected line currents

(id, iq) are inputted into PI controllers. Then, we obtain the commands (V*a, V*

b, V*c) which are used as

references signals of PWM control strategy. Thus, The PWM control strategy determines switching levels to control switches in the main circuit of the five-level rectifier. In this paper, we use the triangulo-sinusoidal strategies with four bipolar carriers. The network power factor is controlled by imposing the quadrature line current i*

q=0A.

Fig. 2. Control circuit of five-level NPC rectifier

4.3 DC link voltage stabilisation

In order to remedy the problem of fluctuation due to the inverter input DC voltages drift, an optimized DC link voltage stabilization system, shown in Fig. 3, is proposed. We suggest a solution which consists in establish a balancing bridge between the rectifier and the intermediate filter. Capacitor voltage equalization control should be implemented to restrict the charge-discharge current to the allowable cell limitations in the capacitor string. Fig. 3 shows another variant of the balancing circuits called mixed optimized clamping bridge. This circuit is capable of transferring the electric load from the capacitor which has high voltage towards neighbour capacitor that has low voltage. This circuit is placed in parallel with every condenser of the DC link. It consists of a bidirectional semiconductor Tk and an inductance Lck with a resistor in series Rck, which serves to stabilize the DC voltages. This circuit was suggested in a concern reducing the losses by effect Joule even to cancel them. If the voltage Uck gets higher then an imposed reference U*

ck, the switch Tk is opened to slow down the charging of capacitor Ck. The model of the intermediate filter with the stabilization bridge is defined as follow:

dtSiiidtiUC Lddcc )'( 1122111

dtSiiiiidtiUC Lddddcc )'( 22122'1222 (8)

dtSiiiiidtiUC Lddddcc )'( 33344'

4333

dtSiiidtiUC Lddcc )'( 4444444

Vs1

Vs2

Vs3

R L

R L

R L

isa

isb

isc

n

i’d id

ic

Vdc

N

P Vdc

i*c id

i’*d i*

d id

V*d V*

q i*

q=0

iq

isa isb isc

V’a V’

b V’c

Three

phase five level NPC

rectifier

Three

phase five level NPC

inverter

I M

K

V*dc


Fig. 3. Optimized DC link voltage stabilization system

With:

dtiRUSLi LkckckkkLk )(.1 (9)

The switches are controlled as follow:

kcck xUU * with k=1, 2, 3 & 4 (10)

00010kkkk LxLxk iSelseiSthenxif (11)

5. Simulation Results

In the following simulations, the machine works without load, then at the moment t =17s a load torque, equal to 80 % of its nominal torque ( n=130 N.m), is applied. For these essays, we use the triangulo-sinusoidal with four carriers as command strategy for both multi level converters. We show perfectly the instability of the DC input voltages (Fig.4.a) without using voltage stabilization system. This instability increases when a load torque of the induction machine is applied. However, these voltages are practically equal by pair (Uc1 = Uc4, Uc2 = Uc3)(Fig.4.a), then their differences are not very large (Fig.4.a). This property permits to eliminate the homopolar component of the output voltage of the five-level NPC VSI and let it symmetrical (Fig. 4.b). The cascade two three-level PWM five-level NPC rectifier/inverter loads an induction motor. Using vector control (with flux constant), the speed control performances are shown in Fig. 5. The speed and the electromagnetic torque follow their references. However the electromagnetic torque has very important undulations (Fig.5.b). The results show the importance of the stability of DC input voltages of the rectifier-inverter system to have good performances for the induction machine speed control.

Fig. 4. a) DC Link voltage without stabilization system, b) Output voltage of five-level VSI without use voltage stabilization system

i’d2

i’d1

i’d0

i’d3

i’d4

U’c1

U’c2

U’c3

U’c4

The rectifier

side

The inverter

side

DC

Vol

tage

Ucx

(V)

Time t(s) Time t(s)

Out

put v

olta

ge o

f fiv

e-le

vel V

A(V

)


Fig. 5. a) Representation of the induction machine speed, b) Representation of the induction machine Torque

In order to demonstrate the feasibility of the proposed control method, the closed loop with the voltage stabilization system has been tested by simulations. The Fig. 6, 7, and 8 show the performances of the feedback control of the three-phase five-level NPC rectifier/inverter with voltage stabilization system. In the following simulations, the machine works without load, then at the moment t =15s a load torque, equal to 80 % of its nominal torque ( n=130 N.m), is applied. We note that, the controlled voltage Vdc (Fig. 6.a.) follow perfectly its reference (V*

dc=20000 V). Therefore, the different input DC voltages of the five-level NPC inverter become constant as it is shown in Fig.6.b, and practically equal. Their differences are very small (Fig.7). In consequence, the output voltage of the five-level NPC VSI is symmetrical and stable as depicted in Fig. 8.

The Fig. 9 show direct id and quadrature iq line input current and their references (i*d , i*

q ). We note that the direct current id follow its reference around 2000 A (Fig 9.a). Although, the quadrature line current iq oscillates around zero (Fig 9.b). Thus, the phase voltage and its corresponding line network current are shown in Fig.10.a. The two waves are practically in phase.

Finally, the speed control of the induction machine, fed by this system, is represented on the Fig.10-b. We remark that the undulations on the performances (torque , currents id & iq) of the machine disappear and those performances are improved by using the mixed optimized clamping bridge.

Fig. 6. a) Controlled DC voltage with closed loop, b) DC Link voltages with the stabilization bridge

6. Conclusion

In this paper, a control theory for DC link voltage balancing of three-phase five-level rectifier-inverter system has been presented. Simulations results were shown to verify the analysis and to demonstrate the following advantages of the proposed control:

(rp/mn) *(rp/mn)

(N.m) *(N.m)

Indu

ctio

n M

achi

ne S

peed

(r

p/m

n)

Indu

ctio

n M

achi

ne T

orqu

e (N

.m)

DC

Vol

tage

Vdc

& V

* dc(V

)

Time t(s)

DC

Vol

tage

UcK

(V)

Time t(s)


- While it can generate a five level staircase waveform without the use of transformers, the five-level converter generates almost sinusoidal voltage and current waveforms even at fundamental switching frequency. - The voltages on the DC link capacitor are well balanced with very small ripple. - The system has low harmonic in the line network current and also this current is in phase with the corresponding phase voltage. - In this system, the switching stress is low. - The use of the voltage stabilization system permits a economic and simple electronic implementation, whereas in the space vector modulation control the computational burden, the complexity of the algorithms and the number of instructions are drastic. - They are an ideal interface between a utility and renewable energy sources such as photovoltaic or wind energies.

Fig. 7. a) Error Uc1-Uc2 & Uc3-Uc4, b) Error Uc1-Uc4 & Uc2-Uc3

Fig. 8. a) Output voltage of five-level VSI voltage stabilization system, b) Line current of the induction machine

Fig. 9. a) Direct line input current id and its reference i*d , b) Quadrature line input current iq and its reference i*

q

The

DC

vol

tage

err

or

Time t(s)

The

DC

vol

tage

err

or (V

)

Time t(s)

Out

put v

olta

ge o

f fiv

e-le

vel N

PC

Time t(s)

The

quad

ratu

re in

put l

ine

iq&

i*q(

A)

Time t(s)Time t(s)

The

dire

ct in

put l

ine

id &

i*

d(A

)

Time t(s)

Line

cur

rent

of t

he in

duct

ion

mac

hine


gy ( )

,

Fig. 10. a) Line input current isa and the corresponding phase voltage Vsa, b) Representation of the induction machine torque (for r=100KN.m at 15s)

References

[1] N.Kimura, T. Morizane, K.Taniguchi, T.Oono, “Multi-modulation signal PWM control for multi-level converter”, The 11th International Power Electronics and Motion control conference, EPE-PEMC 2004, Riga, Latvia, 2-4 September 2004. [2] Z. Pan, F. Zheng Peng and all, “Voltage Balancing Control of Diode-Clamped Multilevel Rectifier/Inverter Systems”, IEEE transactions On Industry Applications, Vol -41. N° 6, November/December 2005, pp. 1668-1706. [3]L. M Tolbert, F.. Zheng Peng, “Multilevel converters as utility Interface for Renewable Energy Systems”, Power Engineering Society Summer Meeting, augaust-2000. IEEE, Vol°. 2, pp.1271 – 1274. [4] T. Ishida, K. Matususe, “Fundamental Characteristic of Five-Level Double Converters With Adjustable DC voltages for Induction Motor Drives”, IEEE Transactions On Industrial Electronics, Vol.49, N°4, August 2002, pp:775-7782. [5] O. Bouhali, B. Francois, E.M. Berkouk, C. Saudemont , “DC Link Capacitor Voltage Balancing in a Three-Phase Diode Clamped Inverter Controlled by a Direct Space Vector of Line-to-Line Voltages”, IEEE Transactions on Power Electronics, Vol. 22, Issue 5, Sept. 2007. [6] R.Guedouani, B. Fiala, E. Berkouk, M. Boucherit , “A New Control Algorithm for Four Three-Phase AC/DC PWM Voltage Source Rectifiers-Five-Level Neutral PoinT Clamped Inverter System.”, Journal the Mediterranean Journal of Measurement and Control , Vol.4, N°4,ISSN 1743-9310,Octobre 2008, pp.161-173. [7] R.Guedouani, B.Fiala, E.M. Berkouk, M.S. Boucherit “Modeling and Control of MultileveL Three - Phase Current Source nverter” International Aegean Conference on Electrical Machines and Power Electronics, ACEMP’11 & Electromotion’11 joint

conference, Istanbul Turkey, 08-10 September 2011, pp. 465-470.. [8]R.Guedouani, B.Fiala, E.M. Berkouk, M.S. Boucherit, " Feedback Control of Two PWM VS Rectifiers – Five-level NPC Voltage Source inverter. Application of Induction Machine drive." special issue N° 2 November 2010, Journal of Electrical Systems ISSN 1112-5209, pp.71-81

Appendix A. The input and output filters parameters are given in the following table.

Input filters parameters Output filters parameters Clamping bridge parameters

L = 1 mH C1= C2= C3= C4=80 mF

Lc=80 mH

R = 0.25 Rc=1

Time t(s)

Torq

ue o

f the

indu

ctio

n m

achi

ne (N.m)

*(N.m)

Time t(s)

Inpu

t lin

e cu

rren

t(A) &

the

phas

e vo

ltage

(V)

Vs1(V) isa(A)

Documents

New Control Strategy of Three-Phase Five-Level NPC ...core.ac.uk/download/pdf/82678266.pdf2. Analysis of Five-level NPC Rectifier/Inverter System 2.1. Topology Fig.1 shows the main