performance analysis of series z -source four switch three ... · four switch three phase inverter...

Performance Analysis of Series Z-Source

Four Switch Three Phase Inverter Fed

Induction Motor Drive 1R. Arivuselvi,

2T.K.S. Sathyanarayanan,

3A.V. Sindhuja and

4Srinivasan

1,2,3,4 Dept. of Electrical & Electronics Engineering,

Tagore Engineering College,

Rathinamangalam, Chennai, India.

Abstract This paper investigates the performance of a reduced switch three phase

series Z-source inverter fed cost effective induction motor (IM) drive

system. The objective of the paper is to minimize the THD and reduces the

overall cost of the system. This paper explores the series Z-source inverter

(SZSI) topology for induction motor (IM) with solar photovoltaic (PV) as

source in drive application. Series Z-source inverter has single stage power

conversion with buck-boost capability. Shoot through ratio is used for

boosting dc link voltage in series Z- source inverter. In this paper, for

enhancing the performance of SZSI improved shoot through envelops

technique along with sinusoidal pulse width modulation control technique

.The major benefits of modified boost control technique are current

reduction and harmonics reduction in output voltage as compared to

simple boost control scheme. The validity and feasibility of modified boost

control technique for shoot through envelop to control SZSI fed induction

motor is verified by simulation results.

Index Terms: Series z source inverter, four switch three phase inverter,

voltage sags.

International Journal of Pure and Applied MathematicsVolume 119 No. 12 2018, 2945-2960ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

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1. Introduction

Series Z-Source Inverter (ZSI), shown in Fig. 1, has been proposed to overcome

the limitations of traditional voltage source inverter (VSI) and the traditional

current source inverter (CSI). In a traditional voltage source inverter, the two

switches of the same phase leg can never be gated on at the same time, due to

the reason of short circuit. SPV fed converter system requires buck-boost

capability to satisfy the load requirement. It requires two stage power

conversion. This two-stage power conversion increases volume, cost and

degrades the reliability of system. In series ZSI, LC-network and inverter bridge

are in series. This series combination is connected across dc source, which

shows the reduced voltage across both capacitors. which is one of the suitable

topology in photovoltaic system. The Z-source inverter beneficially infuses the

shoot through states to step up the dc bus voltage by gating on both the switches

of a same leg. Therefore, the series Z-source inverter can step up step down the

voltage to a desired output voltage that is greater than the given input dc bus

voltage. The reliability of the inverter is improved to protect the circuit. Thus it

offers high reliable, low-cost and single-stage structure for buck boost power

conversion. To reduce the cost of the inverter and to boost up the voltage, this

switched inductor z-source four switch inverter which gives the chopper

operation includes and provides the good solution. These switched inductors are

mainly used to high boosting the voltage operation and protect the switches

during the shoot through period. Since, modified boost control technique is

presented which gives high dc link voltage with lower THD.

Fig 1. Series Z source inverter

2. Drive System Modelling

The block diagram of the proposed system is shown in Fig 2. The drive system

consist of a solar panel, series Z source network, 3 phase four switch inverter , 3

phase induction motor and control circuits. DC is supply obtained from the solar

panel and it is given to the inverter where the pulse is generated by the

sinusoidal PWM generator. According to the switching pattern inverter switches

turned on. The output of FSTPI is fed to 3 phase induction motor. The whole

drive system modeling involves the modelling of the inverter, Induction motor

International Journal of Pure and Applied Mathematics Special Issue

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and Z-source inverter, which are discussed in the following subsections,

Fig 2.Block diagram of series z source inverter

The above fig.2 shows the block diagram of 4 switch 3 phase inverter induction

motor drive. It consists of front end rectifier, Impedance network, Sinusoidal

PWM generator, 4 switch 3 phase inverter. In the conventional z-source inverter

contains two inductors and two capacitors and one split link capacitor leg for

the 3rd

phase with four switches. In the proposed scheme, the split link capacitor

in the 3rd

arm of the existing model is replaced by the z-source capacitor link.

A. Modelling of Photovoltaic Arrays

Photovoltaic’s is the direct conversion of light into electricity. Some materials

exhibit a property known as the photoelectric effect that causes them to absorb

photons of light and release electrons. Each individual solar energy cell

produces only 1-2 watts. To increase power output, cells are combined in a

weather-tight package called a solar module. The basic equation from the theory

of semiconductors that mathematically describes the I-V characteristics of the

ideal photovoltaic cell.

I=Iph –Ir(exp[ ]-1) – (1)

Fig 3. PV Equivalent Circuit

Where, ph is the current generated by the incident light (it is directly

proportional to the Sun irradiation), is the reverse saturation or leakage

current of the diode is the temperature of the p-n junction and “a” is the diode

ideality constant, “q” is the electron charge [1.60217646 * 10−19C], “k” is the

Boltzmann constant [1.3806503 *10−23J/K]. The equivalent circuit of a PV cell


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is as shown in Fig.3. A single cell has a rated voltage of 0.51V and rated power

of 0.3 W .In practical arrays, which are composed of several connected

photovoltaic cells. If the array is composed of Np parallel connections of cells

the photovoltaic and saturation currents may be expressed as

Ipv=Ipv,n+KI∆T( ) (2)

Where, pv,n is the light-generated current at the nominal condition (usually

25°C and 1000W/m2), T= T – Tn (being T and Tn the actual and nominal

temperatures [K]), G[W/m2] is the irradiation on the device surface and Gn is

the nominal irradiation.

B. Four Switch Three Phase Inverter Model

In the analysis, the inverter switches are considered as ideal power switch

q1,q2,q3,q4. The topology employs four switches and four diodes to generate

two line–line voltages, V13 and V23, whereas V12 is generated according to

Kirchhoff’s voltage law from a split-capacitor bank in the dc-link. Due to the

circuit configuration, the maximum obtainable peak value of the line–line

voltage equals Vdc/2.

Fig.4. Inverter switching state (0,0)

It is assumed that the conduction states of the power switches are associated

with binary variables q1 to q4. Therefore, a binary “1” will indicate a closed

switch, while “0” will indicate the open state. Pairs q1-q3 and q2-q4 are

complementary and as a consequence q3=1-q1 and q4=1-q2. It will assumed

that a stiff voltage is available across the two dc-link capacitor , and Vc1=Vc2=

Vdc/2, Vdc corresponds to a stiff dc-link voltage . Pole voltage V1,V2,V3

depend on the states of the power switches and its expressed in terms of the

binary variables q1 and q2 and the dc- link voltage as follows

V1= (2q1 -1) Vdc/2 (3)

V2= (2q2 -1) Vdc/2 (4)


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V3 =0 (5)

In the four switch configuration, there are four switching status such as (0, 0),

(0, 1), (1, 0), and (1, 1), in which. In the case of the six switch converter,

switching status (0, 0) and (1, 1) are regarded as zero-vectors, which the motor

load is replaced by a resistive load cannot supply the dc-link voltage to the load,

so that current cannot flow through the load. However, in the four switch

converter, one phase of the motor is always connected to the midpoint of the dc-

link capacitors, so that current is flowing even at the zero-vectors, as shown in

Fig.4. Moreover, in the case of (0, 1) and (1, 0), the phase which is connected to

the midpoint of dc-link capacitors is uncontrolled and only the resultant current

of the other two phases flow through this phase. If the load is ideally symmetric

and capacitors voltage are equal, there is no current in uncontrolled phase in the

(0, 1) and (1, 0) vectors. Large variations of the voltage across the two dc-link

capacitors caused by one phase current circulating through the capacitive bank,

will cause significant ripples, distortions, and unbalances in inverter output

currents. Two of the inverter switching states (1, 1),(0, 0) cause unequal loading

of the split dc-link capacitors. This causes one half of the link to discharge at a

faster rate than the other, resulting in the generation of a voltage imbalance. The

possible value of pole voltages V1 and V2, depend on the state of the switches

q1 and q2. The induction motor load is replaced by the resistive load for the

understanding the modes operation. There are four switching states with the

following pole voltages

V1 = V2 = , when q1 = q2 = 0 ;

V1 = , V2 = when q1=1, q2 = 0 ;

V1 = , V2 = when q1 =0, q2 = 1

V1 = V2 = , when q1 = q2 = 1

The phase to neutral voltage can be defined as

V01= V1- Vn0 (6)

V02= V2 - Vn0 (7)

V03= V3- Vn0 and V3= 0 (8)

where “n” is the centre point of split capacitors, “o” is the neutral point of IM

windings and Vno is the voltage between the centre point of split capacitors and

the neutral point of IM windings.

Normally the induction motor load phase voltage are balanced


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Van+Vbn+Vcn = 0

Vn0= 2q1+2q2-2) (9)

The phase to neutral voltage can be derived as follows

Substituting Equations (7) and (1) in (4)

V01= ( (4q1-2q2-1) (10)

V02= ( (4q2-2q1-1) (11)

V03= ( (-2q1-2q2+2) (12)

Table 1 : Switching function and output voltages from inverter

Switching Function

Switch ON

Output voltage

q1 q2 T1 =q1 T3=q2 V01 V02 V03

0 0 T2 T4

0 1 T2 T3

0

1 0 T1 T4

0

1 1 T1 T3

For better realization, the three phase quantities are transformed to αβ

quantities.

= 123 (13)

K= (14)

Where =[ , ]T

, 123 =[ 01 02 03]

The induction motor is assumed to be symmetric with its neutral wire

disconnected. The voltage components are given by

= (q1Vdc-( Vdc- ( )) (15)


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= ( ( q2Vdc-( ) Vdc) (16)

Table 2 : The combinations of switch states

Q1 Q2 V= +j

0

0

V1= ( )

1

0

V1= ( )

1

1

V1= ( )

0

1

V1= ( )

C. Induction Motor Model

The mathematical model of a three phase y connected induction motor and the

load is given by the following equations in the d-q synchronously rotating

reference frames as [6] .A change of variable that formulates a transformation

of the 3 phase variables of stationary circuit elements to the arbitrary reference

frame is expressed as

fqd0s

=Ks

fabcs

Where (fqd0s)T =[fqs fds f0s], (fabcs)

T =[fas fbs fcs]

The voltage equations in machine variable is expressed as

Vabcs=rsiabcs+p abcs (17)

Vabcr=rriabcr+p abcr (18)


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Te = (3P/4) Lm[iqs idr-ids iqr] (19)

Te = Jm(dωr/dt)+Bmωr+TL (20)

dθr/dt = ωr (21)

Where:

Vqs, Vds = q, d-axis stator voltages

iqs, ids = q, d axis stator current

Iqs, Ids = q, d axis rotor current

Rs, Rr = The stator and rotor resistances per phase

Ls, Lr = The self inductances of the stator and rotor respectively

Lm = The mutual inductance

Ωr = The rotor speed

p = The number of poles

p = The differential operator

Te = The electromagnetic developed torque

TL = The load torque

Jm = The rotor inertia

Bm = The rotor damping co-efficient

θ = The rotor position

D. Series Z Source Model

Fig.1 shows the main circuit of the proposed solar photovoltaic based series ZSI

fed induction motor drive system consist of inductors (L1, L2), capacitors

(C1,C2), diode D, decoupling capacitor Cp, feedback diode Ds and solar PV

source. SZSI utilises shoot through state for boosting dc voltage. Shoot through

is the state where upper and lower devices of the same phase leg of inverter are

turn on intentionally. In simple boost control, shoot through is achieved by

comparing dc reference line with triangular wave. By changing the level of dc

reference or line, shoot through duty ratio is changed which results in boosting

of dc link voltage. Shoot through can be achieved in seven different ways by

turning on two switches of the phase a or b or c or ab or bc or ca or abc.. The


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change in magnitude of Vt provides boosting of dc link voltage. In tradition

there are two control signals of Vt . i .e. Vt1 and Vt2 with positive magnitude and

negative magnitude. For reducing the size, cost of the PV panel and LC network

in series Z-source network, modified boost control with improved shoot envelop

technique is used. SZSI is operated in two modes: shoot through mode and

active mode (non shoot through mode)

Mode 1(shoot through mode):

In shoot through mode inductors charge and capacitors discharge, diode goes in

to reverse bias due to negative voltage appear across it. Fig.5(a) shows

equivalent circuit of SZSI in shoot through mode. The variables and their

direction of current are shown in the fig. 5(a)

Fig.5(a) Equivalent circuit of SZSI in shoots through mode

Mode 2 (non shoot through mode):

In non shoot through state inductors discharge and capacitors charge, diode

operates in forward bias mode. Fig. 5 (b) shows equivalent circuit of SZSI in

non shoot through mode. The variables and their direction of current are shown

in the fig. 5(b).

Fig.5(b) Equivalent circuit of SZSI in non shoots through mode

E. Analysis of the Z-source network

The Fig.1.Show equivalent circuit of the impedance source inverter. Assume the

inductor (L1, L2,L3 andL4) and capacitors (C1 and C2) have the same

inductance and capacitance values respectively. From the above equivalent

circuit:

Vc1=Vc2=Vc (22)


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VL1=VL2=VL (23)

VL=VC,Vd=2VC

Vi=0

During switching cycle T;

VL=V0-VC (24)

Vd=V0

Vi=VC-VL=VC-(V0-VC) (25)

Vi=2VC-V0

Where, V0 is the dc source voltage &

T=T0+T1 (26)

The average output voltage of the inductors over one switching period (T)

should be zero in steady state

VL=(T0/T)vc+(T1/T)(V0-VC)=0

VL=(T0VC+T1V0-T1VC)/T=0

(T0-T1)VC+T1V0=0

(T1-T0)VC=T1V0

VC/V0=T1/(T1-T0) (27)

Similarly the average dc link voltage across the inverter bridge can be found as

follows. From equation (25)

Vi=2Vc(T1/T)-V0(T1/T)=0 (28)

2VC(T1/T)=V0(T1/T)

2VC=VO

From equation (27)

VC=V0T1/ (T1-T0)

The peak dc link voltage across the inverter bridge,

Vi=2VC-V0


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Vi = ( / 1 − 0)V0

Vi=B.V0 (29)

Where B= T/(T1 –T0)≥1

B – Boost factor

The output peak phase voltage from the inverter

Vac=M(Vi/2) (30)

Where, M – Modulation index

Vac=M(BV0/2) (31)

The output voltage can be stepped up and down by choosing an appropriate

buck boost factor BB:

BB= B.M (it varies from 0 to α )

The capacitor voltage can be expressed as:

Vc1=Vc2=Vc=V0=(1-(T0/T))/(1-(2T0/T)) (32)

The boost factor BB is determined by the modulation index m and the boost

factor B. The boost factor B can be controlled by duty cycle of the shoot

through the zero state over the non-shoot through the states of the PWM

inverter. The shoot through zero state does not affect PWM control of the

inverter because it equivalently produces the same zero voltage to the load

terminal. The available shoot through period is limited by the zero state periods

that is determined by the modulation index.

3. Need of Inductors and Capacitors in Z Source Network

For the traditional voltage source inverter, the dc capacitor is the sole energy

storage and filtering element to suppress voltage ripple and serve temporary and

in the current source inverter the current ripples are suppressed. The series Z-

source network is a combination of four inductors and two capacitors. In this

combined circuit, the Z-source network is the energy storage/filtering element

for the series Z source inverter. This series Z-source network provides a second-

order filter and is more effective to suppress voltage and current ripples than

capacitor or inductor used alone in the traditional inverters. Therefore, the

smaller inductors and capacitors are required compare than the traditional

source inverters. Considering additional filtering and energy storage provided

by the inductors, the series Z source network should require less capacitance


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and smaller size compared with the traditional voltage source inverter.

Similarly, when the two capacitors (C1 and C2) are small and near to zero.

Considering additional filtering and energy storage by the capacitors, the Z-

source network should require less inductance and smaller size compared with

the conventional current source inverter.

4. Result and Discussions

The proposed system is verified and performance of the inverter configuration

and its control strategy is developed by Matlab software. IM current waveform

and voltage waveform of the four switch three phase series Z-source inverter are

identical conditions with traditional six switch three phase inverter. Peak value

of line – line voltage is 220 V is shown in fig.6 In simple boost control

technique for achieving peak line voltage as in proposed modified control

technique, shoot through ratio must be increased which results in reducing the

modulation index.

Fig. 6 Inverter output voltage with time (sec)

The starting phase current in the acceptable range. The steady state three phase

current waveform shown in fig.7 indicate almost balanced conditions

Fig. 7 Inverter output current with time (sec)

Performance curves response of induction motor for full load condition (Tm=

10.5 N-m). From the result , it can be observed that the speed reaches the steady

state value that is 1500 rpm with in 0.6 sec when the motor running at full rated

load condition . so when the motor is fed by z-source inverter then its speed

increases and settling time decrease .The Total Harmonic Distortion (THD) of

is found as 3.41 where as the THD of 6 switch three phase PWM inverter is

found as 6.61% shown in Fig. 12. It is found that the performance of the four


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switch three phase inverter based drive is much close to that of the traditional 6

switches three phase inverter. The analysis and simulation results show that this

inverter can dramatically reduce the complexity of the control algorithms and

cost.The waveform for current, speed, electromagnetic torque and THD are

shown in figures 8-12.

Fig 8 Stator Current with Time (sec)

Fig .9 Rotor Current with Time (sec)

Fig.10 Electromagnetic torque with Time (sec)

Fig.11 Speed with Time (sec)


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Fig.12 Harmonic Spectrum of the SZSI

5. Conclusion

This paper had been demonstrated that the reduced switch series Z-source

inverter topology is a good alternative technology to traditional inverter for

more efficient, more reliable and less cost conversion systems. The performance

parameters of three-phase induction motor such as stator current, rotor current,

rotor speed and electromagnetic torque was investigated for different load

conditions. In summary, the reduced switch series Z-source inverter ASD

system has several unique advantages that are very desirable for many

applications: It can produce any desirable output ac voltage, even greater than

the line voltage Reduces in-rush and harmonic current. Unique drive features

include buck-boost inversion by single power-conversion stage, improved

reliability, strong EMI immunity and low EMI.

References

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[2] F.Z.Peng, “Z-source Inverter,” IEEE Transactionson Industry Applicati on, 39(2), pp.504-510, March/April 2003. Wuhan, China.

[3] Peng,F.Z.,Miaosen shen and Zhaoming Qian, 2004. “Maximum boost control of the Z-source inverter”. Proceeding of the IEEE PESC.

[4] Peng,F.Z.Joseph , Wang,et al: “Z-source inverter for motor drives”, IEEE Trans.power electron., vol.20, No.4 ,pp 857-863,2005.

[5] Poh Chiang Loh, D.Mahinda Vilathgamuwa, Yue Sen Lai,Geok Tin Chua and Yunwei Li “ Pulse-Width Modulation of Z-Source Inverters” IEEE Transaction on Power Electronics, Vol.20, No.6, November 2005.

[6] Nasir Uddin, M., T.S. Radwan and M.A. Rahman, 2004. “Performance analysis of four switch 3-phase inverter fed IM Drives”. Proceeding of the 3rd IEEE International Conference.


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[7] K.Srinivas and Dr S.S.Dash “ Performance Of a Reduced Switch Z Source Inverter Fed IM Drive “ International Journal Of Computer And Electrical Engineering , vol.2 , no .4, 4 aug 2010

[8] Vani Vijay, Shruthi .K J.P.Giridhar Kini“Modified Z-Source Inverter Based Three Phase IM Drive For Solar PV Application” International Conference On Power Signals Control And Computation (EPSCICON ) 8 , on 10 aug 2014

[9] Xu Peng Fang, Xu Guang Wang , and Zhi Qiao chen “A Single-phase AC power supply based on modified quasi –z-source inverter” IEEE Transcation on applied Superconductivity , Vol.24 , no.5 , oct 2014

[10] A.A.Apte , V.D.Malwade “Simulation of a 4-switch , 3-phase inverter fed induction motor drive system” International journal of innovations in engineering research and technology vol2 , issue 2 Feb-2015


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