z source inverter and its application

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Simulation of Z-Source Inverter Using MaximumBoost Control PWM Technique

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IJSSVol.7 No.2 July-December 2013, pp.49-59

Serials Publications, (India)

49

Simulation of Z-Source Inverter Using Maximum Boost

Control PWM Technique

Suresh L., G.R.S. Naga Kumar, M.V. Sudarsan & K.Rajesh

Department of Electrical & Electronics EngineeringVignans Lara Institute of Technology & Science, Vadlamudi - 52213, INDIA

Email: [email protected]

Abstract:-Z source inverters have been recently proposed as an alternative power conversion concept asthey have both voltage buck and boost capabilities. These inverters use a unique impedance network,

coupled between the power source and converter circuit, to provide both voltage buck and boost properties,

which cannot be achieved with conventional voltage source and current source inverters. To facilitate

understanding of Z source inverter, this paper presents a detailed analysis, showing design of impedancenetwork, implementation of Maximum Boost control (MBC) PWM technique and simulation of Z source

inverter.

Key words:- PWM Technique, MBC, Z source inverter

I. INTRODUCTION

There exists two traditional converters, voltage-source (or voltage-fed) and current-source (or

current-fed) converters, either rectifier or inverter depending on power flow directions. There are some

limitations in those two inverters.

A. Voltage Source Inverter (Vsi)

VSI is a 3- bridge inverter fed from DC voltage source (or) AC voltage source with diode

rectifier as shown in fig 1. A large capacitor is connected at the input terminals tends make the input DC

voltage constant. Six switches are used in the main circuit; each composed of power transistor and an

anti parallel diode to provide bidirectional current flow and unidirectional voltage blocking capability. It

has eight switching states. In those eight states, six are active states and two are zero states. VSI can beoperated as a stepped wave inverter or pulse width modulated (PWM) inverter.

Fig .1: Voltage Source Inverter

It has the following conceptual and theoretical barriers.

The AC output voltage is limited below and cannot exceed the DC input voltage. External equipment is needed to boost up the voltage, which increases the cost and lowers the

overall system efficiency.

There is a possibility for the occurrence of short through which destroys the devices. An output LC filter is needed, which causes additional losses and control complexity.


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B. Current Inverter (Csi) Source

CSI is 3- bridge inverter fed from current source i.e. a voltage source in series with large

inductor as shown in fig 2. Six switches are used; each composed of Insulate Gate Bipolar Transistor

(IGBT) or Metal Oxide Semiconductor Field Effect Transistor (MOSFET) with series diode to provide

unidirectional current flow and bidirectional voltage blocking. Unlike VSI, CSI has nine switching states

in those six are active states and three are zero states. The AC output voltage is greater than DC inputvoltage.

Fig 2: Current Source Inverter

However, the current-source inverter has the following conceptual and theoretical limitations: It is a boost inverter The cost of CSI is high. The operating power factor is poor on line side. CSI is vulnerable to EMI noise in terms of reliability.

2. Z SOURCE INVERTER

The main objective of static power converters is to produce an AC output waveform from a dc

power supply. Impedance source inverter is an inverter which employs a unique impedance network

coupled with the inverter main circuit to the power source. This inverter has unique features in terms ofvoltage (both buck & boost) compared with the traditional inverters. A two-port network that consists of

a split-inductor and capacitors that are connected in X shape is employed to provide an impedance

source (Z-source) coupling the inverter to the dc source, or another converter. The DC source/load can

be either a voltage or a current source/load. Therefore, the DC source can be a battery, diode rectifier,

thyristor converter, fuel cell, PV cell, an inductor, a capacitor, or a combination of those [1]. Switches

used in the converter can be a combination of switching devices and anti-parallel diode as shown in Fig.

3

Fig. 3: ZSI Using the Anti parallel Combination of Switch and Diode

Six switches are used in the circuit; each is traditionally composed of a power transistor and an

anti parallel (or freewheeling) diode to provide bidirectional current flow and unidirectional voltage


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blocking capability. The commonly used switches are Metal Oxide Semi-Conductor Field Effect

Transistor (MOSFET), Insulated Gate Bipolar Transistor (IGBT), Bipolar Junction Transistor (BJT),

Silicon Controlled Rectifier (SCR), and Gate Turn off Thyristor (GTO) etc. Here we employed IGBT as

the switch as it combines the advantages of both BJT and MOSFET.

A.Impedance NetworkThe Z-source concept can be applied to all DC-to-AC, AC-to-DC, and AC-to-AC and DC-to-DC

power conversion. It consists of voltage source from the DC supply, Impedance network, and three phase

inverter and with AC motor load. AC voltage is rectified to DC voltage by the three phase rectifier. In

the rectifier unit consist of six diodes, which are connected in bridge way. This rectified output DC

voltage fed to the Impedance source network which consists of two equal inductors (L1, L2) and two

equal capacitors (C1, C2).The network inductors are connected in series arms and capacitors are

connected in diagonal arms .The impedance network is used to buck or boost the input voltage depends

upon the boosting factor .This network also act as a second order filter .This network should require less

inductance and smaller in size. Similarly capacitors required less capacitance and smaller in size. This

impedance network, constant impedance output voltage is fed to the three phase inverter main circuit.

Depending upon the Gating signal, the inverter operates and this output is fed to the 3-phase AC load or

AC motor.

B.Equivalent Circuit and Operating PrincipleThe Z-source inverter is analyzed using voltage source inverter. The unique feature of the Z-

source inverter is that the output ac voltage can be any value between zero and infinity regardless of the

input DC voltage. That is, the Z-source inverter is a buckboost inverter that has a wide range of

obtainable voltage. The traditional V- and I-source inverters cannot provide such feature.

The main feature of the Z-source is implemented by providing gate pulses including the shoot-

through pulses. Here how to insert this shoot-through state becomes the key point of the control methods.

It is obvious that during the shoot-through state, the output terminals of the inverter are shorted and the

output voltage to the load is zero. The output voltage of the shoot-through state is zero, which is the

same as the traditional zero states, therefore the duty ratio of the active states has to be maintained to

output a sinusoidal voltage, which means shoot-through only replaces some or all of the traditional zero

states.

Let us briefly examine the Z-source inverter structure. In Fig. 3, the three-phase Z-source inverter

bridge has nine permissible switching states (vectors) unlike the traditional three-phase V-source inverter

that has eight. The traditional three-phase V-source inverter has six active vectors when the DC voltage

is impressed across the load and two zero vectors when the load terminals are shorted through either the

lower or upper three devices, respectively. However, three-phase Z-source inverter bridge has one extra

zero state (or vector) when the load terminals are shorted through both the upper and lower devices of

any one phase leg (i.e., both devices are gated on), any two phase legs, or all three phase legs. This

shoot-through zero state (or vector) is forbidden in the traditional V-source inverter, because it would

cause a shoot-through. We call this third zero state (vector) the shoot-through zero state (or vector),

which can be generated by seven different ways: shoot-through via any one phase leg, combinations ofany two phase legs, and all three phase legs.

The Z-source network makes the shoot-through zero state possible. This shoot-through zero state

provides the unique buck-boost feature to the inverter. The Z-source inverter can be operated in three

modes which are explained in below.

Mode I:

In this mode, the inverter bridge is operating in one of the six traditional active vectors; the

equivalent circuit is as shown in figure 4.


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Fig.4: Equivalent Circuit of the ZSI in one of the Six Active States

The inverter bridge acts as a current source viewed from the DC link. Both the inductors have an

identical current value because of the circuit symmetry. This unique feature widens the line current

conducting intervals, thus reducing harmonic current.

Mode II:

The equivalent circuit of the bridge in this mode is as shown in the fig. 5

Fig. 5: Equivalent Circuit of the ZSI in one of the Two Traditional Zero States

The inverter bridge is operating in one of the two traditional zero vectors and shorting through

either the upper or lower three device, thus acting as an open circuit viewed from the Z-source circuit.Again, under this mode, the inductor carry current, which contributes to the line currents harmonic

reduction as shown in below fig 6.

Fig. 6: Equivalent Circuit of the ZSI in the Non Shoot-Through States.

Mode III:

The inverter bridge is operating in one of the seven shoot-through states. The equivalent circuit of

the inverter bridge in this mode is as shown in the below figure 7. In this mode, separating the dc link

from the ac line. This shoot-through mode to be used in every switching cycle during the traditional zero

vector period generated by the PWM control. Depending on how much a voltage boost is needed, the

shoot-through interval (T0) or its duty cycle (T0/T) is determined. It can be seen that the shoot-through

interval is only a fraction of the switching cycle.


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Fig. 7: Equivalent Circuit of the ZSI in the Shoot-Through State.

C.Analysis Of Impedance NetworkThe equivalent circuit of the impedance network [3] is shown in fig. 8

Fig. 8: Equivalent Circuit of Impedance Network

For simplicity, assuming that the inductors L1 and L2 and capacitorsC1 and C2 have the same

inductance and capacitance respectively, the Z-source network become symmetrical.

From the symmetry and the equivalent circuits, we have

(1)

(2)

Given that the inverter bridge is in the shoot-through zero state for an interval ofT 0, during a

switching cycle, T and from the equivalent circuit, Fig. 8, one has

2 , 0;

Now consider that the inverter bridge is in one of the eight non shoot-through states for an

interval of T1, during the switching cycle. From the equivalent circuit, Fig. 8, one has

3

2 4)

Where Vois the DC source voltage and

5


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The boost factor B is determined by the modulation index M. The boost factor B can be

controlled by duty cycle of the shoot-through zero state over the non-shoot through 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 modulation index.

D.Advantages Of Z-Source InverterThe following are the advantages of Z-source inverter when compared to the two traditional

inverters i.e. voltage source inverter and current source inverter.

Secures the function of increasing and decreasing of the voltage in the one step energyprocessing.

Improve resistant to failure switching and EMI distortions. Relatively simple start-up. Provide ride-through during voltage sags without any additional circuits. Improve power factor reduce harmonic current and common-mode voltage. Provides a low-cost, reliable and highly efficient single stage for buck and boost conversions.

3.PWMTECHNIQUESThe number of control methods to control Z-source inverter, that include the sinusoidal PWM

techniques, three types of PWM control algorithms: simple boost control (SBC), maximum boost control

(MBC), constant boost control (CBC).

The modulation index also called as amplitude modulation ratio (M) which is the main control

factor is defined as the ratio of amplitude of reference wave to the amplitude of carrier wave

The linearity between the modulation index and the output voltage is achieved by under

modulation index (M < 1).

Maximum Boost Control [5, 8]

Maximum Boost Control (MBC) turns all traditional zero states into shoot-through zero state.

The PWM signals with the use of the MBC control is shown in Fig 10. MBC maintains the six active

states unchanged and turns all zero states into shoot-through zero states.

The implementation of maximum boost control method [7] is illustrated in Fig. 9. Two straight

envelopes are employed to realize the shoot through duty ratio (Do). The first one is equal to the peak

value of the three-phase sinusoidal reference voltages while the other one is the negative of the first one.

When the triangular carrier waveforms is greater than the upper envelope, Vp,or lower than the bottom

envelope, Vn, the circuit turns into shoot-through state. Otherwise it operates just as traditional carrier-

based PWM.

Fig 9: Implementation Diagram of MBC


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4.SIMULATION&RESULTS

The Z-source inverter can be operated in both boosts and buck operations depending on values of

M. If M is greater than 0.5 it acts as boost inverter, if M is less than 0.5 then it acts as buck inverter.

The following block diagram figure 11 shows the SIMULINK implementation of Z Source inverter.

Fig. 11: Implementation Diagram of Z source inverter

Boost Operation Results

By considering inverter output voltage we can say boost or buck operation.The corresponding input to inverter circuit is output of diode bridge rectifier is fig 12.

Fig 12: Diode Bridge Rectifier Output Voltage for M=0.8

The voltage across the capacitor is shown in fig 13. Initially the capacitor voltage rises to

maximum value after it reaches to constant value.

Fig 13: Voltage across Capacitor for M=0.8.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-50

0

50

100

150

200

250

Time (secs)

VoltageVo(Volts)

Rectifier output voltage Vo for Ma =0.8

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-20

0

20

40

60

80

100

120

Time (secs)

VoltageV

(Volts

)

Voltage across Capacitor for Ma =0.8


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Fig 14: Current flowing through the inductor

The inverter output voltage is shown in fig 15, for M=0.8.

Fig 15: Inverter Output Voltage for M=0.8

Fig 16: Line Line voltage after filter for M = 0.8

Here a 3-phase RL series load is connected to ZSI. The 3-phase output voltage across load is

shown in fig 17.

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-2

0

2

4

6

8

10

12

14

16

Time (secs)

Current(A)

Inductor current for Ma =0.8

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-250

-200

-150

-100

-50

0

50

100

150

200

250

Time (secs)

Voltage

(Volts)

Inverter output voltage for Ma =0.8

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-100

-80

-60

-40

-20

0

20

40

60

80

100

Time (secs)

VoltageV(

Volts)

Line - Line Load Voltage after filter for Ma =0.8


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Fig 17: Three Phase Load Voltage across Load for M=0.8.

5.CONCLUSION

This paper presents, the theoretical analysis and design of Z-source inverter is studied. The Z-

source inverter employs a unique impedance network to couple the inverter main circuit to the powersource and thus providing unique feature. The control methods with the insertion of shoot-through states

of Z-source inverter have been studied. The proposed scheme under maximum boost control is simulated

with the help of MATLAB/SIMULINK and the simulation results are obtained. The simulation results

shows that boost operation can be obtained with Z-source inverter.

REFERENCES

1. F. Z. Peng, "Z-source inverter", IEEE Transactions on Industry Applications, vol. 39, pp.504-510, Mar-Apr 2003.

2. F.Z. Peng, M. Shen, and Z. Qiang, Maximum Boost Control of the Z-Source Inverter, IEEETransactions on Power Electronics, vol. 20, no.4, pp. 833-838, July 2004..

3. Tran Q. V., Chun T. W., Ahn J.R. and Lee H. H., Algorithms for Controlling Both the DC Boostand AC output Voltage of Z-source Inverter, IEEE Transactions on Industrial Electronics, Vol.

54, No. 5, pp. 2745-2750, 2007.

4. Bindeshwar Singh, S. P. Singh, J. Singh, and MohdNaim, Performance evaluation of three phaseinduction motor drive fed from z-source inverter, International Journal on Computer Science

and Engineering (IJCSE).

5. AtulKushwaha, Mohd. Arif Khan, AtifIqbal and Zakir Husain, Z- Source Inverter Simulationand Harmonic Study, Global Journal of Advanced Engineering Technologies-Vol1-Issue1-2012.

6. B.Y. Husodo, M. Anwari, and S.M. Ayob, Analysis and Simulations of Z-Source InverterControl Methods, IEEE Transactions on Industry Applications, vol. 42, pp. 770 778, May-Jun

2006.

7. S. Thangaprakash and A. Krishnan, Implementation and Critical Investigation on ModulationSchemes of Three Phase Impedance Source Inverter, Iranian Journal of Electrical & ElectronicEngineering, Vol. 6, No. 2, June 2010.

8. PankajZope, K.S. Patil, Dr. PrashantSonare, Z-source Inverter Control Strategies, InternationalJournal of Computational Intelligence and Information Security, August 2011 Vol. 2, No. 8.

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-60

-40

-20

0

20

40

60

Time (secs)

VoltageV

(Vo

lts)

Three phase Load Voltage for Ma =0.8

z source inverter and its application

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