DESIGN AND DEVELOPMENT OF Z SOURCE INVERTER IN DGS …

DESIGN AND DEVELOPMENT OF Z SOURCE INVERTER IN

DGS ENVIRONMENT B.SatheeshPrabu

1, S.Muralidharan

2, M.Muhaideen

3

PG Scholar1, Professor2, Assistant Professor3

1,2,3,Department of Electrical and Electronics Engineering,Mepco Schlenk Engineering College,Sivakasi [email protected],

[email protected],

3 [email protected]

Abstract — This paper focuses on the Power extraction from Solar

PV panel which fed to the DC bus through MPPT charge

controller, then conversion of DC to AC is done by using power

inverter. The DG sources are interfaced with grid through special

power converters. As ZSI having both buck-boost property in single

stage is quite better for DG system. This inverter use an impedance

network, coupled between source and converter circuit. The output

from PV system using MPPT technique is feed to the ZSI for the

conversion of AC, and to make the reduced THD and better output.

In this paper the design and simulation using MATLAB Simulink

is presented.

Keywords— PV module, Z-source inverter, P&O MPPT, Harmonic

reduction.

I. INTRODUCTION

In earlier days, the power is generated by conventional

resources. But due some demerits such as need of large

amount reserves, impact on aquatic life by oil spills, non

renewable, rising prices, fossil fuels present in these

resources, there is need for renewable energy resources. Since

the prominent source of renewable energy is ultimately solar

energy that can be collected from sunlight. Though the

generation of electricity using PV Module [12] is a good

selection of renewable sources, it is an intermittent source of

energy due the variable temperature and irradiance condition [8]. It was The Maximum power is obtained by incorporating

P&O MPPT Technique [5] to the PV Module. This method

can be used at different insolation level and load conditions

for standalone systems [9]. The P &O step time can be can be

done by small signal modeling [10].To make the use of this

energy in a reliable manner, storage of energy is required. So

battery is used to store the energy, and the reliability of supply

of energy according to the demand [6]. The charge control

method for standalone system is discussed in [11]. For the

conversion of DC into AC Voltage Source Inverter is used. In

voltage source converters AC output voltage cannot exceed input DC voltage [1]. Additional stages are needed to boost

the voltage. Shoot through fault occurs. So a new inverter

topology, called Z Source Inverter is used. The Various

PWM techniques for ZSI are presented in [2], [3]. The steady

state analysis and designing are referred of ZSI are referred

from [4].

The objective of this paper is to extract Maximum power

from PV Module using MPP Technique and to store the

energy in the Battery. The battery act as source to ZSI which

produce AC output with less distortion.

II. PROPOSED SYSTEM

Fig.1.Block diagram of the Proposed System

The Block diagram of the proposed system is shown in

Fig.1. The system is consisting of PV Module as a source, DC

converter for MPPT technique, Boost converter to increase the

voltage to the charging voltage of the battery for uninterrupted

supply. The ZSI is used to produce AC supply with reduced

THD at the output. The ZSI overcomes the demerits of VSI

and CSI, such as shoot trough fault, distortion in waveform,

and stage of conversion. Most of above discussed papers use

PV Module as a source which directly feed the load. But In

the Proposed system the PV Module is incorporated with

Battery will supply the load continuously.

III. MODELING OF PV MODULE

A PV Module is the combination of PV cells. It is

connected in the manner with proper matching characteristics.

PV systems are well suited for distributed resource

applications. A photovoltaic PV generator is the whole

assembly of solar cells, connections, protective parts, supports

etc.

The Equivalent circuit of the PV cell is shown in Fig.2. It is

modelled as current source with diode, and series and shunt

resistance. The current from current sources provide current to

the diode and to the shunt and series resistance. The equation

(1) represents the photon current. In actual practice both resistances have finite values , which would alter the

characteristics .

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 10, Number 9 (2015)© Research India Publications ::: http://www.ripublication.com

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Fig.2.Circuit diagram of PV model

(1)

IPh= photo current

ISc= short circuit current

Rs= series resistance

Rsh= shunt resistance

VT=(KT)/q

VT= voltage of temperature at room Io= reverse saturation current

q= 1.602*10 ^-19J/V

K= Boltzmann constant (1.381*10^-23 J/k)

The PV Module Specifications are VOC=44.816 V,ISC=7.244

A,PMPP=255W,VMPP=36.69V IMPP=6.97 A.

IV. Z SOURCE INVERTER

Fig.5. Z Source Inverter

The configuration of three phase Z-source inverter is shown

in Fig .2. It is the combination of impedance network and six

switches to convert low DC voltage to the required rated AC

voltage .The impedance network consists of two identical

inductors (L1 & L2) and capacitors (C1 & C2) connected in X-shape to avoid short circuit when the devices are in shoot

through state. Since it has the property to, improve resistant to

failure switching and EMI distortions, improve power factor

reduce harmonic current it is more preferable than VSI.

A. Principle of Operation

To explain the working and control of the Z-source inverter

in Figure.2 let us briefly examine the Z-source inverter

structure. This inverter has six switches same as in traditional

VSI and nine switching states such as six active states ,two zero states and one additional zero state called shoot through

zero state.VSI has eight switching states. However, the three-

phase ZSI bridge has one extra zero state.Figure.3. shows the

Equivalent circuit of the Z-source inverter viewed from the dc

link when the inverter bridge is in the shoot-through zero state.

Figure.4. Point out Equivalent circuit of the Z-source inverter

viewed from the dc link when the inverter bridge is in one of

the eight non shoot-through switching states.

Fig.3 Non shoot through state Figure.4 Shoot through state

The shoot through state is inserted in the OFF state of the

PWM pulse without change ON-state time interval. So the dc

link voltage of the inverter is boosted. It is noticeable here that

the equivalent switching frequency viewed from the Z-source network is six times the switching frequency of the main

inverter, which greatly reduces the required inductance of the

Z-source network. The simple boost control (SBC) method is

used to generate gate pulses which employs two straight

envelope lines equal to or greater than the peak value of the

three phase references to control shoot-through duty ratio in a

traditional sinusoidal PWM. Based on the mentioned

limitation for shoot-through duty ratio in SBC technique,

output voltage can be expressed by

/2)MB(VV DCAC (2)

Thus for any desired boost factor B, the modulation index can

be used is

(3)

Based on (1.4) and (1.5), the voltage gain of Z-source inverter

in the simple boost control method can be described as

(4)

V. MAXIMUM POWER POINT TRACKING

While using PV module there is need of MPPT controller to

extract Maximum power otherwise the power will be wasted

due to varying irradiance conditions. There are various MPPT

methods for tracking the maximum power of PV Module.

For simplicity and reliability Perturb and Observe Method is

used in this paper. For maximum power point tracking a DC-

DC converter is included with the PV panel, according to the nature of requirement. In the P & O MPPT, the previous

power is compared with present power according to varying

irradiance conditions. If the change in power is positive, the

voltage perturbation is increased in correct direction to reach

the maximum point. If the change in power is negative, then

the voltage perturbation is decreased. Since the power is away

from the maximum point, there is a need of reduced voltage to

RSh

IRS)(V]}

VT

IRS)(V{[IISCIPh 1exp0

/T))(2(T1

1B

o

2

1

2

*B

V

VBMG

dc

ac


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reach the maximum power point. When the change in power

is zero, then the maximum power point is achieved. The

power at this point is denoted as PMPP and voltage and current

as VMPP and IMPP. This operation is shown as flowchart in

Fig.6.

Fig.6. Flow chart of P&O Method

VI. SIMULATION RESULTS

A. Simulation of ZSI

Fig.7. Simulation waveform of Z source inverter

In the proposed Z Source inverter designed for, the input

Es=48V,Vac=415V,L1=L2=1.103mH,C1=C2=215.50µF,

Modulation Index M=0.51, Duty cycle (Ds)=0.49.Fig.7.

shows the simulation waveform of Z Source inverter.The RL

load is connected to the output of the Inverter. The Gating

signals to the Inverter switching sircuit is applied with PWM generated with Simple Boost Control Technique. The first

waveform decribes the voltage across the Impedance Network

which attains the steady state gradually. The second waveform

explains the output voltage across the Inverter. The third

waveform explains the output current in the Inverter with RL

Load. It is in sinusoidal shape with 5% ripple.

B. Simulation of MPPT

The waveform in Fig.8.shows the Maximum power of PV.

The first waveform shows the duty cycle obtained from the

MPPT Technique is compared with carrier triangular signal

which is at 50 KHz. The duty cycle acts as reference signal to carrier wave to PWM signal. This signal is applied as gating

signal for boost converter switch. According to the applied

pulse the output of converter obtained at maximum power.

The third waveform shows power profile of PV Module. The

power is constant when it reaches the Maximum power. The

Table 1 shows the comparison of Max power with Specified

Power of PV and nearly Equal. In the specification 255 W is

given, by using P & O MPPT algorithm the Maximum power

is reached at 255 W. The Voltage (VPV) and Current (IPV) also

nearly as the given.

Fig.8. Simulation of Maximum power of PV

TABLE 1

COMPARISION OF MAXIMUM POWER

DESCRIPTION DESIGNED

PARAMETE

R VALUE

PARAMETE

R VALUE

ACHIEVED

VMPP 36.838 V 36.09 V

IMPP 6.925 A 7.079 V

PMPP 255 W 255 W

C.SIMULATION OF PV WITH MPPT & BATTERY

CHARGING CIRCUIT


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Fig.9. Simulation of PV with MPPT and Battery charging circuit

Fig.10.Simulation of charge controller circuit

The Fig.9.shows the simulation of PV interfaced with

MPPT and the output of the MPPT converter is connected to

the battery through boost converter. The rating of the battery

used is 48V, 125Ah. Lead acid battery type is used as battery

due to good high-rate performance, Good float

service, and Easy state-of-charge indication. Good

charge retention for intermittent charge. The

battery is charged through Battery charge controller. The

simulation of battery charge controller is shown in Figure.9.

In the simulation the voltage (VPV) and the Current (IPV) is fed

to MPPT block, to track Maximum power. The duty cycle

from MPPT block is fed as gating signal to the MPPT

converter. The output of the converter is given to the boost

converter which is used to charge the battery (48V, 125Ah) through charge controller. The Fig.10shows simulation of

battery charge controller consists of charge controller block

with current controller and selector switch block. Charge

controller block is designed with Matlab coding. The

Parameters of Boost converter are Vin=36V, Vout=48V,

L=67.49µH,C=216.67µF, Duty Cycle(D)=32.5.

In the Fig.11, the battery charging profile is shown. The

voltage from MPPT converter is sufficient to charge the

battery. So a boost converter is used for increase the voltage to

a level to charge the battery with the help of charge controller.

The charge controller is used charge the battery in constant

voltage method. The reference current is set according capacity and the voltage. In the simulation the first one shows

the state of charge (SOC) of the battery. While charging the

current gradually increases and attain the steady state is shown

in second part. The voltage of the battery charges to steady

state slowly according to the current charging. The battery is

charged through a charge controller. There is also the

indication for fully charged condition, and the battery is

isolated from the charger when it is charged. The reference

current is used maintain the current for charging.

Fig.11. Simulation charging battery profile

D. SIMULATION OF PV WITH ZSI

Fig.12.Simulation of PV with ZSI in Open loop

In Fig.12 shows the open loop control of PV with ZSI. Here

the PV with MPPT converter is created as block called PV

panel and MPPT and battery with boost converter is integrated

in the battery block. Since the current from PV panel is not

sufficient to meet the ZSI with load, its voltage is stored in

battery which has the rating 48V, 125Ah. While connecting this battery with ZSI it can supply the required current.

The PWM method for inverter is SBC Technique with

switching frequency in 10 KHz. It is created as a block and

fed as gating signals to the switches of the inverter. The


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sinusoidal output voltage across the load is nearly 415 V

obtained. Fig.13 shows the simulation waveform of PV with

ZSI in open loop. The first waveform shows the voltage

across the impedance network which gradually boosts the

input voltage. The boosted peak input voltage DC is converted

into AC. The output voltage is shown in second waveform and output which is sinusoidal by placing the LPF across the load

and output current is shown in the third waveform.

Fig.13. Simulation waveform of PV with ZSI in Open loop

Fig.14. Simulation of PV with ZSI in closed loop

Fig.15.Simulation waveform of PV with ZSI in Closed loop

In the Fig.14 the Z Source Inverter which is connected to

the PV with MPPT block and Battery block is connected with

PI controller in closed loop manner. Here reference voltage is

fixed at 415 V. The output voltage across the load is compared

with the reference voltage and the resultant voltage signal is

given to PI controller block. The value of Kp = 0.3 and Ki =

0.5 is used in controller. The output from controller block is

fed to the SBC PWM block to generate the required gating

signal. This gating pulse then given to the switches of the inverter. In the simulation in Fig.15.shows the waveforms of

closed loop simulation. The first waveform shows the gradual

boosting of input voltage across the impedance network. The

second waveform shows the voltage across the load. Due to

the PI controller the steady state is attained with some slow

settling time. The third waveform shows the current in load

with reduced ripple content better than as shown in open loop.

VII. FFT ANALYSIS

A. FFT Analysis of Open loop

Voltage FFT Current FFT

Fig.16. FFT analysis of open loop

FFT analysis of the output voltage and output current is

shown in Fig.16. It is used to find the total harmonic

distribution (THD) in the AC signals.The FFT anaysis is

applied to the output waveform of the openloop control of

ZSIis shown in Figure 10. It shows that the Voltage (THD) is

in 3.83 % and Current THD is in 4.95 %.

B. FFT Analysis of Closed loop

Fig.17. explains the FFT analysis output in closed loop

condition. The voltage THD is in 3.68% and the current is in

4.34% which is better than the THD in openloop. Hence it is

inferred that the output in closed loop with reduced harmonic

content by using the PI Controller.

Table 2 explains the comparison of THD in output

waveform of the Open loop and Closed loop. It shows that

THD of voltage in closed loop is less than that of open loop,

hence harmonic content is reduced. Also current in closed

loop is less than current in open loop that is waveform of the

closed loop manner with reduced ripple. It is obtained that


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better THD in the closed loop condition. Thus the output is

analysed by FFT analysis.

Voltage FFT Current FFT

Fig.17 FFT analysis of closedloop

TABLE 2

COMPARISION OF THD

DESCRIPTION (V )THD (I )THD

ZSI OPENLOOP 3.83 4.95

ZSI CLOSED LOOP

3.68 4.34

VIII.CONCLUSION

In this Paper, the Power from PV system is connected to the

inverter through battery. Here Z source inverter (ZSI) is used,

for converting DC into AC. Because comparing with VSI and

CSI topologies, it is better. To extract the maximum power

Perturb and Observe MPP technique is used. The voltage across the PV system is boosted through boost converter and

stored in the battery with the help of battery charge controller.

The Maximum power is obtained by P&O method and it is

nearer to the required specification. The characteristics of PV

system are improved in this inverter topology. The Z Source

Inverter is directly connected to the Battery. The Battery acts

as source to Z Source Inverter. The control technique used in

ZSI is Simple Boost Control. The THD value of the output

waveform of the inverter is better in closed loop than open

loop of the inverter. The analysis can be extended to other

control techniques of source inverter in future.

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