Design and Performance Analysis of QZSI Implemented in Grid … · 2019-11-18 · Design and Performance Analysis of QZSI Implemented in Grid Integrated PV System 1B. Kavya Santhoshi,

Design and Performance Analysis of QZSI Implemented in Grid Integrated PV System

1B. Kavya Santhoshi, 2 K.Mohana Sundaram

1Research Scholar, Department of Electrical and Electronics Engineering, Anna University, Chennai, India.E - mail: [email protected] 2Professor, Department of Electrical and Electronics Engineering, Vel Tech Multi Tech Dr.

Rangarajan Dr. Sakunthala Engineering College, Chennai,India.

E-mail: [email protected]

Abstract

Considering today’s world energy scenario, PV based power generation system plays a

vital role throughout the world. This necessitates gazing towards the design of suitable

converter for feeding PV power to grid. As numerous converters/PV inverters are so far

discussed, flexibility of the quasi impedance source inverter (QZSI) for changing

irradiance and then its boost factor control topology has made it more suitable for grid

system. Thus this work presents a novel of qZSI for grid-linked PV arrangement with

suitable control technique. The Magnetically Coupled Boost (MCB) control technique

minimizes the low order ripples and stress in voltage across the switches and thereby

enriches the performance of the qZSI. Fuzzy logic control (FLC) is implemented to have

the DC-link voltage under control. Both Sinusoidal PWM (SPWM) and Space vector

PWM (SVPWM) techniques are tailored for generating switching pulse for the inverter.

SVPWM results show superiority compared to SPWM. Finally, the performance of the

proposed controller technique under constant irradiance condition is simulated with

MATLAB and the results are summarized as a comparative analysis.

Keywords: Grid connected PV, PWM, Quasi Z Source Inverter, Solar.

1 Introduction Nowadays, diminishing fossil results in increased installation of Renewable

energy system (RES) based power generating stations. While considering the power

generation using PV system, power converters plays a vital role in it [1]. The converters

employed in PV systems are categorized as either single/two stage inverters. Among

those, the former offers a feasible and economical solution due to its compact nature,

reliability and lower price. Conversely, its topology must be tailored so as to couple

,312-331, Alpha Publishers

This is an Open Access publication. © 2019 the Author(s). All rights reserved.

Journal of Green Engineering (JGE)

Volume-9, Issue-3, October 2019

Journal of Green Engineering, Vol. 9_3

313 B. Kavya Santhoshi et. al.

with PV system. As a result, the size of the inverter gets increased [2]. But the

application of two stage converters reduces Kilo-volt-ampere (KVA) rating of inverter

and enhances the boost operation for an extensive choice of input voltage. However, the

number of switching devices is more, causing high switching loss. So the efficiency

becomes poor. In order to overcome this aforementioned drawback, an impedance

source inverter (ZSI) was introduced with a single stage topology to get buck & boost

conversion. This will easily manage the variations the PV system without overloading

the inverter [3]. However, in order to enrich, the operation of ZSI, qZSI is introduced

with some attractive changes and is made more suitable for PV application. Thus the

advantages of qZSI over PV system are as follows [4]

1) As it draws a constant current (CC) from PV, no extra filtering action (capacitor

filter) is needed.

2) The rating of the components implemented in qZSI is very low as compared to

ZSI

3) It reduces the ripple in the PV system.

Thueyushan et al (2014) formulated a qZSI with cascaded multilevel inverter

implemented in grid connection for a single phase PV system. Li et al (2013) formulated

a qZSI for distribution and generation system. All these studies exhibit various topology

of qZSI for different applications. Hence, the foremost contribution of the following

work is to formulate the dynamic model and to design a control topology for qZSI to

achieve PV-grid synchronization. The novel SVPWM technique is implemented to

control the qZSI. In this topology, FLC is utilized to regulate the boost factor to keep

up a constant DC link voltage. Proposed work is finally verified using MATLAB

simulation.

2 Related Works Since 20th century, ZSIs have gained popularity. In the year 2006, Miaosen

Shen et al, in his paper, elaborated two methods for achieving boosted voltage gain

without causing low frequency ripple [6]. Switching frequency is the key component to

control ZSI. This work was most suited for variable speed applications although there

was an increased stress in voltage across the switches. In 2010, Gajanayake et al

introduced four novel topologies using converters [3]. The broad classification of them

is based on whether the boost in output is obtained with the help of diode or capacitor.

Further, topologies include continuous current producing converter topology and

discontinuous current producing converter topology. All four have been developed from

QZSIs. The primary benefit of using QZSI is the lower voltage stress on capacitor

compared to other topologies.

In the year 2011, Yandong Chen et al proposed the concept of dual-loop control

strategy for power in a grid connected system [2]. Only single phase systems were

considered. The aim was to achieve Power control with nil error and it was successful.

The use of LCL filter increased the component count but helped to reduce the THD in

output current fed to current. In this paper, an attempt to reduce THD with the help of

LC filter alone is made thus reducing the filtering component. In the same year, Yuan

Li et al presented the modeling of QZSI for distributed generation (DG) and elaborated

the associated control issues [16]. Based on the small signal analysis carried out,

dynamical characteristics of QZSI were investigated. Both open loop and closed loop

control methods were depicted. Since M and D are linked, a constant capacitor voltage

control scheme was introduced as two separate stages of control. In this work, this

inference is taken and the control methods have been incorporated. Based on the choice


314

of capacitor voltage, lesser stress on switching devices was achieved. In 2013, Baoming

Ge et al proposed a unique single stage PV system incorporating energy storage and

providing a constant dc link peak voltage [11]. The power compensation has been

improved. Also, constant dc link peak voltage was obtained and maximum power due

to use of MPPT was accomplished. This power was injected to grid after mitigating the

stochastic fluctuations.

X. Zong et al, in 2015, proposed an important study based on IEEE standard for

limiting harmonic injection in grid [7]. The harmonics were calculated using Harmonic

Admittance Matrix otherwise known as frequency coupled matrix (FCM). The limit on

the ideal value of current alone was considered and the method of obtaining minimal

harmonics by measurement using FCM was illustrated. But the algorithm was complex

and due to digital methods of calculating harmonics, the tedious procedure did not seek

attention. Hadeed Ahmed Sher et al, in 2016, proposed a PV system with high tracking

efficiency [1]. He used short current pulse and P & O for achieving maximum power.

The converter used was a modified flyback inverter but the main drawback was that the

converter operated in discontinuous mode. For the proposed work P & O method has

been utilized and QZSI has been used for achieving boost voltage. In 2017, Vivek

Nandan Lal et al presented control and his analysis on a single stage grid-PV system

[8]. In this paper, a high power PV system using three phase converter was considered.

Due to modified control strategy compensation of dip in grid voltage was achieved. The

reactive power injection capability was also improvised. A modified voltage controller

was proposed with feedback linearization. P & Q were controlled by use of dq

components of grid current. From this inference, in the proposed work, dq components

were used for control instead of three phase quantities. In 2019, Kavya Santhoshi et al

provided a critical review on different topologies of inverters used in grid-tied pv

systems and the associated grid standards, grid codes, popular intelligent algorithms

used and reactive power injection to grid [18]. Among these the topologies were keenly

studied and q-zsi inverter is chosen for this work due to its numerous advantages.

Keeping in mind, grid codes and standards mentioned in the paper, this work aims at

providing lesser harmonics at the output for smooth power injection to grid.

3 Proposed Q-ZSI with Grid Integrated PV Scheme The topology of the circuit for proposed work is revealed in figure 1. The

system has a PV arrangement, qZSI, LC filter and utility for testing. In this proposed

system, Perturb & Observe (PO) method of Maximum power point tracking (MPPT) is

executed for standardizing the power from PV. A 500 W grid structure is considered for

performance evaluation using MATLAB Simulink.

3.1 Design of PV system - Modeling of PV Array

PV cells joined in series & Parallel combinations forms a PV array. Figure 2

depicts the Model of a PV cell using semiconductor (Diode). Solar cell representation

is with a current source and an inverted diode [8].

Here, the shunt resistance is neglected. Thus, the diode and series resistance forms

a PV model and the output current for the above circuit is derived as follows.

– sc dI I I (1)

/

1 1Vdv kT

d JI Io eq (2)

Where

Io1 - Reverse saturation current of diode,


q - Charge of an Electron,

Vdv –Diode Voltage,

k - (1.38 * 10-19 J/K) -Boltzmann constant

TJ - Temperature at the Junction (K)

Using the equations 1 and 2, I can be written as

/

1 – 1 qVdv kT

sc o JI I I e (3)

It can also be written as,

( )/

1 – ( 1)q V IRs nkT

sc o JI I I e

(4)

I – PV Current,

V - PV voltage,

N - Ideality Factor of a Diode.

From the above equations, it is concluded that the variations in the weather

condition (i.e. Changes in the temperature) will affect the productivity of PV. Therefore,

MPPT is implemented so as to achieve maximum power from PV [9,10].

Figure 1. PV powered q-ZSI fed grid system circuitry

Figure 2. Model of a PV cell using semiconductor (Diode)


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3.2 MPPT Algorithm P&O technique for MPPT tracking is used and the same is depicted in figure 3.

According to this algorithm, the increase in operating voltage of an array results in

increased output power and vice versa. If the power increases, the next perturbation

should proceed in the exiting direction or else if it is reversed, it will be in the opposite

direction to that of present position.

Figure 3. Algorithm of Perturbation & Observation

3.3 QZSI Network

QZSI circuit is different from conventional ZSI. It has an LC and diode network

configuration [12]. The dc-link voltage gets boosted and shoot through protection is

given to the circuit [5,17] and the same is depicted in figure 4.

NO YES

YES NO NO YES

YES

ΔV>0 ΔV>0

Inputs: V(t), I(t), V(t-Δt), I(t-Δt) P(t)

& P(t- Δt) calculated from the inputs

ΔV=V(t)- V(t- Δt)

ΔP=P(t) – P(t- Δt)

ΔP=0

ΔP>0

Decrease

Vref

Increase

Vref

Decrease

Vref

Increase

Vref

Return

Stop

Start


Figure 4. QZSI circuit

3.3.1 Operating Principle of qZSI Thus approaches of operation of qZSI are as follows.

(1) Non-shoot through mode (active mode). (NSTM)

(2) Shoot through mode (STM)

NSTM

Figure 5 depicts the equivalent circuit of non-shoot through mode. Under this, switching

arrangement of VSI and qZSI are analogous. The dc voltage is fed to inverter acting as

a conventional VSI[6].

Figure 5. Equivalent circuit of QZSI in Active mode

In the STM, for the short period, switches present in the same phase of the inverter

remains ‘ON’. With LC, the switches are not short circuited and thus the voltage gets

boosted up[13]. Thus, the boosted dc link voltage depends upon the boost factor. This

boost factor may be influenced by the shoot through ratio for specified Ma and the

equivalent circuit STM is shown in figure 6


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Figure 6. Equivalent circuit of QZSI in Shoot through Mode

With the following assumptions,

T – One complete switching cycle

T0 – period of STM

T1 – period of NSTM

Hence,

Total T =T0 +T1

Shoot-through duty ratio, D =T0 /T1.

During T1

𝑉𝐿1 = 𝑉𝑖𝑛 − 𝑉𝑐1, 𝑉𝐿2 = −𝑉𝑐2 (5)

During T0,

𝑉𝐿1 = 𝑉𝑐2 + 𝑉𝑖𝑛, 𝑉𝐿2 = 𝑉𝑐1 (6)

𝑉𝑃𝑁 = 0, 𝑉𝑑𝑖𝑜𝑑𝑒 = 𝑉𝑐1 + 𝑉𝑐2 (7)

At steady state condition, the average VL becomes zero.

𝑉𝑃𝑁 = 𝑉𝑐1 − 𝑉𝐿2 = 𝑉𝑐1 + 𝑉𝑐2, 𝑉𝑑𝑖𝑜𝑑𝑒 = 0 (8)

From (3.1) and (3.3),

{𝑉𝐿1 = 𝑉𝐿1 =𝑇0(𝑉𝑐2+𝑉𝑖𝑛)+𝑇1(𝑉𝑖𝑛−𝑉𝑐1)

𝑇= 0 (9)

{𝑉𝐿2 = 𝑉𝐿2 =𝑇0(𝑉𝑐1)+𝑇1(−𝑉𝑐2)

𝑇= 0 (10)

Thus,

𝑉𝑐1 =𝑇1

𝑇1−𝑇0𝑉𝑖𝑛 𝑉𝑐2 =

𝑇0

𝑇1−𝑇0𝑉𝑖𝑛 (11)

From (5), (6) and (7), the peak dc-link voltage across the inverter is

𝑉𝑃𝑁 = 𝑉𝑐1 + 𝑉𝑐2 =𝑇

𝑇1−𝑇0𝑉𝑖𝑛 =

1

1−2𝑇0𝑇1

𝑉𝑖𝑛 = 𝐵𝑉𝑖𝑛 (12)

where

B -Boost Factor

Then the I avg across the inductors (L1, L2) can be given as

𝐼𝐿1 = 𝐼𝐿2 = 𝐼𝑖𝑛 =𝑃

𝑉𝑖𝑛 (13)

AS per KCL,

𝐼𝑐1 = 𝐼𝑐2 = 𝐼𝑃𝑁 − 𝐼𝐿1 𝐼𝐷 = 2𝐼𝐿1 − 𝐼𝑃𝑁 (14)


Thus, from the above equation, it is observed that qZSI has more advantageous

over ZSI.

3.3.2 Inductor Design For traditional operating condition, capacitor voltage is same as input oltage,

Hence inductor voltage becomes zero. During STM, inductor current (IL) increases and

hence the VL=VC[14]. Thus the ILavg is depicted as,

𝐼𝐿 =𝑃𝑉𝑑𝑐⁄ (15)

Where

P -Total Power

Vdc - Input Voltage.

The Iavg for the proposed design can be calculated. When both IL and ST are at

their maximum, then it results in high ripple current. However, only 30% of current

ripple through inductor is allowed. Thus the ripple should be below 4A and maximum

current is about 10.67A. Hence, for fs about 10kHz, the VCavg is calculated as

𝑉𝑐 = (1 −𝑇0

𝑇⁄ ) ∗ 𝑉𝑑𝑐/(1 −2𝑇0

𝑇⁄ ) (16)

Thus by replacing the values in this equation, the VCavg and inductance is

calculated

3.3.3

The most important purpose of capacitor is minimization of voltage ripple and

maintaining voltage at the constant value. During ST, the capacitor will charge the

inductor, therefore IL=IC.

The ripple at the capacitor is calculated using

VC = ILavgTS/C (17)

4 Control Scheme Figure 1 has two control variables, namely shoot through duty ratio and

modulation index. The first is for MPPT control and next is for control of grid.

Considering grid, the injected P and Q is measured using d-q components. Thus the

measured voltage and current are the grid side are sent to the voltage controller and thus,

M is calculated. Thus, these two parameters (m and D) combined together and produce

PWM signals.

4.1

FLC is an intelligent control system. In this work, it is used for maintenance of

constant dc link voltage [11]. They are characterized using fixed linguistic statements

which are mainly based on expert knowledge / experience.

Thus, the FLC implemented in this system has,

Implications by means of Mamdani’s ‘min’ operator.

Seven fuzzy sets for each input (e, Δe) and output (Δu) with

triangular membership functions.

Defuzzification with the ‘centroid’ method

Continuous universe of discourse for Fuzzification.

Design of a Capacitor

Design of Fuzzy Controller


320

Figure 7a, 7b and 7c shows the triangular MF of input & output variables.

Figure 7a. Membership functions of e

Figure 7b. Membership functions of Δe

Figure 7c. Membership functions of Δu

4.2 Generation of PWM Signals Numerous works [15,18,19,and 20] have depicted the use of PWM techniques

for better performance of QZSIs. SVPWM technique is widely used in many

applications as it provides more refined sine wave with less THD. In this technique, 3ϕ

quantities are transformed into 2ϕ quantity using SRFT topology. From these 2ϕ

components, magnitude of reference vector is calculated and PWM signals are

generated.

Thus the realization of SVPWM is carried out using the steps depicted below

1. Determine, an angle (α) ,Vd, Vref and Vq

2. Determine time required for each sector


3. Finally, Compute switching time of each switch (S1 to S6)

Thus, whenever either upper/lower switch of a specific phase is ON, then it can

be denoted as ‘1’ or ‘-1’. Correspondingly, when the switch is in OFF state, then it can

be considered as ‘0’. Hence, based on these combinations, eight switching vectors, line

to neutral voltage and output line-to-line voltages are produced in SVPWM topology.

5 Simulated Results & Discussion The proposed qZSI is simulated in MATLAB. The Output voltage of PV is fed

to inverter through a z source network. Thus the voltage from the PV is boosted using

shoot-through-mode and it is converted into AC. The output grid voltage and current

are fed back using dq reference frame theory. These signals activated the SVPWM

generator and MOSFET switches are switched ON. Here the filter elements are inductor

and capacitor of ZSI. Thus the obtained output voltage is given to the grid.

Figs. 8a–b show the grid voltage, the grid current and the dc link voltage

respectively. Voltage and current are in phase and hence with the help of the results,

here it is concluded that the PV grid-connected system has a unity power factor. It can

be observed that the distortion is lesser while using fuzzy controller.

Figure. 8a. Simulation results with PI controller

Figure. 8b. Simulation results with fuzzy controller


322

Table 1. Comparison of Proposed methods based on PWM topology

PWM Method Used Voltage THD

(%)

Current THD

(%)

SPWM (Open loop) 1.84 1.46

SPWM (Closed loop) 1.67 1.61

Table 2. Comparison of Proposed methods based on control scheme

Control Scheme Used Voltage THD

(%)

Current THD

(%)

PI 1.17 1.3

Fuzzy 0.23 1.2

From tables 1 and 2, proposed topology under the control of fuzzy controller

with SVPWM techniques exhibits lower THD and is about 0.23% which satisfied the

IEEE Standard.

According to IEEE standard, the value of THD should be less than 5% for an

inverter output voltage waveform[7].

Figure 9 gives input V and I provided by panel. The voltage is about 11.5 volts

in magnitude and the current is about 4.2 amps. Figures 10-13 show the simulation

results obtained under different schemes as tabulated.

Figure 9. Solar voltage and current

Figures 10 a, 10 b, 10 c and 10 d are simulation results for SPWM under open

loop control. From figure 10 a it can be noted that, the output current obtained is about

3.8 amps. In Figure 10 b, the THD of output current is seen as 1.46%. In figure 10 c the

output voltage is shown and measures around 230 volts. On the other hand, the THD of

output voltage is found to be 1.84% from 10 d. Figures 11 a, 11 b, 11 c and 11 d are


simulation results for SPWM under closed loop control. In 11 a, the output voltage is

shown and measures around 200 volts. The THD of output voltage under this closed

loop scheme is shown in 11 b. It is about 1.67%. From 11 c, it is evident that

the output current obtained is nearly 4 amps and the THD of output current is 1.61% as

shown in figure 11 d.

Figure 10 a. Output Current measured while using Sine PWM in open loop mode

Figure 10 b. THD of output current measured while using Sine PWM in open loop mode

Figure 10 c. Output Voltage measured while using Sine PWM in open loop mode


324

Figure 10 d. THD of output voltage measured while using Sine PWM in open loop mode

Figure 11 a. Output voltage measured while using Sine PWM in closed loop mode

Figures 12 a, 12 b, 12 c and 12 d present the simulation results obtained using

SVPWM with PI controller. From 12 a, it can be observed that, the output voltage is

measured as 230 volts. THD of output voltage is 1.17% as shown in 12 b. The output

current reaches a maximum of 10 amps in the beginning and settles at 4 amps. It is

displayed in 12 c. 1.30% is the THD of output current obtained in this scheme. It is

showcased in 12 d. Figures 13 a, 13 b, 13 c and 13 d are the simulation results obtained

by using fuzzy controller. The output voltage in figure 13 a is measured as 330 volts.

THD of output voltage measures as low as 0.23 % while using fuzzy controller. From

figures 13 c and 13 d, the output current settles at 4.5 amps after reaching a peak of 5.2

amps initially and 1.20 % is the THD of the output current.


Figure 11 b. THD of output voltage measured while using Sine PWM in closed loop mode

Figure 11 c. Output current measured while using Sine PWM in closed loop mode

Figure 11 d. THD of output current measured while using Sine PWM in closed loop mode


326

Figure 12 a. Output voltage measured while using SVPWM with PI controller

Figure 12.b THD of output voltage measured while using SVPWM with PI controller

Figure 12 c. Output current measured while using SVPWM with PI controller


Figure 12 d. THD of output current measured while using SVPWM with PI controller

Figure 13 a. Output voltage measured while using Fuzzy Controller

Figure 13 b. THD of output voltage measured while using Fuzzy Controller


328

Figure 13 c. Output current measured while using Fuzzy Controller

Figure 13 d. THD of output current measured while using Fuzzy Controller

Hence, it can be said that SVPWM technique is more suitable in grid

configurations utilizing PV. Simultaneously, on the controller design, fuzzy controller

exhibits faster response compared to other controllers.

6 Conclusion Design and performance analysis of q-ZSI for grid integrated topology under

open and closed loop control is proposed. The main objective of this work is to compare

and conclude the best PWM control for generation of pulses between SPWM and

SVPWM and also the best control strategy between PI and Fuzzy control for grid

connected PV systems. Also, both, open loop and closed loop controls for PI and Fuzzy

have been tested and the harmonics obtained in each method have been carefully

observed and tabulated. The best method was found to be fuzzy for which THD obtained

was 0.23%.When FLC is implemented to control the duty cycle of the STM , BF is also

regulated. Therefore, DC link voltage is kept constant. Also, the P&O method provides

a satisfactory output. From the simulation results, the variations in power factor, real/

reactive power and harmonics are determined. The results of THD measured under

different control schemes are tabulated as a comparative analysis. Therefore, this study

helps researchers to identify the best method of control in grid tied configurations.


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Biographies

B. Kavya Santhoshi completed B.E in Electrical and Electronics Engineering from

Saveetha Engineering College (Affiliated to Anna University) in 2010. She pursued M.E in

Power electronics and drives from Jeppiaar Engineering College (Affiliated to Anna

University) in the year 2015. She also completed Master of Business Administration in

Human Resource Management from Alagappa University in the year 2012. She was given

a cash award of Rs. 75,000 for her academic excellence during her undergraduate course

and secured university rank in her M.E. Her research interests include Power electronics and

Renewable energy. She is currently pursuing Ph.d under the guidance of Dr. K. Mohana

Sundaram in Anna University.

Dr. K. Mohana Sundaram received B.E degree in Electrical and Electronics Engineering

from University of Madras in 2000 and M.Tech degree in High voltage Engineering from

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[18] Kavya Santhoshi, B.; Mohana Sundaram, K.; Padmanaban, S.; Holm-Nielsen, J.B. and Prabhakaran, K. K, "Critical Review of PV Grid-Tied Inverters", Energies, vol.12, no.10, pp. 19-21, 2019.

[19] Kavya Santhoshi, B., and Mohana Sundaram, K., "Hybrid Converter with Simultaneous AC and DC Output for Nano-Grid Applications with Residential System", J. Eng. Appl. Sci., vol.13,no.3, pp. 3289 -3293, 2018.

[20] Kavya Santhoshi, B.; Kuppusamy, M.; Sivasubramanian, M.; and Akila, S., "A Novel Multiport Bidirectional Dual Active Bridge Dc-dc Converter for Renewable Power Generation Systems", Indian J. Sci. Technol., vol.9, no. 1, pp. 85701, 2016.


SASTRA University in 2002 and Ph.D degree from Anna University, India in 2014. His

research interests include Intelligent controllers, Power systems, Embedded system and

Power electronics. Currently he is working as a Professor in EEE department at Vel Tech

Multi Tech Dr. Rangarajan Dr. Sakunthala Engineeering College, Tamil Nadu, India.

Documents

Design and Performance Analysis of QZSI Implemented in Grid … · 2019-11-18 · Design and Performance Analysis of QZSI Implemented in Grid Integrated PV System 1B. Kavya Santhoshi,