Design and Development of Grid-connected Quasi-Z-Source PV Inverter

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 9, No. 4, December 2018, pp. 1989~2005
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v9.i4.pp1989-2005  1989
Journal homepage: http://iaescore.com/journals/index.php/IJPEDS
Design and Development of Grid-connected Quasi-Z-Source PV Inverter
Zulhani Rasin1
, Muhammed Fazlur Rahman2
, Maaspaliza Azri3
, Md Hairul Nizam Talib4
,
Auzani Jidin5
1,3,4,5
Power Electronics and Drives Research Laboratory, Centre for Robotics and Industrial Automation (CeRIA),
Fakulti Kejuruteraan Elektrik, Universiti Teknikal Malaysia Melaka
2
School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, Australia
Article Info ABSTRACT
Article history:
Received Sep 7, 2018
Revised Oct 18, 2018
Accepted Oct 23, 2018
An inverter system applied with the PV source typically has a problem of
lower input voltage due to constraint in the PV strings connection. As a
countermeasure a DC-DC boost converter is placed in between to achieve a
higher voltage at the inverter DC link for connection to the grid and to realize
the MPPT operation. This additional stage contributes to losses and
complexity in control thus reducing the overall system efficiency. This work
discussed on the design and development of a grid-connected quasi-Z-source
PV inverter which has different topology and control method compared to
the conventional voltage source inverter and able to overcome the above
disadvantages. Modelling and performance analysis of the voltage and
current controller to achieve a good power transfer from the PV source, as
well sycnchronization with the grid are presented in detail. Results from both
simulation and experimental verification demonstrate the designed and
developed grid-connected qZSI PV inverter works successfully equivalent to
the conventional voltage source inverter system.
Keyword:
Distributed generation system
Grid-connected
PV inverter
Quasi-Z-source inverter
Voltage and current control
Copyright © 2018 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Zulhani Rasin,
Power Electronics and Drives Research Laboratory,
Centre for Robotics and Industrial Automation (CeRIA),
Fakulti Kejuruteraan Elektrik, Universiti Teknikal Malaysia Melaka,
Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia.
Email: zulhani@utem.edu.my
1. INTRODUCTION
The Z-source inverter (ZSI) [1] as shown in Figure 1(a) is a topology proposed to overcome the
drawbacks identified in the existing traditional voltage source inverter (VSI). The main characteristic of the
topology is the existence of impedance network formed by the inductors and capacitors between the source of
input and the inverter stage. The existence of this impedance network will allow for a situation which is not
allowed in the traditional VSI; the shoot-through or short-circuiting of the switches of the same leg at the
inverter stage. By intentionally creating the shoot-through situation, the ZSI inverter able to boost the input
voltage to produce an output voltage that cannot be achieved by the traditional voltage source inverter.
Among all the newly developed topologies based on ZSI, the Quasi Z-Source Inverter (qZSI) [2]
shown in above Figure 1(b) is based on the drawbacks which are available with the original ZSI (specifically
the voltage fed ZSI) where the input current is discontinuous during the boost period (shoot-through) and
both the capacitors must sustain the high voltage stress. These problems are improved with qZSI, which
produces a continuous input current at the input inductor, and a lower voltage stress on one of the capacitor
with the same boost ratio. Furthermore, the qZSI topology itself has a common DC rail between the inverter
and the input source. In terms of number of components it has the least number of components compared
with the other topologies, maintaining two inductors and two capacitors as the original ZSI. This topology
has been widely discussed for application in photovoltaic distributed generation system [3]-[8].

 ISSN: 2088-8694
Int J Pow Elec & Dri Syst, Vol. 9, No. 4, December 2018 : 1989 – 2005
1990
(a)
(b)
Figure 1. Topology of (a) Z-source inverter and (b) Quasi-Z-Source inverter
In grid-connected PV inverter system, there are several functions that the system must fulfil; ability
to track the maximum power point (MPPT) in order to harvest the maximum power from the solar PV and
the ability to realize a voltage and current control for smooth power flow to the load and grid. Li and
Anderson [9] are the first to present basic guidelines on the design of parameters for the qZSI which includes
the calculation for components rating in dealing with wide input voltage range of the PV for connection with
the grid. Based on the provided results through simulation and experiment, it is shown that the inverter able
to deal with variation range of input voltage and producing an output voltage suitable for grid connection.
However, there is no control element involved for actual implementation of MPPT and to facilitate current
flow into the grid.
The work by Park et al. [10]–[11] suggests a new algorithm of MPPT implementation for the grid-
connected qZSI and with current control at the point of common coupling (PCC). The MPPT is realized
through the adjustment of shoot-through duty ratio, with capacitor voltage controlled at a fixed value. Current
control is implemented based on the reference signal given from the difference in capacitor voltage control
but no detailed modelling or explanation is provided. The produced simulation and experiment results are
also very limited with no indication that the current control is functioning well in grid-connected
environment.
Then again Li et al. in [12] – [13] comes with more detailed works on the controller design where
the PV inverter system able to function in both standalone mode and grid-connected mode. During
standalone mode the output voltage of the inverter is controlled similar to the voltage control in the
conventional VSI where the SPWM signal is adjusted according to the feedback from the PI voltage
controller. At the DC side, the transfer function relation between capacitor voltage and the shoot-through
duty ratio is first established which enable the capacitor voltage to be controlled at a fixed value to stabilize
the DC voltage across the inverter switches. In order to improve the controller dynamic with variation in the
input voltage, a feed forward component which defines the duty cycle according to the input voltage and
capacitor voltage values is added to the controller. The shoot-through signal is combined with the SPWM
signal during the zero stages.
In grid-connected mode, similarly the DC side and the AC side are controlled separately. The
dynamic relation between capacitor voltage and grid current is established and current is controlled based on
the reference signal from the capacitor voltage controller. The inner loop current controller and
L1
D
C1
S1 S3 S5
S4 S6 S2
Vin Vdc
+
-
to load/grid
L2
C2
PV
L1 D
C1
Vpv=Vin
L
C2
(a)
S1 S3 S5
S4 S6 S2
to load/grid
PV
L1 D
C1
S1 S3 S5
S4 S6 S2
Vpv=Vin Vdc
+
-
to load/grid
L2
C2
(b)

Int J Pow Elec & Dri Syst ISSN: 2088-8694 
Design and Development of Grid-connected Quasi-Z-Source PV Inverter (Zulhani Rasin)
1991
synchronization with the grid is implemented based on the explored conventional method available in the
VSI. From the simulation and experiment results, the MPPT works well to track the maximum power with
the current flowing to the grid is synchronized with the grid voltage and the capacitor voltage is controlled at
constant. There are no results however shown for a variation of input power and how the internal current loop
functioning to feed current into the grid.
Other than the implementation in a 3-phase system, implementation in a single phase system is
presented by Chun et al. in [14]. It has a similarity with the above works in the sense that the capacitor
voltage is used as a reference to control the current to the grid and by making it constant MPPT is
implemented by controlling the shoot-through duty ratio. The difference is on the use of resonance controller
for the current controller which sufficiently able to track the AC reference grid current while eliminating the
steady state error at the chosen resonance frequency. Furthermore, by introducing a modified space vector
PWM for insertion of shoot-through during zero states, the number of switching per cycle is reduced and
reduction in switching loss can be expected.
All the above works provide the foundation for the implementation of qZSI in grid-connected
environment. The use of capacitor voltage to implement the current control to the grid is a better option
rather than using the pulsating voltage across the inverter switches which requires a more complex sensor
circuitries. Having a separate control for the AC and DC side makes it possible to implement the MPPT and
in overall the qZSI based topology works similarly as the other conventional VSI PV inverter system.
In the next section 2, the methodology used in carrying out the work is detailed. Section 3 provides
the verification results using both the simulation tool and experimental hardware and the work is concluded
in section 4.
2. RESEARCH METHODOLOGY
Figure 2 shows the overall system block diagram which consists of several main components. The
PV source is the main power supply for the quasi-Z-source inverter which is connected to the grid via a
transformer which is optional depending on the level of output from the inverter. Resistive load is used to
demonstrate a local power demand and the inverter system is also equipped with a battery energy storage to
provide backup for the power demand and store the excess power obtained from the PV array.
L1 D1
S1 S3 S5
S4 S6 S2
L2
C2
Lf
Cf
load
3-phase
grid
415V
C1
Sa
Sb
C3
Battery
L3
PV source
Digital processing platform dSpace1104
Vg_a
Vg_b
Vg_c
VC1
Vpv
Ipv
Ipv Vpv Ibat
VC1 Io_abc Vg_abc
battery
PWM
inverter
PWM
Ig
I
load
Ig_abc
Transformer
Vo_a
Vo_b
Vo_c
Vdc
Vi_a
Vi_b
Vi_c
Io
Figure 2. Overall block diagram of the grid-connected qZSI PV inverter system
2.1. Small Signal Model of qZSI Network
Small signal model analysis of the qZSI network is necessary to obtain a transfer function that
relates various variables in the network to be used for the controller design. Figure 3 shows the small signal
model of the qZSI used for the analysis. There are four state variables from the asymmetric qZSI network;
current through two inductors iL1 and iL2 and the voltage across two capacitors vC1 and vC2. Here it is assumed
that C1=C2=C, L1=L2=L, the stray resistances of inductors r1=r2=r and the equivalent series resistances of

 ISSN: 2088-8694
1992
the capacitors R1=R2=R. Switch S represents when the switches of any leg in the inverter is in shoot-through
condition.
L1
C1
Lf
Cf
Rload
vg
C2 L2
r1
vin
r2
R2
R1
va
vb
vc
vc1
vc2
vdc
vL2
vL1
+ -
+
-
+
-
+ -
+
-
Rf
Rg Lg
ig
io
iLf
iCf
(a)


1
L
v
in
v
1
L
i
1
r 2
r
2
R
1
R
1
c
v
2
c
v
load
i
S
 




2
L
v
 
2
L
i
1
c
i
2
c
i
in
i


1
L
v
in
v
1
L
i
1
r 2
r
2
R
1
R
1
c
v
2
c
v
load
i
S
 




2
L
v
 
2
L
i
1
c
i
2
c
i
in
i
(b) (c)
Figure 3. (a) Model of grid-connected qZSI (b) during shoot-through operation and (c) during non-shoot-
through operation
Previous work [4] has discussed on the analysis, and skipped due to space constraint. Transfer function
relating between the capacitor voltage vc1 and iload is obtained as follow:
 
 
   
 
     
   
ˆ 0 0 0
ˆ ˆ 2
0 2
ˆ 0 0
ˆ 1 1 2 1
ˆ 1 2
c
load
in
c
v
i d s
load v s
v s R d d d Ls r R
G s
i s LCs C r R s d


     
 
   
(1)
2.2. Modelling of the Voltage and Current Control
The grid-connected qZSI PV inverter system operates in two modes, the standalone mode and the
grid-connected mode. The standalone occurs when there is interruption with the grid supply, forcing the
inverter system to be cut-off from the grid or if the PV system is installed in no-grid area. During this mode,
it is necessary to control the voltage and frequency of the inverter output and the current flow is naturally
based on the demand from the load. When the inverter is connected to the utility grid, the inverter output
voltage follows the grid voltage and only current control is necessary to ensure a balance power flow
between the PV source, load, grid and the battery storage. Expressing the AC side of the qZSI inverter in
state space based on Figure 4 below, the following state space equations (2) can be established. vi and iI are
the inverter output voltage and current, Lf, Cf and rf are the LC filter components and its inductor resistance,
vo and io are the voltage and current at the point of common coupling (PCC) with the grid.
In synchronous d-q frame, equations (2) can be expressed as equations (3) and by using the state
feedback decoupling control, the inverter output voltage and current control can be described as (4) and
Figure 5. As an explanation, when the inverter is in a standalone mode, the voltage at the load side vod needs
to be controlled at reference voltage vod* and using the PI controller the reference inverter output current iId*
is produced. The flowing inverter output current iId is controlled to follow the reference current by the PI

1993
controller which produces reference command voltage vid* for generating the PWM reference signal. The
above explanation also applies for the q component of the d-q frame. In grid-connected mode the voltage at
the PCC does not need to be controlled and only current control is implemented to generate the reference
command voltage vid*.
Figure 4. AC side of qZSI inverter
Ia ia Ia oa
f Ib ib f Ib ob
Ic ic Ic oc
oa Ia oa
f ob Ib ob
oc Ic oc
i v i v
d
L i v r i v
dt
i v i v
v i i
d
C v i i
dt
v i i
       
       
  
       
       
       
     
     
 
     
     
     
(1)
Id
f id f Id od f Iq
Iq
f iq f Iq oq f Id
od
f Id od f oq
oq
f Iq oq f od
di
L v r i v L i
dt
di
L v r i v L i
dt
dv
C i i C v
dt
dv
C i i C v
dt




   
   
  
  
(2)
 
 
 
 
* *
* *
* *
* *
I
id f Id od f Iq P Id Id
I
iq f Iq oq f Id P Iq Iq
I
Id od f oq P od od
I
Iq oq f od P oq oq
k
v r i v L i k i i
s
k
v r i v L i k i i
s
k
i i C v k v v
s
k
i i C v k v v
s




 
     
 
 
 
     
 
 
 
    
 
 
 
    
 
 
(3)
vi
iI
rf Lf
Cf
vo
io
Grid
Vdc
PCC

 ISSN: 2088-8694
1994
Figure 5. (a) Voltage control and (b) current control of the AC side
According to the instantaneous power theory for balanced three phase system by Akagi [15], a
balanced three phase system under sinusoidal condition is defined as a three phase system with equal
amplitudes and has 120º phase difference between all the phases. The instantaneous voltages and currents in
the abc frame with amplitude Vm and Im then can be defined as follow.
2 sin
2
2 sin
3
2
2 sin
3
a m
b m
c m
v V
v V
v V






 
 
 
 
 
 
 
 
m
m
m
2 sin
2
2 sin
3
2
2 sin
3
a
b
c
i I
i I
i I






 
 
 
 
 
 
 
 
(4)
The active power P and reactive power Q can be defined as equations (6) with its synchronous d-q
frame representation in (7).
     
1
3
a a b b c c
b c a c a b a b c
P v i v i v i
Q v v i v v i v v i
     
 
        
  (6)
 
 
3
2
3
2
q q d d
q d d q
P v i v i
Q v i v i
   
   
(7)
Since the vq component is set to zero at the PLL control, equation (7) becomes equations (8) which
implies that the active and reactive power can be controlled independently through iId and iIq as in the
Figure 5.
 
 
3
2
3
2
d d
d q
P v i
Q v i
 
  
(8)
ωLf
iIq ωLf
-
+
viq*
+
-
+
+
+
-
+
vid*
iId
iIq*
+
PI
PI
iId*
vod
voq
ωCf
vod
voq ωCf
-
+
+
-
+
+
+
-
+
voq*
+
PI
PI
vod*
iod
ioq
vod
(a) (b)

1995
Figure 6 shows the capacitor voltage and current control mechanism during grid-connected
operation. At the DC side of the system, based on the controlled value of capacitor voltage VC1, the shoot-
through duty ratio d is controlled by the MPPT algorithm in order to regulate the voltage across the PV
terminal Vpv to achieve the maximum power point according to the following equation (9).
1
1 2
1
pv c
d
v v
d



(9)
The VC1 is controlled at the AC side with a two loops control mechanism; outer loop voltage control
of VC1 and the inner loop current control. The design of the capacitor voltage PI controller is based on the
small signal transfer function relation between VC1 and iload as in equation (1). iload is obtained through its
ratio G with output current at PCC io. The output of VC1 controller is the current reference signal iId_ref* which
is relayed to the inner loop current control. As for the inner loop current control strategy, a synchronous
decoupling current control in two-phase rotating (d-q) frame is applied. The advantage of this control strategy
is the three phase symmetric grid voltage and current that can be transformed into a DC variables and by
adopting a PI controller, it is possible to realize a no-steady state error adjustment. A feedforward d
component of the grid voltage Vd is used to improve the controller’s dynamic performance. Figure 7 shows
the block diagram of the inner loop current control in s-domain.
Figure 6. Block diagram of capacitor voltage and inverter output current control in grid-connected mode
Figure 7. Block diagram of inner loop current control
GPI(s) are the PI controller transfer function, Gf(s) is the filter transfer function and Gd(s) is the delay caused
by the digital computation with Ts is the sampling period.
   
1 1
1
d f
s f f
G s G s
sT sL r
 
 
(10)
ωLid
abc/
dq
II_abc
iId
iIq
ωLiq
abc/
dq
PLL
Vg_abc
θ*
iId_ref
*
PI
PI
+
-
iIq_ref
*
dq /
abc
Vd
Vq=0
Vd
vq
SVPWM
generator
+
-
+
+
+
VC1
VC1_ref* PI iId_ref
*
gate drive
signal S1 – S6
+
-
ˆ
ˆ
c
load
v
i
G
+
-
θ*
d
vd
MPPT
Vpv
Ipv
shoot-through
d
G io
iload
iId_ref*
GPI (s) Gd(s) Gf(s)
iId
+
-

 ISSN: 2088-8694
1996
In designing the controller, it is required to have a stable system and the inner current control is
designed to have a shorter settling time to achieve a fast correction and the outer capacitor voltage controller
with a slower response. Figure 8 shows the open loop frequency responses analysis for both current and
voltage control based on parameters values in Table 1. Both show a stable system with phase margin bigger
that 45º. Figure 9 shows the step response analysis and it is shown that the outer loop voltage response is
slower compared to the inner loop current response.
(a)
(b)
Figure 8. Open loon bode plot frequency response analysis for (a) inner loop current control and (b) outer
loop capacitor voltage control in grid-connected mode
Table 1. Parameters Values for Capacitor Voltage and Current Control Analysis during Grid-connected Mode
L C d r R
4.7 mH 1100 V 0.3 0.2 Ω 1 mΩ
(a)
(b)

1997
(a)
(a)
(b)
(b)
Figure 9. Step response analysis of the (a) outer loop voltage control and (b) inner loop current control
2.3. Grid Synchronization
The phase lock loop (PLL) system is commonly used in grid connected inverter system to ensure
synchronization between the grid and the inverter output. This is to make sure a good power factor is
achieved in the range specified by the regulation. A basic PLL configuration is shown in Figure 10 [16]. The
sensed grid phase voltages vga, vgb and vgc are firstly transformed into synchronous d-q frame to allow the AC
variables represented in DC form, thus the magnitude and phase control can be easily implemented to
achieve zero steady state error. The q component of the transformed voltages is controlled at zero and the PI
controller produces the frequency ω*
that is integrated to obtain the θ*, which drives the feedback voltage Vq
to the reference value Vq
*
_ref = 0.
The value θ* is continuously used for the d-q transformation of the grid voltage which means if the
frequency ω*
is identical to the utility grid frequency, the produced waveform is a DC where the q
component is zero. As a result the θ* is always locked to the utility frequency. A feed forward reference ωff =
2πfg where fg is the grid frequency, is added to the controller output to improve the initial dynamic
performance which helps to decrease the starting time of the PLL.

 ISSN: 2088-8694
1998
Figure 10. Configuration of PLL
3. RESULTS AND ANALYSIS
Based on the system designed and modelled in the previous sections, both the simulation using the
Matlab®
software and the hardware experiment are carried out with the objective of verifying the operability
of the designed grid-connected qZSI PV inverter system. For the simulation, a platform of 5 kW system is
configured to show that it is operable over wide range of power level under residential PV installation
category. The PV source configuration is set at Vmpp 235.9 V with the capacitor voltage VC1 controlled at 680
V. This makes it possible for direct connection to the 3-phase 415 Vl-l grid. For the experimental platform as
shown in Figure 11, the system is scaled down at 0.5 kW power level with the PV source is configured at
Vmpp 141.1 V, with capacitor controlled at 200 V and connection to the 3-phase 415 Vl-l grid is made through
a transformer. Table 2 shows the parameters and specifications used for both platforms.
Figure 11. Experimental platform setup at 0.5 kW power level
PI
1
s
abc
dq
Vq*_ref = 0
Vq
Vd
+
-
+
+
Kff
ωff
θ*
abc
dq
2π
vga
vgb
vgc
ω ω*

1999
Table 2. Parameters/Specification used for the Simulation and Experimental Setup for Grid-connected PV
Parameters/Specification 5 kW configuration (simulation) 0.5 kW configuration (experimental)
1. Max power from PV 5 kW Vmpp = 235.9V Impp = 21.36A 0.5 kW Vmpp = 141.1V Impp = 3.6A
2. Max power to grid 5 kVA 0.5 kVA
3. DC bus voltage
VC1 controlled at 680 V , Vdc 1126 - 1148 Vpk
shoot-through d : 0.39 – 0.41
VC1 controlled at 200 V , Vdc 258 - 279
Vpk
shoot-through d : 0.2 - 0.3
4. Switching frequency fsw 10 kHz
5. Grid connection transformerless 3-phase 415 Vl-l_rms
with transformer
3-phase 104 Vl-l_rms : 3-phase 415 Vl-l_rms
6. Grid filter Lf = 10 mH, Cf = 8 uF
7. qZSI network L1 = L2 = 4.7 mH, C1 = C2 = 1000 uF L1 = L2 = 4.7 mH, C1 = C2 = 800 uF
8. Battery storage unit Lead Acid 20 x 12 V in series, 42 Ah Lead Acid 6 x 12 V in series, 7 Ah
3.1. Simulation Results at 5 kW
Figure 12(a) shows the successful control of capacitor voltage VC1 at 680 V while power from the
PV source is varied between 1 kW to 5 kW. This is achieved through the current control operation shown in
Figure 12(b) where the inverter output current Isd is tracking the reference value from the capacitor voltage
controller Iref. Since all the output current is fed to the grid without a load in between, the reactive component
Isq is controlled at 0. The three phase output current of the inverter is well proportional to the power fed to the
inverter.
(a)
(b)
(b)
Figure 12. (a) The power fed by the PV source Ppv (top) and the control of capacitor voltage VC1 (bottom) (b)
The current control of the inverter (top) and the corresponded 3-phase inverter output current (bottom)
As shown in Figure 13(a), the 3-phase grid voltage is detected using the PLL algorithm and the
angle θ between 0 to 2π radian is produced, with time length synchronized with one cycle period of the grid
voltage. For synchronization of the inverter output voltage with the grid voltage, the θ value is applied in the
(a)

 ISSN: 2088-8694
2000
current control algorithm. As shown in Figure 13(b), the produced active phase current is in-phase with the
grid voltage.
(a)
(a)
(b)
(b)
Figure 13. (a) 3-phase grid voltage Vgrid (top) and the angle θ produced for synchronization (b) active
inverter output phase current of the inverter in-phase with the grid voltage
3.2. Experimental Results at 0.5 kW
In grid-connected mode, as shown in Figure 14, capacitor C1 voltage VC1 is controlled at 200V to
realize the MPPT operation through the control of the inverter output current. Based from results, when
power from the PV source is increasing from 300W to 500W, the active current d component of the inverter
output current Isd is successfully controlled to follow the VC1 controller’s output reference signal Iref shown in
Figure 14(a). The same goes for the case when PV power is decreasing as shown in Figure 14 (b). For both
situation since only active power is flowing to the grid, the reactive current q component of the inverter
output current Isq is controlled at 0. The power produced by the PV source goes into the inverter and output
current of the inverter Iinv is proportional to the input power from the PV source output in Figure 14 (a) – (b),
as shown in Figure 15.

2001
(a)
(b)
Figure 14. (a) Capacitor voltage VC1 and current d-q control with Ppv increasing from 300 W to 500 W (b)
Capacitor voltage VC1 and current d-q control with Ppv decreasing from 500 W to 300 W

 ISSN: 2088-8694
2002
(a)
(b)
Figure 15. Variation of inverter output phase current Iinv as PV source is increased and decreased (a) 300 W
to 500 W (b) 500 W to 300 W
In grid-connected mode, it is important the inverter output is well synchronized with the grid
voltage. The grid line-to-line voltage Vl-l of 103.92 Vpk is firstly measured through the sensor and the
corresponding 30° shifted value of each phase voltage Vphase-pk of 60 Vpk is calculated and produced as shown
in Figure 16(a). Inside the PLL algorithm the synchronous d-q transformation is carried out with the q
component controlled at zero and integration of controller’s output is the angle theta θ which is synchronized
with the grid phase voltage shown in Figure 16(b) (top). This θ is used in the formation of the 3-phase

2003
sinusoidal control signal as in Figure 16(b) (middle) before producing the SVPWM reference signal for
triggering the inverter switches as in Figure 16(b) (bottom). Based from the results, the PLL is operating
satisfactorily. With the inverter is injecting power to the grid, the produced inverter output current is almost
in phase with the grid voltage at power factor 0.95 as shown in Figure 17.
(a)
(b)
Figure 16. (a) The line-to-line voltage Vl-l of the grid measured by sensor at 103.92Vpk and the produced
phase voltage Vphase at 60 Vpk (b) The theta θ produced by PLL (top), the produced 3-phase sinusoidal control
signal (mid) and the SVPWM reference signal (bottom)

 ISSN: 2088-8694
2004
Figure 17. Grid phase voltage (25V/div) at 60 Vpk and current (2A/div) in grid-connected mode
4. CONCLUSION
The paper presents the design and development of the grid-connected quasi-Z-source PV inverter
system. The qZSI inverter system has an advantage over the conventional VSI of boosting the lower PV
voltage to higher level DC link for connecting with the grid. The voltage and current control of the inverter to
achieve a balance power transfer is modelled and analysed to achieve stability and good dynamic response. A
description on the PLL technique use for synchronization with the grid is also presented. Verification with
both simulation at 5 kW power level and lab scale experiment at 0.5 kW is carried out and based on the
results, all the controllers work satisfactorily and the inverter integrate well with the grid to supply the
obtained power from the PV source. A verification on the inverter performance in complying with the grid-
connection standard for distributed generation system will be conducted in the future works.
ACKNOWLEDGEMENTS
The author would like to thank the Universiti Teknikal Malaysia Melaka for supporting this work
under research grant PJP/2015/FKE(10B)/S01447.
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