 Open Access
 Total Downloads : 318
 Authors : Huaqiang Zhang, Caijuan Qi, Jian Zhang, Lingshun Liu
 Paper ID : IJERTV3IS051211
 Volume & Issue : Volume 03, Issue 05 (May 2014)
 Published (First Online): 27052014
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Isolated ShootThrough ZSource Inverter
Huaqiang Zhang1, Caijuan Qi1, Jian Zhang1, Lingshun Liu2

Department of Electrical Engineering, Harbin Institute of Technology at Weihai, Weihai Shandong 264209, China;

Department of Control Engineering, Naval Aeronautical and Astronautical University, Yantai Shandong 264001, China
Abstract The boost factor and modulation index of traditional Z source inverter restrains each other and the inserted shootthrough duty ratio must be less than the zero vector in a switch cycle. These defects not only limit boost capability but also reduce the flexibility of control strategy. An isolated shootthrough Zsource inverter (ISTZSI) topology structure which adding a diode, a capacitor and an entirelycontrollable device in traditional impedance network is proposed. The inverter reduces the startingup inrush current and the voltage stress on capacitors greatly, separates the control of shootthrough duty ratio from the topology, which realizes the decoupling control between boost factor and modulation index, and obtains higher voltage transfer ratio and good dynamic performance. On the base of theoretical analysis, the simulation and experiment on ISTZSI have been done. The research results verify the correctness and the superiority of the topology.
Key words: Zsource inverter; isolated; decoupling control; boost capability; shootthrough zero vector
. INTRODUCTION
In 2002, the topology of Zsource inverter was proposed. The uniqueness of this topology lies in the introduction of Zsource network which can make the upper and lower devices of each phase leg gate on simultaneously. This shootthrough state provides a singlestage buckboost function. Therefore, Zsource inverter has many important research and application values, such as in renewable energy connected to power grid [13], in electric vehicles [4, 5], in motor drive [6, 7] etc. In recent years, different aspects of Zsource inverter have been studied by scholars from all over the world. [89] presented a threelevel Zsource inverter, suitable for high voltage and high power occasion. A doubleoutput Zsource inverter was summarized in [10], which can increase the number of loads, and fit the conditions of multiple loads and large gap of output voltage. In view of the shortcomings of Zsource inverter, a quasiZsource
inverter was proposed in [11]. On the basis of quasiZsources,
inverter was proposed in [14], however, the proposed inverter leaded to the problems of more devices, larger volume and higher cost.
To some extent, though the performance of Zsource inverter is improved by many improved programs, there are still a number of substantive issues unsolved. For example, due to the pinning and coupling of boost factor (B) and modulation index (M) in traditional Zsource inverter, the way of improving boost capability is restricted. But, the high boost inversion abilities of Zsource inverter must be required in the condition of lowvoltage input. In this paper an isolated shootthrough Zsource inverter (ISTZSI) is proposed for this problem. Compared to traditional Zsource inverter, this topology has many advantages. It can be summarized in the following.

The control of shootthrough duty ratio is separated from the topology, which can obtain high boost capability by increasing shootthrough duty ratio.

Avoiding the problem of serious startingup inrush current and high capacitor voltage stress.

Overcome the mutual coupling of boost factor and modulation index, and realize the flexible control of shootthrough duty ratio from 0 to 50% without the limitation of zero vectors in SPWM or SVPWM algorithm.
On the foundation of Zsource inverter and their families, theoretical analysis and experimental verification of ISTZSI have been done in this paper. The research results verify the aforementioned advantages.
" COMPARISON OF SEVERAL ZSOURCE INVERTERS BOOST CONTROL
As shown in Fig. 1, the impendence network of traditional Zsource inverter connected in Xshape couples input source and inverter, which has buckboost function.
D
L1
a new Tsource inverter, introducing a coupling transformer and reducing a 1capacitor, was proposed in [12], which not only had good boost capability, but used fewer devices and had small size and low cost. A tapped inductor quasiZsource inverter was put forward in [13], which provided high inversion gain, but was difficult to avoid the adverse effects of
Vdc
C1 C2
L2
S1 S3 S5
S2 S4 S6
Load
leakage inductance. In order to increase the boost capability of traditional Zsource inverter, a switch inductor Zsource
The authors thank the support both the Foundation of National Natural Science (51377168) and the Foundation of Shandong Province Science and Technology Development Planning (2011GGH20411), which enabled the achievement of the mentioned research results.
Fig.1. Topology of traditional Zsource inverter.
The operation principles of Zsources inverter have been analyzed in [15], which achieve boost capability by controlling the shootthrough duty ratio. The boost factor of Zsource network can be expressed as
B 1
1 2D
(1)
Vdc
S1 S3 S5
L1
Load C C
where D is shootthrough duty ratio.
The capacitors voltage of Zsource network and the output peak phase voltage from the inverter can be respectively expressed as
1 2
S2 S4 S6
L2
V 1 D V
(2)
(a)
C 1 2D dc
V MB V GV
(3)
L L L
g 2 dc dc
1 2 3 C4
L4
where M is the modulation index, and G is the voltage gain of Zsource inverter.
The Zsource inverter overcomes the limitations of traditional voltagesource inverter and currentsource inverter and provides a novel power conversion concept. However, the traditional Zsource inverter still shows some drawbacks, such as: the discontinuous input current, serious startingup inrush current, high capacitor voltage stress of the impedance network, limited shootthrough duty ratio and the mutual coupling relationship between B and M etc. The reason why the boost factor B coupled with modulation index M is that the boost ratio of Zsource inverter is determined by shootthrough duty ratio which is determined by the zero vector in a switching cycle, while the modulation index determines the remaining zero vector duty ratio. Therefore, B increases with the decrease of M, which also reduces the flexibility of the control strategy. Here, we take the traditional synchronized pulse width modulation (SPWM) control strategy as the example, for fixed modulation index M, the remaining zero vector duty ratio of a switching cycle is 1M, so the maximum shootthrough duty ratio can be expressed as D=1M, namely, to get a larger shootthrough duty ratio D is bound to reduce the modulation index M [16].
For these problems, scholars have proposed varies of quasiZsource inverters (qZSI). As shown in Fig. 2(a), singlestage qZSI was proposed in [1719]. The difference of
C1 C2
Vdc
C1
L1 L2
C4 C5
Vdc
C4
L1 L2
C1 C2
Vdc
S1
C3
S2
(b)
C2 C3
L3 L4
S1
C6
S2
(c)
L3 C5
L4
S1
C3
S2
(d)
S3 S5
S4 S6
S3 S5
S4 S6
S3 S5
S4 S6
Load
Load
Load
the improved Zsource inverter is that the positions of th inverter bridge and diode are exchanged, which effectively solve huge startingup inrush current and high capacitor voltage stress, however, the biggest drawback is the limited boost ratio, which is hard to meet the occasion of wide input voltage. Based on the aforementioned, improved multistage
qZSI was put forward in [20, 21], which can be divided into
Fig.2. Improved quasiZsource inverter. (a) Singlestage quasiZsource inverter; (b) Diodeassisted extendedboost qZSI; (c) Capacitorassisted extendedboost qZSI; (d) Hybrid extendedboost qZSI.
As shown in Fig. 2 (b), the boost factor can be expressed
as
three categories: diodeassisted extendedboost, capacitorassisted extendedboost and hybrid extendedboost, as shown in Fig. 2(b), Fig. 2(c) and Fig. 2(d), respectively. For diodeassisted extendedboost qZSI, a inductor, a capacitor and two diodes are added while boost ratio becomes original 1/(1D)2 times. For capacitorassisted extendedboost qZSI, a inductor, two capacitors and a diode are added while boost ratio becomes 1/(1(2+N)*D) (where N is the number of increased stages).
B 1
(1 2D)(1 D)2
In Fig. 2 (c) the boost factor can be expressed as
B 1
(1 4D)
(4)
(5)
As we can see from (4) and (5), no matter diodeassisted
extendedboost or capacitorassisted extendedboost, it could improve boost factor by cascade, and need less shootthrough duty ratio when achieving the same boost ratio. Although it improves boost capability greatly, but simultaneously, more devices are applied. Furthermore, with the increasing number
of extendedstage, more devices are needed, thus it leads to the problem of complex structure, high cost and large volume etc.
In addition, too small shootthrough duty ratio is susceptible to be interfered by system, which will cause system instability. More importantly, these structures do not solve the drawback of the coupling relationship between B and M, and the control flexibility is inadequate, thus the
equivalent circuit is shown in Fig. 4(a). In this state, ISTIGBT, DC power source and the two inductors L1 and L2 form a closed circuit. Meanwhile, the two inductors get charged, then, the charged inductors can be regard as DC source. Assuming that the inductors L1 and L2 and the capacitors C1 and C2 have the same inductance (L) and capacitance (C), respectively, we have
application areas of Zsource inverter are limited, especially, in high boost ratio occasion.
. ISOLATED SHOOTTHROUGH ZSOURCE
VC1 VC2 VC
VL1 VL2 VL
(6)

Circuit Topology
INVERTER
When in the shootthrough state, the inductors of Zsource network get charged and the capacitors discharge, we can get
The topology of isolated shootthrough Zsource inverter (ISTZSI) is shown in Fig. 3. Its basic idea is that the traditional Zsource network and the inverter are separated by an independent circuit, which consists of an IGBT, a diode and a large capacitor. This circuit realizes the decoupling control of boost factor and modulation index.
VL Vdc VC
0
Uo
IL1
(7)
L1
L1
Vdc
C1 C2
D0
Vdc
C C
1 2
ISTIGBT
ultra_C
D_S
S1
L2
S3 S5
(a)
L2
IL2
IL1
L1
S2 S4 S6
Load
Vdc
C1 C2
Fig.3. Topology of the isolated shootthrough Zsource inverter (ISTZSI).
Isolated shootthrough Zsource inverter adds a fullcontrolled device IGBT in traditional Zsource inverter, which individually controls the shootthrough duty ratio, the
U
o
(b)
L2
IL2
IGBT is designated as ISTIGBT. Meanwhile a large capacitor (ultra_C) is connected in DClink, which is used to gentle the fluctuation of DClink voltage after joining shootthrough duty ratio and has the ability to provide instant high current. In order to avoid ultra_C through ISTIGBT discharges in shootthrough state, it is necessary to add a diode between ISTIGBT and ultra_C. This diode mainly plays the role of separating ISTIGBT and ultra_C, so we call it D_S. This topology decouples boost factor and modulation index, and the shootthrough duty ratio is separated from physical structure, thus, the inverter is termed isolated shootthrough
Zsource inverter.
Fig.4. Equivalent circuits of isolated shootthrough Zsource inverter. (a)
Shootthrough state; (b) Nonshootthrough state.
2) NonShootThrough State: ISTIGBT is off, and its equivalent circuit is shown in Fig. 4(b). In this state, Vdc and equivalent DC source of the inductors supply loads together.
It can be seen from Fig. 4(b) that during nonshootthrough state, the inductors of Zsource network discharge, the capacitors get charged and the diode D0 is positive onset. By applying KVL, the following steadystate relationships can be observed

Operation Principles
From the view point of the switching states of the main circuit, the operation principles of ISTZSI are similar to those
VL VC
Uo Vdc VC VL Vdc 2VC
(8)
traditional Zsource inverters. The substates of the proposed topology are classified into the shootthrough state and the nonshootthrough state, respectively.

ShootThrough State: ISTIGBT is on, and its
Considering the fact that the average voltage of the
inductors over one switching period should be zero in steady state, thus, the following relationship can be derived
VC
D
1 2D
Vdc
(9)
when D is zero, the corresponding capacitor voltage is also zero. As for the steady increase of shootthrough duty ratio D from zero to expected value, it could effectively reduce
Combining expression (8) with expression (9), the DClink voltage can be described as
damages to capacitors caused by instantaneous high voltage, thus it achieves the goal of softstart and small startup current.
D. Boost Capability Analysis of Inverter
U 1 V
BV
(10)
The boost inversion ability of a whole Zsource is
o 1 2D
dc dc
determined by the interactions of Zsource impedance and the PWM control method applied to the main circuit. As
If under the condition of SVPWM, the output peak phase voltage from the inverter can be expressed as
U B
described in [22], two kinds of common modulation strategy which termed as the simple boost control method and the third harmonic injection control method have been introduced. The simple boost control method is convenient and simple, and the
o
Vg
Vdc
3 3
(11)
third harmonic injection method can increase the range of M, both of which are widely used in threephase inverter system. Among, the sketch map of third harmonic injection control is
In Fig. 3, the ISTIGBT realizes the independent control of
Zsource shootthrough vector, making the control of inverter output is more flexible. Compared to the aforementioned Zsource inverters, ISTZSI not only has its excellent characteristics, but also realizes the decoupling control of B and M, which makes B can adjust in a large range so that it can get a higher voltage transfer ratio. Whats more, the value of shootthrough duty ratio will be not too small and not easy to be interfered by system, so it can obtain stable boost factor
shown in Fig. 6.
Vp
Va
Vb
Vc
S

Vn
ap

Capacitor voltage stress and the Ability of Softstart
From (1) (2) and (9), we can get the relationship between the ratio of capacitor voltage stress and Vdc (Vc/Vdc) and boost factor (B) in traditional Zsource inverter and ISTZSI.
In traditional Zsource inverter
Sbp Scp San
Sbn
Scn
Fig.6. Sketch map f third harmonic injection control.
V /V
B 1
(12)
The relationship of shootthrough duty ratio and
c dc 2
modulation index in simple boost control can be expressed as
In ISTZSI
M 1 D
(14)
V /V
B 1
(13)
Combining expression (1) with expression (14), the
c dc 2
voltage gain G can be described as
The relationship curve is shown in Fig. 5.
4
I
ional ZS
Tradit
3
G Vg
Vdc
2
BM 1 D
1 2D
(15)
Vc/Vdc
The relationship of D and M in third harmonic injection is
TZSI
IS
2
1
0
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
B
Fig.5. Comparison of capacitor voltage stress between traditional Zsource inverter and ISTZSI.
The voltage gain G is
M 2 3(1 D)
3
(16)
It can be seen from Fig. 5 that the capacitor voltage stress of traditional Zsource network is much higher than ISTZSIs in the same boost factor. Therefore, to achieve the same gain,
G BM 2 3(1 D)
3(1 2D)
(17)
ISTZSI can choose smaller capacity capacitors, which is beneficial to reduce cost and volume.
The capacitor voltage of impedance network is determined by shootthrough duty ratio D. According to expression (9),
From (11), the voltage gain of ISTZSI can be obtained as follows
G Vg
V dc
2
2 3(1 2D)
(18)
of shootthrough in a switch cycle, and there is only one shootthrough vector in a switch cycle for ISTZSI, so n=1. IL denotes the mean value of inductor current, as for the inductor current is equal to the load current, thus the value of IL can be determined by the maximum load current.
Based on expression (15)expression (17) and expression
(18), the relationship of voltage gain and shootthrough duty ratio is shown in Fig. 7. It can be seen that the voltage gain of ISTZSI is bigger in the same shootthrough duty ratio. Taking D=0.4 as example, in simple boost control the voltage gain of traditional Zsource inverter is G1=3, in third harmonic injection the voltage gain is G2=3.46 while the ISTZSIs is G=5.77. In actual, the shootthrough duty ratio of traditional


The Design of capacitors for impedance network

The capacitors of impedance network are mainly determined by the tipple of capacitor voltage. If the value of capacitor is too big, its cost and volume will increase. If the value of capacitor is too small, it will not suppress the voltage ripple. Thus, the following equation can be used to select
dV
Zsource inverter is impossible to achieve 30% in general. Because in order to achieve a larger shootthrough duty ratio, the modulation index must be a lower one, which will seriously affect the quality of inverter output voltage. When D=0.3, G=2, i.e. the maximum voltage gain of traditional
where
I C C
C dt
(23)
Zsource inverter is around 2. To ISTZSI, it has realized the decoupling control of B and M, so we need not consider the negative impacts of low modulation index brought in when shootthrough duty ratio is high.
12
ZSI simple boost control
dVC VC x2 %VC
dt t DT / n
From (23) to (25), we can get
(24)
(25)
10 ZSI third harmonic injection control
ISTZSI
8
C ICdt ILt
IL DT
(26)
G
6 dVC
4
VC
x2 %nVC
2
0
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
D
Fig.7. Voltage conversion gain comparison of traditional topology and ISTZSI in the same shootthrough duty ratio.
E. The Parameters Design of ISTZSI
The design of related parameters in ISTZSI mainly includes the design of Zsource impedance network and the
where x2% is the percentage of voltage ripple.
(3) The Design of ultra_C
The main function of ultra_C is used to gentle the DClink voltage ripple, which is caused by the shootthrough of ISTIGBT, simultaneously it also provides instantaneous high current.
When capacitor get charged, it satisfies (27)
1 t
design of ultra_C.
V V
(V V ) (1 e
R0C )
(27)
1) The Design of inductors for impedance network
The design of inductors mainly considers if the current continuous or not. If the value of inductors is too small, it will not guarantee the continuity of current. The circuit will enter nonnormal state in discontinuous current. If the value of inductors is too big, it will be easy to form resonant with capacitors. The following formula can be selected
t 0 1 0
where, V0 is the initial voltage of capacitor. V1 is the final voltage of capacitor, i.e. the charged voltage of capacitor. Vt is the capacitor voltage at time t. R0 is the resistance of charging circuit.
In the extreme case, the initial voltage of ultra_C is always zero at the beginning of charge, so V0 =0. It can be considered that the charging process is over when the value of
V L dIL
L dt
(19)
ultra_C is up to 95% of the DClink voltage. Hence,
where
Vt 0.95
1
1 2D
Vdc
(28)
dIL IL x1 %IL
dt t DT / n
(20)
(21)
While the final value of charge is equal to DClink voltage,
so
Hence, it can be deduced that
V1
1
1 2D
Vdc
(29)
L VLdt VCt VC DT
dIL IL x1 %nIL
(22)
The charging of ultra_C should be completed during the shootthrough time in a switch cycle. Therefore the charging time can be expressed as
where, x1% is the percentage of current ripple. n is the number
t DT
From formula (27) to (30), we can get
(30)
V
/V
c
50
0
U /V
100
o
50
ultra _ C
DT
(1 ln 0.05) R0
(31)
0
/V
60
30
V
g
0
30
60
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
SIMULATION ANALYSIS
To verify the aforementioned theoretical results, a
(b)
t/s
simulation example for isolated shootthrough Zsource inverter is given in openloop mode under the condition of SVPWM. The corresponding parameters are shown in Tab. 1.
Tab.1. Parameters of isolated shootthrough Zsource inverter. (where,
R1 is the protect resistance of ultra_C. R is the load. fs is the frequency of switch)
Parameter Value 
Parameter Value 
Vdc/V 35 L/mH 19.2 C/F 700 Ultra_C/F 470 
R1/ 0.1 R/ 10 fs/kHz 10 M 0.84 
The comparison of waveforms between ISTZSI and traditional Zsource inverter with same output voltage can be seen in Fig. 8. Under the condition of third harmonic injection, the same simulation parameters as ISTZSI are chosen by traditional Zsource inverter. When shootthrough duty ratio D=20%, the simulation waveforms of ISTZSIs Zsource capacitor voltage (Vc), DClink voltage (Uo) and inverter output phase voltage (Vg) are shown in Fig. 8(a). It can be seen that capacitor voltage (Vc) is 10.5V, the DClink voltage (Uo) is 57V and inverter output phase voltage (Vg) is 32.7V, respectively, which coincide well with the theoretical value. The negative voltage of capacitor represents that Zsource capacitor voltage is negative in upper and positive in lower, as shown in Fi. 3. Comparing Fig. 8(a) with Fig. 8(b), the DClink voltage of traditional Zsource inverter is 66.5V. According to Uo=Vdc/(12D), we can get D=24%, i.e. the needed shootthrough duty ratio of ISTZSI is smaller than the traditional one in the same inverter output. The capacitor voltage of traditional Zsource network is 50V and the maximum fluctuation at start can achieve 65V, which is much higher than ISTZSIs.
V /V
c
20
0
20
40
U /V
o
90
60
30
0
V
g /V
50
0
50
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
t /s
(a)
Fig.8. Comparison of waveforms between two topologies with the same output voltage. (a) ISTZSI; (b) Traditional Zsource inverter.
c /V
20
V
0
20
40
o /V
100
U
50
0
g /V
100
V
0
100
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
t/s
Fig.9. Simulation waveforms when D=30%.
20
c /V
0
U /V V
20
40
60
150
o
100
50
0
g /V
100
V
0
100
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
t /s
Fig.10. Simulation waveforms when D=35%.
If continue increasing the shootthrough duty ratio of ISTZSI, it is easy to get Fig. 9 and Fig. 10 which show the simulation waveforms of the capacitor voltage of Zsource network (Vc), the DClink voltage (Uo) and threephase voltage of inverter output (Vg) when D=30% and D=35%, respectively.
The comparison between simulation results and theoretical values of Vc, Uo and Vg in different shootthrough duty ratio can be seen in Tab. 2.
Tab.2. Comparison between simulation results and theoretical values in different shootthrough duty ratio
D Vc/V Uo/V Vg/V 20% Theoretical value 11.67 58.33 33.68
Simulation value 10.5 57 32.7
30% Theoretical value 26.25 87.5 50.52
Simulation value 24 84.5 48.5
35% Theoretical value 40.83 116.67 67.36
Simulation value 38 112 64
It concludes, from Tab.2 and Fig. 8 to Fig. 10, the increasing shootthrough duty ratio results in the ascending of Zsource capacitor voltage, yet it is much lower than the capacitor voltage of traditional Zsource inverter. The DClink voltage slowly increases, which becomes steady state in final. As boost factor and modulation index has realized decoupling
control, the phase voltage of invert output is 3 / 3 times of DClink voltage, which is consistent with the actual
simulation results. Due to the impact of switching loss and the parasitic parameters, simulation value is lower than theoretical one after reaching steady state. However, compared with traditional Zsource inverter, the boost capability of ISTZSI has improved a lot.
. EXPERIMENTAL VERIFICATION
To further verify the correctness of isolated shootthrough Zsource inverter, a testing hardware circuit has been constructed. The same parameters as simulation are chosen to test the steady output waveforms of the DClink voltage (Vo), the capacitor voltage of Zsource network (Vc) and the phase voltage of inverter output (Vg) when D=20%, D=30% and D=35%. Figs. 1112 and 13 correspond to experimental results, respectively.
(b)
(a)
(a)
(b)
(c)
Fig.11 Experimental results when D=20%. (a) DClink voltage; (b) Capacitor voltage stress; (c) Phase voltage of inverter output.
(c)
Fig.12. Experimental results when D=30%. (a) DClink voltage; (b) Capacitor voltage stress; (c) Phase voltage of inverter output
(a)
(b)
(c)
Fig.13. Experimental results when D=35%. (a) DClink voltage; (b) Capacitor voltage stress; (c) Phase voltage of inverter output
It can be seen from Fig. 11 to Fig. 13, when D=20%, D=30% and D=35%, the DClink voltage (Uo) is 56.6V, 84.4V and 110V, and capacitor voltage of impedance network (Vc) is 10.5V, 23.4V and 37.2V, and the peak value of inverter output phase voltage is 64.8V, 96.0V and 127V, respectively. All experimental curves have a good agreement with the previous simulation and theoretical analysis results.
. CONCLUSIONS
This paper has presented a novel Zsource inverter topology: isolated shootthrough Zsource inverter. The proposed inverter avoids the problem of serious startingup inrush current and high capacitor voltage stress in traditional Zsource inverter. To overcome the mutual restriction of boost factor and modulation index, the control of shootthrough duty ratio is separated from the topology, which makes shootthrough duty ratio achieve flexibility control from 0 to 50% without the limitation of zero vectors in SPWM or SVPWM algorithm and improves the boost capability greatly. Therefore, the proposed inverter could be widely used in low input occasion. Both the simulation and experimental results demonstrate its feasibility and effectiveness.
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