Modulation Method For A Dual-Buck Inverter Which Minimizes Zero Crossing Distortion And Circulating Current

Modulation Method For A Dual-Buck Inverter Which Minimizes Zero Crossing Distortion And Circulating Current
IRJPEE
Modulation Method For A Dual-Buck Inverter Which
Minimizes Zero Crossing Distortion And Circulating
Current
1Doddabasappa N* and 2Sanjay Lakshminarayan
1School of EEE, Reva University, Bangalore, India
2School of EEE, Ramaiah Institute of Technology, Bangalore, India
This paper presents a low cost dual buck inverter with high efficiency without any reverse
recovery issues for the MOSFET. It is ideal for super junction Power MOSFET application, when
used for bipolar PWM in case of PV applications, the duty cycle will be around 50% at zero
crossing and there will be distortion of current. As low cost, high reliability, high efficiency and
low output distortion of alternating current are very crucial in the design of inverters. This paper
proposes a novel method for modulation of bipolar PWM, which will minimize the current
distortion at zero crossing and achieves low circulating current. The simulation results validate
the method.
Keywords: Inverter, High Efficiency, Zero Crossing Distortion.
INTRODUCTION
With the advances in the renewable sector, there is a large
potential for renewable sources like wind solar, PV etc.
There is a great growth in PV sector with the intervention of
government policies, subsidies for using PV for generation
of electricity. There are many products using PV, one of
them is the topology without a transformer less to eliminate
the 50Hz transformer to achieve high efficiency and low
cost. The inverters without a transformer are low cost as
they use silicon device and have high efficiency as they
have less switches and use super junction Power
MOSFETs
The efficiency of the inverter can be improved by many
ways, such as by designing a new inverter circuit, by using
advanced devices like super junction power MOSFET, SiC
devices. These devices are widely used in power factor
correction circuits, battery chargers etc because of their low
conduction and switching losses. In this paper a dual buck
inverter without reverse recovery problem for MOSFETs is
introduced and it is also useful for the application in super
junction power MOSFET. (S. K. Chowdhury and M. A.
Razzak, 2014; Xiangdong Zong, 2011; Frede Blaabjerg et
al., 2004; Zhilei Yao et al., 2009; Soeren Baekhoej Kjaer et
al., 2005; Tamas Kerekes et al., 2009).
In case of dual buck PV inverter, bipolar PWM method is
used. This is because of the leakage current in transformer
less PV inverters. This paper is organized as (A)-
Introduction of high reliability and efficiency dual-buck full
bridge transformer less inverter. This uses a conventional
bipolar PWM method (CB-PWM).
(B)-Introduces an improved bipolar PWM with distortion at
zero crossing current, but solves the inverter circulating
current and also the leakage current of the PV panel. (IB-
PWM) (C)-Takes care of the distortion at zero crossing
current, by introducing a novel nonlinear bipolar PWM. (D)-
All the inverter topologies are analysed and compared
based on the THD values of each topology.
*Corresponding author: Doddabasappa N, School of
EEE, Reva University, Bangalore, India. Email:
ndg.eee28@gmail.com
International Research Journal of Power and Energy Engineering
Vol. 3(2), pp. 061-066, November, 2017. © www.premierpublishers.org, ISSN: 3254-1213x
Conference Paper

Doddabasappa and Lakshminarayan 062
DUAL BUCK INVERTER
The Fig.1 shows a Dual-buck full-bridge PV inverter. It is
very reliable as it will eliminate the reverse recovery
problems, dead time and shoot-through. This dual buck full
bridge inverter is proposed so that the properties and
advantages of power MOSFETs can be utilized. The size
of the passive components, resistive nature of the
conduction voltage drops, lower switching losses, instant
switching speed are all the benefits of power MOSFETs. As
seen from Fig.1, it has unidirectional inductor current and
works for positive and negative half cycle current. But it has
a drawback that four separate inductors are required. (B. F.
Chen, 2012; Henry Benedict Massawe et al., 2013; Zhilei
Yao et al., 2009; Ahmed Abdalrahman et al., 2012; S. K.
Chowdhury and M. A. Razzak, 2013; Marco Liserre et al.,
2005).
Fig.1: High efficiency transformer-less dual-buck full
bridge PV inverter
SIMULATION AND RESULTS
A-TRADITIONAL BIPOLAR PWM METHOD
The block diagram of the conventional bipolar PWM (CB-
PWM) is shown in Fig.2.In order that the proposed dual
buck full bridge inverter must adopt to the CB-PWM for
applications of PV,the dual to common mode voltage from
CB-PWM is always fixed to Vdc/2.
Fig. 2: Conventional bipolar PWM (CB-PWM) method
Fig. 3(a) and 3(b) show the operating mode and equivalent
circuit for the CB-PWM when switches S1 and S4 are
turned ON and when switches S2 and S3 are turned ON
respectively.
When S1 and S4 are conducting, freewheeling current of
L2 and L3 would circle back to DC bus through D1 and D4.
Similarly, When S2 and S3 are conducting, freewheeling
current of L1 and L4 flow back to DC bus through D2 and
D3, it leads to a circulating current and decrease efficiency.
The simulation results for dual buck inverter using CB-
PWM are shown in Fig.4 without filter. The highlighted
circle in the waveform indicates the presence of circulating
current in the negative half cycle.
Fig. 3(a): Operating mode and equivalent circuit during S1
and S4 turn on.
Fig. 3(b): Operating mode and equivalent circuit during S2
and S3 turn on.

Int. Res. J. Power Energy Engin. 063
Fig. 4: Simulation results with CB-PWM method without
Filter
B. IMPROVED BIPOLAR PWM METHOD
The improved bipolar PWM (IB-PWM) is shown in
Fig.5.Here the current polarity is taken into consideration
(Frede Blaabjerg et al., 2004; Zhilei Yao et al., 2005;
Soeren Baekhoej Kjaer et al., 2005) and each inductor
operates under both positive and negative half cycle
currents. Hence PWM is fed to the switches considering the
polarity.
Fig. 5: Improved bipolar PWM (IB-PWM) method
The Fig.6 (a),6 (b), 6(c), 6 (d) all show the operating model
and equivalent circuits for the IB-PWM .During the positive
half cycle, the switches S1 and S4 will be switching and the
freewheeling current will pass through D2 and D3. During
the negative half cycle, the switches S2 and S3 will be
switching and the freewheeling current will pass through D1
and D4.
The simulation results for IB-PWM are shown in Fig.7.We
see that the circulating current is eliminated and switching
power loss is also reduced. But, the current in zero-
crossing region would have an obvious distortion, which
originates from the discontinuous-conduction mode (DCM)
condition.
Fig.6 (a) Charging Interval during Positive Half Cycle
Fig.6 (b) Freewheeling Interval during Positive Half Cycle
Fig.6 (c) Charging Interval during Negative Half Cycle
Fig.6 (d) Freewheeling Interval during Negative Half
Cycle

Fig. 7: Simulation results with IB-PWM method
The Volt-second balance equation on inductor is,
When inductor current is under continuous-conduction
mode (CCM) condition, TB-PWM method and IB-PWM
method have same Volt-second balance equation, which is
[5],
(2)
However, during DCM condition at unity PF operation, The
Volt-second equation for IB-PWM method will be
So, if Vref = 0, switching on & off time would be:
(4)
And ripple current & average current would be:
(5)
Assume Vref & output current Iac are in the phase, when
current is positive and S1-S4 are conducting, the
relationship between the duty cycle (D) and Vref is:
When Vre = 0, D will be 50%, it will let current in boundary
mode [ibm] which depends on current ripple.
When current is negative and S2-S3 are conducting, the
relationship between the duty cycle (D) and Vref is:
When Vref = 0, D will be 50%, it will let current in boundary
mode [-ibm ] which depends on the current ripple.
The current changes from +ibm and –ibm at the zero
crossing over of the current, the duty cycle will be 50%.
Due to the distortions at the zero crossing over the THD of
the output AC current is 29.66%. These simulation results
are shown in Fig.7.
C. NOVEL PWM MODULATION METHOD FOR
CURRENT ZERO-CROSSING
The proposed method combines the advantages of both
CB-PWM without distortion and IB-PWM with high
efficiency. The Fig.8 shows the proposed method, which
uses the unipolar method.
Fig. 8: Proposed Novel PWM Modulation Method in Dual
Buck Inverter
In this paper a new modulation method is proposed to
provide a duty cycle of 0% at zero cross over. This in turn
makes the inductor current to enter DCM instead of
boundary mode.
Though the PWM gating pattern is bipolar, but this
proposed modulation method will adopt a unipolar method.
This method makes the duty cycle zero at the point of zero
cross over, which improves the distortion in current at zero
cross over.

Int. Res. J. Power Energy Engin. 065
Fig. 9: Simulation Results with Proposed Bipolar PWM
Method in Dual Buck Inverter
THD ANALYSIS AND COMPARISON FOR ALL THE
INVERTER TOPOLOGIES
THD calculations can be obtained from SIMULINK. The
switching pattern that is used in this project for all the
inverters topologies are harmonic elimination method.
Where,
V1= Fundamental Voltage magnitude
V2 = Magnitude of 2nd Harmonic
V3 = Magnitude of 3rd Harmonic
V4 = Magnitude of 4th Harmonic
Vn = Magnitude of nth Harmonic
TABLE 1: Total Harmonic Distortion Analysis
METHODS TOTAL HARMONIC DISTORTION
IB-PWM 29.66%
NOVEL-PWM 3.77%
Fig.10: THD comparison between Novel PWM method
And IB-PWM method
CONCLUSION
Nowadays the inverters are without transformers with high
efficiency and less cost. This paper introduces a dual buck
inverter to reduce the distortion and circulation current at
zero cross over.
Detail discussion and analysis point out:
• In case of Traditional bipolar PWM technique, there
will be circulating current and current distortion will not
be present.
• With the Improved bipolar PWM technique, there will
be no circulating current, but significant current
distortion will be present.
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Accepted 23 October, 2017
Citation: Doddabasappa N and Lakshminarayan S
(2017). Study H and Estimation of Energy Transfer to the
Active DC-Link Capacitor Due to Harmonic Current.
International Research Journal of Power and Energy
Engineering, 3(2): 061-066.
Copyright: © 2017. Doddabasappa and Lakshminarayan.
This is an open-access article distributed under the terms
of the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original author and source are
cited.

Modulation Method For A Dual-Buck Inverter Which Minimizes Zero Crossing Distortion And Circulating Current

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