4th Power Electronics, Drive Systems & Technologies Conference (PEDSTC2013), Feb l3-14, 2013, Tehran, Iran

Dual-Input Single-Output DC-DC-AC Converter

Mehdi Azizi, Mustafa Mohamadian, Reza Beiranvand, AmirHossein Rajaei Faculty of Electrical and Computer Engineering

Tarbiat Modares University Tehran, Iran

m-azizi@modares.ac.ir , mohamadian@modares.ac.ir, Beiranvand@modares.ac.ir, amirhrajaei@gmail.com Abstract-In this study, a new multi-input converter with two dc In order to generate a high reliable electric energy, the inputs and one single phase ac output is proposed, which dc/dc system requires an energy saving part (e.g. battery pack), converters are able to operate in buck and boost modes. Both mainly because of requirement to meet load variation. It has dc/dc and dc/ac part works simultaneously with minimum several advantages such as improving reliability, quality and limitation on the operating function of the other parts. The dynamic of output power. It also and decrease manufacturing propos

.ed converter structure and o�erating princip�e in all cost of system as the system can be designed according to

operatIOn modes are presented and dIscussed. A carner-based average power consumption instead of peak power [12 \3]. PWM switching method is also proposed for the converter. The

'

proposed converter is simulated and the results are illustrated in Instead of employing several converters, a multi input the paper. The simulation results verified operation and converter can be considered in a multi energy source system. theoretical analysis of the system. Interchangeable input is also desirable so the converter can be

Keywords- multi input dc/dc converter, renewable energy system, inverter, pulse width modulation, dc/ac converter

I. INTRODUCTION

According to a report of U.S. department of energy which is published in 2006, average increasing rate of global energy consumption will be 2% from 2003 to 2030, while electric energy demand is even more and will be about 2.7% [1]. This magnifies the importance of renewable energy resources and justifies day to day increase for the investment on this type of electric sources [2,3]. Since about 40% of people round the world live in areas which is difficult and costly to install energy transmission lines, small stand-alone electric energy generators employing renewable energy resources is one of the best choices for providing energy [4].

Terminal voltage of renewable energy sources varies inherently, therefore employing voltage converters as the interface between the source and consumer/grid is unavoidable to control the output voltage [3]. Fuel cells are good examples as the voltage variation is 0.5-1 p.u. and has a low level output voltage which is not applicable. A dc/dc converter should be used to improve the dc voltage and a dc/ac inverter is required to change the type of voltage from dc to ac [5].

Similar to fuel cell, grid connection of solar systems requires a dc/dc voltage converter because of low voltage output [6]. Although series connection solar cells can step up the panel terminal voltage but increase the shadow effect which is undesirable [7,8].

Dependency of some types of renewable energy resources such as wind and solar to climate, causes tendency for combining various type of sources to provide a high reliable and high quality power feeding a load [9]. Wind and solar are good choices for combination as high speed wind are mostly available in cloudy weather and at nights, while calm days are sunny and solar energy is available. Therefore, such system improve reliability compare to systems employing one of the sources [10,11].

978-1-4673-4484-5/13/$31.00 ©2013 IEEE 315

used disregarding the type of available energy sources [13]. In some cases, bidirectional power flow is also necessary (e.g. battery), therefore a bidirectional multi input converter is more suitable [15,16].

In this paper, a novel dual input topology is proposed. The operating principles are described in detail. The topology is based on using three switche leg which has been used as a dual output inverter and ac/ac inverter [17].

This paper is organized as follows. The proposed system and operating principles are described in section II. Boost, buck and inverter operation modes of the converter are explained in section III, IV, and V respectively. In section VI, power balancing of input systems is explained. The results of simulations for the proposed system are shown and discussed in section VII.

II. CONFIGURA nONS, SPECIFICA nONS, AND

BASIC OPERA nON PRINCIPLES OF THE PROPOSED

TWO DC INPUT AND ONE AC OUTPUT CONVERTER

The main structure of the proposed dual input converter is shown in fig. 1. One of the inputs is connected to a battery pack as energy saving system and the other is connected to a dc voltage terminal of a renewable energy source. As shown in Fig.l ,lower output terminals of the legs are assumed to be dc voltage terminals, although the upper terminals can be also used. As the inputs are similar, according to the structure, they are interchangeable and bidirectional. Battery connected terminal has two modes of operation: 1- buck operation during battery charging, 2- boost operation, during battery discharging. The other dc voltage terminal works as a boost converter.

According to the topology, two lower switches (S3 and S6) and four upper diodes (0)02 and 0405) of the legs are used for boosting voltage, also two upper switches (SIS2 and S4SS) and

+

C

Vne

Figure 1. Proposed Dual-Input Single-Output DC-DC-AC converter

two lower diodes (D3 and D6) are employed for buck

operation. All switches and diodes are required for ac/dc

voltage inverting. The limitation imposed on switching

according two three operation modes, and how to control the converter to minimize the limitation, are described in the

following.

III. BOOST OPERATION OF THE PROPOSED TWO INPUT

CONVERTER

As shown in Fig. 1. While S3 and S6 are switched on, the respective inductors would be charging by V) and V z and the inverter is able two apply any desired voltage vector at the output terminal similar to a conventional full bridge single phase inverter. It should be considered that the inductor current passes through S3/S6, while the state of other switches are determined according to the required output voltage vector. The ON state of SiS6 last for O)Ts and OzTs for V) and Vz voltages respectively in a switching period.

Turning off S3/S6 causes the inductor currents flow through D],DzID4,Ds and output DC bus capacitor. In this time interval the zero vector is applied to the load. The time interval of this operating mode is (l-DI)TS and (l-Dz)TS for S3 and S6 respectively.

Normally Two input dc voltages are not the same. Therefore two other modes are defined: I) S3 is on and S6 is off. 2) S6 is on and S3 is off. The simplest way to control the inverter in these modes is applying the zero vectors to the load. In this paper, it is assumed that SiS6 are turned on simultaneously and turned off considering the required duty cycle.

Similar to conventional boost converter, voltage transfer function is given as,

V DC -- --

VI I - DI (1)

V DC 1

V2 I - D

2

Ignoring the converter losses, (2) is applicable to converter,

316

(l-DJ)IJ (l-D2)I2

C

+

VDC

Figure 2. Equivalent circuit ofthe proposed converter when both inputs are working in boost mode.

where, II and Iz are average source currents and 10 is the average current of converter dc bus feeding the load. Using (1) and (2) the converter is modeled as shown in Fig.2. Obviously, equal 0) and Oz result in I) and Iz to be equal.

According to above discussion, inverter can apply nonzero voltage vector only when both switches S3 and S6 are turned on and two inputs are charging the inductors. Therefore, maximum modulation index of inverter will be,

IV. BUCK OPERATION OF THE PROPOSED TWO INPUT

CONVERTER

(3)

The connected leg to energy saving element (e.g. battery) may operate in buck mode, and this happens when the battery is getting charged. Here, V) in Fig.1 is assumed to be the terminal voltage of a battery pack. The least limitation on inverting operation of the converter is achieved while the buck converter is working in continuous conduction mode (CCM). This criteria should be considered in the design procedure of converter elements. In this conditions, the buck operation can be divided to two intervals. First interval in which the current is flowing to battery , when S I and Sz are conducting for D 1 Ts and Sz is off. The inverter part is not able to apply all the voltage vectors to the output load.

In the second interval which lasts for (I-O))Ts, S3 is ON and Sl and Sz is switched on/off according to the required output voltage vector and there is no limitation for the inverter part to apply the vectors. The Inductor current of the Ll is flowing through 03.

Similar to a conventional buck converter, input to output ratio of the buck converter is,

VI = DrDC (4)

The other switching leg is working as a boost converter. In order to optimize the inverter operating mode, the buck and boost operation of the converter should be synchronized as follows: while the inductor is charging in boost part (S6 is

+

c VDC

Figure 3. Equivalent Circuit of the dc/dc converter when the battery is charging.

conducting for an interval of D2 Ts), S3 should be ON (for a

time interval of (I-D\)Ts), and when boost inductor is discharging through the output capacitor, (SI, and S2 are switched ON), S3 and S4 goes to ON state and zero vector is

applied to the load. In practical applications, duty ratios, D\ and Dz are different

because of different values of V 1 and V 2. Therefore, two other modes are defined according to the state of S3 and S6. In these operating conditions zero vector is assumed to be applied in this paper.

The available time interval for applying nonzero vector in a switching period is the minimum value of DzTs and (I-D\)Ts and the maximum modulation index is,

(5)

Input and output power balancing in the buck converter is as follows,

V. INVERTER MODE OPERATION OF THE PROPOSED

CONVERTER

All switching states of the proposed converter considering dc/dc converter operation are shown in table I.

Considering the structure of the proposed topology, the inverter is able to apply the desired voltage vectors to the load when both lower switches (S3, S6) are switched on (see Table. t). This happens in two conditions: 1) both dc/dc converters are working in boost mode (input inductors are charging) which lasts for D\Ts. 2) One of the dc/dc converters works in buck mode, and the other works in boost mode (while boost converter is charging the input inductor and lasts for D\ Ts and the buck converter does not charge the battery which lasts for (I-D\)Ts).

The following other three states are defmed:

• Both S3 and S6 are opened: The inverter can apply zero voltage vector (Table.l raw 5).

• S3 is ON and S6 is OFF: zero and negative voltage vectors are applied by the inverter (Table.l row 8-9).

• S3 is OFF and S6 is ON: zero and positive voltage vectors are applied by the inverter (Table. 1 row 6-7).

The last two operation modes are not applied while the dc input voltages and duty cycles are the same and S3 and S6 are switched on simultaneously. Different input voltages and therefore different duty cycles cause one of the lower switches to be ON while the other is switched off. In this conditions, nonnal operation of the inverter is interrupted while it is working in The same statement is true for negative half cycle while S3 and S6 are OFF and ON respectively. In this paper zero vector is assumed to be applied to the load in these operating modes. This means that nonzero voltage vectors are generated by the inverter part only when both lower switches are turned on. The above discussion are summarized in Table II.

The switching method used in this paper for the proposed

TABLE I. INVERTER SWITCHING STATES CONSIDERING DC/DC CONVERTER OPERATION.

State S, S2 S3 S. S, S" Inverter Legt Leg2 Voltage

Buck Boost Buck Boost

1 OFF ON ON OFF ON ON 0 (I-O)Ts OTs (I-O)Ts OTs

2 ON OFF ON ON OFF ON 0 (l-D)Ts DTs (l-D)Ts DTs

3 ON OFF ON OFF ON ON + (l-D)Ts DTs (l-D)Ts DTs

4 OFF ON ON ON OFF ON - (I-O)Ts OTs (I-O)Ts OTs

5 ON ON OFF ON ON OFF 0 OTs (1-0)Ts OTs (1-0)Ts

6 ON ON OFF ON OFF ON 0 DTs (l-D)Ts (l-D)Ts DTs

7 ON ON OFF OFF ON ON + DTs (l-D)Ts (l-D)Ts DTs

8 ON OFF ON ON ON OFF 0 (I-O)Ts OTs OTs (1-0)Ts

9 OFF ON ON ON ON OFF - (I-O)Ts OTs OTs (1-0)Ts

317

TABLE II. INVERTER VOLTAGE POLARITY ACCORDING TO THE STATE OF LOWER SWITCHES.

State S3 S. Inverter voltage

1-2-3-4 ON ON +/-/0

8-9 ON OFF -/0

6-7 OFF ON +/0

5 OFF OFF 0

dual input converter is analogous to carrier-based modulation

technique. Considering the duty cycles for buck and boost converters, a certain offset is added to the reference voltage

waveforms, therefore all parts (two dc/dc and one inverter) are

able to work together without any interruption as discussed before. The required offset is given by (7) while battery is

charging (buck operation) and both dc/dc converters are in boost operation mode.

Offset = max( (1- D Roost)' DRuck) battery ch arg ing (7)

Offset = max((I- D Boost!) , (1- D BooltZ)) battery disch arg ing

In this switching method, the switching commands for lower switches (related to dc/dc converters) are generated by comparing the value of -D and -(l-D) with the carrier waveform in boost and buck operating modes respectively. The logical compliment value of the comparison result are considered as the switching commands for the lower switches (S3 and S6).

The logical result of the comparison between minimum value

of -D and -(l-D) and the carrier waveform is logically ORed

with the logical result of the comparison between inverter sinusoidal reference voltage value and carrier waveform.

Using OR gate is required buck mode operation of converter and providing a current path in boost mode with RL load. If

one of the dc/dc converters is working in buck mode and the

other is working in boost mode, the interval of DITs in buck

leg and (I-Dz)Ts in boost leg should be equal and the modulation algorithm should be able to provide it. The

switching method for three switches of a leg in the proposed

-D or >--------'-1.:»- S I.

Carrier

Figure 4. Block diagram of switching command generation for three

switches of a leg in the proposed multi input converter

318

multi input converter is shown in Fig. 4.

VI. SIMULATION RESULTS

The simulation parameters are shown in table III. Both DC/DC converters are working in boost mode and the duty cycle are adjusted such that the DC linl( voltage is 200 V. Using (7) offset value of 0.12 is added to voltage reference waveform. In order to control the output ac voltage a simple two loop controller system is employed which the inner and outer loops are the current and voltage controllers respectively. [t should be mentioned that the modulation index of the inverter part is 0.8 in this operating condition.

DC bus voltage is shown in Fig.5 which demonstrates the ability of the dc/dc converter to follow the reference value. Both input currents and powers and delivered power to the load are shown in Fig.6 and Fig.7 respectively. The load voltage and current which are shown in Fig.8 and Fig.1 0 verifies the proper operation of inverter. The output voltage THD and the low order harmonics are within the acceptable range which is illustrated in Fig.9. It should be also noted that the amplitude of fundamental harmonic shown in Fig.9 is 155.6 V that demonstrates the ability of voltage controller to follow its reference value.

TABLE III. THE SYSTEM PARAMETER USED IN THE SIMULATION.

25 V

17V

110 V

60Hz

20kHz

input inductance 0.85 pH

dc link ca acitance 1 mF

Out ut ac filter inductance 5 mH

Output ac filter capacitance 60 pF

dc link voltage 200 V 210'---:==;:::::::::;:==:;::::::::;::=�-'------':''---'--_

> i 190 .:;

� 180 � U 170 �

160

0.5 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58 0.59 0.6 Time (S)

Figure 5. DC bus voltage

12

0.5 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58 0.59 0.6 Time(S)

Figure 6. Input currents

�300�------------�PL�--------------------------�

� � 250

t 200 � � 150 5-o 100 �

! 50� _____________ Q�L ___________________________ �

� OJ)

" Q >-

0.5 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58 0.59 0.6 Time(S)

Figure 7. Input and output power to the converter

g -50 ..-<

-100

-150

_200"-----"---..L.----'-----'-----'--"-----'-----'----"--� 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58 0.59 0.6

Time(S)

Figure 8. Output ac voltage

VII. CONCLUSION

In this paper a new dual DC input single AC input converter was proposed and operation principle of the converter in all operating modes (buck, boost, and dc/ac inverting) was described. As discussed in the paper, the proposed converter is able to perform buck, boost and AC conversion. The maximum modulation index is limited due to the buck and boost operation of dc/dc converter parts. This limitation is not a major concern as the maximum value of modulation index is improved by increasing the voltage deference between input and dc bus voltages. The proposed system was simulated and the results were illustrated which demonstrated the theoretical analysis and performance of the converter.

319

Fundamental (60Hz) - 155.6, TI-ID- 3.56010

0.5

°oL---���---I-��-L�sL-��-�--L---L----'· Harmonic order

E ; 0

u '"

g ..-< -2

-4

Figure 9. Harmonic spectrum of output ac voltage

0.5 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58 0.59 0.6 Time(S)

Figure 10. Output current

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