Scholarly article on topic 'Individual Step-up Converter with Active Recovery Stage for High Efficiency Conversion of Photovoltaic Energy'

Individual Step-up Converter with Active Recovery Stage for High Efficiency Conversion of Photovoltaic Energy Academic research paper on "Materials engineering"

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Energy Procedia
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{"DC-DC converters" / "renewable energy" / "MCB-RS boost" / "HVDC Bus" / Smart-grids.}

Abstract of research paper on Materials engineering, author of scientific article — P. Petit, M. Aillerie, J.P. Sawicki, T.V. Nguyen, J.P. Charles

Abstract The incessant involving of smart grids associated with renewable energies increases the demand in high efficiency converters. So it is important to consider all possible improvements involving reliability and simple structures. In this study we demonstrate the improvement potentiality of a DC/DC step-up converter, connected to a HVDC bus, based on a MCB-RS model (Magnetically Coupled Boost with Recovery Stage). We suggest a new possibility of improvement of the converter by mastering its active switch allowing an easier control of the internal voltage inherent to the recovery stage. The suggested solution permits a saving of the internal energy and prevents the over heating of certain components, like recovery diode. In the MCB-RS converter, the losses are for a main part dissipated in the recovery diode, representing a non-negligible amount of power. Thus, we propose to save this energy by the insertion in the recovery circuit of an additional parallel switch. This solution offers an additional advantage by making possible the flow back of a certain amount of energy by keeping in the on state the parallel switch. Finally we compare the simulated results with a real system and validate this new performing structure.

Academic research paper on topic "Individual Step-up Converter with Active Recovery Stage for High Efficiency Conversion of Photovoltaic Energy"

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Energy Procedia 50 (2014) 479 - 487

The International Conference on Technologies and Materials for Renewable Energy, Environment

and Sustainability, TMREES14

Individual step-up converter with active recovery stage for high efficiency conversion of photovoltaic energy

P. Petit, M. Aillerie, J. P. Sawicki, T. V. Nguyen, and J. P. Charles

Lorraine University, LMOPS-EA 4423, 57070 Metz, France Supelec, LMOPS, 57070 Metz, France

Abstract

The incessant involving of smart grids associated with renewable energies increases the demand in high efficiency converters. So it is important to consider all possible improvements involving reliability and simple structures. In this study we demonstrate the improvement potentiality of a DC/DC step-up converter, connected to a HVDC bus, based on a MCB-RS model (Magnetically Coupled Boost with Recovery Stage). We suggest a new possibility of improvement of the converter by mastering its active switch allowing an easier control of the internal voltage inherent to the recovery stage. The suggested solution permits a saving of the internal energy and prevents the over heating of certain components, like recovery diode. In the MCB-RS converter, the losses are for a main part dissipated in the recovery diode, representing a non-negligible amount of power. Thus, we propose to save this energy by the insertion in the recovery circuit of an additional parallel switch. This solution offers an additional advantage by making possible the flow back of a certain amount of energy by keeping in the on state the parallel switch. Finally we compare the simulated results with a real system and validate this new performing structure.

© 2014ElsevierLtd.Thisisanopenaccessarticle under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) Keywords: DC-DC converters, renewable energy, MCB-RS boost, HVDC Bus, Smart-grids.

1. Introduction

The Magnetically Coupled Boost with Recovery Stage (MCB-RS) was already presented in literature as an interesting solution to improve the global efficiency of the basic step-up converter [1-3]. Such a converter, individually associated to a PV panel, is intended to deliver all the extracted energy thanks to a convenient tracker routine, toward a high voltage DC bus (HVDC bus) with a minimal rate of losses [4, 5].

*Also at Lorraine University, IUT de Thionville-Yutz, 57970 Yutz; Tel.: +33-387-378-565; fax: +33-387-378-559 E-mail address:pierre.petit@univ-lorraine.fr, aillerie@metz.supelec.fr, vinhnt@qui.edu.vn

1876-6102 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) doi: 10.1016/j.egypro.2014.06.058

Nomenclature

HVDC High voltage direct current MCB-RS Magnetically Coupled Boost with Recovery Stage MCB-RS Magnetically Coupled Boost with assisted Recovery Stage PLC Power Line Communication

We report in Fig.1 the global architecture of such a distributed installation involving several PV panels connected together by the HVDC bus [6]. It is to be noted that all the converters can communicate information and data by using the energy transport HVDC bus as a physical data way support PLC (Power Line Communication) constituting what is commonly named a smart systems [7, 8]. Some advantages of such distributed energy production system used as renewable energy generator can be underlined. We can cite the possibility to take into account various types of energy sources, sources having various behaviors or excited under different conditions as in case of shading or aging in a photovoltaic installation. We can also note other advantages as the simplicity of the command and the relative few electronic active components involved for the realization. It is well known that in a classical boost, a high output-input voltage ratio presents an important duty cycle dissymmetry implying some specific problems in the energy transfer [1, 9, 10]. As a consequence, the MCB converter presents an interesting performing compared to the classic basic step-up [6, 12].

Fig.1. Illustration of a global installation involving several PV panels to a HVDC bus via DC/DC converters.

In a previous paper [13] it was demonstrated that the recovered energy presents some uncontrollable increases of the recovered energy as function of the voltages applied to the converter for some specific values of the duty cycle driving the switch and for some values of the coupling coefficient of the transformer. For these raisons, this basic MCB-RS converter is not fully satisfactory in its basic form. To increase the global efficiency of the MCB-RS converter, we show in the present contribution a solution based on the realization of a smart energy recovery stage that meeting this objective without excessively increasing the structure architecture and its cost.

2. The basic MCB-RS converter

Some studies [1, 11-14] describe very precisely the global behavior of a MCB DC/DC converter intended to manage the energy provided by a photovoltaic panel. A first improvement yielding to the MCB-RS, i.e. a boost MCB with recovery stage is reported in Fig. 2 [4]. It is constituted by Li-Lki-Mi-D0-C, on which a secondary boost voltage has been added constituted by L2-Lk2-D1. L1 and L2 are the two main inductances whereas Lk1 and Lk2 are the leakage inductances.

Lkl _TY_

Vds =r

L2 , /YYT\

V20 -r~ Vout

Fig.2. The MCB-RS boot.

This configuration is very well adapted for high Vout/Vin ratio allowing values around 10 and above, depending only of the turns ratio, i.e. m=Vout/Vin lying the secondary and primary windings of the transformer represented by (L1-Lk1)-(L2-LK2) in Fig.2. In an optimal configuration, the duty cycle applied to the switch M1 is very close to an ideal value of '/2, which is the best value for a fly-back converter like the one studied here. As a consequence, one of the main advantages of such a structure is the low voltage Vds applied to the MOSFET switch M1. Nevertheless a complete analysis of the behavior of this converter has been described very precisely in a previous study [13] and lets appear that this voltage Vds depends directly of the voltage on the capacitor Vc which is a non constant value depending on several external parameters. It has been demonstrated that Vc is lied to the input and output voltages, Vin and Vout, respectively but also that Vc depends of the coupling factor of the two inductors k. The relations between these various parameters are given by the following set of equations [13].

(m+fc)-( ^+fc) a

xm( 1-fc2) Vin

(1-œ) kV

x = £ -x) O"^)-*(à+") P]

«m (1-k2) Vin( 1+x) £ = [(1-a)AV-k m( 1 - k2)Vin{ 1 + x)]

(2) (3)

To precisely predict the behavior of the converter, a preliminary measure of the transformer-coupling coefficient was necessary. The method used for the determination of the inductance of the two inductors consists in two sequential measurements based on the consideration of the additive and subtractive contribution of the magnetic flux

of the inductor windings connected in series. So we obtained two values of inductors, named respectively L12M and L12m defined by

L12M = Lt + L2 + 2M (4)

L12m = Li + L2 - 2M (5)

Thus, we can deduce the value of M representing the coupling inductor as

M = Llm~Ll2m (6)

In the realization of our experiments L12M = 168 mH and L12m = 75 mH. Thus, we deduce the value for M = 23.25 mH representing a coupling factor k given by the classical formula:

k = = 0.981 (7)

This high value of k corresponds to a high quality of transformer and allows a good performance of the converter for duty cycle switching up to a = 0.8. For upper values of a, it had been demonstrated [13] that the Vc voltage increases rapidly applying as a consequence a high voltage on the switch M1. It results an unfavourable working condition for the switch M1. Thus, this preliminary study points out the fact that high duty cycle of the switching control of M1 and/or low values of the coupling factor makes the voltage quite unstable and a decrease of the efficiency of the converter. Moreover, this drawback of the basic MCB-RS converter is not alone as it has been observed that some additional losses also occur in the intermediate diode D0 [13].

3. The MCB-ARS converter

Following the preliminary characterizations, as presented above, of the basic MCB-RS converter and their conclusions, a specific analysis is done to find a clever solution for a better control of the Vc voltage than in the basic system. So it is evident that a controlled switch in place of the recovery diode can resolve the both drawbacks of the basic MCB-RS converter, as reported above. This solution yields to the realization of a magnetic coupled boost converter with assisted recovery stage, that we named MCB-ARS converter.

—I— V out

Fig.3. The MCB-ARS converter with a switch control added on the recovering diode D0.

Thus, to build this new MCB-ARS converter, we suggest a modified architecture based on the basic MCB-RS converter to insure a new function allowing the shunt of the diode D0 by a MOSFET during some part of the cycle, as detailed below. The corresponding schema is reported in Fig.4 where a MOSFET M2 is added in parallel to D0. This parallel solution constitutes a functioning security considering some possible switching delays of M2. Within

this architecture, during the time when Mi is switched off, the MOSFET M2 flows back all the exceeding energy stores in the C capacitor down to the Vin generator via the inductor L1.

In order to insure the viability of such a solution, we have simulated using Orcad software environment a configuration shown in Fig. 3. In Fig. 4 we see that two switches, S1 and S2, replace the two MOSFETs M1 and M2 of Fig. 4, respectively. To insure that the behavior of S1 is the same as M1, it has been necessary to add in parallel an equivalent diode representing the natural intrinsic substrate diode existing in a MOSFET. The Rds-on has been adjusted to 1 mQ and the Rds-off to 1MQ. For simulation facilities, in Fig. 4, the inductor L3 replaces the previous L2 inductor in Fig. 3 but takes the same place and role. The HVDC bus is represented by the voltage generator V3, which imposes a constant value to the converter output. The simulation has been driven for various values of Vout and duty cycle, but the coupling factor was always kept to a constant value, corresponding to that of the transformer of the laboratory converter prototype.

K _Linear

COUPLING = 0.98

V 1 24V dc

R2 0.031 LI

V2 = 2 V TD = 0 TR = In TF = In PW = 50u PER = lOOu

L3 R1 D1

Fig.4. The MCB-ARS converter used for simulation with Orcad software.

4. Simulation of the MCB-ARS converter

The chronograms of the currents in Li and L2 as obtained by simulation are represented in Fig. 5. We observe in a first step that the current inside L1 decreases down with a linear slope, and then takes a negative value confirming the restitution of the excess stored energy in the capacitor C. At the same time, the energy transfer observable in L3 operates during all the half period. That means that the energy is transferred in good conditions with minimum stress in the output diode D1.

Considering the voltages shapes, we observe a constant low value for the Vc voltage. That implies a low voltage applied to the MOSFET M1 (to S1 during simulation - see comments above and Fig.5). The voltage on the anode of D1 is characterized by a quasi-symmetrical square oscillation and presents some pseudo oscillations at the D1 switching off. As a consequence, the total reverse voltage applied on D1 is two times the output peak-voltage, i.e. 240V in our simulation. That means also that the diode D1 must be chosen to have a correct maintain reverse voltage adapted to the output converter voltage.

For the full evaluation of the behavior of the converter when the input voltage fluctuates, we perform some additional simulations. In Figs. 6, we report the dependence of the voltage with the duty cycle when the converter is connected to a resistive load. We clearly observe a nonlinear dependency of the recovery voltage Vc and output voltage Vout versus the duty cycle. It is to be noted that the typical value for the duty cycle is 50%, and for this particular value, the Vc voltage is close to Vc = 2 Vin, and the output voltage Vout = (m + 1)Vin.

^-Vt-R

: | ! !

1 1

¡5 3:T)~

J 1—r

V( LI 2) -J

A 1 —--i

r ✓ ! V

u | J

Fig.5. Chronograms: a) Current in Li and L3, b) voltages of the MCB-ARS in red VDl-anode, in dash blue Vds, continuous Vc voltage.

Fig.6. Dependence of voltages with the duty cycle, on resistive load.

5. Realization and characterizations of the MCB-ARS converter

Fig.7. Execution schema of the MCB-ARS converter.

In Figs. 7 and 8 we show the schematic and the corresponding picture of the laboratory prototype used for the tests.

Fig.8. Pictures of the testing MCB-ARS converter prototype. a) View to the board and ICD programmer b) view to the oscilloscope and duty

cycle switching command.

Fig.9. (a) Comparison between simulated and experimental variation voltage of Vc versus the duty cycle in the MCB-ARS converter; (b) Measurements of Pout and global efficiency as a function of Duty cycle.

A PIC Demo2 board, involving a PIC16F877 and the debugging circuitry, controls the system. It generates an adjustable PWM signal at the CCP1 output. A specific delay control is implemented on the test board including also the MOSFET driver. The structure looks like a half bridge converter, but our system works at lower voltage than common Push-Pull devices they are used to. In the same way, the conductive times are especially adjusted by a NOR gates RC delay hardware timer in order to avoid a simultaneous conduction in the two MOSFETs.

We managed experiments with various values for the output voltage and duty cycle and with a fixed resistive load of 141Q. The input generator consists in two 12V Pb batteries connected in series. The input current was limited to 10A respecting the maximum allowable current for the ampere meter and protection fuses. Figs. 8, also presents a phase of test in which the variations of Vc and Vout are measured as a function of the duty cycle. The resistive load of 141 Q was used to extract the converted power. We can also see in Fig.9.a the extremely high increases of Vc for duty cycle upper than about 70%.

Another simulation was made to compare the variation of Vc versus the duty cycle with the real conditions i.e. on a resistive load of 141Q. The simulation results Fig.9.b red shape is superposed with the real measurements in black. The experiments must be stopped when the duty cycle reached 60% because the input current overloaded the acceptable limit of the input current of 10A.

A resistive load of 141Q on the output of the converter allows a theoretical power of 150W. It is what we observe on the Fig.9.b for a 60% duty cycle. Because of a non-efficient cross section of the transformer windings, we see that the global efficiency reaches 95% and then falls down for increasing output power.

6. Conclusion

New performing converter MCB-RS with recovery assistance, the MCB-ARS converter is an attractive solution to improve the global efficiency to control and stabilize the recovery voltage inside the converter. Our experiments show that two components are stressed and implicated in the losses, the transformer and the MOSFFET Mi, that can be easily resolved by adequate dimensioning. The global circuitry remains still very simple to drive with classical and cheap devices and can be easily integrated in a small compact set. A nest goal can be defined now for performing the global efficiency by dimensioning specifically the transformer for higher output power idem the MOSFET M1.

Acknowledgements

The authors gratefully acknowledge the Industrial Engineering and Maintenance (GIM) department, particularly its chief

Dr. Yves Gillet, of the University Institute of Technical (IUT) of Thionville-Yutz (France) particularly its Director, Dr. Patrick Klein, for the financially support and for the facilities offered during their researches.

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