Magnetic Dual Coupled Boost with Recovery Stage DC–HVDC Converter for Renewable Energy Generator

T.V. Nguyen; M. Aillerie; P. Petit; T.K. Bui

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Energy Procedia 74 (2015) 499 - 506

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

Sustainability, TMREES15

Magnetic Dual Coupled Boost with Recovery Stage DC-HVDC Converter for Renewable Energy

Generator

T.V. Nguyen1'2'3, M. Aillerie2'3,P. Petit2,3, T.K.Bui1

1Quang Ninh University of Industry, Qui, Quang Ninh, Vietnam 2Lorraine University, LMOPS-EA 4423, 57070 Metz, France 3Supelec, LMOPS, 57070 Metz, France

Abstract

This paper presents a new high-efficiency-high-step-up based converter integrating two non-isolated secondary interleaved coupled inductors with recovery stages dedicated to smart HVDC distributed architecture in renewable energy production systems. Appropriate duty cycle ratio assumes that the two recovery stageswork with parallel charge and discharge to achieve high step-up voltage gain. Besides, the voltage stress on the main switch is reduced with a passive clamp circuit and thus, low on-state resistance Rdson of the main switch can be adopted to reduce conduction losses. In addition, the coupled inductors alleviate the reverse-recovery problem of the diode. Thus, the efficiencyof a basic DC-HVDC converter dedicated to renewable energy production can be further improved with such topology. A prototypeconverter is developed, and experimentallytested for validation.

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) Keywords:DC-DC converters, boost, high step-up voltage gain, sustainable energy system.

1. Introduction

Many topologies of DC-DC, or DC-HVDC converters dedicated to renewable energy conversion have been proposed in literature to improve the conversionefficiency and achieve high step-up voltage gain [1-3].High step-up gain can be achieved by switched capacitor or voltage-lift technique [4].However, themain switch suffers of high

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) doi: 10.1016/j.egypro.2015.07.735

transient current inducing huge increase of conduction losses[5]. Converters based onmagnetic coupled-inductor technique can achieve high step-up gain by adjusting the value of the turns ration but influence to the coupling factor of the transformer[6]. However, this kind of converter is generally based on pulse width modulation (PWM) techniques driving a switch, mainly a MOSFET, placed between the two inductors. This architecture allows an adjustment of the drain-source voltage to the nominal one optimizing losses in the MOSFET [5].By else, during intermediary modes at the switch-on/switch-off and switch-off/switch-ontimes, the leakage inductor induces losses, degrading the global conversion efficiency. For this reason, converters using coupled inductors with passive or active clamp circuit have been proposed [7, 8]. In such a system, the energy of the leakage inductor is recycled into the output stage during the switch-off period limiting the parasitic voltage peak across the switch. To achieve large high step-up gain, converter using secondary side of magnetic coupled inductor used as flyback or forward types has been proposed in Ref. [9]. Magnetic coupled boost(MCB) converterand output stacking techniques have been also proposed in Ref. [10-12].MCB converters increase the voltage gain by increasing the turn ratio and/or adding extra winding stages. As a consequence, compared to basic step-up converters, the family of MCB converters offermany possibilities of gain, adaptability and availability, associated to a possible large conversion efficiency when careful attention is brought to the sizing of the various elements, adapted to the intended application [13].

Nomenclature

C1, C2, C3 Capacitors (nF)

D1, D2, D3 Diodes

HVDC High Voltage Direct Current

L1, L2, L3 Principal inductors ( H)

Lk1, Lk2, Lk3 Leakage inductor ( H)

M MOSFET switch

MCB Magnetically Coupled Boost

MCB-RS Magnetically Coupled Boost with intermediary Recovery Stage

MDCB-RS Magnetically Dual Coupled Boost with intermediary Recovery Stage

In a recent publication [10], we have analyzed by simulation, modeled and tested a prototype of an original MCB converter with intermediary recovery stage (MCB-RS) having a single secondary stage dedicated to renewable energy conversion for middle power installation [10]. The recovery stage is a passive clamping circuit allowing the recovery of the energy lost in the leakage inductor and the clamping of the voltage in the switch. In the current contribution, we present a new step-up converter with two secondary stages developed to improve the voltage gain while limiting the voltage across the MOSFET. This converter is named MDCB-RS for Magnetic dual coupled boost with recovery stage converter. Besides, the secondary-stage of the coupled inductor can alleviate the reverse-recovery problem of diodes rectifier, as present in the previous MCB-RS converter.

In the following of this publication, we will consider the special care bring in the sizing of the MDCB-RS in the light of its working behavior, analyzed during a full cycle with special consideration of the currenttransients at the intermediary states of the cycle. Finally, we will discuss the influence of the coupling factor existing between the primary and the dual secondary windings on the voltage, mainly on the voltage between primary and secondary stages of the coupled inductors i.e. on the gain and the efficiency of the overall converter.

2. General presentation of the magnetic dual coupled boost with recovery stage (MDCB-RS)converter

Fig. 1 shows the circuit topology of the proposed Magnetic Dual Coupled Boost with Recovery Stage (MDCB-RS) DC-HVDC converter.The MDCB-RS is based on a boost converter with one primary and two secondary coupled inductors, each secondary stage associated with its own energy storage capacitor, named C2 and C3 in Fig. 1. The switch is represented by a MOSFET M, driven by a PWM signal generated to delivered the maximum possible power generated by the input variable energy source (from a photovoltaic panel, as example). The equivalent circuit model of the MDCB-RS converter includes the inductor L1 and the leakage inductor Lk1 of the

primary coil, and the corresponding L2, L3, Lk2 and Lk3for the two secondary coils. The recovery stage is constitutes by the diode D1 and the capacitor C1 (Full explanation of the working modes of this recovery stage are fully described in Ref. [10]).

Fig. 1. Diagram Boost coupled magnetic of a converter with two stages RS.

As we can see, the architecture of the MDCB-RS converter remains quite simple with one switch, three diodes, three capacitors; the main elements being the interleave two secondary inductors, having adapted transformation or turn ratio. The turn ratio of the first coupled secondary inductor determines the voltage across the capacitor C2. The second secondary inductor allows a complementary adjustment of the total gain of the converter to the nominal voltage expected at the output of the converter. Within this architecturewith both amplification levels, one can expect an additional significant decrease of the voltage across the MOSFET compared to basic converter and as a consequence a limitation of the losses due to the resistance Rdson, losses having a sub-linear proportionality to the voltage Vds [5]. Thus, compared to a basic boost converter, or to a MCB-RS converter, the MDCB-RS converter seems to be a simple solution to increase the efficiency while keeping a high output voltage.

3. Sizing and working mode simulations of the MDCB-RS

The various working modes and the behaviors of the voltages across the inductors of the MDCB-RS were simulated under the applicative OrCad software environment allowing the determination of its theoretical efficiency. The corresponding simulation schema of the converter is shown in Fig. 2.

V1 = 0 V2 = 15 TD = 0 TR = 1 n TF = 1 n PW = 30u

QK K1 K_Linear

COUPLING = 0.95

0.01 50uH —i

Vin M1 1—

R2 L2 0.2 2500uH

15p L3a< 2500uH

PER = 60u

MURf ■ v

Fig. 2. Simulation Boost coupling magnetic-two recovery stages to secondary coil.

For the sizing of the MDCB-RS converter, we have considered a 12-40V variable voltage source V^, as providing from a single photovoltaic panel and a DC high voltage of 400V corresponding to a standard voltage level in a HVDC distributed energy production system in a smart grid approach [10, 14]. For simulations, we have chosen for Vin an intermediary voltage Vta = 24V.

All the inductors, i.e. L1 for the primary and L2 and L3 for the two secondary stages are represented with their respective resistors, R1, R2 and R3, and their respective parasitic capacitors C4, C5 and C6. We have chosen a realistic value for the coupling factor equal to 0.95, currently achieved in practice.

For the modeling of the behavior of the converter, we have chosen an equilibrium setup with same amplification ratio for the two stages, i.e. a setup having equal secondary windings, L2 = L3.

3.1. Simulation result

We report in Fig. 3 simulation curves representative of the two intermediary voltages VD2 and VD3. The voltage on the distribution and the voltage stress on the D2 and D3 diodes reduced around 200V corresponding to the parameters: input voltage Vin = 24V, output voltage Vout = 400V.

303V 200V

V(D2:1)

IflA- -

SEL» 0V

76.0482ms 76.0800ms

a V(Vout:2) t V(D3:1)

76.1200ms

76.1600ms

76.2000ms

76.2341ms

Fig. 3.Voltage stress rectifier of the diodes D2 and D3 in the two RS stages and output voltage Vout of the converter.

The voltage shapes in Fig.4 show clearly the energy recovery phase show the curves of voltage VCi and VC2 of C1 and C2 followed by first and second RS stage. These two stages in turn the energy recovery leaking primary winding, corresponding to the ratio of transformation by effect of the magnetic Boost to increase performance for this converter. Add the voltage on Mi is improved to reduce by dividing the secondary windings of the transformerwhen the low value as 0.7 of the coupling coefficient previously generated high voltage, the voltage on the transistor does not exceed the maximum permissible value, which ensures its protection.

64.7659ms 64.8000ms 64.8400ms 64.8800ms 64.9200ms 64.9600ms

□ V(C1:2) « V(Ml:d) Time

Fig. 4. Simulated capacitor voltage of the C1 and C2 in two RS stages. For this simulation, the transformation ratio is fixed at a standard value

m=10, duty ratio = 0,5, k=0.95.

3.2. Operation

In the schema reported in Figure 1, leakage inductors are not represented but exist at all stages of the converter, usually designed by Lk1, Lk2 and Lk3 at the primary and secondary stages, of the coupling inductor, respectively. Figure 5 shows the topological mode of this converter in one complete switching cycle Ts with a, a', a'' and a''' the duration of the five modes.

The explanation of each mode is as follow:

Mode 1 [t0, t1]: During this time interval, the switch M is turned on while the diodes D1, D2 and D3 are turned off. The primary-side current of the coupled inductor iLk1 increases linearly. The magnetizing inductor L1 stores its energy from the dc source Vin. Due to the leakage inductor Lk1, the secondary-side current of the coupled inductor iL2 equals zero. The output voltage equals the voltage Vout.

Mode 2 [t1, t2]: During this time interval, the switch M is turned off while diodes D1, D2 and D3 are turned on. The primary-side current of the coupled inductor iLk1 linearly decreases. The energies of the leakage inductor Lk1 and magnetizing inductor L1 are released to the clamp capacitor C1. The primary and secondary windings of the coupled inductor, DC sources Vin, transfer their energies to the output load. This operating mode ends when capacitor C1starts to discharge at t = t2. In which, the capacitor C2 is still charge and more slowly than C1.

Mode 3 [t2, t3]: The switch M and D1 are turned off and diodes D2 and D3 are turned on. The primary-side current of the coupled inductor iLk1 is continuously linearly decreasing but at a slower rate than mode 2. The primary-side and secondary-side windings of the coupled inductor, DC sources Vin, and clamp capacitor C1 transfer their energies to the load. The capacitor C2 is still charge by capacitor C1.

Mode 4 [t3, t4]: The switch M and D1 are turned off and D2, D3 are turned on. The primary-side current of the coupling inductor equals zero. The secondary-side current equals the current of the capacitor C1. Clamp capacitor C1 transfers their energy to the load. The capacitor C2 is still charge by capacitor C1.

Mode 5 [t3, t4]: The switch M and D1 are turned off and D2, D3 are turned on. The primary-side current of the coupling inductor equals zero. The secondary-side current equals the current of the capacitor C1 and C2 (starts to

I\ f\

\ V

■ \ V V, \

\ V , \ \

\ \

- X \

= V(C2:1)

w y y

discharge). Clamp capacitor C1 transfers their energy to the load and C2 transfers their energy to the L2. When capacitor C1 is discharge return zero, C2 transfers their energy to the load.

Figure 5: Key waveforms of the converter MCB-TRS at operation. See above explanations of the various modes

4. Influence of the coupling factor in the efficiency of the MDCB-RS

We have chosen to present, in Fig.6.a the influence of the coupling coefficient k on the voltage on the clamped capacitor when a variable input voltage Vm= (12-40)V is considered and for a transformation ratio fixed at a standard value m=10. This consideration is related to the coupling quality inductors and is often neglecting with a usual consideration of the unit value for the coupling coefficient (k=1). Nevertheless, it is a key factor influencing the global efficiency of the converter and, thusits global behavior as the value of k straightforwardly induces huge changes in the voltage VCi.

Therefore it directly affects the voltage on the switch M. As an example, for a fixed value of the input voltage, Vin=24V, we compared between the two converter MCB-RS Ref. [10] and MDCB-RS that we propose in this work. We can see that the voltage VC1 in the first recovery stage decrease around 32 V.

260 240 220

_ 160 UJ

3 140 >

o 120 >

100 80 60 40

0.70 0.75 0.80 0.85 0.90 0.95 1.00 Coupling factor, k

Fig. 6: Voltage on the clamped capacitor CI versus the coupling coefficient k of coupling inductors of the converter, for a variable input voltage

Vin = 24V.

This value VC1 is lower in the MDCB-RS converter, corresponding to a reduced voltage on the switch. This offers the opportunity tohave a reduction of its stress and offer the possibility to chose a MOSFET with lower Rjson. As shown in Ref. [10], the decrease of the resistive losses in the transistor willsignificantly improvethe efficiency of the converter.

We report in Fig. 7, the evolution of the power efficiency of the MDCB-RS converter as function of the coupling factor k.

100 98 96 94

t,92 o

§ 90 o

^ 88 86 84 82 80

0.70 0.75 0.80 0.85 0.90 0.95 1.00 Coupling factor, k

Fig. 7:Measurements of Pout and global efficiency as a function of coupling factor. For this simulation, the transformation ratio is fixed at a

standard value m=10, duty ratio = 0,5.

We can see that, with the parameters considered in this study, especially an input voltage Vjn = 24V and an output voltage Vout = 400V, and the technological choices made for electronic components, coils having a coupling factor k = 0.95 and a report identical transformation between the primary coil and two secondary coils is similar, the overall performance reached 0.96%.

5. Conclusion

Such a structure, besides the fact that it imposes a particular embodiment of the windings, with significant isolation constraints, poses no problem to get very high elevation reports keeping the benefit of an optimal duty cycle around 50%. Furthermore, this scheme has the advantage of reducing the tension on the switch and thereby allow a choice of a MOSFET with low Rdson resistance, so low losses contributing to improved performance. This converter architecture may be a track in the development of the distributed architecture of HVDC bus.

Acknowledgements

The authors gratefully acknowledge the Director of the University Institute of Technical (IUT) of Thionville-Yutz (France), Patrick Klein and the Head of the Industrial Engineering and Maintenance (GIM) department, Yves Gilletand the Head of the Quang Ninh University of industry (QUI), Duc Tinh Nguyen for the financially support and for the facilities offered during these researches.

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Magnetic Dual Coupled Boost with Recovery Stage DC–HVDC Converter for Renewable Energy Generator Academic research paper on "Materials engineering"

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

Similar topics of scientific paper in Materials engineering , author of scholarly article — T.V. Nguyen, M. Aillerie, P. Petit, T.K. Bui

Academic research paper on topic "Magnetic Dual Coupled Boost with Recovery Stage DC–HVDC Converter for Renewable Energy Generator"