Partial Power DC-DC Converter for Photovoltaic ... - IEEE Xplore

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stages. The advantages of the converters with a DC-stage are mainly the distributed maximum power point tracking algorithm per PV string or PV module and, ...
Partial Power DC-DC Converter for Photovoltaic Microinverters Jaime W. Zapata∗ , Hugues Renaudineau∗ , Samir Kouro∗ , Marcelo A. Perez∗ and Thierry A. Meynard† ∗ Electronics

Engineering Department, Universidad Tecnica Federico Santa Maria, Valparaiso, Chile National Polytechnique de Toulouse, University of Toulouse, Toulouse, France email: [email protected], [email protected], [email protected] † Institut

Abstract—In order to increase the conversion efficiency in photovoltaic (PV) systems, different configurations and topologies were developed. Depending on the application, the converters used for grid connection are built using one or two conversion stages. The advantages of the converters with a DC-stage are mainly the distributed maximum power point tracking algorithm per PV string or PV module and, when required for grid connection, the possibility of voltage elevation. However, the conversion efficiency is lower than configurations with a singlestage as the central inverter. Therefore, the proposed work presents a Partial Power DC-DC converter (PPC) which process part of the entire system power, and the remaining power is directly supplied to the output side. A topology is proposed and the details of its operation are explained based on the operating principle. Simulations are performed in order to evaluate the converter performance.

I. I NTRODUCTION In the last decade the interest given to photovoltaic (PV) microinverters increased significantly, following the realize of the Enphase M175 microinverter in 2008. In comparison with the others PV inverter architectures - the centralized, string and multistring configurations - the microinverter is known to be the most efficient architecture against non-uniform conditions on the PV field since it is the most distributed solution [1]. Indeed, with this configuration, the MPPT is realized at the PV module level. Among other advantages, the microinverter does not require any expensive DC power distribution line [2], its mass production allows to reduce the manufacturing costs [3], and it is supposed to be more reliable with commercial products warrantied up to 25 years. On the other hand, the microinverter is the PV inverter configuration that presents the lower efficiency. This drawback is mainly due to the requirement of a high voltage elevation ratio so that the grid connection is possible. For this purpose, the microinverter architectures that can be found in the literature can be divided into two groups: single-stage solutions where a single inverter realize both grid connection and voltage elevation, e.g. [4]–[6]; and two-stage solutions with a first DCDC stage for voltage elevation purpose and a DC-AC converter for grid connection such as [3], [7], [8]. Both in the academic literature and among commercial microinverters, the two-stage architecture is the most common. Various DC-DC converter configurations can be found. The most classical consists in interleaved flyback converters such as in [9], or in the commercial Enphase microinverter. Other

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solutions have been proposed such as boost converter [10], half-bridge DC-DC converter [11], or resonant converter [12], [13], which is the commercially solution adopted by APS. The DC-DC conversion stage realize high voltage elevation to enable the connection between a single PV module (around 30Vdc) to the grid. If isolation is required, high frequency transformers are included in the DC-stage, which is found in most commercial topologies. In this paper, a two-stage microinverter architecture is proposed. Based on the concept of partial power converter (PPC), a high efficiency DC-DC stage is developed. Section II describes the concept of partial power converters. For comparison, a structure with a classical DC-DC converter stage and the proposed microinverter are analyzed in section III. The modeling of the losses of the studied converter, its components sizing and the control scheme are given in section IV. Simulation results are provided in section V to validate the improvement achieved with the proposed microinverter architecture. Finally section VI gives the conclusions and perspectives of this work. II. PARTIAL POWER CONVERTERS Due to the additional stage, the global conduction, switching and magnetic losses increase. Therefore, the efficiency presented in two-stage configurations is lower compared with single-stage configurations. Moreover, the whole power generated by the system is handled by the converter as shown in Fig. 1 (a), which also reduce the conversion efficiency. In order to overcome the low efficiency, some authors have proposed different solutions based on interleaved connections [14]. Moreover, the concept of partial power converter, as shown in Fig. 1 (b), has been introduced in [15], which uses the isolated converters in order to make the connection and avoid a short circuit. The technique of partial power processing, where only a portion of the total power is required to elevate the input voltage, can significantly reduce the converter size and power loss [16]. Moreover, some solutions have been published in order to handle a small portion of power for different applications, such as in PV powered electric aircraft [17], and distributed architectures [18]. In order to analyze the power processed by the converter, a variable called partial power ratio Kpr is established as,

Fig. 1. DC-DC Converters. (a) Full power converter (FPC). (b) Partial power converter (PPC).

Kpr =

Ppc , Ppv

(1)

where, Ppc and Ppv are the power processed by the converter and the power delivered by the PV module, respectively. Then, the converter working as a PPC corresponds to a partial power ratio below unity Kpr < 1. III. M ICROINVERTER CONFIGURATIONS A. Classical microinverter The microinverter configuration uses a dedicated grid-tied converter per PV module and it presents a small size and low power rating. One of the most common microinverter configurations found in the literature and practice is based on Flyback/Hbridge topology as shown in Fig. 2 (a). The Flyback converter is essentially an isolated Buck-Boost topology which uses the transformer to store magnetic energy. It not only provides galvanic isolation, it also allows a voltage elevation depending on the turns ratio. The Flyback converter is the simplest and most common of the isolated topologies for low-power applications, because a large transformer core is needed for higher power levels in order to avoid the saturation [19]. The voltage is controlled by the duty cycle D, and the transfer function is represented as, D vpc = , vpv n(1 − D)

(2)

where the turns ratio is represented by n = n1 /n2 and, in case of the full power converter the output corresponds to the DC-link voltage vpc = vdc .

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Fig. 2. Microinverter configurations. (a) Traditional microinverter. (b) Proposed partial power DC-DC converter for a grid connected PV system.

The converter is designed depending on the power processed, therefore the switch and diode stresses also change if it works as full or partial power architecture. The MOSFET and diode voltage stress vsm , vdm and, MOSFET and diode current stress ism , idm are calculated as, vsm = vpv + nvpc

(3a)

vdm = vpv /n + vpc

(3b)

ism = ipc + io /n

(3c)

idm = nipc + io

(3d)

In case of the full power converter the current ipc = iin , and the voltage vpc = vdc . B. Proposed microinverter Based on the structure shown in Fig. 1 (b) some topologies can be proposed. The proposed microinverter topology is shown in Fig. 2 (b). It is composed by a Flyback-based PPC DC-DC converter connected to a H-bridge inverter realizing grid connection. Due to the configuration, the converter is only able to work as a boosting stage, fitting well with the considered application since the required DC-voltage vdc for

the grid connection must be greater than the PV voltage vpv . Using Kirchhoff laws, the following equations are derived, vdc = vpv + vpc

(4)

iin = ipc + io

(5)

The power handled by the converter is expressed as, Ppc = vpv · ipc

(6)

The conversion efficiency of the DC-DC stage can be calculated as, vdc · idc (7) η= vpv · ipv Including (5), (4), (6) and (7) in (1), the partial power ratio is expressed as shown in (8) where, G = vdc /vpv , corresponds to the voltage gain, Kpr =

1 1 − η G

(8)

The variation of the partial power ratio of the proposed converter under different voltage elevation ratio is shown in Fig. 3 (a). The shadowed area represents the region where the converter works as a PPC. If the PV voltage is lower than the DC-link voltage, the power flows toward the PV modules which in PV applications is not allowed. Fig. 3 (b) shows the P-V curve under different solar irradiation. If the DC-link voltage keeps a fixed value, the changes of solar irradiation results in variations of the converter voltage vpc , due to the change of the PV voltage needed to perform the MPPT algorithm. In section V, the effects of this variation on the efficiency and partial power ratio Kpr are analyzed. IV. S IZING , LOSSES AND CONTROL In order to evaluate the efficiency of the analyzed structures, the semiconductors and transformer losses are considered in the simulation. 1) Semiconductor losses: Both the transistor and diode losses are incorporated to the model using the thermal modR in which, the eling tool found in the software PLECS switching and conduction characteristics of a certain device, are added to the model with the information provided by the manufacturers. The diode losses are calculated using the following equations [20], Pcond = vd · iF

(9a)

= Err · f (9b) 1 Psw of f = · Qrr · vr · f (9c) 4 1 Qrr = · trr · irr (9d) 2 where, vd is the diode voltage drop, iF is the diode forward current, Err is the reverse recovery energy losses, f switching frequency, Qrr is the reverse recovery charge, vr is the reverse Psw

of f

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Fig. 3. Operational features of the proposed converter. (a) Partial power ratio under different voltage gain. (b) P-V curve under different solar irradiation at 25o C.

blocking voltage, trr is the reverse recovery time, and irr is the reverse recovery current. To analyze the MOSFET and IGBT losses, the model is made using the following equations. Pcond = vce,ds · ic,d

(10a)

Pswon = Eon · f

(10b)

Pswof f = Eof f · f

(10c)

Eon = vce,ds · ic,d · ton

(10d)

Eof f = vce,ds · ic,d · tof f

(10e)

where, vce,ds is the collector-emitter/drain-source voltage, ic,d is the collector/drain current, Eon is the turn-on energy losses, Eof f is the turn-off energy losses, ton is the turn-on time, and tof f is the turn-off time.

2) Transformer losses: In order to estimate the transformer losses, some authors have proposed different techniques. One of them is made by obtaining the conduction and core losses. The last one is obtained by the Hysteresis and Eddy losses as a function of the frequency, flux density and temperature [21]. The conduction losses are calculated as, Fig. 4. DC-DC control scheme

Pcond = rp · i2p,rms + rs · i2s,rms ,

(11)

where, rp and rs are the primary and secondary winding resistances, ip,rms and is,rms correspond to the winding rms current values. The hysteresis losses are modeled using (12) which is an extension of the Steinmetz equation for non-sinusoidal magnetizing currents [22].  Vcore K2 −1 ˆ K3 Ct2 τ 2 − Ct1 τ + Ct , Physt = ρm K1 feq B Tsw (12) with Tsw the switching period, ρm the density of the core ˆ the peak flux density, K1 , material and Vcore its volume, B K2 , K3 loss coefficients, Ct , Ct1 , Ct2 temperature coefficients, τ the temperature, and feq the equivalent switching frequency defined as (13) considering Bmax and Bmin the maximum and minimum values of the flux. 2 Z Tsw  dB 2 feq = dt (13) 2 dt π 2 (Bmax − Bmin ) 0 The eddy current losses are calculated as, Peddy =

π 2 Vcore ˆ e )2 , (fsw Bλ 6ρρm

(14)

where λe is the thickness of the ribbon, and ρ is the electrical resistivity of the core. 3) Control scheme: The control scheme is shown in Fig. 4, and is similar for the two studied structures. In order to extract the maximum power, a Perturb and Observe (P&O) MPPT algorithm is performed because of the simple implementation and effective tracking [23]. A cascaded control loop is imple∗ mented where the given voltage reference vpv , coming from the MPPT algorithm, is controlled by an external PI controller, and the current reference i∗c , is controlled by an internal PI controller. The output signal represents the duty cycle D going to a classical PWM, which gives the required signal to drive the converter. The grid tied inverter is controlled through classical singlephase voltage oriented control (VOC) algorithm. It controls the DC-link voltage with a fixed reference, performs grid synchronization and active and reactive power control [24].

TABLE I S IMULATION PARAMETERS Variable

Parameter

Value

PV power under STC

275 W

Maximum PV voltage under STC

31 V

vg

Grid voltage

110 Vrms

fg

Grid frequency

60 Hz

Lg

Filter at AC-side

3 mH

fac

DC-AC switching frequency

3 KHz

Cpv

Input capacitor

220 µF

Cdc

DC-link capacitor

3300 µF

fdc

DC-DC switching frequency

25 KHz

Ppv Vmpp

The DC-stage results are shown in Fig. 5. Fig. 5 (a) shows the PV voltage, where it can be observed the classical oscillations around the MPP voltage vmppt . Due to the vpv variations, the converter voltage vpc also varies so that vdc remains constant as shown in Fig. 5 (b). The voltage results show that vdc is always greater than vpv and matches with (4), which is suitable for the microinverter due to the voltage elevation requirement. The DC-currents at the converter side are shown in Fig. 5 (c), Fig. 5 (d) and Fig. 5 (e), the results match with (5). It is important to notice that io is always positive, then the power is flowing toward the inverter as expected. The AC-stage results shown in Fig. 6 are similar for both analyzed structures. The grid voltage and current are shown in Fig. 6 (a), where it is verified that the converter works with unitary power factor. The three levels of voltage generated by the inverter are shown in Fig. 6 (b). The losses per converter stage are shown in Table II for two operating points. The results show that the total conversion losses working with a partial power convert are lower than the full power converter, and the conversion losses of the DC-AC stage are similar between the two configurations. TABLE II C ONVERTER LOSSES

V. S IMULATION RESULTS In order to validate the proposed converter and control R schemes, simulations in software PLECS have been performed. The simulation parameters are given in Table I. The results are shown in Fig. 5. In order to evaluate the dynamic performances, a step in the irradiation is simulated a time t=0.5 s, from 1000 W/m2 to 800 W/m2 .

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DC-stage Input Power

AC-stage

PPC

FPC

PPC

FPC

275 W

2.085 %

2.583 %

0.315 %

0.317 %

222 W

2.5 %

2.93 %

0.304 %

0.306 %

Fig. 7 (a) shows the variation of the power ratio Kpr and

Fig. 6. AC-stage results. (a) Voltage and current at the grid-side. (b) Voltage generated by the inverter.

Fig. 5. DC-stage results for the proposed converter. (a) PV voltage. (b) Converter and DC-link voltage. (c) Input current. (d) Current at the MOSFET side. (e) Current at the diode side.

the power handled by the converter Ppc according to the input power. When the input power decreases, the power ratio increases due to the vpv reduction, i.e when Ppv =250W, the converter handle Ppc =207W and when Ppv =150W, the converter handle Ppc =125W, which correspond to Kpr =82.8% and Kpr =83.3% respectively. Fig. 7 (b) shows the PPC-based microinverter and FPCbased microinverter efficiencies depending on the input power variations, it reaches a high value when the converter works

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Fig. 7. Simulation results under different input power. (a) Partial power ratio and power handled by the converter. (b) Converter efficiency.

at the nominal values, then it decreases when the irradiation also decreases. In order to compare the performance of the proposed configuration, a simulation of a full power converter is made for the same operating point. Both converters work at the same parameters, with the exception of the voltage and current sizing for the semiconductors and the transformer power rating, so that they are optimized in order to reach their maximum efficiencies. Table III resumes the main differences between both converters. It is noted that the partial power

converter not only has a better efficiency, but the requirements for the semiconductors are lower as well. This aspect may lead to an improved reliability of the microinverter TABLE III C OMPARISON BETWEEN PARTIAL AND FULL POWER CONVERTER AT NOMINAL POWER

Parameter Input power (W)

PPC

FPC

275

275

Processed power (W)

226.9

275

Power Ratio (%)

82.51

100

Efficiency (%)

97.6

97.1

Mosfet Drain-Source Voltage (V)

56.67

62

Mosfet Drain Current (A)

16.50

18.05

Diode blocking voltage (V)

340

372

Diode forward current (A)

2.75

3.0

VI. C ONCLUSIONS The work presents a DC-DC converter for photovoltaic microinverters which handles less power than the given by the PV module. The topology is based on the Flyback converter, which is commonly used for small power applications. Several aspects of the design and operation are improved in comparison to a full power converter, such as a higher efficiency of the DC-DC conversion and a small power rating for the converter is achieved. Despite of the variations in solar irradiation, the power handled by a partial power converter remains lower compared with a full power converter, it leads to smaller conversion losses and a greater efficiency. VII. ACKNOWLEDGMENT The authors acknowledge the support provided by FONDECYT 1151426, by SERC Chile (CONICYT/FONDAP/15110019) and by AC3E (CONICYT/FB0008) of Universidad Tecnica Federico Santa Maria. R EFERENCES [1] S. Kouro, J. Leon, D. Vinnikov, and L. Franquelo, “Grid-connected photovoltaic systems: An overview of recent research and emerging pv converter technology,” IEEE Ind. Electron. Mag., vol. 9, no. 1, pp. 47– 61, March 2015. [2] Y.-H. Kim, J.-W. Jang, S.-C. Shin, and C.-Y. Won, “Weighted-efficiency enhancement control for a photovoltaic ac module interleaved flyback inverter using a synchronous rectifier,” Power Electronics, IEEE Transactions on, vol. 29, no. 12, pp. 6481–6493, Dec 2014. [3] Z. Zhang, X.-F. He, and Y.-F. Liu, “An optimal control method for photovoltaic grid-tied-interleaved flyback microinverters to achieve high efficiency in wide load range,” Power Electronics, IEEE Transactions on, vol. 28, no. 11, pp. 5074–5087, Nov 2013. [4] H. Hu, S. Harb, N. Kutkut, Z. Shen, and I. Batarseh, “A single-stage microinverter without using eletrolytic capacitors,” Power Electronics, IEEE Transactions on, vol. 28, no. 6, pp. 2677–2687, June 2013. [5] N. Sukesh, M. Pahlevaninezhad, and P. Jain, “Analysis and implementation of a single-stage flyback pv microinverter with soft switching,” Industrial Electronics, IEEE Transactions on, vol. 61, no. 4, pp. 1819– 1833, April 2014.

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