Boost Inverter Topology with High-Frequency Link

IEEJ Journal of Industry ApplicationsVol.8 No.5 pp.849–856 DOI: 10.1541/ieejjia.8.849

Paper

Boost Inverter Topology with High-Frequency Link Transformer for PVGrid-Tied Applications

Hamdy Radwan∗ Non-member, Mahmoud A. Sayed∗∗ Non-member

Takaharu Takeshita∗∗∗ Member, Adel A. Elbaset∗∗∗∗ Non-member

G. Shabib∗ Non-member

(Manuscript received Sep. 6, 2018, revised May 31, 2019)

This paper proposes a new topology for single-phase photovoltaic PV grid-tied applications. The whole system con-sists of a two-stage, high-frequency boost inverter cascaded by rectifier–inverter system. A single-phase high-frequencytransformer is used to link both stages and provide galvanic isolation between the AC and DC sides. A single-stagehigh-frequency boost inverter (HFBI), in the first stage, boosts and converts the DC output voltage of the PV array toa high-frequency single-phase square waveform and achieves maximum power point tracking (MPPT). In the secondstage, the rectifier-inverter system (RIS) interfaces HFBI to the grid. The proposed topology has many advantagessuch as increasing the inverter output voltage level, MPPT, high reliability, small size, and light weight. In addition,a proportional integral current control (PI) is used to inject a sinusoidal current into the grid at unity power factor.The proposed topology has been verified analytically using PSIM software and experimentally by using a laboratoryprototype.

Keywords: photovoltaic, grid connected, boost inverter, high frequency transformer

1. Introduction

In the last few years’ renewable energy has the greatestgrowth compared to other energy resources due to its relia-bility, availability, maintainability and safety (1)–(3). One of thepromising sources of renewable energy is photovoltaic en-ergy. Therefore, the research is driven in this direction toimprove the reliability of photovoltaic energy resources.

The proper PV grid-connected system should performsome functions such as maximum power point tracking(MPPT), voltage boosting, galvanic isolation for safety pur-poses, injection of low harmonics high quality AC power tothe grid with unity power factor, and using high efficient im-plementation (4)–(7). Several topologies for PV grid connectedinverter have been presented; generally, there are two types ofgrid-connected PV systems, those with and without galvanicisolation.

Galvanic isolation can be implemented by using a line fre-quency transformer (LFT) or a high frequency transformer(HFT). By contrast, topologies without galvanic isolation aretransformerless topologies.

Transformerless topologies (8)–(12) are lighter, more efficient,

∗ Faculty of Energy Engineering, Aswan UniversityAswan, Egypt

∗∗ Department of Electrical Power and Machines Engineering,Faculty of Engineering, South Valley UniversityQena, Egypt

∗∗∗ Dept. of Electrical and Mechanical Engineering, Nagoya In-stitute of TechnologyJapan

∗∗∗∗ Dept. of Electrical Engineering, Minia UniversityEl-Minia, Egypt

less costly, and less footprint than the galvanic isolated in-verters. However, the main drawback that must be overcomein non-isolated PV inverters is the leakage ground currentsthrough the solar module parasitic capacitance, in addition todc current injected to the grid (13).

Dangerous leakage current increases system losses, re-duces the grid-connected current quality, induces severe con-ducted and radiated electromagnetic interface and causes per-sonal safety problems. To keep the leakage and dc currentsinjected to the grid under control, complex solutions are re-quired.

In order to interface the low output voltage of the PVmodule to the grid, high voltage boosting technique is re-quired; therefore, the use of a line frequency transformer iswidespread (14) (15). In addition to voltage stepping up, it pro-vides galvanic isolation between the grid and the PV system,that plays an important role in safety purpose and personalprotection. Thus avoiding dc current injection into the gridand eliminating leakage current. Nevertheless, the line fre-quency transforms are large, heavy, and expensive, the wholesystem is bulky and hard to install as a result of its low fre-quency (16) (17). Therefore, the topology with line frequencytransformer is considered as a poor solution, which is bet-ter to replace by high-frequency transformers (HFT). UsingHFT (18)–(20) guarantees galvanic isolation between the grid andthe PV system, in addition to overcoming the disadvantagesof using conventional line frequency transformer (21) (22). How-ever, there is a rarity in scientific research for using HFT withPV systems in a way that performs all the required functions,especially MPPT.

This paper presents a new topology for interfacing PV

c© 2019 The Institute of Electrical Engineers of Japan. 849

Boost Inverter Topology with High-Frequency Link Transformer（Hamdy Radwan et al.）

Fig. 1. The proposed system

array with the grid. The topology is designed to provide a10-kHz square wave voltage that enables HFT to be used forgalvanic isolation between the AC and DC sides and con-figure a multi-featured system such as MPPT and boostingthe DC voltage of the PV module in one stage with reducedpower switches. therefore, the overall system is more effi-cient compared with the conventional topologies.

This paper is organized as follows; first, the circuit config-uration of the proposed system is described. Second, the op-eration modes of the proposed topology are presented. Third,MPPT and PI controller are discussed. Finally, simulationsand experiments consider the fundamental operation wave-forms of the proposed system.

2. Proposed System

2.1 Proposed Topology (basic version) The pro-posed system consists of two stages, High-frequency boostinverter (HFBI) cascaded by rectifier-inverter system (RIS)as shown in Fig. 1. In addition, the implemented switchingcontrol strategies are shown in the figure. The first stage isa redesign of the topology given in (6) to obtain a 10-kHzsquare wave output voltage instead of the fundamental gridvoltage. The topology consists of two buck-boost convertersconnected, as shown in Fig. 2, as the second stage is simplyapproximated by a resistor. Each of these converters oper-ates sequentially in discontinuous conduction mode (DCM)for one half cycle of the targeted 10 kHz square waveform.DCM operation prevents the circulating currents between theinductor and the parallel-connected switch in the next oper-ating half cycle. The power MOSFETS SW1 and SW3 areswitched at high frequency of 100 kHz while SW2 (or SW4)is continuously turning ON during the positive half cycle(or negative half cycle) of the targeted 10 kHz square wave-form. Switches SW1 and SW2 operate to provide the positiveboosted half-cycle, whereas SW3 and SW4 operate to providethe negative boosted half-cycle.

When SW1 is ON (or SW3), energy is stored in the inductor

Fig. 2. Configuration of the single-stage HFBI

Fig. 3. The switching pulses at the gates of controllableswitches

“L1” (or L2) by the PV source. When SW1 (or SW3) isOFF, D1 (or D2) gets forward biased, discharging the induc-tor stored energy into capacitor Cbi, which continuously feedscurrent to the load. The switched gate signals for SW1, SW2,SW3 and SW4 are shown in Fig. 3.2.2 Modified Proposed Topology The target of the

proposed topology is a 10 kHz square waveform output volt-age that is linked to RIS by HFT. Therefore, the topology isdesigned to achieve many features of the complete system as

850 IEEJ Journal IA, Vol.8, No.5, 2019


Fig. 4. The modified switching pulses at the gates ofswitches

mentioned previously. But at the instant of turning the opera-tion between the two buck boost, the polarity of the capacitorvoltage VCbi cannot change instantaneously. Although thistime is very short (2 ns), two paths of surge current appeardue to high value of VCbi compared to the input voltage. As-suming the polarity of VCbi changed from the positive halfcycle to negative half cycle and by referring to Fig. 2, thefirst path of surge current flows through capacitor Cbi, switchSW4 and body diode of switch SW2. The second path ofsurge current flows through capacitor Cbi, switch SW4, inputcapacitor Cp and body diode of switch SW3. In order to limitthis surge current, two stages of modification have been pro-posed. The first one considers reverse blocking IGBT (23) forSW2 and SW4 instead of conventional IGBT and adding se-ries diode in opposite direction of the body diode of SW1 andSW3.

Consequently, the capacitor current ICbi is limited by flow-ing through inductor L2. Although, the surge current is lim-ited, it is added to the source current in inductor L2 (SW3 isON) at the instant of changing the polarity of Vcbi, resultingin rising the output voltage at the begging of each half cycle.In order to obtain proper square wave shape, the second stageof modification was done by keeping SW1 and SW3 in OFFstate at this instant. The modified switched gate signals forSW1 and SW3 are shown in Fig. 4.

3. Operation Modes and Parameters Design

The operation modes of the boost inverter are similar forthe basic version and the modified version of the proposedtopology. The basic version topology has three modes of op-eration based on the switching of SW1 during the positivehalf-cycle, since the switch SW2 is always ON during thesethree modes. In Mode1, switch SW1 is ON and energy isstored in the buck boost inductor L1 by the PV source. InMode2, switch SW1 is OFF and D1 is forward biased, dis-charging the inductor stored energy into capacitor Cbi, whichfeeds current to the load (R). In Mode3, both SW1 and D1 areOFF as a result of DCM operation. The operation modes areshown in Fig. 5. As a result of DCM operation during eachcycle of the output voltage, the stored energy in the buck-boost inductor L1 (or L2) is completely discharged into ca-pacitor Cbi. Then the capacitor Cbi feeds the stored energyinto the load. Therefore, the energy delivered into the loadEo(t) is equal to the energy drown from the source Epv(t) dur-ing the switching time period Ts.

According to Mode 1, the extracted energy from the PV

(a) Mode 1

(b) Mode 2

(c) Mode 3

Fig. 5. Operation modes (a) when SW1 ‘on’, (b) whenSW1 ‘off’, D1 ‘on’, (c) when SW1 ‘off’, D1 ‘off’

source is the following:

Epv(t) =12

L1I2L1· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (1)

The peak value of the inductor current can be formulated asfollows;

IL1 =Vpv

L1DTs · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (2)

Substituting by (2) in (1) yields:

Epv(t) =V2

pv

L1D2Ts

2 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (3)

The energy transferred into the load during switching periodTs is given by:

Eo(t) = VoIoTs =V2

o

RTs · · · · · · · · · · · · · · · · · · · · · · · · · · (4)

Equalizing (3) and (4) yields the formula of the boost con-verter voltage gain as follows,



Vo

Vpv=

√(R

2L1D2Ts

)· · · · · · · · · · · · · · · · · · · · · · · · · · · · (5)

As a result of DCM operation, the energy stored of the in-ductor L1 is completely transferred into capacitor Cbi whichfeeds it into the load during each switching period. There-fore, (5) is used to determine the value of the inductor L1,which results the following expression:

L1 ≤V2

pv

2PD2Ts · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (6)

where P is the rated power transferred into the load.To determine the value of Cbi, the energy stored in the in-

ductor L1 during the ON mode can be equated to the changein capacitor energy during the OFF mode, yields the follow-ing expression:

Cbi ≤L1I2

pk L1

4VoΔV≤ VoTs

2RΔV· · · · · · · · · · · · · · · · · · · · · · · · · (7)

where ΔV is the ripple of the capacitor voltage.In the second stage the DC-link capacitor (Cdc) is sized ac-

cording to (17)

Cdc =Pg

2ωVdcΔvdc· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (8)

Where Pg is the average active power injected into the grid,ω is the line angular frequency in rad/sec and Δvdc is the am-plitude of the DC-link voltage ripple.

4. Control Strategies

The control of the proposed system is divided in two majorstrategies; PV side control and grid side control. The func-tion of the PV side control is extracting the maximum powerof the PV source. The grid side control is assigned to injectsinusoidal current into the grid with minimum total harmonicdistortion and unity power factor. The grid side control isachieved by Current control.4.1 PV Side Control The operating point of the PV

sources may change randomly during the operation of thesystem according to the environmental conditions. There-fore, MPPT algorithm is needed to extract maximum instan-taneous power. Several MPPT techniques have been pro-posed in the last decades. P&O MPPT algorithm (24) is oneof simple hill-climbing algorithms, which extensively usedin practical PV systems because of its simplicity. Moreover,prior study or modeling of PV characteristics is not required.

Although the implementation of the algorithm is simple, ithas some drawbacks such as the oscillation of the operatingpoint around the MPP at steady state, which raises the wasteof some amount of available energy. In addition, the P&Oalgorithm can be confused by rapidly changing atmosphericconditions. In some literatures (25) (26), the negative effects ofthe P&O algorithm drawbacks are limited by optimizing theP&O algorithm parameters by customizing them to the dy-namic behavior of the PV system.

The flow chart of P&O MPPT algorithm is depicted onFig. 6. The algorithm starts by reading PV output voltage andcurrent to calculate PV output power P(k). Then comparesthe calculated power with that of the previous perturbation

Fig. 6. Flowchart of the P&O algorithm

cycle P(k-1). Depending on the result of the comparison, thealgorithm perturbs the PV output voltage by increasing or de-creasing. If the perturbation causes an increase in PV power,the subsequent perturbation is made in the same direction.Otherwise, the subsequent perturbation is made in the oppo-site direction. When the perturbation of the algorithm hasthree-level at steady state, it indicates the algorithm is stableand swings around the MPP.4.2 Grid Side Control Current control is more ef-

ficient than voltage control for controlling grid inverter (27).Current controller has fast response and less sensitive to dis-tortion in grid voltage. Linear proportional-integral (PI) con-troller is widely used in current control; it provides proper re-sponse low harmonic content, constant switching frequency.PI controller calculates the error between a sensed inverteroutput current and a desired injected current to the grid, andthen the controller minimizes this error. The control schemeof the inverter connected with the grid is shown in Fig. 1. Theinverter connected with the grid through L filter that is usedto eliminate the current ripple.

The reference grid current Ig∗ is obtained by multiplyingthe unity grid voltage signal with the maximum value of thereference current, which is determined from the input powerand the grid voltage in order to achieve unity power factor inaddition to synchronizing the inverter output voltage and cur-rent with the grid. Therefore, the grid voltage and current aredetected. The actual grid current is subtracted from the ref-erence grid current and the error between them is minimizedby using conventional PI controller.

5. Simulation Results

In order to validate the operation of the proposed system,it has been carried out in PSIM software (ver. 10.0). 250 WPV module is simulated at 25◦C temperature and 1000 W/m2

radiation. The simulated circuit parameters and the electricalcharacteristics of PV module at MPP are listed in Table 1.Switches SW1 and SW3 are responsible of boosting the inputvoltage. Therefore, they switched by 100 kHz and their dutycycles are modulated by MPPT algorithm. Switches SW2

and SW4 are responsible of inverting process; hence they are



Table 1. Proposed simulated circuit parameters

Fig. 7. Simulation Results of Ipv, Vpv and Ppv of PVmodule at MPP and the duty cycle perturbation

switched by 10 kHz 0.5 duty cycle.Figure 7 shows the PV outputs (Vpv, Ipv and Ppv) at MPP

and the duty cycle perturbation (output of MPPT algorithm).It is clear that the duty cycle perturbation has three-level atsteady state, which indicates that P&O MPPT algorithm isstable and swings around the MPP.

Switches signals of H-bridge grid inverter are generated bycomparing the controlled signal, which is obtained by con-ventional PI controller with a 20 kHz saw tooth carrier sig-nal to generate Sinusoidal Pulse width modulation (SPWM)pulses. In order to decrease switching losses without affect-ing the performance of the inverter output, a Hybrid PulseWidth Modulation (HPWM) is used. The strategy consistof a simple combinational logic circuits that generate gat-ing signal for switches with two different frequencies, Twoswitches (SW5, SW7) are modulated at switching frequency20 kHz and the other two switches (SW6, SW8) are operatedat 60 Hz. Thus switching losses of switches SW6 and SW8

can be neglected and switching losses of switches SW5 andSW7 can be reduced by half (each one is operated at one halfof the fundamental grid voltage. In order to show HPWMoperation, the grid voltage is sensed and reduced to a unitamplitude sine wave then multiplied by 0.8 modulation index(open loop case) before comparing with a 1 kHz saw toothsignal. The operation is illustrated in Fig. 8.

Figure 9 shows the grid voltage and current at steady-stateMPPT algorithm. It is clear that the injected grid current is

Fig. 8. Waveforms describing the HPWM switch oper-ation

Fig. 9. Simulation Results of grid voltage, grid current,dc link voltage & current and primary voltage of HFT

in-phase with the grid voltage. The figure also shows Vdc

and Idc of RIS. The frequency of the primary voltage Vp ofHFT is high frequency, as shown in Fig. 9, which is the out-put of the proposed HFBI topology therefore, Fig. 9 is mag-nified to show this waveform and other high frequency signalwaveforms in suitable view, as shown in Fig. 10. Switch gatesignal SW1 is 100 kHz and Switch gate signal SW2 is 10 kHz0.5 duty cycle. The boost inverter capacitor voltage VCbi (Vp)is square waves at 10 kHz. The inductor current IL1 is DCMto avoid the circulating current between the inductor and theparallel-connected switch in the next operating half cycle.

6. Experimental Results

A200-W experimental prototype has been carried out in thelaboratory to verify the operation of the proposed configura-tion. A photograph of the experimental set up used is shownin Fig. 11. The circuit parameters are listed in Table 2.

For the first stage HFBI, (STP42N60M2-EP) Power MOS-



Fig. 10. HFBI switches gate signals, HFBI filter capac-itor voltage, and inductor current and primary voltage ofHFT

Fig. 11. Photograph of the experimental prototype

Table 2. Proposed experimented circuit parameters

FET was used as the controllable switching device for SW1

and SW3. For switches SW2 and SW4, reverse blockingIGBT (85G60RB) was used. For H-bridge, grid inverterIGBT (50G60HD) was used.

The control circuit uses the PE-Expert 3 system, whichconsists of DSP board, PEV board, AD board and FPGAboard. The control program is created in DSP by C language.The program inputs are the grid voltage and current sensedsignals through PEV board. The output-controlled signals ofDSP are sent to FPGA board to generate the gate signals withdead time. Then, the gate signals converted into an opticalsignal before transmitted to the gate drive circuit.

Fig. 12. Experimental results of the proposed systemwith resistive load

Fig. 13. Experimental results of the proposed systemwith grid connected

Two cases of test were performed, one of them the inverterconnected to a standalone resistive load of 50Ω and the sec-ond, the inverter connected to Grid. The PV source is re-placed by dc source.

Figures 12 and 13 show the experimental results of 10%of the rated power of the prototype in case of connectingto resistive load and connecting to grid, respectively both inclosed loop operation. In both cases, the output voltage andcurrent are in-phase with law THD. In addition, the figuresdemonstrate a boosting of the HFBI more than 4 times.

Figure 14 is the magnification of Fig. 13 to show the highfrequency signal waveforms of HFBI in suitable view. Asshown in Fig. 14, the capacitor voltage Vcbi (the primary volt-age of HFT) and primary current Ip of HFT are square waveat 10 kHz. Also, The inductor current L1 of HFBI is DCM toguarantee discharging all its energy and no circulated currentbetween the inductor and the parallel-connected switch in thenext operating half cycle.



Fig. 14. The magnification of Fig. 13

7. Conclusion

This paper has proposed a new single-stage high-frequencyboost inverter cascaded by Rectifier-inverter system for PVgrid-tie applications. The topology has been analyzed, de-signed, simulated and implemented in laboratory. The topol-ogy performs many features such as MPPT, boosting PVvoltage in addition to the high frequency square wave out-put voltage that allows the use of HFT to guarantee gal-vanic isolation between the grid and the PV system, in ad-dition to overcoming the drawbacks of conventional line fre-quency transformer. The proposed HFBI topology is onestage with reduced power switches compared with the con-ventional topologies. The functionality and switching opera-tion of the proposed system in closed loop have been depictedwith simulation and experimental.

The presented experimental results demonstrate a boostingof more than 4 times, therefore, low PV array voltages (typi-cally 40–100 V range) can be stepped up to levels commensu-rate with the grid voltage (110–230 V ac). Consequently, theuse of a few series connected solar panels is sufficient. Thisavoids environmental changes such as shadow, which reducesthe utilization of solar panels. Simulation and experimentalresults, which emphasizing the performance of the proposedtopology of proposed system, have been validated.

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Hamdy Radwan (Non-member) was born in Luxor, Egypt, in 1982.He received the B.Sc. degree in electrical engineeringfrom Faculty of Energy Engineering, Aswan Univer-sity, Egypt in 2005. From 2010–2013, he was withAswan Power Electronic Application Research Cen-ter (APEARC), as a research Assistantand receivedthe M.Sc. degree in electrical engineering in 2013. Heis currently working toward the Ph.D. degree. Since2014, he has been with Aswan University, Aswan,Egypt, where he was assistant lecturer in the Depart-

ment of Electrical Engineering, faculty of Energy engineering. In 2016, hejoined Nagoya Institute of Technology, Japan as a Ph.D. special researchstudent. His current research interests include digital Control, renewableenergy, and PV Grid-Tie Applications.

Mahmoud A. Sayed (Non-member) was born in Qena Prefecture,Egypt, in 1974. He received the B.Sc. and M.Sc.degrees in electrical engineering from Minia Univer-sity, Minya, Egypt, in 1997 and 2001, respectively,and the Ph.D. degree in electrical engineering fromNagoya Institute of Technology, Nagoya, Japan, in2010. Since 1999, he has been with the Departmentof Electrical Engineering, Faculty of Energy Engi-neering, Aswan University, Aswan, Egypt, first as anAdministrator and since 2001 as a Lecturer. Since

2010, he has been with the Faculty of Engineering, South Valley University,Qena, Egypt, first as an Assistant Professor and since 2015 as an AssociateProfessor. His research interests include voltage regulation and loss mini-mization of electrical distribution systems using series and shunt pulse-widthmodulation (PWM) converters, PWM techniques for bidirectional ac/dc anddirect ac/ac converters, modular multilevel converters (MMxC), machinedrives for electrical vehicles applications, in addition to renewable energyapplications and machine drives. Dr. Sayed is a senior member of the IEEEPower Electronics and Industry Application Societies.

Takaharu Takeshita (Member) was born in Aichi, Japan, on August23, 1959. He received the B.S. and M.S. degreesin Electrical Engineering from Nagoya Institute ofTechnology, Nagoya, Japan, in 1982 and 1984, re-spectively, and the Ph.D. degree in Electrical Engi-neering from Nagoya University, Nagoya, Japan, in1990. Since 1991, he has been with Nagoya Instituteof Technology, where he is currently a Full Profes-sor and is involved in research on power convertersand motor drives. Dr. Takeshita is a member of the

Society of Instrument and Control Engineers (SICE), Society of Signal Pro-cessing Applications and Technology of Japan (SSPATJ), and Institute ofElectrical Engineers of Japan (IEEJ).

Adel A. Elbaset (Non-member) was born in Nag Hamadi, Qena-Egypt, on October 24, 1971. He received the B.Sc.,M.Sc., and Ph.D. at the Faculty of Engineering, De-partment of Electrical Engineering, Minia University,Egypt, in 1995, 2000 and 2006, respectively. He is astaff member of the Faculty of Engineering, Electri-cal Engineering Dept., Minia University, Egypt. Hewasvisiting assistant professor at Kumamoto Univer-sity, Japan, until August 2009. Presently, he is Profes-sor at the Department of Electrical Engineering. His

research interests are in the area of power electronics, power system, neuralnetwork, fuzzy systems and renewable energy and Optimization.

G. Shabib (Non-member) received his B.Sc. degree in electrical en-gineering from Al Azhar University. In October1982, he joined the electrical engineering depart-ment, King Fahad University of Petroleum and Min-erals, Dhahran Saudi Arabia as research assistant.In December 1985, he received his M.Sc. degreein electrical engineering at King Fahad Universityof Petroleum and Minerals. In November 1987, hejoined Qassim Royal Institute, Qassim, Saudi Ara-bia aslecturer. He received his Ph.D. degree from

Menoufia University, Egypt, in 2001. He joined Aswan High Institute ofEnergy, South Valley University, Aswan, Egypt in 1999. He joined DigitalControl Laboratory, Tsukuba University, Japan asvisiting Professor in 2006–2007. His research interests are power system stability, control, Self-tuningcontrol, Fuzzy logic techniques, digital control techniques, all as applied topower systems.


Documents

Boost Inverter Topology with High-Frequency Link