322 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 62, NO. 1, JANUARY 2015

Modulation Techniques to Reduce LeakageCurrent in Three-Phase Transformerless

H7 Photovoltaic InverterTan Kheng Suan Freddy, Nasrudin A. Rahim, Senior Member, IEEE , Wooi-Ping Hew, Member, IEEE , and

Hang Seng Che, Member, IEEE

AbstractRecently, reduced common-mode voltage(CMV) pulsewidth modulation (RCMV-PWM) methods havebeen proposed to reduce the leakage current in three-phasetransformerless photovoltaic (PV) systems. However, mostof these studies only focus on leakage current eliminationand neglect the overall performance of the PV systems onissues such as cost, voltage linearity, dc-link current rip-ples, and harmonic distortion. In this paper, a three-phasetransformerless inverter, adapted from the single-phase H5topology, is investigated. Since the H5 topology has beenconventionally developed for a single-phase system, itsadaptation to the three-phase system requires the develop-ment of corresponding three-phase modulation techniques.Hence, modulation techniques are proposed based on con-ventional PWM. The performances of the proposed PWM,in terms of CMV, leakage current, voltage linearity, out-put current ripples, dc-link current ripples, and harmonicdistortion are studied and discussed via simulation andexperiment. It is proven that the proposed topology is ablereduce the leakage current without sacricing the overallperformance of the system.

Index TermsCommon-mode voltage (CMV), leakagecurrent, photovoltaic (PV) system, transformerless.

I. INTRODUCTION

THE rapid increase in human population and the fast growthof industries have shifted the attention of the researchcommunity toward photovoltaic (PV) energy. PV energy isfree, green, and inexhaustible. Recently, PV power systemshave become widespread due to the government incentives,

Manuscript received November 24, 2013; revised March 13, 2014;accepted May 12, 2014. Date of publication June 2, 2014; date of currentversion December 19, 2014. This work was supported in part by theCampus Network Smart Grid for Energy Security under Grant H-16001-00-D000032 and by the Peruntukan Penyelidikan Pascasiswazah (PPP)under Grant PV105-2012A.

T. K. S. Freddy and W.-P. Hew are with the Power Energy DedicatedAdvanced Center (UMPEDAC) and the Department of Electrical Engi-neering, Faculty of Engineering, University of Malaya, Kuala Lumpur59990, Malaysia (e-mail: freddytan.86@gmail.com; wphew@um.edu.my).

N. A. Rahim is with the Power Energy Dedicated Advanced Center(UMPEDAC), University of Malaya, Kuala Lumpur 59990, Malaysia,and also with King Abdulaziz University, Jeddah 21589, Saudi Arabia(e-mail: nasrudin@um.edu.my).

H. S. Che is with the Power Energy Dedicated Advanced Center(UMPEDAC), University of Malaya, Kuala Lumpur 59990, Malaysia(e-mail: hsche@um.edu.my).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIE.2014.2327585

Fig. 1. H7 topology for three-phase transformerless PV systems.

reduction in PV arrays prices, and the advancement of powerelectronics and semiconductor technology [1][6].

Generally, there are two types of grid-connected power sys-tems, i.e., with a transformer and without a transformer. Thetransformer used can be a high-frequency transformer on thedc side or a low-frequency transformer on the ac side [7][10]. In addition to stepping up the input voltage, it plays animportant role in safety by providing galvanic isolation, whicheliminates the leakage current and avoids dc injection intothe grid. Nevertheless, the transformers are bulky, heavy, andexpensive. Hence, transformerless PV systems are introducedto overcome these issues. They are smaller, lighter, lower incost, and highly efficient [9][12].

However, safety is the main concern for the transformerlessPV systems due to high leakage current. Without galvanicisolation, a direct path can be formed for the leakage currentto flow from the PV to the grid. When the PV is grounded,stray capacitance is created. The fluctuating potential chargesand discharges the stray capacitance, which generates highleakage current [13][15], [26]. In addition to the safety issue,the high leakage current will degrade the performance of thePV system by increasing the grid current ripples, losses, andelectromagnetic interference.

Conventional pulsewidth modulation (PWM), either space-vector PWM (SVPWM) or discontinuous PWM (DPWM), arenot suitable for three-phase transformerless PV applicationsdue to high leakage current. In order to reduce the leak-age current to meet the requirement of the standard, severalconversion structures and modulation techniques have beenproposed recently. In [5], the connection between the neutral

0278-0046 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

FREDDY et al.: MODULATION TECHNIQUES TO REDUCE LEAKAGE CURRENT IN H7 PV INVERTER 323

TABLE IPULSE PATTERNS FOR VARIOUS PWM METHODS

point of the grid to the middle point of the dc link ensuresconstant voltage across the stray capacitance, which leads tozero leakage current. However, this is not practical becausesuch connection creates the inductance in the neutral line. Thisinductance generates high-frequency potential between the PVarray and the ground, which leads to leakage current higherthan the permissible level recommended in the standard [17].A new conversion topology has been proposed in [19] withextra two switches and 12 diodes. These excessive additionalcomponents significantly increase the cost, the losses, and thesize of the entire PV system. Moreover, the leakage current isstill relatively high (280 mA) [19].

Reduced CMV PWM (RCMV-PWM) methods such as activezero-state PWM (AZPWM) [20], near-state PWM (NSPWM)[21], and remote-state PWM (RSPWM) [22], have been pro-posed recently. Without zero vectors, RCMV-PWM methodsare able to reduce the CMV to reduce the leakage current.Nevertheless, the overall performances in terms of voltagelinearity, dc-link and output current ripples, line-to-line outputvoltage pattern, losses, and total harmonic distortion (THD)of the output current are degraded. Considering all criteria forthe transformerless PV systems, this paper investigates a three-phase transformerless inverter, as shown in Fig. 1, by adaptingthe popular single-phase H5 topology [28] for three-phase op-eration. As shown in Fig. 1, an additional switch is added to theconventional three-phase inverter structure to provide galvanicisolation. Given that a total of seven switches are utilized, thistopology is referred to as the H7 topology hereafter.

H5 topology was originally proposed in [28] to reduce theleakage current in single-phase transformerless PV systems.Although H7 is a simple extension from the H5 inverter, studyon this topology is yet to be reported and is thus attempted herein this paper. Since the H5 topology has been conventionallydeveloped for a single-phase system, its adaptation to the three-phase system (i.e., the H7 topology) requires the developmentof a corresponding three-phase modulation technique. Hence,modified DPWM (MDPWM) based on conventional DPWMis proposed here. The performance of the H7 topology withthe proposed modulation techniques is compared with otheravailable RCMV-PWM methods [20][22] in terms of CMV,leakage current, dc-link current ripples, output current ripples,and the THD of the output current. Discussions are donebased on Matlab/Simulink simulations and further validatedvia experimental works. It is proven that the H7 topologywith the proposed modulation techniques gives the best overall

Fig. 2. Voltage vector states for the RCMV-PWM methods with differentways of partitioning the space-vector sectors. (a) Type A. (b) Type B.performance and is suitable for transformerless PV applicationsfor 230-V (RMS) grid system.

This paper is organized as follows. Leakage current reductionmethods via RCMV-PWM are first discussed in Section II. TheH7 conversion structure with the proposed modulation tech-niques and the operation principles are presented in Section III.Simulation and experimental results are shown in Sections IVand V, respectively, to validate the performance of the varioustopologies. Finally, conclusion is made in Section VI to sum-marize the results and findings.

II. COMMON-MODE BEHAVIOR AND LEAKAGECURRENT REDUCTION METHODS

Leakage current minimization is one of the most importantconsiderations in transformerless PV inverters. In the past,various transformerless PV inverter topologies have been in-troduced, with leakage current minimized by the means ofmodulation techniques and conversion structure, which will bediscussed in this section.

A. Modulation TechniquesSeveral modulation techniques, which are termed as RCMV-

PWM, have been proposed recently as listed in Table I. Themodulation techniques can be classified into two types: A [20]and B [21], [22], based on the way the space-vector sectors arepartitioned, as shown in Fig. 2.

In AZPWM, the active vectors are complemented with twoopposing active vectors to create zero vectors with equal time.In RSPWM, a group of odd active vectors (V1, V3, and V5) oreven active vectors (V2, V4, and V6) is used to generate desiredoutput voltage with constant CMV. NSPWM employs onlythree adjacent active vectors. Without using any zero vectors,

324 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 62, NO. 1, JANUARY 2015

RCMV-PWM reduces the CMV to reduce the leakage current.The CMV (VCM)of a three-phase PV inverter is expressed as[5], [18], [26]

VCM =VAN + VBN + VCN

3. (1)

However, issues such as voltage linearity, harmonic dis-tortion, and output voltage arise due to the elimination ofthe zero vectors. One common problem for RCMV-PWMmethods is its bipolar line-to-line output voltage. In everyswitching transition, the voltage changes across the inductorsfrom +VDC to VDC. This doubles the voltage stress acrossthe inductors to twice of the input voltage. Moreover, suchoutput voltage pattern generates overvoltage transients [21],[23], [24], large current ripples across the inductors (eventuallyrequires larger filters), and high switching losses [1], whichreduce the overall efficiency of the entire system. Moreover,NSPWM and RSPWM are only applicable in limited modu-lation range. NSPWM operates linearly only for 0.61 < m /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages false /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 600 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False

/Description > /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [ > /FormElements false /GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks false /IncludeInteractive false /IncludeLayers false /IncludeProfiles false /MultimediaHandling /UseObjectSettings /Namespace [ (Adobe) (CreativeSuite) (2.0) ] /PDFXOutputIntentProfileSelector /DocumentCMYK /PreserveEditing true /UntaggedCMYKHandling /LeaveUntagged /UntaggedRGBHandling /UseDocumentProfile /UseDocumentBleed false >> ]>> setdistillerparams> setpagedevice