A Single-Stage Single-Phase Transformer-Less Doubly Grounded Grid-Connected PV Interface

IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 1, MARCH 2009 93

A Single-Stage Single-Phase Transformer-LessDoubly Grounded Grid-Connected PV Interface

Hiren Patel and Vivek Agarwal, Senior Member, IEEE

Abstract—A transformer provides galvanic isolation andgrounding of the photovoltaic (PV) array in a PV-fed grid-connected inverter. Inclusion of the transformer, however, mayincrease the cost and/or bulk of the system. To overcome thisdrawback, a single-phase, single-stage [no extra converter for volt-age boost or maximum power point tracking (MPPT)], doublygrounded, transformer-less PV interface, based on the buck–boostprinciple, is presented. The configuration is compact and uses lessercomponents. Only one (undivided) PV source and one buck–boostinductor are used and shared between the two half cycles, whichprevents asymmetrical operation and parameter mismatch prob-lems. Total harmonic distortion and dc component of the currentsupplied to the grid is low, compared to existing topologies andconform to standards like IEEE 1547. A brief review of the ex-isting, transformer-less, grid-connected inverter topologies is alsoincluded. It is demonstrated that, as compared to the split PVsource topology, the proposed configuration is more effective inMPPT and array utilization. Design and analysis of the inverterin discontinuous conduction mode is carried out. Simulation andexperimental results are presented.

Index Terms—Double grounding, IEC 1547, IEC 61727, max-imum power point tracking (MPPT), NEC 690, partially shadedconditions, photovoltaic.

I. INTRODUCTION

D EMAND for clean, economical, and renewable energyhas increased consistently. Among a variety of renewable

energy sources available, photovoltaic (PV) appears to be a ma-jor contender on account of its abundance, easy availability,and pollution-free operation. Increasing interest in sustainableenergy production through PV [1], however, demands atten-tion on various issues such as maximum power point track-ing (MPPT) [2], [3], personal safety, grid integration, stabilityand reliability, power quality, power electronic interface of PVwith the grid, and operation under various environmental con-ditions [4]–[7]. With increased level of penetration of PV-basedsystems into the existing grid, these issues are expected to be-come more critical with time, since they may affect the perfor-mance of the other grid-connected systems [8]. This implies thatthere is a need for a set of rules and regulations to govern thegrid-connected PV systems.

At present, there are no such globally accepted standard rulesand regulations. However, in some countries, it is mandatory

Manuscript received February 19, 2008; revised June 8, 2008. Current versionpublished February 19, 2009. Paper no. TEC-00048-2008.

H. Patel is with the Department of Electrical Engineering, Indian Institute ofTechnology—Bombay, Mumbai 400 076, India, and also with the SarvajanikCollege of Engineering and Technology, Surat 395 001, India.

V. Agarwal is with the Department of Electrical Engineering, Indian In-stitute of Technology—Bombay, Mumbai 400 076, India (e-mail: [email protected]).

Digital Object Identifier 10.1109/TEC.2008.2006551

that the grid-connected PV systems must comply with certainstandards like IEEE 1547 [9], IEC 61727, and NEC 690 [10].IEEE 1547 has provisions related to the performance, operation,testing, safety considerations, and maintenance of the grid–PVinterconnection. IEEE 1547 and IEC 61727 impose limitationson the maximum allowable amount of injected dc current into thegrid, which is 0.5%–1% of the rated output current of the source.NEC 690 requires that the grid-connected, PV-based, dc–acinverter topologies must be appropriately earthed (grounded).

The grounding of a PV system, referred to as “system earth-ing,” needs special consideration due to safety reasons [11], andto minimize the effects of lighting and other surges. It refers toan intentional connection to earth of one of the current-carryingconductors in the PV system. In view of this, in certain coun-tries (e.g., USA), it was mandatory to provide earthing to thePV system when its output dc voltage exceeded a certain level,typically 50 V. According to the revised NEC 690 (revised in2005), PV inverters with ungrounded conductors are now al-lowed in USA. However, the ungrounded PV inverters have tofulfill a number of additional requirements (e.g., disconnectsand overcurrent protection in both the conductors, provision forminimization of the effects due to surges, etc.) [12]. It is impor-tant to note that even though ungrounded PV systems are nowallowed, system earthing is still advisable.

In addition to the PV string’s grounding, the neutral conduc-tor of the 1 − φ inverter feeding the utility or the neutral of theutility itself is grounded. This necessitates a PV inverter topol-ogy that allows double grounding [10]. The inverter topologiesconsisting of a transformer can easily realize double ground-ing. In fact, double grounding can also be achieved with thetransformer-less topologies employing split PV sources. As dis-cussed later in Section II, most of the transformer-less topolo-gies, which have double grounding feature, employ a split PVsource (i.e., a PV source split into two halves PV1 and PV2).However, they suffer from the following drawbacks.

1) They generate alternate half cycles of the grid current fromdifferent halves of the split PV source. If the two halves(PV1 and PV2) of the PV source operate in unidenticalconditions like different solar insolation levels or differ-ent shading patterns, it results in a distorted grid currentwith high total harmonic distortion (THD) and/or high dccomponent.

2) Grid current may have high THD and/or dc component ifthe modules used for PV1 and PV2 have some mismatch.

3) Only one of the two halves of the PV source is utilizedin a given ac half cycle. This leads to more ripple in thevoltage across PV1 and PV2 . As a result, the averageoutput power extracted from the PV decreases.

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94 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 1, MARCH 2009

Fig. 1. Schematic of the circuit for the proposed system.

4) As explained later in Section V, when operating undernonuniform conditions, inverters with split PV source areless effective in the utilization of the PV array.

This paper reviews various transformer-less topologies andproposes a new single-stage, 1 − φ, transformer-less, PV-fedgrid-connected system. Fig. 1 shows the proposed configura-tion employing double grounding. The salient features of theproposed topology are as follows.

1) It meets the double grounding requirement.2) Even under partially shaded conditions, there is no prob-

ability of injecting dc component into the grid.3) It has a single buck–boost inductor and uses the same

operating principle (as that of a buck–boost converter) inboth the halves of the ac cycle. As a result, a symmetricalgrid current with low THD and dc component is obtained.

4) It uses only one PV string as the input source, which isused in both halves of the ac cycle. Due to this, the ripplein the voltage of the input capacitor Cin (Fig. 1) is less,leading to the extraction of more average output power.

5) The proposed topology is more suitable in effective uti-lization of the PV array.

6) It can be used irrespective of the PV voltage being greateror smaller than the grid voltage amplitude. This featurerenders it suitable for places where the environmentalconditions vary over a wide range or the array is likely toreceive nonuniform solar insolation (e.g., partially shadedconditions).

7) All the features like MPPT control, inversion, and voltagetransformation are encompassed into a single stage.

8) It uses fewer components, and is therefore, more compact.

II. REVIEW OF EXISTING TOPOLOGIES

Over the years, researchers have presented many topologiesthat are based on the galvanic isolation provided by the trans-former [10], [13], [14]. Another advantage of the transformer isthat it can provide voltage transformation. Some single-stage in-verter configurations based on high-frequency transformer arealso reported [10] to overcome the drawbacks such as lowerefficiency, higher cost, and bulky size of the configurations em-ploying line frequency transformer.

Several researchers have worked with the PV-fed grid-connected, transformer-less dc–ac inverter topologies [15]–[18],[20]–[22] and the issues related to their operation [23], [24].

To offer a compact and economic design, the transformer-lesstopologies may have to compromise on the double groundingaspect [15], [16]. A few topologies, which do offer doublegrounding feature, are shown in Fig. 2. Kasa et al. [17] havepresented an inverter based on half-bridge buck–boost configu-ration [Fig. 2(a)]. It employs two parallel-connected buck–boostconverters, each with its own PV string (PV1 or PV2 of the splitPV source). It can operate over a wide input voltage range, andhas low switching and conduction losses. But, as only one PVstring is used over a half cycle, ripple in the voltage across de-coupling capacitors is very large, leading to a decrease in thenet output power obtained from the PV strings. This could beminimized by having a large electrolytic capacitor, but at anincreased cost. In addition, in case of unbalanced power gen-eration from the two PV strings (PV1 and PV2), the currentinjected into the grid may not be symmetrical (highly distortedwith high THD), and may have a dc component that does notconform to IEEE 1547.

The configuration shown in Fig. 2(b) utilizes generation con-trol circuit (GCC) [18], which comprises two switches (S1 andS2) and an inductor L. With GCC, MPPT can be applied to boththe PV strings independently. The disadvantage with the con-figuration is that it is inherently a half-bridge, exhibiting buckcharacteristics. Hence, this topology is applicable only whereinput voltage is greater than the output voltage. Due to this, a PVstring of large number of PV modules would be required, whichis more likely to get affected by partial shading conditions. Insuch a case, the PV characteristics of these strings may havemultiple peaks [19], which makes it difficult to track the globalpeak, unless each PV module is supported by its own GCC.

Another transformer-less topology that can provide a solutionto the dual grounding problem is a multilevel (three levels) half-bridge diode-clamped inverter (HBDC) shown in Fig. 2(c) [20].It produces three different voltage levels: 1) VC1 when switchesS1 and S2 are ON; 2) zero when switches S2 and S3 are ON;and 3) −VC2 when switches S3 and S4 are ON. Here also, asin the topology of Fig. 2(a), each of the PV strings is loadedonly for half the ac cycle. This results in the requirement of alarge electrolytic capacitor to reduce the ripple in the capaci-tor voltage. This topology does not provide voltage boosting,and when the two PV strings generate unequal power, it maygenerate asymmetrical output current.

Kusakawa et al. [21] have presented an inverter topology[Fig. 2(d)] based on the buck–boost principle for an ac module,with a low power rating of around 50 W. Unlike other configu-rations, this configuration uses only one PV string. The circuitis operated in the continuous conduction mode (CCM), whichrequires a large inductor value. Further, the PV is not actu-ally grounded; thus, the topology does not truly provide doublegrounding. Nevertheless, during alternate half cycles, either thepositive or the negative conductor of the PV remains groundedthrough the switches S1 and S4 .

All the inverter topologies, shown in Fig. 2(a)–(d), operateon the same principle in both the ac half cycles. Schekulin [22]has proposed a topology [Fig. 2(e)] derived from the basic Zetaand Cuk (fourth-order converters) configurations. This topol-ogy uses minimal number of devices. However, during positive


PATEL AND AGARWAL: SINGLE-STAGE SINGLE-PHASE TRANSFORMER-LESS DOUBLY GROUNDED GRID-CONNECTED PV INTERFACE 95

Fig. 2. Existing transformer-less, single-stage, 1 − φ PV-based inverter topologies with double grounding. (a)–(c) Topologies based on split PV-source [17],[18], [20]. (d)–(e) Configurations with single PV source [21], [22]. (f) Proposed topology.

half-cycle, it transfers the power to the grid on the principle of abuck–boost converter, while during negative half-cycle, it usesthe boost principle to transfer the power to the grid. This asym-metrical operation not only makes the control scheme complex,but may also result in asymmetrical current and may inject dccomponent into the grid current.

III. CIRCUIT CONFIGURATION AND WORKING

The circuit schematic of the proposed 1 − φ PV inverteris shown in Fig. 1. Unlike some other topologies, which usetwo PV strings, the circuit of Fig. 1 employs only one PVstring as the energy source. In addition, the negative conductorof the PV source always remains grounded, thereby providingdouble grounding. The circuit uses five switches (S1 through S5)and three diodes (D1 through D3). It uses only one buck–boostinductor L. Inductor Lf and capacitor Cf form the low-passfilter, which allows only the 50-Hz component of the inverteroutput current to enter into the grid. The various modes in whichthis circuit operates are shown in Fig. 3.

During the negative half cycle of the ac grid voltage, switchesS1 , S2 , and S5 along with the diode D1 form a buck–boost con-verter. The switches S3 and S4 always remain OFF. Fig. 3(a)–(c)shows modes I through III, respectively, in which the converteroperates during the negative half cycle. In the negative half cy-cle, switches S2 and S5 are always kept ON, while the switchS1 is triggered with the sine-triangle pulsewidth modulation(PWM) technique. When switch S1 is ON, energy is stored ininductor L1 [mode I, Fig. 3(a)]. When S1 turns OFF, the storedenergy is transferred to the grid [mode II, Fig. 3(b)].

During the positive half of the ac cycle, switches S1 ,S3–S5 ,along with diodes D2 and D3 , form a buck–boost converter.Switch S2 always remains OFF. The buck–boost converter is nowoperated by controlling the switches S1 and S5 using the signalderived from the sine-triangle PWM. Fig. 3(d)–(f) shows modes

IV through VI, respectively, in which the converter operatesduring the positive half cycle. During these modes, switches S3and S4 are always ON. When switches S1 and S5 are ON, energyis stored in inductor L1 [mode IV, Fig. 3(d)]. When S1 and S5turn OFF, the stored energy is transferred to the grid [mode IV,Fig. 3(e)].

The amplitude of the “reference” sinusoidal waveform usedfor the sine-triangle PWM (mentioned before) is controlled totrack the maximum power point (MPP). The controller imple-ments the perturb and observe (P&O) method, and identifieswhether to increase or decrease the amplitude of the referencewaveform to achieve MPP. It then increases/decreases the am-plitude of the sinusoidal template, which is derived from thegrid voltage.

Unlike the operating modes shown in Fig. 3, where the opera-tion in both the half cycles is based on the buck–boost principle,it is possible to operate the proposed configuration even withasymmetric operating principle. During the negative half cycle,the operating modes are similar to those shown in Fig. 3(a) and(b), i.e., employing the buck–boost principle. But in the positivehalf cycle, instead of operating the configuration as a buck–boostconverter, it is operated as a buck converter for that period ofthe half cycle where VPV > vg and as a boost converter for theremaining period. This feature of operating the circuit distinctlyas a buck and a boost converter for the positive half cycle canreduce stress on the components. This is a desirable yet unac-ceptable proposition on account of the asymmetry it introducesbetween the positive and negative half cycles because the topol-ogy does not support distinct buck and boost operations duringthe negative half cycle.

IV. DESIGN OF THE COMPONENTS

The design of various components used in the proposed in-verter is very crucial for generating a sinusoidal grid current,



Fig. 3. Circuit diagrams for various operating modes of the proposed configuration. (a)–(c) Operation during negative half cycle: modes I–III. (d)–(f) Operationduring positive half cycle: modes IV–V. Bold lines show the active current paths. (c) and (f) Operation after all the energy stored in the buck–boost inductor istransferred to the grid.

which is in phase with the grid voltage. The design also decidesthe discontinuous conduction mode (DCM) or CCM operation.The analysis leading to the design of the various componentsfor the DCM operation is presented next.

A. Design of Inductor L1

The design of the buck–boost inductor should ensure DCMoperation of the inverter under all conditions of temperature andpeak insolation levels. The inductor should also be able to handlethe maximum energy corresponding to the peak power rating ofthe PV array. If DCM is ensured for peak power conditions, theinverter will operate in DCM for all other conditions. The valueof the inductance is obtained for the boundary condition (criticalconduction mode), i.e., for the operation on the boundary ofDCM and CCM.

As the current injected into the grid is in phase with thegrid voltage (vg = Vgm sinωt, where Vgm is the amplitude,ω is the angular frequency, and t is the time), the maximum(instantaneous) power is injected into the grid at the time whenthe grid voltage is at its peak (i.e., ωt = 90◦). Thus, the design forthe inductor should be in accordance with the current, voltage,and power values at that instant. The maximum power obtainedfrom the PV string is given by

PPV max = VPV mpp maxIPV mpp max (1)

where VPV mpp max and IPV mpp max are the output voltageand current of the PV array, when working at MPP under max-imum insolation and uniform conditions. Assuming a losslessinverter, the maximum power injected into the grid is

Pg max =Vgm√

2Igm max√

2= VPV mpp maxIPV mpp max . (2)

Here, Igm max is the amplitude of current injected into thegrid under maximum insolation and uniform insolation condi-tions. If the switching frequency (fs) of the converter is veryhigh for a given switching period (Ts), the grid voltage (vg ) andthe grid current (ig ) can be considered constant. Under theseconditions, at an instant ωt = 90◦, vg = Vgm and ig = Igm max .The energy transferred to the grid during this period is

Emax = Vgm Igm maxTs. (3)

During the ON period

TON =LcritIpeak

VPV mpp max(4)

and during the OFF period

TOFF =LcritIpeak

Vgm(5)

where Ipeak is the peak inductor current and Lcrit is the in-ductance value for the critical conduction mode. From (4) and(5)

Ts = TON + TOFF = LcritIpeak

[1

Vgm+

1VPV mpp max

]. (6)

Using (6), the energy stored in the inductor during this periodis given as follows:

LcritI2peak

2=

T 2s

2Lcrit

[1

Vgm+

1VPV mpp max

]−2

. (7)

From (3) and (7),

Lcrit =Ts

2Vgm Igm max

[1

Vgm+

1VPV mpp max

]−2

. (8)



From (2) and (8),

Lcrit =0.25Ts

VPV mpp maxIPV mpp max

[1

Vgm+

1VPV mpp max

]−2

.

(9)Thus, the value of Lcrit (for the critical conduction mode) is

obtained from the PV string parameters, the grid voltage, andthe switching frequency. To ensure DCM operation, L1 (Fig. 1)should be smaller than Lcrit .

B. Design of Filter Capacitor Cf

The value of filter capacitor Cf is obtained by equating theenergy released by the inductor L1 and the energy receivedby the capacitor Cf . If ∆V is the ac voltage ripple on Cf , atωt = 90◦, the energy balance between Cf and L1 is given bythe following:

LcritI2peak

2=

Cf

2[(Vgm + ∆V )2 − (Vgm − ∆V )2 ]. (10)

Hence,

Cf =I2peakLcrit

4Vgm ∆V. (11)

From (6) and (11),

Cf =T 2

s

4LcritVgm ∆V

[1

Vgm+

1VPV mpp max

]−2

(12)

which gives the value of Cf in terms of L1 , grid voltage, PVstring parameters, switching frequency, and acceptable high-frequency ac voltage ripple, superimposed on the sinusoidalcapacitor voltage.

C. Design of Filter Inductor Lf

The filter inductor can be obtained as follows:

Lf =1

ω2Cf=

1(2πfc)

2Cf

(13)

where fc is the cutoff frequency, which is much less than theswitching frequency (fs) at which the switch S1 is operated.

D. Design of Decoupling Capacitor Cin

The size of Cin can be decided as follows [10]:

Cin =PPV

4πfVPV mpp max ∆VPV. (14)

Hence,

Cin =IPV mpp max

4πf ∆VPV(15)

where f is the frequency of the grid voltage and ∆VPV is theamplitude of the ripple voltage across the decoupling capacitorconnected across the PV array.

V. SIMULATION RESULTS

In this section, various cases are simulated to highlight theissues, arising due to partial shading, control scheme used,

Fig. 4. Performance of the configuration shown in Fig. 2(a) under nonuniformconditions. MPPT is implemented based on the information sensed from arrayPV1 . (a)–(c) Output power, voltage, and current of arrays PV1 and PV2 ,respectively. (d) THD of grid current. (e) Voltage across filter capacitor. (f) and(g) Inductor current. (h) and (i) Grid current under uniform and nonuniformconditions, respectively.

configuration, improper design of the circuit components, etc.Case a discusses the effect of partial shading on the split-PV-source-based configurations. Case b presents the effect of con-trol scheme employing two different operating principles in thetwo half cycles, while case c shows the effect of improper selec-tion of the buck–boost converter on the grid current waveform.

A. Results With a Split Source Topology

1) Case a (Operation of a Split PV Source in Partially ShadedConditions Where MPPT is Used With an Unshaded Array):Fig. 4 shows the response of the circuit shown in Fig. 2(a) [17]when operating under nonuniform conditions. The PV array,PV1 , is receiving an insolation of 0.5 kW/m2 . PV2 receives0.5 kW/m2 until t = 2 s; 0.1 kW/m2 for the interval 2–3.7 s;and 0.3 kW/m2 after t = 3.7 s.

Hill-climbing method is used for MPPT by sensing volt-age and current of the array PV1 . Fig. 4(a)–(c) shows that the



maximum power from both the arrays is being tracked until t =2 s due to uniform conditions on both the arrays. However, asthe MPPT is done by sensing the parameters of PV1 , after t =2 s, when PV2 starts receiving lower insolation, an optimumutilization of the array PV2 is not achieved. Fig. 4(d) shows thatthe THD of the grid current is less than the permissible limit un-til t = 2 s. However, after t = 2 s, the THD of the grid current isabout 90% for the interval 2–3.7 s and is 48% beyond t = 3.7 s.

Simulations were also performed with the MPPT imple-mented by sensing the parameters of PV2 in place of PV1 , withthe other conditions remaining the same (results not explicitlyshown due to paucity of space). It was observed that the THD ingrid current after the occurrence of sudden shading increased to85%, but in steady state, it reduced to within 10%–15% range.However, the steady-state power tracked in this case is muchsmaller than the previous case (325 W), leading to ineffectiveutilization of the PV arrays, especially PV1 .

B. Results With the Proposed Topology

1) Case b (Performance of an Inverter With Asymmetri-cal Operating Principles in the Two Half Cycles): The resultsshown in Fig. 5 are for a PV inverter employing different prin-ciples of operation in the positive and negative half cycles. Theproposed circuit (Fig. 1) is operated as a buck–boost converterin the negative half cycle [Fig. 3(a)–(c)]. However, in the pos-itive cycle of grid voltage, instead of operating the circuit as abuck–boost converter, the circuit is operated as a buck converterwhen the PV voltage is greater than the grid voltage and as aboost converter when the grid voltage is greater than the PVvoltage.

Fig. 5(c) shows the asymmetry in the grid current when thecircuit is operated with “bang-bang control” to control the in-ductor current in CCM. The amplitude of grid current during thepositive half cycle is 2.5 A, while that in the negative half cycle,it is about 2.1 A. Hence, the grid current has a dc componentof 0.10 A (about 6.44% of the fundamental component) and itsTHD is about 15% in CCM. Fig. 5(d)–(f) shows the results whenthe same circuit is operated in DCM. The grid current is moredistorted as compared to the CCM operation, with a THD of22% and a dc component of 0.18 A (10.15% of the fundamentalcomponent). Fig. 5(f) shows that the distortion is higher duringthe positive half cycle of grid current. This is on account of theswitching from the buck to boost mode and vice versa.

2) Case c (Effect of Buck–Boost Inductor Value on thePerformance of the Inverter): Fig. 6 shows the results whenan inductor of inappropriate value is used with the proposedtopology (Fig. 1). Following values are selected for the variouscomponents: L1 = 210 µH, Cf = 20 µF, Lf = 2.3 µH, Cin =4500 µF. ∆V and ∆VPV are taken as 20 and 2.5 V, respec-tively, while fc = 750 Hz. Two parallel-connected PV stringsare used as the source. Each PV string has six series-connectedPV modules. IPV mpp max and VPV mpp max values for this ar-rangement are approximately 7 A and 95 V, respectively. Withfs = 10 kHz and Vg = 230 V (rms), the critical value for induc-tor Lcrit , using (9), is obtained as 203 µH. Hence, the selectedvalue of L1 (=210 µH) results in CCM operation. Fig. 6(a)

Fig. 5. Performance of a circuit operating on different principles in the posi-tive and negative half cycles. (a)–(c) With CCM operation. (d)–(f) With DCMoperation.

shows that when the generated PV power is near its full capac-ity, the system operates in CCM. As a result, grid current getsdistorted [Fig. 6(b)] and injects a distorted grid current with alarge dc component [Fig. 6(e)]. Fig. 6(c) shows a 100-Hz ripplein the PV current. This results in a ripple in the capacitor (Cin )voltage. The more the current ripple, the more is the ripple inthe voltage and less is the average output power obtained fromthe PV array. The voltage ripple can be minimized by using ahigher value decoupling capacitor (Cin ).

Fig. 7 shows the response of the proposed converter (Fig. 1)while operating in DCM (with L1 < 203 µH). Fig. 7(b) showsthat as there is only one PV array and only one inductor, theproblem of asymmetry in the grid current does not arise. Theinsolation received by the array before t = 2 s is 1 kW/m2

and 0.5 kW/m2 after t = 2 s. Fig. 7(d) shows the amplitude ofthe reference sinusoidal waveform used for the sine-triangularPWM technique. The amplitude reaches the steady-state valueand oscillates about this value. The oscillations are a result ofintentional perturbations due to the application of P&O method



Fig. 6. Results with the proposed topology with improper inductor size.(a) Inductor current. (b) Grid current. (c) PV string’s output current. (d) Ampli-tude of the reference waveform. (e) THD and dc components of grid current.

for MPPT. Fig. 7(h) and (i) demonstrates the effectiveness of thetopology, and the control scheme to limit the THD and the dccomponent to a much lower value of 0.02 and 0.015 A (about 1%of the fundamental component at low power output under thegiven partially shaded conditions), respectively. Also, as shownin Fig. 7(e)–(g), unlike the split PV-source-based topologies, theMPP is tracked continuously. This ensures effective utilizationof the PV array.

VI. EXPERIMENTAL RESULTS

The experimental setup comprises three parallel-connectedPV strings, each with six series-connected PV modules. Thespecifications of the PV module (output power, current andvoltage at MPP, short-circuit current, and open-circuit voltage)at an insolation level of 1 kW/m2 and 25 ◦C temperature arePmax = 38 W, Impp = 2.29 A, Vmpp = 16.6 V, ISC = 2.55 A,and VOC = 21.5 V. Partial shading, using the transparent gelatinpapers, is applied artificially by shading the two modules in thefirst PV string and three modules in the second PV string.

A. Results With the Two-Inductor Topology [16]

Figs. 8 and 9 show the results obtained with the circuit con-figuration that has two parallel-connected buck–boost convert-ers [16] and does not have a double grounding. To study theeffect of mismatch in buck–boost inductor values, the circuit isimplemented with buck–boost inductors of values L1 = 0.32 mHand L2 = 0.29 mH. Fig. 8 shows that due to the mismatch, thegrid current amplitudes are not same in the positive and negativehalf cycles (visible in the power waveform). Fig. 9 shows thevariation in the potential of the positive and negative bus withrespect to the grid neutral. The absolute maximum voltage at thePV bus is the sum of the grid voltage amplitude and the PV busvoltage. This may lead to an electric shock to a person touchingthe PV array.

Fig. 7. Results when appropriate inductor value is used with the proposedconfiguration. (a) Inductor current. (b) Grid current. (c) Voltage across the filtercapacitor. (d) Amplitude of reference wave. (e)–(g) Output current, voltage, andpower of the PV array. (h) THD of the grid current. (i) DC component of thegrid current.

Fig. 8. Results with an inverter that uses two buck–boost inductors [16].

B. Results With the Proposed Topology

Fig. 10 shows the performance of the proposed inverter(Fig. 1) when operating at the global peak power point. Theoperating voltage, current, and power at this point are 92 V,2.1 A, and 193.2 W, respectively. The grid voltage is adjusted(using a variac) to about 100 V with a peak of 141 V. The powersupplied to the grid is 168 W. Fig. 10 shows a symmetrical,



Fig. 9. Variation in the potential of PV bus conductors.

Fig. 10. Results with the proposed single-inductor buck–boost inverter whenoperating at the global peak.

Fig. 11. Results with the proposed inverter at low insolation level. Voltagewaveforms have been attenuated ten times with the attenuator probe. (a) Currentand voltage waveforms for the PV array, grid, and filter components. (b) THDof grid current at two different low solar insolation levels, λ1 and λ2 (whereλ1 > λ2 ): case (i) for λ1 and case (ii) for λ2 .

distortion-free, sinusoidal grid current that is in phase with thegrid voltage. Fig. 11 shows the operation of the proposed inverterwhen the array is receiving low solar insolation. Fig. 11(a) showsthe grid-side and PV-side waveforms along with the voltage

across the filter capacitor and the buck–boost inductor current.Fig. 11(b) shows that for a grid current of 0.82 A, the THD ingrid current is 4.9% (case i), which conforms to the standardIEEE 519 (THD ≤ 5%), while for a grid current of 0.43 A(corresponding to a very low power level), the THD is 9.2%(case ii).

VII. CONCLUSION

Single-phase, single-stage, transformer-less grid-connectedPV interfaces, capable of resolving the double grounding prob-lem, have been reviewed. Most of the existing transformer-lesstopologies achieve double grounding by using a split PV source.Such topologies, when operating under nonuniform conditions,face problems such as inefficient array utilization and dc cur-rent injection into the grid. Even inverters sourced by a singlePV string, but which operate on different principles in the twohalves of the ac cycle, inject a significant dc component into thegrid current.

A compact PV–grid interface, which operates with a singlePV source and has the capability of double grounding has beenproposed, analyzed, designed, and developed. It is observed thatthe maximum voltage that can develop on the ungrounded con-ductor is limited to the PV array output voltage, and hence, thetopology exhibits a good safety feature. The topology uses onlyone PV source, a single buck–boost inductor, and a decouplingcapacitor that are shared in both the half cycles. This elimi-nates the problems arising out of asymmetrical operation andmismatch in the components.

The THD and dc component of the injected grid current aremuch lower as compared to other topologies. In addition, dueto its inherent nature, it can work over a wide input voltagerange. The effectiveness of the proposed inverter configurationto operate in nonuniform insolation conditions is demonstratedwith the help of simulation and experimental results.

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Hiren Patel received the B.E. degree in electricalengineering from the S.V. Regional College of En-gineering and Technology (now S.V. National In-stitute of Technology), South Gujarat University,Surat, India, in 1996, and the M.Tech. degree in en-ergy systems in 2003 from the Indian Institute ofTechnology—Bombay (IITB), Mumbai, India, wherehe is currently working toward the Ph.D. degree inelectrical engineering.

His current research interests include computer-aided simulation techniques, distributed generation,

and renewable energy, especially energy extraction from photovoltaic arrays. Heis an Assistant Professor at Sarvajanik College of Engineering and Technology,Surat.

Mr. Patel is a Life Member of the Indian Society for Technical Education.

Vivek Agarwal (S’92–M’93–SM’01) received theBachelor’s degree in physics from St. Stephen’s Col-lege, Delhi University, Delhi, India, the Master’s de-gree in electrical engineering from the Indian Instituteof Science, Bangaluru, India, and the Ph.D. degree inelectrical and computer engineering from the Univer-sity of Victoria, Victoria, BC, Canada.

He was a Research Engineer with Statpower Tech-nologies, Burnaby, BC, Canada. In 1995, he joinedthe Department of Electrical Engineering, IndianInstitute of Technology—Bombay, Mumbai, India,

where he is currently a Professor. His current research interests include powerelectronics, modeling and simulation of new power converter configurations,intelligent and hybrid control of power electronic systems, power quality is-sues, electromagnetic interference (EMI)/electromagnetic compatibility (EMC)issues, and conditioning of energy from nonconventional sources.

Prof. Agarwal is a Fellow of the Institute of Electronics and Telecommuni-cation Engineers (IETE) and a Life Member of the Indian Society for TechnicalEducation.


A Single-Stage Single-Phase Transformer-Less Doubly Grounded Grid-Connected PV Interface

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