Transformerless Photovoltaic InvertersConnected to the Grid

T. KerekesInstitute of Energy Technology

Aalborg UniversityPontoppidanstraede 101

9220 AalborgDENMARK

R. TeodorescuInstitute of Energy Technology

Aalborg UniversityPontoppidanstraede 101

9220 AalborgDENMARK

U. BorupPowerlynx A/SJyllandsgade 286400 SonderborgDENMARK

Abstract- Renewable energy sources are getting more andmore widespread, mainly due to the fact that they generateenergy by keeping the environment clean. Most of these systemshave an isolation transformer included, which if excluded fromthe system would increase the efficiency and decrease the size ofPV installations, furthermore it would lead to a lower cost for thewhole investment. But there are some safety issues regarding themissing galvanic isolation. This paper is aiming to analyze andcompare the most common single-stage transformerless PVinverter topologies for single-phase and three-phase with respectto the leakage current generation. The best results, both forsingle-phase and three-phase systems, are obtained when themiddle point of the input capacitors is connected to the neutralpoint, thereby minimizing the voltage fluctuations present at theterminals of the PV panel.

I. TRANSFORMERLESS PV INVERTERS

As photovoltaic (PV) module prices become cheaper, thereduction of manufacturing costs of PV inverters becomes amust. PV inverters that have an isolation transformer on thegrid side, are big in size, therefore making the whole systembulky and hard to install. Topologies that use high frequencytransformer in the DC-DC converter have a reduction in theoverall efficiency, due to the leakage in the transformer.[1][2][3]A higher efficiency, smaller size & weight and a lower price

for the inverter is possible in case the transformer is left out.These transformerless solutions offer all the before mentionedadvantages, but there are some safety issues due to the solarpanel parasitic capacitance. [1]

This resulting leakage capacitance value depends on manyfactors; some of these are enumerated below: PV panel andframe structure, surface of cells, distance between cells,module frame, weather conditions, humidity and dust coveringthe PV panel. [5]

In [5] it is mentioned that a typical value of 100-200pF wasmeasured between the PV cells and the grounded palm of aperson. But in case the surface of the panels was covered withwater, this capacitance increased to 9nF, 60 times its previousvalue. In case of a solar array having a considerable surface,

the resulting capacitance has values between 50-150 nF/kW,depending on the weather conditions and panel structure.

This leads to leakage currents between the panel terminalsand ground, depending on inverter topology and switchingstrategy. The level of the leakage current depends mostly onthe amplitude and frequency content of the voltagefluctuations that are present at the PV panel terminals, but italso depends on the value of the parasitic capacitance. [5]

TABLE 1: Leakage current mean levels and correspondingdisconnection times (DIN VDE 0126- 1-1)Leakage current Disconnectaverage value time (s)

(mA)

30 0.360 0.15100 0.04

According to the German DIN VDE 0126-1-1 standard, incase of transformerless PV inverters connected to the gridthere needs to be a Residual Current Monitoring Unit(RCMU), which is sensitive for DC and AC currents and cansense DC fault currents. In case the leakage current to ground(peak value) is greater than 300mA, then disconnection isnecessary within 0.3s. Otherwise, in case of an instantaneousfault current TABLE 1 should be followed. [8]

Elimination of the transformer may lead to DC currentinjection into the grid which might saturate the powertransformers. Injected DC currents are not fault currents, butare due to a small asymmetry between the positive andnegative half-wave of the injected AC current. Most standardsdealing with utility interconnection have a section regardingthe level of the DC current that is allowed to be injected (seeIEEE 929-2000, IEC 61727, IEEE 1547, EN 61000-3-2) andthis varies between 0.5 and 1% of the rated current.

1-4244-0714-1/07/$20.00 C 2007 IEEE. 1733

11. SINGLE-PHASE TRANSFORMERLESS PV INVERTERS

In order to verify the level of leakage currents of differenttopologies simulations and experimental measurements havebeen done. Simulations were done using Simulink and PLECStoolbox, used for simulation of electrical circuits within theSimulink environment [7].

All simulation results are based on the general simulationmodel presented on Fig. 1 for the single-phase topologies.Same filter and grid parameters have been used throughout thesimulations, these are listed below:* LCL filter parameters: Lfi=0.7mH - filter inductance

inverter side, Lgi=2mH - filter inductance, Cf=2.2pF filtercapacitance, Rcfs= 1 mQ -capacitance ESR

* Inverter parameters: fsw = 1OkHz* Grid parameters: Vg=22OVrms -grid RMS voltage,

f=5OHz -grid frequency, Zg=0.5+1 1e-3 -grid impedance;

DCC

- Ground

Fig. 1: Single-phase system with parasitic capacitances

The leakage capacitances are added to the model in order tobe able to simulate the leakage current. Both capacitancevalues are chosen to be lOOnF [5]. The leakage current ismeasured between N and Ground.The most widespread single phase topology is the full-

bridge one. This topology can be chosen to have bipolar or aunipolar PWM controlled switches. Using unipolar switchingstrategy the inverter pulses will have twice the frequency thatis in case of bipolar switching, therefore the output filter canbe smaller, than in case of the bipolar. [6]

Fig. 2 presents the full-bridge topology and two of its newerdevelopments that consist in an added DC or an ACdecoupling part (grey background).

DC+

DC -

Filter Gric

LL

Fig. 2: Full-Bridge topology, showing the added DC or AC decoupling

[6][9][10]

DC decoupling has the advantage, that during the free-wheeling phases the AC voltage circuit gets disconnectedfrom the DC voltage circuit using a switch (S7). [9]

In case of the AC decoupling, switches S5 and S6 are usedduring freewheeling, thereby increasing the efficiency of theinverter, because the freewheeling through the DC link isavoided. [10]

TekRun Triq'd

Chl RMS4 9 V

: : - -= ~~~~~~Ch13 RMeS[ . . ., ~~~~~~~~26lmA4.Cii 1 0kvl1 .O0ms Al Lire f 0.00 VE

12Jun 2006gg150.0°°0% 13:53:18

Fig. 3: Experimental data for FB with bipolar switching.

TekRun Ii Trig'd

Chl RMS320 v

1h001.OkV M2.00 1 C O kX OOmsl Al Line F 0.00 VI2Jun 2D06

iii;0.;;00% 1 3:2849

Fig. 4: Experimental data for the FB topology with unipolar switching.

For the experimental measurements a three-phase setup wasused, made up of a DC power supply, a three-phase Danfossinverter and filter with a direct connection to the utility grid.In case of the single-phase setup only 2 legs of the inverterhave been used. For these single-phase measurements the1.7kWp PV installation has been used, having the frame of thepanels connected to ground, thereby creating a path for theflow of the leakage current.As seen on Fig. 3, representing the experimental scope data

in the case of the full-bridge topology with bipolar switching,the PV terminals are fluctuating with the grid frequency andthe fluctuations have half the amplitude of the grid voltage.Due to this sinusoidal fluctuation and the low frequencycontent of the fluctuation, the leakage current towards groundis small.

Fig. 3 shows the measurements, where: the inverter voltagesare with blue; voltage fluctuations of PV terminal with cyanand ground leakage current with magenta.

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The experimental results for the case of the full-bridgetopology with unipolar switching are presented on Fig. 4. Itcan be observed that the PV terminals have a high frequencyfluctuation (Vgrid/2± Vdc/2). The leakage current in this casereaches peak values up to 5A.

Also in the case of Fig. 4 the measurements shown are: theinverter voltage with blue; voltage fluctuations of PV terminalwith cyan and ground leakage current with magenta.Comparing these previous two cases it can be concluded,

that the full-bridge topology with unipolar switching is notsuitable for transformerless PV systems due to the largeleakage current flowing towards ground.

Much better results regarding the voltage fluctuationspresent at the PV panel terminals (DC+ and DC-) can beachieved in case the middle point of the input capacitors isconnected to Neutral, thereby minimizing the voltagefluctuations. Such topologies are the half bridge (HB) andneutral point clamped (NPC) topology, presented on Fig. 5.The NPC is another widely used single phase topology.

Simulation based on this topology have shown promisingresults, which are presented on Fig. 6.

It can be seen that using the NPC topology the voltagefluctuations on the PV panel are very small and the leakagecurrent is below 30mA, which was the lowest level of leakagecurrent where disconnection was necessary, as stated in theVDE 0126 standard.

III. THREE-PHASE TRANSFORMERLESS PV INVERTERS

Most of the single-phase installations were small scale PVsystems up to 5-6 kWp. Being a single-phase system meantthat there was a pulsating power output, which required bigcapacitors that increased the cost and decreased the lifetimeand reliability of the whole system. With a three phase systemon the other hand there is constant power output, which meansno large capacitors, leading to improved cost, reliability andlifetime of the whole system. Also the power output of thesesystems can be higher, reaching up to 10-15 kWp in case ofrooftop applications.

DC+

Fig. 7: Grid connected three-phase full-bridge topology

Fig. 5: PV arrays connected to grid through a NPC inverter [1]

400

+ 3000

E 200 l

100

Fig. 7 shows the three-phase full bridge system, which is themost simple and widely used topology in case of three-phasesystems. To control the switches a simple sinusoidal PWM isused having a modulation technique using 1 triangular waveand three symmetrically displaced sinusoidal signals whichare synchronized with the three-phase grid.

In the experimental setup, the PV panels have been replacedby DC power supplies, due to weather conditions.TekSto [

.n. tag

Grid current

0.07 0.08Time [s]

Vokage measuredbetween

DC+ and Ground

0.05lll

0.025

(D -0.025

-0.05 AAAAAAAAF 6ia.06 0.07 0.08Time [s]

Fig. 6: Simulation result for NPC inverterChi 2.00 A m250 V m s Al C f-145 VI

Ch4E:j5AV X ]27 Nov 2006960.000jts 02:42:26

Fig. 8: Scope data for DC to earth voltage (channel 4) and grid voltage(channel 2) in case of a transformerless three-phase full-bridge topology

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,I0.06 0.09 O.

0.09 O.

This is the reason why the leakage current has not beenmeasured and only the voltages of the DC+ and DC- terminalshave been measured.As seen on Fig. 8, the DC-to-earth voltages, shown by

channel 4 with color green, has a high frequency content(switching frequency) that can lead to large leakage currentsflowing towards ground. Therefore the three phase full-bridgeinverter is not suitable for PV inverter solutions due to itslarge leakage current and the problem with the DC-to-earthvoltage.

Furthermore two other topologies will be investigated usingthe same simulation tools as described for the case of thesingle phase topologies.The first topology is made up of a combination of three

NPC legs (3xNPC): to each one of the three phases a single-phase NPC leg is connected and controlled individually. Fig. 9presents the equivalent circuit, the NPC leg is detailed on Fig.5. The filter is a three-phase LCL filter with capacitorsconnected in triangle.

Fig. 9: 3xNPC used for a three phase system

individually connected to the three-phase grid, having themiddle point of the input capacitors connected to the Neutral.Each phase is individually controlled, using a sinusoidal

PWM strategy.

Fig. I11: Half-bridge legs used to supply a three phase system

500lll

400 -_

+300 _

E 200 -_

100 _

wZ 0.025

230

(D -0 .025 F

-0.050.06 0.07 0.08 0.09

Time [s]

Fig. 12: Simulation results for 3-full-bridge split capacitor topology

Also in this case the simulation results, presented on Fig. 12,show that the voltage fluctuation on the PV panel terminals isvery small and the leakage ground current is again smallerthan 30mA.

IV. COMPARISON

wZ 0.025-

230

CD -0.025-

-0.05,0.06 0.07 0.08

Time [s]

Fig. 10: Simulation result for 3 x NPC

As seen on Fig. 0, the simulation results are very

promising, with only a few Volts of ripple on the PV terminalsand a leakage current below 30mA.

The other topology (3xHB) is similar to the previously

presented solution, but in this case three half-bridge legs are

All the simulations were done using the same filterparameters, switching-frequency and the grid was set to bepure sinusoidal. The DC input voltage was set to be at 400Vfor the single-phase topology and 700V for the three-phasetopologies.

In case of the experimental results, for the single-phasesetup the PV panels were supplying the DC power, while incase of the three-phase setup DC power supplies were

changed for the PV panels.As detailed in TABLE 2, the single phase full-bridge

topology (FB) is only suitable in a transformerless PV systemin case the applied PWM strategy is the bipolar one, becauseusing unipolar switching, the PV panel terminals are jumping

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500 _

400-

+ 300

E 200

'O_

0.06

0.0b1

O,_0.06 0.07 0.08

Time [s]0.09 0.1

0.1

0.07 0.08Time [s]

0.09 O.

0-05 F

0.09 O.

between ±Vdc with the switching frequency, that wouldgenerate high leakage currents through the parasiticcapacitance of the PV panel. Much better results are achievedin case the middle point of the input capacitors is connected toNeutral. This was the case for the NPC topology, in whichcase the voltage fluctuations present at the PV panel terminalsis reduced to a few Volts, leading to very low leakage current.

In case of the three-phase topologies, the experimentalresults have shown that the normal three-phase inverter (3FB)would generate high leakage ground currents, due to the high-frequency voltage fluctuation between ±Vdc. But there aretwo other topologies which would be suitable for three-phasetransformerless PV inverters. These are the 3xNPC and 3xHB,in which case simulations have shown promising results,leading to low leakage current level that was within thestandard requirements.

V. CONCLUSION

Based on the previously detailed comparison and thesimulation and experimental results, it can be concluded thatthe single phase full-bridge topology with bipolar switching issuitable for transformerless PV inverter because the leakagecurrent is much lower than in case of the unipolar switching.The NPC is also a good choice for transformerless PVinverters, due to its grounded middle point, which minimizesthe voltage fluctuations present at the PV panel terminals.

In case of the three-phase topologies the full-bridge inverteris not suitable for transformerless inverter, due to the largevoltage fluctuations, which lead to high leakage current. But

in case of the 3xNPC and 3xHB topologies simulation resultshave shown that using these topologies the leakage current isvery small, below the 30mA which is set to be the smallestlevel required for disconnection based on the DIN VDE 0126-1-1.

REFERENCES

[1]. M. Calais, V. Agelidis; Multilevel converters for single-phase gridconnected photovoltaic systems, an overview, IEEE 1998

[2]. M. Calais, J. Myrzik, T. Spooner, V.Agelidis; Inverters for single-phase grid connected photovoltaic systems - an overview; PowerElectronics Specialists Conference, 2002; 2002 IEEE 33rd Annual, Volume4, 23-27 June 2002

[3]. J. Myrzik, M. Calais; String and module integrated inverters forsingle phase grid connected photovoltaic systems - a review; Power TechConference Proceedings, 2003 IEEE Bologna, Volume 2, 23-26 June 2003

[4]. S. Kjaer, J. Pedersen, F. Blaabjerg, A review of single-phase gridconnected inverters for photovoltaic modules, IEEE Transactions onIndustry Applications, Vol.41, Nr.5. Sep/Oct 2005

[5]. H. Schmidt, B. Burger, Chr. Siedle; Gefahrdungspotenzialtransformatorloser Wechselrichter- Fakten und Geruchte, 18 SymposiumPhotovoltaische Sonnenenergie, Staffelstein, Germany 2003.

[6]. N. Mohan, T.M. Undeland and W.P. Robbins, Power ElectronicsConverters, Design and Application, 3rd edition, John Wiley and Sons, 2003

[7]. J.H. Allmeling and W.P. Hammer, PLECS- Piecewise LinearElectrical Circuit Simulator for Simulink, PEDS'99, Hong-Kong, July 1999,Vol.l. pp.355-360

[8]. DKE Deutsche Kommission Elektrotechnik ElektronikInformationstechnik im DIN und VDE, DIN VDE 0126-1-1, 2005

[9]. United States Patent Application Publication, US 2005/0286281Al, Publication date: 29.12.2005.

[10]. European Patent Office, EP 1 369 985 A2, Publication date:10.12.2003.

TABLE 2: Comparison of transformerless inverter topologies

Topology FB bip FB unip NPC 3FB 3xNPC 3xHB

Nr. of input capacitors I 1 2 1 2 2

Nr. of switches 4 4 4 6 12 6

Bypass diodes - 2 - 6

DC to Ground voltage Vgrid/2, +Vdc, 10kHz 1% Vdc +Vdc, 10kHz 1% Vdc 1% Vdc(peak value and frequency) 50Hz

Leakage current < 30mA >> 5A < 3OmA >> < 3OmA < 3OmA(peak values)

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