Voltage Source Inverters for Grid ConnectedPhotovoltaic Systems

Hinz, H.; Mutschler, P.Darmstadt University of Technology,

Institute for Power Electronics and DrivesLandgraf-Georg Straße 4 D-64283 Darmstadt

Tel.: +49-6151-162166 Fax: +49-6151-162613email: hhinz@srt.tu-darmstadt.de pmu@srt.tu-darmstadt.de

ABSTRACT: This paper describes a novel concept of an inverter for grid connected photovoltaic arrays. It is shown that theuse of a three level voltage source inverter without transformer is a reasonable solution for the input to grid in the lowerpower range (< 5kW). The structure of the photovoltaic power system is presented. Each component of the system will bediscussed in detail. The demands of photovoltaic inverters and special design procedures are presented. Simulation resultsdemonstrate the suitability of the suggested solution for a grid connected photovoltaic system which is currently underdevelopment.Keywords: Inverter - 1: Grid-Connected - 2: Devices - 3

1. INTRODUCTION

In photovoltaic applications the grid interface betweensource (solar array) and load (utility grid) consists of theinverter. To maximize the system efficiency the invertermust be optimized in design and control. For a 2.5kWphotovoltaic power system a single phase voltage sourceinverter is developed which requires only a minimumnumber of components.Most commercial inverters for photovoltaic applicationsinclude a transformer and several sections of powerconversion ([1],[2]). To reduce the degree of complexity itis proposed to omit the transformer and to use only onesection of power conversion. Thereby system losses, sizeand costs decrease.

2. PHOTOVOLTAIC INVERTER

Fig. 1 shows the main structure of the photovoltaicsystem, which consists of the solar array, transformerlessinverter, ac-filter and utility grid (line). The renunciationof the transformer brings up fundamental characteristics:1. There exists a galvanic connection between solar arrayand grid. This admits a leakage current through theparasitic capacitance between solar array and ground([3]), if the mid-point of the solar array is not connectedto ground. Thereby the voltage between array and groundmay heavily oscillate and exceed acceptable levels (over1000V, [4]).2. By the mode of operation of a voltage source inverter(step down), the solar array voltage is not free eligible.To guarantee the operation for various environmentalconditions the right number of series connected solarmodules has to be selected. In the worst case (hightemperature) the array voltage must always exceed theamplitude of the line voltage. If a string of the solar arrayis designed by this requirement the array voltage reacheshigh levels at low temperatures.

For this high dc-voltage the three level half bridge seemsto be a promising approach for the transformerlessinverter, since in the three level inverter each IGBT hasto block only half of the voltage compared with aconventional two level inverter. Fig. 2 shows thestructure of the investigated system. The solar array issplitted into two strings. The mid-point is connected tothe ground, so that the described influence of thecapacitance between solar array and ground will beeliminated. The relatively high levels of the array voltageare no problem for the power section using IGBTs with ablocking capability of 1200V. Also the permissible arrayvoltage of the applied modules (max 1000V) will not beexceeded. For the investigated system one solar arrayconsists of 25 series connected 55W modules. For a lowtemperature of e.g. ϑ=−25°C the open-loop voltage of onemodule is approximately 28V. So in the worst case (lowtemperatures) the maximum array voltage (≈700V) willalways below the permissible value.

3. CONTROL

Fig. 2 shows the basic elements of the control-loop blockwhich have been implemented in a digital signalprocessor (software).

3.1 AC-current controlFor the inverter output current I0 a hysteresis control witha variable hysteresis width is used. To supply a linecurrent IL1 with low distortion the connection to grid is

Fig. 1: Main structure of the PV-system

made via an ac-filter, which consists of an L-C-L-combination. By a superimposed control of all statevariables, described in [3], the ac-filter is activelydamped. Fig. 3 shows the block diagram of the currentcontrol. The error of the instantaneous values of the statevariables (filter capacitor voltage uC and the line currentiL1) and the calculated setpoints (uC,sp, iL1,sp) are fed back.The coefficients of the state feed back rC, rL aredetermined by the selection of poles [3]. The switching

thresholds I0* ± HY are generated by the DSP (see Fig. 2)

3.2 DC-voltage controlTo achieve a steady-state operation the available dc-power Pdc supplied by the solar arrays and the ac-powerPac fed into the grid must be balanced. The available dc-power depends on the environmental conditions(temperature and insolation) and the array voltage. Fig.4a) shows the power voltage characteristic of one solararray. The inverter should always operate in the MPP tomaximize the efficiency. To find the Maximum PowerPoint for all conditions a tracking method is used, whichis based on the fact that in a single phase system theinstant power oscillates with twice the line frequency(fl=50Hz). This oscillation in ac-power also leads to a100Hz ripple in the dc-voltage and dc-power.It can be observed that operating in area I (left side of theMPP) the power voltage slope is positive (dPdc/dUdc>0),operating in area III the slope is negative (dPdc/dUdc<0).This leads to a fixed phase relationship between the arrayvoltage and output power as shown in Fig. 4b). Operatingin area III the voltage maximum and power minimumoccurs at the same time (phase opposition), operating inarea I the maximum of voltage and power occurs at thesame time (in phase). If the system operates in the areaaround the MPP, the ripple of the dc-power is minimized.This features can be used to detect in which part of thepower-voltage characteristics the system operates. TheMPPT adjusts the array voltage by a secondary dc-voltagecontroller to track the load (grid connected inverter)towards the MPP.

Fig. 2: Three level half bridge without transformer for grid connected pv-systems

Fig. 3: Block diagram of the ac-current control

4. SYSTEM OPTIMIZATION

As mentioned above a sinusoidal line current with lowdistortion should be supplied. In the grid connectedapplication it must be considered that the systemperturbation must be limited according internationalstandards (EN 60555-2, DIN VDE 0875 T1). Theharmonic components of the line current depends on thechoice of switching frequency and ac-filter elements. Tomaximize the efficiency of the system an optimizationprocedure as described in [5] has been used.The goal is to find the ac-filter elements and switchingfrequency at which the harmonic components can belimited and the overall inverter losses can be minimized.To estimate the inverter losses which consists of theIGBT- (switching and conducting losses) and filter-losses,approximation formula have been employed. Calculatingpv-inverter losses it must be considered that the suppliedsolar array power is not constant. It depends on thetemperature and insolation and is extremely variable fordiverse operating points in one year. Thereby appearinglosses are variable too. To find the right parameters atypically, yearly energy production of the location of theinverter (Germany) has been considered.Fig. 5 shows the calculated efficiency of the inverter forvarious switching frequencies (fS=4...24kHz) and filterinductors (L0=1...5mH). A maximum efficiency can beachieved by using a switching frequency of fS≈6kHz andfilter elements L0=4mH, L1=3.1mH and C=23µF. With

this parameters in theory over 97.3% of the solar arraysannual energy can be fed into the utility grid.

5. RESULTS

5.1 Maximum Power Point TrackingTo demonstrate the dynamic behaviour of thephotovoltaic system using the described MPPT anunrealistic abrupt change of temperature was simulated.Fig. 6 shows the simulation results of the systemoperating at a constant insolation of E=1000W/m2 and achange of temperature from ϑ1=25°C to ϑ2=15°C att=0.6s. It is assumed that both solar arrays have the sameenvironmental conditions. Fig. 6b) shows the two p-u-characteristics of one solar array sa1 valid for the chosentemperatures before (ϑ1) and after (ϑ2) the change. InFig. 6a) (zoom of Fig. 6b) ) the crosses display the changeof the mean power Pdc1,m supplied by sa1 at eachoperating point. The simulation starts in area III. TheMPPT reduces the setpoint of the array voltage Udc1,sp bya value of ∆U =1V. In this situation the setpoint Udc1,sp islower than the mean value Udc1,m. The dc-voltagecontroller increases the setpoint of the ac-power Pdc,sp.This leads to a continuously discharge of the dc-linkcapacitor until the system reaches area II (t≅0.4s). In IIthe setpoint is given by Udc1,sp=Udc1,m. Under steady stateconditions the voltage in the MPP is not found precisely,because in reality measurement errors occur.At t=0.6s the abrupt rise of temperature occurs. Now thesystem operates in area I (Fig. 6a) curve ϑ2). The MPPTincreases the setpoint of the array voltage Udc1,sp by ∆U.For the new temperature ϑ2 the supplied power of arraysa1 is higher than for ϑ1 (Fig. 6a). The setpoint of the ac-power Pac,sp changes only slightly. Until t≅0.85s thesupplied dc-power of the solar arrays is higher than theac-power fed into the grid. The resulting excess energycharges the dc-link capacitor until the new MPP is foundat t≅1s. This simulation shows that the dc-voltagecontroller is capable to track the load by an abrupt changeof temperature, which results in a visible variation of theMPP-voltage. In reality only the insolation can change

Fig. 4: Principle of the MPPT

4kHz 8kHz 12kHz 16kHz 20kHz 24kHz

94,5%

95,0%

95,5%

96,0%

96,5%

97,0%

effic

ienc

y

switching frequency fS

L0=1mH

L0=2mH

L0=3mH

L0=4mH

L0=5mH

Fig. 5: System optimization

suddenly (caused by passing clouds). In this case theMPP-voltage varies only slightly. The suggested methodstands out for its good control response and an accuratetracking without a periodically (forced) variation of thearray voltage compared to the conventional perturb andobserve method. Thereby a superfluous power swing canbe avoided. The tracking algorithm and dc-voltagecontroller can be easily implemented in a µ-controllerwhich is required for the ac-current control anyway.

5.2 Current controlFig. 7 shows the simulation results of the inverter usingthe described ac-current control. In the simulation anideal sinusoidal line voltage uL was assumed. The powerwas set to P=2,5kW=PN at cosϕ=1, for the filterelements and switching frequency the optimizedparameters were chosen. By the hysteresis control of theinverter output current I0 a sinusoidal line current IL1 canbe fed into the grid. The filter capacitor voltage uC isalmost sinusoidal. The inverter has been realised with the

optimized filter elements. In a first step of experimentalanalyses the dc-link is supplied by two line-commutatedrectifiers. The inverter is connected to a 230V,50Hz grid.Fig 8 shows the experimental results for a net input ofP=2.5kW at cosϕ=1 . The dc-link voltage (mean value,index m) was set to Udc1,m=Udc2,m=435V. This valuecorresponds to the MPP-voltage of one solar arrayoperating at the nominal power point.

6. CONCLUSION

A concept of an inverter without transformer for gridconnected photovoltaic systems has been presented.Using a three level half bridge parasitic capacitancebetween solar array and ground can be eliminated andtolerable levels of blocking voltage of IGBTs can beachieved. A suited control has been developed. Anoptimization procedure results in an optimal combinationof switching frequency and filter elements to minimizeinverter losses.

REFERENCES

[1] Steigerwald, R. et. al.: Investigation of a family ofpower conditioners integrated into utility grid, SandiaNational Labatories Report, SAND 81-7031, 1981

[2] Lohner, A; Meyer T.; Nagel A.: A new panel-integrateble inverter concept for grid-connectedphotovoltaic systems, ISIE 1996 pp 827-831

[3] Meinhardt, M.: Stromeinprägender, transformator-loser Photovoltaik-Wechselrichter mit konkurrierendenSchaltreglern für gleich- und wechselstromseitigeZustandsgrößen, Thesis Darmstadt 1997

[4] Hinz H.; Mutschler P.: Single phase voltage sourceinverters without transformer in photovoltaicapplications, PEMC 1996 pp 3/161-165

[5] Hinz H.: How to choose switching frequency and filterelements for a maximum efficiency photovoltaic inverter,PCIM 1997 pp 429-438

0,0s 0,2s 0,4s 0,6s 0,8s 1,0s 1,2s

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Fig. 6: Maximum Power Point Tracking

Fig.8: Experimental results

-15A

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-15A

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Fig.7: Simulation results