Paper presented at the 16th European Solar Energy Conference and Exhibition, Glasgow, 1.-5.May 2000 1

EMC and Safety Design for Photovoltaic Systems (ESDEPS)

T. Degner1, W. Enders2, A. Schulbe3, H. Daub4 et.al.

(1) Project co-ordination: ISET e.V., K¨onigstor 59, D-34119 Kassel, GermanyPhone: ++49–(0)561–7294–224, Fax –200 Email:abt-a@iset.uni-kassel.de

Project page:http://www.iset.uni-kassel.de/esdeps(2) OFPZ Arsenal (arsenal research), Faradaygasse 3, Objekt 221, A-1030 Wien, Austria(3) Elektrizitats AG Mitteldeutschland, Monteverdistraße 2, D-34131 Kassel, Germany(4) EnEtica S.L., Calle Nuestra Senora del Carmen 1 Ba A, E-35640 Corralejo, Spain

AbstractWithin the framework of the project ESDEPS (EMC and Safety Design for PV Systems) electromagnetic compatibility(EMC) and safety aspects of PV systems are investigated in detail. The findings from these investigations shall be thebasis for the improvement and/or creation of standards concerning the EMC and safety of PV systems.Topics covered by the project are investigations regarding the electromagnetic environment, like the effect of lightningon PV systems and the effect of transients on the mains on PV inverters, as well as investigations with respect toemissions from PV systems on the mains and DC lines and radiated emissions at radio frequencies.Keywords: Safety - 1: Qualification and Testing - 1: Grid-Connected - 2

1 Introduction

The aim of this project is to make a contribution to im-prove the reliability, safety and quality of PV systems inorder to achieve competitiveness in future electricity mar-kets. Therefore the complementary parts of system designi.e. electromagnetic compatibility (EMC) and safety aspectsare considered together to ensure compliance with all essen-tial requirements. The results and conclusions of the projectwill be submitted to the relevant standardisation committeesto further the European harmonisation process.

The project consortium consist of:

ISET e.V., Germany (Project co-ordination): A non-profit re-search institute, associated with Kassel University.arsenal research, Austria. An independent demand orientedservice enterprise for applied research and testing.EAM, Germany. A central German energy provider.EnEtica S.L., Spain. A SME which projects and installs so-lar and wind energy systems at the Canary Islands.

Currently the main activities in the project are the investi-gations regarding the effect of lightning on PV systems, theeffect of transients on the mains on PV inverters, emissionsfrom PV inverters into the mains and emissions at radio fre-quencies from PV systems.

2 Influence of the electromagnetic environmentand immunity tests of PV-inverters

2.1 Influence of lightning

PV-systems may be effected by lightning in a very high de-gree. The parameters of the usual tests approach from mea-suring data which are about 40 years old and were madewith devices with not enough bandwidth. But the electronic-components inserted in recent state of the art systems of-ten are very sensible for high frequency inductions. For asystematic analysis of the effect of lightning strokes in PV-systems, arsenal research has installed a measuring stationfor the automatic recording of induced voltages and currentsoccuring in PV-systems and special PV-components in the

frequency range up to 50Mc. Fig.1 and2 give an impres-sion about the experimental set-up. The station is located ontop of the mountain ”Gaisberg” (with an altitude of 1300m)near Salzburg. In a distance of about 150m there is an ORF-broadcasting station with a measuring system for the light-ning current of a direct stroke into the antenna mast (witha height of 100m). These measurements are performed incooperation between ALDIS (Austrian Lightning Detection& Information System,OVE) and TU Vienna.

Figure 1: Measuring station of arsenal research

The measuring station of arsenal research consists of someelements (e.g. PV-panels and antennas, an automatic high-speed camera oriented to the top of the antenna mast) mountedon a scaffold and some data recording systems located insideof a shielded cabin which is mounted on a trailer. Furtherona lightning rod with a height of about 14m is mounted be-side the scaffold. Recorded data are the voltage of some

Paper presented at the 16th European Solar Energy Conference and Exhibition, Glasgow, 1.-5.May 2000 2

shielded cabin

“field mill”

high speed camera

fibre optic transmissionfrom broadcast stationfor trigger and network

insolated overhead line

lightning rod

PV-modulesand

antennaGPS-antenna

UPS

PV inverter

3 x 230 V

mains connection

PV-modules

Rec. 2 (200MS/s)

PC

Rec. 1 (10MS/s)

data acquisition

230 V~

HP

HP

HP

HP

earthingconnectionHP: high voltage probe

slow

Figure 2: Measurement setup on the station Gaisberg.

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Figure 3: Measurement of induced voltages caused by alightning stroke.

PV-generators connected with PV-inverters, currents of PV-panels in short circuit-operation (screened and not screened),voltage of an overhead-line and an underground-cable, E-field and B-field and the current of direct stroke into thelightning rod. The Arsenal-measuring-stations of ALDIS/TUVienna and arsenal research are connected via fiberoptic-transmission lines. This gives, in the case of a direct light-ning stroke into the antenna mast an unique opportunity ofhaving recorded the inducing current in a known directionand distance and to the same time the induced voltages andcurrents in different elements which will give the base ofbetter theoretical analysis of the relevant parameters of in-duced quantities. Fig.3 shows an example for near lightningstroke induced voltages into a overhead line with a peak-voltage of 15kV and a voltage gradient of 1MV=µsand intoa single PV-panel with a amplitude of 700V.

The aim of the tests will be the definition of immunity levelsfor PV-inverters, definition of test parameters and develop-ment of suited protection-concepts.

2.2 Influence of transients in the mains, causedby switching and failures

Utility interactive inverters are influenced in a high degreeby transient phenomena in the grid. The most importantare switching phenomena and disturbances caused by earth-faults and short circuits. A very high stress for the utility-connected inverters occur, if a short circuit in a connectedbranch is interrupted by a fuse or a circuit-breaker. In thiscase first the voltage will break down and the short-circuit-current in the utility will increase. In the moment of the fuseinterruption the energy stored in the inductive componentsof the utility will be free and generate transient overvoltages.Recently in international standards there do not exist equiv-alent test-pulses for this kind of stress though it can happenrelative probable and can generate defects in PV-inverters,as some tests in arsenal research showed. The energy ofthese transients depend on many parameters like the mo-ment of the short circuit, net impedance at the short-circuit-point, kind of the fuse or circuit-breaker. The experienceshowed, that fuses with ”fast characteristic” cause the high-est steepness of the occuring transients. Arsenal researchhas developed a special test-setup for a systematic analysisof these phenomena with the aim of generation of new stan-dard test definitions. Fig.4 shows the response of a smallinverter (130W) to a short circuit in a device connected tothe mains interrupted by a 6,3A fuse with fast characteristic.The voltage reaches a maximum of 750V and in this mo-ment the current flowing into the inverter has a maximum of48A. But in this case it caused no inverter damage.

Figure 4: Response of a small inverter (130W) to a shortcircuit in a device connected to the mains interrupted by a6,3A fuse with fast characteristic.

The switching of devices connected to the same utility asthe inverter can lead to transient overvoltages and voltagedips. These transients can be simulated on the arsenal-PV-inverter-test-stand and the behaviour PV inverters of differ-ent construction-concepts were tested there under variationof different parameters like moment, duration and ampli-tude. The behaviour of the single inverters was very dif-ferent dependent of their individual construction concept.Some of the inverters were very robust against such distur-bances, but one type was so sensible, that even voltage dipsof 5% lead always to disconnection, and another type of PV-

Paper presented at the 16th European Solar Energy Conference and Exhibition, Glasgow, 1.-5.May 2000 3

inverters in the power class of 100W sinks a peak-current of48A after a half-wave voltage-dip to zero (Fig.5).

Figure 5: Response of a small inverter to a half-wavevoltage-dip.

3 Emissions at radio-frequencies

One important topic of the project is the investigation ofemissions from PV systems at radio-frequencies. These in-vestigations aim at the developments of methods, measuresand procedures to ensure the EMC of the PV systems. Theyimply (i) the investigation of the source of signals at radiofrequencies (e.g. the PV inverter) and (ii) the investigationof the propagation mechanisms.

3.1 Sources of radio-frequency signals: EMC Testson PV inverters

Comprehensive tests concerning the emissions of signals inthe frequency range 150kHz up to 1 GHz have been per-formed on eight up-to-date PV inverters for grid parallel op-eration.

Table 1 gives an overview about the tests. The test havebeen performed according to the requirement for devices inhousehold applications. For applications in an industrial en-vironment the required tests are different:

measurements of rf voltage and power on DC linesare not required.

measurements of the rf field strength requires a greaterdistance (30m instead of 10m), so the results from themeasurements cannot easily adapted to an industrialenvironment

measurements of rf voltage on AC lines is required.Here the limit values are higher than for householdapplications.

The test results were rated according to the limit lines forhousehold applications: Inverters below the limits lines wererated ”good”, less than 10dB above the limit lines ”critical”and more than 10dB above the limit with ”bad”. The resultis shown in Figure6 and may be summarised as follows:

many inverters exceed the limits.

Only one out of eight inverters tested is below thelimit lines for radio field strength.

Half of the tested inverters exceed the limits for rfvoltage on DC lines.

For an industrial environment the rating for the emissions onAC lines is: 5 inverters are ”ok”, none are ”critical” and 3are ”bad”. As mentioned above in an industrial environmenta rating for the other emissions is not required (emissions onDC lines) or not easy to derive (radio field strength).

Other results from the tests are, that the amplitude of theemissions may show a strong dependency on the workingpoint of the inverter.

The emissions on the DC lines of the PV inverter are veryimportant, because the PV generator and the wiring may actsas an antenna and lead to radiated disturbances. This topicis subject of the next section.

freq. range test appliedstandard

0.15–30MHz rf voltage on AC lines EN50081-10.15–30MHz rf voltage on DC lines EN55014-130–300MHz rf power on DC lines EN55014-130–1000MHz rf field strength EN50081-1

Table 1: EMC tests performed on eight PV inverters

0

1

2

3

4

5

6

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rf voltage on ac

lines

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lines

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lines

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rf field strength

No

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rte

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critical

bad

Figure 6: Summary of PV-inverter tests: The emissions arecompared to the limit lines which apply for household ap-plications. Rating:”ok”: below the limits”critical”: limits exceeded by less then 10dB”bad”: limits exceeded by more than 10dB

3.2 Propagation of radio signals in PV systems

The PV inverters tests presented in the previous section showed,that there are signals on the DC side of the inverters whichmay have a significant amplitude in a frequency range up toabout 100 MHz. The question of how do these signals prop-agate in the PV generator and of how strong is the radiatedsignal is the topics of this section.

Therefore fundamental investigations of the rf impedanceand radiation properties of PV cells and modules were per-

Paper presented at the 16th European Solar Energy Conference and Exhibition, Glasgow, 1.-5.May 2000 4

formed first in order to characterise these components froman rf engineering point of view /2/. At present measurementson an experimental PV system (about 400Wp rated) are per-formed. The topics under investigation are here the radiationcharacteristics of the PV plant, e.g. the antenna factor of thesystem. The investigations are currently performed and theresults shown here should be considered preliminary.

Fig. 8, top shows the radiated magnetic field measured infront of the PV generator at a distance of 3 m. The PV gen-erator was connected to a PV inverter. The common modedisturbance current measured at the system is shown in themiddle of the figure. For comparison purposes the radio fre-quency voltage measured on the DC lines of the inverteris shown. Here the inverter was connected to an artificialmains network with 150Ω rf terminating resistor.

It should be noted that the disturbance current measured atthe PV system is in the same order of magnitude comparedto the disturbance current calculated from the disturbancevoltage at the artificial mains network. Disturbance currentand radiated magnetic field show a frequency dependent re-lation.

The figures show, that if the signals on the DC lines exceedthe limit lines, the limits for the radiated field must not beexceeded. However, the opposite is true also. Further inves-tigations aim at the determination of the ”antenna factor”for the PV system to enable the estimation of radiated fieldsfrom the amplitude of the conducted signals.

Fig. 7 shows, that even at higher frequencies a PV generatormay cause radiated electric fields with significant amplitude.

Figure 7: Electric field strength measured in front of our ex-perimental PV system. The PV generator is connected to acommercially available PV-inverter. For comparison shownis a measurement with the PV system not in operation.

4 Conclusion

A significant high share of PV inverters currently availableon the market produce signals at radio frequencies above thelimit lines for household devices.

Figure 8: Top: Radiated magnetic field measured in front ofa PV generator. Middle: Common mode disturbance currentmeasured at the system. Bottom: Radio frequency voltage(inverter connected to artificial mains network)

Radio frequency signals on DC lines from PV inverters maylead to significant radiated electric fields.

The experiments on the lightning station in Austria will formthe basis to define immunity levels for PV inverters, to de-velop suited protection concepts and to define test parame-ters.

The investigations at PV inverters with respect to the re-sponse to various transient current and voltage peaks willhelp to develop suited test set-ups and criteria for PV invert-ers.

References

/1/ H. Wilk, G. Schauer, H. Harich, W. Enders: ”TestingInverters for utility interactive operation”. 2nd PhotovoltaicWorld Conference, Vienna, 6.-10. July 1998

/2/ N. Henze, J. Kirchhof, B. Rothauge. ”Hochfrequen-zeigenschaften von photovoltaischen Generatoren - Die Aus-breitung von Funkst¨orspannungen in Photovoltaikanlagen”.15th Symposium Photovoltaische Solarenergie, Staffelstein2000.

Acknowledgement

This work is funded in part by the European Commissionin the framework of the Non Nuclear Energy ProgrammeJOULE III under contract No. JOR3–CT98–0246.