EMC Issues in High-Power Grid-Connected Photovoltaic Plants

EMC Issues in High-Power Grid-ConnectedPhotovoltaic PlantsRodolfo Araneo,Member,IEEE,Sergio Lammens,Marco Grossi,and Stefano BertoneAbstract—This paper outlines the main electromagnetic compat-ibility issues frequently encountered in high-power grid-connected photovoltaic(PV)systems,and proposes several guidelines that al-low to obtain an effective and highly efficient layout of the plant. Atfirst,with reference to a1-MWp PV plant of Sorgenia Solar, Italy,the paper focuses on the peculiar aspects concerning the radio-frequency behavior of a large-size PV plant:the great ex-tent of the direct-current cabling,the capacitance toward earth of the PV source,the common-mode(CM)disturbance generated by inverters,the galvanic isolation ensured by dedicated low-voltage power transformers,and the presence of resonant circuits for CM leakage currents.The dominant electromagnetic phenomena and coupling mechanisms are pointed out by means of an equivalent circuit and through measurements.In the light of the matured ex-perience,several guidelines are proposed and a new reliable layout is implemented on a second1-MWp PV plant.New measurements confirm the validity of the proposed design strategy.Index Terms—Electromagnetic conductive interference,electro-magnetic coupling,electromagnetic radiative interference,leakage currents,photovoltaic(PV)power systems.I.I NTRODUCTIONI N RECENT years,the need for clean power generation isbeing recognized more and more worldwide.Environmen-tal concerns correlated with conventional energy supply and use(e.g.,global warming caused by greenhouse gas emissions, air pollution,acid precipitation,ozone depletion,and possible radioactive substance emissions)have boosted the research for possible solutions,which today mainly include improved energy efficiency and increased renewable energy supplies[1].The energy production from self-renewing environment-friendly sources has become one of the most appealingfields of research for engineers and scientists.Among them,the solar photovoltaic (PV)source is considered as one of the leading potential sources of electricity for the21st century due to several reasons[2]:it utilizes an abundant energy source(the sun),has no emissions, can be easily integrated in buildings,and the cost of the in-stalled kilowatt peak(kWp)is decreasing and becoming more affordable.The worldwide-installed PV power capacity shows a nearly exponential increase,which is mostly dominated by grid-connected applications[3].In Europe,subsidy programsManuscript received November28,2008;revised April2,2009.First published August4,2009;current version published August21,2009.R.Araneo is with the Department of Electrical Engineering,“La Sapienza”University of Rome,Rome500185,Italy(e-mail:rodolfo.araneo@uniroma1.it). mmens and M.Grossi are with Sorgenia Solar S.r.l.,Rome121-00125, Italy(e-mail:mmens@sorgenia.it;marco.grossi@sorgenia.it).S.Bertone is with Finmotor S.r.l.,Milan20019,Italy(e-mail:info@finlab.it). Color versions of one or more of thefigures in this paper are available online at .Digital Object Identifier10.1109/TEMC.2009.2026055and changes in the energy laws promulgated by several coun-tries in the last years in accordance to European Union(EU) recommendations have been pushing constant developments of PV systems,and have made the PV market grow at rates of40% and more.The spreading of grid-connected systems had a significant impact on several engineering research issues,ranging from an industrialfield concerning the inverter topologies[4]and the optimization of the maximum power point tracker(MPPT)[5] to a powerfield concerning the power quality of the energy injected into the grid(power factor characteristics,harmonic distortion,and possible direct-current injection)[6],systems preventing possible islanding[7],[8],and grid stability under distributed energy generation[9],[10].Several issues worthy of investigation also arise in the elec-tromagnetic compatibility(EMC)field,especially when dealing with high-power grid-connected PV plants.The main topics are radiated and conducted emissions,and immunity problems in electronic equipment.The long direct-current(DC)cabling,the common-mode(CM)voltages generated by inverters,the pres-ence or absence of line frequency power transformers,and the grounding of PV components are all the aspects that affect the radio-frequency(RF)behavior of the PV system,especially as it concerns conducted and radiated disturbances in the frequency range from150kHz to30MHz.These problems are subtle since the PV plant components are usually chosen on the basis of energy performance and efficiency criteria,and on-site mea-surements are usually unavoidable for the characterization of such complex systems.Despite the wide spreading of PV systems,to the best of our knowledge,little effort has been spent to investigate EMC aspects of high-power grid-connected PV plants,as the main contributions present in the literature focused on low-power systems.Two EU projects[11],[12](the research in[11]and [12]was funded in part by The European Commission in the framework of the Non Nuclear Energy Programme,JOULE III) comprehensively investigate numerous PV plants in terms of RF engineering by means of measurements and numerical simula-tions.The main focus is on the emissions at RF on the DC-side of the system.Impedance characteristics and antenna factors for PV systems whose powers range from1to40kWp are an-alyzed and,although the physical values may vary significantly from system to system,common properties are identified.Mea-surements are made on a2.9-kWp grid-connected PV plant for the investigation of the CM current on the ground connection of the single-phase inverter in[13].Other works concern the development of appropriate equipment and setups for the RF measurements of the DC side such as the DC-line impedance0018-9375/$26.00©2009IEEEFig.1.Grid-connected PV plant of Molfetta (Sorgenia Solar property)of 1MWp.stabilization network (LISN)[14],or the examination of possi-ble electromagnetic emission limits [15].In this paper,the RF behavior of high-power grid-connected PV plants is investigated and specific guidelines are proposed for the design of a layout that were reliable form an EMC point of view.Special attention is focused on the possible coupling mechanisms between the DC field and the alternating-current (AC)side of the PV system,which may provide favorable paths for the flow of CM disturbance currents.The paper is orga-nized as follows.In Section II,the main design aspects of high-power PV plants are presented with reference to a first 1-MWp grid-connected PV plant of Sorgenia Solar,Molfetta,Italy:an equivalent circuit and a simple CM model are proposed to shed light on the dominant disturbance coupling mechanisms.Section III presents several frequency domain measurements of CM disturbances encountered in the Molfetta plant under dif-ferent operating conditions.The experimental data allow us to understand the effect of the conducted and radiated disturbances caused by the flow of CM currents into the ground connection,especially on electronic equipment that uses the same connec-tion for grounding or earthing purposes.The effect of Enerdoor passive filters introduced at some critical points of the plant to solve the conducted interference problem is also reported.In the light of the matured experience,in Section IV,general guidelines are given that allow to design a EMC reliable layout of large-area plants.The proposed layout is implemented on a second successive PV plant of Sorgenia Solar,Benevento,Italy.Finally,new measurements show the validity of the proposed design strategy.II.H IGH -P OWER G RID -C ONNECTED PV S YSTEMSA.Design AspectsThe high-power grid-connected solar PV systems are com-posed of several components and subsystems.The basic de-vices are the PV modules,which are generally field-installed in arrays,assemblies with their own support structure and foun-dation,as shown in Fig.1.The PV arrays,generally,consist of one or more strings,i.e.,panels connected in series,which constitute the basic DC power producing units.The arrays are grouped in a PV power subsource,whose output is connected to an appropriate three-phase inverter.More subsources com-pose the whole PV power source.Appropriate switchboards are field-installed throughout the PV source to allow the parallelconnection of strings and arrays.The inverters are then paral-leled on the AC side in an appropriate switchboard,where the grid-interface device is placed,and the whole output is deliv-ered to the medium voltage (MV)distribution network through a line-frequency power transformer to form a grid-connected interactive system.PV systems are also usually equipped with electronic equipment for the acquisition and transmission of environmental data and for security purposes.A simplified one-line diagram of the PV system of Molfetta is shown in Fig.2.It is composed of 4704polycrystalline panels grouped in four PV subsources:the first subgenerator consists of 1184Solon of 210Wp,the second one of 1168Solon of 205Wp,the third one of 1200SEM of 220Wp,and the fourth one is composed of 448SEM of 210Wp and 688SEM of 215Wp.The total rated power of the PV system is 99804kWp.The modules are aggregated in strings composed of 16series-connected pan-els,so that the total number of strings is 294.The PV system extends over two hectares,and is isolated from the ground.The DC voltage level of the PV source is 600V .The four inverters are SunWay TG290of Elettronica Santerno,with maximum input power of 286kWp.The output rms AC voltage of the inverters is 202V .The four inverters are parallel-connected on the AC side on an appropriate low-voltage (LV)switchboard at 202V ,and an LV/medium voltage (MV)202V/20kV power transformer is used to deliver the produced power to the network.An LV transformer is then used to feed the auxiliary services of the plant.B.RF CharacterizationThe EMC characterization of a large PV system is a diffi-cult task,because several aspects influence its RF properties,and generally,all the main parameters of the plant are exactly not known in advance (e.g.,grounding and height of the mod-ules/cabling above the ground).However,the modularity of these plants and some peculiar characteristics that differentiate the PV sources from any other DC voltage source allow us to recognize some common RF characteristics.The main CM disturbances that can be encountered in the frequency range 150kHz–30MHz in a large-sized PV sys-tem are conducted disturbances (generated by the commuta-tions of the power electronic switches utilized by the invert-ers)and the radiated interferences (due to the radiation of CM currents and/or coupling with external electromagnetic fields).ARANEO et al.:EMC ISSUES IN HIGH-POWER GRID-CONNECTED PHOTOVOLTAIC PLANTS641Fig.2.Simplified one-line diagram of the PV system of Molfetta.Since high-power PV systems are generally placed in rural en-vironments,the last issues (coupling with electromagnetic field originated external to the plant)is often negligible.In Fig.3,an equivalent circuit representation of a single PV subgenerator of the Molfetta plant is reported.In the following,with reference to this plant,the RF aspects specifically related to PV systems are discussed,pointing out the dominant disturbance mechanisms that will be eliminated next by the proposed layout in Section IV.1)PV DC Side:The RF behavior of the DC side of a PV system is dominated by the capacitance between generator and earth,and the DC cable inductance.Steady-state characteristics of a PV panel can be modeled by means of the equivalent circuit shown in Fig.3,composed of a connection of a current source,two diodes,a parallel and a se-ries resistance,and a capacitance [16].The most important point is that the single PV panel represents a capacitance toward the earth due to the large area of the modules.The capacitance usu-ally depends on several factors,e.g.,the size of the module,the height above the ground,and the relative humidity that affects the dielectric permittivity of air.For large ungrounded PV ar-rays,typical values of the capacitance have been found to range between 50and 150nF/kWp for glass-faced modules,and up to 1µF /kWp for thin-film modules in damp environments or rainy days.Moreover,in large PV systems,the DC side may extend over several hectares while DC lines may extend in length up to 150m,thus increasing the effect of the distributed transversal capacitance.This capacitance usually provides a path for CM leakage currents.The CM impedance of the DC side is also affected by the grounding of the module frames (equipment grounding):grounding leads to a significant reduction of the CM impedance at lower frequencies (below 1MHz).Grounding of the PV mod-ule frames is an interesting matter of discussion,concerning not only EMC aspects,but also the protection of personnel and property.Although all commercial PV modules are made with reinforced insulation (class II components),their grounding is allowed because it usually helps to ensure maximum system642IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY ,VOL.51,NO.3,AUGUST2009Fig.3.Equivalent circuit of a PVsubsource.Fig.4.(a)CM voltage v cm .(b)Its harmonics.safety over the entire life of the plant (25years and more).Fur-thermore,a live electrical conductor (usually the negative one)of the DC two-wire PV field may be solidly grounded to operate the PV generator as a solidly earthed system (system ground-ing).This leads to a further reduction of the CM impedance at lower frequencies.DC circuit grounding is another interesting issue:it is a common practice in the U.S.where it is recom-mended by the National Electric Code (Section 690)[17],while the common European practice is to operate the PV generator ungrounded.As concerns the DC cabling,its length influences the ef-fect of not only the distributed transversal capacitance but also of the distributed longitudinal inductance,which is part of a resonant circuit,as it is evident from Figs.3and 4.Further-more,its length is quite often comparable with the wavelength of the disturbance so that,in general,wave propagation effects cannot be neglected.Thereby,the well-known multiconductor transmission line model shown in Fig.3is used [18].Since the PV generator may act as an antenna,RF radiation due to CM currents can be large,especially close to possible resonant frequencies of the CM circuit.Therefore,the coupling among different lines via radiated fields must be seriously accounted for.To this purpose,distributed voltages and current sources must be introduced to model excitation effects.2)Inverters:The core of the PV plant are the inverters that are required to feed energy into the grid,to invert the DC current into an AC one with the lowest harmonic content,and to boost up or chop down the DC voltage of the PV generator to the rms value of the grid voltage.To fulfill all these requirements,the inverters make use of several components:an input EMC DC filter,a DC/DC converter,a full-bridge DC/AC inverter,and an output EMC AC filter.The single-stage DC–AC conversion topology is quite often used in common PV inverters.In this topology,the full-bridge inverter,shown in Fig.3,is the core part,which is usually controlled by means of unipolar or bipo-lar pulsewidth modulation (PWM)techniques,as described in well-known textbooks [19].The inverter makes use of power semiconductor switches (bipolar junction transistors (BJTs),ARANEO et al.:EMC ISSUES IN HIGH-POWER GRID-CONNECTED PHOTOVOLTAIC PLANTS643 insulated gate bipolar transistors(IGBTs),or MOSFETs)thatcommutate at high switching frequencies(>16kHz)to avoidlow-order harmonics and minimize the cost of EMCfilters.From an EMC point of view,the full-bridge inverter is a sourceof a CM voltage v cm that can be calculated from the voltages ofthe three midpoints of the three legs with respect to the negativeDC bus,v RO,v SO,and v TO asv cm=13(v RO+v SO+v TO).(1)Fig.4(a)shows the v cm generated by a full-bridge inverter controlled by a synchronous double PWM,with modulating frequency f1=50Hz,carrier frequency f s=1050Hz(fre-quency modulation ratio m s=21),and amplitude modulation ratio m a=1.The low value of the latter parameter has been chosen for the sake of clarity.It is evident that v cm contains high-order harmonics that are clearly shown in Fig.4(b),where its Fourier transform is reported.Because of the presence of harmonics of order higher than200,it follows that if the carrier frequency is16kHz or more,the whole RF range from150kHz to30MHz has to be considered.3)Line-Frequency LV Transformers:Grid-connected PV systems may include line-frequency LV transformers in the con-version stage.These may be necessary to boost up to400V the full bridge output voltage that is usually in the range between 202and270V if no high step-up DC/DC converter is present and/or to guarantee a galvanic isolation between the grid and the PV system.This isolation provides personal protection and avoids the injection of continuous current into the grid.How-ever,since the line-frequency transformer is heavy,expensive, and increases the complexity of the inverter,a large number of inverter topologies without transformer and with multistage conversions have been developed in the literature and are avail-able today[20].They have a low side effect on personal safety and grid integration,and are particularly advantageous for their high efficiencies.However,the removal of the transformer,and hence,its isolation capability must be carefully considered.In fact,avoiding the transformer results in a galvanic connection between the AC side of the PV system and the PV source.This problem is well known when dealing with low-power single-phase inverters connected to the low-voltage grid[21],[22]. C.CM Disturbance PatternsThe four paths available for CM disturbances toflow inside the PV plant are shown in Fig.3:they consist of the capacitance between each module and the ground of the DC subsource, the capacitance toward the ground connection of the inverter, the galvanic connection on the AC side with other subsources, and the radiation mechanism.The CM equivalent circuit of the system is shown in Fig.5: it consists of the stray capacitances,the DC circuits,and the inverters and the switchboard for the parallel connection of the inverters on the AC side.This circuit can be highly resonant, due to the presence of the inductances and capacitances of the DC and ACfilter elements of the inverters,together with the PV array earth capacitance and cabling inductance.The CM volt-ages v cm generated in the full-bridge inverters can excitethis Fig.5.Equivalent circuit for the CM disturbances.circuit and make CM leakage currents i cmflow.Due to the high values of the stray capacitance and to the fact that all the compo-nents of the PV system are optimized to improve the overall effi-ciency,the damping elements in this resonant circuit can be very small so that the CM currents can reach amplitudes well above the permissible levels,such as those allowed by the standards [23],[24]for the AC side.Furthermore,the problem is made more tricky by the fact that the resonant frequencies of the CM circuit are notfixed,due to the varying PV array earth capac-itance that depends on environmental conditions.The leakage currents can be reduced by including damping passive compo-nents orfilters in the circuit;however,these elements reduce the overall efficiency of the energy conversion system.The CM currents can be source of severe conducted interfer-ence that can cause defaults in the operation of inverter elec-tronic circuits of the inverters(MPPT and devices that super-vise the interaction with the grid)and/or in sensitive electronic equipment(e.g.,a security camera system).These disturbances are enhanced by the fact that electronic components are usu-ally grounded to the same ground connection.Furthermore,as previously explained,the source-to-victim coupling can also be radiated.The CM radiated disturbances can be further en-hanced if the power cables feeding electronic equipment and/or their signal cables run near the DC power cables in parallel raceways.D.EMC Requirements and LimitsAs concerns the conducted RF emissions of the inverters,in-ternational standards for industrial[23],[25]and domestic[24] environments enforce the maximum levels on the AC side.How-ever,for the DC side,no standard is available(although in the European project[11],possible limits have been examined and proposed).A simple(and still common)approach is to use for the DC side the same RF limits as in the standards for AC con-nections.After afirst period,when inverters emitted high levels of RF disturbances on the DC side,manufacturers soon realized the importance of reducing them;today,modern inverters from644IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY,VOL.51,NO.3,AUGUST2009experienced manufacturers comply with AC standard limits.In any case,when several inverters are used,parallel-connected in the same PV system,the disturbances generated by single inverters can add up so that the limits may be exceeded.An im-portant issue to be pointed out here is that,for the main terminals disturbance voltage in the frequency band150kHz–30MHz, the Standard EN-61800-3(that specifies EMC requirements for power drive systems(PDSs)and to which industrial PV inverter can be assimilated)imposes limits higher than those imposed by the Standard EN-55011for industrial,scientific,and medi-cal(ISM)equipments.The disturbance voltage limits(average and quasi-peak)for PDS of category C3(i.e.,PDS of rated voltage less than1kV)intended for use in the second envi-ronment(industrial),with rated nominal current higher than 100A,are shown in Fig.6,while the limits for Group1-Class A equipments(suitable for use in all establishments other than domestic)are shown in Figs.9and10.It is,thus,evident that, roughly speaking,the power inverters are allowed to produce more disturbance.III.O N-S ITE M EASUREMENTSThe measurements have been performed in situ according to the Standard EN55001,by using a voltage probe and a receiver in accordance to the publication CISPR16-1[26]:the volt-age probe is a PMM SHC-1and the voltage receiver is a PMM 9000,with average and quasi-peak detectors.The standards pre-scribe measuring the voltage disturbances in the frequency range 150kHz–30MHz at each main terminal of the equipment under test(EUT).In the following,only the CM voltage disturbances will be shown for the sake of conciseness and because they are more pertinent to the subject of this paper.Fig.6(a)and(b)shows the v cm disturbance level at the DC input terminals of inverter 1,when the inverter’s output power is40and17kW,respectively.The point of measurement is indicated with the letter A in Fig.2.Both the quasi-peak and average measurements are reported together with the limits pre-scribed by the Standard EN-61800-3.It can be noted that the disturbance level does not depend on the power supplied by the inverter,but it is intrinsically generated by the working of the inverter.Even if the disturbances meet the specified limits,they are characterized by a high level,especially in the lower part of the frequency range of interest,and they will be the cause of incorrect operation of electronic devices,as will be seen in the following.The effect of Enerdoorfilters connected in parallel between the positive and negative conductors,and ground at the DC input port of the inverter is shown in Fig.6(c).It can be observed that the disturbance level significantly decreases especially in the low-frequency range(below1MHz).The CM disturbance level on the AC side at the three-phase output port of the inverter is similar to the disturbance shown in Fig.6(a) and(b),although it is not reported for brevity.What is worth emphasizing here is that a high level of voltage disturbance is generated by each single inverter on both the DC and AC side; this can propagate through the plant by means of the coupling mechanisms describedpreviously.Fig.6.CM disturbance voltage measured at the DC input terminals of Inverter 1(point A in Fig.2)when the inverter’s output power is(a)40kW and (b)17kW.Limits prescribed by EN61800-3are also reported.(c)The effect of Enerdoorfilters connected in parallel between the positive and negative conductors and ground.To investigate,in more detail,the behavior of the CM dis-turbances,v cm has been measured at the DC output terminals of a panelboard for parallel of PV panels in the subsource 1 (point B in Fig.2).In Fig.7(a),the v cm level from9to150kHz at this point is shown,while in Fig.7(b),the v cm level from 150kHz up to30MHz is reported.The measurement corrob-orates the fact that the conducted disturbances propagate along the DC cabling.Furthermore,in order to point out that these dis-turbances return through the ground connection,v cm has been measured at the ground connection of inverter 1,between the ground system of the MV/LV substation(to which the inverter’s ground conductor is connected)and the ground system of theARANEO et al.:EMC ISSUES IN HIGH-POWER GRID-CONNECTED PHOTOVOLTAIC PLANTS645Fig.7.CM disturbance voltage measured at the DC output terminals of a panelboard for parallel of PV panels in the subsource 1(point B in Fig.2)in the frequency range (a)9–150kHz and (b)150kHz–30MHz.Fig.8.CM disturbance voltage measured at the ground connection of inverter n ◦1,between the ground system of the MV/LV substation to which the inverter’s ground conductor is connected and the ground system of the DC side of the PV plant (point C in Fig.2).DC side coming from the PV plant (point C in Fig.2).The measurement results are shown in Fig.8.This last measurement confirms that the CM disturbances delivered by the inverter,af-ter propagating across the DC and AC side of the PV system,couple again through the ground connections (paths A and B in Fig.3).To point out more clearly that the voltage disturbances prop-agate on the AC side of the plant,v cm has been measured at the AC input terminals of the subsource 1auxiliary services panelboard (point D in Fig.2),and is shown in Fig.9.It isworth Fig.9.CM disturbance voltage measured at the AC input terminals of the subsource n ◦1auxiliary services panelboard (point D in Fig.4).Fig.10.CM disturbance voltage measured at the AC input terminals of the UPS (point E in Fig.2)when (a)the inverters are working and (b)the inverters are disconnected.noting that the auxiliary services of the system are fed by a line-frequency LV delta–star power transformer with voltage ratio 400/202V .The latter does not block out the RF disturbances,since its isolating effect is overtaken by radiation mechanisms and conductive mechanisms through the common ground con-nection,as shown in Figs.3and 5.Moreover,it is clear that the disturbance level is higher than the limits prescribed by the standard [23].This means that the inverters do not disturb each other,but their whole effect is dangerous for sensitive equip-ment fed by the auxiliary service panelboard (e.g.,intrusion646IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY,VOL.51,NO.3,AUGUST2009Fig.11.One-line diagram of the proposed EMC reliable PV power generating system.detection system,fire detection system,and grid and PV protec-tion devices).The AC disturbances have also been measured at the input terminals of the uninterruptible power supply(UPS)(point E in Fig.2),and the results is shown in Fig.10(a).It is also evident that in this case,the disturbance level is higher than the allowable limits.To reduce it,Enerdoor RFfilters have been installed between the auxiliary service panelboard and the UPS. The positive effect is shown in Fig.10(b),which clearly shows that now the limits are met.IV.D ESIGN G UIDELINES AND P ROPOSED L AYOUT Accumulated experience through the design and operation of more than15PV plants in the last two years,with nominal power ranging from1to15MWp,has allowed us to identify some guidelines and advisory recommendations for the design of PV industrial electric power systems that will satisfy the general EMC operational requirements,in general.The proposed EMC reliable scheme is reported in Fig.11.Equipment grounding of the PV module frames and support, which are connected to the ground reference point,isfirst sug-gested since it makes the overall CM impedance of the PV power generating system toward earth more controllable.Although equipment grounding reduces the impedance at low frequen-cies,it allows to obtain a more stable value of the impedance in different weather conditions and dramatically reduces danger-ous resonance effects.Moreover,bonding of the PV module frames is also rec-ommended:the module frames are interconnected with low-impedance paths(ground conductors or support buses)and con-nected to the ground electrode system that has to be extended over the entire PV area with buried ground wires.This prac-tice reduces voltage differences within the PV power generating system(especially at lower frequencies),resonance effects,and fluctuations in the CM impedance.Power transformers should be used at the AC output of the inverters to introduce a galvanic isolation between the inverter that is the source of the disturbances and the AC side of the sys-tem.The type of three-phase-transformer connection is mainly dictated by the distribution system grounding of the AC side and coordination of protective devices.Delta–wye transformers are usually adopted with middle-power-ranged inverters,whereas solidly grounded primary wye–wye transformers are used with high-power inverters to minimize the possible problem of fer-roresonance.From an EMC point of view,the most effective solution should be seeked together with the distribution system of the PVfield,looking at the plant in its whole.A solidly grounded neutral AC distribution system(the neutral is con-nected to the same ground electrode system used for the PV field)with delta–wye connected LV/LV transformers,and with the PVfield operated ungrounded(with electronic ground fault detectors)turns out to be a reliable solution.In fact,it allows the control of transients and temporary overvoltages that may affect sensible AC equipment operating performance,while it does not provide a closed loop for the CM disturbance currents that may circulate thought the AC and DC side of the plant passing through the ground electrode system.Special care must also be paid to protection against ually,an industrial PV power generating system does not introduce a lighting protection level(LPL),so that an ex-ternal lighting protection system(LPS)is not usually required. However,a set of surge protection devices(SPDs)properly se-lected,coordinated,and installed are required to suppress both common-and differential-mode transients,and consequently, reduce failures of inverters.SPDs(class II)are generally in-stalled in the string parallel boxes,and at the DC and AC termi-nals of the inverters.Finally,attention should be focused on the inverter immunity against fast conducted transients.High-quality inverters from experienced manufacturers should be used.They are usually provided with good RF EMCfilters(which are interior parts of the inverter)that are useful to increase the reliability of the inverter since they enhance immunity against common-and differential-mode transients that could cause inverter upsets or, in some cases,failures.Additional RFfilters could be added to reduce the CM disturbances when the layout of the plant has not been properly designed,but they should be avoided because they affect the overall energy performance of the plant.In fact, power dissipation is always present in these components,usually associated with ohmic losses and leakage currents.Also,when using high-qualityfilters,it should be kept in mind that these components should work efficiently for20–25years in an open-air(industrial)environment,where performance degradations can occur more rapidly.The proposed layout has been tested on a new1-MWp PV plant in Benevento,Italy.The PV generator is composed of 5916Solarfun SET-NY crystalline modules grouped in four PV subsources divided as follows:1368170-Wp modules in the subgenerator1,1392170-Wp and132180-Wp modules。