Lightning Class1

8/2/2019 Lightning Class1

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X International Symposium on

Lightning Protection9 th-13 th November, 2009 Curitiba, Brazil

Surge protection for PV generators: Requirements, testing procedures andpractical applications

Josef Birkl1, Peter Zahlmann2, Ottmar Beierl3

1,2DEHN + SHNE GMBH + Co. KG, 92318 Neumarkt, Germany - E-mail: [email protected] Georg Simon Ohm University Nuremberg, Germany

Abstract The general methods of lightning and surgeprotection, as laid down in the lightning protectionstandards of the IEC 62305 series [1] can be applied alsoin photovoltaic installations. This paper describes thegeneral requirements of a coordinated surge protection inPV-applications. We mainly focus on the specificapplication of SPDs on the DC side of a PV generator. Upto now, both the installation standards, such as IEC60364-5-53 and IEC 61643-12 [2] [3]. Also the productstandards for SPDs, such as IEC 61643-1 [4] and UL 1449[5] cover only AC-applications of SPDs. In this paper thespecial requirements on SPDs in PV applications will beevaluated by computer simulation and laboratory testing.Also the implications on the specific testing methods forsuch SPDs will be discussed..

1 INTRODUCTION

Due to the exposed arrangement and the extendedsurface of photovoltaic systems, a high risk of directand indirect lightning strikes arises for such systems.The lighting current of a lightning discharge caninfluence PV installations by inductive and capacitivevoltages as well as lightning surges in the upstreampower supply system. Avoiding system failures due todamage caused by lightning surges requires a goodcoordination of a lightning and surge protectionsconcept. The general methods described in thelightning protection standards IEC 62305 can also be

applied for photovoltaic systems. In general thelightning protection system for a PV generator alsoincludes

- External Lightning Protection System (LPS)- Earthing and bonding- Magnetic shielding and line routing- Coordinated SPD protection

Figure 1 gives an example of the principal lightningand surge protection measures of a grid connected PVgenerator in an installation with an external LPS. Inthis example the separation distance s is kept by an

external LPS, isolated from the structure. Such a

system is also described as an insulated LPS. When aPV array is protected by a LPS, the minimumseparation distance between the conductive parts of thePV array and the external lightning protection systemsshould be kept to prevent partial lightning currentsflowing through the metal parts of the PV-array. ForPV arrays installed on the roof of buildings thenecessary separation distance can not be fulfilled in allcases, due to the limited space. In this case a directconnection between the LPS and the metal PV moduleframe is necessary. This has also implications on thepossible installation of SPDs. Depending on thepresence of an external LPS, the future EuropeanApplication Standard on SPDs connected toPhotovoltaic installations [6] describes different cases

for the possible installation of SPDs:- Building without external LPS- Building with external LPS separation distance s

is kept- Building with external LPS separation distance s

is not keptThis results in the different protection schemes, as laiddown in table 1.

Table 1: Possible Installation of SPDs depending on the LPS.LightningProtection

SPDs on AC-output of converter

SPD on DC-output of converter

No externalLPS

SPD Class IItestedrecommended

SPD Class IItestedrecommended

External LPS;Separationdistance kept

SPD Class Itested

SPD Class IItested

External LPS;separationdistances notkept

SPD Class Itested

SPD Class Itested

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Fig. 1 Lightning and surge protection measures of a grid-connected PV generator Protection by an external, insulated LPS [6]

2 COORDINATED SURGE PROTECTIONMEASURES OF A GRID-CONNECTED PV-GENERATOR

In general the coordinated surge protection measures of agrid-connected PV generator include, as shown in theexample of figure 1, Surge protective devices, both on theDC side and AC side of the PV installation. If the lengthof the circuit between the SPD and the DC-AC-converteris too long, propagation of surges can lead to anoscillation phenomenon. This can increase theovervoltage at the input terminals of the inverter up totwo times of the SPDs protection level. Moreoverlightning flashes close to the PV installation induce anovervoltage in the wiring between the SPD and theinverter, that adds to the protection level of SPD andreduces the protection efficiency of the SPD. Thereforeadditional SPDs as close as possible to the PV inverterare recommended, if the distance between

- PV array and DC-AC converter or- DC-AC converter and origin of installation

exceeds 10 meters.

If the PV-system is equipped with data acquisition andcontrol equipment it is necessary to protect also thecommunication circuits by suitable SPDs [7].Up to now, both the installation standards, such as IEC60364-5-53 and IEC 61643-12 and also the productstandards for SPDs such as IEC 61643-1 and UL 1449cover only AC-applications of SPDs. Therefore this papermainly focuses on the specific application of SPDs on the

DC side of a PV generator.

3 LIGHTNING REQUIREMENTS ON SPDSON THE DC-SIDE OF PV APPLICATIONS

The special requirements on SPDs installed on the DC-side of PV applications will be evaluated by computersimulation and laboratory testing. Also the implicationson the specific testing methods for such SPDs will bediscussed.

3.1 Impulse current ( I imp ) for Class I tested SPDsAs already pointed out, when a PV array is protected by aLPS and when the separation distance cannot be keptdirect bonding of the LPS and the metal part of the PVarray is necessary. In this case partial lightning currentsare flowing via the metal parts of the PV system. Figure 2shows the possible lighting current distribution in such asystem. In case of a direct lightning flash to the LPS asubstantial voltage difference between the equipotentialbonding at the roof level and the remote earth of theplus- and minus-line occurs. Due to this potentialdifference the SPDs installed at the DC terminals of thePV-array, become activated and partial lightning currentsflow via the SPDs and plus- and minus-line towardsremote earth of the DC-AC converter. Apparently theSPDs, installed at both ends of the DC line have to carrypartial lightning currents. The impulse current rating I imp for these lightning current arresters, according to theterminology of IEC 61643-1 classified as Class I testedSPDs has to withstand the expected stress at the point of installation. The lightning current sharing can becalculated according to the methods, described in part 1and part 4 of IEC 62305. Computer simulations can be ahelpful tool to evaluate the lightning current dispersion.

Therefore it is necessary to convert the real world system,as shown in the example of figure 2 into an equivalent

= ~

ss

low voltagepower line

signal line

SPD: Surge Protective Device

ACU: Acquisition and Control Unit

SPD Type 2 for PV application

SPD Type 2

SPD Type 1

SPD (test impulse D1) for signal lineacc. to EN 61643-21

Air termination system

Down conductor

Earth termination system

Combined SPD for low voltagepower supply and signal line acc. toEN 61643-11 and EN 61643-21

s: separation distance is kept(insulated LPS)

a.c.output

d.c.input

meter/maindistribution

equipotentialbonding bar

mainearthing

bar

additionalearthing

connector

windsensor

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electrical circuit diagram. This results for the example of figure 2 in the equivalent circuit diagram of figure 3.When conducted to earth, the lightning current is dividedbetween the down conductors and the electrical lines viathe SPDs connected to them. In order to calculate thelightning current distribution within the system of figure2, following paths to earth have to be considered:- Four down conductors of the LPS- The earthing connection of the PV-array- The two DC lines

Fig.2 Lightning current distribution in PV-system

Fig.3 Equivalent circuit diagram to evaluateI imp

Neglecting the impedance of the SPDs and assuming thatthe length of the LPS down conductors and the electricallines is the same, the current sharing can be evaluated byfollowing simplified approach [8]:

n

I I totalimp

(1)

n being the total number of parallel paths.

Based on this approach the values of Iimp for the SPDs,given in table 2 can be determined for different LightningProtection Levels (LPL) and differentn.

Table 2: Determination of I imp of SPDson the DC side of PV generatorsn=7 (4 down

conductors andearthing

n=4 (2 downconductors; no

additional earthing )

LPL

I imp I imp

I(200kA)

28,6kA 50 kA

II(150kA)

21,4kA 37,5 kA

III/IV(100kA)

14,3kA 25 kA

3. 2 Discharge current ( I n ) for Class II tested SPDsAs pointed out it is the basic aim of a lightning protectionsystem for PV-systems to prevent any partial lightningcurrent flowing within the PV-system. In a system,equipped with an insulted LPS only the induction effects

due to lightning electromagnetic impulse (LEMP) have tobe considered. Therefore it is sufficient, according table1, to install Class II tested SPD , on the DC side of the PVgenerator. According to figure 4 the magnetic field Hcreated by the lightning currents flowing in theneighboring LPS can induce considerable voltages in anyconductor loop, which result in corresponding impulsecurrents, if these conductor loops are closed. Theseinduced surges are the primary threat parameters for theClass II tested SPDs, installed at the terminal of the PVarray. Laboratory tests on a PV module with shuntedoutput terminal have been carried out to evaluate thenecessary surge current rating I n of the SPDs [9].

Fig.4 Induced surges in PV-arrays

due to the magnetic field of a nearby lightning current

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Figure 5 shows the main results of these induction testson a single PV module, which are summarized below:a) The induced current decreases almost proportional to

the distance between PV module and air-terminationconductor and increases with steepness of theprimary lightning current impulse.

b) The induced current has a shorted wave shape whichcorresponds almost to an 8/20 impulse current at a10/350 primary lightning current.

Based on this measurement of a single PV-module thetotal load of a PV array has been estimated with amaximum induced current of 10kA. It can be concludedthat for Class II tested SPDs installed at the DC-side of PV systems a nominal discharge current In = 20 kA(8/20) per mode of protection is sufficient.

-0.2

0.0

0.20.4

0.6

0.81.0

Iinduced [kA]-10

0

10

20

30

40

50I10/350 [kA]

0 0.25 0.5 0.75 1.0 1.25 1.5 1.75Time [ms]

Distance 0,5 mDistance 1 mDistance 2 m

Induced impulsecurrents at

different distances

Primary lightning currents in copper wire10/350 s at different distances

Fig.5 Induced surge current in a single PV-module

Figure 6 shows the further development of these tests oninduced surges in PV-modules. In a second test not onlythe behavior of single module but a complete PV arrayhas been examined. In this case the lightning current wasshared between several paths of the module frame. Thesetests confirmed the general finding of the previous tests.Considerable surges damaged the equipment installed inPV sub-array junction box, which was not protected bySPDs. The decisive test parameter for surges due to

induction effects is the steepness of the lightning currentflowing in air-termination conductors of the LPS. Due tothe impedance of this relatively large test object, theaverage current steepness was limited to skAdt di 10 ,which represents the current steepness of LPL III for thefirst short stroke as described in IEC 62305-1.To evaluate the induced surges for the subsequent shortstrokes, which are defined with 200kA/s for LPL I inPart 1 of IEC 62305 further computer simulations will beperformed. First results of these computations on inducedsurges in PV arrays due to subsequent short strokes willbe presented at the X SIPDA conference.

Fig.6 Test set-up on induced surges in PV arraysdue to the magnetic field of nearby lightning currents

3.3 Coordination SPD protection level U pand immunity of DC-AC-converterThe protective performance of an SPD is described by itsvoltage protection levelU p, as given in IEC 61643. Thesurge immunity of equipment is described by itsimmunity level according IEC 61000-4-5 [10] and itswithstand level according IEC 60664-1 [11]. In order toidentify the needed protection levelU p of the SPDs it isnecessary to establish the surge immunity of the DC-AC-converter. It has to be considered that not only themaximum voltage levelU max to be expected across theterminals of the SPD has to be compared with thewithstand of the equipment. A number of furtheradditional parameters might be relevant for effectiveprotection, such as- Maximum impulse current Imax flow into equipment- Maximum energyW max transferred into equipment- Maximum voltage-time integral dt u - Maximum voltage change

dt du .

These criteria have to be fulfilled, both for SPDs installedat the AC and DC input of the converter. Theserequirements also have to be fulfilled both by Class IItested SPD, when only inductive effects are considered,but also by Class I tested SPDs in case lightning flash

directly into the LPS of the PV installation are taken intoaccount. A lightning current test of SPD and DC-ACconverter under real service has been proposed in [12].The basic idea of such a test is to combine the standardtest philosophy of an equipment immunity test with theincreased stress parameters, needed for advancedlightning and surge protection. Figure 7 shows the testassembly of the impulse current test of a central DC-ACconverter, with Class II tested SPDs installed on the DCside and Class I tested SPDs on the AC side of theconverter.The results of such system tests deliver also an importinput for the design of new and advanced surge protectiontechnologies.

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Fig.6 Test set-up on Induced surges in PV-arraysdue to the magnetic field of a nearby lightning currents

4 SYSTEM REQUIREMENTS ON SPDS ONTHE DC-SIDE OF PV APPLICATIONSIn the previous paragraphs important parameters for theselection of SPDs on the DC-side of PV-generators havebeen described, which are all related to the lightning andsurge events. For a save and reliable operation of SPDalso some important system requirements have to beobserved, which take into account also the special sourcecharacteristic of a PV array.

4.1 Continuous operating voltage U C of the SPDsAn important parameter for the selection of an SPD is themaximum continuous operating voltage U C . The

following general rule also applies for SPDs on the DC-side of PV-applications: U C must be higher than themaximum open circuit voltage of the PV-generator. Thiscriteria has to be fulfilled under normal operationconditions and for all modes of protection (plus to minus,plus to earth and minus to earth). In order to describe theopen circuit voltage of a PV-array, the future IEC-standard CD-IEC 62548 Installation and safetyrequirements for PV generators introduced the termU OC

Arra y, the open circuit voltage of a PV array at standardconditions [13]. These standard conditions, used fortesting and rating of photovoltaic modules are definedwith a PV cell temperature of 25C, an irradiance in theplane of the PV module of 1000W/m and light spectrumcorresponding to an atmospheric air mass of 1,5.However these standard conditions are in conflict withthe basic requirement, that theU c must be higher than theopen-circuit voltage of the PV array under all normaloperation conditions. Figure 7 show the voltage-current-characteristic of a PV-cell at 1000W/m with the PV-celltemperatures as a variable parameter. The open circuitvoltage of a PV-cell increases with a decreasing cell-temperature. In the given example the voltage of ~ 42 Vat a standard temperature of 25C increases to ~ 52 V at- 25C. Therefore this voltage rise due to lower cell

temperatures also has to be considered when selectingU c of a SPD installed PV array.

Fig.7 Voltage-current characteristic of a PV moduleat different cell temperatures

It is also important to notice that the real maximum open-circuit voltage of PV modules differs to its nominal value.Especially in PV application with thin film module thetolerance of the open-circuit voltage has to be observed.Thus the selection of U c for SPDs installed on the DCside of PV application, based onU OC Arra y, seems notreasonable. A new definitionU PVmax , describing theadmissible maximum PV voltage under normal operationconditions has been introduced for the new design of surge protective devices for photovoltaic systems [14].Based on this value, the design engineer of the PV-systemis able to select the correctly dimensioned SPDs, as asimilar definition is also used for the correct selection of the admissible input voltage of DC-AC converters [15].

4.2 Overload behavior of SPD in PV-ArraysClause 4.1 describes criteria for the correct selection of SPDs under normal operation conditions. However SPDsalso have to cope with abnormal conditions such as:- Earth fault on the DC-side of an unearthed PV-

system- Excessive number of lightning strikes or impulse

currentsIn these cases, the SPDs might turn into an overloadstatus. This status has to be save, without any conflict interms of fire protection and personal protection.

a) Earth fault on the DC-side of an unearthed PV-system

Figure 8 describes the basic conditions in case of an earthfault on the DC side of an unearthed PV-System. ManyPV systems require, that neither plus nor minus areconnected to earth, especially if DC-AC converterswithout transformer are applied. In this case, the"maximum system voltage of the PV system" is appliedplus to minus. If none of both DC lines is earthed, thevoltage line to earth is only 50 % of the "Maximumsystem voltage of the PV system". SPDs are connected ingeneral line to earth as lightning surges are in general

surges to earth. Under normal operation conditions for the

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mode of protection line to earth aU c which is abovethis "50 % of the "maximum system voltage of the PV-system" would be sufficient. However in case of aninsulation fault, the full "maximum system voltage of thePV system" is applied across this SPD. So if themaximum continuous operating voltage is chosenaccording to this 50%-criteria, as explained above, SPDsmight be overstressed. SPDs including a non-linearprotection-component, might create an inadmissible faultcurrent due to the V-I-characteristic of the MOV.Doubling theU C of the SPD results in a higher protectionlevel U p at the same discharge current, which might be inconflict with the immunity level of the converter.

=

Fig.8 Insulation fault in an unearthed PV system

The scenario described in figure 8 was the backgroundfor the SPD design with a so-called fault-resistant Y-configuration ,as shown in figure 9. This design ensures,that the SPD is not overstressed even in case of aninsulation fault. On the other hand it provides a lowerprotection level between plus and minus and ensures thecoordination with immunity of the DC input of theconverter.

Fig.9 SPD with fault-resistant Y-configuration

b) Excessive number of lightning strikes or impulsecurrents

Despite the advanced fault-resistant Y-configuration SPDson the DC side of PV Systems might be overloaded due to aexcessive number of partial lightning or impulse currents.

During the Class I and II preconditioning and operating dutytests SPDs have to withstand in total 20 impulses. Followingthis complete test sequence the protection levelU P shall stillwithin the tolerance of a new SPD. A higher, excessive numberof lightning strikes or impulse currents, not exceeding thecharacteristic of SPD might lead to a slow degradationand finally destruction of its internal components. Thismight result in a so called end of life scenario. Incontrast to applications of SPD on the AC side thedisconnection of SPDson the DC side of PV systemsbyupstream overcurrent protection device is difficult, as thenominal current and the short circuit current in PV-arraysis within the same range. The usual SPDs disconnector,

which provide a reliable disconnection in case of thisend-of-life-scenario are designed for AC applications.The disconnection mechanism in DC circuits is differentto AC applications. It is necessary to implement a specificDC disconnector,The general rule to interrupt the DC-fault current is thatthe arc-voltage of the disconnector has to be above theopen circuit voltage of the DC source. For the specificapplication in PV circuits additionally the specificvoltage-current-characteristic of the PV source as shownin figure 10 has to be considered .

U [V]

I [A]

PV-Generator

U DC

Conventional DC-source with two-pole characteristic

ISCx

U arc = f (i)

Fig.10 Comparison of the static voltage-current characteristic

of a conventional DC source and a PV generatorA PV-array has in contrast to a conventional DC-sourcewith two-pole characteristic almost the behavior of constant-current source. A conventional DC-source hasa linear decreasing voltage-current-characteristic. Thedifferent static voltage-current-characteristic of thesedifferent DC- sources results in the different oscillogramsof figure 11. In this figure the voltage across adisconnection device and the corresponding DC currenthave been recorded for a comparison test of aconventional DC source with the described two-polecharacteristic and a real PV generator. The real PV

generator delivers the above described constant current

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