HOW TO VERIFY LIGHTNING PROTECTION EFFICIENCY FOR ELECTRICAL SYSTEMS? TESTING PROCEDURES AND PRACTICAL APPLICATIONS

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1 IX International Symposium on Lightning Protection 26 th -3 th November 27 Foz do Iguaçu, Brazil HOW TO VERIFY LIGHTNING PROTECTION EFFICIENCY FOR ELECTRICL SYSTEMS? TESTING PROCEDURES ND PRCTICL PPLICTIONS Birkl Josef Zahlmann Peter DEHN + SOEHNE DEHN + SOEHNE Josef.Birkl@technik.dehn.de Peter.Zahlmann@technik.dehn.de Hans-Dehn-Strasse 1, D Neumarkt, Germany bstract There are increasing numbers of applications, installing Surge Protective Devices (SPDs), through which partial lightning currents flow, and highly sensitive, electronic devices to be protected closely next to each other due to the design of electric distribution systems and switchgear installations which is getting more and more compact. In these cases, the protective function of the SPDs has to be co-ordinated with the individual immunity of the equipment against energetic, conductive impulse voltages and impulse currents. In order to verify the immunity against partial lightning currents of the complete system laboratory tests on a system level are a suitable approach. The proposed test schemes for complete systems have been successfully performed on various applications. Examples will be presented. 1 INTRODUCTION The protective effect of SPDs is verified in tests according to the relevant product standard such as IEC and IEC [1], [2]. The verification of immunity or surge immunity against conductive impulse voltages or impulse currents of electrical and electronic devices is carried out according to IEC [3]. However, the tests described in these two standards mainly refer to the SPD itself or the equipment to be protected. The necessity to combine both test levels within one system level test is stated by both parties. Figure 1 shows the basic protective criterion for the coordination of the individual surge immunity of a equipment and the "protective performance of SPDs". In general, the protective performance of a SPD is described by stating the voltage protection level U P, which is determined according to the test procedures specified in IEC The immunity of equipment from voltage and current surges is described by the magnitude of the applied voltage test level (installation class). Considering the protection of equipment by means of an upstream SPD, the following basic criteria applies: CWG 1.2/5 // 8/2 CWG Testing of a SPD according to IEC SPD Voltage protection level Up U max < Testing the surge immunity of equipment according to IEC Test level Combined system test: Testing the SPD and equipment to be protected SPD i DUT I max* U max* W EUT* CWG 1.2/5 // 8/2 - Coupling network I max < I max* U max < U max* W EUT < W EUT* Fig. 1 - Protective criterion for the co-ordination SPD and equipment to be protected. The voltage protection level U P, of an upstream SPD has to be below the verified immunity level of the equipment to be protected. U P should be also co-ordinated with the withstand voltage of the equipment Uw according to IEC [4]. However, when applying this basic protective principle, not only the maximum voltage level U max to be expected across the terminals of the SPD has to be compared as generally assumed. Furthermore, a number of additional parameters might be relevant for ensuring effective protection of equipment by means of external SPDs: Maximum impulse current I max flow into equipment Maximum energy W max transfered into equipment Maximum voltage-time integral u dt at equipment, Maximum voltage change du. dt In order to evaluate the above parameters following basic questions are to be taken into account and will be discussed further on: Different protection performance of SPDs depending on the functional principle of individual SPDs Comparability of different test parameters according to IEC and IEC DUT I max U max W EUT

2 2 LIMITTION PERFORMNCE OF SPDs ] Iimpedance Depending on their design SPDs are subdivided according IEC into following three functional principles: Voltage switching type SPD: "can have a sudden change in impedance to a low value in response to a voltage surge". Typical examples of such components are spark-gap-based SPDs. Voltage limiting type SPD: " will reduce impedance continuously with increased surge current and surge voltage". Typical examples are MOVs or diodes. Combination type SPD: " incorporates both voltage switching type and voltage limiting type components" The protection performance of SPDs may significantly differ depending on the design features described. In Figure 2.1 and 2.2, this matter is outlined based on two basic examples. In both examples it is assumed, that the equipment to be protected contains a MOV directly at the equipment input terminal. Furthermore it is supposed that the equipment is protected by an upstream SPD at a voltage protection level of U P = 1,5kV. In example 2.1, the SPD is designed as spark gap, in figure 2.2 a MOV type SPD is assumed. Based on a total load of 1k 1/35µs, the two pictures show the impulse current, which flows into the equipment, and the voltage across equipment. In this simplified examples the primary values I max and U max have almost the same value. However, when comparing the integral values W max and u dt, it is obvious that the let-through energy generated in the equipment to be protected and the voltage-time integral at the equipment may differ considerable in value when using two different SPDs which have the same voltage protection level U p, but different functional principles. varistor-based SPD did result in a higher let-through engery, especially at impulse currents with a long time to half-value, such as the 1/35µs current waveshape. However at a spark-gap-based SPD it can be expected that the load on the equipment to be protected is reduced due to the switching performance particularly if impulse currents with a long time to half-value occur [6] i[k] Impulse current generator Impedance Spark gap External voltage-switching type SPD total current current into SPD u[v] 8 4 MOV Voltage-limiting componet - in equipment to be protected voltage across SPD current into equipment voltage at equipment t[µs] t[µs] Figure Protection of equipment by an external spark gap Impulse current generator i[k] total current current into SPD MOV External voltage-limiting SPD current into equipment 8 4 U [V] MOV Voltage-limiting component in equipment to be protected voltage across equipment voltage across SPD t[µs] t[µs] Figure Protection of equipment by an external MOV When assessing whether equipment can be protected by an upstream SPD, the protection performance, considering also let-through energy and voltage-time integral, which depends o the functional principle of the SPD, may be decisive factors. Table 1: Comparison of load parameters for the calculation examples in figures 2 Load Parameter at equipment Maximum voltage U max Maximum current I max Voltage-time integral u dt Maximum energy W max transfer Voltageswitching Voltage-limiting SPD SPD < 8V < 8V < 2 < 2 35mVs 2 Ws 1mVs 7 Ws 3 COMPRISON OF TEST PROCEDURES CCORDING TO IEC ND IEC In the scope of IEC it is clearly stated, that direct lightning strikes, are not considered. Furthermore the difference of equipment level immunity and system level immunity is pointed out: "The manufacturer should test his equipment to confirm the equipment level immunity" and those responsible for the installation should then apply measures necessary to ensure that the interference voltage caused by lightning strokes does not exceed the chosen immunity level. Comparability of the parameters determined in various tests is a basic requirement that equipment with a specific immunity according to IEC can be protected by a SPD with specific voltage protection level according to IEC Comparability is in some cases difficult due to different test philosophies and test methods, on which the two standards are based. Some of the basic differences between these two standards are described below.

3 3.1 Different source impedances Equipment immunity according to IEC is determined with a Combination Wave Generator (CWG) with an internal impedance of 2 Ω, a short circuit current of wave form 8/2µs and an open-circuit voltage of wave form 1.2/5µs. When testing SPDs Type III, which are mainly used for the protection of equipment, such a CWG is also required according to IEC However, when testing the immunity of equipment, different coupling elements are used. For example, when testing low-voltage power supply lines against earth, an additional resistor of 1 Ω is connected in series. The example in Figure 3 shows that different parameters might be obtained from both tests due to this additional series impedance. In the assumed example a MOV based SPD is tested according to IEC with an opencircuit voltage U OC =1kV. In this example, a voltage protection level of U P =1kV is achieved. n equipment immunity of 2kV was determined according to IEC In order to ensure this surge withstand it is assumed that varistor is integrated inside the equipment. This MOV is charged with a total energy of 1 Joule when testing immunity according the EMC-standard. However, if the SPD and equipment to be protected are tested in combination with a 2Ω-CWG of 1kV 1.2/5µs, the energy produced in the internal MOV of the equipment is 2 times higher due to the different source impedance. This may result in an energetic overload of this component although the basic criterion, i.e. the immunity level of equipment has to be greater than the maximum voltage protection level of the SPD, is met in this example. 3.2 Different waveforms and threat values IEC specifies different installation classes based on the installation conditions. Vlass 4 installations are defined as power installation which can be subjected to inteference voltages generated by the installation itself or by lightning. The C-power supply input of equipment in installation class 4 will be tested with a hybrid impulse of the waveshape 1.2/5µs (8/2µs). Whereas partial lightning currents are simulated in a laboratory by means of an energetic current impulse with the waveform 1/35µs according to the primary threat values, which are part of the lightning current standards Therefore, SPDs ClassI, often called lightning current arresters, are used for this purpose. These are installed where cables enter buildings for protection against lightning currents. These SPDs, are tested according to IEC with this energetic 1/35µs test impulse. comparison of the different waveshapes, as given in figure 4, shows that a general statement whether equipment can be protected by an external SPD also in case of lightning currents with a long current wave is sometimes difficult, due to the different current waveforms. s pointed out above, spark-gap-based SPDs with a wave-breaking performance reduce the actual stress parameters for the equipment, due to it s switching characteristics. The remaining stress for the equipment then corresponds to the current waveform used to evaluate the test level of the EMC-surge immunity test. i [k] 1 Case 1: Test of a single SPD according to EN U [kv] I [k] i.5 voltage above SPD current through SPD 2. test voltage 1 kv SPD u t [µs] 4 U max = 1 kv, I max = 4 k W max = 6 J Case 2: Test of a single SPD according to EN U [kv] I [].8 1 1Ω 9µF i.4 5 DUT voltage above terminal equipment test voltage u 2 kv current in terminal equipment t [µs] 4 U max = 6 V, I max = 1 W max = 1 J Case 3: System test of an SPD in combination with the terminal equipment U [kv] I [k] 1. 1 i *.5 voltage above terminal equipment test SPD DUT U * current in terminal equipment voltage 1 kv t [µs] 4 U max* = 85 V, I max* = 1.1 k W max* = 2 J Figure 3 - Comparison of the threat values when co-ordinating SPD and equipment to be protected 5 2 Partial lightning current 1/35 Short circuit current of a CWG 8/2 2 1 t [µs] Figure 4 - Comparison of different wavevforms for 1/35 partial lightning current and 8/2 surge current 3.3 doption of the various test philosophies For several years, experts of the IEC 77B and IEC 37 committees responsible have been trying to adapt the various test philosophies, on which the installation and device regulations are based [5]. description of this problem of different test procedures was added to the latest standard version of IEC Ed. 2. in an additional informative annexe. In this article, possible test procedures for such a cross-standard system test are outlined based on practical examples.

4 4 SYSTEM TEST In general the following statement of IEC applies: "In order to ensure system level immunity, a test at the system level is recommended to simulate the real installation. Therefore, when carrying out a system test, the actual installation conditions have to be simulated as realistically as possible. test installation set up in the laboratory includes for example the following: SPDs required dditional protective equipment installed, such as overcurrent protective devices and RCDs ctual length and type of the connecting cables between the individual system components Equipment and terminal equipment which have to be protected against surges The response of upstream SPDs result in secondary effects such as change of wave form. Carrying out a system test also verifies that these effects do not have impermissible effects on the function of the equipment. 4.1 Energetic coordination It has to be observed that immunity of the total system cannot be increased by means of uncoordinated adding of SPDs. Moreover, it has to be ensured in all cases that the Surge protective components installed in the terminal equipment do not make the high-performance, upstream SPDs ineffective. This "Blind Spot" can be verified in the laboratory by carrying out tests, which are referred to as "coordination tests". The test current has to be increased gradually for the most critical case of this coordination test is often not the maximum impulse current load to be expected. In case of maximum test current, low line impedance of the connecting cable often results in sufficient coupling. The high current steepness di/dt ensures that the upstream SPD is activated before the surge protection integrated in the terminal equipment to be protected is overloaded. For reproducable coordination tests special requirements for the test generators apply. Impulse current generators with a "fictive" internal impedance 1 Ω have proven to be sufficient. Thus, the requirement that lightning currents are to be regarded as "ideal" power source is met with adequate accuracy [7]. 4.2 Lightning current test of SPD and equipment under real service conditions The protection of equipment in the case of a direct lightning strike can be verified by means of carrying out a Lightning current test under real operating conditions. Equipment and SPDs are tested in a combined system test under operating conditions, which have to be as real as possible. The basic idea of such a lightning current test under operating conditions is to combine the standard test philosophy of an immunity test according to IEC with the increased requirements of an impulse current test or lighting current test according to IEC Example I: Protection of a central inverter for solar power plants Figure 5 shows the basic circuit diagram of the individual surge protection measures for a solar power plant. There are two possible ways how lightning currents and surges can couple into the central DC-C-converter in the operation building: The collectors and their connecting cables with the DC-input of the DC-C-converter form a wide conductor loop. In this conductor loop high impulse currents will be induced even at distant strikes. If, however, the DC-conductors are run in a steel conduit only a low energy impulse current loading have to be taken into account. In such cases the installation of Class II SPDs according IEC directly at the DC-input of the DC-C-converter and close to the solar generator is necessary and sufficient [8]. In the operation building, which includes the DC-C-converter all metal systems shall be connected directly and all systems under operating voltage shall be connected indirectly via Class I SPDs to the lightning equipotential bonding. Partial lightning currents in C-lowvoltage-power-supply Earthing Central building with DC-C-converter Induced surge into DC-input of converter Figure 5 Basic protection scheme of the surge protection for a solar power plant Testing procedure: In the following an example of such a system level test, called "lightning current test under service conditions" will be presented. The DC-C-converter to be protected is tested under operating conditions, i.e. the device will be loaded with lightning partial currents at live state while being connected with a DC supplying voltage. Figure 6 shows the basic circuit diagram of this system level test. This circuit diagram shows how 8/2µs impulse currents are coupled into the DC input of the DC-C-converter under operating conditions. It has also been verified that during and after the feeding of lightning partial currents of wave form 1/35 into the C connection of the DC- C-converter via an C supply transformer, electrical energy has been supplied into the general low-voltage mains.

5 S2 25 -DC ~ + DC - C-converter ~ L1 L2 L3 L1 L2 L3 N S1 C-power-transformer 23 / 4 V 5 Hz PEN I C I DC DC-Power supply 6 V Impuls current generator 8/2 I Impuls + IImpuls - I total - U protect V - Multipole surge arrester - Class II SPD- for PV system External Lightning current arrester - Class I SPD - for C-power supply Wh N L1 L2 L3 Figure 6 Circuit diagram lightning current test of a central DC-C-converter under real operating conditions 4.4 Example II: Lightning and surge protection of electrical systems in nacelle and hub of a windturbine The verification of the effectiveness of lightning and surge protection for electrical and electronic systems in the nacelle and hub of wind turbines by laboratory testing, will be described on the example of a pitch drive control system. Pitch systems in the rotor hub are used for adjusting the rotor blades. If the wind exceeds a critical value, the turbine will be moved out of the wind. Description of pitch drive control system The examined pitch drive system did include several Cmotors, C converter, for communication a serial link (Profibus DP) and several multi turn position sensors. ll power-supply lines were protected by Class 2 SPDs according IEC The data lines were protected by multi-stage data line protectors tested according IEC These type of arresters include in the first stage a powerful gapped arrester and downstream of the decoupling element a diode element, ensuring the low protection level. Lightning current parameters It is assumed that the pitch drive control system is located with Lightning Protection Zone 1, that means no direct lightning currents but surge currents are flowing in the electrical lines within the system. Furthermore the complete pitch drive system, consisting of a control unit, C-motors, position sensors and the complete cabling between these different components is also stressed by the magnetic field, which is generated by direct lightning currents flowing in the surrounding metallic hub. ) Induction effects due to lightning Impulse currents up to 1k (1/35) were injected into a defined metal, mounting plate, in order to examine the behavior of the complete system within an electromagnetic field generated by lightning currents. The resulting induced impulse currents within the cabling of the complete system were monitored. The characteristic values I max, Q Strok e and W/R of the lightning currents were determined for every test impulse. The functional endurance of the pitch drive system during the injection of direct lightning currents into the mounting plate was monitored in order to verify any influence of conducted interference to the control unit caused by effects of closeby lightning currents. Figure 7 shows the laboratory test set-up for this test series. Measurement of impulse currents Impulse current generator 2 k 1/35 Injection of impulse current into metal frame Distribution board including control system Figure 7 System test of a wind turbine pitch control system B) Impulse test 8/2 of low voltage power supply and Surge immunity test of data line Impulse currents 8/2 were injected directly both into the power supply and into the data line conductors in order to check the surge withstand of the connected equipment. During this test also the co-ordination of the installed external SPDs both for data lines and power lines and surge protection components, installed already inside the

6 connected equipment was verified. In the described example the test of the surge current carrying capability onto the power supply of the pitch drive control unit under service conditions was performed with discharge currents up to 4 k 8/2. The impulse current was injected in this case in the line conductor to PENconductor, while the system was connected with 255 V mains voltage. dditionally a surge immunity test of the data line type Profibus DP under service conditions has been performed. It was the aim to examine the interference on the connection lines of the Profibus system as they are caused by effects of lightning. Therefore 8/2 impulse currents up to 5k have been directly injected into the data bus. The injection of the 8/2 discharge currents has been done in two different coupling modes: Line-to earth-coupling Screen-to-earth-coupling During the test the pitch drive system was running in test mode. The data transfer between the pitch controller and an external computer has been monitored. The correct function of the complete pitch control system could be successfully verified during and following the complete test series. No data interruption or any damage to connected the pitch drive control system has been observed. C) Lightning current withstand of pre-wired connection unit for low voltage power supply This test is used to check for the cumulative effects that occur when multiple modes of protection of a multi-pole SPD conduct at the same time. The basic test procedure of the total discharge current test for multiple SPDs according IEC is applied. The distribution of the impulse currents and it's characteristic parameters, such as peak current I peak, total charge Q and specific energy W/R are monitored during the test, as IEC assumes a balanced impulse current distribution. In the laboratory, this balanced current distribution is ensured by series inductances and resistances. ssuming a balanced surge current distribution amongst the phase lines and the neutral line represents a "worst-case" analysis. Different earthing practices in different parts of the world have a very huge influence onto the actual lightning current distribution. However surge-protection systems tested under these conditions can be applied in all applications regardless the specific earthing conditions at the individual site. dditionally the equipment to be protected has been connected to the output terminals of the surge protective unit. So this test combines again the stress parameter of the lightning protection standard with the immunity verification of equipment and therefore exceeds the standardized requirements considerably. However, it offers the user of the SPDs the most realistic proof about the actual lightning current carrying capability and the protection of downstream equipment. D) Proof of continuity of supply The above test sequences mainly focussed on the lightning current behaviour of the systems. But also the the reliability of the low-voltage C power supply are becoming more important for the user. Therefore an additional test for the selectivity of backup fuses and SPDs was included. This was done by laboratory testing using the basic test procedure of a duty-cycle test, described in the SPD-standard, but selecting the real overcurrent protective element, prospective short-circuit current and system voltage for the specific application. The frequency of follow currents and the follow current limitation of a lightning current arrester are the decisive parameters for a reliable power supply of a system. It was ensured that low-energy overvoltages are suppressed to a low protection level without leading to any 5Hz-mains follow currents. Should impulse currents arise with higher energies and possibly lead to follow currents, these should be limited to ensure that an upstream overcurrent protective element will not respond. 5 SUMMRY It is not possible in every case to compare all parameters determined due to different test philosophies specified in standards concerning immunity tests of terminal equipment and test requirements specified in the product standards for SPDs. Therefore, the system test presented in this article is a method for verifying immunity on system level, which has been tried and tested in various applications. 6 REFERENCES [1] IEC Ed. 2: 25-3 Low-voltage surge protective devices - Part 11: Surge protective devices connected to low-voltage power systems; Requirements and tests. [2] IEC Ed. 1.: 2-9 Low voltage surge protective devices - Part 21: Surge protective devices connected to telecommunications and signalling networks - Performance requirements and testing methods [3] IEC Ed. 2.: Electromagnetic Compatibility (EMC)- Part 4-5: Testing and measurement techniques - Surge immunity test [4] IEC : 22 Insulation coordination for equipment within low-voltage systems- Part 1: Principles requirements and tests [5] H. Bachl "Überspannungsschutz Koordination Geräte - SPDs; IEC/EN versus IEC/EN " [Surge protection coordination devices; IEC/EN versus IEC/EN ] on the occasion of the D--CH conference 8/24, Rostock Wannemünde, Germany [6] IEC : 26: Protection against lightning - Part 4: Electrical and elctronic systems within structures [7] J.Birkl, P. Hasse "EMV-Testverfahren zur bleiterkoordination" [EMC test procedures for coordination of SPDs], in EMC Kompendium 1998 [8] H. Pusch., B. Schulz "Blitz- und Überspannungsschutz für Solarkraftwerke" in TB 7-8/23, S