DETERMINATION OF THE DIELECTRIC STRENGTH OF LPS COMPONENTS BY APPLICATION OF THE CONSTANT-AREA-CRITERION

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X International Symposium on Lightning Protection 9 th -13 th November, 29 Curitiba, Brazil DETERMINATION OF THE DIELECTRIC STRENGTH OF LPS COMPONENTS BY APPLICATION OF THE CONSTANT-AREA-CRITERION

, Neumarkt, Germany ralph.brocke@technik.dehn.de, claudia.rother@technik.dehn.de Abstract - A comparison between the dielectric strength of insulators during the application of the standardised voltage impulse 1.

1 X International Symposium on Lightning Protection 9 th -13 th November, 29 Curitiba, Brazil DETERMINATION OF THE DIELECTRIC STRENGTH OF LPS COMPONENTS BY APPLICATION OF THE CONSTANT-AREA-CRITERION Ottmar Beierl 1, Ralph Brocke 2, Claudia Rother 3 1 Georg Simon Ohm University Nuremberg, Germany ottmar.beierl@ohm-hochschule.de (corresponding author) 2,3 DEHN + SÖHNE GmbH + Co. KG., Neumarkt, Germany ralph.brocke@technik.dehn.de, claudia.rother@technik.dehn.de Abstract - A comparison between the dielectric strength of insulators during the application of the standardised voltage impulse 1.2/5 µs and steep voltage impulses in the sub-µs range was made. By measuring the breakdown voltage of a comparison arrangement in air and comparison with the dielectric strength of different insulators, it can be shown that steep voltage impulses e.g. caused by subsequent strokes mainly determine the dielectric strength of the whole insulation arrangement. Furthermore a theoretical analysis of the voltage / time characteristics of the comparison arrangement is presented, which is based on the constantarea-criterion (CAC) used for the determination of the k i factor in the IEC standards. With the publication of the new IEC by definition of a new waveshape which takes the lightning current 1/2µs into account consideration, some new approaches have to be considered. By a comparison of the breakdown characteristic of different insulators at steep voltages with the characteristic of a comparison arrangement different ranges of the impulse flashover voltage-time characteristic can be found where the LPS is safe and functional according to the standard and other which may be endangered by a breakdown or flashover between the LPS and other parts of the building to be protected. Therefore the present paper on the one hand tries to take up the basics of the calculation according to the current standards of the separation distance impulse-voltage-time characteristic and the CAC and on the other hand tries to describe their background regarding current standards in consideration of the historical development. Based on these facts it is to be examined whether these basics can be accepted and are valid for future extensions. This includes: The determination of the coefficient k i and the dielectric strength of a proximity distance based on the lightning current waveform The influence of the geometry of the down conductors The influence of materials in the area of proximity distances (e.g. construction materials) With regard to the special needs of isolated lightning protection systems it is furthermore planed to clarify some points for their physically correct integration into future standards. This concerns for example: The determination of the equivalent separation distance using impulse-voltage-time characteristics and the CAC The qualification of experimental test methods, which ensure a safe functionality of the tested isolated LPS The correlation of theoretical and experimental results is discussed in order to develop suitable lab test procedures which represent the requirements given in the standards. The aim of this contribution is to help to provide an acceptable, simple and physically correct basis for current and future problems when calculating separation distances. 1 INTRODUCTION A LPS has to be designed and dimensioned in such a way that the lightning impulse current is safely discharged and the separation distances required for preventing undesired flashovers to the metal installation within a structure is maintained. In the past both isolated and non-isolated LPS were installed based on the lightning protection standards of the IEC 6235 series. Due to the ever increasing complexibility of electrical and electronic systems installed in structures and the high availability requirements new protection goals have to be developed. The IEC [1] standard specifies criteria for the correct dimensioning of the separation distances required for the installation of an isolated LPS. Both lightning current components with maximum current steepness and lightning current components, which determine the electric strength of insulating arrangements due to their exposure duration, have to be considered for dimensioning the separation distance. Lightning currents induce high voltages into installation loops, which can reach values of several 1 kv and can cause flashovers / breakdowns at proximity points if the separation distance is not maintained. 273

2 The minimum distances between the LPS and the metal installation, which ensure safe isolation at the point of proximity, are called permissible proximities. These minimum distances are defined by the induced voltage u ind and the electric strength of the insulation at the point of proximity. This strength depends on the time characteristic of the induced voltage. The isolation capability of insulators for isolated or partly isolated LPS depends not only on the dimension. It can be shown that the isolation capabilities depend on the dielectric strength of the insulator material, on the surface discharge behaviour but also on the steepness of impulse voltages relevant during lightning. 2 SEPARATION DISTANCE To prevent dangerous flashovers as a result induced voltages, the separation distance s has to be maintained between the metal installation in a structure and the LPS in accordance with IEC ki kc s = l (1) k m It is obvious that the value of the separation distance is determined by the length of the down conductor. The k- coefficients represents the selected class of LPS, the division of the lightning current into the different down conductors and the insulation material in the isolating distance. 3 ELECTRIC STRENGTH OF ISOLATING DISTANCES The electric strength of the insulation near a proximity between the LPS and the metal installation is determined by both the puncture and the flashover strength of the existing insulating medium. The electric strength depends on the time characteristic of the induced voltage. 3.1 Impulse withstand voltage and constant-areacriterion The dimensioning of the impulse withstand voltage is still based on the impulse breakdown voltage of air. For this purpose, the different shapes of voltage impulses are considered by the CAC: t 2 t1 [ u t) U ] dt = A ( (2) The time characteristic of the impulse voltage is integrated via the time interval t 1 t < t 2, in which the voltage u(t) exceeds the static breakdown voltage U. The determination of the parameters of the CAC is based on tests carried out on long spark gaps according to [2]. The following parameters were determined for a rod-rod spark gap with a clearance d in air: U 5 = 6 1 d[ m] V; A =,6 d[ ]Vs (3) m This assumption is only justified for practical applications if the insulation is mainly established by means of an air gap. The tests in [3] show that a material factor k m =.5 has to be used for solid materials such as concrete or brickwork, which takes the reduced electric strength of these materials into account. If insulating materials such as rods or pipes made of high-voltage-proof material e.g. GRP are used, a material factor k m =.7 has proven its worth in many practical applications. [3, 5] 3.2 Use of the constant-area criterion in the standard The standard uses a square pulse as time characteristic for the induced voltage. This square pulse corresponds with the ideal voltage characteristic U ind induced by the magnetic field of a lightning current impulse i B (t) in a conductor loop if it is assumed that the lightning current impulse increases in a ramp-shaped manner. For the calculation in the standard a front time T 1 =.25 µs (negative subsequent stroke (NSS)) is used for all classes of LPS. The peak value I Bmax of the lightning current varies depending on the class of LPS. The mutual inductance M describes the magnetic coupling between the lightning current carrying down conductor and a conductor loop. dib ( t) I Bmax Uind = M M (4) dt T1 The mutual inductance of an "elongated" down conductor with regard to a conductor loop in the immediate vicinity is approximately used for the calculation in the standard. It is common to specify the mutual inductance per metre loop height M. The induced voltage is calculated using (4) as follows: I B max U ind = M l (5) T1 The evaluation of the CAC for the square pulse leads to the impulse voltage-time curve according to (6), details are given in [5]. The parameters according to (3) and the evaluation of (5) form the basis for the calculation of the k i factors in accordance with the standard. The impulse voltage-time characteristic for t C = T 1 used in the standard is calculated as follows: ˆ d 1 U = 6kV 1 [m] + 1 / [µs] (6) T 274

3 When determining the k i factors it has to be considered that for a single conductor which carries lightning current I B with an adjacent loop an induced voltage U ind according to (5) does not cause failure of the dielectric strength according to (6) during a time period T 1. The following equation is used for the determination of the k i factors: if t C = T 1 ; Û = U ind and k m = 1; k C = 1 then: 1 M I B max 1 (7) ki = 6 [μh/m] [ka] T1 1+ [μs] The values T 1 =.25 µs and I Bmax = 5, 37.5 and 25 ka remained unchanged in the course of the last modification of the standard. The mutual inductance per metre loop height M', however, was discussed and was changed from 1.5 µh/m to 1.2 µh/m. This led to a reduction of the k i factors from k i =.1;.75;.5 to.8;.6;.4. Fig. 1 illustrates this procedure by means of the impulse voltage-time curve according to (6) for a separation distance d = 1 m. Here the CAC is exactly fulfilled for a square pulse with a duration of t C = T 1 =.25 µs in case of a peak value of Û = 3 kv. 4 kv 3 2 Different voltage waveforms and insulation arrangements, which simulate the situation in proximity distances, were experimentally evaluated according to the CAC. The impulse voltage-time curves for the different insulation arrangements were determined in the laboratory. U d5 was approximately assumed as static breakdown voltage U for the calculation of the voltagetime areas A of the arrangements and determined in highvoltage tests. In a first step the parameters of the standard according to (3) were compared with the experimentally determined parameters of three insulation arrangements (comparison arrangements). 4.1 Comparison arrangements Both a rod-rod arrangement and two arrangements consisting of crossed round wires with a diameter of 8 and 16 mm were used as comparison arrangement. The clearance was s V = 45mm. High-voltage pulses with a 1.2/5µs or.4/45µs pulse shape were used for all comparison arrangements. 4.2 Impact of the comparison arrangement The impulse voltage-time curves of different comparison arrangements were determined in a variety of measurements and compared with one another (Fig. 2) Measurement: s V =45mm; 1.2/5µs X 8mm X 16mm t C in ns Fig. 1 - Impulse voltage-time curve according to [2] for d = 1 m The procedure shown above for the description of the dielectric strengths of proximity paths by means of impulse voltage-time curves has not only become established for spark gaps in air, it forms also the basis for the description of the impulse withstand voltage of insulating arrangements and materials. 4 EXPERIMENTAL STUDIES t [µs] 1 Fig. 2 - Comparison of the measured impulse voltage-time curves at the tested comparison arrangement The following degrees of homogeneity η were calculated for the arrangements as relation between the average and maximum field intensity for different clearances d (Table 1). Table 1: Degree of homogeneity η for different clearances d Comparison Degree of homogeneity η arrangement d =.45 m d = 1 m crossed rods (8 mm).8.43 crossed rods (16 mm) rod-rod (16 mm)

4 The values shown in Table 2 are the result of the evaluation of the parameters of the CAC of the different test arrangements for different pulse shapes. Table 2: Comparison of the measured constants of the CAC Comparison.4/45µs 1.2/5µs arrangement s V = 45mm U [kv] A CAC [Vs] U [kv] A CAC [Vs] Rod-rod Crossed rods (8 mm) Crossed rods (16 mm) It is revealed that the theoretical values of U = 27 kv and A =.27 Vs calculated from (3) slightly differ from the measured values. The results according to [2] are based on tests performed on spark gaps in air with clearances d > 1 m. As the degree of homogeneity increases in case of lower clearances, lower clearances tend to have higher values for U. This deviation decreases with higher clearances d. No significant influence of the geometry of the comparison arrangement was discovered since U and A have almost the same values. The impulse voltage-time curve generated from the CAC also shows that there are only insignificant differences between the geometries tested and that they do not play an important role for the qualification of a test method. 4.3 Impact of the pulse shape Fig. 3 shows the comparison between impulse voltagetime curves at different pulse shapes. 1 To be able to evaluate the impact of the pulse shape on the determination of the parameters of the CAC, the impulse voltage-time curves illustrated in Fig. 3 were determined for the crossed-rod comparison arrangement. The measurements on crossed-rod arrangement with 16 mm diameter give comparable results. The comparison of the impulse voltage-time curves measured at the comparison arrangement crossed rods with different diameters shows that the measured impulse voltage-time curves depend on the selected pulse shape. It can be observed that higher values for the voltage u respectively shorter chopping times t C are achieved with a considerably steeper.4/45µs test impulse in the front section of the measured impulse voltage-time curve. However, the determined parameters of the CAC are almost identical and can be determined by means of any pulse shape. 4.4 Comparison with the impulse voltage-time curve in accordance with the standard Since the measured values were determined by means of double exponential test pulses with a rise time t 1 >, they differ from the generated impulse voltage-time curve for square pulses. They can, however, be converted since the determined parameters of the CAC (U and A), as shown in 4.3, do not depend on the pulse shape (Fig. 4). 1 8 X 8mm.4/45 (measurement) X 8mm 1,2/5 (measurement) X 8mm.4/45 (gen. from CAC) X 8mm 1.2/5 (gen. from CAC) 8 X 8mm.4/45 X 8mm 1.2/5 6 s V = 45 m 6 s V = 45 mm t [µs] 1 Fig. 3 - Comparison of the impulse voltage-time curves of the crossed rod comparison arrangement (8mm) for.4/45µs and 1.2/5µs (measurement) t [µs] Fig. 4 - Comparison between the measured impulse voltage-time curve (.4/45 and 1.2/5) and the conversion to the impulse voltage-time curve for square pulses (generated impulse voltage-time curve) 276

5 The calculation of the separation distance according to (6) is based on the parameters specified in (3). They are based on the assumption of a square pulse with a rise time t 1 =. The associated impulse voltage-time curves for square test pulses are generated by means of the measured parameters of the CAC (U and A). In Fig. 5 the impulse voltage-time curves (s V = 45 mm) which were determined by means of.4/45 µs respectively 1.2/5µs test pulses are compared with the impulse voltage-time curve with the same clearance d in accordance with the standard gen. with CAC from.4/45µs Measurement gen. with CAC from 1,2/5µs Measurement VA 8mm gen. with CAC from.4/45µs Measurement VA 8mm gen. with CAC from 1,2/5µs Measurement VA 16mm gen. with CAC from.4/45µs Measurement VA 16mm gen. with CAC from 1,2/5µs Measurement Impulse voltage-time curve in accordance with the standard (s V = 45mm) Neg. subsequent stroke (,25/1µs) Pos. first stroke (1/35µs) t [µs] 5 Fig. 5 - Comparison of the impulse voltage-time curve generated from the CAC with the impulse voltage-time curve in accordance with the standard (d=45mm) It is clearly visible that both the practically determined impulse voltage-time curves and the impulse voltage-time curve based on the standard slightly just touch the NSS impulse whereon the standard is based. In case of the positive first stroke (PFS) the practically determined impulse voltage-time curves as well as the impulse voltage-time curve based on the standard is always higher than the PFS. A comparison arrangement (CA) meets the standard requirements for the separation distance s V, the theoretically determined values are verified by laboratory measurements. 4.5 Measuring method for the determination of the equivalent separation distance of insulation arrangements Since the theoretical values are almost identical with the practical values, the following consistent measuring method for the determination of the equivalent separation distance of insulation arrangements can be defined. Determination of U (corresponds to U d5% in good approximation) and A from a sufficient number of measurements for different test voltage amplitudes ,2,2,4,6,8 t /µs Fig. 6 - Determination of U and A Determination of the impulse voltage-time curve for square pulses using the measured parameters U and A Comparison with the pulses in accordance with the standard (NSS and PFS) After that the material factor k m can be evaluated by finding an impulse voltage-time curve in air (standard) for a clearance s V, which is below the impulse voltage-time curve generated from the measurements for all pulse times t. The relation between the determined clearance s V and the insulation length l iso can be regarded as material factor for the tested material / insulation arrangement NSS U Fig. 7 - Comparison between measurements and standard impulses 4.6 Use of this measuring method for an insulation arrangement made of GRP The test method presented in this article was verified for an insulation arrangement made of GRP (glass fibre reinforced plastic), which represents a random insulation arrangement, to check if the CAC can be used and if an effective material factor k m can be determined. The static breakdown voltage U and the parameter A were measured at an insulation arrangement made of GRP with a length of the insulating material of l = 6 mm in dry and wet condition and in the same way as the tests A,2,4,6,8 1, 1,2 t /µs A Impulse voltage-time curves: u mess = f(u, A) u Norm = eq. 6 pos. first stroke 277

6 performed on the comparison arrangements. It was tested if the CAC can be applied to the impulse voltage-time curves. The parameters U and A could be determined both for.4/45µs and 1.2/5µs pulse shapes and were comparable. Fig. 8 shows the impulse voltage-time curve generated by means of the CAC for the GRP arrangement. GRP arrangement (6mm-dry-.4/45); k m =.56 GRP arrangement (6mm-dry-optimized-.4/45); k m =.63 Impulse-voltage-time curve acc. (6); NSS and NFS acc. (4); d = 34mm Impulse-voltage-time curve acc. (6); NSS and NFS acc. (4); d = 38mm NSS NFS M =1.2µH/m t [µs] 2 Fig. 8 - Evaluation of the impulse voltage-time curve for a GRP arrangement and comparison with pulses in accordance with the standard for different k m factors The result of the comparison with the pulses defined in the standard and impulse voltage-time curves is a material factor k m, which is considerably lower that the equivalent air gap s V. Taking also into account that in the new revision of IEC :29 the waveshape and the parameters for the negative first stroke (NFS) are defined with 1/2 µs (1kA for LPL I) it can is obvious that a comparison with on lightning current impulse is not sufficient. While in case of long pulse durations (e.g. PFS) the tested insulation arrangement always has a higher electric strength than a comparison arrangement in air (for all k m 1), it is obvious that the determined impulse voltagetime curve achieves or exceed the value based on the standard for the NSS or the NFS already in case of a material factor k m = SUMMARY This article picks up and analyses the basic principles for the calculation of the separation distance, which is currently based on the IEC :26 standard. The k i factor and the breakdown strength of a proximity distance are determined by means of the lightning current characteristic which is taken as a basis. The design of isolated LPS requires components which will reliably withstand all loads and which were properly inspected. In future it will be necessary to define detailed test procedures for isolated LPS and their components. This article verifies the basic principles of the calculation of the separation distance in accordance with the standard by means of high-voltage tests performed on comparison arrangements. It can be shown that the impulse voltage-time curve for square pulses which is required for determining the necessary separation distance in accordance with the standard is almost independent of the selected test pulse shape, that the CAC can be used for insulating clearances in air and creepage distances e.g. GRP, that the consideration of the definition of a single factor k m is not sufficient for insulation arrangements, that only the comparison of the impulse voltage-time curve generated by means of the CAC with the lightning current components to be considered gives reliable information on the electric strength of the insulation arrangement. The aim of this article is to pick up the basic principles of the current calculation of the separation distance in accordance with the standard -the impulse voltage-time curves and the CAC, to analyse whether these basic principles will be still acceptable in the future and will remain unchanged and to make a contribution to the provision of an acceptable, easy and physically correct basis for present-day and particularly future issues concerning the calculation of the separation distance. 6 REFERENCE [1] IEC6235-3:26 Protection against lightning Part 3: Physical damage to structures and life hazard [2] Thione, L: The Dielectric Strength of Large Air Insulation; in K. Ragaller: Surges in High-Voltage Networks. Plenum Press, New York, 198. [3] W. Zischank,. J. Wiesinger, P. Hasse: "Insulators for isolated of partly isolated Lightning Protection Systems to verify safety Distances" Proceedings of the 23. ICLP, Florenz 1996, pp [4] R. Brocke, A. Wechsler and O. Beierl, "Verification of test procedures for testing insulators for isolated lightning protection systems", Proceedings of the 29 th International Conference on Lightning Protection, ref. 1-1, pp. 1-12, Uppsala, Sweden, June 28. [5] O. Beierl, R. Brocke and A. Wechsler, "Testing creeping discharge behaviour of insulators for isolated lightning protection systems", Proceedings of the IX SIPDA, pp , Foz do Iguacu, Brazil, Nov

NECESSARY SEPARATION DISTANCES FOR LIGHTNING PROTECTION SYSTEMS - IEC REVISITED

X International Symposium on Lightning Protection 9 th -13 th November, 29 Curitiba, Brazil NECESSARY SEPARATION DISTANCES FOR LIGHTNING PROTECTION SYSTEMS - IEC 6235-3 REVISITED Fridolin H. Heidler 1,