S.3.4 The Use of Isolated Downconductors to keep Separation Distances

13/03/2014
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S.3.4 The Use of Isolated Downconductors to keep Separation Distances

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The Use of Isolated Downconductors to keep Separation Distances Application, Experiences and Tests Brocke, Ralph DEHN + SÖHNE GmbH + Co. KG. Hans-Dehn-Str. 1 92318 Neumarkt, Germany ralph.brocke@technik.dehn.de Reeb, Régis DEHN FRANCE Sárl 30, route de Strasbourg 67550 Vendenheim, France regis.reeb@dehn.fr Abstract—Lightning protection of buildings and electrical installations includes also the controlling of separation distances to avoid uncontrolled side flashes between the different systems. The use of isolated downconductors is a proper way to ensure reliable isolation between the Lightning Protection System (LPS) and the electrical installation in a building to be protected [1]. Experiences in application, examples for installation and the requirements on this isolated downconductor are shown. The paper describes the application and the design rules for the use of isolated downconductors. An appropriate test method is applied to determine an equivalent separation distance. By using calculations as well as experimental verifications different arrangements have been approved to allow an efficient protection of antenna systems or tall structures like silos. It can be shown that such an isolated downconductor opens new possibilities to protect buildings. Keywords—h.v.-testing, isolated downconductor, lightning protection, separation distance I. INTRODUCTION The efficiency of the lightning protection system of buildings and their electrical installation depends on several facts. The protection against direct strikes by installing an adequate air termination system and an appropriate downconductor system which ensures the necessary separation distance is important to avoid partial lightning currents entering the building. Fig. 1 shows the distribution of lightning currents in a building where the LPS (non-isolated LPS) is connected to equipotential bonding, metal parts of the construction and via SPDs to electrical installation. It can be shown that partial lightning currents will enter the building. In conjunction with the resulting transient magnetic fields interference or damage with the installed electric or electronic equipment may occur. Fig. 1. Lightning current distribution in a building with a non-isolated LPS To avoid these possible interferences an isolated LPS can be erected, which is electrically isolated from conductive constructions and electrical installations inside the building. The electrical installation and the isolated LPS are connected only at one common point, usually at the MEB at ground level. To ensure reliable isolation between LPS and conductive installations inside the building uncontrolled and dangerous sparking between downconductors and earthed parts of the building construction have to be avoided in any case. Fig. 2 illustrates the relation between lightning current steepness, induced voltage and a possible breakdown at the proximity between downconductor and electrical installation. This uncontrolled breakdown is avoidable if the necessary separation distance along the downconductor path is ensured. For more than 10 years isolated downconductors are successfully used to erect isolated LPS. This system uses a h.v. coaxial cable with a semi-conductive shielding. The principle design as well as tests and requirements on such isolated downconductors were presented in [1]. proximity voltage u at the point of proximity induced by lightning impulse current i 250 ns u i 25 kA 1 MV t MEB electrical installation Fig. 2. Breakdown between LPS and electrical installation inside a building with inadequate separation distance II. SEPARATION DISTANCE The required separation distance for avoiding proximities may be estimated acc. to [2], [3] by: L k kk s m ci      with s: Separation distance ki: Factor - protection level of the LPS kc: Factor - lightning current distribution km: Factor - material of the electrical isolation L: Vertical distance from the point where the separation distance s has to be determined to the next point of the equipotential bonding. The length of the separation distance depends on the downconductor length, the protection level, the division of the lightning current over different downconductors and the material [3] in the isolating distance (Fig. 3). simplified equivalent circuit (ideal earth termination system) strike into the structure required separation distance s earth termination system air termination downconductor s h Fig. 3. Keeping the required separation distance to avoid impermissible proximities The following calculations in accordance with [3] show examples to illustrate the range of necessary required separation distances s Example 1: Detached residential building Protection level (LPL) III  ki = 0,04 two downconductors  kc = 0,66 length of each downconductor  L = 10 m required separation distance  s = 0,27 m Example 2: Tower for mobile phone communication Protection level (LPL) II  ki = 0,06 two downconductors  kc = 0,66 length of each downconductor  L = 20 m required separation distance  s = 0,8 m The values are based on the assumption that the max. steepness of lightning currents is caused by negative subsequent strokes. The required separation distance s was calculated for air. For solid materials like concrete or bricks the necessary distance is doubled [3]. This rough approximation shows very clearly that required separation distance s easily exceeds the thickness of common brick or concrete walls. In practice, the control and the permanent keeping of the separation distances have proved to be difficult and partly impossible. Electrical installations are flush-mounted or covered so that the exact position can often not be located. If later installations are carried out, the required separation distance is often not controlled. In a first approximation the voltage stress at the top of a downconductor against nearby earthed parts depends only on the inductive voltage drop along the downconductor. Using this simplification the voltage stress for the above mentioned examples can be estimated. Taking di/dt = 100 kA/μs and 150 kA/μs, L = 1,2 μH/m, and the above mentioned length of both examples, the voltage at the top of the conductor can easily reach 600 kV and 900 kV respectively. It has to be taken into account that this voltage stress occurs only during the very short period of time of the impulse current rise (< 1 µs). III. COAXIAL CABLES WITH SEMI-CONDUCTIVE SHIELD AS ISOLATED DOWNCONDUCTORS By coating metallic downconductors with insulating materials with a high electrical withstand capability, the separation distance s can basically be reduced. However, the electrical strength of the total system “downconductor” is still depending on the arrangement itself and on the coming up of creeping discharges [5]. In order to fulfil the requirements on an isolated downconductor and to avoid partial lightning currents via the earthed conductive cable sheath, special coaxial cables with a semi-conductive cover were developed [4], [5]. The special cable entrance fitting is realised via an adjusted connection to the air termination (feeding point) and a special designed connection to the equipotential bonding mounted in a corresponding distance (Fig. 4). Fig. 4. Electrical field of an isolated downconductor with field control (schematic diagram) The semi-conductive sheath leads to an equalised electrical field distribution at the surface of the isolated downconductor and avoids the onset of creepage discharges (Fig. 5). The semi- conductive sheath enables an equalised potential and field distribution on the surface of the cable entrance fitting area of the isolated downconductor. Fig. 5. Construction of the isolated downconductor (HVI® with field control) Relying on the required separation distance s, the max. length Lmax of such an isolated downconductor can be calculated as follows: s kk k L ci m   max  During the last years isolated downconductors were designed which are able to ensure equivalent separation distances in air up to s ≤ 0.90 m. To achieve this electric strength the dielectric strength of the cable insulation as well as the creepage discharge strength in the cable entrance fitting area must be adjusted carefully. The semi-conductive sheath is connected to the air-temination system and to the equalpotential bonding system via special designed terminals. IV. TEST SET-UP TO DETERMINE THE EQUIVALENT SEPARATION DISTANCE An appropriate test set-up was described in different contributions [6], [7], [8] and is currently proposed to IEC [9] to be implemented as a standardised test procedure that allows a comparison of the dielectric strength of the isolated downconductor at a certain impulse voltage with the one of a comparably inhomogeneous spark gap in air (used as a comparison arrangement, CA) (Fig. 6). 1 1. High voltage impulse generator 2. High voltage impulse divider 3. Impulse measuring device 4. Comparison arrangement (spark gap) 5. Specimen under test 5 4 2 3 s Fig. 6. Test set up for dielectric strength using a comparison arrangement For dielectric testing two sections of potential grading at the beginning and the end of the insulated down conductor are required and tested. The volume of the insulation material under test is well defined by the length l2 of the grounded metallic pipe (Fig. 7). 1 Insulating down conductor 2 Grounded metallic pipe with length l2, 3a Area of potential grading with length l1 3b GND connection used for potential grading 4 Connection to the impulse generator Fig. 7. Insulated down conductors - preparation of test samples The CA behind the test sample is used as a chopping gap to generate steep impulses with an impulse width of about 1 µs. This test setup allows the test of the isolated downconductor with very short impulse voltages and simultaneously a direct comparison with the electrical strength of the CA [10]. Fig. 8 and Fig. 9 show the test setup and voltage applied to an isolated downconductor (equivalent separation distance 2 l2 4 l1 3a 3b 1 1 0,75 m) tested with impulse voltages up to 900 kV whereas the flashover takes place at the CA (Fig. 6). Fig. 8. Test setup for the dielectric test on an isolated downconductor Impulse voltage testing DEHNconductor HVI® 15/05/2013 14:31 8.06.04.02.00.0 0.0 -0.2 -0.4 -0.6 -0.8 µs MV Fig. 9. Impulse voltage test on an isolated downconductor It can be shown that such a load is controlled safely and repeatedly by both the semi-conductive coating and the conductor itself. In addition to the dielectric test isolated downconductors including their fittings need to be proved if the designated lightning current carrying capability is given. Fig. 10 shows the the test arrangement in the lab and Fig. 11 a schematic view of the test setup. Fig. 10. Test arrangement in the lab to test the lightning carrying capabilty Fig. 11. Schemativ view of the test setup of the lightning carrying capabilty Fig. 12 shows the successful test of an isolated down conductor system HVI® including its terminals and connectors declared for 150kA (10/350µs). p01_150kA: I_10_350 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 kA 0.0 0.5 1.0 1.5 2.0 2.5 ms Fig. 12. High impulse current applied at the lightning carrying capabilty test V. APPLICATION OF ISOLATED DOWNCONDUCTORS In the meantime the isolated downconductor has been installed several thousand times. One main field of application is the protection of electrical installations like ventilation/air condition, smoke and heat exhaust ventilation systems on building roofs (Fig. 13). By using isolated downconductors the isolation between the LPS and the electrical power cables on roof remain effective even if they have to cross. 1. Isolating down conductor 2. Plate made of insulating material 3. Rigid fastener 4. Head piece 5. Head piece fastener 6. Min. bending radius isolating down conductor 7. Conductor holder l l 3 2 1 45 6 4 5 3 flexible lead 7 7 Fig. 13. Protection of roof installations by isolated downconductors connected to the air termination system of the LPS One other field of application is the protection of mobile phone base stations on host buildings, where the uncontrolled and dangerous sparking between downconductors and earthed parts of the building construction is not acceptable (Fig. 14). interception rod antenna cable entrance fitting area insulated downconductor insulating support tube connection to the equipotential bonding of the BTS connection to the earth termination system   corresponds to s = 0,75m in air Fig. 14. Protection of a mobile transmission antenna by an an isolated downconductor (HVI® ) If there are several installation parts to be protected, it would not be recommendable to lead the isolated downconductors individually from each air-termination system to the earth-termination system. The individual isolated downconductors coming from the air-termination system can be connected with an isolated ring conductor (Fig. 15). isolated downconductor isolated distance holders s isolated ring conductor cable entrance fitting area Fig. 15. Connection of an isolated downconductor to a bare metallic ring conductor From this isolated ring conductor, several downconductors can then be led to the earth-termination system. This opens the way to a more equal current distribution and therefore to a reduction of the coefficient of current distribution kc in eq. (1). The current sharing over several downconductors, e.g. by installing multiple isolated downconductors in parallel, can reduce the required separation distance s as well as the current carrying capability significantly (Fig. 16). As magnetic interactions can come up from the parallel installation of conductors, it has to be ensured that a minimum distance is kept. The coefficient of current distribution kc can be calculated in accordance with [3]. If the connections to the isolated ring conductor are as far away as possible, e.g. at the corners of the roof, an approximately equal current sharing is achievable. Fig. 16. Enlargement of the actual separation distance s by lightning current sharing Behind the cable entrance fitting area the isolated downconductor may be connected to earthed but not lightning current carrying parts of the structure. 2 HVI® cables inside & outside a support tube 3 HVI® cables inside & outside a support tube 1 HVI® cable & interconnected air terminations The isolated downconductor must be installed within the protective area of the air-termination system of the external lightning protection system (Fig 16). cable duct metal attic cover in the protective area of the air-termination system foundation earthing electrode air-termination system reinforcement separation distance s kept by the isolated downconductor  equipotential bonding protective angle cable duct Fig. 17. Installation of the isolated downconductor conductor within the protective area of the air-termination system of the external lightning protection system according to [3]. Isolated downconductors can also be used to protect omnidirectional antennas (antennas emitting their signals in an angle of 360°) with a minimal shielding effect. Also in this application the unit to be protected has to be situated in the protective area of the air-termination system. An example of this application is shown in figure 17. Fig. 18. Protection of an omni-directional antenna Another application for high voltage isolated downconductors is the protection of PV generators. The use of isolated down conductors allow a reliable isolation between the PV installation and the LPS in conjunction with a minimum of sun shading reducing the power generation (Fig. 19) Fig. 19. Isolated downconductor used to protect a PV generator VI. CONCLUSION Lightning protection standards clearly require the control of proximities, the realisation and permanent keeping, however, is often difficult in practice. Partly isolated lightning protection systems are proven and highly developed, however in many cases not applicable (e.g. architecture, acceptance and technical limitations like lateral thrust due to wind). In large complex lightning protection systems the control of the separation distances has proved to be difficult and partly impossible. For insulated down conductors it is not necessary to specify a minimum diameter for the inner conductor. This can be verified by the suggested high current impulse test. The insulated down conductors are tested during the type tests to prove mechanical firmness which is determined by the metallic inner conductor, the insulating layer and the cover. Using the published and well established test set-up with a comparison arrangement in air [8] a well known and accepted method for the determination of the equivalent separation distance is given. The presented applications of isolated downconductors with semi-conductive sheaths provide a wide range possibilities for planning engineers and constructors to keep the required separation distances s between the LPS and electric installations or conductive building construction parts permanently and almost independent from the laying. REFERENCES [1] Beierl, O., Brocke, R., Hasse P., Zischank W.: “Controlling Separation Distances with Isolated Down-Conductors”, 27rd International Conference on Lightning Protection (ICLP), Avignon (2004) [2] IEC 62305-1, Ed.2: Protection against lightning – Part.1: General principies [3] IEC 62305-3, Ed.2: Protection against lightning – Part.3: Physical damage to structures and life hazard [4] D’Alessandro, F.: “On the applicability of isolated downconductors”, VIII International Symposium on Lightning Protection (SIPDA), Sao Paulo (2005), pp. 300-306 [5] Brocke, R., Zahlmann, P.:”Keeping Seperation Distances by Use of Insulated Downconductors-Application and Experiences”, Proceedings of the 28th International Conference on Lightning Protection (ICLP), Kanazawa (2006), pp. 763-768 [6] Zischank W., Wiesinger J., Hasse P.: “Insulators for Isolated or Partly Isolated Lightning Protection Systems to Verfiy Saftey Distances”, 23rd International Conference on Lightning Protection (ICLP), Firenze (1996), pp. 513-518 [7] Brocke, R., Zahlmann, P.: “Requirements on Isolated Downconductors” Proceedings of the VIII International Symposium on Lightning Protection (SIPDA), Sao Paulo (2005), pp. 295-300 [8] Beierl, O, Brocke, R., Rother, C.: “Simplified electrical test procedure for components of isolated LPS”, Proceedings of the 30th International Conference on Lightning Protection (ICLP), Cagliari (2010), pp. 1109-1–1109-9 [9] IEC TS 62561-8, Ed 1.0 Lightning Protection System Components (LPSC) Part 8: Requirements for components for isolated LPS [10] O. Beierl, R. Brocke and C. Rother, "Determination of the dielectric strength of LPS components by application of the constant-area- criterion", Proceedings of the X International Symposium on Lightning Protection (SIPDA), Curitiba, (2009), ref. IV-06.