SELECTED PROBLEMS OF LIGHTNING PROTECTION OF SATELLITE DIGITAL TV CENTRES

Transcription

1 29 th International Conference on Lightning Protection 23 rd 26 th June 2008 Uppsala, Sweden SELECTED PROBLEMS OF LIGHTNING PROTECTION OF SATELLITE DIGITAL TV CENTRES Karol Aniserowicz Bialystok Technical University, Faculty of Electrical Engineering BTU, Wiejska 45 D, Bialystok, Poland Miroslaw Zielenkiewicz Centre of Protection Against Overvoltages and Electromagnetic Interferences RST sp.j., Mysliwska 2, Bialystok, Poland Abstract Selected problems concerning the lightning protection of satellite digital TV centres are considered. The means of lightning and overvoltage protection are discussed. Examples of incorrectly and correctly made installations are presented. The examples concern the design and realization of lightning protection systems, bonding, conducting contacts and surge arresters. 1 INTRODUCTION Digital television stations broadcasting their programmes via satellite links are equipped with lots of modern apparatus. The electronic devices must be of high quality and of very high level of reliability. In regard to large number of emitted programmes the stations of this type are called the satellite television and radiodiffusion digital platforms. Many TV channels are permanently on-line, monitoring for any news or special events, many channels are interactive. Numerous satellite antenna dishes are mounted on roofs of these objects (even more than 60 antennas) or at the ground level. Large number of power supply, signal and RF cables run inside and outside buildings to antennas, transmitting/receiving and control units. In order to keep the operational reliability the satellite digital TV platforms are also equipped with air conditioning systems, fully redundant power supply and other systems. This makes the objects especially vulnerable to damages caused by lightning electromagnetic pulse (LEMP). The modern communication signal processing devices are digital, working with weak signals in wide frequency bands, so the susceptibility not only to LEMP, but also to other electromagnetic threats should be considered while planning the lightning protection of these objects. Any technical problem, concerning especially the communications system (uplink satellite downlink), relates also to millions of recipients. A direct or nearby atmospheric discharge can cause heavy losses in electronic hardware or serious electromagnetic interferences. Among possible consequences of improper protection system are: a broken link to a satellite, a broadcast interruption, and loss of an operator s reliability as a transmitter of commercial programs and advertisements. Many research works and standards have been published concerning the evaluation of overvoltage threats. The parameters describing levels of hazard have been specified in the IEC standards and other publications [1-4]. While thinking about the broadcasting equipment security one should consider how to ensure the survival during the hazards of the highest levels. The highest direct stroke currents specified in [1 2] are as follows: 200 ka, 10/350 µs the first positive lightning stroke; 50 ka, 0.25/100 µs the subsequent negative strokes; 100 ka, 1/200 µs the first negative lightning stroke. The secondary-effect current and voltage surges are standardized to be of several kiloamperes, 8/20 µs and several kilovolts, 1.2/50 µs. The electromagnetic field can also cause damages and disturbances. The electric field strength close to the lightning channel reaches hundreds of kv/m and the magnetic field several ka/m. In a distance of some hundreds of meters the field strength is still of order of kilovolts per meter or hundreds of amperes per meter, respectively. LEMP can cause damages in electronic circuits in a distance exceeding 1.5 km from the discharge channel. Unfortunately, shielding against this threat is not sufficiently described in the IEC standards [1]. Formulas concerning only the magnetic field shielding can be found in the IEC Formulas for shielding of the electric field are omitted. ICLP2008 Local Organisation Committee 7a-1-1

2 29th International Conference on Lightning Protection The consciousness and technical knowledge related to the LEMP and other electromagnetic threats is still insufficient, and this can be one of the reasons for serious technical and organizational problems. Several aspects should be considered during planning the lightning and overvoltage protection. The most important aspects are: estimation of the possible electromagnetic threats, design of the lightning protection air terminations, grounding, equipotentialization (bonding), shielding, surge protective devices in coordination with the AC power supply system, overvoltage protection of RF cables and signal lines, filters. All systems and circuits should be made according to the best engineering practice. Some typical examples are discussed below. 2 ESTIMATION OF RISK OF DIRECT LIGHTNING STROKE A risk of the direct lightning strike can be evaluated using standard formulas [1]. Such analysis can be helpful e.g. at the stage of planning the facility location. It is good to have in mind that it is not necessary to locate the considered TV centre in a high rise building. The equivalent direct stroke collection area A e is calculated as shown in Fig. 1. A e l h w 3h Fig. 1. Equivalent collection area of an individual building Consider the risk of a direct lightning hit to a relatively low and relatively high building of the dimensions as follows: l = 60 m, w = 40 m, h 1 = 15 m for the case of a low building or h 2 = 100 m for a high one. Hence, the collection area is A e1 = m 2 or A e2 = m 2, respectively. Let the lightning density N g be of 2 strikes per km 2 and year (e.g. it concerns some countries in the region of the Baltic Sea). The expected average annual number N d of lightning strikes to the building is equal to: N d1 = N g A e1 = flashes per year (1) N d2 = N g A e2 = flashes per year (2) Quotient 1/N d can be considered as an average time period between direct lightning flashes to the building. So, the analyzed lower building can be hit once per 28 years (1/N d1 = 28.2), and the higher one once per about one year and a half (1/N d2 = 1.45). Hence, the lowest possible building is the reasonable choice for location of the facility. It is good to place large uplink antennas at the ground level not only because of their weight but also from the point of view of lightning protection (Fig. 2). However, in many other respects, often a decision is taken to locate a satellite TV centre in a tall building in a downtown. In such a case it is necessary to situate the antenna field at the top of the building. For this reason an effective lightning protection system becomes one of the most critical problems during a design process. Fig. 2. It is easy to provide an appropriate lightning protection zone for antenna mounted at the ground level (the metal mast working as a lightning protection rod) 7a-1-2

3 ICLP 2008, June 23-26, Uppsala, Sweden 3 LIGHTNING PROTECTION SYSTEM Very often lightning protection systems on roofs with antennas and other apparatus are erroneously designed. Some examples are presented in this section. A roof with antennas, ventilation outlets, air conditioners, other equipment and cabling is shown in Fig. 3 before and after of a new LPS design and assembly. In this case the best choice was to make an elevated conducting grid. a) b) Fig. 3. Antennas on a roof before (a) and after (b) of the LPS assembly Examples of misconception of air termination design are presented in Fig. 4. In both cases the lightning rods affixed to the parabolic reflectors would guide the possible surge current directly to the protected antennas and their electronic circuits. Besides, there are also other errors visible. The antennas and their cables are not at a safe distance from the roof metal fringe (Fig. 4a). The lightning rod shown in Fig. 4b is too short to secure the antenna from being directly hit. a) b) Air termination Air termination Fig. 4. Misconceptions of air termination design. Air terminations attached to the back (a) or to the top (b) of antennas The electronic or electric circuits of the antennas presented in Fig. 4 can be easily damaged by flashovers from the reflectors. One of the possible surge paths is shown in Fig. 5 the wires of the reflector heating system. It is necessary to assure the proper lightning protection not during normal operation, but also during assembling works, to make a temporary LPS when elements of a final construction could disturb. Such a system is required for protection of already installed systems and because of safety of people working on the roof. An example of a temporary LPS is demonstrated in Fig. 6. 7a-1-3

4 29th International Conference on Lightning Protection Fig. 5. Reflector heating wire a possible way for flashover Fig. 6. Temporary air termination 4 SHIELDING It follows from the concept of lightning protection zones (LPZ) [1] that shielding grids against LEMP are required. Of special importance for the planning of shields are the existing metal components of the building (e.g. steel reinforcements in concrete). Shielding is essential around rooms containing electronic transmitting, receiving and signal processing apparatus. Usually these rooms are classified to the LPZ 2 [1], which is presented in Fig. 7. LPZ 0 A LPZ 0 B LPS LPZ 1 d s/1 = w OPB 0/1 OPB 1/2 LPZ 2 Overlapping of LPZ 1 and LPZ 2 Shield 0/1 Shield 1/2 Fig. 7. Realization of the lightning protection zone concept [1] in satellite TV station (OPB overvoltage protection box) Electronic systems shall be located with respect to so-called safety distances from the shield of the LPZ [1]. The reason for this distance is the relatively high strength of electric and magnetic field close to the wire grid, due to partial lightning currents flowing in the shield. The safety distances are dependent on the nature of the currents (being parts of a direct strike or being induced), hence on the level of the LPZ. For the LPZ 1 this distance is denoted as d s/1 and it is equal to the shielding mesh width w. If the outer shielding grid is formed by the conductors of the lightning protection system, then d s/1 is of the order of meters. In practice technical rooms are widespread and at least one wall of these rooms is the outer wall of the building. So, it is not possible to ensure the proper distance between the walls of LPZ 1 and LPZ 2. This problem is illustrated in Fig. 7. Additional protective measures are required in such situations. 7a-1-4

5 ICLP 2008, June 23-26, Uppsala, Sweden 5 EQUIPOTENTIALIZATION A next set of difficulties is connected with the equipotentialization (or bonding). The often met practice in apparatus rooms is a bonding network made of wires running from bolt to bolt (Fig. 8a). This is a wrong concept, because disconnection or bad quality of one contact leads to loose of protection for many racks. a) WRONG AC supply box equipotentialization (bonding) wires b) BETTER AC supply box equipotentialization (bonding) wires apparatus racks apparatus racks elevated floor elevated floor equipotentialization (bonding) bars c) BEST AC supply box apparatus racks elevated floor multipoint bonding equipotentialization grid Fig. 8. Examples of different concepts for equipotentialization: a) wrong, b) better, but resonant, c) equipotentialization grid b) a) Fig. 9. Details of equipotentialization grid (during assembling): braided-wire strips (a), and copper bar (b) 7a-1-5

6 29th International Conference on Lightning Protection A better idea is illustrated in Fig. 8b: the use of copper equipotentialization bars assures connections which are short and easy to maintain. However, the one-point bonding is not a good way for wideband devices first resonant effects occur typically about 10 MHz, still within the LEMP spectrum band. The best, non-resonant, concept is the equipotentialization grid (Fig. 8c). The conducting elements of the elevated floor should be bonded to this grid. Fragments of bonding network are presented in Fig. 9. These photos were made during assembling process. Fig. 10 contains the examples of incorrect bonding. Both of them are the kinds of the one-point bonding concept. At least two wires terminate at each bolt. Multiple wires in Fig. 10a form many resonant connections to each rack in the room and they are probably not cheaper than the grid presented in Fig. 8c. The connection of many (five) wires to one bolt has the same disadvantage as shown in Fig. 8a. a) b) Fig. 10. Incorrect bonding below the elevated floor. Lots of potentially resonant wires instead of bonding grid, two wires terminate at each bolt (a). Five wires attached to one bolt (b) Taking care of the best engineering practice includes the basic knowledge about corrosion at the contacts of dissimilar metals. In places of high humidity the fast growth of corrosion is observed (Fig. 11). Fig. 11. Corrosion at direct contacts between copper and galvanized steel. Rust under an elevated floor (a), and zinc hydroxide rings around bolts in a low-voltage switchgear (b) 7a-1-6

7 ICLP 2008, June 23-26, Uppsala, Sweden 6 SURGE ARRESTERS Often cables enter the building in many places. This fact causes difficulties and additional costs of lightning protection. It is necessary to minimize the number of cable entries to the building (as shown in Fig. 7). At the entries the cables should be protected against overvoltages. The best place for mounting of surge arresters is a bonding copper panel (Fig. 12a) or a set of bonding bars (Fig. 12b). Fig. 12. Overvoltage protection box (OPB) at antenna cable entries 7 CONCLUSION Some problems concerning the lightning protection of digital satellite TV centres have been discussed. It follows from the authors experience that the most important issues met in practice concern: design and realization of lightning protection systems, bonding techniques, protection against corrosion, shielding effectiveness, location of surge arresters. Many problems could have been avoided if all the electric and electronic installations had been made according to appropriate standards. 8 REFERENCES [1] IEC 62305:2006. Series of standards: Protection against lightning. Part 1: General principles. Part 2: Risk management. Part 3: Physical damage to structures and life hazard. Part 4: Electrical and electronic systems within structures. [2] KTA 2206, Auslegung von Kernkraftwerken gegen Blitzeinwirkungen, Fassung 6/00. German standard. [3] NFPA 780, Standard for the Installation of Lightning Protection Systems. National Fire Protection Association, USA, [4] MIL-HDBK-419A, Grounding, Bonding and Shielding for Electronic Equipments and Facilities, 29 Dec a-1-7