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LIGHTNING PROTECTION for BROADCASTING STATIONS

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LIGHTNING PROTECTION for BROADCASTING STATIONS LIGHTNING PROTECTION for BROADCASTING STATIONS by Phillip R Tompson BE(Hons) CPEng MIE(Aust) MIEE MIEEE NOVARIS PTY LTD Abstract - Broadcasting transmitting stations and Thunderday maps are published by meteorological indeed all high power MF, HF and VHF transmitter organizations worldwide. As may be expected the sites are particularly prone to lightning strikes and number of thunderdays is generally greatest subsequent damage. This paper examines the reasons for this phenomenon and discusses protection in tropical regions around the equator and falls off as techniques which may be applied to all high power one progresses north and south towards the poles. transmitting sites whether they are used for civilian broadcasting or military communications. Another commonly used statistic to record lightning activity is the “Lightning flash density”. This is defined as the number of lightning flashes to ground occurring on or over unit area in unit time. This is commonly expressed as per square kilometer per year -2 -1 INTRODUCTION (km year ). It is not difficult to understand why broadcasting As may be expected there is a relationship between stations are so prone to lightning strikes. Medium thunderdays and ground flash density. Figure 1, frequency stations, MF, consist of tall, slender vertical reproduced from BS6651 shows this relationship. radiators generally located in a flat, often swampy area. The radiating mast is therefore highly prominent and being the tallest structure around, will be highly Thunderdays Mean flashes per sq km susceptible to receiving direct lightning strikes. per year per year Ng High frequency stations, whether used for shortwave broadcasting or military communications 5 0.2 generally consist of vast antenna farms with numerous 10 0.5 antennas supported by tall masts. It is again easy to see 20 1.1 why these structures are frequently struck by lightning. 30 1.9 Very high frequency, VHF, stations whether 40 2.8 broadcasting television or FM radio are located on 50 3.7 mountain tops and other prominent sites. 60 4.7 80 6.9 In addition to direct strikes to transmitting 100 9.2 structures, strikes both direct and induced to power lines feeding transmitters must also be considered. Source: BS6651 Fig 1. Thunderdays vs Ground flash density STRIKE INCIDENCE To assess the susceptibility of transmitting There are two common statistics used to measure structures to lightning, the number of likely strikes per the incidence of lightning strikes. The first is the term annum can be readily calculated. The attractive radius “thunderday”. This term is defined as a calender day of tall, slender structures of height greater than 60m, during which thunder is heard at a given location. The can be found by use of an equation for Ra given by international definition of lightning activity is given as Ericsson in reference 3. the number of thunderdays per year. This is also called the “isoceraunic level”. Novaris Pty Ltd 72 Browns Road, Kingston, TAS. 7050 AUSTRALIA Section #: SK09 Revision: 1 Tel: + 613 6229 7233 FAX: + 613 6229 9245 Date: 22.04.97 Page: 1 of 1 Email: [email protected] URL: www.novaris.com.au 0.64 (0.66 + 2I x 0.0001) Ra = I x h potential is essentially caused by the self inductance of the tower. Block in ref 6 presents a formula for where approximating the inductance of a typical slender tower. The self inductance of a 100m tower is 77 Ra = the attractive radius for the structure, in microhenries. meters I = the prospective lightning stroke current The potential at the top of the tower may be amplitude, in kiloamperes calculated from the following formula: h = the structure height, in meters V = L x dI/dt Using the above equation, a transmitting mast with where a height of 100m and a prospective lightning stroke current of 50KA, has an attractive radius of 267m. L = Self inductance in microhenries dI/dt = Rate of rise of current in amps per The collection area is then given by: microsecond 2 -6 For a 50KA current rising in 1 microsecond, the Ac = π x Ra x 10 potential at the top of the tower will be approximately where 3.8MV. Ac = the collection area for the structure, in square kilometers. A 100m high transmitting mast will have a collection area of 0.224 km2. Finally the prospective number of strikes per annum can be calculated from: P = Ac x Ng where Fig 2. Self inductance of 100m slender tower P = the prospective number of strikes per annum -2 -1 Ng = the ground flash density km year Earth Potential Rise In an area with 80 thunderdays, the mean ground flash density is 6.9. So a 100m high transmitting tower As the current pulse flows to ground a rise in will receive on average 3 direct strikes every 2 years. ground potential will also occur. By ignoring the effects of inductance and considering resistance of the earthing system alone this earth potential rise can be easily calculated. PROTECTION PRINCIPLES For example a 50KA impulse flowing to ground Direct Strike with a 10 ohm earth resistance will raise the earth potential by 500KV. When lightning strikes a tower that is either directly grounded or grounded via a spark gap arrester Since the local ground potential rises, any cables the current pulse, which typically may have a rise time leaving the vicinity of the tower will carry this of 1 microsecond and a decay to half amplitude of 50 potential to the transmitter building. Current will flow microseconds, will flow down the tower to ground. along coaxial cable sheaths and create a potential between the inner and outer conductors of these It is important to be aware that no matter what cables. form the lightning protection takes there will be a potential gradient developed up the tower. This Novaris Pty Ltd 72 Browns Road, Kingston, TAS. 7050 AUSTRALIA Section #: SK09 Revision: 1 Tel: + 613 6229 7233 FAX: + 613 6229 9245 Date: 22.04.97 Page: 2 of 2 Email: [email protected] URL: www.novaris.com.au At many stations the transmitting tower is often located some distance from the transmitter building so it cannot even be assumed that both the tower and building earth will rise to the same potential. Surge Protection Whether the tower is struck by lightning or lightning strikes the incoming power line, surge protection on all incoming services is essential. The aim is to reference all incoming services to the local ground either directly or via surge diverting components such as metal oxide varistors, spark gaps etc. Fig. 3 Spark gap incorporating Jacob’s ladder DIRECT STRIKE PROTECTION Typical spark gap dimensions may be set depending upon transmitter power, base impedance A direct strike to a transmitting tower is unlikely to and altitude. Figure 4 below from ref. 7 shows the damage antennas unless the antenna itself is struck or relationship between breakdown voltage and spark gap correct earthing and bonding principles have not been for various gap geometries. adhered to. The peak RF antenna voltage is given by: Antennas which form the highest point of the structure and are not at tower potential are particularly Vpeak = 2.83 x Za x Ia vulnerable and it is difficult to protect these effectively. High gain whip antennas mounted at tower where top are typical examples. The best form of protection is to carry some spares. Vpeak = peak antenna voltage Za = antenna base impedance, ohms Where antennas are mounted on the lower faces of Ia = antenna current, amps RMS the tower, it is usual to erect a vertical spike, or Franklin rod, at the top of the tower to act as the air Whilst it is preferable to minimise the spark gap, terminal. To be effective the top of the rod needs to be dimensions below 5mm are impractical where a build at least 3 metres above the highest point of the up dirt etc may cause the gap to arc over with the antenna. presence of RF power alone. Furthermore the gap must be sufficient to allow the arc to extinguish once No special precautions with regard to triggered by the lightning strike. downconductors on all steel towers are necessary. The four legs of a self supporting tower provide an At MF sites where a single feed wire connects the excellent path for the lightning impulse current. antenna to a tuning hut, a common method of reducing Special air terminals and proprietary downconductors lightning current in the antenna feed wire is to form it consisting of custom made coaxial cables etc are into one or more loops to produce a low but finite totally unnecessary. They do nothing to reduce the series inductance. potential rise at the top of the tower and cannot possibly be insulated to the level required to prevent HF systems incorporating balanced feeders to high flashover to the tower itself. power baluns employ spark gap protection across the balun terminals. MF and HF antennas of which the mast itself is the radiating structure pose a special problem. When the mast is mounted on a base insulator it is usual to provide a spark gap across the insulator to conduct the lightning to impulse to ground. The spark gap may consist of either spheres, points or a Jacob’s ladder to assist in extinguishing the arc. A typical spark gap with Jacob’s ladder is shown in figure 3. Novaris Pty Ltd 72 Browns Road, Kingston, TAS. 7050 AUSTRALIA Section #: SK09 Revision: 1 Tel: + 613 6229 7233 FAX: + 613 6229 9245 Date: 22.04.97 Page: 3 of 3 Email: [email protected] URL: www.novaris.com.au Figure 5.
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