LIGHTNING PROTECTION for BROADCASTING STATIONS

Transcription

1 LIGHTNING PROTECTION for BROADCASTING STATIONS by Phillip R Tompson BE(Hons) CPEng MIE(Aust) MIEE MIEEE NOVARIS PTY LTD Abstract - Broadcasting transmitting stations and indeed all high power MF, HF and VHF transmitter sites are particularly prone to lightning strikes and subsequent damage. This paper examines the reasons for this phenomenon and discusses protection techniques which may be applied to all high power transmitting sites whether they are used for civilian broadcasting or military communications. INTRODUCTION It is not difficult to understand why broadcasting stations are so prone to lightning strikes. Medium frequency stations, MF, consist of tall, slender vertical 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 susceptible to receiving direct lightning strikes. High frequency stations, whether used for shortwave broadcasting or military communications generally consist of vast antenna farms with numerous antennas supported by tall masts. It is again easy to see why these structures are frequently struck by lightning. Very high frequency, VHF, stations whether broadcasting television or FM radio are located on mountain tops and other prominent sites. In addition to direct strikes to transmitting structures, strikes both direct and induced to power lines feeding transmitters must also be considered. Thunderday maps are published by meteorological organizations worldwide. As may be expected the number of thunderdays is generally greatest in tropical regions around the equator and falls off as one progresses north and south towards the poles. 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 (km -2 year -1 ). As may be expected there is a relationship between thunderdays and ground flash density. Figure 1, reproduced from BS6651 shows this relationship. Thunderdays Mean flashes per sq km per year per year Ng Source: BS6651 Fig 1. Thunderdays vs Ground flash density STRIKE INCIDENCE There are two common statistics used to measure the incidence of lightning strikes. The first is the term thunderday. This term is defined as a calender day during which thunder is heard at a given location. The international definition of lightning activity is given as the number of thunderdays per year. This is also called the isoceraunic level. To assess the susceptibility of transmitting structures to lightning, the number of likely strikes per annum can be readily calculated. The attractive radius of tall, slender structures of height greater than 60m, can be found by use of an equation for R a given by Ericsson in reference 3. Date: Page: 1 of 1

2 R a = I 0.64 x h ( I x ) R a = the attractive radius for the structure, in meters I = the prospective lightning stroke current amplitude, in kiloamperes h = the structure height, in meters Using the above equation, a transmitting mast with a height of 100m and a prospective lightning stroke current of 50KA, has an attractive radius of 267m. The collection area is then given by: potential is essentially caused by the self inductance of the tower. Block in ref 6 presents a formula for approximating the inductance of a typical slender tower. The self inductance of a 100m tower is 77 microhenries. The potential at the top of the tower may be calculated from the following formula: V = L x di/dt L = Self inductance in microhenries di/dt = Rate of rise of current in amps per microsecond A c = π x R a 2 x 10-6 For a 50KA current rising in 1 microsecond, the potential at the top of the tower will be approximately 3.8MV. A c = the collection area for the structure, in square kilometers. A 100m high transmitting mast will have a collection area of km 2. Finally the prospective number of strikes per annum can be calculated from: P = A c x N g P = the prospective number of strikes per annum N g = the ground flash density km -2 year -1 In an area with 80 thunderdays, the mean ground flash density is 6.9. So a 100m high transmitting tower will receive on average 3 direct strikes every 2 years. Direct Strike PROTECTION PRINCIPLES When lightning strikes a tower that is either directly grounded or grounded via a spark gap arrester the current pulse, which typically may have a rise time of 1 microsecond and a decay to half amplitude of 50 microseconds, will flow down the tower to ground. It is important to be aware that no matter what form the lightning protection takes there will be a potential gradient developed up the tower. This Fig 2. Self inductance of 100m slender tower Earth Potential Rise As the current pulse flows to ground a rise in 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. For example a 50KA impulse flowing to ground with a 10 ohm earth resistance will raise the earth potential by 500KV. Since the local ground potential rises, any cables leaving the vicinity of the tower will carry this potential to the transmitter building. Current will flow along coaxial cable sheaths and create a potential between the inner and outer conductors of these cables. Date: Page: 2 of 2

3 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. DIRECT STRIKE PROTECTION A direct strike to a transmitting tower is unlikely to damage antennas unless the antenna itself is struck or correct earthing and bonding principles have not been adhered to. Antennas which form the highest point of the structure and are not at tower potential are particularly vulnerable and it is difficult to protect these effectively. High gain whip antennas mounted at tower top are typical examples. The best form of protection is to carry some spares. Where antennas are mounted on the lower faces of the tower, it is usual to erect a vertical spike, or Franklin rod, at the top of the tower to act as the air terminal. To be effective the top of the rod needs to be at least 3 metres above the highest point of the antenna. No special precautions with regard to downconductors on all steel towers are necessary. The four legs of a self supporting tower provide an excellent path for the lightning impulse current. Special air terminals and proprietary downconductors consisting of custom made coaxial cables etc are totally unnecessary. They do nothing to reduce the potential rise at the top of the tower and cannot possibly be insulated to the level required to prevent flashover to the tower itself. 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. Fig. 3 Spark gap incorporating Jacob s ladder Typical spark gap dimensions may be set depending upon transmitter power, base impedance and altitude. Figure 4 below from ref. 7 shows the relationship between breakdown voltage and spark gap for various gap geometries. The peak RF antenna voltage is given by: V peak = 2.83 x Z a x I a V peak = peak antenna voltage Z a = antenna base impedance, ohms = antenna current, amps RMS I a Whilst it is preferable to minimise the spark gap, dimensions below 5mm are impractical a build up dirt etc may cause the gap to arc over with the presence of RF power alone. Furthermore the gap must be sufficient to allow the arc to extinguish once triggered by the lightning strike. At MF sites a single feed wire connects the antenna to a tuning hut, a common method of reducing lightning current in the antenna feed wire is to form it into one or more loops to produce a low but finite series inductance. HF systems incorporating balanced feeders to high power baluns employ spark gap protection across the balun terminals. Date: Page: 3 of 3

4 Figure 5. Earthing and bonding points Fig. 4 Spark gap dimensions EARTHING AND BONDING Since a direct strike to a transmitting tower will raise earth potentials and cause current to flow in feeders and coaxial cable sheaths, it is essential to pay particular attention to correct earthing and bonding practices. 1. Ensure that the antenna system is securely bonded to the tower structure. 2. Bond the sheath of the feeder cable to the tower structure at the antenna. 3. Bond the sheath of the coaxial feeder to the tower structure at the point it leaves the tower. Do this just prior to the bend in the feeder. 4. Ensure that the tower is securely earthed. For a 50KA lightning impulse every one ohm reduction in earth resistance will reduce the earth potential rise by 50KV. BUILDING LAYOUT The geometry of the interconnections in and around the transmitter building are of vital significance to the effectiveness of the lightning protection system. The objective is to provide a path for the potentially destructive lightning current flowing from the antenna to the AC supply line via a path that does not include the interior of the building. The ideal building layout would be one the coaxial feeders, AC supply and other services enter the building at one point. At this point all services are connected to the building ground either directly in the case of coaxial cable sheaths, water pipes etc or via surge protection devices in the case of AC power, telephone, programme line etc. It is impossible to reduce earth resistances to zero so there will always be earth potential rises developed when lightning strikes. By carrying out the above procedures a single earth will be produced such that an equipotential earth rise will occur and current flow in the station through vital equipment will be eliminated. 5. Bond the sheath of the cable to the station ground at the point of entry to the transmitter building. AC Power SURGE PROTECTION The AC supply line to the transmitter building usually represents the lowest impedance to remote grounds and will therefore carry much of the lightning current flowing away from the site. The surge Date: Page: 4 of 4

5 protection installed must thus have sufficient capacity to carry these currents. This situation is altogether different from the case of induced voltages in power lines for which many surge protection devices are designed. It is usual to choose mains power filters in preference to shunt connected surge diverters. The filters provide multistage protection and redundancy in the event of a component failure. Figure 6 shows the configuration of a typical mains power filter. This is not always reliable if the gas device is installed at the end of a long feeder and the arcing path has some finite impedance. Fig 7. Gas arrester coaxial protector L1 L2 MOV s LOW PASS FILTER E1 E2 A recent development in the protection of high power broadcasting transmitters has been the development of a coaxial cable spark gap incorporating an optical sensor to detect an arc. Once the arc is detected, an optical fibre signals the information to the transmitter and can initiate a brief interruption to transmission to allow the arc to extinguish. Fig 6. Mains power filter Generally three phase versions with surge ratings no less that 70KA would be employed. Current ratings depend upon the station s requirement but for high power transmitters, 630 amps per phase at 220V is not uncommon. The spark gap can be made very small as it no longer has to be wide enough to extinguish the arc so firing voltages are low and equipment is protected. Such a device with an adjustable spark gap is shown in figure 8. This device is designed for 3 1/8 cable using EIA flange type connectors. These filters would be installed at the building point of entry of the AC power supply. Coaxial Cable Protection At high frequencies the components used in power filters are unusable. Metal oxide varistors have a high self capacitance which shunt RF energy. The only useable device is the gas filled arrester which may be connected between the inner and outer sheaths of coaxial cables to clamp differential voltages. Such devices are readily available for low power transmitters up to a few hundred watts. At higher powers gas filled arresters also become unusable unless special precaution are taken to ensure that, once fired, they will not be held in a conductive state by the RF energy. It is possible to utilise some types of arresters for high power use assuming that the transmitter will automatically shutdown once the arrester fires when its reverse power rises. Fig 8. Adjustable spark gap protector for 3 1/8 coaxial cable. These coaxial cable protectors are installed at the building point of entry. Signal Line Protection Incoming programme lines and telephone lines also require protection at the point of entry. Typical Date: Page: 5 of 5

6 protection devices that may be utilised include, gas arresters and multistage protection in configurations similar to figure 9. L1 E1 L2 E2 Fig 10. Mast guy static drain gas arrester MOVs Fig 9. Multistage signal line protector STATIC PROTECTION suppressor diodes It has long been recognised that static voltages can build up on the insulated guy wires of MF and HF transmitting masts, ref 8. This phenomenon can be caused by the electric field build up prior to a lightning strike or can be caused by friction due to wind in very dry conditions. Up until the advent of fully solid state transmitters this problem caused little inconvenience. Transmitters employing valve power amplifier stages were robust enough to withstand the momentary impedance mismatch as the guy insulators arced over discharging the static. CONCLUSION The protection from lightning of high powered broadcasting stations poses some special problems. There is no doubt that broadcasting structures are highly prone to direct lightning strikes. With proper attention being paid to earthing, bonding and correct station layout in addition to the correct choice of surge protection, effective lightning protection can be provided. Even at sites building layout is poor, ie services enter from different parts of the building, earthing is incomplete, it is still possible to retrofit protection and earthing schemes which can provide effective lightning protection Modern solid state transmitters with sensitive reverse power detection actually shutdown under such conditions. The solution is to provide a form of static drain across the insulators so that charge build up is slowly dissipated. A common approach is to connect inductors across the guys and across the base insulator in the tuning hut. One problem with this solution is the presence of unwanted resonances. An alternative approach is to connect high resistance elements across the guys to dissipate the charge. Such a device is shown in figure 10. It consists of a conductive material with a resistance of a few megohms concentric with a spark gap to carry the current from a lightning strike. These devices can be connected across guy insulators without the problems associated with resonance. Date: Page: 6 of 6

7 REFERENCES 1. NZS/AS , Australian, New Zealand Standard Lightning Protection 2. BS6651, British Standard on Lightning Protection. 3. A.J Eriksson An improved electrogeomagnetic model for transmission line shielding analysis Trans. IEEE, July 1987, Vol. PWRD-2, No M. M. Frydenlund Lightning Protection for People and Property. Van Nostrand Reinhold. 5. J.L Norman Violette, Donald R. J. White, Michael F. Violette Electromagnetic Compatibility Handbook. Van Nostrand Reinhold. 6. Roger R Block The Grounds for Lightning and EMP Protection Polyphasor Corporation 7. Reference Data for Radio Engineers Howard W Sams & Co. 8. George H Brown A Consideration of the Radio- Frequency Voltages Encountered by the Insulating Material of Broadcast Tower Antennas Proc of I.R.E September Henry Jasik Antenna Engineering Handbook Mc Graw- Hill Phillip R Tompson graduated from the University of Queensland with an honours degree in Electrical Engineering in His early experience was gained as a communications engineer with Telecom Australia, then with power utilities specialising in communications and control systems design and management. His work now involves consultancy in the field of lightning protection, power quality as well as product design work for surge and overvoltage protection products. He is a chartered member of IE(Aust), IEE, and IEEE as well as a member of the Australian Standards Committee on lightning protection. Date: Page: 7 of 7