Lightning and Surge Protection of Cellular Transmit Sites

Transcription

1 IX-19 Lightning and Surge Protection of Cellular Transmit Sites D. Kokkinos, N. Kokkinos, I. Vlaseros, ELEMKO SA, K. Dimas, M. Koulizos, P. Garoufalis, J. Sotiriadis TIM STET HELLAS Abstract-- This paper aims to analyze the design, installation and the components/devices used in a lightning protection system of a cellular transmit site base station. Additionally a wide number of difficulties that were faced during the implementation of the above will be presented. The experience over 10 years of continues installations (more than 200) all over Hellas and statistical data recorded during these years will also be presented emphasizing the important and sensitive parts of a station and showing various techniques and devices that can be used to successfully minimize the damage and how to overcome specific difficulties. Due to the long term experience and the variety of difficulties that were detected various case studies will be presented (i.e. isolated stations, high earth resistance, high lightning activity etc). Index Terms-- Bonding, Earthing, GSM, Surge protection devices, TOV I. INTRODUCTION The erection of the first mobile telecommunication base stations in Hellas started in STET HELLAS (TIM) was one of the first two companies in Hellas, which created their own network. There are two general types of stations those used in rural (known as GSM base stations) and those in urban areas. The ones used in rural areas are of the interest of this paper since they are isolated tall antennas, which make them an easy target to lightning flashes. Also in order to succeed efficient and wide coverage on an area, the GSM antenna is installed on high altitudes, although nowadays this is limited due to environmental issues. Contact Address: Dr. Nicholaos Kokkinos ELEMKO SA, Tatoiou 90 str, GR,Metamorphosis, Attiki,, Hellas nkokkinos@elemko.com Considering that Hellas has the highest lightning activity in Europe and the high altitude of the GSM stations many of them are facing severe damages either due to direct or indirect lightning flashes. The only protection measure against lightning surges was an isolation transformer after the electricity supply meter with T2 [1] surge protective devices on the secondary site. In 1996 a collaboration between STET HELLAS and ELEMKO SA was established by installing a complete Lightning Protection System (LPS) on the GSM base stations that were suffering the most. The first base station that was protected has suffered continues and excessive damages from the beginning of its operation and was in Akarnanika mountains (East Hellas with T d = 50 days per year). The LPS design and the statistical data that will be described in this paper were based on this base station, which is successfully protected since In total more than 200 GSM base stations were also successfully protected by applying similar design. II. GENERAL LAYOUT OF A GSM BASE STATION AND REPORTED DAMAGES In general the GSM base stations are quite complex but for the purpose of this paper only parts that require protection will be described. The exterior of a common GSM base station covers a total area of approximately 200 m 2 and is composed out of three main parts, which are all surrounded by a concrete fence. The three main parts are: the antenna tower, the diesel generator shelter and the main shelter, which contains all the necessary electrical and electronic equipment for the receiving and transmitting of the signal. The antenna tower height varies according to the purpose and the usage of the station; usual heights are between 15 and 45 meters. The antenna and the shelters are th International Conference on Lightning Protection

2 supported on a reinforced concrete base, which can be either one large base for all or smaller individual for each one (see Figure 1). Rod for antenna protection Cellular Antennas Wave-guides LPZ1 LPZ0 A Building containing equipment for the operation of the site LPZ2 Interconnected earthing grids for tower footing support and foundation earthing for the equipment cabinets building (additional electrodes may be used) Fig. 1. Layout of the exterior of a common GSM base station MV/LV substation detail analysis of the surge protection is explained further is the paper. Figure 2 shows a damage caused by poor equipotential bonding inside the main shelter. In the event of a direct lightning strike on the tower of the station the local earth potential will raise causing high potential difference between earthed and non earthed equipment. Due to the small area inside the shelter the equipment were installed near to each other. If the equipment were only earthed through the PE conductor having as a reference point to earth the equipotential bonding bar, which is usually installed at the electric supply meter, the time needed to equalize the potential between two neighbouring equipment will be high and therefore the probability of a flashover between the two is high. The tower supports all the antennas, which usually are covered by a plastic enclosure to protect them from ice and snow. Wave guides are running along the tower connecting the antennas with the receiving / transmitting equipment, which are installed in the main shelter. In the main shelter all the necessary electrical and electronic equipment that are used for receiving and transmitting are installed. Generally inside the main shelter the equipment that are installed are, the receivers and transmitters, the multiplexers, alarm panel EXALI, A/C units, main electric board and various secondary electric panels such as fire protection, security lights etc. Coaxial, telecom and power cables are running in metallic ducts interconnecting the antennas with the equipment. The GSM base station is usually connected to the MV electric distribution network through an isolated MV line having a local MV/LV substation supplying only the station. The isolated part of the MV line supplying the station may experience lightning flashes also causing damages to the electrical and electronic equipment of the station. The main electric supply is connected via a standby diesel generator, to DC power supply units, since most of the receiving / transmitting equipment are using DC supply. Common operating DC voltage is 48 VDC. All the equipments have sensors, connected to a main alarm panel, known as EXALI that transmits the operating conditions of the equipment to the main control which is based in the headquarters of the GSM company. The most sensitive to lightning surges equipment in a GSM base station are, the standby diesel generator, the DC power supplies, the EXALI board and the input output of the receiving and transmitting equipment. Figure 10 shows a schematic configuration of the cabling routing with the equipment and the position and type of the surge protection devices that were used. A more Fig. 2. Damages due to poor bonding and incoming lightning surges III. EXTERNAL LPS OF GSM BASE STATIONS In rural areas the risk is high since the site is exposed to either direct lightning stroke on the tower or on the connecting service facilities. The fact that rural areas may have difficulty to be approached especially during winter months requires a higher efficiency LPS design. Usually the most common protection level of GSM base station is level I with additional measures [2]. Figure 1 shows a general layout of the external LPS for a simple GSM base station. The vulnerable points of the tower are the antennas, which can be protected either be a metallic ring surrounding the antenna or if this is not allowed due to operating limitations a protection rod may be installed according to IEC [3]. The shelters situated underneath the tower in most of the cases do not require any external LPS since the protection angle of the tower is sufficient to protect them. To Rx - Tx Collar Fig. 3. Protection of microwave antennas To Rx - Tx Rod th International Conference on Lightning Protection

3 IV. EARTHING AND BONDING OF GSM BASE STATIONS Installing a foundation earthing in every part that is composed out of reinforced concrete will provide a low earthing system impedance (see Figure 4). Usually parts of the station that are made out of reinforced concrete are the fence of the station and three bases (tower, main shelter and generator shelter). To succeed a better equipotential and to reduce the total earthing impedance all of individual foundation earths shall be interconnected having at least four connection points with each other. Additionally the foundation earthing will help to have awaiting conductors at the points where a direct connection with the earthing system is required. Common points where direct bonds with the earthing system are: the tower base and the metallic stairway, inner equipotential bonding rings and generally wherever an equipotential bonding bar is required. Epsilon is composed have much lower impedance than typical rod type electrodes. Antenna Mast Main Shelter with all the transmitting / receiving equipment with metallic enclosures installed inside the main shelter Standby diesel generator shelter M Equipotential bonding bars provide a star bonding method for the waveguides, which should be earthed at various points along their body. Important points of waveguide earthing are at the base of the tower, before entering the main shelter and before connection to the equipment (see Figure 5 Figure 7). Local MV/LV substation 1:1 Isolation Transformer (delta star) with local Neutral All the metallic equipment situated in the shelters are bonded with each other either direct or through the inner equipotential bonding ring performing a meshed bonding method, which has one bond with the foundation earthing system. The inner ring will have an easier access if it is installed about 0,5m above the equipment of the shelter since if it is at the bottom after the installation of the equipment access will be almost impossible. All the metallic elements such as cable ducts, doors, air condition ducts are also bonded to the inner equipotential bonding ring in order to minimize any potential difference between metallic surfaces. It is important to avoid the generation of long earth leads connecting equipment to either to the inner ring or to an equipotential bonding bar. Therefore if two equipment are at a close distance with each other, instead of having as a common reference point to earth the inner ring, which can be 1m away from both, it is better to have an additional bond between them with a sort length lead. 30 x 3mm St/tZn tape used for foundation earth electrode Ø8mm St/tZn or Aluminium conductor used for interconnections of the individual earthing systems 30 x 3mm Cu tape used equipotential bonding inner ring Clamp used for connecting conductors/tapes/metallic surfaces 1.5m long rod earth electrode E TM Plate earth electrode Fig. 4. General layout of the earthing system Neutral is left O/C For earthing system shown in Figure 4 and at a soil resistivity between Ωm a DC resistance of 30 Ohms may be possible to be achieved. But in Hellas the soil resitivity may be significantly higher and for that reason the DC resistance may well be above 100 Ohms. It is therefore required to improve the foundation earthing system. By using awaiting conductors additional earth electrodes can be connected to reduce further the earthing system impedance. The Epsilon (E TM ) earth electrode is a flexible and effective solution. It is equivalent to six 1,5m rod earth electrodes and additionally the plates that the Fig. 5. Waveguide earthing at the base of the tower th International Conference on Lightning Protection

4 In electrical installations there is common problem to have multiple earthing systems (i.e. LPS earthing system, electrical earthing system PE, telecom earthing system). All the earthing systems should have one common reference point, unless due to operating issues one should remain isolated (i.e. telecom), then this should also be bonded through a low sparkover voltage isolating spark gap. Fig. 6. Waveguide earthing before entering the main shelter A common mistake that was found in numerous cases was that the SPD to have as an earth the LPS earthing system where the equipment had a reference to earth only the electrical earthing system through the protective conductor. Obviously this means that the SPD will not protect the equipment unless the two earthing systems are bonded at the SPD and only one earth conductor is then connecting the SPD and the under protection equipment. As a practice is useful to have at the electric supply main distribution board one equipotential bonding bar, where all the individual earthing systems should be bonded (see Figure 9) Fig. 7. Waveguide earthing after entering the main shelter Equipotential Bonding Bar Under Protection PE Earthing system LPS Earthing system Fig. 9. Effective bonding of individual earthing system Fig. 8. Bonding of metallic equipment inside the main shelter through the inner equipotential bonding ring V. INTERNAL LPS OF GSM BASE STATIONS Bonding of all the metallic equipment through the shortest path is very important as mentioned in the previous paragraph. Additionally cable screening is also important and therefore all cables shall be driven inside metallic ducts, which should be bonded with the inner equipotential bonding ring. Cables and waveguides that change from one LPZ to another LPZ should have a reference to the earthing system either direct or through Surge Protection Devices SPDs. Waveguides are usually bonded as described in the previous paragraph and RF type SPDs are installed at the end of the waveguide before the input of the equipment that they are connected. The earth leads of the SPDs should be as short as possible and the earth of the SPD should be the same with the earth of the under protection equipment. The selection of the appropriate SPDs is also very important in order to have technical and economically effective solution. ELEMKO has started installations of SPDs in GSM sites before the implementation of the new IEC and EN standards series [1], [4]. Therefore many of the SPDs that were and are still in service are only tested with the 8/20µs waveshape. But with the introduction of the new IEC and EN standards the design was adjusted of course to the new requirements. Figure 10 shows a schematic layout of the cabling the refers to the SPDs that are installed at various points of the installation. Additionally Figure 4 shows a detail regarding the isolation transformer that is used in many GSM base stations in order to isolate the PE coming from the electricity supply and also the Neutral. Having the configuration of Delta to Star connection and by having SPDs in both primary (25kA, 10/350µs) and in the secondary (40kA, 8/20µs) it is possible to have a local Neutral conductor isolating the one coming from the th International Conference on Lightning Protection

5 electricity supply. The earthing system in every GSM base station was set to be TN S after the secondary of the isolation transformer. MV/LV For the purpose of this paper the results of one of the first GSM base stations will be presented for the first two years ( ) since during that time this station was set by TIM-STET Hellas as a test station for the lightning protection companies to prove the efficiency of their design and equipment since the IEC and EN standards were not yet available. Electric Supply Standby Diesel Generator Iimp = 25kA, 10/350µs Uc < 2,5 kv Imax = 40kA, 8/20µs Uc < 2 kv The test station is in Akarnanika mountain and it is one of the highest lightning activity areas of Hellas facing the Ionian sea on the west coast of Hellas on an altitude of approximately 1000m. - Receiving Transmitting DC Power Supply Units + Microwave Antennas Security lights Receiving Transmitting Imax = 6kA, 8/20µs Uc < 1,5 kv Imax = 10kA, 8/20µs Uc < 150V EXALI Imax = 20kA, 8/20µs Uc < 150V Inputs from various monitoring sensors To GSM headquarters Fig. 10. Schematic configuration of the cabling network in a GSM base station A major problem that was and still occurs is the high TOVs arriving from the electricity supply. Due to the poor earthing system of the electricity grid especially in rocky rural areas in case of a short circuit the generated overvoltages and the time needed to clear the fault is long enough to cause high TOVs in the LV network causing damages LV equipment. A temporary solution was to use spark gaps for primary protection and high MCOV (550V) for MOV type SPDs for secondary protection. Additionally all SPDs should be sealed in a metallic enclosure to protect the other equipment in case of a failure. VI. LPS EFFICIENCY RESULTS In total 212 GSM base stations all over Hellas are protected by similar designs as described in the previous paragraphs for about ten years now. Depending on the station alterations are always necessary. Every one or two years there was an inspection on the site and any damages or failures of SPDs were recorded. In the first two years (January 1996 December 1997) and after various alteration of the initial design the efficiency of the LPS of the Akarnanika GSM station was very high since it has not suffered any damage even after 57 surges having an magnitude between 500A and 100kA, which were recorded by an analogue surge counter. In the beginning of 2006 the counter was reading 290 surges, which is an average of 30 surges per year! The only SPDs that needed replacement after these two years were some MOV type (50kA, 8/20µs) since at that time such SPDs were used for primary protection as well. But none of them was ever exploded or excessively damaged due to high lightning or surge current in order to justify the requirements for the current IEC and EN standards [1], [4]. None of the data or RF type SPDs has ever been damaged in any of these 212 stations over the period of ten years. The extensions of the foundation earthing system needed to be replaced after five years since the material that was used was galvanized steel and has suffered corrosion although protective actions have taken when connecting them with the foundation earthing in order to avoid electrochemical corrosion. In such high technological installations the equipment can be upgraded or replaced by new ones quite frequently, which require inspection of the equipotential bonding. VII. REFERENCES [1] EN , Low voltage surge protective devices Part 11: Surge protective devices connected to low voltage power distribution systems Requirements and test [2] EN-IEC , Protection against lightning Part 2: Risk management [3] EN-IEC , Protection against lightning Part 3: Physical damage to structures and life hazard [4] IEC , Low voltage surge protective devices Part 1: Surge protective devices connected to low voltage power distribution systems Requirements and test th International Conference on Lightning Protection