IRANIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 4, NO. 1, WINTER-SPRING 2005

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1 IRANIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 4, NO. 1, WINTERSPRING Study of Overvoltage Protection of a Microwave Repeater Station P. Valsalal, S. Usa, and K. Udayakumar Abstract Ever since inception, the electric power and communication systems are exposed to dangerous overvoltage surges starting from lightning to service frequency temporary overvoltage waves with disastrous consequences. The operation of electronic devices in this environment had led to a situation wherein they are severely affected. All sorts of electronic equipment are at risk: this calls for an effective protection against transient overvoltages. It includes adequate equipment protection, inherent capability of the equipment to withstand overvolage surges, good installation and a sound earthing arrangement. This paper describes an incident involving the exposure of wireless equipment and the operating personnel to frequent lightning activities in a hilly place and the need of a good installation practice, better earthing and adequate protection against direct lightning strokes. Index Terms Earth resistance, microwave tower, overvoltage, repeater station, surge suppressor. T I. INTRODUCTION ODAY S modern electronic systems such as those designed for highspeed communication rely heavily on very sensitive, high performance electronic components. These devices are generally exposed to lightning, power network transients and electromagnetic pulses [1]. Lightning activity can cause transient overvoltages on both main power supply lines and data communication signal lines through direct/indirect strikes. In the case of indirect strikes, it enters into signal/data cables that are coupled into electrical services through resistive, inductive and capacitive effects. This mode of transfer of overvoltage surges from electrical mains to communication cables is called coupling. The power lines and the interface circuits also suffer from the impact of the intruding surges. In this paper, an attempt has been made to describe the events that had led to significant damages to the electronic equipment, power lines and the building that houses the electronic devices in a microwave communication center by lightning surges. The remedial measures undertaken are also elaborated. II. PROBLEM STATEMENT There is a microwave radio link located at Melpet in a hilly place (altitude 1088 meters) in Tamil Nadu State of India. The software based telephonic exchange, primary multiplexer (MUX), battery charger and other communication equipment located at this place suffered damages caused by flashover and overheating during Manuscript received February 18, 2004; revised November 17, The authors are with the Department of Electrical and Electronics Engineering, College of Engineering, Anna University, Chennai, Tamil Nadu, , India ( valsalal@yahoo.com). Publisher Item Identifier S (05) /05$ JD summer months, when the lightning was severe. The equipment were damaged even when they were in off position. The building and feeding electric power lines also suffered extensive damages. The energy meters provided at the main supply for the building went up in flames during the lightning strikes. In view of this situation, urgent effective measures were needed to find out the severity of problem sources, possible protective steps to safeguard the electronic devices and building concerned from the impact of lightning surges. III. SYSTEM DESCRIPTION The repeater station consists of a building containing telephonic exchange and other electronic equipment. Also, there are two communication towers namely Very High Frequency (VHF) and microwave towers with connecting communication cables. Fig. 1 shows the single line diagram of repeater station. The data on lightning occurrence of repeater station for the past four years is given in Table I. It is based on the field records available at site. No special instruments have been provided on the said towers to measure the magnitude of lightning current. A. Existing Protection The existing protections are as follows: Lightning masts at a height of one meter were placed at the top of both VHF (height of 20 m) and microwave (height of 30 m) towers, connected to a separate earth pit through a copper strip. Feeding electric power lines were provided with silicon carbide gapped arrester. Separate earthing was provided for the communication equipment and the battery charger The communication cables used have been provided with a protective metal screen. Surge suppressors were not provided for the electronic equipment and the interconnecting cable was not provided with any overvoltage protection. The protective arrangements in place were mainly for the VHF and microwave towers; no protection was given for the building and the equipment housed inside. IV. PROBLEM SOURCES From the severity of the damages noticed, it is felt that the existing surge protection is inadequate and it requires reinforcement. Lightning discharges, a natural phenomena, is the main source of the traveling wave. When it hits the building in point, the surge current found a path to earth through the building and its fabric, in an erratic and unpredictable manner, causing fireballs at various locations

2 58 IRANIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 4, NO. 1, WINTERSPRING 2005 Fig. 1. Single line diagram of a repeater station (VHF tower not shown). Not to the scale. Fig. 2. Photo sketch of towers a view. TABLE I DATA ON THE OCCURRENCE OF LIGHTNING IN THE REPEATER STATION Month Jan Nil Nil Nil Nil Feb Nil Nil Nil Mar Nil Nil April May June July Aug Sep Oct Nov Nil Nil Dec Nil Nil Nil Nil of the building [2]. Strikes anything up to a kilometer away can cause massive transient overvoltages (or surges) on mains power, data communication, signal or telephone lines [3]. As regards the power lines, the main source of trouble is direct/indirect lightning strokes when it impinges on the line very close to the building under study. As regards the cables and the electronic equipment, the transient overvoltages, which are transmitted through resistive, inductive and capacitive couplings, are the main origin of the problem. Inside the building, screened cables can offer protection against voltages induced from current carrying lightning conductors. Outside the building it is susceptible to resistively coupled transients. Added to all above sources, the earth resistance at the location in point was found to be at a higher level and it required adequate measures to lower the earth resistance values. V. POSSIBLE REMEDIAL MEASURES A. Communication Towers The existing protection is found to be adequate. Lightning masts at a height of 1 m at the top of both VHF and microwave towers that are earthed with copper strip and connected to a separate earth pit. As per the details given, area covered by the lightning masts with a shielding angle of 30 is quite adequate to protect the communication towers. For 20 m VHF tower: The base of tower with square side of length = 3.2 m Area of tower (assume, basecircle) = m 2 Area covered by the lightning mast (shielding angle, o 2 θ = 30 ) = π ( h tanθ ) = m 2 where h is the height of the tower For 30 m microwave tower: The base of tower with square side of length = 2.1 m Area of tower (assume, basecircle) = 6.92 m 2 Area covered by the lightning mast (shielding angle 30 o ) = m 2. Since towers are within the coverage area of the respective lightning masts, it can be concluded that the given protection is sufficient for both VHF and microwave towers. Fig. 2 shows a view of communication tower. B. Building There is a wrong perception that the building itself can protect the electronic equipment housed inside. In reality, it requires a welldesigned structural lightning protection [4]. Absence of good grounding will cause a high electric field to envelop the lightning current s travel route. Sparking or side flashing probably will occur currentcarrying rebars and nearby grounding objects [5]. In this case, as the building did not have any structural protection, it is suggested that an appropriate protection may be provided to its structure. Further, reinforced concrete structures

3 VALSALAL et al.: STUDY OF OVERVOLTAGE PROTECTION OF A MICROWAVE REPEATER STATION 59 Fig. 3. Transient protectors installed on underground/overhead incoming/ outgoing lines. (a) (b) Fig. 4. Overvoltage estimation for existing arrangements, (a) at LVDB and (b) at electronic equipment. require an additional system of down conductors embedded in or attached to exterior walls. So the building protection is to be carried out in accordance with the existing standards [6]. Building earthing protection should be implemented as follows: A horizontal air termination should run along the building s parapet. It should be a copper strip of 25 3 mm or a GI strip of 25 6 mm run over an insulator support provided in such a way that no air termination point should touch the building. The distance the adjacent insulator supports should be as minimum as possible. Vertical air termination (4 m) should be provided at the higher most portion of the building and it should be connected to the horizontal air termination strip. Fig. 5. Overvoltage estimation with proposed arresters (interface and LVDB). The down conductor are provided with separate earth pit in order to have a low impedance path from the air termination to the earth electrode so that the lightning current can be safely conducted to earth. It is necessary to avoid sharp bends. As far as possible, the number of joints in down conductors must be minimum. It is essential to provide minimum of two down conductors by considering different height level of the building. A test point should be given to check the continuity of earth. All the data and telephone lines must be away from power supply and lightning protection systems. The coaxial cable from the tower also separated by a minimum distance of 50 cm from the lightning protection system. The body of the tower is also considered as lightning protection system. The outer conductors of all the coaxial cables which run from the structure are bonded at three points: at their upper ends, at the point of exit from the structure and at their point of entry into the equipment building or cubicle. The earth pit of lightning protection system should be away from other earth points by at least 10 feet. C. Data Communication Cables These cables when attached to the antenna should be routed through the mast so as to prevent exposure to direct lightning surges. Since the data lines are susceptible to transient overvoltages, as a general rule, all the incoming and outgoing data communication, signal and telephone lines should be provided with suitable transient protectors [2] as shown in Fig. 3, which do not exist at present. D. Mains Power Protection Surges and spikes from nearby lightning strikes, arcwelders and high voltage cables can destroy or disrupt unprotected electronic equipment. These destructive forces enter mains power circuits within buildings by a variety of methods. At this point, the surges should be stopped in order to prevent them from propagating further. Most low voltage power system (230/415 V) and the electronic and electrical equipment with which they are associated can withstand voltage surges of two or three times normal peak

4 60 IRANIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 4, NO. 1, WINTERSPRING 2005 TABLE II DIRECT LIGHTNING IMPINGES ON OHL RESULTS Electromagnetic pulse / interference Occurrence of lightning surges (external) Occurrence of switching transient (internal) SI. No Lightning Hits Close to Interface Voltage at Interface (pu) Voltage at LVDB (pu) Voltage Entering into Electronic Equipment (pu) 1 No arrester Arrester at interface Arrester at LVDB Arrester at interface and LVDB operating voltage for the duration of a typical lightning surge. During a lightning surge, the voltage is well in excess of these values and therefore surge protection is required. In this case, the feeding power lines suffer from an inadequate protection and it is evident from the fact that there were occasions, the complete heavyduty threephase energy meters goes up in flames when heavy lightning strikes. The prediction of overvoltages by digital simulation using Electromagnetic Transient Program (EMTP) will help to arrange for the effective protection to the affected equipment. The simulations are carried out when a lightning surge (8/20 µs) hits the overhead line (OHL). The OHL, underground cable (UGC) is modeled as distributed parameters and the electronic equipment is considered as resistive load. Fig. 4 indicates the estimation of overvoltages for existing arrangement of repeater station. The simulated value of voltages at OHLUGC interface, LVDB (low voltage distribution board) and entering into electronic equipment, when lightning impinges on the overhead line is tabulated in Table II. In order to protect UGC, LVDB and other connected equipment from the overvoltage, it is suggested to put surge suppressors both at OHLUGC interface and LVDB. Fig. 5 shows the simulated value of reduced voltage at electronic equipment after connecting surge suppressors. The system with poor grounding may not be protected even if surge suppressors are provided properly. So it requires a good multipoint earthing systems. E. Electronic Equipment Inside the Building Surges in the power system induced by a lightning stroke can cause equipment malfunction or the introduction of false data or commands. Also, lightning generated electrical energy levels are extremely high and can cause significant damage to electronic equipment. If the voltage transient is large enough to cause surge current to flow in the equipment, the stored energy is released to cause damage. Semiconductor junctions can be damaged by energy of the order of microjoules. For example, the energy in a 2.5 mm 2 cable of 1 m and 10 m length, for a peak current of 1 ka is 0.65 J and 9.0 J, respectively. So this energy is ample to damage many electronic components [7]. It is important to note that any cable entering an electronic device is also an easy path for lightning induced current to enter and cause damage. The factors that contribute to the failure of electronic equipment when exposed to overvoltage surges are shown in Fig. 6. The degradation and the damages suffered by these equipment Electrical system Induction voltage Mains power supply Cause short circuit damaged Power, data, communication, signal and telephone lines Fast transient (lightning) surges Failure of electronic equipment Building Earthing potential rise Internal interface circuit damaged Microelectronic circuit Electrical system Fig. 6. Factors that contribute to the failure of electronic equipment when exposed to overvoltage surges. are mainly due to flashover and heating since their voltage withstand and heat resisting capabilities are very much limited. Hence effective measures and protective devices for this kind of equipment are essential. The shunting current, equalizing voltage, shielding, grounding and surge suppression are among the effective surge protections for the electronic systems [1]. However, in view of the fact that electronic devices are very sensitive to voltage surges, a voltage clamping device is generally preferred for them. Further, a common surge arrester or isolation component provided for both supply lines and electronic devices cannot offer reliable surge protection since the operating frequencies of electronic devices are far higher than that of power supply equipment. Hence, separate surge suppressors are to be provided for them. F. Electronic Equipment Placement Electronic equipment must not be placed on the top floor of the building where it is adjacent to the rooftop air terminations and conductor mesh of the building s lightning protection system. Also, these equipment should be away from tall lightning attractive structures such as masts, towers, etc. and outside walls especially corners of the building. This shows that the electronic equipment should not be located where it will be close to large current flows and the threat of induced transient overvoltages [2]. VI. PROTECTION COMPONENTS Lightning induced voltage surges can rise from zero to 6 kv in about 1 µs. Surge diverting components must therefore operate quickly. No single surge protection component can combine all the necessary features to prevent excess energy from sensitive parts of electronic equipment. For this reason, multicomponent networks or hybrid circuits incorporate the best features of several surge protection components. The ideal protection network is, one that is fast acting, capable of carrying large currents for short periods and capable of limiting the voltage across and the current through protected equipment to levels below which damage can occur. The main parameters of

5 VALSALAL et al.: STUDY OF OVERVOLTAGE PROTECTION OF A MICROWAVE REPEATER STATION 61 TABLE III TECHNICAL DETAILS OF SURGE SUPPRESSORS TO BE USED FOR PROTECTING VARIOUS EQUIPMENT SI No. Item LVDB Electronic Equipment Coaxial Telephone Line Battery Primary Power Line Cable Power Signal Charger MUX 1. Nominal system 415 V 230 V 230 V 415 V 230 V <20 V voltage (3ph) (1ph) (1ph) (3ph) (1ph) 2. Max. 10% above 10% above 10% above 10% above 10% above <90 V operating voltage nominal nominal nominal nominal nominal 3. Surge current 18 ka per 18 ka per 18 ka 5 ka 10 ka 20 ka rating* mode** mode** per mode** 4. Leakage current <0.3 ma 35 µa <0.3 ma <0.3 ma 5. Type of Surface/DINrail Surface Crone DINrail Surface/DINrail Mounting mounting Mounting mounting MDF mounting 6. Inline impedance 50/75 Ω 7. Shunt capacitance <5 nf 8. Bandwidth <2 GHz 9. Voltage standing wave ratio 1.2:1 10. Ringing voltage <290 V * Based on IEEE/ANSI C62.41 (8/20 µs) standard. ** 18 ka for live neutral, 18k A for neutral earth, 18 ka for line earth. TABLE IV EARTH RESISTANCE MEASURED VALUES Name of earthing Point Earthing Resistance (Ω) Lightning mastvhf tower Lightning mastmicrowave VHF & microwave tower body Equipment Power supply Few equipment in parallel Time Division Multiplexing (TDM) the surge suppressor that require attention are residual voltage, current capacity, time delay of action and connecting capacitance. The residual voltage of the voltage limiters must be below the withstand voltage of the electronic devices. The connecting capacitance of surge suppressors requires a greater attention as it may lead to the failure of electronic circuits. Local protection can be achieved with either wirein protectors at local power distribution board or plugin protection. If protection is required for a number of electronic devices, it may be more cost effective to install a wirein protector at the LVDB rather than individually protecting each electronic devices with a plugin protector. The most data/telecommunication cabling systems and the equipment with which they are associated can safely withstand voltage surges of their normal peak operating voltage [7]. Since each electronic devices are rated for different voltages and damages noticed are severe, it is suggested to put separate surge suppressors for each electronic equipment. For the given application, the details of surge suppressors are given in Table III. All surge suppressors should be mounted and connected as close to the protected electrical equipment as possible in order to reduce the risk of picking up voltage surges in cable runs caused by inductive and capacitive coupling. Moreover when analyzing a system for the surge protection the effective distances must be taken into consideration. The effective distances are the physical separation any two pieces of apparatus. As a general rule, surge protection must be applied when this distance is greater than 100 m horizontally or 10m vertically [7]. TABLE V VALUES OF RESISTIVE AND INDUCTIVE VOLTAGE [7] Cross Length Resistance Section (m) (mm 2 (Ω) ) Inductance (µh) Peak Resistive Voltage (V) Peak Inductive Voltage (V) VII. EARTHING ARRANGEMENTSSUGGESTED MODIFICATIONS The VHF and microwave towers are earthed with copper strip and connected to a separate earth pit. The protective earth wire provided in the building wiring circuit has been connected to a separate earth pit. The earthing electrode arrangement itself has resistance and inductance. For low frequencies, such as 50 Hz mains supplies, inductance is usually negligible in practice and only resistance needs to be considered. In this case there are seven earth points. The values of earth resistance are measured using four wire method and tabulated in Table IV. An earth, which is adequate for the normal mains supply frequency, may not be so for surge protection. Therefore, it is suggested that all earth points should be connected in parallel to reduce the overall resistance and there are many types of earthing methods available in standards to reduce the earth resistance value in hilly areas. Inductance of an earthing system is more complicated than resistance. Table V shows the value of peak resistive and inductive voltages for different wire dimensions. In order to reduce peak inductive voltage, keep all surge earth cables as short as possible. VIII. CONCLUSIONS The transient overvoltages caused by lightning in a microwave repeater station, the damages suffered by the building and electronic equipment are described. The selection of various protective devices is furnished and the essentiality of voltage limiters for surge protection of

6 62 IRANIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 4, NO. 1, WINTERSPRING 2005 electronic devices is stressed. The suggested measures are effective structural protection for the building, provision of transient protectors on all incoming and outgoing data communication lines and appropriate surge suppressors for both electronic devices and LVDB. P. Valsalal received B.E., and M.E., degrees in Electrical Engineering from Bharathiar University and Anna University in 1990 and 1993, respectively. Currently she is pursuing Ph.D. degree in Anna University. She worked as Lecturer in University of Madras during In 1998, she joined as Lecturer in Anna University and subsequently she was promoted as Senior Lecturer in Her research interest includes estimation of overvoltage and lightning arrester modeling. REFERENCES [1] Q. Chen, J. He, W. Zhou, and Q. Su, "Surge protection for interface circuits of communication system," IEEE Trans. on Power Delivery, vol. 18, no. 1, pp. 8589, Jan [2] The Electronic System Protection Handbook, W. J. Furse & Co. Ltd. [3] "Lightning protection it can strike twice," Asian Electricity, vol. 13, no. 6, pp. 6875, Sep [4] T. Bird, H. Karmazyn, B. Barker, and B. O Neill, "Guarding against lightning: a protection racket?," Electrical Review, vol. 222, no. 7, pp. 2627, Apr [5] M. M. Frydenlund, "Lightning protection systems must fit both the purpose and the structure," EMC Technology, pp. 1924, Nov./Dec [6] Indian Standard IS 2309:1989, Protection of Buildings and Allied Structures against Lightning Code of Practice. [7] Application Note MTL Instruments Pvt. Limited. S. Usa received B.E, M.E., and Ph.D., degrees in Electrical Engineering from College of Engineering, Anna University in 1986, 1989, and 1995, respectively. From 1992 to 2000, she worked as Lecturer and since 2000 as Assistant Professor at the College of Engineering, Anna University. Her research interest includes Electromagnetic field computation and High Voltage Engineering. She is a member in IEE, UK. K. Udayakumar received B.E., M.E., and Ph.D., Degrees in College of Engineering, Guindy, Anna University in 1972, 1974, and 1987, respectively. He started his career as Lecturer in Anna University and subsequently promoted as Assistant Professor and currently he is the Professor, in the Department of Electrical and Electronics Engineering, Anna University. He also guided number of Ph.D. scholars and has several publications in distinguished international journals. He also served as Director of various centers of the University and is a pioneer in Engineering and IT education in the country. His research interest is High Voltage Engineering. Dr. Udayakumar is an IEEE Member and was the Chairman of Institute of Electrical and Electronic Engineers, Madras Chapter.