Lightning is a natural phenomenon involving transient high current discharge in the atmosphere. Cloud-to-ground lightning, wherein the discharge occurs between the cloud and the ground is quite hazardous to systems on the ground. Apart from threat to life, the devastating effects of lightning can be mainly of thermal, mechanical and electromagnetic origin. Many a times, thermal and electromagnetic effects are of main concern. A direct hit, wherein the system under consideration becomes a part of the lightning path, could be quite catastrophic to many vulnerable systems like oil rigs, chemical factories, missile/satellite launch pads. From the safety and operational point of view, lightning is of serious concern for electrical systems including transmission lines and substations, nuclear power stations, telecommunication station and data banks. Lightning cannot be avoided, however, by employing a suitable Lightning Protection System (LPS), adequate protection against a direct hit can be provided to ground based systems. A typical lightning protection system involves: 1) Air termination network, which is responsible for stroke interception, 2) Down conductor system, which provides to the stroke current a minimal impedance path to the ground and 3) Earth termination network, for safe dissipation of current into the ground. Similarly, for the indirect effects, which are basically of electromagnetic origin, suitable protection can be designed. The key factors in a protective action involve interception of the dangerous strokes, minimization of the consequential potential rise on down conductors, as well as, at earth termination and keeping the field in the protective volume within an acceptable level. The last aspect can be generally categorized into secondary level protection. For critical systems, the lightning protection system is generally isolated from it. In such designs, potential rise on LPS governs the physical isolation required between the protected and protection system. For a given level of bypass strokes, cost of the LPS increases with the amount of physical separation employed. All most all of the earlier works have concentrated on lightning surge response of power transmission line towers. Apart from their relatively moderate heights, the intention was to arrive at a model, which can be incorporated in circuit simulation software like EMTP. Consequently, they envisage or approximate the mode of propagation to be TEM. In reality, for down conductors of height greater than say 30 m, only TM mode prevails during the initial critical time period. Hence the earlier models cannot be extended to general lightning protection schemes and for down conductor of larger lengths. Only limited literature seems to be available on the characteristics of general down conductor configurations. The problem in hand is very important and some serious research efforts are very much essential. In view of the above, the present work aims to evaluate the rise in potential as well as current injected into the soil at the base for: (i) practical range of down conductor configurations involving single down conductor (with height exceeding 30 m) and (ii) pertinent values of stroke current parameters. The protection schemes considered are isolated vertical down conductor, isolated tower (both square and triangular cross-section) and, tower with insulated lightning mast carrying ground wires. The parameters under consideration are: (i) height and cross section for the down conductor, (ii) clearance between the down conductor and the protected system, (iii) channel geometry, wherein only inclination is to be considered, (iv) velocity of current along the channel and (v) wave shape and rise time for the stroke current. For the evaluation of lightning surge response of transmission line towers, many theoretical and experimental approaches are found in the literature. However, works considering the TM mode of current propagation is relatively limited. In that both experimental and theoretical approaches have been adopted. Theoretical approach invariably adopted numerical field computation in frequency domain using Numerical Electromagnetic code (NEC-2). Fourier Transform techniques are employed to extract the time domain quantities. This approach is very economical, free from experimental errors and least time consuming. Hence it is selected for the present work. However, there are certain limitations in this approach. In NEC simulation, there is a restriction on the size and the arrangement of individual elements. Therefore, although fairly complex tower structures can be simulated, some simplification in the geometry is unavoidable. Such an approximation has been reported to cause insignificant error. NEC is not accurate for calculations in low frequency regime. But in the present work, the initial time regime is of concern wherein the high frequency components dominate. Therefore the above said limitation is not of any serious concern. In order to validate the approach, potential rise is computed for 120 m tall cylindrical down conductor and tower. Results are compared favorably with earlier works, which are based on potential lead wire method. A careful re-look into the ’potential rise’ on the down conductors reveal several things. The electric field in the region between the protection system and protected system is the root cause for the breakdown/flashover. For a given geometry, the integral of the electric field along the shortest path between the two systems must be representing the overall stress on the air gap. Further, for the later time periods, this integral coincides with the well-known quasi-static potential. All the available data and models for breakdown of long air gaps are basically in terms of this quasi-static potential. In view of this, the above path integral is defined as ’equivalent potential rise’ (which will be hereafter termed as ’potential rise’), and taken as the index for surge response. Further, observation of the computed spatio-temporal radial electric field around the down conductor reveals some additional features, which are not common in the quasi-static regime. Electric field reverses its polarity in space, which is due to the opposite current flowing in the lightning channel. Therefore, ’potential rise’, which is taken as the representative for the dielectric stress on the air, should not be evaluated for larger distances. Considering this and noting that the protected system generally lies well within a distance of 50% of the H, height of the down conductor, potential rise is evaluated by integrating electric field within this distance (12.5%H, 25%H, 50%H). Three heights (100%H, 75%H, 50%H) are considered for the evaluation of the potential. The influences of various down conductor and lightning channel parameters are analyzed. Finally vertical channel with full velocity for current propagation is arrived for the investigations. Also, the influence of neighboring conducting objects is briefly studied. It is argued that it needs to be ignored for the general study. Analysis is carried out for a range of down conductor configurations of heights ranging from 45 m to 120 m. Cylindrical down conductor is selected for the detailed study on the overall characteristics and its dependency on pertinent parameters. The characteristics of potential rise are found to be significantly different from that given by the commonly employed uniform transmission line model. In the regime of very fast front currents, down conductor of comparable heights have comparable potential rise. For the larger time to crest, behavior tends more to wards that for quasi-static regime. The dependency of the potential rise on radius of the down conductor seems to be logarithmic in nature. Surge response of isolated towers of both square and triangular cross sections is studied for heights ranging from 45 m to 120 m. The overall characteristics are found to be similar to cylindrical down conductor. Dispersive propagation is found to exist on towers. As a result, the base currents are slightly lower and potential rise exhibits less oscillations. Data curves on potential rise at three different heights and for three different spatial extents are generated for the range of down conductor heights with rise time of the stroke current as the variable. Several interesting observations have been made. Next the investigation is taken up for the insulated mast scheme. The parameters of the study are taken as the number of ground wires, grounding location of ground wires and length of the insulation cylinder. Potential across the insulation, tower base currents, and ground wire end currents are deduced. The basic characteristics of the potential rise are shown to be quite similar to that for the transmission line. For fast front currents the temporal variation is bipolar with a smooth decay. In other words, oscillations are sustained for considerably longer duration. Voltage stress across the insulation surface for one ground wire design is found to be higher by 1.4 - 2.4 times than that for isolated tower. The highest amplification of the ground end current, which occurs for fast front currents, is about 1.8 times. Potential difference across the insulation for two-ground wire design is higher by a factor of 1.3 - 1.85 than that for isolated tower. For the design with four ground wires, potential across the insulation is comparable with that for the tower. However, the mechanical strength of the insulating support should also be considered in the selection of number of ground wires. There exists, especially for fast front strokes, significant induction to the supporting tower. The height of the insulation seems to possess no appreciable influence on the potential rise and base currents. Several issues need to be considered before selecting this design. The contribution made by the present work can be summarized as follows. It basically deals with lightning surge response of isolated down conductors of height in the range 45 - 120 m. The configurations considered are, cylindrical down conductor, tower with both square and triangular cross section and insulated mast scheme. It makes a careful study on the ’potential rise’ on down conductors and a suitable definition for the same is proposed. Basic characteristics of potential rise and ground end currents are studied for the above-mentioned designs. Their salient features are enumerated. For the towers, design data curves are provided for relevant range of stroke current rise time. The issues that need to be considered in the insulated mast scheme are discussed along with the data on potential rise and base currents. The findings of this work are believed to be very useful for the design of lightning protection scheme involving isolated down conductor. Further the results are useful in analyzing the consequential lightning generated threat of being close to tall towers.

Electromagnetic effects of lightning currents during a direct hit to tall communication towers, other instrumented towers and chimneys can be hazardous to associated cables, as well as, electrical and electronics systems. The standard practice in telecommunication and other related fields is to bond the cable sheath to the tower and ground connection is made before it enters the base station. However, in some specific cases when power, signal and data logging cables are to be supported on the same tower, isolation of power cables is demanded. In a totally different situation, attempts are also made to have a dedicated isolated down conductor. A critical review of the situation demanded a more quantitative answer to the following questions: (i) whether it is possible to electrically isolate a dedicated down conductor, (ii) is it possible to electrically isolate the cables and their terminal equipment both mounted on towers serving as down conductor and if so, what will be the nature of current induced in the cables and (iii) as per the standard practice, if the cable sheaths are connected to the tower/structure, what will be the nature of the current shared by them. Addressing these important issues formed the scope of the present work. For the tall structures considered in this work, for the critical time periods, wave nature of the current dominates. This called for electromagnetic modeling covering Transverse Magnetic (TM) mode of the wave propagation. Owing to the complex geometrical features involved with the problem, both experiments on electromagnetically scaled laboratory models, as well as, theoretical simulation is attempted. An electromagnetically scaled laboratory model is employed for the time domain experimental investigation. This approach, which has been validated earlier, is further scrutinized to ensure its adequacy. In order to achieve generality and noting the fact that the associated parameters are rather difficult to be varied in the experimentation, theoretical investigation is also employed. For this, both NEC-2, as well as, an in-house thin wire time domain code developed for this work is employed. NEC-2 could handle multi-wire multi-radius junctions, while in-house time domain code could handle proximity and non-cylindrical shapes encountered with tower lattice elements. The investigation of induction to isolated cables on simple down conductors and towers is considered first. The induced current is shown to be bipolar oscillatory with the period of oscillation governed by the length of the cable. It is shown that the level of induction for good earth termination is below 5 – 10 % while that with moderate inductance in the earth termination can enhance the induction to higher levels. The level of induction is shown to be not critically dependent on the length of the cable, gap between cable and down conductor/tower. When multiple cables are mounted, they seem to influence each other and individually carry currents of lower amplitude. Also, the effect of shape and proximity of the tower lattice elements on induction is investigated. If the cable is housed inside a metallic tray, the amplitude of induced current is shown to be quite small. Subsequently, the evaluation of electrical stress between the isolated down conductor on tower and simplified representation of the structure is considered. A suitable definition of the electric stress for the wave regime is evolved and then it is shown that, at present, the voltage difference defined by the path integral of electric field across shortest path between the two entities is the best indicator for the stress. The electrical stress in the case of isolated down conductor on tower, as well as, down conductor with isolated cable is shown to reach very dangerous levels. On the other hand, the stress on the isolated cables on towers also serving as down conductors is shown to be relatively moderate. Interestingly, it is shown that the electrical stress and the voltage difference is dependent on the gap and for the critical time period, can be much lower than that calculated as a product of equivalent tower surge impedance and the stroke current, even before the arrival of ground end reflections. Finally, the current shared by cables connected to the down conductor is investigated. For the case of simple cylindrical down conductor with cable connected to it at the top, it is shown that the amount of current shared by the cable is not dependent on its length and the relative radii (cross section) have only a weak influence. For the case with down conductor formed by L and + angles, it is shown that the placement of cable at their interior corner can reduce the initial current shared by the cable. In order to model best possible situation with towers, experiments are conducted with cable inside an aluminum pipe. Even in this case, cable current builds up with successive reflections to become comparable with the current through the pipe itself. Subsequent investigation with 1:40 and 1:20 tower models lead to several interesting observations. Cables running along leg/face of the tower whether placed inside or outside the tower, always shares good amount of current. Further, frequent bonding of the sheath to the tower increases the current shared by the cable. Cable when housed in a metallic tray shares less than 50% of the current shared without the tray. Even though a complete quantification is not to be achieved in this work, it has made a good beginning with some significant contribution towards lightning protection issues pertaining to tall towers and structures.