Ο κεραυνός είναι μία ηλεκτρική εκκένωση ατμοσφαιρικής προέλευσης απλή ή πολλαπλή μεταξύ νέφους και γης. Αποτελεί ένα φυσικό φαινόμενο με επικίνδυνα αποτελέσματα για την ασφάλεια των ανθρώπων, των κτιρίων και των ηλεκτρικών συστημάτων. Αν και είναι τόσο παλιό όσο και ο πλανήτης μας, μπόρεσε να εξηγηθεί μόνο κατά τους τελευταίους δύο αιώνες. Από το 1753 όπου ο B. Franklin απέδειξε την ύπαρξη ατμοσφαιρικού ηλεκτρισμού μέχρι σήμερα, με πολλές μοντέρνες μεθόδους και πειράματα έχει αποκτηθεί αρκετή γνώση επάνω στο φαινόμενο αυτό και κατά συνέπεια έχουν αναπτυχθεί αρκετά ασφαλείς τρόποι προστασίας από τους κινδύνους ενός κεραυνού. Όταν ένας κεραυνός πλήξει το έδαφος, το ρεύμα του εκφορτίζεται στη μάζα της γης. Η τελική κατανομή του ρεύματος εξαρτάται κατά πολύ από την ανομοιογένεια του εδάφους, επειδή το ρεύμα θα ρέει όπου είναι πιο εύκολο δηλαδή κατά μήκος αγώγιμων κοιτασμάτων ή γεωλογικών σφαλμάτων που είναι διάχυτα σε έδαφος με μικρή αγωγιμότητα. Εξ αιτίας αυτού του γεγονότος, όταν υπάρχει ένας μεταλλικός αγωγός μεγάλου μήκους , η μεταλλική του επιφάνεια θα βραχυκυκλώσει τμήμα του ηλεκτρικού πεδίου που δημιουργείται στο έδαφος από πλήγμα κεραυνού. Το ρεύμα του κεραυνού συγκεντρώνεται στην επιφάνεια του αγωγού, ενώ το βάθος διείσδυσης του κεραυνού είναι κυρίως συνάρτηση της επικρατούσας ειδικής αντίστασης του εδάφους. Επίσης, ένας κεραυνός που πλήττει το έδαφος παράγει ένα μεταβαλλόμενο ηλεκτρομαγνητικό πεδίο, το οποίο επάγει τάσεις σημαντικού ύψους στους υπόγειους μεταλλικούς αγωγούς που βρίσκονται σε γειτνιάζουσα περιοχή με το σημείο του πλήγματος, οι οποίες θα καταπονήσουν τους υπόγειους αγωγούς προκαλώντας ακαριαία καταστροφή ή ισχυρή καταπόνηση οδηγώντας σε βαθμιαία φθορά. Σκοπός αυτής της διπλωματικής εργασίας είναι η μελέτη και η ανάλυση των επαγόμενων υπερτάσεων οι οποίες εμφανίζονται σε υπόγειους μεταλλικούς αγωγούς λόγω των κεραυνικών πληγμάτων. Για αυτούς τους λόγους κάνουμε μια εκτενή ανάλυση της δημιουργίας κεραυνών καθώς και των επιπτώσεών τους στη γήινη επιφάνεια. Ακόμη γίνεται αναφορά στο είδος των υπερτάσεων που δημιουργούνται από τους κεραυνούς καθώς και στο τρόπο με τον οποίο προστατεύονται οι υπόγειοι αγωγοί από τις υπερτάσεις. Στη συνέχεια με την χρήση του λογισμικού ATP-EMTP γίνεται ανάλυση των υπερτάσεων που δημιουργούνται στους αγωγούς όταν ένας κεραυνός πλήξει μια γειτονική γραμμή μεταφοράς. Η εργασία ολοκληρώθηκε με παρατηρήσεις επάνω στα αποτελέσματα των εξομοιώσεων. / Lightning is an electrical discharge atmospheric origin single or multiple between cloud and earth. It is a natural phenomenon with dangerous results for the safety of people, buildings and electrical systems. Although it is as old as our planet, could be explained only in the last two centuries. Since 1753 when the B. Franklin proved the existence of atmospheric electricity up to today, with many modern methods and experiments has been acquired enough knowledge on this phenomenon and accordingly have been developed several safe ways to protect against the risks of lightning. When a lightning struck the ground, his current discharged to the mass of earth. The final distribution of current depends by far from the heterogeneity of the soil, because the current flows where it is easier, that is to say at length of conductive deposits or geological faults that are diffuse in the soil with low conductivity. Because of this, when there is a long metal pipeline, his metal surface will fuse department of electric field that is created in the ground by hurt of lightning. The lightning current is concentrated on the surface of the pipeline, while the depth of infiltration of lightning is mainly a function of the prevailing soil resistivity. Also, a lightning that affects the ground produces a altered electromagnetic field, which induces voltages considerable amount of underground metallic pipelines located in nearby locations to point to the shock, which stresses the underground pipes, causing immediate destruction or high stress leading to a gradual deterioration. Aim of this work is the study and analysis of induced voltages that occur in underground metallic pipelines due to lightning strikes. For these reasons, we do an extensive analysis of the creation of lightning and its impact on the earth's surface. Also refers to the type of over voltages caused by lightning and the way protect underground pipes from the over voltages. Afterwards, with the use of ATP-EMTP software becomes analysis of the over voltages that are created in the pipelines when a lightning struck a nearby transmission line. The work was completed with observations on the results of the simulation.

In the recent years, there has been a trend to run metallic pipelines carrying petroleum products and high voltage AC power lines parallel to each other in a relatively narrow strip of land. The case of electromagnetic interference between high voltage transmission lines and metallic pipelines has been a topic of major concern since the early 60’s. The main reasons for that are: • The ever increasing cost of right-of-ways, suitable for power lines and pipelines, along with recent environmental regulations, aiming to protect nature and wildlife, has forced various utilities to share common corridors for both high voltage power lines and pipelines. Therefore, situations where a pipeline is laid at close distance from a transmission line for several kilometers have become very frequent. • The rapid increase in energy consumption, which has led to the adoption of higher load and short circuit current levels, thus making the problem more acute. Due to this sharing of the right-of-way, overhead AC power line field may induce voltages on the metallic pipelines running in close vicinity leading to serious adverse effects. This electromagnetic interference is present both during normal operating conditions as well as during faults. The coupling of the field with the pipeline takes place either through the capacitive path or through the inductive or conductive paths. In the present work, the induced voltages due to capacitive and inductive coupling on metallic pipelines running in close vicinity of high voltage power transmission lines have been computed.The conductor surface field gradients calculated for the various phaseconfigurations have been presented in the thesis. Also the electric fields under transmission lines, for both single circuit and double circuit (various phase arrangements) have been analysed. Based on the above results, an optimum configuration giving the lowest field under the power line as well as the lowest conductor surface gradient has been arrived at and for this configuration induced voltage on the pipeline has been computed using the Charge Simulation Method (CSM). For comparison, induced voltages on the pipeline have been computed for the various other phase configurations also. A very interesting result is that the induced voltage on the pipeline becomes almost negligible at a critical lateral distance from the center of the powerline and beyond which the induced voltage again increases.This critical distance depends on the conductor configuration. Hence it is suggested that the pipeline be located close to the critical distance so that the induced voltage would be close to zero. For calculating the induced voltage due to the inductive coupling, electromotive force (EMF),induced along the pipeline due to the magnetic field created by the transmission line has been calculated. The potential difference between the pipeline and the earth, due to the above induced EMFs, is then calculated. As the zones of influence are generally formed by parallelism, approaches, crossings as well as removals, the computation involves subdividing the zone into several sections corresponding to these zones. The calculation of voltages is carried out at both the ends of the sections. Each section is represented by an equivalent π electrical network, which is influenced by the induced EMF. The induced EMF is calculated during faulted conditions as well as during steady state conditions. Inductive coupling calculations have been carried out for the following cases: •Perfect parallelism between powerline and pipeline. •zone of influences formed by parallelism, approaches, crossings and removals. It has been observed that when the pipeline is approaching the HV transmission line at an angle, then running parallel for certain distance and finally deviating away, the induced voltage is maximum at the point of approach or removal of the pipeline from the transmission line corridor.The induced voltage is almost negligible near to the midpoint of the zone of influence. The profile of the induced voltage also depend on whether the pipeline is grounded or left open circuited at the extremities of the zone of influence. Effect of earth resistivity and anti-corrosive coatings on induced voltage has also been studied. For mitigating the induced voltage on the pipeline,numerous low resistive earthings have been suggested. Results show that significant reduction in induced voltage can be achieved as the number of earth points is increased.