A power system will inevitably be subjected to transient overvoltages, most often produced by switching operations and lightning strikes. These transient overvoltages may harm sensitive equipment without sufficient protection, but it is not an easy task to predict what transients may occur or how they affect the system. A fault occurred in a low voltage system at Forsmark 2012 and overvoltages from lightning were concluded as the probable cause. The three aims with this thesis are to develop and test a model of the power system at Forsmark 3, analyse the transient behaviour of the system when subjected to lightning surges and identify critical parameters, and lastly to provide a tool for investigating the protection of the system. The modelling and simulations were performed in the freely available simulation program LT Spice. Challenges and difficulties have been to obtain parameter values for components that are relevant for the high frequencies produced by the short rise time in lightning surges. The main conclusions are: the maximum current in the lightning discharge has the largest impact on the system voltages and surges in the external grid can produce standing waves in the system, causing significant overvoltages even in low voltage systems. The simulations produced larger overvoltages than would occur in a real system, which also would dampen the high frequency signals to a larger extent. Further work to obtain models better suited for high frequency simulation is suggested.

This study aims to examine the hosting capacity (the maximum amount of distributed generation possible to add to a current grid) of Mälarenergi’s distribution networks. The three areas examined are a rural network, a modern suburban grid and an older suburban grid. The networks are modelled in PowerWorld Simulator with data mainly from Mälarenergi’s NIS (Network Information System). The basic models include calculated minimum loads based on load profiles, combined with 0 kW, 2 kW, 4kW or 5 kW installed photovoltaics (PV) systems at each consumer. The compensating models are based on the previous ones but with reduced transformer voltages to lower the risk of grid over-voltages. A high load case is also examined to make sure there are no under-voltages for these models. The results show that the rural network is strong enough to handle the biggest available PV system at 5 kW, if the transformer voltage is lowered from 1.03 p.u. to 1.005 p.u. The modern suburban grid can host 4 kW solar panels together with a lowered voltage level of 1.005 p.u. The larger package of 5 kW leads to overloading at the transformer when used. The older suburban grid has the largest issues with both overvoltages and overloading and can only handle 2 kW distributed generation with a voltage reduction to 1.005 p.u. The models are fairly sensitive because of assumed transfomer parameters and, in the case of the modern suburban grid, some lines being removed due to limitations in the software. Regardless, the results are robust enough that they can be considered correct.

Solar Bora is a company that works to bring electricity to small villages in Africa to help them save time in cooking and other things that we nowadays forget about. Africa is the continent with the most thunder strikes in the entire world, and it is therefore common that electronics get affected by this. Our work helps to protect against the surges caused by the electromagnetic pulses that are made by nearby thunder strikes. A surge protection is made using two surge protection devices, a metal oxide varistor and a transient voltage suppression diode in parallel. These are made to short-circuit large transients to ground to protect a circuit from the damaging transient. To test the circuit, we charged a capacitor up to 720 volts and then short-circuited it against the protective circuit to create a high voltage transient. The protective circuit managed to protect against the created transient, but a larger transient would be preferable to see the circuit’s full potential. / Solar Bora är ett företag som jobbar med att skapa energi till de små byar i Afrika som inte har tillgång till det, för att hjälpa dem spara tid vid matlagning och andra vardagssysslor som vi tar för givet. Afrika är den kontinent som har mest åsknedslag i hela världen och det är därför vanligt att elektronik skadas utav åsknedslagen. Arbetet som utförs kommer hjälpa skydda den elektroniken tillhörande Solar Boras system mot överspänningar orsakade av de elektromagnetiska pulserna från närliggande åsknedslag. Ett överspänningsskydd skapas genom användningen av en metal oxide varistor och en transient voltage suppression diode parallellkopplat. Dessa kortsluter stora transienter till jord för att skydda kretsen mot den skadande transienten. För att testa kretsen användes en uppladdad kondensator på 720V som sedan kortslöts mot kretsen för att skapa en högspänningstransient. Skyddet lyckades att dämpa transienten som tänkt men större transienter skulle behövts för att visa kretsens fulla potential.

Många elektriska apparater är i behov av reaktiv effekt för att kunna fungera. Transport av reaktiv effekt tar onödig plats av den tillgängliga kapaciteten i elnätet, därför används kondensatorbatterier nära slutanvändaren för att generera reaktiv effekt. Därmed genereras och förbrukas den reaktiva effekten i en avskild del av nätet. Tillkoppling av kondensatorbatterier ger upphov till transienter som kan skada andra närliggande komponenter. Utifrån det verkliga ställverket Stallbacka i Trollhättan har denna rapport analyserat de transienter som uppstår efter tillkoppling av ett kondensatorbatteri inom mellanspänningsområdet. Analysen har innefattat hur stora transienterna blir efter olika förutsättningar och scenarier. Resultatet visade att transienterna i detta fall aldrig nådde upp till några allvarliga nivåer, och därmed klarade komponenterna sig med god marginal. Huruvida transienterna påverkar elkvaliteten är oklart, då det inte finns några definierade krav. Slutligen skulle ett beräkningsverktyg för förutspådda transienter tas fram. Beräkningsverktyget blev aldrig fullständigt på grund av den ohanterliga lösningen som erhölls. / Many electrical devices need reactive power to operate. Transmission of reactive power occupies a proportion of the available capacity in the power system and therefore capacitor banks are used near to the end user to generate reactive power. Thus, the reactive effect is generated and consumed in a separate part of the power system. Capacitor banks switching causes transients which may damage the surrounding components. Based on the real substation Stallbacka in Trollhättan, this study has analysed capacitor banks switching transients within the medium voltage level. The analysis has covered the size of these transients according to different conditions and scenarios. The result showed that the transients in this case never reached serious levels, thus there was no impact on the components. It is not clear whether the transients affect the power quality because there are no defined limits.  A calculation tool to the predicted transients was supposed to be created in the process. This calculation tool was never completed due to the unmanageable solution that was obtained.