The most widespread system of an earthing neutral point of a supply transformer in a MV network is an earthing through an arc-suppression coil. In this case of a resonant-earthed power system a fault current is compensated by the suppression coil and only residual current flows through a fault point. A value of the residual current is much smaller then a value of an original capacitive current of the network and is not dependent on the fault location. According to that reasons it is really complicated to fast and accurately determine the fault point location. Since nowadays used methods for the ground fault location inside wide distribution networks have a lot of disadvantages, the thesis is focused on a proposal of a new method for the ground fault location especially inside of a compensated network. The main reasons for a consecutive installation of power quality meters into distribution transformer substations are a power quality monitoring and a global evolution of an electrical network onto the “SmartGrids”. In case that all results from the meters are properly synchronized and centralized, new possibilities of control or of an evaluation of a network are enabled. This doctoral thesis proposes the possibility for the ground fault detection with an aid of synchronized records recorded on the low-voltage side of the distribution transformers 22/0.4 kV into the network equipped with automatics for short-time increasing of an active part of a faulty current. The described method uses a voltage drop evoked by connecting of the auxiliary resistor for location of the faulty section. The proposed method is tested with help of numerical model which presents a part of the distribution network Series of simulation are executed and correct function and force function of the method is reviewed. Finally the thesis describes real experiment and also a confrontation between simulation and real measuring, and also possibilities of method usage are commented.

Meshed high-voltage direct current grids are becoming an increasingly important technology for integrating renewable energies into the power system. To control the grids in the best possible way, optimal converter and grid control strategies are needed. This project studies how a four-terminal high-voltage direct current grid is operated and controlled by implementing different grid and converter control strategies. The grid control strategies examined are centralized voltage control and distributed voltage control with and without deadband. Simulations are made in the software PSCAD. Different fault types on the grid are studied to investigate how the power flow and voltage level are affected. An optimal value for both the deadband width and droop constant has been identified. Moreover, the results indicate that centralized droop control is not a suitable grid control strategy, whereas distributed voltage control with and without deadband are. The fault study indicates no differences between distributed voltage control with and without deadband. The power flow and voltage levels are identical for all fault types. / Högspända likströmsnät spelar en allt större roll med att integrera förnyelsebar energi i våra elnät. För att styra dessa nät på bästa möjliga sätt krävs optimala omvandlar- och nätkontrollstrategier. I detta projekt studeras hur ett fyrterminalt högspänt likströmsnät kan styras och drivas genom att implementera olika omvandlar- och nätkontrollstrategier. De nätkontrollstrategier som studerats är centraliserad spänningskontroll och distribuerad spänningskontroll med och utan ett spänningsintervall. Alla simuleringar har utförts i programmet PSCAD. Olika fel i nätet har även studerats för att undersöka hur effektflödet och spänningsnivån påverkas. Ett optimalt värde på både spänningsintervallet och droop konstanten har identifierats. Dessutom har resultat som indikerar att centraliserad spänningskontroll inte är en lämplig nätkontrollstrategi erhållits, medan distribuerad spänningskontroll med och utan spänningsintervall är det. Felsimuleringarna påvisar ingen skillnad mellan distribuerad spänningskontroll med och utan spänningsintervall. Effektflödet och spänningsnivån är identiska för alla fel.