With the growing load demand, Ultra high Voltage (UHV) transmission lines are utilized in many countries around the world for bulk power transmission from remote locations over long distance. UHV transmission lines have typical features and it poses a challenge to the power system design with respect to protection, insulation and reactive power compensation, etc. Protection is a key issue in UHV transmission since a relay failure can interrupt and damage the power system. There are distance and differential protection schemes in the transmission line which account for security of the power system.  This thesis report is based on analysis carried out to find out the typical features associated with the UHV transmission systems. Also the impacts of the UHV transmission line characteristics on line distance protection scheme are observed. The traditional distance relays based on the lumped line parameters are not suited for the UHV transmission lines of very long distances. In this case a simulation is carried out for a 765 kV transmission system modeled in PSCAD. In such a case the non-linearity is even more prominent and the relay is less dependable. In line with the simulation and the analysis for the challenges in UHV transmission system, it is observed that the fault impedance of the line is non-linear and this non-linearity causes the failure of relay operation for a fault location at the boundary of the zone of protection. The fault simulation was carried out in PSCAD and the quadrilateral distance relay characteristics were plotted using MATLAB. From the simulation and results, it is finally concluded that traditional distance protection relays with lumped parameter line modeling is not suitable for UHV transmission liens due to non-linearity in fault impedance and it leads to relay failure. / Ultra high voltage (UHV) transmissionsledningar används i många länder till följd av ett växande behov av överföra hög effekt från avlägset belägna produktionsanläggningar till konsumenter. UHV-transmissionsledningar har speciella egenskaper som innebär utmaningar vid designandet av kraftsystem. Några utmaningar är systemskydd, isolation, och reaktiv effektkompensering. Systemskydd är en viktig aspekt för UHV-transmission eftersom haveri av reläskydd kan orsaka driftstopp och även skada ett kraftsystem. Det finns distans- och differentialskydd i transmissionsledningar som utgör skydd för kraftsystemet.  Denna avhandling är baserad på analyser som har utförts för att åskådliggöra de typiska egenskaperna som är sammankopplade med UHV-transmissionssystem. Även inverkan på distansskydd orsakad av karaktäristiken av UHV-transmissionsledningar utvärderas. De traditionella distansreläskydden som baseras på de sammanslagna ledningsparametrarna är inte lämpade för UHV-transmissionsledningar som stäcker sig över långa avstånd. I detta fall har en simulering utförts i PSCAD för ett transmissionssystem med spänningen 765 kV. I ett sådant fall är karaktäristiken ännu mer olinjär och reläskydden ännu mindre pålitliga. Det observeras att felimpedansen för ledningen är olinjär och till följd av detta orsakas problem med reläskydden då ett fel uppkommer vid utkanten av den skyddade zonen. Denna observation överensstämmer med simuleringarna och de förväntade utmaningarna kopplade till UHV. Simuleringar av felfall utfördes i PSCAD och karaktäristiken av reläskydden plottades med hjälp av MATLAB. Från resultat presenteras i rapporten, konkluderas det att konventionella distansskyddsreläer med modellering av sammanslagna ledningsparametrar inte är lämpliga för UHV-transmissionsledningar på grund av att den olinjära felimpedansen leder till att reläskydden havererar.

Ever increasing demand for electricity, exploitation of large hydro and nuclear power at remote location has led to power evacuation by long EHV/UHV transmission systems. This thesis concentrates on transient analysis of EHV/UHV transmission systems for improved planning and protection. In this thesis, the uncontrolled and controlled switching methods to limit the switching surges during energization of 765kV and 1200k VUHV transmission lines are studied. The switching surge over voltages during the energization of series compensated line are compared with uncompensated line. A Generalized Electromagnetic Transients Program has been developed. The program incorporates specific models for studying the effectiveness of various means for control of switching surge over voltages during UHV transmission line energization and also simulation of various types of faults. Since power grids may adopt next higher UHV transmission level 1200kV, these studies are necessary for insulation coordination as well as transmission line protection relay settings. A new fault detection/location technique is presented for transmission line using synchronized fundamental voltage and current phasors obtained by PMUs at both ends of line. It is adaptive to fault resistance, source impedance variation, line loading and fault incidence angle. An improved Discrete Fourier Transform (DFT) algorithm to estimate and eliminate the decaying dc component in a fault current signal is proposed for computing the phasors. The settings for digital distance relays under different operating conditions are obtained. The relay should operate faster and be more sensitive to various faults under different conditions without loosing selectivity. An accurate faulted transmission line model which considers distributed shunt capacitance has been presented. The relay trip boundaries are obtained considering transmission line model under realistic fault conditions. For different loading conditions ideal relay characteristic has been developed. The obtained trip boundaries can be used for proper settings of practical relay. An adaptive relaying scheme is proposed for EHV/UHV transmission line using unsynchronized/synchronized fundamental voltage and current phasors at both ends of line. For fault location, the redundancy in equations is achieved by using two kinds of Clarke’s components which makes the calculations non-iterative and accurate. An operator for synchronization of the unsynchronized measurements is obtained by considering the distributed parameter line model. The distance to fault is calculated as per the synchronized measurements. Support Vector Machine(SVM) is used for high speed protection of UHV line. The proposed relaying scheme detects the fault and faulted phase effectively within few milli seconds. The current and voltage signals of all phases at the substation are fed to SVM directly at a sampling frequency of 1.0kHZi.e20 samples/cycle . It is possible to detect faulted phase with in 3msec, using the data window of 1/4th cycle. The performance of relaying scheme has been checked with a typical 765kV Indian transmission System which is simulated using the developed EMTP.