Many different Flexible AC Transmission System (FACTS) devices have been studied in recent years in order to control the flow of power through transmission lines and reduce the overall burden on the power grid. The net results of these devices are decreased utility costs, increased system stability, and improved system flexibility. The main issues with most currently available FACTS controllers are the high costs of installation, and operation. The Sen Transformer (ST) is a new FACTS device that decreases these costs by using relatively inexpensive and industry familiar transformer technology to independently control the active and reactive power in a transmission line.<p> This thesis introduces the first full transient model for the ST developed in an ElectroMagnetic Transients Program (EMTP) using a hybrid transformer modeling approach. This technique handles all the non-linearities of the core, including losses and saturation effects, as well as inter-phase coupling, and zero sequence effect with an attached topographically correct core model. This new model can be used in a variety of power system studies such as transient and dynamic simulations, and protection analysis. The flexibility of the hybid ST model allows for different core and winding configurations as well as response to very fast transients with little modification. Fault analysis studies are presented to showcase the capabilities of the transient ST model developed.<p> The first ST transient model using the Finite Element Analysis (FEA) technique is also created for comparison with the hybrid ST model. This method uses Maxwells equations, material non-linearities and coupled electric circuits to obtain a precise transient solution for the ST. There is good agreement between the two models in a test system for multiple types of fault scenarios. The hybrid ST model is therefore the preferred model to use for fault analysis since it reduces simulation time drastically when compared to the FEA ST model.<p> The hybrid ST model is then used to develop and test differential, and ground protection schemes that ensure device safety during faulted scenarios. The protection schemes are analyzed and compared with analogous Phase Angle Regulator (PAR) schemes that have been implemented for many years.

Many different Flexible AC Transmission System (FACTS) devices have been studied in recent years in order to control the flow of power through transmission lines and reduce the overall burden on the power grid. The net results of these devices are decreased utility costs, increased system stability, and improved system flexibility. The main issues with most currently available FACTS controllers are the high costs of installation, and operation. The Sen Transformer (ST) is a new FACTS device that decreases these costs by using relatively inexpensive and industry familiar transformer technology to independently control the active and reactive power in a transmission line.<p> This thesis introduces the first full transient model for the ST developed in an ElectroMagnetic Transients Program (EMTP) using a hybrid transformer modeling approach. This technique handles all the non-linearities of the core, including losses and saturation effects, as well as inter-phase coupling, and zero sequence effect with an attached topographically correct core model. This new model can be used in a variety of power system studies such as transient and dynamic simulations, and protection analysis. The flexibility of the hybid ST model allows for different core and winding configurations as well as response to very fast transients with little modification. Fault analysis studies are presented to showcase the capabilities of the transient ST model developed.<p> The first ST transient model using the Finite Element Analysis (FEA) technique is also created for comparison with the hybrid ST model. This method uses Maxwells equations, material non-linearities and coupled electric circuits to obtain a precise transient solution for the ST. There is good agreement between the two models in a test system for multiple types of fault scenarios. The hybrid ST model is therefore the preferred model to use for fault analysis since it reduces simulation time drastically when compared to the FEA ST model.<p> The hybrid ST model is then used to develop and test differential, and ground protection schemes that ensure device safety during faulted scenarios. The protection schemes are analyzed and compared with analogous Phase Angle Regulator (PAR) schemes that have been implemented for many years.

In this thesis, design and implementation of a distribution-type, voltage source converter (VSC) based static synchronous compensator (D-STATCOM) having the simplest converter and coupling transformer topologies have been carried out. The VSC STATCOM is composed of a +/- 750 kVAr full-bridge VSC employing selective harmonic elimination technique, a low-pass input filter, and a &amp / #8710 / /Y connected coupling transformer for connection to medium voltage bus. The power stage of VSC based STATCOM is composed of water-cooled high voltage IGBT modules switched at 850 Hz for the elimination of 5th, 7th, 11th, 13th, 17th, 19th, 23rd,and 25th voltage harmonics. Special care has been taken in the laminated busbar design to minimize stray inductances between power semiconductors and dc link capacitor. Reactive power control is achieved by applying the phase angle control technique. The effect of input filter on total demand distortion has been investigated theoretically by mathematical derivations. The proposed VSC STATCOM has been implemented for reactive power compensation of Coal Preparation System in Kemerk&ouml / y Thermal Power Plant. The field test results have shown the success of the implemented system in view of fast response in reactive power compensation, and minimum input current harmonic content, and compliance with the IEEE Std. 519-1992 even for the weakest power systems. The application of selective harmonic elimination technique and phase angle control to VSC STATCOM has led to optimum switching frequency and device utilization for high voltage IGBTs at the expense of slower response as compared to other PWM techniques.

Η παρούσα διπλωματική εργασία έχει ως αντικείμενο τη διερεύνηση της λειτουργίας και το σχεδιασμό συστήματος ελέγχου του δυναμικού αποκαταστάτη τάσης (DVR - Dynamic Voltage Restorer, όπως αναφέρεται στη διεθνή βιβλιογραφία) που χρησιμοποιείται στα δίκτυα διανομής. Η συγκεκριμένη συσκευή ανήκει στην κατηγορία των FACTS (Flexible ac Transmission Systems), παρέχει εν σειρά αντιστάθμιση, και ο σκοπός λειτουργίας της είναι η βελτίωση της ποιότητας της παρεχόμενης ισχύος και η αύξηση της αξιοπιστίας του συστήματος. Εξειδικεύεται στις βυθίσεις τάσεως. Πιο συγκεκριμένα, ο δυναμικός αποκαταστάτης τάσης έχει στόχο, όπως μαρτυρά και το όνομά του, να διατηρεί την τάση ενός φορτίου κατά το δυνατόν σταθερή στην τιμή που αυτή είχε πριν συμβεί η όποια βύθιση. Έτσι, θα καταβληθεί προσπάθεια προκειμένου να προσομοιωθεί η λειτουργία μιας τέτοιας συσκευής σε ένα απλό δίκτυο με τη βοήθεια του λογισμικού PSCAD. Θα κατασκευαστεί ο DVR καθώς και το σύστημα ελέγχου του κι αφού συνδεθεί σε ένα φορτίο, θα δημιουργήσουμε διάφορα είδη σφαλμάτων και θα μελετήσουμε την απόκρισή του και την ικανότητά του να αποκαθιστά την τάση. / This diploma thesis refers to Dynamic Voltage Restorer (DVR), a series compensator used in transmission systems. It is a device that belongs to FACTS and its main function is the mitigation of volatge sags and swells.

With limited enhancement or expansion of the transmission infrastructure, the contemporary power systems are operating under more stressed conditions. It becomes important to fully utilize the existing transmission system to supply load demand as much as possible, thus eliminating or reducing the need for new transmission investment. Flexible AC Transmission System (FACTS) technology provides an alternative to fully utilize the existing transmission lines as well as new and upgraded lines, by controlling power and also enhancing the power transfer capability of transmission lines. However, the implementation of FACTS controllers in the transmission system has introduced new power system dynamics that must be addressed in the area of power system protection, such as rapid changes in line impedance, power angle, line currents, transients introduced by the occurrence of fault and associated control action of the FACTS controller. Therefore, the performance of the protection system must be carefully analyzed in the presence of FACTS controllers. The thesis aims at evaluating the performance of distance relays when different types of FACTS controllers, in particular Voltage Source Converter (VSC) based FACTS controllers, are incorporated at the midpoint of the transmission system to achieve voltage profile improvement and power transfer capability. The detailed models of these controllers and their control strategies are described. The presence of FACTS controllers in the loop affects both steady state and transient components of voltage and current signals. The rapid response of FACTS controllers to different power system configurations significantly affects the apparent impedance seen by distance relays. The apparent impedance seen by distance relays would be different from that of the system without FACTS controller. Due to this, the distance relay may malfunction, resulting in unreliable operation of the power system during faults. Furthermore, the effect of FACTS controllers on distance relay operation depends on the type of FACTS controller used, the application for which it has been installed and its location in the power system. The distance relay is evaluated for different loading conditions and for various fault conditions. Simulation studies are carried out using PSCAD/EMTDC based transient simulation package.

Small signal oscillation has been always a major concern in the operation of power systems. In a generator, the electromechanical coupling between the rotor and the rest of the system causes it to behave in a manner similar to a spring mass damper system. Following any disturbance, such as sudden change in loads, actuations in the output of turbine and faults etc. it exhibits an oscillatory behaviour around the equilibrium state. The use of fast acting high gain AVRs and evolution of large interconnected power systems with transfer of bulk power across weak transmission links have further aggravated the problem of these low frequency oscillations. Small oscillations in the range of about 0.1Hz to 3.5Hz can persist for long periods, limiting the power transfer capability of the transmission lines. These oscillations can be reduced by incorporating auxiliary controllers on generator excitation system. Power System Stabilizers (PSSs) were developed to produce additional damping by modulating the generator excitation voltage. Designing effective PSS for all operating conditions especially in large interconnected power systems still remains a difficult and challenging task. More and more power electronic based controllers have been and will be used in power systems. Many of these controllers such as Static Var Compensators (SVCs), Static Synchronous Compensators (STATCOMs) and Unified Power Flow Controllers (UPFCs) etc., are incorporated in power transmission networks to improve its operational capability. In addition, some of the energy storage systems such as Battery Energy Storage systems (BESS), Super conducting Magnetic Energy Storage System (SMES) as well large non-conventional energy sources are also increasingly being integrated with the power grid. With large integration of these devices, there is a significant impact on system stability, more importantly on small signal oscillatory instability of the power system. This thesis primarily focuses on impact of such devices on small signal oscillatory stability of the power systems. More specifically in this thesis small signal stability analysis of a Single Machine Infinite Bus (SMIB) system with a grid connected wind powered Permanent Magnet Synchronous Generator (PMSG) has been presented. A SMIB system has been purposely chosen so that general conclusions can be obtained on the behaviour of the embedded STATCOM/Energy Source (ES) system on system stability. With a better understanding of the impact of such a system it would be probably possible to analyze more complicated multimachine power system and their impact on system stability. Small signal model of the complete system which comprises the generator, transmission network, inter connecting STATCOM, the wind power generator and all associated controllers has been developed. The performances of the system following a small disturbance at various operating conditions have been analyzed. To obtain quantitative estimates of the damping and synchronizing torques generated in the system, expressions for damping and synchronizing torque clients have been developed. With these analyses, the relative impact of the STATCOM and STATCOM with ES on system performance have been assessed. It is shown that with active and reactive power modulation capabilities effective and efficient control of small signal oscillations in power systems can be achieved.