Investigation of FACTS devices to improve power quality in distribution networks

Flexible AC transmission system (FACTS) technologies are power electronic solutions that improve power transmission through enhanced power transfer volume and stability, and resolve quality and reliability issues in distribution networks carrying sensitive equipment and non-linear loads. The use of FACTS in distribution systems is still in its infancy. Voltages and power ratings in distribution networks are at a level where realistic FACTS devices can be deployed. Efficient power converters and therefore loss minimisation are crucial prerequisites for deployment of FACTS devices. This thesis investigates high power semiconductor device losses in detail. Analytical closed form equations are developed for conduction loss in power devices as a function of device ratings and operating conditions. These formulae have been shown to predict losses very accurately, in line with manufacturer data. The developed formulae enable circuit designers to quickly estimate circuit losses and determine the sensitivity of those losses to device voltage and current ratings, and thus select the optimal semiconductor device for a specific application. It is shown that in the case of majority carrier devices (such as power MOSFETs), the conduction power loss (at rated current) increases linearly in relation to the varying rated current (at constant blocking voltage), but is a square root of the variable blocking voltage when rated current is fixed. For minority carrier devices (such as a pin diode or IGBT), a similar relationship is observed for varying current, however where the blocking voltage is altered, power losses are derived as a square root with an offset (from the origin). Finally, this thesis conducts a power loss-oriented evaluation of cascade type multilevel converters suited to reactive power compensation in 11kV and 33kV systems. The cascade cell converter is constructed from a series arrangement of cell modules. Two prospective structures of cascade type converters were compared as a case study: the traditional type which uses equal-sized cells in its chain, and a second with a ternary relationship between its dc-link voltages. Modelling (at 81 and 27 levels) was carried out under steady state conditions, with simplified models based on the switching function and using standard circuit simulators. A detailed survey of non punch through (NPT) and punch through (PT) IGBTs was completed for the purpose of designing the two cascaded converters. Results show that conduction losses are dominant in both types of converters in NPT and PT IGBTs for 11kV and 33kV systems. The equal-sized converter is only likely to be useful in one case (27-levels in the 33kV system). The ternary-sequence converter produces lower losses in all other cases, and this is especially noticeable for the 81-level converter operating in an 11kV network

Small-Signal Modelling and Analysis of Doubly-Fed Induction Generators in Wind Power Applications

The worldwide demand for more diverse and greener energy supply has had a significant impact on the development of wind energy in the last decades. From 2 GW in 1990, the global installed capacity has now reached about 100 GW and is estimated to grow to 1000 GW by 2025. As wind power penetration increases, it is important to investigate its effect on the power system. Among the various technologies available for wind energy conversion, the doubly-fed induction generator (DFIG) is one of the preferred solutions because it offers the advantages of reduced mechanical stress and optimised power capture thanks to variable speed operation. This work presents the small-signal modelling and analysis of the DFIG for power system stability studies. This thesis starts by reviewing the mathematical models of wind turbines with DFIG convenient for power system studies. Different approaches proposed in the literature for the modelling of the turbine, drive-train, generator, rotor converter and external power system are discussed. It is shown that the flexibility of the drive train should be represented by a two-mass model in the presence of a gearbox. In the analysis part, the steady-state behaviour of the DFIG is examined. Comparison is made with the conventional synchronous generators (SG) and squirrel-cage induction generators to highlight the differences between the machines. The initialisation of the DFIG dynamic variables and other operating quantities is then discussed. Various methods are briefly reviewed and a step-by-step procedure is suggested to avoid the iterative computations in initial condition mentioned in the literature. The dynamical behaviour of the DFIG is studied with eigenvalue analysis. Modal analysis is performed for both open-loop and closed-loop situations. The effect of parameters and operating point variations on small signal stability is observed. For the open-loop DFIG, conditions on machine parameters are obtained to ensure stability of the system. For the closed-loop DFIG, it is shown that the generator electrical transients may be neglected once the converter controls are properly tuned. A tuning procedure is proposed and conditions on proportional gains are obtained for stable electrical dynamics. Finally, small-signal analysis of a multi-machine system with both SG and DFIG is performed. It is shown that there is no common mode to the two types of generators. The result confirms that the DFIG does not introduce negative damping to the system, however it is also shown that the overall effect of the DFIG on the power system stability depends on several structural factors and a general statement as to whether it improves or detriorates the oscillatory stability of a system can not be made

Apparatus for providing a servo drive signal in a high-speed stepping interferometer

An analog voltage approximately linearly proportional to a desired offset from the present null position of a moving mirror in an interferometer is applied to the mirror moving means. As the mirror moves to the next null position, as determined by the analog voltage, the fringes of a laser reference interference pattern are detected. At the occurrence of each fringe the analog voltage is reduced proportionally so that when the next null position is reached, this driving analog is effectively zero. A binary up/down counter, by its internal count, causes a digital/analog converter to supply the analog voltage to the mirror moving means. Fringe detection and direction of movement logic cause the binary up/down counter to be decremented from its offset count as the mirror is moved to the new null position. Undesirable movement of the mirror due to vibration or other sources causes a correcting drive signal to be applied to the mirror moving means that is proportional to the distance of movement

Medium Voltage Generation System with Five-level NPC Converters for Kite Tidal Power

Offshore power generation has emerged as a prominent source of energy and the installed capacity of new plants has been steadily increasing in recent years. Tidal power specifically is a promising renewable energy source which has not been highly exploited yet, despite its distinctive advantages of being predictable and independent of weather conditions. The main objective of this Licentiate thesis is to analyze and propose solutions for two common problems in offshore power production, which are the power variations due to the non-steady speed profile of the water speed flowing through the turbine and the efficient transportation of the produced power to the shore.The tidal power application utilized in this thesis is the subsea kite, which is a recently developed tidal energy conversion technology that can increase the generated power compared to a traditional static tidal turbine. A turbine is mounted on a submerged kite and the kite moves inside the sea following a predefined trajectory and generating electric power from the tidal currents. The speed and torque of the turbine varies periodically due to the periodic movement of the kite in the sea and, therefore, the control of the generator needs to be able to handle this variable generated power. The kite studied in this thesis has rated active power of 500 kW.In the first part of the thesis, the power generation system of the subsea kite is modelled and the profile of the generated power is extracted given a specific tidal current and turbine geometry. The control of the power converters is described and tested for the specific profile of the generated power. The speed of the generator is controlled by a properly designed Maximum Power Point Tracking algorithm, which ensures that the generator extracts the maximum power from the tidal stream. Experimental verification of the model of this innovative system is also conducted on a laboratory 35 kVA emulator of the tidal power generator.The second part of the thesis deals with the design of a medium voltage generator\ua0drive. The use of medium voltage in the power generation system is highly advantageous for the tidal kite application, since it can reduce the current flowing through the undersea cables connecting the tidal plant to the local grid. Therefore, the size of the cables can be reduced. The drive proposed here uses two 5-level Neutral Point Clamped (NPC) converters connected back-to-back. The 5-level NPC converters can operate with high voltage, while using multiple low-voltagerated power switches. Contrarily, the typical 2-Level converters have limited voltage capability, since they would require more expensive high-voltage-rated power switches. The increased operating voltage of the power conversion system results to lower current and losses in the cables. Another advantage of the NPC converter is the low harmonics at the ac side, which reduces the requirements for passive grid filters. However, the voltage balancing of the dc-link capacitors in this converter topology is a challenge which has not been effectively solved in previous studies. Therefore, a novel voltage balancing strategy is proposed here that uses advanced Space-Vector-Modulation techniques and hardware-based voltage balancing schemes with reduced number of components and lower power losses. Finally, a laboratory prototype of the NPC-converter-based power conversion system is developed with rated power 50 kVA. SiC MOSFETs are used on theconverters to further increase the system’s efficiency and voltage capability.This thesis presents the model, control and laboratory emulator of a kite-based tidal power generator. The experimental set-up can be utilized for conducting research on other renewable sources, such as wind power, that have similar performance. Also, the developed multilevel drive is suitable for various applications where medium voltage grid-connected drives are used and particularly in distributed renewable power generation

Inverter Design for SiC-based Electric Drive Systems with Optimal Redundant States Control of Space Vector Modulation

The need for inverters with ever increasing power density and efficiency has recently become the driving factor for research in various fields. Increasing the operating voltage of the whole drive system and utilizing newly developed SiC power switches can contribute towards this goal. Higher operating voltage allows the design of drives with lower current, which leads to lower copper losses in cables and machine, while SiC switches can drastically increase the inverter efficiency. Offshore renewable power generation, such as tidal power, is a typical application where the increase of operating voltage can be highly beneficial. The ongoing electrification of transportation calls also for high power electric powertrains with high power density,where SiC technology has key advantages.In the first part of the thesis, suitable control schemes for inverters in synchronous machine drive systems are derived. A properly designed Maximum Power Point Tracking algorithm for kite-based tidal power systems is presented. The speed and torque of this new tidal power generation system varies periodically and the inverter control needs to be able to handle this variable power profile. Experimental verification of the developed control is conducted on a 35 kVA laboratory emulator of the tidal power generation unit.Electric drives using multilevel inverters are studied afterwards. Multilevel inverters use multiple low-voltage-rated switches and can operate at higher voltage than standard two-level inverters. The Neutral Point Clamped (NPC) converter is a commonly used multilevel inverter topology for medium voltage machine drives. However, the voltage balancing of its dc-side capacitors and the complexity of its control are still issues that have not been effectively solved. A new method for the optimal utilization of the redundant states in Space Vector pulse-width-Modulation (SVM) is proposed in this thesis in order to control its dc-link voltages. Experimental verification on a 4-kV-rated prototype medium-voltage PMSM drive with 5-level NPC converters is conducted in order to validate the effectiveness of the proposed control technique.Low switching and conduction losses are typical characteristics of SiC switches that can be utilized to build inverters with high power density, due to the increased efficiency and smaller form-factor. Due to the above, SiC power modules have been particularly attractive for the automotive industry. The design approach of 2-level automotive inverters has been studied in this project. Moreover, a new design approach for the cooling system of automotive inverters has been developed in this thesis, which fine-tunes the inverter heatsink utilizing standard legislated test routines for electric vehicles. Multiple conjugate-heat-transfer (CHT) computation results showcase the iterative optimization procedure on a test-case 250 kW (450 A) automotive SiC inverter.Finally, the experimental testing of high power machine drives in order to verify the control and the hardware design is an important step of the development process. Thus, the performance of the prototype 450 A SiC 2-level inverter has been been experimentally validated in a power hardware-in-the-loop (P-HIL) set-up that emulates an automotive drive system. Several challenges have been addressed with respect to the accurate modelling of the motor and the control of the circulating power in the system. A new control technique utilizing the redundant states of the SVM has been developed for this set-up to effectively suppress the zero-sequence current to 3.3 % of the line current at rated power

Static shunt and series compensations of an SMIB system using flying capacitor multilevel inverter

The flying capacitor multilevel inverter (FCMLI) is a multiple voltage level inverter topology intended for high-power and high-voltage operations at low distortion. It uses capacitors, called flying capacitors, to clamp the voltage across the power semiconductor devices. A method for controlling the FCMLI is proposed which ensures that the flying capacitor voltages remain nearly constant using the preferential charging and discharging of these capacitors. A static synchronous compensator (STATCOM) and a static synchronous series compensator (SSSC) based on five-level flying capacitor inverters are proposed. Control schemes for both the FACTS controllers are developed and verified in terms of voltage control, power flow control, and power oscillation damping when installed in a single-machine infinite bus (SMIB) system. Simulation studies are performed using PSCAD/EMTDC to validate the efficacy of the control scheme and the FCMLI-based flexible alternating current transmission system (FACTS) controllers

An all monolithic MOS A/D converter - Low power clocks, multiplexers, registers, and A/D converter Final report

Research and developments of monolithic, MOS, ten bit, analog to digital converte