Strukturno promjenjivo upravljanje momentom asinkronog motora bez mjernog člana brzine vrtnje

Induction motor speed sensorless torque control, which allows operation at low and zero speed, optimizing both torque response and efficiency, is proposed. The control is quite different than the conventional field-oriented or direct torque controls. A new discontinuous stator current FPGA based controller and rotor flux observer based on sliding mode and Lyapunov theory are developed, analyzed and experimentally verified. A smooth transition into the field weakening region and the full utilization of the inverter current and voltage capability are possible. The reference tracking performance of speed and rotor flux is demonstrated in terms of transient characteristics by experimental results.Predloženo je upravljanje momentom asinkronog motora bez mjernog člana brzine vrtnje, koje omogućuje rad na malim brzinama i u stajanju te pritom optimira i odziv momenta i učinkovitost. Predloženo je upravljanje prilično drugačije od konvencionalnog upravljanja poljem i neposrednog upravljanja momentom. Razvijeni su, analizirani i eksperimentalno potvrđeni novi diskontinuirani regulator struje statora implementiran u FPGA i estimator rotorskog magnetskog toka zasnovan na kliznim režimima i Ljapunovljevoj teoriji. Omogućeni su glatki prijelaz u područje slabljenja polja i puna iskoristivost strujno-naponskih mogućnosti napojnog pretvarača. Performanse slijeđenja referentne veličine brzine vrtnje i rotorskog magnetskog toka pokazane su eksperimentalno dobivenim prijelaznim pojavama

PERFORMANCE STUDY OF DSTATCOM WITH PI CONTROLLED SVPWM AND HYSTERESIS CURRENT CONTROLLER FOR POWER FACTOR IMPROVEMENT

ABSTRACT A Distribution STATCOM (DSTATCOM) is a current controlled Voltage Source Converter (VSC) used for reactive power compensation, when connected to the power system. The DSTATCOM using PI controlled Space Vector Pulse Width Modulation (SVPWM) technique has high peak overshoot and large settling time. SVPWM technique requires Phase Locked Loop (PLL) for measurement of varying frequency that is required for Park's transformation and for control. This paper presents a DSTATCOM for reactive power compensation using space vector based Hysteresis Current Controller (HCC) and compares its performance with that of the PI controlled SVPWM method. The HCC technique is robust and has faster transient performance compared to SVPWM control technique. In addition, this technique does not require PLL for measurement of frequency, has reduced switching losses, and is easy to implement. The performance of DSTATCOM with HCC is studied in MATLAB/SIMULINK environment

Microgrids/Nanogrids Implementation, Planning, and Operation

Today’s power system is facing the challenges of increasing global demand for electricity, high-reliability requirements, the need for clean energy and environmental protection, and planning restrictions. To move towards a green and smart electric power system, centralized generation facilities are being transformed into smaller and more distributed ones. As a result, the microgrid concept is emerging, where a microgrid can operate as a single controllable system and can be viewed as a group of distributed energy loads and resources, which can include many renewable energy sources and energy storage systems. The energy management of a large number of distributed energy resources is required for the reliable operation of the microgrid. Microgrids and nanogrids can allow for better integration of distributed energy storage capacity and renewable energy sources into the power grid, therefore increasing its efficiency and resilience to natural and technical disruptive events. Microgrid networking with optimal energy management will lead to a sort of smart grid with numerous benefits such as reduced cost and enhanced reliability and resiliency. They include small-scale renewable energy harvesters and fixed energy storage units typically installed in commercial and residential buildings. In this challenging context, the objective of this book is to address and disseminate state-of-the-art research and development results on the implementation, planning, and operation of microgrids/nanogrids, where energy management is one of the core issues

Enhanced control of DFIG-based wind power plants to comply with the international grid codes

A review of the latest international grid codes shows that large wind power plants are stipulated to not only ride-through various fault conditions, but also exhibit adequate active and reactive power responses during the fault period in order to support the network stability. In particular, modern grid codes require wind power plants to: (1) ride-through various voltage sag and swell conditions, (2) inject reactive current into the grid during the fault period, and (3) attain swift active power restoration after the fault clearance. This thesis proposes a transient control scheme for DFIG-based wind power plants to comply with these requirements.In the first part of this thesis, the latest regulations enforced on large wind power plants are studied and compared. This study identifies the most stringent regulations defined by the international grid codes, to be further investigated in the following chapters. In the second part of this thesis, extensive simulation studies are carried out to examine the transient response of DFIG-based wind turbines under various symmetrical and asymmetrical fault conditions. Supplementary theoretical analyses are also presented to justify the observations made in the time-domain simulations results. For the first time, the impacts of phase-angle jump, voltage recovery process and sag parameters on the DFIG response are explored. The results of this study can assist researcher to identify the difficulties that hinder successful fault ride-through response of DFIG-based wind turbines, as requested by the international grid codes.In the third part of the thesis, an enhanced hysteresis-based current regulator (referred to as VBHCR) is proposed to be implemented in the rotor-side and grid-side converters of DFIG-based wind turbines. The main advantages of this current regulator are very fast transient response, simple control structure and insensitivity to the machine parameters variations. Simulation results show that on one hand the VBHCR has very good steady-state performance and on the other hand, it presents very fast/robust tracking response. Therefore, the DFIG equipped the proposed current regulator can fulfill the most stringent low-voltage ride-through requirements imposed by the international grid codes, i.e., those stipulated by the Australian grid code. In the fourth part of the thesis, a new hybrid current control scheme is introduced to enhance both low and high voltage ride-through capabilities of DFIG-based wind turbines. The proposed control scheme uses the standard PI current regulators under steady-state conditions but upon a voltage sag or swell occurrence, the supervisory control unit transfers the switching strategy of the rotor-side and grid-side converters to the hysteresis-based method. The VBHCR remains in action until the oscillation in the rotor current and dc-link voltage of DFIG suppress below the safety limit and then, the PI current regulator are activated through a re-initialization process.Finally, the conventional vector control scheme of DFIG-based wind power plants is modified to fulfill the regulations imposed on the active and reactive power responses of wind farms subject to various faults. New design strategies are suggested and their corresponding P-Q capability curves are thoroughly studied. Simulations results show that the proposed control scheme can meet the Australian regulations as the most demanding grid code. The best design strategy, with enhanced active and reactive power responses, permits the rotor-side and grid-side converters of DFIG to be temporarily overloaded during the fault period and also exploits the free capacity of the GSC to inject further reactive power to the grid. As a result, the active power generation of DFIG-based wind power plant can be retained during the fault period while its reactive power injection capacity of DFIG is also increased to further support the grid

Controlo das potências activa e reactiva fornecidas à rede eléctrica por conversores CC/CA fontes de tensão

Tese de mestrado. Engenharia Electrónica e de Computadores. 2004. Faculdade de Engenharia. Universidade do Port

Application of SMES Unit to improve the performance of doubly fed induction generator based WECS

Due to the rising demand of energy over several decades, conventional energy resources have been continuously and drastically explored all around the world. As a result, global warming is inevitable due to the massive exhaust of CO2 into the atmosphere from the conventional energy sources. This global issue has become a high concern of industrial countries who are trying to reduce their emission production by increasing the utilization of renewable energies such as wind energy. Wind energy has become very attractive since the revolution of power electronics technology, which can be equipped with wind turbines. Wind energy can be optimally captured with wind turbine converters. However, these converters are very sensitive if connected with the grid as grid disturbances may have a catastrophic impact on the overall performance of the wind turbines.In this thesis, superconducting magnetic energy storage (SMES) is applied on wind energy conversion systems (WECSs) that are equipped with doubly fed induction generators (DFIGs) during the presence of voltage sags and swells in the grid side. Without SMES, certain levels of voltage sags and swells in the grid side may cause a critical operating condition that may require disconnection of WECS to the grid. This condition is mainly determined by the voltage profile at the point of common coupling (PCC), which is set up differently by concerned countries all over the world. This requirement is determined by the transmission system operator (TSO) in conjunction with the concerned government. The determined requirement is known as grid codes or fault ride through (FRT) capability.The selection of a SMES unit in this thesis is based on its advantages over other energy storage technologies. Compared to other energy storage options, the SMES unit is ranked first in terms of highest efficiency, which is 90-99%. The high efficiency of the SMES unit is achieved by its low power loss because electric currents in the coil encounter almost no resistance and there are no moving parts, which means no friction losses. Meanwhile, DFIG is selected because it is the most popular installed WECS over the world. In 2004 about 55% of the total installed WECS worldwide were equipped with DFIG. There are two main strategies that can be applied to meet the grid requirements of a particular TSO. The first strategy is development of new control techniques to fulfil the criterion of the TSOs. This strategy, however, is applicable only to the new WECS that have not been connected to the power grid. If new control techniques are applied to the existing gridconnected WECSs, they will not be cost effective because the obsolete design must be dismantled and re-installed to comply with current grid code requirements. The second strategy is the utilization of flexible AC transmission system (FACTS) devices or storage energy devices to meet the grid code requirements. This strategy seems more appropriate for implementation in the existing WECS-grid connection in order to comply with the current grid code requirements. By appropriate design, the devices might be more cost effective compared to the first strategy, particularly for the large wind farms that are already connected to the grid.A new control algorithm of a SMES unit, which is simple but still involves all the important parameters, is employed in this study. Using the hysteresis current control approach in conjunction with a fuzzy logic controller, the SMES unit successfully and effectively improves the performance of the DFIG during voltage sag and swell events in the grid side; thus, this will prevent the WECS equipped with DFIG from being disconnected from the grid according to the selected fault ride through used in this study. The dynamic study of DFIG with SMES during short load variation is carried out as an additional advantage of SMES application on a DFIG system. In this study, the proposed SMES unit is controlled to compensate the reduced transfer power of DFIG during the short load variation event. Moreover, the SMES unit is also engaged in absorbing/storing some amount of excessive power that might be transferred to the grid when the local loads are suddenly decreased. Finally, the studies of intermittent misfires and fire-through that take place within the converters of DFIG are carried out in order to investigate the impact of these converter faults on the performance of DFIG. In this part, the proposed SMES unit is controlled to effectively improve the DFIG’s performance in order to prevent it from being disconnected or shut down from the power grid during the occurrence of these intermittent switching faults

An investigation of harmonic correction techniques using active filtering

This thesis presents an investigation of techniques used to mitigate the undesirable effects of harmonics in power systems. The first part of this research develops an effective and useful comparison of alternative AC-DC converter topologies. In particular, a full evaluation of the circuit first proposed by Enjeti (known here as the Texas circuit) with a capacitively smoothed output voltage is made, specifically for operation as a 'clean power' supply interface for a variable speed drive (VSD). This mode of operation has not previously been reported in research literature. Simulation and experimental results verify the performance of the circuit and demonstrate that it draws a current with low harmonic content, but the circuit has a number of problems. This part of the research concludes that the six-switch rectifier is the most viable circuit for operation as a supply interface for a VSD due to its bidirectional power flow capability and its excellent versatility of performance. The second part of this research exploits the versatility of the six-switch rectifier and develops the current control strategy for operation of the circuit as a sinusoidal frontend and as a shunt active filter. It is found that the 'traditional' current control method suffers a significant drop in performance when the switching frequency is constrained to 2kHz due to high power levels. The major development in this thesis was an advanced current control strategy, where additional rotating frames of reference are introduced, thereby converting previously oscillatory current values to d.c. values. This is demonstrated to result in vastly improved immunity to disturbances such as supply distortion and a greatly improved steady state performance. In addition, the new controller requires no additional circuitry (apart from current transducers on the load current) and can be applied to an existing sinusoidal front end. Simulation results confirm the operation of the controller with the circuit operating as both a shunt active filter and as a sinusoidal front end. The new controller has been implemented on an experimental rig exhibiting the features of a high power inverter, i.e. low switching frequency and significant device turn-on and turn-off times, and the results confirm the superior performance of the new current controller

Active Control of Voltage Ripples in Power Electronic Converters

Two major challenges, i.e., bulky electrolytic capacitors and isolation transformers, remain as critical obstacles for further improvement on reliability, power density and efficiency of power electronic converters, which are mainly used to reduce low-frequency voltage ripples and high-frequency common-mode voltage ripples, respectively. In order to overcome the two challenges, the most straightforward way is to simply combine existing solutions developed for each of them. However, this would considerably increase system complexity and cost, which should be avoided if possible. In this thesis, these two challenges are innovatively addressed in a holistic way by using active control techniques. This thesis first focuses on the reduction of low-frequency voltage ripples in conventional half-bridge converters, after adding an actively-controlled neutral leg. As a direct application of this strategy, a single-phase to three-phase conversion is then proposed. After that, a ρ-converter with only four switches is proposed to significantly reduce both low-frequency ripples and high-frequency common-mode ripples in a holistic way. It is found that the total capacitance can be reduced by more than 70 times compared to that in conventional full-bridge converters. As a result, there is no longer a need to use bulky electrolytic capacitors and isolation transformers. Then, the ρ-converter equipped with the synchronverter technology is operated as an inverter for PV applications. Another converter is also proposed for the same purpose but with reduced voltage stress. In order to further reduce the total capacitance and to reduce the neutral inductor in the ρ-converter, a new type of converter, called the θ-converter, is proposed. Finally, two actively-controlled ripple eliminators are proposed to reduce low-frequency ripples in general DC systems while the aforementioned research is focused on some specific topologies. Extensive experimental results are presented to validate most of the developed systems while the rest are validated with simulation results

An investigation of harmonic correction techniques using active filtering

This thesis presents an investigation of techniques used to mitigate the undesirable effects of harmonics in power systems. The first part of this research develops an effective and useful comparison of alternative AC-DC converter topologies. In particular, a full evaluation of the circuit first proposed by Enjeti (known here as the Texas circuit) with a capacitively smoothed output voltage is made, specifically for operation as a 'clean power' supply interface for a variable speed drive (VSD). This mode of operation has not previously been reported in research literature. Simulation and experimental results verify the performance of the circuit and demonstrate that it draws a current with low harmonic content, but the circuit has a number of problems. This part of the research concludes that the six-switch rectifier is the most viable circuit for operation as a supply interface for a VSD due to its bidirectional power flow capability and its excellent versatility of performance. The second part of this research exploits the versatility of the six-switch rectifier and develops the current control strategy for operation of the circuit as a sinusoidal frontend and as a shunt active filter. It is found that the 'traditional' current control method suffers a significant drop in performance when the switching frequency is constrained to 2kHz due to high power levels. The major development in this thesis was an advanced current control strategy, where additional rotating frames of reference are introduced, thereby converting previously oscillatory current values to d.c. values. This is demonstrated to result in vastly improved immunity to disturbances such as supply distortion and a greatly improved steady state performance. In addition, the new controller requires no additional circuitry (apart from current transducers on the load current) and can be applied to an existing sinusoidal front end. Simulation results confirm the operation of the controller with the circuit operating as both a shunt active filter and as a sinusoidal front end. The new controller has been implemented on an experimental rig exhibiting the features of a high power inverter, i.e. low switching frequency and significant device turn-on and turn-off times, and the results confirm the superior performance of the new current controller