A New Fast Peak Current Controller for Transient Voltage Faults for Power Converters

Power converters are the basic unit for the transient voltage fault ride through capability for most renewable distributed generators (DGs). When a transient fault happens, the grid voltage will drop suddenly and probably will also suffer a phase-jump event as well. State-of-the-art voltage fault control techniques regulate the current injected during the grid fault. However, the beginning of the fault could be too fast for the inner current control loops of the inverter, and transient over-current would be expected. In order to avoid the excessive peak current of the methods presented in the literature, a new fast peak current control (FPCC) technique is proposed. Controlling the peak current magnitude avoids undesirable disconnection of the distributed generator in a fault state and improves the life expectancy of the converter. Experimental and simulation tests with high power converters provide the detailed behaviour of the method with excellent results

A Virtual Space Vectors based Model Predictive Control for Three-Level Converters

Three-phase three-level (3-L) voltage source converters (VSC), e.g., neutral-point clamped (NPC) converters, T-type converters, etc., have been deemed to be suitable for a wide range of medium- to high-power applications in microgrids (MGs) and bulk power systems. Compared to their two-level (2-L) counterparts, adopting 3-L VSCs in the MG applications not only reduces the voltage stress across the power semiconductor devices, which allows achieving higher voltage levels, but also improves the quality of the converter output waveforms, which further leads to considerably smaller output ac passive filters. Various control strategies have been proposed and implemented for 3-L VSCs. Among all the existing control methods, finite-control-set model predictive control (FCS-MPC) has been extensively investigated and applied due to its simple and intuitive design, fast-dynamic response and robustness against parameter uncertainties. However, to implement an FCS-MPC on a 3-L VSC, a multi-objective cost function, which consists of a term dedicated specifically to control the dc-link capacitor voltages such that the neutral-point voltage (NP-V) oscillations are minimized, must be designed. Nevertheless, selecting proper weighting factors for the multiple control objectives is difficult and time consuming. Additionally, adding the dc-link capacitor voltages balancing term to the cost function distributes the controller effort among different control targets, which severely impacts the primary goal of the FCS-MPC. Furthermore, to control the dc-link capacitor voltages, additional sensing circuitries are usually necessary to measure the dc-link capacitor voltages and currents, which consequently increases the system cost, volume and wiring complexity as well as reduces overall reliability. To address all the aforementioned challenges, in this dissertation research, a novel FCS-MPC method using virtual space vectors (VSVs), which do not affect the dc-link capacitor voltages of the 3-L VSCs, was proposed, implemented and validated. The proposed FCS-MPC strategy has the capability to achieve inherent balanced dc-link capacitor voltages. Additionally, the demonstrated control technique not only simplifies the controller design by allowing the use of a simplified cost function, but also improves the quality of the 3-L VSC output waveforms. Furthermore, the execution time of the proposed control algorithm was significantly reduced compared to that of the existing one. Lastly, the proposed FCS-MPC using the VSVs reduces the hardware cost and complexity as the additional dc-link capacitor voltages and current sensors are not required, which further enhances the overall system reliability

Control Based Soft Switching Three-phase Micro-inverter: Efficiency And Power Density Optimization

In the field of renewable energy, solar photovoltaic is growing exponentially. Grid-tied PV micro-inverters have become the trend for future PV system development because of their remarkable advantages such as enhanced energy production due to MPPT implementation for each PV panel, high reliability due to redundant and distributed system architecture, and simple design, installation, and management due to its plug-and-play feature. Conventional approaches for the PV micro-inverters are mainly in the form of single-phase grid connected and they aim at the residential and commercial rooftop applications. It would be advantageous to extend the micro-inverter concept to large size PV installations such as MW-class solar farms where threephase AC connections are used. The relatively high cost of the three-phase micro-inverter is the biggest barrier to its large scale deployment. Increasing the switching frequency may be the best way to reduce cost by shrinking the size of reactive components and heat-sink. However, this approach could cause conversion efficiency to drop dramatically without employing soft switching techniques or using costly new devices. This dissertation presents a new zero voltage switching control method that is suitable for low power applications such as three-phase micro-inverters. The proposed hybrid boundary conduction mode (BCM) current control method increases the efficiency and power density of the micro-inverters and features both reduced number of components and easy digital implementation. Zero voltage switching is achieved by controlling the inductor current bidirectional in every switching cycle and results in lower switching losses, higher operating frequency, and reduced size and cost of passive components, especially magnetic cores. Some iv practical aspects of hybrid control implementation such as dead-time insertion can degrade the performance of the micro-inverter. A dead-time compensation method that improves the performance of hybrid BCM current control by decreasing the output current THD and reducing the zero crossing distortion is presented. Different BCM ZVS current control modulation schemes are compared based on power losses breakdown, switching frequency range, and current quality. Compared to continuous conduction mode (CCM) current control, BCM ZVS control decreases MOSFET switching losses and filter inductor conduction losses but increases MOSFET conduction losses and inductor core losses. Based on the loss analysis, a dual-mode current modulation method combining ZVS and zero current switching (ZCS) schemes is proposed to improve the efficiency of the micro-inverter. Finally, a method of maintaining high power conversion efficiency across the entire load range of the three-phase micro-inverter is proposed. The proposed control method substantially increases the conversion efficiency at light loads by minimizing switching losses of semiconductor devices as well as core losses of magnetic components. This is accomplished by entering a phase skipping operating mode wherein two phases of an inverter are disabled and three inverters are combined to form a new three-phase system with minimal grid imbalance. A 400W prototype of a three-phase micro-inverter and its hybrid control system have been designed and tested under different conditions to verify the effectiveness of the proposed controller, current modulation scheme, and light load efficiency enhancement method

Model Predictive Control Technique of Multilevel Inverter for PV Applications

Renewable energy sources, such as solar, wind, hydro, and biofuels, continue to gain popularity as alternatives to the conventional generation system. The main unit in the renewable energy system is the power conditioning system (PCS). It is highly desirable to obtain higher efficiency, lower component cost, and high reliability for the PCS to decrease the levelized cost of energy. This suggests a need for new inverter configurations and controls optimization, which can achieve the aforementioned needs. To achieve these goals, this dissertation presents a modified multilevel inverter topology for grid-tied photovoltaic (PV) system to achieve a lower cost and higher efficiency comparing with the existing system. In addition, this dissertation will also focus on model predictive control (MPC) which controls the modified multilevel topology to regulate the injected power to the grid. A major requirement for the PCS is harvesting the maximum power from the PV. By incorporating MPC, the performance of the maximum power point tracking (MPPT) algorithm to accurately extract the maximum power is improved for multilevel DC-DC converter. Finally, this control technique is developed for the quasi-z-source inverter (qZSI) to accurately control the DC link voltage, input current, and produce a high quality grid injected current waveform compared with the conventional techniques. This dissertation presents a modified symmetrical and asymmetrical multilevel DC-link inverter (MLDCLI) topology with less power switches and gate drivers. In addition, the MPC technique is used to drive the modified and grid connected MLDCLI. The performance of the proposed topology with finite control set model predictive control (FCS-MPC) is verified by simulation and experimentally. Moreover, this dissertation introduces predictive control to achieve maximum power point for grid-tied PV system to quicken the response by predicting the error before the switching signal is applied to the converter. Using the modified technique ensures the iii system operates at maximum power point which is more economical. Thus, the proposed MPPT technique can extract more energy compared to the conventional MPPT techniques from the same amount of installed solar panel. In further detail, this dissertation proposes the FCS-MPC technique for the qZSI in PV system. In order to further improve the performance of the system, FCS-MPC with one step horizon prediction has been implemented and compared with the classical PI controller. The presented work shows the proposed control techniques outperform the ones of the conventional linear controllers for the same application. Finally, a new method of the parallel processing is presented to reduce the time processing for the MPC

Source Grid Interface of Wind Energy Systems

Wind power is one of the most developed and rapidly growing renewable energy sources. Through extensive literature review this thesis synthesizes the existing knowledge of wind energy systems to offer useful information to developers of such systems. Any prototyping should be preceded by theoretical analysis and computer simulations, foundations for which are provided here. The thesis is devoted to an in-depth analysis of wind energy generators, system configurations, power converters, control schemes and dynamic and steady state performance of practical wind energy conversion systems (WECS). Attention is mainly focused on interfacing squirrel cage Induction generators (SCIG) and doubly-fed induction generators (DFIG) with the power network to capture optimal power, provide controllable active and reactive power and minimize network harmonics using the two-level converter, as a power electronic converter. Control of active and reactive power, frequency and voltage are indispensable for stability of the grid. This thesis focuses on two main control techniques, field oriented control (FOC) and direct torque control (DTC) for the SCIG. The dynamic model of induction generator is non-linear and hence for all types of control, the flux and the torque have to be decoupled for maintaining linearity between input and output for achieving high dynamic performance. FOC is used for decoupled control for rotor flux and electromagnetic torque . The stator current is decomposed into flux and torque producing components and they both are controlled independently. FOC uses three feedback control loops generate gating signals for the converter. DTC also achieves high dynamic performance by decoupling of rotor flux and electromagnetic torque without the intermediate current loops. DTC asks for the estimation of stator flux and torque and like FOC has 2 branches which have flux and torque comparators. The errors between the set and the estimated value are used to drive the inverters. The two methods are valid for both steady and transient state. Their validity is confirmed by simulating the systems on MATLAB/Simulink platform and comparing them the results obtained by hand calculations. Further DFIG’s are introduced. The dynamic model is developed using the machines equivalent circuit and is expressed in the stationary, rotor and the synchronous reference frames for evaluating the performance of the machine. The stator of the DFIG is directly interfaced to the grid and by controlling the rotor voltage by a two level back-to-back converter the grid synchronization and power control is maintained. The DTC and the direct power control (DPC) methods are used to control the rotor side (RSC) and the grid side converter (GSC). The RSC generates the 3-ph voltages of variable frequency in order to control the generator torque and the reactive power exchanged between the stator and the grid. The GSC exchanges active power with the grid injected by the RSC with a constant frequency. The steady and transient behavior of the machine is investigated through simulations

Multilevel Converters: An Enabling Technology for High-Power Applications

| Multilevel converters are considered today as the state-of-the-art power-conversion systems for high-power and power-quality demanding applications. This paper presents a tutorial on this technology, covering the operating principle and the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in-depth and self-contained information, while recent advances and state-of-the-art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not-familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.Ministerio de Ciencia y Tecnología DPI2001-3089Ministerio de Eduación y Ciencia d TEC2006-0386

Power Converters in Power Electronics

In recent years, power converters have played an important role in power electronics technology for different applications, such as renewable energy systems, electric vehicles, pulsed power generation, and biomedical sciences. Power converters, in the realm of power electronics, are becoming essential for generating electrical power energy in various ways. This Special Issue focuses on the development of novel power converter topologies in power electronics. The topics of interest include, but are not limited to: Z-source converters; multilevel power converter topologies; switched-capacitor-based power converters; power converters for battery management systems; power converters in wireless power transfer techniques; the reliability of power conversion systems; and modulation techniques for advanced power converters