Shehab Ahmed

High-Performance Adaptive Perturb and Observe MPPT Technique for Photovoltaic-Based Microgrids

Solar photovoltaic (PV) energy has witnessed double-digit growth in the past decade. The penetration of PV systems as distributed generators in low-voltage grids has also seen significant attention. In addition, the need for higher overall grid efficiency and reliability has boosted the interest in the microgrid concept. High-efficiency PV-based microgrids require maximum power point tracking (MPPT) controllers to maximize the harvested energy due to the nonlinearity in PV module characteristics. Perturb and observe (P&O) techniques, although thoroughly investigated in previous research, still suffer from several disadvantages, such as sustained oscillation around the MPP, fast tracking versus oscillation tradeoffs, and user predefined constants. In this paper, a modified P&O MPPT technique, applicable for PV systems, is presented. The proposed technique achieves: first, adaptive tracking; second, no steady-state oscillations around the MPP; and lastly, no need for predefined system-dependent constants, hence provides a generic design core. A design example is presented by experimental implementation of the proposed technique. Practical results for the implemented setup at different irradiance levels are illustrated to validate the proposed technique.

Detection of Rotor Slot and Other Eccentricity-Related Harmonics in a Three-Phase Induction Motor with Different Rotor Cages

Detection of rotor slot and other eccentricity-related harmonics in the line current of a three-phase induction motor is important both from the viewpoint of sensorless speed estimation as well as eccentricity-related fault detection. It is now clear that not all three-phase induction motors are capable of generating such harmonics in the line current, however. Recent research has shown that the presence of these harmonics is primarily dependent on the number of rotor slots and the number of fundamental pole pairs of the machine. While the number of fundamental pole pairs of a three-phase induction motor usually is within one to four (higher pole pairs are generally avoided due to increased magnetizing current), the number of rotor slots can vary widely. The present paper investigates this phenomenon further and obtains a hitherto nebulous theoretical basis for the experimentally verified results. Detailed coupled magnetic circuit simulation results are presented for a four-pole, three-phase induction motor with 44, 43, and 42 rotor slots under healthy, static, dynamic, and mixed eccentricity conditions. The simulation is flexible enough to accommodate other pole numbers also. These simulations are helpful in quantifying the predicted harmonics under different combinations of load, pole pair numbers, rotor slots, and eccentricity conditions, thus making the problem easier for drive designers or diagnostic tools developers. Data from three different induction machines-namely, a four-pole, 44-bar, 3H; a four-pole, 28-bar, 3HP; and a two-pole, 39-bar, 100 HP motor-have been used to verify the results experimentally.

Quasi Two-Level Operation of Modular Multilevel Converter for Use in a High-Power DC Transformer With DC Fault Isolation Capability

DC fault protection is one challenge impeding the development of multiterminal dc grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and dc fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode dc component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a dc to dc transformer, where two converters operating in the proposed mode are coupled by an ac transformer for voltage matching and galvanic isolation. The proposed dc transformer is shown to be suitable for high-voltage high-power applications due to the low-switching frequency, high efficiency, modularity, and reliability. The dc transformer facilitates dc voltage regulation and near instant isolation of dc faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.

Multiple-Module High-Gain High-Voltage DC–DC Transformers for Offshore Wind Energy Systems

Renewable energy sources, such as offshore wind farms, require high voltage gains in order to interface with power transmission networks. These conversions are normally made using bulky, complex, and costly transformers and high-voltage ac-dc converters with unnecessary bidirectional power flow capability. Multiple modules of single-switch single-inductor dc-dc converters can reach high gains without transformers in these applications due to low semiconductor conduction loss in high-power devices. This paper describes a new approach for high-gain high-voltage dc-dc converters using multiple modules of single-switch single-inductor transformerless converters. Results for low-voltage experimental prototypes show gains of up to 29 p.u. and demonstrate the potential of the approach as high-gain dc-dc converters for offshore wind farms. This paper then demonstrates the viability of multiple-module converters compared to a conventional high-voltage dc converter and a theoretical full-bridge converter due to fewer devices and valves, comparable isolation levels, and ease of interleaving for increased reliability.

Selection criteria of induction machines for speed-sensorless drive applications

Induction motors, both three and single phase, are used extensively for adjustable speed drive applications. These machines are structurally very robust and are primary source of motive power and speed control where DC machines cannot be used. For closed loop control of these machines, sensorless speed estimation is usually preferred. Among the current estimation techniques available for speed-sensorless induction motor drives, speed measurement based on rotor slot related harmonic detection in machine line current happens to be a prominent one. While these harmonics can be strong in certain kinds of machine, some other machines may exhibit very weak rotor slot harmonics that can be obscured by noise. Skewing, slot shapes and types, structural unbalances etc. also have a prominent effect on the detectability of these harmonics. The present paper attempts to investigate this problem based on the interaction of pole pairs, number of rotor bars and stator winding. Though the analysis and experimental results have been mainly provided for three phase squirrel cage induction motors, single phase and slip ring induction motors have also been addressed. Further, it has been shown that eccentricity related fault detection could also be easily accommodated with this kind of speed detection technique at no or negligible extra cost when certain motors are selected.

Evaluation of a Multilevel Cascaded-Type Dynamic Voltage Restorer Employing Discontinuous Space Vector Modulation

In this paper, the application of a cascaded multilevel inverter as a dynamic voltage restorer (DVR) is investigated. Two discontinuous multilevel space vector modulation (SVM) techniques are implemented for DVR control and are shown to reduce inverter switching losses while maintaining virtually the same harmonic performance as the conventional multilevel SVM at a high number of levels. This paper also presents a mathematical relationship for computing the distortion at the point of common coupling (PCC) as a function of the distortion of the DVR. This enables the selection of the number of levels required for a certain application. An extended sag duration support compared to the two-level DVR is another advantage of the DVR with a cascaded multilevel inverter. The common-mode voltage (CMV) at the PCC has been evaluated for the three SVM techniques (the conventional multilevel SVM and the two discontinuous SVM), presenting a lower CMV for the second discontinuous SVM. A design example is presented for an 11-kV 5-MVA DVR multilevel cascaded inverter for up to 17 levels, employing the conventional multilevel SVM and the two discontinuous SVM techniques.

Model construction of single crystalline photovoltaic panels for real-time simulation

Real-time simulation and fast prototyping with power electronics, critical loads, and control systems have prompted recent interest in accurate electrical terminal models of photovoltaic (PV) panels and array systems. Advancement in computing technologies have allowed the prototyping of novel apparatus to be investigated in a virtual system under wide range of realistic conditions repeatedly, safely, and economically. This paper accesses numerical iteration methods, selects appropriate techniques, and combines them with model construction methods well suited for boosting the computation speed of an electrothermal dynamic model of a PV panel. Significant improvements resulting from the proposed modeling approach in computation time and numerical convergence speed are verified using experimental results published for the target PV panel and Opal RTs RT-Lab Matlab/Simulink based real-time engineering simulator.

Effect of Stator Winding Connection on Performance of Five-Phase Induction Machines

This paper studies the effect of stator winding configuration on the performance of five-phase induction machines under healthy as well as faulty conditions. The study compares two connections, namely, star and pentagon connections. The comparison is conducted using both simulation and experimental results. The steady-state model based on symmetrical components theory is introduced for both connections with one-line open due to a converter fault, and the corresponding machine characteristic curves are estimated. During faults, two alternatives for machine operation are possible, namely, open-loop control and optimal current control. While the first alternative corresponds to higher torque ripple and unbalanced winding currents, the second option necessitates unbalanced phase voltages and typically an increased dc-link voltage to source the required optimal currents. Consequently, an increase in the employed semiconductor device rating is required, which is a critical design factor particularly in medium-voltage applications. A new V/f control technique is proposed to ensure disturbance-free operation with one-line open for both winding connections. Based on the unbalanced machine model and experimental verification, the derating factors that ensure safe machine operation for both winding connection alternatives are calculated. The comparison between the two connections shows the superiority of the pentagon connection under fault conditions in terms of efficiency, average torque, torque ripples, and derating factor.

Multilevel Medium-Frequency Link Inverter for Utility Scale Photovoltaic Integration

A multilevel topology with medium-frequency ac link for medium-voltage grid integration of utility photovoltaic (PV) plants is discussed in this paper. A megawatt-scale PV plant is divided into many zones, each comprising of two series-connected arrays. Each zone employs a medium-frequency transformer with three secondaries, which interface with the three phases of the medium voltage grid. An insulated-gate bipolar transistor full-bridge inverter feeds the MF transformer. The voltages at the transformer secondaries are then converted to three-phase line frequency ac by three full-bridge ac-ac converters. Second line frequency harmonic power does not appear in the dc bus, thereby reducing the dc capacitor size. Cascading several such cells, a high-quality multilevel medium-voltage output is generated. A new control method is proposed for the cascaded multilevel converter during partial shading while minimizing the switch ratings. The proposed topology eliminates the need for line frequency transformer isolation and reduces the dc bus capacitor size, while improving the power factor and energy yield. This paper presents the analysis, design example, and operation of a 10-MW utility PV system with experimental results on a scaled-down laboratory prototype.

Optimum Power Transmission-Based Droop Control Design for Multi-Terminal HVDC of Offshore Wind Farms

Power generation through wind is expected to play a major role in the worlds future energy portfolio. Nevertheless, wind power integration remains a challenging research area due to the special characteristics of wind power generation. Specifically, offshore wind has received significant attention worldwide due to the vast generation potential available. The electrical infrastructure of offshore wind farms is thus of significant importance. The multi-terminal HVDC solution represents a preferable solution and has shown promise in solving wind farm interconnection problems. Droop control techniques have been proposed as a means to regulate the DC voltage and facilitate the automatic coordination between different converters without the need for fast communication between units. Different methodologies have been developed to select the droop gains to satisfy the system performance specifications. In this work, a control design methodology is proposed for power sharing among the multi-terminal HVDC feeders providing that the power transmission efficiency is optimized. A simulation study on a 400-kV/1000-MW four-terminal HVDC transmission topology is conducted to ensure the validity of the proposed methodology.