Emad M. Ahmed

Variable Step Size Maximum Power Point Tracker Using a Single Variable for Stand-alone Battery Storage PV Systems

The subject of variable step size maximum power point tracking (MPPT) algorithms has been addressed in the literature. However, most of the addressed algorithms tune the variable step size according to two variables: the photovoltaic (PV) array voltage (VPV) and the PV array current (I PV ). Therefore, both the PV array current and voltage have to be measured. Recently, maximum power point trackers that are based on a single variable (I PV or V PV ) have received a great deal of attention due to their simplicity and ease of implementation, when compared to other tracking techniques. In this paper, two methods have been proposed to design a variable step size MPPT algorithm using only a single current sensor for stand-alone battery storage PV systems. These methods utilize only the relationship between the PV array measured current and the converter duty cycle (D) to automatically adapt the step change in the duty cycle to reach the maximum power point (MPP) of the PV array. Detailed analyses and flowcharts of the proposed methods are included. Moreover, a comparison has been made between the proposed methods to investigate their performance in the transient and steady states. Finally, experimental results with field programmable gate arrays (FPGAs) are presented to verify the performance of the proposed methods.

New Three-Phase Symmetrical Multilevel Voltage Source Inverter

This paper presents a new design and implementation of a three-phase multilevel inverter (MLI) for distributed power generation system using low frequency modulation and sinusoidal pulse width modulation (SPWM) as well. It is a modular type and it can be extended for extra number of output voltage levels by adding additional modular stages. The impact of the proposed topology is its proficiency to maximize the number of voltage levels using a reduced number of isolated dc voltage sources and electronic switches. Moreover, this paper proposes a significant factor (FC/L), which is developed to define the number of the required components per pole voltage level. A detailed comparison based on FC/L is provided in order to categorize the different topologies of the MLIs addressed in the literature. In addition, a prototype has been developed and tested for various modulation indexes to verify the control technique and performance of the topology. Experimental results show a well-matching and good similarity with the simulation results.

Scaling Factor Design Based Variable Step Size Incremental Resistance Maximum Power Point Tracking for PV Systems

Variable step size maximum power point trackers (MPPTs) are widely used in photovoltaic (PV) systems to extract the peak array power which depends on solar irradiation and array temperature. One essential factor which judges system dynamics and steady state performances is the scaling factor (N), which is used to update the controlling equation in the tracking algorithm to determine a new duty cycle. This paper proposes a novel stability study of variable step size incremental resistance maximum power point tracking (INR MPPT). The main contribution of this analysis appears when developing the overall small signal model of the PV system. Therefore, by using linear control theory, the boundary value of the scaling factor can be determined. The theoretical analysis and the design principle of the proposed stability analysis have been validated using MATLAB simulations, and experimentally using a fixed point digital signal processor (TMS320F2808).

Novel three-phase multilevel voltage source inverter with reduced no. of switches

Recently, great attention has been addressed for multilevel inverters, as they exhibit low total harmonic distortion in the output voltage and low electromagnetic interference. In this paper a new configuration of three-phase multilevel voltage source inverter is presented. This structure consists of series-connected sub-multilevel inverters blocks. The number of utilized switches, insulated gate driver circuits, voltage standing on switches, installation area and cost are considerably reduced compared to the conventional topologies. With the selected inverter DC sources, high-frequency pulse-width modulation control methods can be effectively applied without loss of modularity. Low-frequency and sinusoidal PWM techniques were successfully applied. Hence, high flexibility in the modulation of the proposed inverter is demonstrated.

MPPT schemes for single-stage three-phase grid-connected photovoltaic voltage-source inverters

Single-stage grid-connected PV systems provide many advantages such as simple topology, high efficiency, high power density, and lower cost. However, achieving MPPT, while conditioning the output power and synchronizing with the power grid, is a big challenge in such systems. In this paper two MPPT schemes are investigated for single-stage three-phase grid-connected photovoltaic voltage-source inverters. MPP is tracked using either the load angle or the inverter modulation index.

Study and Analysis of New Three-Phase Modular Multilevel Inverter

This paper proposes a new three-phase modular multilevel inverter (MMLI). The proposed inverter consists of repeated modular primary cells that are connected in series configuration. Therefore, the proposed topology can be extended to get more output voltage levels by adding additional cells without increasing voltage stresses across power switches. Both sinusoidal pulsewidth modulation (SPWM) and staircase modulation are successfully applied to the proposed inverter. The proposed inverter features many advantages such as reduced number of semiconductor power switches, driver circuits, dc-voltage sources, simplified control algorithm, and reduced voltage stresses across the switches independent of output voltage level. Furthermore, the performance of the proposed inverter is studied under both open-loop and closed-loop operations for highly inductive load. Beside the obtained simulation results, a laboratory prototype has been carried out and tested to verify the control techniques and performances of the topology. Simulation and experimental results are compared together to highlight the similarity and consistence of them.

Thermal and Reliability Assessment for Wind Energy Systems With DSTATCOM Functionality in Resilient Microgrids

Different functionalities can be incorporated into wind energy conversion systems (WECSs) in resilient microgrids in order to reduce the total system cost and to assist their self-healing capability. Meanwhile, wind industry-based reliability surveys have addressed that power electronics components are the most vulnerable parts in WECS. Therefore, thermal stresses and lifetime consumption of WECS are critical factors for evaluating the added functionalities and for developing new control strategies as well. Almost all of the previous methodologies in the literature tackle the problem of lifetime assessment of WECS using only the behavior of wind generation profiles and reactive power injection, which is limited by grid codes. Subsequently, these approaches cannot efficiently achieve lifetime assessment in case of resilient microgrid operation and different load demands. This paper proposes a more convenient approach for thermal behavior, and lifetime assessment for WECSs that considers the influence of the added DSTATCOM functionality, and various modes of resilient microgrids operation. Moreover, the proposed approach utilizes a joint probability distribution function (JPDF) that combines both of the collected field data of wind generation and load demand levels as well. The feasibility of the proposed approach has been verified analytically and compared to the previously addressed approaches. It can be concluded that thermal behavior and reliability assessment of WECS are highly impacted by the added DSTATCOM functionality.

Thermal Stresses Relief Carrier-Based PWM Strategy for Single-Phase Multilevel Inverters

Enhancing power cycling capability of power semiconductor devices is highly demanded in order to increase the long-term reliability of multilevel inverters. Ageing of power switches and their cooling systems leads to their accelerated damage due to excess power losses and junction temperatures. Therefore, thermal stresses relief (TSR) is the most effective solution for lifetime extension of power semiconductor devices. This paper presents a new TSR carrier-based pulse width modulation (TSRPWM) strategy for extending the lifetime of semiconductor switches in single-phase multilevel inverters. The proposed strategy benefits the inherent redundancy among switching states in multilevel inverters to optimally relieve the thermally stressed device. The proposed algorithm maintains the inverter operation without increased stresses on healthy switches and without reduction of the output power ratings. In addition, the proposed algorithm preserves voltage balance of the dc-link capacitors. The proposed strategy is validated on a single-phase five-level T-type inverter system with considering different locations of thermal stresses detection. Experimental prototype of the selected case study is built to verify the results. Moreover, comparisons with the most featured strategies in literature are given in detail.

Distributed cooperative control with lower generation cost for DC microgrid

This paper presents a distributed cooperative control that ensures reliable operation of DC Microgrid with lower generation cost without centralized controller. Centralized hierarchical multilevel controller is the commonly used controller in Microgrids, however it requires heavy communication network and present single point of failure. Therefore, the proposed controller is a distributed cooperative control that provide simple communication network infrastructure. The controller of each source exchanges data with only its neighbors on the communication network. The distributed cooperative controller that compare the generation cost of each source with a weighted average of its neighbors in the network. Accordingly, it generates a voltage correction term in order to adjust the local voltage set point and to provide lower generation cost in the microgrid. The performance of the proposed controller has been investigated and tested using MATLAB simulation program. Moreover, the efficacy of the proposed controller to changes in operational conditions and its resiliency against the loss of converters is also investigated.

Grid-connected single-phase multi-level inverter

Recently, great attention has been addressed for multilevel inverters, as they exhibit low total harmonic distortion (THD) in the output voltage and low electromagnetic interference (EMI). This paper proposes a single-phase five-level pulse width modulation (PWM) inverter for grid connection. The proposed PWM technique has some switches operated at fundamental line frequency and the others operate at inverter switching frequency. The proposed inverter has advantages of less number of components and low THD compared with the conventional multilevel inverter. The inverter operational principles and the switching functions are analyzed in this paper. Moreover, a proportional integral (PI) current control algorithm is designed and implemented in DSPACE DS1103 to keep the current injected into the grid sinusoidal and in-phase with the grid voltage for unity power factor. Simulation and experimental results prove the powerful merits of the proposed multi-level inverter and the capabilities of the proposed PWM technique.