Mohamed Elshaer

Bi-directional AC-DC/DC-AC converter for power sharing of hybrid AC/DC systems

In this paper, some of the aspects related to the connectivity of DC microgrids to the main grid are investigated. A prototype system has been designed and implemented to address these aspects. The described system is dependent mainly on sustainable energy sources. Hence, a special care has been given to dealing with this kind of sources while designing different components of the system. Certain features had to be maintained in the system in order to assure efficient integration of different sources such as, efficient and reliable load-feeding capability and full controllability of voltage and power flow among various buses in the system. Two different converters have been investigated; firstly, a fully controlled rectifier has been designed to tie the DC grid with the AC one. A vector decoupling controlled sinusoidal pulse width modulation (SPWM) technique has been used to allow the designed rectifier to maintain a constant output voltage while being able to control the active and reactive power drawn from the grid independently. Hence, this controlled rectifier acts as a voltage regulator for the DC microgrid and has a uni-directional power flow capability from the AC grid to the DC microgrid. Moreover, in order to allow bi-directional power flow, a bi-directional AC-DC/DC-AC converter has also been designed. The Bi-directional AC-DC/DC-AC converter controls the active power transferred from the DC grid to the AC grid while operating at unity power factor. In addition, it controls the active power transferred from the AC grid to the DC grid while operating at unity power factor. Both simulation and experimental results verify the validity of the proposed system.

Smart optimal control of DC-DC boost converter in PV systems

Proportional integral derivative (PID) controllers are usually used to control DC-DC boost converters in PV systems. However, they have to be tuned based on certain defined operating range using averaged mathematical models. Loading conditions have great effect on PI controllers; PI controllers are subjected to failure under dramatic load changes. This limits the PI controllers operating range. Moreover, transient and steady state response both get affected by changing the operating range. This paper presents a novel smart-PID controller for optimal control of DC-DC boost converter used as voltage controller in PV systems. This proposed controller maximizes the stable operating range by using genetic algorithms (GA) to tune the PID parameters ultimately at various loading conditions. Then, a fuzzy logic approach is used to add a factor of intelligence to the controller such that it can move among different values of proportional gain (Kp), derivative gain (Kd) and integral gain (Ki) based on the system conditions. This controller allows optimal control of boost converter at any loading condition with no need to retune parameters or possibility of failure. Moreover, the paper presents a novel technique to move between the PI and PID configurations of the controller such that the minimum overshoot and ripple are obtained, which makes the controller very applicable for PV systems supplying sensitive loads. The controlled boost converter is used as an interface between photovoltaic (PV) panels and the loads connected to them. It converts any input voltage within its operating range into a constant output voltage that is suitable for load feeding. The proposed smart controller adapts the duty cycle of the boost converter based on input voltage and loading conditions such that it outputs a constant output voltage. A prototype system has been developed to verify the applicability of the proposed controller. Moreover, simulation and experimental results both confirm its validity as an effective and reliable controller for boost converters in PV systems and the possibility to use it in different applications.

DC bus voltage control for PV sources in a DC distribution system infrastructure

This paper proposes a design of a controlled voltage bus for a PV source to be used in a hybrid DC distribution system infrastructure. Load centers, boost converter, and distribution panels combine to link the solar collectors with multiple loads and Backup battery systems add to the complexity of a PV installation. The controlled voltage bus is constructed based on the design of a DC-DC boost converter. A Traditional DC-DC boost converter is limited to get a constant DC voltage as an input and boosting it to a certain voltage level as long as the load is fixed. The boost converter described in this paper is capable of receiving variable DC voltage as an input and give a constant boosted DC output voltage. The significant advantage of this device is that it is capable of producing a fixed voltage, with a very small ripple that can be neglected, while operating with variable load. The parameters of the DC-DC boost circuit were calculated such that it gives the device the ability of controlling a wide range of input voltage and be operated at a wide range of load variations. Closed-loop control of the boost converter utilizes a conventional proportional integral (PI) controller. Since the system has two variables which are the input voltage and the load, the duty ratio is not allowed to be constant to get a fixed output voltage. The PI controller is used in a closed loop system to obtain a suitable duty cycle to keep the output voltage constant according to the reference level of the DC-bus. Simulation and experimental results for different disturbance conditions show good performance of this proposed control system. The results verify the validity of the proposed model and show its practical use in renewable energy applications.

Grid connected DC distribution system for efficient integration of sustainable energy sources

In this paper, some of the aspects related to the design and implementation of grid connected DC microgrids are investigated. A prototype system has been designed and implemented to address these aspects. The described system is dependent mainly on sustainable energy sources. Hence, a special care has been given to dealing with these kinds of sources while designing different components of the system. Certain features had to be maintained in the system in order to assure efficient integration of different sources, efficient and reliable load-feeding capability and full controllability of voltage and power flow among various buses in the system. Moreover, a fully controlled rectifier has been designed to tie the DC grid with the AC one. A vector decoupling controlled sinusoidal pulse width modulation (SPWM) technique has been used to allow the designed rectifier to maintain a constant output voltage while being able to control the active and reactive power drawn from the grid independently. Both simulation and experimental results verify the validity of the proposed system.

Integration of sustainable energy sources into DC zonal electric distribution systems

In this paper, converter topologies, and their control, that can be used as an interface between fuel cells and the DC bus in a DC zonal electric distribution system (DC ZEDS) have been investigated. Performance of the conventional DC-DC boost converter has been discussed. Moreover, a modification has been applied to it in order to enhance its performance. The proposed converters performance has been compared to that of a conventional boost converter. The proposed topology gives a continuous output current and some other advantages over the conventional one. However, it is more complex. A comparative study of both techniques presented in this paper have been conducted from cost as well as performance point of view. A prototype system has been designed and simulated in MATLAB/SIMULINK to validate the proposed techniques. Moreover, they have been examined experimentally to verify the results and conclusions deduced out of the study. The constraints that have been taken into consideration the most while designing the prototype system are performance, weight and cost, respectively. Both simulation and experimental results show the effectiveness of the proposed technique and its validity as a DC-DC converter for fuel cells integration to DC ZEDS.

Reactive power compensation control for stand-alone synchronous generator-based wind energy conversion system

This paper presents a controlled reactive power compensator (RPC) for a stand-alone synchronous generator (SG)-based wind energy conversion system (WECS). The proposed controlled RPC consists of a synchronous condenser (SC), an AC/DC converter whose output supplies the excitation circuit of the SC, and a control scheme that adapts the converter output to supply the field voltage required to fit the power factor to its desired value. A lab-scale system consisting of a WECS, an induction motor acting as lagging power factor load connected to the WECS in addition to the proposed controlled power factor corrector (PFC) was setup in hardware to examine its response using the DSpace 1104. The obtained results justify the validity and applicability of the proposed control scheme as a reliable tool to control the operation of an RPC.

High-quality integration of fuel cells energy into electric grids

This paper investigates different converter topologies, and their control, that can be used as an interface between fuel cells and the DC bus in a power system. As an example, the integration of fuel cells energy to the DC bus in a DC zonal electric distribution system (DC ZEDS) will be focused on. The DC ZEDS example is reasonable as high-quality power supply is interesting the most for the US navy. Among the two topologies presented, one is new and its performance is compared to that of the conventional boost converter. The proposed topology utilizes a modified boost converter to enhance the integration of sustainable energy sources into the system. It gives continuous output current and some other advantages over the conventional one. However, it is more complex. A comparative study of the techniques presented in this paper will be conducted from performance as well as cost points of view. A prototype system has been designed and simulated in MATLAB/SIMULINK to validate the proposed technique. Moreover, it has been examined experimentally to verify the results and conclusions deduced out of the study. Both simulation and experimental results show the effectiveness of the proposed technique and its validity as a DC-DC converter for fuel cells integration to DC ZEDS and its outperformance over conventional DC-DC boost converters.

Smart optimal control of DC-DC boost converter for intelligent PV systems

This paper presents a novel smart-PID controller for optimal control of DC-DC boost converter used as voltage controller in PV systems. This proposed controller maximizes the stable operating range by using genetic algorithms (GA) to tune the PID parameters ultimately at various loading conditions. Then, a fuzzy logic approach is used to add a factor of intelligence to the controller such that it can move among different values of proportional gain, derivative gain and integral gain based on the system conditions. This controller allows optimal control of boost converter at any loading condition with no need to retune parameters or possibility of failure. Moreover, the paper presents a novel technique to move between the PI and PID configurations of the controller such that the minimum overshoot and ripple are obtained, which increases the controller applicability for utilization of PV systems in supplying sensitive loads. The controlled boost converter is used as an interface between photovoltaic (PV) panels and the loads connected to them. It converts any input voltage within its operating range into a constant output voltage that is suitable for load feeding. The proposed smart controller adapts the duty cycle of the boost converter based on input voltage and loading conditions such that it outputs a constant output voltage. A prototype system has been developed in the laboratory to verify the applicability of the proposed controller. Moreover, simulation and experimental results both confirm its validity of the proposed controller as an effective and reliable controller for boost converters in PV systems and the possibility to use it in practical situations.

Smart Operation for AC Distribution Infrastructure Involving Hybrid Renewable Energy Sources

Abstract This paper presents an effective algorithm for optimizing the distribution system operation in a smart grid from cost and system stability points of view. This algorithm is mainly dependent on forecasted data of the power available from different renewable energy sources as well as the load demand. Hence, full attention was paid to the forecasting process. A non-linear regression technique was applied to build accurate forecasting models for different sources and load conditions. These models help in monitoring and predicting the total power generation and demand online. The main objective of the optimization process is to control the power shared from different sources such that they satisfy the load demand with the least cost while giving higher priority to renewable energy sources. Moreover, batteries are controlled in such a way that they are allowed to discharge only when there is no very big load predicted within a future period so that they become available to act as a buffer for a predicted large load increase which may affect the stability of the system and hence reduce voltage dips. A fuzzy controller was utilized to determine how many hours before a predicted occurrence of load the battery system should be turned off and what percentage of power should be taken out of the batteries and the grid to solve this issue. Different case studies were investigated to verify the validity of the proposed algorithm and define the system performance under several conditions.

Enhancing loading limitations in PV systems

This paper discusses the loading limitations in PV systems resulting from switching the power electronic interfaces and transients associated with large loads. These conditions derate the power generation capability of the PV system. We propose some methods to enhance the loadability of these systems under both steady state and dynamic operations. A PV system for home application purposes, with a rated power of 280 W was designed and built. The proposed enhancements were applied to the experimental setup and the obtained results verified the effectiveness of the proposed methods.