Athanasios Karlis

Fuzzy Cognitive Networks for Maximum Power Point Tracking in Photovoltaic Arrays

The studies on the photovoltaic (PV) power generation are extensively increasing, since it is considered as an essentially inexhaustible and broadly available energy resource. However, the output power of the photovoltaic modules depends on solar radiation and temperature of the solar cells. Therefore, to maximize the efficiency of the renewable energy system, it is necessary to track the maximum power point of the PV array and make the array operate near it. Maximum power operation is a challenging problem, since it requires that the system load is capable of using all power available from the PV system at all times. The I-V characteristic of the load must intersect the focus of maximum power points on the I-V characteristics of the PV array for varying insolation and temperature levels. Fuzzy Cognitive Networks (FCN) have been proposed as an operational extension of Fuzzy Cognitive Maps (FCM), which work in continuous interaction with the system they describe and may be used to control it. In this chapter FCN is used to construct a maximum power point tracker (MPPT), which may operate in cooperation with a fuzzy MPPT controller. The proposed scheme outperforms other existing MPPT schemes of the literature giving very good maximum power operation of any PV array under different conditions such as changing insolation and temperature. Moreover it has the ability to adapt to different changes that might happen during the life cycle of the PV module, such as a destroyed cell of the PV array.

Improving the voltage profile of a medium voltage grid using distributed generation units

In this paper the impact of photovoltaic (PV) system connected to the medium voltage distribution grid using three-phase three-level inverter and step-up transformer will be investigated. The PV inverter is capable to control active and reactive power independently. The impact to the grid voltage profile of both active and reactive power regulation of the PV system will be examined. In order to simplify the control scheme and improve the performance of the converters the p-q theory has been chosen due to the ability to compensate the reactive power of the load. The proposed electric power system was investigated via simulation results during steady state and transient response. The analysis will be focused on the grid voltage profile improvement regarding of the of the inverters active and reactive power injection.

Partial discharge diagnostics in wind turbine insulation

The purpose of this paper is to review work undertaken on partial discharges and their influence on the insulation of wind turbines. No matter whether partial discharges can be considered as the main cause of deterioration of the insulation material, the initial cause of failure or not but an indication of the material degradation, there is no doubt that they are intimately linked to the aging of machine insulation. Material degradation can be detected by non-destructive techniques (e.g., partial discharge measurements, change of tan δ) or by destructive techniques, such as by cutting small pieces of the insulating material and by putting them under the scrutiny of the scanning electron microscope (SEM). Wind generators are a modern subject of research, especially in view of the growing demands of electric energy worldwide and the problems facing the environment all over the globe. Wind turbines are a novel field of research regarding partial discharge diagnostics since they are subjected to a variety of aging factors, which are different from conventional turbines. In this respect, particular attention should be paid to the multi-factor stressing of insulation and their consequences on the partial discharges.

Provision of frequency regulation by a residential microgrid integrating PVs, energy storage and electric vehicle

The scope of this paper is to develop an energy management system (EMS) in order to integrate Vehicle-to-Grid (V2G) technology in a residential scale microgrid. The aforementioned configuration embodies a Photovoltaic plant, a domestic energy storage device, household loads, and a plug-in Electric Vehicle which is connected to the system through an off-board DC charger. The EMS developed ensures a) maximum energy efficiency of the household consumer, b) direct solar power use and c) customer driving needs satisfaction by providing frequency regulation to the utility grid through an autonomous distributed control. Herein, a residential microgrid set up is modeled via Matlab/Simulink and the effectiveness of the control scheme is demonstrated for the considered case studies.

A study on the dynamic behavior of a DFIG with sensorless-based control in cooperation with a fuzzy controlled energy storage system

The present study deals with the combined operation of a doubly-fed induction generator (DFIG) with sensorless based control, which is common to wind energy conversion systems (WECSs) and an energy storage system (ESS). The scope of this paper is to present a coordinative control system to enhance the DFIG ability to recover from a fault condition. Despite the numerous benefits of the sensorless control, this approach suffers from high sensitivity to the dynamics effects. Herein, a three-phase short-circuit is considered as a significant and most common dynamic effect. The newly protection method - combined with the classical protection methods, i.e. crowbar, breakers - refers to a battery based energy storage system connected to the DC link of the back to back converter through a fuzzy-controlled bidirectional DC/DC converter. The aforementioned configuration aims to mitigate the DC bus voltage fluctuations during grid faults and subsequently to ensure the seamless operation of the overall system. The set-up was modeled via Matlab/Simulink and the simulation results obtained, validated the effectiveness of the proposed control system.

The effect of water droplets and salinity on the offshore wind turbines windings insulation: A short review

The purpose of this paper is to review the work that has been done on how humidity, salinity and water droplets affect the offshore wind turbines electrical stator winding insulation. Partial discharges (PDs) could be both the reason of the insulations deterioration as well as the result of this deterioration and they are linked with the aging of the insulation system. The presence of humidity, salt and water environment offshore can exasperate the occurance of PD. Wind turbines, onshore and offshore are one of the most promising areas of research. Offshore wind turbines have to operate in harsh environments such as marine environment that could affect even more their lifetime.

A view on humidity effects in high voltage electric generator's insulation

Partial discharges (PD) are a symptom and at the same time the result of the deterioration and the aging of an electrical machines insulation. Based on experiments the flashover voltage of epoxy resin samples in relation to humidity was studied and the results are discussed.

Comparative study on the crowbar protection topologies for a DFIG wind turbine

This paper presents a study on a doubly fed induction generator (DFIG)-based wind energy conversion system (WECS), emphasizing to the grid connection and protection needs. Today, due to the high wind power penetration into the grid, a protection system mounted on the WECS is definitely essential in order to achieve a better performance during dynamic states. In this study, two types of fault were considered. Firstly, a three-phase short-circuit in a transmission line and secondly a voltage dip were examined. Several of the protection schemes have been presented in literature during the last two decades. In this paper, different topologies of the well-known crowbar protection method combined with or without a chopper circuit mounted parallel to a capacitor in the dc link are presented. Herein, a comparative study in terms of current and voltage fluctuations between these protection methods was held and is presented below giving more attention in the sensitivity of power semiconductor devices.

Electrical machine insulation: Partial discharges, consequences and diagnostic technique

Partial discharges (PDs) are a symptom and at the same time a result caused by the deterioration and the aging of the insulation of a rotating machine. Comments, regarding the usefulness as a fault diagnosis method, the hysteresis method presented in previous publications are offered in this paper. This method points out that the size of a curve and the area that the curve creates as PDs occur may vary and it could be an indicator of the insulations deterioration, the existing defects and the PDs that have taken place over the years.

Modeling, simulation and performance evaluation of a low-speed battery electric vehicle

Electric vehicles (EVs) are considered the solution for energy crisis and environment pollution in individual transportation. However, EVs are still not competitive with conventional vehicles, in terms of limitation on the driving range caused by current power battery technology. Therefore, energy efficiency is one of the most important factors for battery electric vehicles (BEVs) because they are highly dependent on the aforementioned limitations. Hence, low-speed BEVs, which are for urban transportation, can be considered a promising solution. Also, a model-based design of a real low-speed EV is presented. Given that there are no standard simulation models for these vehicle types, a novel modeling approach via Matlab/Simulink is presented and an experimental verification followed. Finally, an innovative optimization strategy is proposed through a comparative study between four basic battery types.