Hamed Taheri

Effect of power system harmonics on transformer loading capability and hot spot temperature

Under harmonic conditions, caused by nonlinear loads, losses and temperature inside the power transformer increase. The hot spot temperature is one of the major limiting factors that affect the transformer life time and its loading capability considerably. This paper presents the determination of field distribution on the transformer components as well as its losses using finite element method (FEM). The hot spot and top oil temperature are computed under harmonic conditions according to two techniques, i.e. the dynamic thermal model and IEEE guide. The effect of harmonics on the useful life of transformer is then analyzed. Finally, an algorithm is proposed to determine the loading capability of transformer under harmonic conditions.

Modified Maximum Power Point tracking (MPPT) of grid-connected PV system under partial shading conditions

PV systems essentially depend on insolation and temperature; but partial shading phenomenon defects characteristics of PV systems significantly. Unlike under ideal condition that PV system has only one maxima, it has several locals under partial shading (with only one global maxima). Conventional Maximum Power Point Trackers (MPPTs) easily fail to track real optimum point and are trapped at locals. It loses considerable energy of PV system. To overcome this problem and draw maximum possible energy from PV system, this paper proposes a Maximum Power Point tracking technique based on Differential Evolution (DE) which is capable of finding global MPP under partial shading conditions. The performance of proposed algorithm is verified by means of simulation in MATLAB/Simulink and is well compared with conventional MPPT. The proposed MPPT is applied to a grid-connected PV system in conjunction with DC-DC boost converter and DC-AC inverter. The simulation results validate that presented MPPT is able to track global MPP under partial shading condition accurately, fast and with zero oscillation.

Nonlinear maximum power point tracking controller for photovoltaic system

This paper proposes a nonlinear maximum power point tracking (MPPT) controller for photovoltaic (PV) systems. The proposed MPPT technique is designed in conjunction with a Z-source dc-dc converter as an interface between the PV array and the load. To improve the system performance in transient regime as well as steady-state condition in both tracking and regulation, a nonlinear MPPT controller is designed. The scheme of the proposed nonlinear MPPT controller consists of the design of a nonlinear MPPT algorithm and a nonlinear controller for the duty cycle generation. The effectiveness of the proposed method is investigated via MATLAB simulation. In addition, the results are compared with the conventional perturb and observe (P&O) method. The simulation results highlight advantages of the proposed technique over the conventional P&O method in terms of an improved response in the transient state, an accurate tracking of the MPP as well as a significant reduction in oscillations around the maximum power point (MPP).

Integration of electric vehicles into a smart power grid: A technical review

Electrification of a transportation system is one of the most promising alternatives to mitigate the dependency of urban life to fossil fuels. However, introducing a large number of grid-connected vehicles reveals technical problems affecting the entire power system, especially the low voltage section. In this context, this paper presents a review of technical challenges associated with the integration of Vehicle-to-Grids (V2Gs). These challenges are studied in several subsections of a power system such as the operation of power electronics equipment, supply-demand imbalance, and impacts on voltage and frequency. In addition, to clarify the concept of smart grid in a power system, a new definition of a smart power grid in the sector of power distribution is elaborated considering effects of V2Gs. This developed definition specifies that the penetration of V2Gs, in fact, establishes an opportunity for implementing the smart power distribution through offering renewable energy storages, two-way communication, and reactive and active power injections to the grid. This review may be regarded as an important basis for the investigation of future challenges in the integration of electric vehicles into a smart grid.

An approach to precise modeling of photovoltaic modules under changing environmental conditions

Equivalent circuit models of photovoltaic (PV) modules are used to investigate the performance of PV systems for different operating conditions. The electrical characteristics of PV modules change as environmental conditions vary. Hence, for an accurate study of the PV modules when insolation and temperature are variable, it is necessary to verify the dependence of the parameters of the PV module model to insolation and temperature. This paper proposes a single-diode model of PV modules based on a novel approach for extracting the climate-dependent parameters. The parameters of the model are updated for given environmental conditions so that the model is able to precisely yield the corresponding real characteristics of the PV module. Comparison between the simulated model and experimentally measured data demonstrates the effectiveness of the developed approach in determining the PV characteristics under variable weather conditions. The proposed model can be helpful to properly design and select PV systems.

Two-Diode Model-Based Nonlinear MPPT Controller for PV Systems

The main objective of this contribution is to present a nonlinear maximum power point tracking (MPPT) controller for photovoltaic (PV) systems based on a two-diode model of a PV module. The proposed MPPT technique operates in conjunction with a Z-source dc-dc converter as an interface between a PV system and a load. The scheme of the proposed nonlinear MPPT controller consists of the design of a nonlinear MPPT algorithm and a nonlinear controller for the generation of duty cycle. The nonlinearity of the PV model as well as the power electronics converter is taken into account in the design of the MPPT controller. Since the PV model parameters vary depending not only on the values of the insolation and the temperature but also on the position of the operating point on the PV characteristics, an adaptation mechanism based on the two-diode PV model is proposed. Thus, these parameters are updated as per real atmospheric conditions. The effectiveness of the proposed method in transient regime as well as steady-state condition is investigated via MATLAB simulation. Furthermore, simulation results are compared with the conventional perturb and observe and incremental conductance methods. The simulation results highlight the capability of the proposed technique over these conventional methods in terms of an improved response in the transient state, an accurate tracking of MPP as well as a significant reduction in the oscillations around the MPP.

Modeling and nonlinear control of multilevel inverter for photovoltaic application

The present work deals with a nonlinear control strategy for the purpose of photovoltaic (PV) systems integration into the power system through cascaded Z-source H-bridge inverters. The topology of the proposed system which is able to obtain a 5-level voltage is discussed. To facilitate a systematic voltage control, an exact nonlinear model of the proposed system is presented. A nonlinear voltage regulator, based on exact input-output feedback linearization scheme, is designed to maintain the terminal voltage of the PV inverter at the point of common coupling. Moreover, to harvest the maximum power from the PV arrays, a nonlinear controller for a dc-dc converter which is in conjunction with the maximum power point tracker (MPPT) is designed. The performance of the proposed Z-source multilevel inverter is evaluated numerically in the MATLAB/Simulink environment. The simulation results validate the performance of the proposed model-based controller in both steady-state and transient conditions, since the nonlinearity of the system is taken into consideration.