Shamsodin Taheri

Novel Control Strategy for Modular Multilevel Converters Based on Differential Flatness Theory

This paper aims to present a novel control strategy for modular multilevel converters (MMC) based on differential flatness theory, in which instantaneous active and reactive power values are considered as the flat outputs. To this purpose, a mathematical model of the MMC taking into account dynamics of the ac-side current and the dc-side voltage of the converter is derived in a d-q reference frame. Using this model, the flat outputs-based dynamic model of MMC is obtained to reach the initial value of the proposed controller inputs. In order to mitigate the negative effects of the input disturbance, model errors, and system uncertainties on the operating performance of the MMC, the integral-proportional terms of the flat output errors are added to the initial inputs. This can be achieved through defining a control Lyapunov function which can ensure the stability of the MMC under various operating points. Moreover, the small-signal linearization method is applied to the proposed flat output-based model to separately evaluate the variation effects of controller inputs on flat outputs. The proficiency of the proposed method is researched via MATLAB simulation. Simulation results highlight the capability of the proposed controller in both steady-state and transient conditions in maintaining MMC currents and voltages, through managing active and reactive power.

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 adaptive neuro-fuzzy inference system-based MPPT controller for photovoltaic arrays

This paper presents a Maximum Power Point Tracking (MPPT) method applying an Adaptive Neuro-Fuzzy Inference System (ANFIS) for a stand-alone photovoltaic (PV) system. The proposed ANFIS-based MPPT technique determines the optimal operating point of a PV system which is designed in conjunction with a Z-source DC-DC converter as an interface between the PV array and the load. In the ANFIS-based MPPT controller, real meteorological data are used to define the two input membership function plots assuming that the PV array is located in Ottawa, Canada. The performance of the proposed MPPT technique in tracking the maximum power point (MPP) is assessed numerically in the MATLAB/Simulink environment. The simulation results highlight the benefits of determining reference voltage and duty cycle as output membership functions of the control system without using the current and voltage sensors. Moreover, in comparison with conventional control systems, the proposed solution can reduce the complexity and cost of the control system by eliminating the PID controller.

Dynamic performance control of modular multilevel converters in HVDC transmission systems

This paper focuses on dynamic performance control of modular multilevel converters (MMC) in high-voltage direct current (HVDC) transmission systems. To achieve this objective, a new mathematical model including six state variables of ac-currents and dc-link voltage of MMC, and circulating currents of converter arms are proposed for MMC in d-q reference frame. In addition, a robust control technique with three sub-control loops is designed to provide the stable operation of MMC. In the overall structure of the proposed controller, three outer, central and inner loops have the duties of 1) making the state variables error zero with changeable convergence rate, 2) adding robustness characteristic to the proposed controller, and 3) generating the appropriate reference values for MMCs currents, respectively. The effectiveness of the proposed control algorithm is investigated via MATLAB simulation. The simulation results highlight the capability of the proposed control algorithm in offering an accurate active and reactive power tracking through the control method of MMC, a stabilized dc-link voltage, capacitor voltage balancing of sub-modules, and minimization of circulating currents of converter arms during dynamic transitions and steady state operation.

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.

Simulation and validation of thermal stability for complex system design high power dissipation

Nowadays, heat is an inevitable issue affecting the performance of electronic devices, used in several power applications. In order to improve the lifetime as well as the productivity of these devices, heat should be reduced or kept at a stable level. The objective of this work is to make a thermal study of the ASIC (4.68 mm × 5.97 mm) under natural and forced air convections. The simulation results carried out with the Finite Element Method (FEM)-based software i.e., COMSOL and NISA tools. The DBC (Dirichlet Boundary conditions) method is applied around the ASIC at 25°C. Through these simulations the relationship between the powers dissipated by ASIC and the difference of temperature in both forced and natural convection is validated. Simulation results show a decrease in temperature of 22°C under forced air convection. This work offers an appropriate tool to model a variety of physical phenomena characterizing a real problem as the heat exchange by convection. The temperature profile obtained from the FEM model can ensure a uniform temperature distribution in both natural and forced convection. This work could be regarded as an important basis for the improvement of the reliability of new microelectronic devices, commonly used in power networks.

A control technique for operation of single-phase converters in stand-alone operating mode

A droop-Lyapunov based control technique using direct-quadrature (d-q) rotating frame dynamic model is presented in this paper for the frequency and voltage magnitude regulation of a stand-alone single-phase voltage-source inverter (SPVSI). Steady-state and dynamic performance of the controller are analyzed based on the d-q frame model and direct Lyapunov method respectively to satisfy control aims and system stability as operation criteria. To further clarify the operation area of the inverter, positive and negative maximum values for d-q components of inverter current are acquired by introducing a capability curve (CC) for entire operating condition. The performance of the proposed control technique is evaluated numerically in the MATLAB/Simulink environment. The simulation results validate the capability of the proposed control method in both steady-state and transient responses.

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.