Kaveh Razi

Analytical Method for Pattern Generation in Five-Level Cascaded H-Bridge Inverter Using Selective Harmonic Elimination

This paper proposes an analytical procedure for computation of all pairs of valid switching angles used in pattern generation in five-level H-bridge cascaded inverters. The proposed procedure eliminates harmonic components from inverter output voltage and, for each harmonic, returns the exact boundaries of all valid modulation index intervals. Due to its simple mathematical formulation, it can be easily implemented in real time using a digital signal processor or a field-programmable gate array. In this paper, after a detailed description of the method, simulation and experimental results demonstrate the high quality of achievable results.

Observer-Based Control of LLC DC/DC Resonant Converter Using Extended Describing Functions

This paper presents theoretical and practical results about dynamic analysis, frequency response, and control of a LLC resonant dc/dc converter operating under wide input voltage and load variations. A nonlinear model for the LLC resonant converter was developed using the extended describing function method; then, based on the derived model, a nonlinear observer-based controller was designed and implemented with a digital signal processor. Transient responses obtained under input voltage and output load variations show that the proposed controller is capable to stabilize the output effectively. Experimental results prove the superiority of the proposed observer-based controller over a conventional PID controller.

Harmonic mitigation technique for multilevel inverters in power systems

A new procedure to mitigate some harmonics is presented for a 5-level cascaded inverter, varying the modulation index. The two switching angles α1 and α2 are computed by using a procedure based on graphycal analysis by using Chebyshev polynomials and the Warings formulae. The proposed mitigation technique is applied to reduce a pair of harmonics among third, fifth and seventh in inverter output voltage according to standard code requirements. Experimental results are also extracted by the prototype built at the laboratory. The measured results confirm the quality of the presented procedure.

Fuzzy-logic-based high accurate fault classification of single and double-circuit power transmission lines

In this paper, a fuzzy logic-based fault classification of transmission lines is proposed. The classification procedure is carried out by only current phasors of three phases. The proposed technique is able to classify all the possible faults including single-phase to ground, two-phases, two-phases to ground and three-phases faults with high accuracy in a wide variety of system pre-fault loading conditions. In addition, this method can identify the faulted phase(s) from non-faulted phase(s). The mutual effect of parallel transmission lines has been investigated in this study, too. The proposed method has a good performance in high fault resistances, high system loading level and high fault distances from relaying point. Large numbers of test cases are generated to verify the performance of proposed technique. The simulation studies have been carried out by using PSCAD/EMTDC and MATLAB fuzzy-logic toolbox.

Comparative transient response analysis of LLC resonant converter controlled by adaptive PID and fuzzy logic controllers

If properly controlled, LLC resonant converters can achieve zero voltage switching (ZVS) over the entire operating range, thus minimizing switching losses of the power MOSFETs and secondary diodes. For such a reason, this paper presents and compare a half bridge LLC resonant converter controlled by both an adaptive PID and a fuzzy logic controller (FLC). For obtaining the DC voltage transfer function, an analysis tool, the so called Fundamental Harmonic Approximation (FHA) method has been adopted. FHA is also used to determine circuit parameters and for predicting the transient response of the converter during sudden changes of input and load, which is then verified through simulations and laboratory measurements.

A grid-connected PV system with LLC resonant DC-DC converter

This paper presents a grid-connected photovoltaic (PV) system with a three level voltage source converter (VSC) using double closed loop control strategy. The outer DC voltage control loop is to regulate the DC bus voltage, and the inner current control loop is to synchronize the output current with the grid voltage, thus ensuring unity power factor. A LLC resonant DC-DC converter is used in the proposed system to step-up the voltage of the PV array and to extract maximum power. It is intrinsically isolated by a high frequency transformer, then the parasitic capacitance of the PV panels to ground could not be of concern; furthermore, because of soft switching technique, it operates at high frequency with low switching losses. Size and cost of the magnetic components and DC-link capacitor are decreased compared with traditional boost converters. An incremental conductance method integrated within PI controller was used to extract maximum power by PV. Simulation results based on MATLAB/Simulink verify the validity and dynamic performance of the proposed system during fast solar irradiation changes and ensure DC bus voltage stability.

Digital control of a half-bridge LLC resonant converter

Recently, LLC resonant converters are gaining more and more interest in DC/DC conversion due to their high efficiency and power density. Usually, they are used in low power applications, i.e. in the range of few hundred Watt and with constant output load, however they are very appealing in high power and renewable energy applications as well as in many other applications affected by wide variations of the load and/or the source voltage. This paper presents and discusses a novel half bridge DC/DC resonant converter with digital control and fuzzy logic. The proposed system is compared with a more traditional PID controller. A variable frequency control algorithms are implemented using a low cost, high performance Texas Instrument TMS320F28069 DSP for the regulation of the output voltage. Simulation results using MATLAB are presented to evaluate the quality of the obtained result. A laboratory prototype has been realized and experimental results obtained using both PID and Fuzzy controllers are shown and discussed.

Linearization of LLC resonant converter model based on extended describing function concept

LLC resonant DC-DC converters are becoming increasingly popular due to their ability to work with zero voltage switching (ZVS) feature. The reduction in switching power losses, especially in high frequencies, greatly enhances the efficiency of these converters in comparison with conventional PWM boost converters. This paper presents theoretical details for modeling the LLC resonant converter, by which its characteristic and dynamic behavior can be analyzed. To obtain an accurate model, the extended describing function concept is used and the approximate nonlinear model derived. Then the derived model is linearized in vicinity of the steady state operating point and the state space matrices of the system are obtained. The accuracy and validity of the model is then verified using MATLAB/Simulink.

A deterministic harmonics mitigation technique for five-level inverters

A deterministic algorithm aiming at reduction of harmonic content in three phase, five-level cascaded inverters is proposed. The procedure starts from a non-linear system consisting of disequations and, using Chebyshev polynomials and Warings formula, for any assigned modulation index computes those switching angles α1 and α2 reducing harmonics amplitude such as to fulfill grid code specifications. Simulation and experimental results demonstrate the achievable results.

Real-time Harmonics Elimination procedures for high-power converters

This paper proposes a Selective Harmonic Elimination algorithm for multilevel power converters. Starting from an analysis of valid modulation index intervals, the algorithm computes those switching angles which eliminate the undesired harmonic. Differently by other approaches, it is based on the use of full analytical and numerical procedures, which can be easily executed in real-time. After a detailed description of the method, simulation and experimental results obtained with a five level cascaded multilevel inverter are shown demonstrating the quality of the achievable results.