Georgios Kampitsis

Direct MPP Calculation in Terms of the Single-Diode PV Model Parameters

In this paper, new expressions are introduced for the determination of the maximum power point (MPP) of photovoltaic (PV) systems as explicit functions of the five parameters of the single-diode model employing the Lambert W function. These equations provide the voltage and current at MPP in a direct and straightforward manner, thus dispensing with any need for iterative solution. They are initially derived for a PV system operating under uniform conditions, and subsequently extended for mismatched conditions at the PV string level. The novelty of these formulae lies in their solid theoretical foundation, which supports their validity in the general case and offers a well-founded symbolic formulation for the MPP evaluation problem. Extended simulations and experimental validation are performed to verify the accuracy and computational efficiency of the proposed equations compared with other methods available in the literature.

Variable Frequency Controller for Inductive Power Transfer in Dynamic Conditions

Inductive power transfer is a highly attractive option for powering unmanned aerial or underwater vehicles, in harsh environments and while in continuous motion. This study presents a variable frequency controller for series-series compensated contactless chargers operating in dynamic conditions. The controller tracks, in real time, the frequency for which the output voltage is load independent. The criterion for that is the zeroing of the phase difference between the secondary current and the primary-side inverter output voltage. Control is performed by a phase-locked loop with optical communication between the two sides. Experimental results on a 1-kW prototype, for power transfer while in motion, show fast frequency response, along with steady output voltage, despite load variations. Comparison is performed with two other fixed frequencies of operation; the natural frequency of the primary resonant circuit and the maximum output power frequency at nominal gap. The proposed control is proven superior in terms of output power level and stability, as well as safety to highly misaligned conditions.

Comparative evaluation of the short-circuit withstand capability of 1.2 kV silicon carbide (SiC) power transistors in real life applications

Abstract In this paper, the response of the new generation silicon carbide (SiC) power transistors under short-circuit faults is examined. The study is focused on real life applications, where a large stray inductance is typically part of the main power loop. Three types of short-circuits are experimentally investigated and the effect of the stray inductance in each case is recorded. A comparative analysis regarding the performance of the SiC junction field effect transistors (JFETs) and SiC metal oxide field effect transistors (MOSFETs), along with the latest technology silicon (Si) MOSFETs is carried out. Maximum instantaneous power, critical energy and saturation current are the main criteria for evaluating the robustness of each power device. A destructive test is performed in order to obtain the short-circuit withstand capability and the failure mechanism of each semiconductor.

Power Reserves Control for PV Systems With Real-Time MPP Estimation via Curve Fitting

In order for a photovoltaic (PV) system to provide a full range of ancillary services to the gird, including frequency response, it has to maintain active power reserves. In this paper, a new control scheme for the dc/dc converter of a two-stage PV system is introduced, which permits operation at a reduced power level, estimating the available power (maximum power point—MPP) at the same time. This control scheme is capable of regulating the output power to any given reference, from near-zero to 100% of the available power. The proposed MPP estimation algorithm applies curve fitting on voltage and current measurements obtained during operation to determine the MPP in real time. This is the first method in the literature to use the nonsimplified single-diode model for the determination of the MPP and the five model parameters while operating at a curtailed power level. The developed estimation technique exhibits very good accuracy and robustness in the presence of noise and rapidly changing environmental conditions. The effectiveness of the control scheme is validated through simulation and experimental tests using a 2-kW PV array and a dc/dc converter prototype at constant and varying irradiance conditions.

Forced Current Balancing of Parallel-Connected SiC JFETs During Forward and Reverse Conduction Mode

In this paper, a thorough investigation on the parallel connection of the latest generation vertical trench silicon carbide (SiC) junction field-effect transistors (JFETs) during forward and reverse conduction is performed. Both enhancement and depletion mode SiC JFETs are examined, regarding their static and dynamic characteristics. A parametric analysis on the current sharing, reliability, and effectiveness of the parallel connection is carried out. Current asymmetry over the different JFETs is monitored via series-connected current transformers. A forced current balancing technique during forward conduction mode is achieved by controlling the time delay of the gate signal through a low-cost digital signal controller. The drain current mismatch during reverse conduction is also studied and addressed by applying similar techniques as in the forward conduction, or by adding an antiparallel power diode. The performance of paralleled JFETs is validated through simulation and experimental results at room temperature and 150 °C.

Wireless Phase - Locked Loop control for inductive Power Transfer Systems

Power transfer and efficiency maximization are the main aims in Inductive Power Transfer Systems (IPTSs) control. Misalignment between the magnetically coupled coils is inevitable in real life conditions and its impact in the power transfer capability of such systems is significant. This work presents a simple, yet robust technique for real-time, wireless tracking of the optimal operational frequency of an IPTS under different coupling conditions. The control is implemented by a Phase-Locked Loop (PLL). The phase of the load current signal is optically transmitted to the primary, where the PLL synchronizes the inverter output voltage to that signal. Experimental results on a secondary-side series compensated and a series-series compensated IPTS prototype show fast and accurate response, unaffected by bifurcation phenomena.

An accurate Matlab/Simulink based SiC MOSFET model for power converter applications

In this paper, an analytical model of a silicon carbide power DMOSFET is developed in Matlab/Simulink environment, intended for high performance converter simulations. The proposed Simulink-based model is a highly useful tool that allows users to study system response to transient phenomena and calculate energy losses in a variety of topologies, control strategies and operating conditions. The static characteristics of the model and its performance during hard switching are validated through experimental testing and comparison with similar models in other commercially available simulation platforms. A 1 kW single phase, high frequency inverter is developed using SiC MOSFETs to investigate the accuracy of the proposed model when simulating a complete power converter.

Utilization of SiC MOSFETs in Voltage Source Inverter of Inductive Power Transfer Systems for Enduring Capacitive Loads

In this work, the use of SiC MOSFETs is proposed for the implementation of the H-bridge Voltage Source Inverter (VSI) of Inductive Power Transfer Systems (IPTSs) in order to successfully support potential capacitive loading. Conventional power semiconductor devices are prevented from feeding capacitive loads because of the occurrence of current overshoots and voltage spikes due to the reverse recovery phenomenon of the antiparallel diodes. Operation of the VSI under capacitive load can be caused in IPTSs during resonant frequency tracking or misalignment between the coupled coils, due to the existence of resonant circuits. Experiments conducted with both Si and SiC MOSFETs show the superiority of the latter in terms of system stability, endurance in high power and avoidance of catastrophic failure.

Experimental Investigation of the Response of Different PLL Algorithms to Grid Disturbances

In this paper, the performance of three different phase locked loop (PLL) algorithms is experimentally evaluated in a grid connected photovoltaic inverter. Robust PLL is a key requirement for ensuring system compatibility with existing grid codes. The double second order generalized integrator (DSOGI) and the decoupled double synchronous reference frame (DDSRF) PLL are compared against the conventional synchronous reference frame (SRF) PLL under grid disturbances. All synchronization algorithms are initially simulated in Matlab and subsequently transformed in discrete time for implementation in the digital controller. Grid distortions, such as phase voltage sags, frequency fluctuations and voltage harmonic distortion are emulated via a controllable three phase generator.

Output power increase of a series-series compensated inductive power transfer system via asymmetric loading

This work implements an asymmetric loading rectification technique in an Inductive Power Transfer System (IPTS) intended for use in electric vehicle wireless chargers. Asymmetric loading is performed by a half-cycle short-circuit / half-cycle loading operation, in the secondary side of a series-series compensated IPTS. The aim is to reduce the value of the load resistance as reflected to the power supply, in order to increase the power intake from a given voltage source. Increase in the power provided to the battery is of great importance in wireless charging systems, as it decreases the duration of charging. Achieving it without intervention on the input voltage source is an additional advantage, since it decreases the number of converters used for the system. Simulations and experiments are carried out in a series-series compensated prototype IPTS for a variety of resistive loads. Results show a significant increase in the output power, especially for high resistance loads, in comparison with the conventional full wave rectification. Equivalent results are obtained by charging a 28.8 V, 20 Ah LiFePO4 battery pack.