Mutlu Boztepe

Global MPPT Scheme for Photovoltaic String Inverters Based on Restricted Voltage Window Search Algorithm

String inverter photovoltaic (PV) systems with bypass diodes require improved global maximum power point tracking (GMPPT) algorithms to effectively reach the absolute maximum power operating point. Several GMPPT algorithms have been proposed to deal with this problem, but most of them require scanning wide voltage ranges of the PV array from nearly zero voltage to open-circuit voltage that increases the scanning time and, in turn, causes energy loss. This paper presents a novel GMPPT method which significantly restricts the voltage window search range and tracks the global power peak rapidly in all shading conditions. Simulation tests and experimental comparisons with another GMPPT algorithm are presented to highlight the features of the presented approach.

Decoupling Capacitor Selection in DCM Flyback PV Microinverters Considering Harmonic Distortion

Electrolytic capacitors as a decoupling reservoir restrict the lifetime of photovoltaic (PV) microinverters. This has led to the development of several improved decoupling circuits that can reduce the capacitor value to allow the use of nonelectrolytic types. In this paper, the minimum decoupling capacitor value for the proper operation of discontinuous conduction mode flyback PV microinverters is analyzed by taking into account the total harmonic distortion (THD) and PV power utilization ratio. The results presented show that the decoupling capacitor value influences the THD more than PV power utilization. A decoupling capacitor selection method for single-stage and two-stage flyback inverters is proposed. Experimental results obtained on an 80-W test bench are presented.

Power Controller Design for Photovoltaic Generation System under Partially Shaded Insolation Conditions

Partially shaded photovoltaic (PV) modules typically exhibit additional difficulties in tracking maximum power point (MPP), since their power-voltage (P-V) characteristics are complex and may have multiple local maxima. For this reason, conventional techniques fail to track MPP effectively, if the PV array is partially shaded. This paper presents a novel power compensation and control system for PV arrays under complicated non-uniform insolation conditions. The proposed system is based on forward biasing bypass diode of shaded modules by monitoring dynamic resistance and voltage of PV modules. For this purpose, the proposed system is used DC/DC converter equipped with each PV string in PV array. The proposed system can identify which modules are shaded. The effectiveness of the proposed system is investigated and confirmed for complicated partially shaded PV array.

Evaluation of two-stage soft-switched flyback micro-inverter for photovoltaic applications

The research on the micro inverters for photovoltaic (PV) applications has been increased in recent years due to its advantages of minimization of module mismatch losses, the elimination of serial diode losses, and suitability for mass production etc. However, the micro-inverters suffer from the low voltage at the input PV terminals that causes high conduction loss and requires large electrolytic capacitors for decoupling purposes. The electrolytic capacitors have relatively low lifetime in comparison with the PV modules, and restricts the micro-inverter service life significantly. Moreover, the high conduction loss decreases the micro-inverters conversion efficiency considerably. In order to mitigate these drawbacks, two stage micro-inverters are generally proposed. In this study, a 200 W two stage soft switched flyback micro-inverter is designed for highest efficiency, and compared with the conventional single stage flyback micro-inverter. By using two-stage soft-switched flyback micro-inverter, the decoupling capacitor value is decreased by 10 times, and maximum efficiency is increased by 13.7%. The experimental results are also presented.

Modified dual active bridge photovoltaic inverter for solid state transformer applications

The solid state transformer (SST) was announced as the one of the most emerging technology by the Massachusetts institute of technology (MIT). An SST generally includes a DC or AC buses in high and/or low voltage side in order to connect the external input sources to the SST. We proposed discontinues conduction mode (DCM) modified dual active bridge inverter topology for integrating photovoltaic (PV) panels to SST. The proposed inverter achieves soft switching, hence it has high efficiency. Moreover, it can be operated by open loop control with eliminating high frequency current sensor. In this paper, the analysis of the proposed inverter is presented, and simulation results are given for validating the proposed inverter.

Design and implementation of programmable PV simulator

Digital controlled SMPSs (Switched Mode Power Supply) have many benefits and advantages. Nowadays they are highly preferred instead of analog (linear) power supplies due to their high efficiency, small size, and flexibility. In this paper a buck converter which has an output voltage of 0-30V and output current of 0-5A is designed step by step. At this process its calculations and simulations are performed. This converter can process in constant voltage and constant current modes. Also a PV (Photovoltaic) Simulator which can simulate the power output characteristics of various PV panels is added to our device. Lastly, a MATLAB GUI interface is designed to communicate with computer for obtaining the desired PV characteristic.

Bi-directional DCM DAB inverter for SST applications

Generally, conventional single phase solid state transformer (SST) topologies includes three stage conversions. In the first stage, AC/DC conversion accomplished for transforming high voltage AC (HVAC) to high voltage DC (HVDC). From HVDC, low voltage DC is obtained at the second stage, and low voltage AC is formed at the third stage. Three stage conversions are suitable for connecting renewable energy sources through low voltage DC (LVDC). However, all of the stages are switched at high switching frequency. Hence, this situation may bring high switching losses, high control complexity and lower efficiency. In this paper, we proposed bi-directional current source discontinues conduction mode (DCM) Dual Active Bridge (DAB) inverter. In the proposed inverter, HVDC is transformed to LVAC in one high frequency switching stage. Hence, it eliminates the third stage of three stage conventional SST. On the other hand, renewable energy sources can be connected to the proposed converter through LVAC bus. In this paper, the analysis of the proposed inverter is presented, and simulation results are given under resistive and inductive loads for validating the proposed inverter.

Trapezoid current modulated DCM AC/DC DAB converter for two-stage solid state transformer

Solid state transformer topologies (SST) have gained great attention because of having features such as reactive power compensation, output voltage regulation and smart grid operation etc. Among SST topologies, three-stage topologies have been mainly focused because there are many appropriate topologies for each conversion stages. However, conventional three-stage topologies may bring complex closed loop control methodologies, high value inductances and capacitances and hard switching losses. In this study, we proposed trapezoid current modulated DCM AC/DC DAB converter for two-stage SST. The proposed converter achieves soft switching; hence it can have high efficiency. Moreover, it can be operated by open loop control with eliminating high-frequency current sensors. In this paper, the analysis of the proposed converter is presented and theoretical analyses are validated by simulations.

Variable switching frequency operation of DCM flyback micro-inverter

Since the dedicated micro-inverters operate individual photovoltaic (PV) modules at its maximum power point, the micro-inverter topology exhibits better performance than the central and string topologies against partial shading. Among the micro-inverter topologies, discontinuous conduction mode (DCM) flyback micro-inverter is the most attractive one due to its simplest structure, easy control and potentially low cost. However it suffers from low input voltage range and high conduction loss. In this paper, a variable switching frequency control method is proposed which allows extending the input voltage range considerably. Additionally, the proposed method can decrease the total losses by 24.82% according to the simulation results.

Loss analysis of trapezoid and triangular current modulated DCM AC/DC DAB converter

Solid state transformer topologies (SST) can be used instead of Low Frequency Transformers (LFT) in the near future because of superior advantages such as; reactive power compensation, output voltage regulation, able to connect renewable energy sources to distribution system and smart control etc. In this study, the loss analysis of mixed current modulated (trapezoid and triangular) DCM AC/DC DAB converter for two-stage SST is introduced. In order to prove the analyses a 100 W SST prototype is set up.