Elizaveta Liivik

Comparison of Performance of Phase-Shift and Asymmetrical Pulsewidth Modulation Techniques for the Novel Galvanically Isolated Buck–Boost DC–DC Converter for Photovoltaic Applications

This paper discusses realization possibilities of the buck mode in the novel magnetically integrated quasi-Z-source series resonant converter (qZSSRC) with secondary resonance. Our aim was to achieve higher power conversion efficiency at different power levels and dc voltage gains of the converter. The phase-shift modulation (PSM) was comprehensively compared with the asymmetrical pulsewidth modulation (PWM). Both control techniques were evaluated for the case of the discontinuous current through the resonant network, which made their realization more challenging. The operating principle of the qZSSRC in the buck mode with two compared control techniques was explained by means of steady-state analysis. To verify the theoretical assumptions, a 250-W prototype of the photovoltaic module integrated converter was assembled and tested. It was experimentally confirmed that in contrast to the PSM, the asymmetrical PWM for the buck mode control of the qZSSRC will lead to higher power conversion efficiency at different power levels and dc voltage gains of the converter, except the operating points close to the boundary with the normal mode of the converter.

Wide Input Voltage Range Photovoltaic Microconverter With Reconfigurable Buck–Boost Switching Stage

This paper presents a novel soft-switching galvanically isolated buck-boost dc–dc converter as a module-integrated converter for photovoltaic (PV) applications. It features three major operating modes: boost, normal, and buck mode. Their proper distribution over the input voltage range enables wide voltage regulation capabilities, i.e., close to those of nonisolated power optimizers. The proposed control algorithm requires only one buck-boost switching stage that performs voltage regulation by means of the switching stage reconfiguration with smooth transition between the modes. The design guidelines and the digital control system for PV applications are provided. The module-integrated converter was verified experimentally with a solar array simulator. The converter showed good maximum power point tracking performance and a peak efficiency of nearly 97%.

Control features of multicell-type current regulator for resistance welding

The features of resistance welding process are considered. A multiceli buck converter as a simplest, efficient and stable topology for resistance welding power supply is proposed. The efficiency of a buck converter cell is analyzed. The particularities of welding pulse generation are described. Relay control as an appropriate method for minimizing the welding pulse curve error is analyzed. The restrictions of relay method use for multiceli converter topology are shown. A modified strategy to reduce the error of welding pulse generation by multiceli converter is proposed. The theoretical aspects of the converter design are verified with MATLAB Simulink.

Low-cost photovoltaic microinverter with ultra-wide MPPT voltage range

This paper presents a novel low-cost photovoltaic (PV) microinverter with an ultra-wide MPPT voltage range. This approach is based on the two-stage energy conversion where the front-end boost half-bridge DC-DC converter is used to step up the fluctuating PV voltage to a stabilized high DC voltage, which is next transformed to a grid compliant AC voltage by the grid-tied inverter. The operation principle of the microinverter is explained by the help of the steady-state analysis. To verify theoretical assumptions and evaluate the performance of the proposed concept, a 250 W experimental prototype of a photovoltaic microinverter was assembled and tested. Finally, its shade-tolerant operation is demonstrated with the partially shaded 60-cell Si PV module.

Reverse power flow control possibilities of galvanically isolated impedance-source DC-DC converters

Galvanically isolated impedance-source (IS) dc-dc converters are a high performance solution proven for emerging power electronics applications. However, studies of their bidirectional operation have been scarce. Focus in this paper is on the reverse power transfer possibilities of the high performance quasi-Z-source (qZS) series resonant dc-dc converter. This enables extension of functionality and application range of the converter to the energy storage and buffering systems. Two novel methods of reverse power flow control were introduced: one that requires no qZS network reconfiguration and is based on the resonant properties of the given topology, while the other is based on the qZS network reconfiguration into LC filter and does not depend on whether an IS converter is resonant or not. Experimental study was conducted for the power level of 250 W within the output voltage range of 20 V to 30 V. Experimental results corroborate the theoretical predictions and show that one of the methods is superior to the other by efficiency values and curve flatness.

Efficiency improvement from topology modification of the single-switch isolated quasi-Z-source DC-DC converter

Single-switch converters can be an attractive solution for low-cost and low-power applications. This paper studies efficiency improvements from topology modifications of the single-switch galvanically isolated quasi-Z-source DC-DC converter. The following approaches were studied: synchronous rectification in the quasi-Z-source network, implementation of a voltage multiplier rectifier and the multidevice switching stage, and elimination of the DC blocking capacitor from the primary winding of the isolation transformer. To evaluate the performance improvement, the efficiency was measured for all approaches in a wide input voltage and power range. It was shown that thanks to the proposed modifications, the peak efficiency of the single-switch galvanically isolated quasi-Z-source DC-DC converter was increased to 95.5%, which includes the auxiliary power and control system losses.

Single-switch galvanically isolated step-up DC-DC converter for residential photovoltaic applications

This paper evaluates a modified single-switch galvanically isolated quasi-Z-source DC-DC converter intended as a topology for photovoltaic module-level power electronics applications. The operation principle of the converter is explained by the help of the steady-state analysis. To verify theoretical assumptions and evaluate the performance of the proposed concept, a 200 W experimental prototype of a photovoltaic microconverter was assembled and tested.

Photovoltaic module characteristic influence on reliability of micro-inverters

Due to an integrated system structure between the Photovoltaic (PV) module and inverter, the characteristic of PV module can strongly influence the operating condition and loading of micro-inverters. The commercially available PV module has a certain variation in terms of power rating and voltage at the Maximum Power Point (MPP). Accordingly, the PV micro-inverter loading conditions, and thereby the reliability, will inevitably be affected by those variations. In this paper, the impact of PV module characteristic on the reliability of micro-inverters is investigated. Six commercial PV modules are used to demonstrate the variation in the PV module characteristic in real-field operation, and the mission profile of the installation site in Arizona is used as a case study. The evaluation results indicate that the voltage at the MPP of the PV module has a strong influence on the thermal loading and reliability of the micro-inverter. Employing a PV module with high voltage at the MPP can effectively reduce the loading of the micro-inverter, since the inverter efficiency is maintained at a high level during the operation. In that case, a high-reliable operation of the micro-inverters can be achieved.

Reliability evaluation of an impedance-source PV microconverter

The reliability of an impedance-source PV microconverter is evaluated based on the real-field mission profile. As part of a PV microinverter, the dc-dc microconverter is firstly described. Then the electro-thermal and lifetime models are built for the most reliability-critical components, i.e., the power semiconductor devices and capacitors. The finite element method (FEM) simulation is used for the thermal impedance extraction. The mission profile, i.e., the ambient temperature and solar irradiance, from Aalborg, Denmark is applied to the built electrothermal model. Finally, the thermal loading profiles and annual wear-out damage accumulation are obtained. In addition, experimental measurements from a 300-W converter prototype are given.

Multiphase galvanically isolated impedance-source DC-DC converter for residential renewable energy applications

In this paper, a novel topology of the high step-up multiphase galvanically isolated impedance-source DC-DC converter is proposed. It was derived by the input-parallel-output-parallel cascading of the asymmetrical quasi-Z-source half-bridge cells. The operating principle of the converter is explained by the steady state analysis. It was also demonstrated how the input current ripple of the converter could be decreased by increasing the number of interleaved phases. To validate our approach experimentally, a two-phase DC-DC converter with the power rating of 300 W was assembled. It was confirmed that the proposed converter is capable of ensuring the six-fold regulation of the input voltage with the maximum DC gain of 40 and peak efficiency of 94.5%.