Characterisation of the cascode gate drive of power MOSFETs in clamped inductive switching applications

This thesis proposes a novel gate drive circuit to improve the switching performance of MOSFET power switches in power electronic converters. The proposed topology exploits the cascode configuration, allowing the minimisation of switching losses in the presence of practical circuit constraints, which enables efficiency and power density improvements. Switching characteristics of the new topology are investigated and key mechanisms that control the switching process are identified. Unique analysis tools and techniques are also developed to demonstrate the application of the cascode gate drive circuit for switching performance optimisation

Climate control of terrestrial carbon exchange across biomes and continents

Peer reviewe

DC-Side Sensorless Control of Battery Interfaced Three-Phase Full Bridge Cascaded H-Bridge Multilevel Grid-Connected Inverter

Grid connected battery systems play an important role in present day power systems. In this paper, a DC-side sensoreless control of battery interfaced three-phase full bridge cascaded H-bridge multilevel grid-connected inverter is proposed. The key achievement of this paper is the use of the DC-link voltage and current estimation proposed in [1] for a three-phase battery storage system and achieving battery SOC balancing using the estimation algorithm. Coulomb counting (Amp-hour balancing) method is used for SOC estimation because of its simplicity and it can utilize the value of the estimated battery current directly. One disadvantage of this method is the need to identify the value of the initial SOC value which was assumed to be known. The balancing between the batteries has been done using the estimated SOC values. Simulated results are presented to show the efficacy of the proposed algorithm. </p

Control of series-cascaded grid-forming microgrid configuration with a storage device

With the increase of inverter-based energy resources in power grids, it is imperative that the frequency stability is maintained and inverters can be controlled to improve frequency damping by using grid-forming inverters (GFM). The microgrid that is formed by several DGs that are connected in series is called a series topology microgrid. The series topology with individual LC output filters benefits from utilizing lower and different dc-link voltages. One of the drawbacks in series topologies is that the series cascaded DG microgrids have rarely been discussed when it comes to a different configuration combinations of DGs such as dispatchable and non-dispatchable DGs. In this paper, a comprehensive study of series-cascaded microgrid configuration consisting of master and slave DGs with a storage device is done. The power circuit, control circuit, and results are shown.</p

Independent Voltage Oriented DC-Side Sensorless Control of Three-Phase Cascaded H-bridge Multilevel Inverter with Decentralized MPPTs

This project aims at designing an efficient and economical photovoltaic system architecture. The PV arrays are broken into groups. Each group is connected to one of the H-Bridges in a three-phase based multilevel inverter. Each H-bridge is controlled by an independent MPPT block in conjunction with the grid controller. The cost is then reduced by eliminating the PV-side current and voltage sensors in each H-bridge. This is done by properly tracking the PV string power using the ac-side sensors. Estimating both PV current and voltage is important to have a plug and play capability for PV strings in the conventional PV grid-connected systems. The control of the reference dc voltages is based on separate Voltage-Oriented Control (VOC). The phase-unbalanced issue caused by partial shading might occur on the PV modules. Therefore, the zero-sequence injection is done by circulating energy among the H-bridges from each phase in the same hierarchal order.</p

Advanced resampling techniques for PWM amplifiers in real-time applications

Regularly sampled pulse width modulation (PWM) has been a mainstay of the power electronics community since the advent of digital controllers. In this form of PWM the modulating signal is sampled only at either the peaks and/or the troughs of the triangular carrier waveform, then held constant until the next sampling instant, which allows ample time for the calculation of switching instants. Unfortunately, it produces phase and amplitude distortion of the modulating signal that can be significant at low pulse numbers. In applications where the desired modulating signal is not known a priori, this phase delay can become a critical limitation. The analogue alternative to regular sampling is naturally sampled PWM, where the modulating signal is permitted to vary as a continuous waveform. This form of PWM does not apply any phase or amplitude distortion to the modulating signal. True natural sampling is, however, impossible to implement on the digital control platforms that are used in modern power electronics applications. In this paper, a type of hold circuit known as a First-Order Hold (FOH) circuit is used in conjunction with the technique known as resampled regular PWM to improve upon the limitations of regular sampling. It is found that this augmentation to the PWM strategy improves both the linearity and phase delay of the modulator as well as (for high pulse numbers) harmonic performance. The FOH circuit has an overshoot in its transfer function which should be avoidable if the modulating signal is sampled at a sufficiently high rate

Optimal stomatal behaviour around the world

Stomatal conductance (g(s)) is a key land-surface attribute as it links transpiration, the dominant component of global land evapotranspiration, and photosynthesis, the driving force of the global carbon cycle. Despite the pivotal role of g(s) in predictions of global water and carbon cycle changes, a global-scale database and an associated globally applicable model of g(s) that allow predictions of stomatal behaviour are lacking. Here, we present a database of globally distributed g(s) obtained in the field for a wide range of plant functional types (PFTs) and biomes. We find that stomatal behaviour differs among PFTs according to their marginal carbon cost of water use, as predicted by the theory underpinning the optimal stomatal model and the leaf and wood economics spectrum. We also demonstrate a global relationship with climate. These findings provide a robust theoretical framework for understanding and predicting the behaviour of g(s) across biomes and across PFTs that can be applied to regional, continental and global-scale modelling of ecosystem productivity, energy balance and ecohydrological processes in a future changing climate

Climate Control of Terrestrial Carbon Exchange across Biomes and Continents

Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predicate future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks1, 2. However, knowledge of even the broad relationships between climate and terrestrial CO2 exchange with the atmosphere on yearly to decadal scales remains highly uncertain. Here we present data describing net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 132 unique sites including various ecosystems over 6 continents with a total of 583 site-years. With respect to controlling factors we find two distinct groupings of sites: (1) a temperature-limited group where NEE has an exponential relationship with mean annual temperature; and (2) a dryness-limited group where NEE has an inverse exponential relationship with the dryness index7. A strong latitudinal dependence emerges, with 92% of the temperature-limited sites located above 42oN, and 77% of the dryness-limited sites located below 42oN. The sensitivity of NEE to mean annual temperature breaks down at a threshold value of ~16oC, above which no further increase of CO2 uptake with temperature was observed and dryness influence overrules temperature influence. Our findings suggest that (1) net ecosystem carbon exchange is highly limited by mean annual temperature at mid- and high-latitudes, and (2) net ecosystem carbon exchange is highly limited by dryness at low latitudes.JRC.H.2-Air and Climat