Volatile organic compounds (VOCs) play a vital role in the global carbon budget and in the regional formation of ozone in the troposphere, and are emitted from both natural and anthropogenic ...activities. They can also serve as a source of secondary organic aerosol (SOA). Field and model studies showed evidences of a strong marine biogenic influence on marine aerosols. Although knowledge of terrestrial VOC emissions and SOA formation mechanisms has been advanced considerably over the last decades, processes constraining marine VOC emissions and marine SOA formation remain poorly understood. Seawater contains an extremely complex, diverse, and largely unidentified mixture of VOCs. Despite the fact that the ocean covers 70% of the Earth's surface, the role of the ocean in the global budget of VOCs is still unclear. The distribution and emission of sea surface VOCs exhibit considerable spatial-temporal variation, with higher concentrations often, but not always, correlated with biological activities. VOCs in surface seawater have been measured in various geographic regions, however, knowledge of the distribution of marine VOCs and the role of the oceans in the global atmospheric chemistry is still insufficient due to the paucity of measurements. This study reviews marine VOCs in terms of current analytical methods, global marine VOCs measurements, their effects on SOA, and future needs for understanding the role of marine VOCs in the chemistry of the atmosphere.
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•Discussed various sample pre-treatment and mass spectrometry-based detection techniques•Reviewed reported important marine VOCs and their spatial-temporal distributions•Reviewed the contribution of marine VOCs to the marine aerosols
Solid-state transformers (SSTs) enable electric energy distribution with higher flexibility, sustainability and efficiency, and the AC/DC hybrid microgrid (MG) is one of the representative ...applications. However, the design and analysis of SSTs and MGs were limited by lack of fast simulation tools that can accurately and efficiently handle systems with massive number of switching devices. This article demonstrates the discrete-state event-driven (DSED) approach which allows an accurate and faster simulation of the converters than the state of the art. The numerical prototyping of a hybrid MG composed of a megawatt SST together with multiple converters (e.g. PVs, EVs and batteries) is presented in this article. The performance is compared with existing simulation approaches. Various operation modes are investigated, including the grid-connected mode, the islanded mode, and the mode transition considering the communication delay. A time cost of several hours is decreased to tens of seconds with same accuracy using DSED, enabling faster simulations for complicated tasks which can benefit the SST and hybrid MG design.
Power electronic systems are intrinsically hybrid systems, consisting of continuous states and discrete events. The hybrid nature makes their accurate and efficient simulation challenging to achieve. ...A novel approach called discrete state event-driven (DSED) is able to solve such hybrid systems efficiently, but it shows unsatisfying simulation speed when calculating circuits containing parasitic parameters, namely stiff systems. Since the effect of parasitic parameters brought by connection lines can be destructive when they produce voltage peak or resonance, it is crucial to evaluate the impact of parasitic elements during the design phase of the converters by simulation. This paper proposes a backward DSED (BDSED) approach that can solve stiff systems efficiently by cooperating with the event-driven framework. The BDSED adopts a semi-variable-step-variable-order (S-VSVO) mechanism for integrating continuous states and uses interpolation method for dealing with discrete events. With this simulation approach, the effect of parasitic elements in power electronic systems can be analyzed more efficiently compared to other commercial software. In a case study, the proposed approach shows 60 times faster in simulation speed compared with ode15s in Simulink and more than 3 times faster compared with stiff solver in a commercial software called PLECS at the same level of accuracy.
Modeling and simulation have emerged as an indispensable approach to create numerical experiment platforms and study engineering systems. However, the increasingly complicated systems that engineers ...face today dramatically challenge state-of-the-art modeling and simulation approaches. Such complicated systems, which are composed of not only continuous states but also discrete events, and which contain complex dynamics across multiple timescales, are defined as generalized hybrid systems (GHSs) in this paper. As a representative GHS, megawatt power electronics (MPE) systems have been largely integrated into the modern power grid, but MPE simulation remains a bottleneck due to its unacceptable time cost and poor convergence. To address this challenge, this paper proposes the numerical convex lens approach to achieve state-discretized modeling and simulation of GHSs. This approach transforms conventional time-discretized passive simulations designed for pure-continuous systems into state-discretized selective simulations designed for GHSs. When this approach was applied to a largescale MPE-based renewable energy system, a 1000-fold increase in simulation speed was achieved, in comparison with existing software. Furthermore, the proposed approach uniquely enables the switching transient simulation of a largescale megawatt system with high accuracy, compared with experimental results, and with no convergence concerns. The numerical convex lens approach leads to the highly efficient simulation of intricate GHSs across multiple timescales, and thus significantly extends engineers’ capability to study systems with numerical experiments.
At present, power electronic transformers (PETs) have been widely used in power systems. With the increase of PET capacity to the megawatt level. the problem of increased losses need to be taken ...seriously. As an important indicator of power electronic device designing, losses have always been the focus of attention. At present, the losses are generally measured through experiments, but it takes a lot of time and is difficult to quantitatively analyze the internal distribution of PET losses. To solve the above problems, this article first qualitatively analyzes the losses of power electronic devices and proposes a loss calculation method based on pure simulation. This method uses the Discrete State Event Driven (DSED) modeling method to solve the problem of slow simulation speed of large-capacity power electronic devices and uses a loss calculation method that considers the operating conditions of the device to improve the calculation accuracy. For the PET prototype in this article, a losses model of the PET is established. The comparison of experimental and simulation results verifies the feasibility of the losses model. Then the losses composition of PET was analyzed to provide reference opinions for actual operation. It can help pre-analyze the losses distribution of PET, thereby providing a potential method for improving system efficiency.
Real-time (RT) hardware-in-the-loop (HIL) simulation aims to speed up the validation process for power electronic systems (PES). The complex PESs with high switching frequency constitute some of the ...most challenging applications in RT-HIL. Conventional RT-HIL relies on adding extra expensive computing hardware to achieve submicrosecond step size, reducing errors caused by unavoidable sampling delays. This article proposes a CPU-based event-driven RT (EDRT) simulation framework by improving the algorithm rather than using additional hardware. The framework consists of two parts: 1) the synchronous-cycle event detection sampling method, which eliminates the delay error by detecting switching events; and 2) the discrete hybrid time-step numerical algorithm, which combines variable and fixed step-size simulation to improve the calculation efficiency and uses the ideal model to improve the modeling accuracy. The proposed framework is applied to a power electronic transformer with 24 switches and a 20 kHz switching frequency as a simulated case. Comparing the proposed simulation results with experimental results and other simulation results, the proposed EDRT framework can achieve the same numerical accuracy as the offline simulation but only requires 1/36 of the computation time. Furthermore, the hardware cost to achieve the same computational scale is reduced to 1/20 of the conventional HIL.
Simulation is an efficient tool in the design and control of power electronic systems. However, quick and accurate simulation of them is still challenging, especially when the system contains a large ...number of switches and state variables. Conventional general-purpose integration algorithms assume nonlinearity within systems but face inefficiency in handling the piecewise characteristics of power electronic switches. While some specialized algorithms can adapt to the piecewise characteristics, most of these methods require systems to be piecewise linear. In this article, a numerical derivative-based flexible integration algorithm is proposed. This algorithm can adapt to the piecewise characteristic caused by switches and have no difficulty when nonlinear nonswitching components are present in the circuit. This algorithm consists of a recursive numerical scheme that obtains high-order time derivatives of nonlinear components and a decoupling strategy that further increases computational efficiency. The proposed method is applied to solve a motor derive system and a large-scale power conversion system to verify its accuracy and efficiency by comparing experimental waveforms and simulated results given by commercial software. Our proposed method demonstrates several-fold acceleration compared to multiple commonly used algorithms in Simulink.
The large-scale adoption of power electronics systems enables a more sustainable and flexible power grid. The offline simulation tools support the analysis of such systems. However, despite of the ...advancement in computer hardware and software, the accurate and efficient simulation of large-scale power converters is still a significant challenge; the simulation is usually time-consuming. In this article, we propose a state-variable-interfacd decoupling (SVID) strategy based on the discrete state event-driven (DSED) approach. The SVID strategy partitions the large-scale circuit into smaller subsystems with state variables as interfaces so that the numerical integration can be performed in a decoupled manner. Meanwhile, it ensures that no accuracy is sacrificed after decoupling by representing the interfaced variables with the high-order Taylor expansion. A 10-kV 2-MW solid-state transformer with 578 power switches is studied as a simulated case, and more than 1000-fold acceleration (from 3 h to 12 s) is achieved compared with commercial software, where the SVID strategy contributes to five- to eightfold. The simulated results are compared with both explicit and implicit solvers in commercial software and experimental results. The DSED approach with SVID strategy makes it possible to analyze and design a high-power converter more reliably and perform studies that are very difficult or impossible to do otherwise.
Accurate and efficient simulation of power electronic systems is challenging due to their hybrid nature consisting of continuous states and discrete events. In this paper, a comprehensive discrete ...state event-driven (DSED) framework is proposed for offline simulation of such hybrid dynamic systems. Two key elements of the proposed comprehensive DSED simulation framework that contribute to the significant improvements in numerical accuracy and computational efficiency are a flexible adaptive algorithm for flexible integration of continuous states and an event-driven mechanism for efficient locating of discrete events. The proposed DSED simulation framework is applied to a 50-kVA solid-state transformer and a multileg full-bridge Class-D amplifier to verify its accuracy and efficiency by comparisons with experimental waveforms and simulated waveforms of Simulink and PLECS. Up to 100-fold and 10-fold improvements in simulation speed are achieved at the same level of numerical accuracy compared with Simulink and PLECS, respectively.
In the analysis of power electronics system, it is necessary to simulate ordinary differential equations (ODEs) with discontinuities and stiffness. However, there are many difficulties in using ...traditional discrete-time algorithms to solve such equations. Kofman and others presented the quantized state systems (QSS) algorithm in the discrete event system specification (DEVS) formalism. The discretization is applied to the state variables instead of time range in QSS. QSS is efficient to solve ODEs, but it is difficulty to be used when simulating actual power electronics systems with controller’s and other events. Based on the idea of this numerical algorithm and discrete event, a Discrete State Event Driven (DSED) simulation method is presented in this paper, which is fit for simulation of power electronics system. The method is developed to deal with non-linearity, stiffness and multi-time scale of power electronics systems. The DSED simulation method includes event definition, module seperation and modeling, event-driven mechanisms, numerical computation based on QSS, and some other operations. Simulation results verified the effectiveness and validity of the proposed method.
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