An experimental study of a high-current vacuum arc (VA) generated between two static CuCr25 contacts spaced 20 or 30 mm apart was conducted to characterize the arc appearance and the cathode spot (CS) distribution.
SiC transistors can achieve blocking voltages of 10kV and more. This makes them especially attractive for energy transmission and distribution. Although SiC devices can in theory operate at high temperature (more than 200°C), the on-state resistance of SiC MOSFETs exhibits a strong dependency on the junction temperature. As a consequence, it is shown that these transistors must actually operate at a relatively low junction temperature (less than 100°C) to increase conversion efficiency and prevent thermal runaway. This requirement for high-performance cooling systems has consequences on the packaging technology: the corresponding power modules must both offer a high voltage insulation and a low thermal resistance. In particular, there is a trade-off in the thickness of the ceramic substrate located between the SiC devices and the cooling system. We propose a new substrate structure, with raised features, which improves the voltage strength of a substrate without increasing its thickness. This structure is demonstrated experimentally.
The role of Modular Multilevel Converters (MMCs) in HVDC grid greatly differs depending on whether it is an offshore or an onshore station. From the common point in their control schemes, an unexploited ability of the MMC—the controllability of the internally stored energy—is identified in both offshore and onshore applications. The virtual capacitor control, previously proposed by the authors, makes use of this degree of freedom to provide energy contribution to the DC grid. The impact of this control is demonstrated by time-domain simulations of a five-terminal HVDC grid.
The objective of this study is to propose a methodology for the transient stability assessment of a simple but representative AC grid in case of DC fault. After validation of the methodology, some HVDC link protection criteria are defined in terms of the Critical Time to Return to Operation. These criteria will be helpful for the design of HVDC protection systems or for the sizing of future HVDC links in order to respect the constraints of the existing protection strategies.
In accordance with their interesting properties, ionic liquids (ILs) have attracted in the recent years a great interest in the material field. Recently, few authors have demonstrated the reactivity of ionic liquids towards epoxy in order to form new epoxy-based networks...
This presentation will provide an update on our current project: designing a cooler for a high power (5 MW) MVDC converter for offshore wind turbines applications. A number of constraints are imposed, mainly related to a limited volume, environmental, safety and health regulations, and of course cooling performance. Indeed, as we presented last year (ATW 2017), the silicon carbide power semiconductors used in this converter should operate at a junction temperature lower than 100 °C for better efficiency.
This paper reviews a few of the major control solutions for AC grid forming and operation of DR-HVDC based OWFs. Then these solutions are compared based on their approach in solving the major challenges involved.
In this paper, a thorough analysis of the converter arm behavior is presented, which gives an analytic expression of the lower limit of the energy as a function of the converter operating point and the connected grid conditions. The relation between the lower energy limit and the operating power is analyzed by using the practical MMC specifications of an HVDC application. An experimental test of a small-scale MMC mock-up demonstrates the validity of the theoretical analysis.
Accurate cable modeling is a recurrent issue for electric architecture evaluation and design, especially in specific contexts, like offshore wind farms. This paper proposes optimal analytical cable models for the technical and economic assessment of offshore wind generation systems.
With a growing number of commercial installations around the world, HVDC technology increased its presence and importance in the power systems. Among various converter topologies, the Modular Multilevel Converters (MMCs) are considered as the most suitable one for HVDC application today. Besides its recognised advantages over conventional converters, the MMC has an interesting extra degree of freedom, which is the energy stored in the distributed cell capacitors. Although the amount of this energy is relatively small, it can provide a significant contribution to the DC system stability when properly used. This paper presents experiment results that demonstrate the effectiveness of virtual capacitor control. This control, previously proposed by the authors, makes use of the above additional degree of freedom to attenuate fluctuations of the DC voltage, which tend to be inherently volatile against power disturbances compared to the frequency of conventional AC systems. Under the virtual capacitor control, the MMC behaves as if there were a capacitor on the DC side of the converter whose size is easily adjusted by the control variable and can be even bigger than the physical capacitor actually embedded in the converter. In practice, the emulation of the capacitor dynamics is realised by the auxiliary control which adjusts the exchange of the energy between the stacked cell capacitors and the DC grid during the transient. Thus, no adverse effect is imposed on the AC grid. Furthermore, the system operator can optionally adjust the equivalent capacitance of the system to achieve desired mitigation level of DC voltage fluctuation during the operation. Therefore, this additional degree of freedom can largely extend the operability of the DC systems. The feasibility and effectiveness of the virtual capacitor control is demonstrated by experimental results obtained by using a small-scale MMC prototype.
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