Why are ceramic materials (when compared to organic materials) necessary for the purpose of electric isolation in power modules, especially for medium voltage applications...
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.
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.
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.
This paper reports the design, the processing, the static characterisation, the switching behaviour and the high current stress test of 10 kV aimed 4H-SiC bipolar diodes. The actual breakdown voltage of the selected devices is between 7 kV and 8 kV. The switching characterisations show a good behaviour with a t rr of only 90 ns. No degradation was observed after the application of 10 000 high current pulses during the stress tests.
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.
This paper presents a novel protection scheme based on converter breaker; its key element, namely a low-speed mechanical DC breaker, is located at each DC converter output.
This paper presents an assessment methodology of protection strategies for meshed grids. It also proposes the computation of two performance indicators to evaluate protection strategies through a reliability and speed perspective. The Monte Carlo method is used to calculate the two indicators proposed. These two indexes can be used as criteria for comparison between protection strategies. Due to the increasing debate around the protection for HVDC grids, three proposals of protection of HVDC grids were chosen as application cases.
This study proposes a non-selective protection strategy for multi-terminal high voltage direct current grids based on resistive-type superconducting fault current limiters (SFCLs). Located at the output of AC/DC converters, the SFCL limits the current contribution from the AC grid in case of DC fault. With this approach, the fault clearing time constraint is relieved allowing the use of mechanical DC circuit breakers for fault current interruption. Furthermore, the breaking capability and energy dissipation requirements of the breaker are greatly reduced. To achieve a fast restoration of the DC grid, a redundant SFCL is introduced in parallel to those in operation, bypassing them when the fault is effectively suppressed. In addition, primary and back-up protection schemes are described and tested using a three-terminal bipolar HVDC grid based on half-bridge modular multilevel converters and cable transmission. Simulations are implemented in EMTP-RV® to analyse and discuss performances of the proposed fault clearing strategy.
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