Abstract-Since passing its commissioning tests in early August 2011, the flexible pulsed dc current source (FPDCS) at ETH's high voltage laboratory has enabled research in different areas of HVDC switchgear as well as its applications with an unprecedented variability and flexibility, establishing many new research opportunities. Over the course of five years of continuous use, a significant number of incremental upgrades were made to hardware, software and to the application practices. Different fields of application were identified and a number of future upgrades were determined, when testing circuit-breaker components, disconnectors and even power semiconductors using FPDCS. In this publication, we strive to share our experiences and recommendations for construction, operation and enhancement of similar current sources for research, development and commercial operation.

Mechanical circuit breakers (MCBs) are the limiting component for current injection HVDC circuit breakers. Improving their interruption performance reduces requirements for capacitance and inductance needed in the injection circuit and thus space use and costs. Higher performance can be achieved by creating a period of low current gradient before zero crossing in the MCB, e.g. by using a saturable inductor (SI). In this paper, the impact of duration and steepness during the low currentgradient phase is linked to arc parameters of the investigated model gas circuit breaker. It is shown in a scaled experimental setup that an optimum design of the SI can be derived from arc time constant and interruption limits for constant current gradients. This optimisation results in a considerable increase of interruption performance. The feasibility of implementing an SI in a full-scale HVDC circuit breaker is demonstrated using simulations. Using an improved injection scheme, the stresses for the MCB can be reduced significantly. Consequently, the injection circuit components can be scaled down, making the topology more economical. The reduced interruption requirements might also make it possible to use a single gas interrupter instead of a series connection of vacuum interrupters, reducing the complexity of the mechanical switch.

Abstract-At the high voltage laboratory at ETH Zurich switching arcs are investigated in order to understand the physical processes that determine the relationship between current and arc voltage in gas blast circuit breakers. Experiments are performed using a breaker prototype with many independently controllable parameters, and very versatile pulsed DC current source. Previous work showed that the gas pressure has a strong influence on the arc voltage, therefore changing the fluid dynamic conditions in which the arc burns can be used to create different dU/dI characteristics. In the presented paper a method to quantify these changes is presented, and the impact of the axial position of the contacts in a model gas circuit breaker on the voltage as function of current are discussed. The influence of the contact position on the average dU/dI curve were small, but the fluctuations around this average value change depending on the nozzle section in which the arc burns. These results will be used to improve the theoretical understanding of the different arc cooling mechanisms, which in turn should enable the design of new geometries that result in a more favorable arc voltage characteristics for passive oscillation HVDC circuit breaker topologies in the future.

DC switchgear using gas circuit breakers (CB) in parallel to a LC-resonant path is used for decades already. This "passive oscillation" topology relies on negative damping of the oscillating current, made possible by the fact that the voltage drop over the CB decreases with increasing current. This concept is cheap and reliable, since it does not require any active components besides the mechanical breaker. However, the achievable breaking times and maximum interruptable current amplitude highly depend on the u(i) characteristic of the axially blown arc inside the breaker. Improvements to the breaking chamber could move the maximal breakable current to higher values, and reduce the time to current zero (CZ). Analyzing which parameters influence the arc voltage is crucial for this optimization. Knowing which axial segments contribute how much to the total arc voltage enables a deeper understanding and possibilities to improve performance. Direct measurements of voltages inside the nozzle of a CB are not feasible, therefore an indirect method was developed. The contact positions in a model circuit breaker were varied systematically, and by evaluating the u(i) curves of 13 different configurations, the voltage drop of eight segments of 2 cm length was calculated. It is shown that most of the voltage drops over the converging nozzle part, where gas density and acceleration is high. These segments also exhibit beneficial behavior, with du/di being negative. The method was validated by comparing the extracted results with suitable direct measurements.

Abstract. The influence of ablation on the du/di behavior of an arc in a model gas circuit breaker was examined. Specifically the transition from a state without ablation in the nozzle towards states with ablation was of interest, since prior work indicated that for high currents the voltage becomes constant or du/di gets even positive if ablation is present. Measurements with different blow pressures and rectangular DC currents of varying amplitude were compared, using PMMA-nozzles and dry air as blowing gas. Ablation was measured by weighing the nozzle, scanning the cross section, and using a coordinate measuring machine. The results agreed well, and confirmed that higher pressure shifts the du/di curve towards more favorable values.

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For the development and optimization of gas circuit breakers and switchgear, a detailed understanding of the arc related processes is of great importance. Ideally, analytical or numerical models with predicitive capability can be found and used during the design process preceding costly and time-consuming experiments.<br /><br />In the present contribution, we report on a novel measurement and evaluation technique to determine the thermal arc time constant ("thermal inertia") that is commonly used in simple black-box models to describe the arc's dynamical properties. The method is introduced and applied to example arcs under varying blow gas conditions in air.<br /><br />

High-speed optical imaging is used in conjunction with fast electrical measurements to advance the understanding of the development of short circuit failures in silicon carbide power MOSFETs. Special samples are manufactured, which are compatible and comparable to TO-247 packages, but do not have any encapsulation. This allows optical observation of die surface during the test. The information on visible processes on the die allows for a better understanding of the sequence of events leading up to a failure. Imagery of destructive drain-source failures is also obtained, as well as post-failure images of surface and cross-sections. Aluminum metal melting is observed even for very short tests, before electrical indications of damage. The onset and completion of melting are used as information on the temperature of the die surface. Using this data for calibration, a detailed electro-thermal model is then used to simulate the temperature distribution and evolution during the short circuit.