Electrical substation | EPFL Graph Search

A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. They are a common component of the infrastructure. There are 55,000 substations in the United States. Substations may be owned and operated by an electrical utility, or may be owned by a large industrial or commercial customer. Generally substations are unattended, relying on SCADA for remote supervision and control. The word substation comes from the days before the distribution system became a grid. As central generation stations became larger, smaller gene

Dimensional and structural control of silica aerogel membranes for miniaturized motionless gas pumps

Eunho Jeong, Bo Jiang, Nayoung Kim, Matthias M. Koebel, Thomas Maeder, Paul Muralt, Shuangyi Zhao

With growing public interest in portable electronics such as micro fuel cells, micro gas total analysis systems, and portable medical devices, the need for miniaturized air pumps with minimal electrical power consumption is on the rise. Thus, the development and downsizing of next-generation thermal transpiration gas pumps has been investigated intensively during the last decades. Such a system relies on a mesoporous membrane that generates a thermomolecular pressure gradient under the action of an applied temperature bias. However, the development of highly miniaturized active membrane materials with tailored porosity and optimized pumping performance remains a major challenge. Here we report a systematic study on the manufacturing of aerogel membranes using an optimized, minimal-shrinkage sol–gel process, leading to low thermal conductivity and high air conductance. This combination of properties results in superior performance for miniaturized thermomolecular air pump applications. The engineering of such aerogel membranes, which implies pore structure control and chemical surface modification, requires both chemical processing know-how and a detailed understanding of the influence of the material properties on the spatial flow rate density. Optimal pumping performance was found for devices with integrated membranes with a density of 0.062 g cm–3 and an average pore size of 142.0 nm. Benchmarking of such low-density hydrophobic active aerogel membranes gave an air flow rate density of 3.85 sccm·cm–2 at an operating temperature of 400 °C. Such a silica aerogel membrane based system has shown more than 50% higher pumping performance when compared to conventional transpiration pump membrane materials as well as the ability to withstand higher operating temperatures (up to 440 °C). This study highlights new perspectives for the development of miniaturized thermal transpiration air pumps while offering insights into the fundamentals of molecular pumping in three-dimensional open-mesoporous materials. Keywords: aerogel membrane; miniaturized gas pump; sodium silicate; low-temperature cofired ceramics; Knudsen flow

Amer Chemical Soc2015

Study on the Reversible DC-DC Converter as Interface between Low and Medium Voltage DC Networks

Carlo Arrioja, Stephan Kenzelmann, Alfred Rufer

This paper introduces an isolated reversible DC-DC converter with a particular topology, which benefits from both the NPC (neutral point clamped) structure and the series-parallel connection of converters. The key property of the proposed topology is the output voltage elevation above the blocking capabilities of each switch, without taking to a delicate synchronization of series-connected semiconductors. The converter is composed by two identical cells, each containing a full bridge, a medium frequency transformer and an NPC converter, connected in parallel at the input and in series at the output. The operation principle of each cell, into which a trapezoidal modulation was implemented, is similar to a DAB (dual active bridge). A new model improves the dynamic performance of the controller. Simulation and experimental results verify the proposed topology, its control and start-up strategy.

2012

Reversible DC/DC Converter as interface between Low and Medium Voltage DC Networks

Carlo Arrioja, Stephan Kenzelmann, Alfred Rufer

This paper introduces an isolated reversible DC/DC converter with a particular topology, which benefits from both the Neutral Point Clamped (NPC) structure and the series-parallel connection of converters. The key property of the proposed topology is the output voltage elevation above the blocking capabilities of each switch, without taking to a delicate synchronization of series-connected semiconductors. The converter is composed by two identical cells, each containing a full bridge, a medium frequency transformer and an NPC converter, connected in parallel at the input and series at the output. The operation principle of each cell, into which a trapezoidal modulation was implemented, is similar to a Dual Active Bridge (DAB). Simulation and experimental results verify the proposed topology, its control and start-up strategy.

2011

Dimensional and structural control of silica aerogel membranes for miniaturized motionless gas pumps

Eunho Jeong, Bo Jiang, Nayoung Kim, Matthias M. Koebel, Thomas Maeder, Paul Muralt, Shuangyi Zhao

Amer Chemical Soc2015