Performance analysis of the RC clampers in switching off no load arc furnace transformers
This paper presents an over-voltage attenuation performance analysis of RC clampers and surge arresters during transients caused by switching off a no load transformer applied to an arc furnace. A Brazilian Industry actual system was simulated using the Matlab/Simulink software. The simulation results indicate that the adequate use of RC clampers can reduce the arresters operations and practically eliminates the re-strikes and repetitive transient over-voltages when using vacuum circuit breakers to switch the arc furnace transformer.
Detailed study of fast transient phenomena in transformers and substations leading to an improved system design
In recent years several unusual failures were reported in medium voltage distribution systems. Some conventional dry-type and oil-filled transformers had clearly been exposed to a specific type of overvoltage. During thorough lab and factory investigations, no design or manufacturing faults could be found. Internal winding resonances were generally accepted to be the cause of the failure. This paper describes typical symptoms, critical factors and some ideas for appropriate protection and total system design, based upon experimental results from laboratory and on-site measurements. Laboratory tests are performed to investigate the high frequency behaviour of layer windings in distribution transformers. It is shown that multiple reignitions in vacuum circuit breakers can initiate internal winding resonances. Impulse tests confirm the resonance patterns at the natural frequencies. Long term high frequency measurements are performed in a medium voltage substation where problems have been detected before. The main purpose of the paper is to make system designers more aware of fast transient phenomena and to show that they represent a larger problem than generally accepted.
Stability of planar PEMFC in Printed Circuit Board technology
Abstract The use of planar PEMFCs in printed circuit board (PCB) technology with a thickness of less than 3.5 mm are presented. This planar design consists of an open cathode side which allows a completely passive, self-breathing operation of the fuel cell. Power densities of 100 mW/cm 2 at 500 mV with hydrogen as a fuel were achieved. A steady operation with this type of fuel cell was demonstrated over a week without any cell flooding. Since the electrical conducting elements of PCBs usually are made of copper, corrosion in the wet environment of a PEMFC is expected. Thus, a significant degradation in performance of fuel cells made of plain copper PCBs was seen in long-term operation. In order to avoid corrosion, the copper layer has to be coated. Fuel cells in PCB design with diverse coatings were tested in long-term operations up to 1000 h under load. Moreover, corrosion currents have been determined by the use of potentiodynamic scans. Promising coatings are electroplated Cr and Ni and a combination of Ni/Au.