Low Current Behavior Research Articles

Compensation of Discontinuous Conduction in Three Phase Voltage Source PWM Inverters

Nowadays the increasing utilization of alternative energy sources demand power converters capable of delivering high quality energy to the low voltage grid. Good dead time compensation and linear operation are essential to keep low order harmonics at acceptable levels. High-current operating conditions are well handled by traditional methods, but low current behaviour and waveforms with multiple zero crossings within a switching period are problematic, especially at low load conditions at or below 10% load. Such conditions often occur in the battery chargers of electric cars. Power quality is also essential for sensorless drives, as the accuracy of the output voltage is very important especially at low rotational speeds. This paper describes the identified cases of discontinuous conduction during dead time. Based on the voltage and current waveforms of the three phase inverter, an improved dead time compensation method is shown for symmetric PWM controlled inverters. The compensator uses already available measurement data, and compensates the resulting error voltage at duty cycle level. The described method can be used for any type of three phase modulation. A similar approach can also be used for two phase modulations.

Analysis and Optimization of Transport Losses in Hydroxide Exchange Membrane Fuel Cells

As an emerging alternative to proton exchange membrane fuel cells (PEMFCs), hydroxide exchange membrane fuel cells (HEMFCs) are more cost-effective due to PGM-free catalysts options, more affordable bipolar plate production, and other potential savings.1 However, unlike PEMFCs, the HEMFC technology is in its early phases and there is a continuous effort to improve the performance and durability through the advancement of materials and optimization of operating conditions. The polarization curve is one of the most common methods of testing a fuel cell. A typical polarization curve for a fuel cell consists of open circuit potential (crossover losses), low-current behavior (kinetics losses), moderate-current behavior (ohmic losses), and high-current behavior (transport losses). Crossover and ohmic losses are determined by electrolytes, kinetics losses depend on catalysts and transport losses involve gas diffusion and water management. In addition, a triple phase boundary comprised of an electrolyte, an electrode and a gaseous reactant is crucial for the electrochemical reactions. Thus, the optimization of all these aspects is critical for HEMFC performance improvement. Here, we focus our work on transport phenomena in a fuel cell. The materials we used are hydroxide exchange membranes (HEMs) and ionomers (HEIs) based on poly(aryl piperidinium) (PAP).2 The specific chemical structure (shown in Figure 1) endows PAP HEMs with excellent chemical stability, high conductivity and mechanical robustness, making PAP HEMs/HEIs one of the best choices for HEMFC electrolyte. Furthermore, the use of PAP HEMs/HEIs enables operating HEMFCs at 95 °C, which accelerates reaction rates and decreases heat rejection. Optimization of fuel cell operating conditions is vital for water management. Water production by hydrogen oxidation reaction (HOR) tends to cause flooding in the anode, while water consumption from oxygen reduction reaction causes cathode to dry-out. In addition, the electro-osmotic drag pulls water from the cathode to the anode (in the reverse direction of PEMFCs), thus worsening the water management problem further. In this work, observable characteristics in the polarization curve for a flooded HEMFC were found, and a model was developed to describe this issue. Then we demonstrate that balanced water management can be obtained by adjusting the anode relative humidity and back pressure. Limiting current technology is widely employed for PEMFCs to characterize the transport resistance in order to reduce mass-transport losses; however, no one has done this systematically to HEMFCs, although there is a difference between limiting current behavior of PEMFCs and HEMFCs. We have found this technique useful to screen catalysts, optimize catalyst loading and adjust ionomer loading to improve the efficiency of gas diffusion, which is very helpful when switching from oxygen to air on cathode. Our optimized HEMFC can achieve a peak power density of 2.02 W cm-2 in H2/O2 and 1.33 W cm-2 in H2/air with platinum group metal (PGM) based catalysts (shown in Figure 2). The current density at 0.663 V for cell voltage is about 1.70 A cm-2, which is suitable for hydrogen fuel cell vehicles. References B. P. Setzler, Z. Zhuang, J. A. Wittkopf, and Y. Yan, Nat Nanotechnol, 11 (12), 1020-1025 (2016).J. Wang, Y. Zhao, B. P. Setzler, S. Rojas-Carbonell, C. Ben Yehuda, A. Amel, M. Page, L. Wang, K. Hu, L. Shi, S. Gottesfeld, B. Xu, and Y. Yan, Nature Energy, (2019) doi:10.1038/s41560-019-0372-8. Figure 1

The Road to Practical E-Textiles Is Smooth as Silk

The work of Zhang et al. describes a method for 3D-printing a triboelectric generator composed of silk fibroin (SF) and a conductive core of carbon nanotubes (CNT). The resulting SF@CNT fibers offer a roadmap toward energy harvesting fabrics within E-textiles.

Substrate cross-conduction effect on the performance of serially connected microbial fuel cell stack

This study presented a new design of scalable, air-cathode microbial fuel cell (MFC) stack that shared a common fuel feed passage. As two individual cells were electrically connected in series by metal wires and hydraulically joined by conductive substrate flow, the performance degradation phenomenon was observed. The open circuit voltage (OCV) and low current behavior of stacked MFC were lower than should be expected. This energy loss was proposed to be a consequence of parasitic current flow due to the substrate cross-conduction effect and can be likely minimized through controlling the distance between the anode electrodes or/and the cross-sectional area of substrate flow. The unique and simple water distribution system of the tubular MFC stack would contribute to the further scale-up and implementation of MFC technologies, especially for wastewater treatment.

The use of commercial thyristors in repetitive high voltage switching devices for plasma sources

This paper presents a commercial high voltage thyristor used as a switch allowing a tank capacitor to discharge in a load. In classical high power pulse technology applications the output voltage pulse has to be characterized mainly by its crest value, its rise-time, the period the thyristor is held in the on-state and the fall-time. These parameters are studied as a function of the power circuit and of the trigger circuit. The thyristor presents two behaviours: the main is either higher or lower than the latching current. The low current behaviour is extensively investigated as it allows repetitive operation of the device. Two pulse power applications triggering electrical discharges are presented. Each one necessitates a specific pulsed power supply using series thyristor stacks or Marx structures. The first pulsed source delivers negative pulses with a crest voltage V oM = -35 kV, a turn on capability of T r = 90 ns and a repetition rate F = 900 Hz. The second is built using Marx structure and is characterized by V oM = 60 kV, T,. = 250 ns, F = 900 Hz.

Quasi-steady state, low current behaviour of a magnetized coaxial plasma source

The Coaxial Plasma Source-1 facility (Mayo R M et al 1995 Plasma Sources Sci. Technol. 4 47) was modified from a short pulse, high current (SPHC) pulse forming network (PFN) with very low inductance (∼200 nH) to a large inductance ladder circuit. This modification allows for a longer, flat top gun current pulse that eliminates the under-damped, sinusoidal behaviour of the gun current with consequent interruptions in plasma parameters. The new PFN was designed to produce a current waveform for a much longer period (∼1 ms). As a consequence of increasing the pulse length, the magnitude of the gun current was reduced as no additional energy storage was added to the PFN. The characterization of the electrical and plasma behaviour of the experiment operated with the long pulse, low current (LPLC) PFN is presented. The gun currents produced by the LPLC PFN are approximately one-fifth in magnitude of the gun currents produced by the SPHC PFN. Axial plasma parameters were measured near the muzzle of the plasma source, and electron densities were found to range from 1 × 1019 m−3 to 7 × 1019 m−3 depending upon the axial location. These values are approximately 1–2 orders of magnitude less than the electron densities produced by the SPHC PFN at the same locations. Electron temperatures range from 30 to 60 eV at these locations and are very similar to those produced by the SPHC PFN. A resistive MHD model was applied as an order estimate of the plasma resistivity and demonstrates reasonable agreement with measured values of the magnetized coaxial gun resistance.

Fundamental studies of Au contacts in MEMS RF switches

Microelectromechanical systems (MEMS) radio frequency (RF) switches hold great promise in a myriad of commercial, aerospace, and military applications. However, there is little understanding of the factors determining the performance and reliability of these devices. Fundamental studies of hot-switched gold (Au) contacts were conducted using a micro/nanoadhesion apparatus as a switch simulator. Experiments were conducted in a well defined air environment under precisely controlled operating conditions. Fundamental properties were connected to performance with an emphasis on the effects of contact force and electric current on contact resistance (R), microadhesion, and reliability/durability. Electric current had the most profound effect on switch performance. Observations at low current (1–10 μA) include: (1) slightly higher R; (2) asperity creep; (3) high adhesion after rapid switching; (4) switch bouncing; and (5) reasonable durability. Conversely, observations at high current (1–10 mA) include: (1) slightly lower R; (2) melting; (3) no measurable adhesion; (4) less propensity for switch bouncing; (5) necking of contacts; and (6) poor reliability and durability due to switch shorting. Low current behavior was dominated by the propensity to form smooth surface contacts by hammering, which led to high van der Waals force. High current behavior was dominated by the formation of Au nanowires that bridge the contact during separation. Data suggest the presence of an adventitious film containing carbon and oxygen. Aging of the contacts in air was found to reduce adhesion.

Pinning phenomena in a superconducting film with a square lattice of artificial pinning centers

We study the transport properties of a superconducting Nb film with a square lattice of artificial pinning centers (APCs) as a function of dc current, at a temperature close to the superconducting transition temperature of the film. We find that, at low dc currents, the differential resistance of the film shows the standard matching field anomaly, that is, the differential resistance has a local minimum at magnetic fields corresponding to an integer number of flux lines per APC. However, at higher dc currents, the differential resistance at each matching field turns to a local maximum, which is exactly opposite to the low current behavior. This effect might indicate that the flux lines in the APC system change their flow mode as the dc current is increased.

Lateral Ionic Conduction in Planar Array Fuel Cells

A performance degradation phenomenon is observed in planar array fuel cells. This effect occurs when multiple cells sharing the same electrolyte membrane are connected in series to build voltage. The open circuit voltage (OCV) and low current behavior of such a series connected planar stack is lower than should be expected. The flow of ionic cross currents between cells in the array, dubbed membrane cross-conduction, is proposed as the likely cause for this loss phenomenon. This hypothesis is confirmed by experimental observations. An equivalent circuit model for a planar double cell is developed which takes into account membrane cross conduction. This model is shown to predict the observed current-voltage behavior of an experimental planar double cell while a simple series model does not. The validated model is used to investigate the impact of various fuel cell parameters on the membrane cross-conduction effect. Design rules are extracted to minimize membrane cross-conduction losses for a linear fuel cell array. It is concluded that the membrane cross-conduction phenomenon primarily affects the OCV and low current density behavior of planar fuel cell arrays. Losses due to membrane cross conduction are minimal for conservative cell spacing, but can be significant for densely packed fuel cell arrays. © 2003 The Electrochemical Society. All rights reserved.

Low-current behavior of silicon transistors

Low-current behavior of silicon transistors