Constant Direct Current Voltage Research Articles

A Communication‐Free Master–Slave Control of Cascaded‐Type DC Microgrids With Nondispatchable Generations

ABSTRACTThis paper addresses the challenge of integrating dispatchable and nondispatchable distributed generators (DGs) in cascaded‐type direct current (DC) microgrids and proposes a communication‐free master–slave control strategy. In the proposed method, all DGs are treated as voltage sources. A master dispatchable DG is primarily responsible for maintaining a constant DC voltage at the point of common coupling (PCC). The rest slave nondispatchable DGs adjust their output power based on a front maximum power point tracking (MPPT) controller. This communication‐free approach enhances system reliability and ease of implementation. Moreover, it ensures load voltage stability, providing a high‐quality power supply for users. Additionally, it enables nondispatchable DGs to achieve MPPT‐based output power and enables power curtailment operation. Subsequently, a small‐signal stability analysis confirms the efficacy of the proposed strategy. Finally, simulation and experiment results further validate its efficacy.

Characteristics of Energy Conversion and Temperature Response of Coal Excited by a DC Electric Field.

This study is aimed at investigating the characteristics of energy conversion and temperature response of coal excited by a direct current (DC) electric field (CEEDCEF). First, factors influencing energy conversion and temperature response of CEEDCEF were theoretically analyzed. Based on the analysis, the temperature distribution law of coal under different excitation conditions was simulated using the COMSOL software. The results of theoretical analysis and numerical simulation were verified through an experiment on the temperature distribution on the surface of CEEDCEF. Finally, the energy conversion mechanism and temperature response characteristics of CEEDCEF were revealed. The research results show that loading time and the loading voltage are the main factors influencing the temperature rise of CEEDCEF. Under the excitation of 6000 V constant DC voltage, the internal temperature at the lower end face of CEEDCEF increased from 29.4 to 92 °C within 20-90 min, the sections of the internal temperature increased from 36 to 94 °C under different voltage excitations of 3000-6000 V. Moreover, the temperature rise response process is divided into three stages, i.e., slow warming, fast warming, and slow cooling into stabilization. The coal shows a "capacitance effect" in the early stage of DC electric field excitation and a "resistance effect" after the charge reaches saturation. In addition, the temperature surges when the free radicals in the macromolecular structure of the coal turn into a current beam. With the increase in excitation time, the electrical parameters of the coal tend to be stable, and the surface temperature slowly decreases and stabilizes accordingly. The research results provide theoretical support for the gas production mechanism of the coal stimulated by the electric field and exploring methods for the monitoring and prewarning of these dynamic disasters.

Nanopipette-Based Nanosensor for Label-Free Electrochemical Monitoring of Cell Membrane Rupture under H2O2 Treatment.

H2O2 is an essential signaling molecule in living cells that can cause direct damage to lipids, proteins, and DNA, resulting in cell membrane rupture. However, current studies mostly focus on probe-based sensing of intracellular H2O2, and these methods usually require sophisticated probe synthesis and instruments. In particular, local H2O2 treatment induces cell membrane rupture, but the level of cell membrane destruction is unknown because the mechanical properties of the cell membrane are difficult to accurately determine. Therefore, highly sensitive and label-free methods are required to measure and reflect mechanical changes in the cell membrane. Here, using an ultrasmall quartz nanopipette with a tip diameter less than 90 nm as a nanosensor, label-free and noninvasive electrochemical single-cell measurement is achieved for real-time monitoring of cell membrane rupture under H2O2 treatment. By spatially controlling the nanopipette tip to precisely approach a specific location on the membrane of a single living cell, stable cyclic membrane oscillations are observed under a constant direct current voltage. Specifically, upon nanopipette advancement, the mechanical status of the cell membrane can be sensibly displayed by continuous current versus time traces. The electrical signals are collected and processed, ultimately revealing the mechanical properties of the cell membrane and the degree of cell apoptosis. This nanopipette-based nanosensor paves the way for developing a facile, label-free, and noninvasive strategy to assay the mechanical properties of the cell membrane during external stimulation at the single-cell level.

Helical micromotor operating under stationary DC electrostatic field.

The direct current (DC) motor is a rotary device that converts DC electrical energy into mechanical energy. However, it is known that, in downsizing the currently available macromotor, rotary motion of DC micromotors cannot work well due to the larger viscous effect. Here, we report simple DC micromotors working under a new principle. We previously revealed that in an oil phase containing an ionic surfactant, non-spherical particles exhibit various types of regular motions such as spinning and circular orbital motions. In this study, we found that a microhelix exhibits a new type of periodic motion, namely, the cork-screw-type rotation, in a specific direction depending on the material of the helix, metal or non-metallic organics. The results show that a left-handed nickel helix rotates in the clockwise direction when viewed from the positive electrode, whereas an organic one rotates in the opposite direction (anti-clockwise) under the same electrode arrangement with stationary constant DC voltage. In addition, we demonstrate that the cork-screw rotation is switched to opposite direction by changing the handedness (chirality). It is to be noted that the micromotors reported here maintain their stable motion without any mechanical support such as rotational axes or electronic switching devices. The invented DC micromotor would be applicable for mechanical and fluidic devices, being useful as a smart device in microrobots and microfluidics.

Dynamo Speed Control and Tectonics—Modeling Earth as a Shunt Wound DC Machine

The Sun’s slow periodic flux transfer to the Earth, the low frequency of Schumann Resonance, and the fixed DC voltage of the capacitor direct us toward direct current (DC) machines for electrical modeling purposes. The Earth exhibits dual characteristics of a motor generator set by motoring the mechanical Earth around its axis, while at the same time generating energy for its spherical capacitor. It follows that electrical and mechanical output of the Earth are powered by the magnitude of the flux transfer events, the constant DC voltage supply and any potential nuclear contribution within the core. Like an induction furnace, powerful magnetic flux from the Sun partially melts the outer iron core of the Earth and magnetizes the inner solid iron core. The solid inner magnetic core acts as a rotating armature similar to a DC machine. All electrical machines experience no load and full load power loss while in operation. Speed control of large rotating DC machines is well understood and has been applied in industry for over a century. Speed can be changed either by varying the field resistance and/or the armature resistance. The characteristic of a constant speed DC machine is such that a change in field resistance will cause a compensatory change in armature resistance to maintain velocity. In the case of the earth, a decrease in armature resistance results in an increase in volume of the iron core, which may result in greater seismic and volcanic activity. Climate change may be the direct result of changes in soil and sea water resistance, which we lump together as field resistance.

Electroremediation of Lead-Contaminated Kaolinite using Cation Selective Membrane and Different Electrolyte Solutions

Electrokinetic extraction, which is an emerging technology, can be used for in situ removal of contaminants by the application of a direct current (DC) electric field across the contaminated subsurface soil. In this study, a kaolinite spiked with Pb (720 mg kg−1) was used to investigate the removal of Pb through electrokinetic extraction in the presence or absence of a cation selective membrane (CSM). The contaminated kaolinite was subjected to a constant DC voltage (2 V cm−1) for 4 days. A low DC voltage applied in absence of CSM developed a high pH interface within the electrokinetic cell. The mobility of Pb thus decreased due to the remarkable rise in the pH values. The inclusion of a CSM improved the removal efficiency but could not achieve the critical surface pH (<pH 3.5). The removal efficiency increased in the presence of Ca(NO3)2, such that 95 % of the spiked Pb was extracted from kaolinite, with 88 % of the Pb being recovered from the cathode chambers. In contrast, only 24 % of the spiked Pb was removed in the presence of ethylenediaminetetraacetic acid (EDTA). Moreover, Pb was detected in the anode chamber solution and 60 % of the spiked Pb accumulated at the anode. In the presence of both EDTA and Ca(NO3)2, Pb migrated and accumulated at the anode, thus indicating that Ca has a negligible effect in the presence of EDTA.