Higher Ohmic Resistances

In grids with resonant grounding, in the event of a current failure the residual ground fault current or the coil current, or parts thereof, can flow for several hours through the resistance. The coil current can be up to 400 A, depending on the size of the grid. Failure currents and voltages on cathodically protected pipes are described in detail in Ref. 8. 

Unnecessary loading of the breakdown fuse through transient overvoltages can be avoided by connection to a r element which consists of a length choke and transverse capacitors. So-called iron core chokes are most conveniently used for series chokes, which are usual in power electronics. A damping element with a 67-juF capacitor is advised at the input and output of the r element. 

Nickel-cadmium cells have a very low ac resistance of 1 mQ. The chaige state of the cells is of secondary importance. Nickel-cadmium cells must have sufficient current capacity and have current stability. They can be used directly as a dc decoupling device (Fig. 14-6) [6]. 

For normal operation, the breakdown fuse is not connected because if it is called upon to act, the cell would be short circuited, which could lead to its destruction. When there is work being done on the cable and the cathodic protection station is switched off, the fuse is connected by closing the switch. The cell (2) can be isolated by removing the connection (4). Finally, by closing the connection (1), direct grounding is established. In installing the cell, the process is carried out in the reverse direction. 

The charge state of the cell must be maintained in operation to have a cell voltage of 0.9 to 1.2 V [6]. Overcharging the cell is to be avoided due to electrolytic decomposition of water and evolution of gas. The cell voltage should therefore not exceed 1.4 V. Cathodic protection stations should be operated so that the cell voltage lies in the desired range. 

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The most recent work comes from Cao et al in 2012 [51]. They developed a poly(methyl vinyl ether- /f-malic anhydride) (PMVMA) membrane by chemical grafting of ammonium groups to the polymer. The membrane was stable up to 150° and showed increasing ion conductivity from room temperature to 60 °C. Their peak power density of 155 mW cm was obtained at 35 °C and on H2/O2 and demonstrated a vast improvement over lower temperatures (and hence higher ohmic resistances) (Figure 2.15). 

With eveiy change in ion concentration, there is an electrical effect generated by an electrochemical cell. The anion membrane shown in the middle has three cells associated with it, two caused by the concentration differences in the boundaiy layers, and one resulting from the concentration difference across the membrane. In addition, there are ohmic resistances for each step, resulting from the E/I resistance through the solution, boundary layers, and the membrane. In solution, current is carried by ions, and their movement produces a fric tion effect manifested as a resistance. In practical applications, I R losses are more important than the power required to move ions to a compartment wim a higher concentration. 

Operating temperature has a significant influence on PEFC performance. An increase in temperature lowers the internal resistance of the cell, mainly by decreasing the ohmic resistance of the electrolyte. In addition, mass transport limitations are reduced at higher temperatures. 

The voltage losses in SOFCs are governed by ohmic losses in the cell components. The contribution to ohmic polarization (iR) in a tubular cell" is 45% from cathode, 18% from the anode, 12% from the electrolyte, and 25% from the interconnect, when these components have thickness (mm) of 2.2, 0.1, 0.04 and 0.085, respectively, and specific resistivities (ohm cm) at 1000°C of 0.013, 3 X 10, 10, and 1, respectively. The cathode iR dominates the total ohmic loss despite the higher specific resistivities of the electrolyte and cell interconnection because of the short conduction path through these components and the long current path in the plane of the cathode. 

The recorded current is caused not only by the heterogeneous electron transfer to the substrate (the Faradaic current ), but also by the current used to charge the electrical double layer, which acts as a capacitor. The measured potentials include the potential drop caused by the ohmic resistance in the solution, the iR drop. Both the charging current ic and the iR drop grows with the sweep rate it is always desirable to compensate for ic and iR drop, but it becomes imperative at higher sweep rates. There exist different ways to compensate electrically for these phenomena, and this makes it possible to operate up to about 103 V sec-1. It is assumed below that the data are obtained with proper compensation. 

Hypochlorous acid formed at the anode by hydrolysis of dissolved chlorine reacts with hypochlorite according to the equation (XII I-14). This undesirable process can be partially suppressed by maintaining a low temperature of the electrolyte. The cooling of the solution is still more important when higher current densities are used. A low temperature is, however, disadvantageous as far as energy consumption is concerned, as the ohmic resistance of the electrolyte increases. 

In analyzing the polarization data, it can be seen that the cathodic reaction on the copper (oxygen reduction) quickly becomes diffusion controlled. However, at potentials below -0.4 V, hydrogen evolution begins to become the dominant reaction, as seen by the Tafel behavior at those potentials. At the higher anodic potentials applied to the steel specimen, the effect of uncompensated ohmic resistance (IRohmk) can be seen as a curving up of the anodic portion of the curve. 

The choice of the anion is most important in anodic reactions. Perchlorates have been found very useful as they are difficult to oxidize and are often soluble both in water and nonaqueous solvents. In anodic (Section VI, F) or cathodic (Section IV, A) substitution reactions the nucleophilicity of the anion is of interest. High concentrations of tetraalkylammonium p-toluenesulfonates in water make the solubility of organic compounds higher than in pure water, and such solutions combine a low ohmic resistance with good dissolving power. 

The higher the applied current is, the more critical becomes the design of the cell the ohmic resistance must be kept low, and it is especially important that the tip of the reference electrode (the Luggins capillary ) ends close to the working electrode. Otherwise, the inevitable potential drop due to the ohmic resistance between the working electrode and the Luggins capillary (the /.R-drop ) becomes intolerably great.58,57... 

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