Primary current

When a battery produces current, the sites of current production are not uniformly distributed on the electrodes (45). The nonuniform current distribution lowers the expected performance from a battery system, and causes excessive heat evolution and low utilization of active materials. Two types of current distribution, primary and secondary, can be distinguished. The primary distribution is related to the current production based on the geometric surface area of the battery constmction. Secondary current distribution is related to current production sites inside the porous electrode itself. Most practical battery constmctions have nonuniform current distribution across the surface of the electrodes. This primary current distribution is governed by geometric factors such as height (or length) of the electrodes, the distance between the electrodes, the resistance of the anode and cathode stmctures by the resistance of the electrolyte and by the polarization resistance or hinderance of the electrode reaction processes. 

Cell geometry, such as tab/terminal positioning and battery configuration, strongly influence primary current distribution. The monopolar constmction is most common. Several electrodes of the same polarity may be connected in parallel to increase capacity. The current production concentrates near the tab connections unless special care is exercised in designing the current collector. Bipolar constmction, wherein the terminal or collector of one cell serves as the anode and cathode of the next cell in pile formation, leads to gready improved uniformity of current distribution. Several representations are available to calculate the current distribution across the geometric electrode surface (46—50). 

The distribution of current (local rate of reaction) on an electrode surface is important in many appHcations. When surface overpotentials can also be neglected, the resulting current distribution is called primary. Primary current distributions depend on geometry only and are often highly nonuniform. If electrode kinetics is also considered, Laplace s equation stiU appHes but is subject to different boundary conditions. The resulting current distribution is called a secondary current distribution. Here, for linear kinetics the current distribution is characterized by the Wagner number, Wa, a dimensionless ratio of kinetic to ohmic resistance. 

Seconday Current Distribution. When activation overvoltage alone is superimposed on the primary current distribution, the effect of secondary current distribution occurs. High overpotentials would be required for the primary current distribution to be achieved at the edge of the electrode. Because the electrode is essentially unipotential, this requires a redistribution of electrolyte potential. This, ia turn, redistributes the current. Therefore, the result of the influence of the activation overvoltage is that the primary current distribution tends to be evened out. The activation overpotential is exponential with current density. Thus the overall cell voltages are not ohmic, especially at low currents. 

Iligh-voltage controllers which regulate prirnaiv input voltage to the rectifier and wiper transformer and house primary current-limiting protection, meters, and instrumentation are designed for local or remote operation. 

Currenl (ratio) error at % of rated primary current... 

This is the point on the magnetic curve of the laminated core of the CT at which the saturation of the core will start. It is defined as the point where an increase of 10% in the secondary voltage will increase the excitation current / i by 50% (Figure 15.19). Beyond this point, a very large amount of primary current would be required to further magnetize the core, thus limiting the secondary output to a required level. 

This is the ratio of instrument limit primary current to the rated primary current. Consequently a high SF will mean a high transformation of the primary current and can damage instruments connected to its secondary. For measuring instruments therefore it is kept low, as it is required to measure only the normal current and not the fault current. 

This is the highest limit of the primary current that can be transformed to the secondary, substantially proportional, complying with the requirement of the composite error (Section 15.6.1). For example, a protection CT 2000/5A represented as 5P10 means that a primary current up to ten times the rated (i.e. up to 2000 x 10 A) will induce a proportional secondary current. The factor 10 is known as the accuracy limit factor as noted below. 

This is the ratio of the rated accuracy limit primary current to the rated primary current. For example, in the above case it is... 

This defines the maximum permissible composite error at the rated accuracy limit primary current, followed by letter P for protection. The standard prescribed accuracy... 

For high set protective schemes, where to operate the protective relays, the primary fault currents are likely to be extremely high, as in the above case. Here it is advisable to consider a higher primary current than the rated for the protection CTs and thus indirectly reduce the ALF and the product of VA X ALF. In some cases, by doing so, even one set of CTs may meet the protective scheme requirement. 

It has been found that, except high set relays, all other relays may not require the ALF to be more than 5. In such cases it is worth while to use two sets of protection CTs, one exclusively for high set relays, requiring a high accuracy limit factor (ALF), and the other, with a lower ALF, for the remaining relays. Otherwise choose a higher primary current than rated, if possible, and indirectly reduce the ALF as illustrated in Example 15.4 and meet the requirement with just one set of protection CTs. 

Table 15.11 Maximum short-time factors obtainable economically corresponding to rated output, accuracy class, accuracy limit factor and rated short-time for wound primary current transformers...

Similarly, for a protection CT from Table 15.11 choose an accuracy class of I OPS with a VA burden of 5 for a second short-time current. If this does not meet the need, the protection CT may also have to be selected with a higher-rated primary current. 

Under energized condition when the CT s secondary is accidentally open circuited, the current will have only the magnetizing path and the voltage Induced across the CT open terminals will be the same as across the magnetizing circuit. Under this situation the magnetizing circuit shall carry the same current as caused by the primary current, which is very high. 

I have decided to use a current transformer to sense the primary current waveform, since resistive methods are impractical in the half-bridge topology. Several transformer manufacturers make current transformers for such a purpose wound on a toroidal core. Coilcraft makes current transformers with 50, 100, and 200 turns on their secondaries. The secondary voltage must be determined in order to have representative current waveforms of the level to work with the control IC. The voltage needed on the output of the current transformer is... 

AVe in this case can be the peak-to-peak voltage of the oscillator ramp if the method of control is voltage-mode, or the maximum peak voltage representing the primary current within the current-mode method of control. The gain can be converted into decibels, which is shown in Equation B.7. 

Appearance potential methods all depend on detecting the threshold of ionization of a shallow core level and the fine structure near the threshold they differ only in the way in which detection is performed. In all of these methods the primary electron energy is ramped upward from near zero to whatever is appropriate for the sample material, while the primary current to the sample is kept constant. As the incident energy is increased, it passes through successive thresholds for ionization of core levels of atoms in the surface. An ionized core level, as discussed earlier, can recombine by emission either of a characteristic X-ray photon or of an Auger electron. 

Ratio and phase angle errors also occur due to the need for a portion of the primary current to magnetize the core and the requirement for a finite voltage to drive the current through the burden. These errors must be small. Current transformers are classified (in descending order of accuracy) into types AT, AM, BM, C and D. Ratio errors in class AT must be within the limits of ( 0.1 per cent for AT and ( 5 per cent for D, while the phase error limit on class A1 is ( 5 minutes to ( 2 minutes in types CM and C. 

In recent years, there has been interest in using zinc as a power-impressed anode for the cathodic protection of steel in concrete. The zinc is flame sprayed onto a grit blasted concrete surface to a final film thickness of approximately 250 m. A primary anode is necessary. Early systems used brass plates as the primary anode, but more recent systems used platinised titanium or niobium wire anodes as the primary current conductor. 

The Haring-Blum cathode is divided into two equal plane areas, distant f 1 and fj from a common anode, and a quantity called the primary current density ratio P is defined as... 

The Hull cell cathode has a continuous variation of current density along its length, and there are equations which give the primary current density at any point not too near the end. If the local thickness is measured at two points for which P is known, Tcan be calculated. The real current distribution is a function of cathode and anode polarisation as well as of the resistance of the electrolyte. The metal distribution ratio will be... 

As A will be a function of current density, T will be a function of electrode area, and comparisons should therefore be made with cells of standard size. Equation 12.12 shows that high throwing indices will result when polarisation rises steeply with current (AE, AEj) and cathode efficiency falls steeply (cj >> f i)- The primary current ratio, P = affects the result because... 

Throwing indices measured in a Hull cell differ from those in a Haring-Blum cell because of the differences in geometry. In a Hull cell several pairs of points can be found which have the same primary current ratio, but for which M and hence T are found to vary because of polarisation changes. 

If the circuit is broken after the e.m.f. has been applied, it will be observed that the reading on the voltmeter is at first fairly steady, and then decreases, more or less rapidly, to zero. The cell is now clearly behaving as a source of current, and is said to exert a back or counter or polarisation e.m.f., since the latter acts in a direction opposite to that of the applied e.m.f. This back e.m.f. arises from the accumulation of oxygen and hydrogen at the anode and cathode respectively two gas electrodes are consequently formed, and the potential difference between them opposes the applied e.m.f. When the primary current from the battery is shut off, the cell produces a moderately steady current until the gases at the electrodes are either used up or have diffused away the voltage then falls to zero. This back e.m.f. is present even when the current from the battery passes through the cell and accounts for the shape of the curve in Fig. 12.1. 

The rotating hemispherical electrode (RHSE) was originally proposed by the author in 1971 as an analytical tool for studying high-rate corrosion and dissolution reactions [13]. Since then, much work has been published in the literature. The RHSE has a uniform primary current distribution, and its surface geometry is not easily deformed by metal deposition and dissolution reactions. These features have made the RHSE a complementary tool to the rotating disk electrode (RDE). 

The primary current distribution is uniform on the hemisphere. Numerical calculations using the potential theory have shown that the current distribution is essentially uniform on the RHSE if the current density is less than 68% of the average limiting current density [47]. 

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