Special-purpose current transformers

Protective current transformers, for special purpose applications Part-4/1992 BS 7626/1991 ... 

Protective current transformer for special purpose applications part-3/1992 part 4/1992 ... 

The high impedance is usually a resistance chosen to limit the earth current to about 20 mA. A current detector is used in conjunction with the resistance to raise an alarm if a line-to-earth fault occurs. A zig-zag transformer, or reactor, is sometimes used with three-wire supplies such as used in drilling rigs and emergency systems. It is specially designed and internally connected to create a very low zero sequence impedance to earth currents. Therefore, the current is limited only by the resistance of the neutral earthing resistor. Some special purpose earth current alarm systems are available that inject a small DC current into the three-phase system, which is used to identify the actual location of the fault. 

Fig. 8.11 - Schematic of the main processes in Fourier transform analysis. In this case the excitation is by pseudo-random noise, but other forms such as multifrequency sine wave excitation can be used. The signal processor is a special purpose computer that carries out timed sampling of a fixed number of points which are transformed to give the real and imaginary components of the cell current as a function of frequency.

Procedures of data treatment were as follows. Firstly, the special integration of experimental voltammograms, transformed into i-t functions, is performed with Eq. (11.9) and Acj [ quantities, as E functions, are obtained. The second step consists in determination of distribution of proton donors at the electrode surface under cathodic polarization conditions. For this purpose, material balance equations are used. When increasing cathodic overvoltage, surface decreases from the bulk value (initial state) to zero (limiting current region). It must be emphasized that the surface concentration of the electrically active substance (hydronium ions) does not fall to zero, in contrast to common redox processes. In the absence of proton donors, a neutral medium is created at the electrode surface ([H+]j = [OH ]j), otherwise an alkalization occurs(see Eq. (11.7)atCH = 0). 

Infrared (IR) spectra of organic compounds are characteristic of various functional groups in the molecules. IR spectral information is somewhat complementary to mass spectral information. Therefore, the combination of GC with IR spectroscopy is, after GC/mass spectrometry, the second most important structural identification tool. Since conventional IR spectroscopy is less sensitive than most GC detectors, the necessary sensitivity enhancement is achieved through the use of Fourier transform techniques. With the advent of refined optical systems and fast computational techniques, the combination of GC with Fourier-transform IR spectrometry is becoming widely used, although its sensitivity is currently less than that of mass spectrometry. Special optical cells were designed for the purposes of this combination. 

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