Temporary voltage

Electrical trees consist of visible permanent hoUow channels, resulting from decomposition of the material, and show up clearly in polyethylene and other translucent soHd dielectrics when examined with an optical microscope. Eresh, unstained water trees appear diffuse and temporary. Water trees consist of very fine paths along which moisture has penetrated under the action of a voltage gradient. Considerable force is required to effect this... 

In actual operation, disturbances on a power system, causing sudden changes in the system parameters, are quite frequent and may generate temporary overvoltages and voltage surges, as summarized above. The system disturbances may be of two types, external or internal, as explained below. 

Table 24.3 Permissible voltage variations during temporary system disturbances...

I Shunt reactors These are provided as shown in Figure 24.23 to compensate for the distributed lumped capacitances, C , on EHV networks and also to limit temporary overvoltages caused during a load rejection, followed by a ground fault or a phase fault within the prescribed steady-state voltage limits, as noted in Table 24.3. They ab.sorb reactive power to offset the charging power demand of EHV lines (Table 24.2, column 9). The selection of a reactor can be made on the basis of the duty it has to perform and the compensation required. Some of the different types of reactors and their characteristics are described in Chapter 27. 

Most electrochemiluminescent systems rapidly decay with time (cf. Fig. 6), even when strenuous efforts are made to exclude impurities and when applied voltages are minimal. Some unusual temporary increases in emission intensity with time have also been noted.63 These have not yet been satisfactorily explained. 

Pocket ionization chambers are small enough to be worn like a ball-pen. They are charged by a temporary connection to a voltage source, and the residual charge after exposure is read on the scale of an electrometer fitted in the ionization chamber. Pocket ion chambers are not sensitive to a and low-energy f radiation. 

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