Transformers short-time rating

Since the transformer will be in the circuit for only 15 to 20 seconds, the approximate short-time rating of the transformer can be considered to be 10-15% of its continuous rating. The manufacturer of the auto transformer would be a better judge to suggest the most appropriate rating of the transformer, based on the tapping and starting period of the motor. 

Note The above example is only for a general reference. The CDF of the transformer, for the short-time rating, should be increased writh the starting time and the number of starts per hour. Refer to the transformer manufacturer for a more appropriate selection. 

Note With the availability of modern circuit breakers with higher short-time ratings of 65 kA/80 kA/100 kA, it is now possible to use even larger transformers up to 2500 kVA, depending upon their other merits. 

This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays will carry any inrush current during a switching operation or a fault. No short-time rating is thus assigned to such transformers. 

As discussed in Section 13.4.1(5). these sections are under the cumulative influence of two pow er sources and may be tested for a higher short-time rating, which would be the algebraic sum of the two fault levels, one of the generator and the other of the generator transformer as noted in Table I 3.8. Also refer to Figures 31,1 and 13.18 for more clarity. 

For the tap-offs, connecting a UAT through the main bus section between the generator and the generator transformer, however, as discussed above, the momentary peak current will depend upon the short-time rating of such tap-offs. The likely ratings are noted in Table 13.8. 

Refer to Figures 15.8 and 15.9. These transformers are quite different from a measuring or a protection transformer, particularly in terms of accuracy and short-time VA ratings. They are installed to feed power to the control or the auxiliary devices/components of a... 

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...

Both time-related failure rates and demand-related failure rates can apply to and be reported for many pieces of equipment. Both types of rates are included in some of the data tables in Chapter 5. If a piece of equipment is in continuous service, such as a transformer, the failure rate is dominated by time-related stresses compared to demand-related stresses. Other failure rates may be dominated by demands. Take a piece of wire and repeatedly bend it. With each bend its probability of catastrophic failure increases. In a relatively short time, if the bending is continued, the wire will fail. On the other hand, the same wire could be installed in a manner that would prevent mechanical bending demands. In this case, the occurrence of catastrophic wire breakage would be remote. In the first instance, the failure rate is dominated by demand stresses and in the second by time-related stresses, such as corrosion. 

A solid compound X is transformed into Y when it is heated at 75 °C. A sample of X that is quickly heated to 90 °C for a very short time (with no significant decomposition) and then quenched to room temperature is later found to be converted to Y at a rate that is 2.5 times that of a sample that has had no prior heating when both are heated at 75 °C for a long period of time. Explain these observations. 

Recently, Darowicki [29, 30] has presented a new mode of electrochemical impedance measurements. This method employed a short time Fourier transformation to impedance evaluation. The digital harmonic analysis of cadmium-ion reduction on mercury electrode was presented [31]. A modern concept in nonstationary electrochemical impedance spectroscopy theory and experimental approach was described [32]. The new investigation method allows determination of the dependence of complex impedance versus potential [32] and time [33]. The reduction of cadmium on DM E was chosen to present the possibility of these techniques. Figure 2 illustrates the change of impedance for the Cd(II) reduction on the hanging drop mercury electrode obtained for the scan rate 10 mV s k... 

The cure of thermoset resins involves the transformation of a liquid resin, first with an increase in viscosity to a gel state (rubber consistency), and finally to a hard solid. In chemical terms, the liquid is a mixture of molecules that reacts and successively forms a solid network polymer. In practice the resin is catalyzed and mixed before it is injected into the mold thus, the curing process will be initialized at this point. The resin cure must therefore proceed in such a way that the curing reaction is slow or inhibited in a time period that is dictated by the mold fill time plus a safety factor otherwise, the increase in viscosity will reduce the resin flow rate and prevent a successful mold fill. On completion of the mold filling the rate of cure should ideally accelerate and reach a complete cure in a short time period. There are limitations, however, on how fast the curing can proceed set by the resin itself, and by heat transfer rates to and from the composite part. 

Analysis In standard applications, the short-time Fourier transform analysis is performed at a constant rate the analysis time-instants / are regularly spaced, i.e. tua =uR where R is a fixed integer increment which controls the analysis rate. However, in pitch-scale and time-scale modifications, it is usually easier to use regularly spaced synthesis time-instants, and possibly non-uniform analysis time-instants. In the so-called band-pass convention, the short-time Fourier transform X (t",Q.k) is defined by ... 

Suppression rules. Let X(p,Qk) denote the short-time Fourier transform of x[ri, where p is the time index, and Qk the normalized frequency index (0t lies between 0 and 1 and takes N discrete values for k = 1,N, Wbeing the number of sub-bands). Note that the time index p usually refers to a sampling rate lower than the initial signal sampling rate (for the STFT, the down-sampling factor is equal to hop-size between to consecutive short-time frames) [Crochiere and Rabiner, 1983]. 

Water uptake in plasticized polyvinylchloride based ion selective membranes is found to be a two stage process. In the first stage water is dissolved in the polymer matrix and moves rapidly, with a diffusion coefficient of around 10 6 cm2/s. During the second stage a phase transformation occurs that is probably water droplet formation. Transport at this stage shows an apparent diffusion coefficient of 2 x 10 8 cm2/s at short times, but this value changes with time and membrane addititives in a complex fashion. The results show clear evidence of stress in the membranes due to water uptake, and that a water rich surface region develops whose thickness depends on the additives. Hydrophilic additives are found to increase the equilibrium water content, but decrease the rate at which uptake occurs. 

Actually, the rate of chemical transformations affects the rate of growth of the layers at all stages from the start of the interaction of initial substances to the establishment of equilibrium in the system. The wide-spread opinion according to which the chemical transformations have no effect on the layer-growth kinetics, except in a very short initial period of time, is thus quite groundless. 

As shown in figure 4.9, the time evolution P f) shows three distinct time dependencies, each characterized by either 7, F, or e (1) The Lorentzian-like envelope, with width F for the complete absorption spectrum transforms into an exponential decay with rate constant F/ft, which is for the short-time decay of PXt)- (2) The set of resonances, separated on average by an energy e, transform into a set of oscillations (i.e., recurrences) whose periods are approximately elh. (3) The envelope of each individual resonance also transforms into an exponential-like decay, characterized by the rate 7/ft, which corresponds to leakage from the sparse i) — /) subspace into the quasi-continuum ). The recurrences described above in (2) are damped out by this slow decay. 

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