Machine surge

For variable speed machines surge is a fine, not a single point The next group of schemes are based on developing an equation for this fine — either from the compressor manufacturer s data or from plant testing. 

The special design of the Latham bowl allows for a specific blood cell separation known as SURGE. This technique makes use of the principle of critical velocity. The Latham bowl is filled until the huffy coat, ie, layer of platelets and white cells, moves in front of the bowl optics. At this point the machine starts to recirculate plasma through the bowl at increasing rates. The smallest particles, ie, platelets, ate the first to leave the bowl. Their high number causes the effluent line to turn foggy. The optical density of the fluid in the effluent line is monitored by the line sensor. A special algorithm then determines when to open and close the appropriate valves, as well as the optimum recirculation rate. 

In addition, it would also help to dampen the amplitude of the arriving surge. The u,se of inductor on the load side to provide an impedance to the arriving surges with a view to suppressing them is not good practice, for it may diminish the p.f. of the circuit and also catise a voltage drop across it, which may affect the machine s performance. 

The impulse test is basically an in-house coil insulation withstand test for surge voltages and forms a part of the test requirement for HT machines with resin-rich formed coils of 2.4 kV and above. Once the machine is assembled, such a test is unnecessary, as it may not be able to reveal deficiencies, if any, in the insulation of the coils deep inside the slots. Moreover, if a failure is noticed on the assembled machine, there is no option but to scrap the whole winding. 

Figure 12.27 Typical scheme for surge protection of a rotating machine...

Protection of rotating machines from switching surges 17/578... 

The need to protect a rotating machine from switching surges, contact bouncing and surge transferences 17/576... 

Figure 17.7(a) Surge impedance of rotating machines as a function of inpuf MVA/VUne kV (from Pretorious and Eriksson, 1982)... 

If the surge is very steep, say, with a front time of 0.2 jjs. or less, and the cable length is short, say, lO m (presumed), it may inflict all its severity to only the entrance coil of the machine and sometimes even only a few entrance turns of this coil. However, with the technological improve-ment of switching devices, the arc prestrikes are now less predominant or non-existent and hence, the switching surge may not be as steep as described here. 

The value of L and C of a machine will determine its surge impedaiice = -jUC and surge frequency /, = LC). A low surge impedance will help to... 

It has been seen that smaller rating HT machines are more susceptible to steeply rising voltage surges, compared to larger ratings, due to their relatively w eaker interturn insulation and require more careful attention to their adequate protection. We attempt to explain this phenomenon as follows ... 

Protection for the machine should be such that the voltage surges and their rise limes, whenever they occur in the system, shall fall within this envelope of the machine. (Refer to Section 17.10 for a total surge protection.)... 

With these data, one can determine the dielectric curve the machine must have when switched with such an interrupting device. This can be compared with the actual dielectric curve of the machine (Figure 17.18) obtained from its manufacturer to decide the compatibility of the interrupting device for the machine or vice versa and the extent of surge protection, if necessary. For more details and results of similar simulation tests, see Central Board of Irrigation and Power (1995). 

For adequate protection of the machine it is essential to know the amplitude, F, and the rise time, /, of the severest voltage surge (FOW) that may occur on the system. It is recommended that the actual field tests be conducted for large installations according to the recommended simulation test circuits, noted above, to ascertain these surges. 

Since the standard insulation level (BIL) of a machine, equipment or a system is already defined, according to Tables 11,6. 14.1, 32.1(A), 13.2 and 13.3. the mtichines are aecordingly designed for this btisic insulation (BIL) only. When the prospective surges are expected to be more severe than this, separate protection becomes imperative. This is particularly important for a rotating machine w hich, besides being a dry equipment, also has only a limited space within the stator slots and hence has the smallest BIL of all, as is evident from Table I 1.6. compared to Tables 14.1, 32.1(A) and 13.2. For its comprehensive protection it can be considered in two parts,... 

This is the winding insulation to the body, whicli is more vulnerable to prospective voltage peaks. V,. as a result of TRVs. When the TRV exceeds the BIL of the machine, it can be dampened to a safe limit with the use of a surge arrester, say, from peak to ih, as illustrated in Figure 17.21. Details of a surge arrester and the procedure for its selection are discussed below. See also Example 17.6. The selection of the arrester w ill also depend upon the method of star (neutral) formation of the stator s... 

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