Machines voltage surges

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.)... 

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. 

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. 

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... 

---