Insulation voltage surges

As discussed in Sections 17.5 and 23.5.1, an electrical network may be exposed to different voltages surges which may be internal or external. The extent of exposure of the connected equipment would determine its level of insulation. lEC 60071-1 has recommended the desired... 

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 ... 

BIL is the basic insulation level of equipment. When the system TOV or voltage surges exceed this level, the equipment may yield. In the latest international and national standards it is defined as follows ... 

The substitution of a gas for part of the polymeric matrix usually changes the electrical properties. The dielectric constant, dissipation factor, and dielectric strength are lowered roughly proportionally to the amount of gas in the foam. The lower the density of the cellular plastic, the lower the dielectric constant and the better the insulation. Because the dielectric strength is also reduced, the insulation is susceptible to breakdown from voltage surges from lightning and short circuits. Polyolefin foams are preferred for low frequency electrical insulation - PVC, PS foams, silicone and fluoropolymer foams are also used [31, 65, 66]. 

MIMS cable systems should be protected by surge arresters to prevent insulation failure if transient voltage surges are hkely. 

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. 

More than one lightning impulse insulation level is indicative of the extent of exposure of equipment to lightning surges. Correspondingly the power frequency withstand voltage can also be more than one. 

The significance of this term can be realized by the fact that the voltage stress of a surge, having a maximum amplitude of 4.5 p.u.. with a front time / of 5 ps. will roughly be the same or even less severe compared to a surge with an amplitude of only 2 p.u. and a front time of 0.2 ps (see Insulation Sub-committee. Rotating Machinery Committee, 1981). 

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... 

Lightning and switching surges that damage motors are related to high-voltage motors. The insulation level of motors is below that of many other types of apparatus such as transformers, switchgear, cables,... 

Anderson, J. G. and K. O. Tangen, Insulation of switching-surge voltages, in EHV Transmission Line Reference Book, Edison Electric Inst., New York, 1968. 

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