Surge protection

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

Therefore a motor protection relay, which may be a normal time ora long lime, will be able to provide through one unit a comprehensive motor protection, generally requiring no more protection, except for a surge protection, which may be needed for an HT motor. 

When the system is provided with a surge protection. 

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). 

Where this is not necessary, it can be assessed by the equipment s exposure to such TOVs and surges and thus determine the appropriate level of BIL. lEC 60071-2 provides guidelines for the most appropriate surge protection scheme and is discussed in Section 18.6. 

Equipment may be designed for more than one BIL values as noted in the various tables referred to above for motors, switchgears and other equipment. The choice of BIL for equipment for a paniciilai application will depend upon the extent of exposure the equipment may be subject to in normal service and the security level required by the system and the surge protection. For more details refer to Section 13.4.1(3). 

When equipment is exposed to direct lightning strikes Equipment connected directly to an overhead line, or even through a transformer, will fall into this category. Select the highest value of BIL and even then a surge protection will become necessary for critical installations. 

When equipment is shielded This is when it is installed indoors, like a generator or motor. Now it may be subject to only attenuated surges. One may now select a lower value of BIL. In most cases surge protection may not be essential for direct lightning strikes. 

These are only basic guidelines. It is difficult to define exposed or shielded equipment accurately. Equipment installed indoors may never be subject to lightning strikes or their transferences, but may be exposed to severe switching surges and require surge protection as for an exposed installation. There is no readymade formula by... 

Figure 18.11 Surge protection of cables, transformer and motor...

Consider segment Xof Figure 18,25 for the purpose of surge protection. The detailed working is provided in a tabular form, for more clarity, as under ... 

Surge travels from higher voltage side of the transformer to the lower voltage side. Consider a surge protection on the primary, with details as follows ... 

Note Even then a surge protection is essential for the tertiary if the tertiary is provided. 

Surge protection on HV side is adequate to absorb direct surges. The transferred surges from HV side to the LT side are too feeble to cause any harm. In case of static switchings, on LT side, however, spike arrester will be essential. Section 6.14.1. 

A lightning surge protective level is found to be more stringent than the switching surge or FOW protective levels. Hence, it is sufficient to check the protective distance requirement for the lightning surge alone. 

Figure 18.25 Surge protection analysis of potential locations for the network illustrated in Figure 18.15...

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