Surge arresters motors

All electrical equipment are designed for a specific BIL, as indicated in Tables 11.6, 13.2, 14.1, and 32.1(A) for motors, switchgears and bus systems respectively, and Tables 13.2 and 13.3 for the main power system (line clearances and insulators). If the actual severity of a prospective surge, i.e. its amplitude and/or rise time or both, is expected to be higher than these levels (higher amplitude and lower rise time) the same must be damped to a safe level, with the use of surge arresters, surge capacitors or both as discussed later. 

Figure 17.5 Oscillograms showing the effectiveness of a surge arrester on a 6.6 kV motor...

A surge arrester is not able to reduce the steepness (r.r.r.v) of a surge. While motors with single-turn coifs may be safe as turn to turn insulation becomes the same as the major insulation with respect to the body, motors... 

Since the amplitude of the FOW is more than permissible (16.2 kV for a normal design and 26.9 kV for a special design motor Table 11,6) it is advisable to dampen the amplitude with the help of a surge arrester. Using a 6 kV station class surge arrester of Example 17,6,... 

Sers ice conditions As for other equipment (e.g. motors, transformers or swdtchgears) a surge arrester is influenced too by unfavourable operating conditiotis such as noted in Table 18.4. 

Selection of gapless surge arrester Classification of arresters Surge protection of motors Pressure relief facility Assessing the condition of an arrester... 

With this particular arrester, the motor has to be specially designed for the higher level of FOW impulse voltage withstand. But the manufacturer can always modify the protective characteristics of the arrester, depending upon the system s requirements. The matter may therefore be referred to the manufacturer for recommendations. The arrester may be fitted with a 0.25, uF surge capacitor in parallel, to reduce the steepness of the FOW (for the arrester of example 17.5, it is 14/0.2 kV/jUS) to a safer value. 

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