Page 611 - Industrial Power Engineering and Applications Handbook

17676 Industrial Power Engineering and Applications Handbook
in the same proportion. This is because of the higher 17.8.4 The need to protect a rotating machine
number of turns per coil which diminish the dielectric from switching surges, contact bouncing
quantity, hence the endurance of the turn insulation, and surge transferences
as witnessed during tests. The higher number of turns
also pose a problem in forming the coils, particularly Switching surges have been seen to be the severest of
at bends, as it is difficult to maintain the same quality all. But it has been a greatly debated subject whether a
of insulation. Table 17.1 (see Pretorius and Eriksson, protection is overemphasized against switching surges,
1982) suggests typical ratios for different voltage particularly when the occurrences of such high TRVs
systems in three smaller frame sizes used for an HT may be rare yet are significant to cause concern, opinions
motor. differ. Technological improvements and application of
Lower ratings have low L and C and therefore have a the latest techniques in the extinction of arc plasma,
higher Z, (Figures 17.7 and 17.8) and do not help in making use of high-speed interrupting devices, using
dampening or taming the surges. SF6 or vacuum as the insulating and the quenching
3 Similarly, high-speed machines too have more coils medium, meticulous design of the arc chamber, design
per slot, subjecting the interturn insulation to higher and material of the making contacts achieve an interruption
stresses. almost devoid of restriking and adopting the latest
4 The length of the entrance coil is another important insulating practices for motor insulation (Section 9.3)
parameter, which may be subject to most of the voltage such as vacuum impregnation and additional bondage
stress up to 70-90%, as noted earlier, for very fast- and bracing of the windings’ end turns have diminished
rising (tl 50.2 pus) TRVs. So are the last few turns of if not totally eliminated the effect of these surges on
the entrance coil, or the first few turns of the second machines. Nevertheless, to take account of the possibility
coil, for fast-rising surges (tl = 0.2 - 0.4 ps), or the of these surges causing damage to the terminal equipment,
last few turns of the last coil making the star point of generally 2.4 kV and above, it is advisable that protection
the windings, when the TRV is almost uniformly be provided as a preventive measure to protect the costly
distributed (tl approaching 0.4 ps). The steepnesses machines and, all the more costlier, against the risk of a
are only indicative to illustrate the severities of surges shutdown of a plant in the event of a possible failure of
and their influence on the turn insulation. For a the machine. More so when a rotating machine has a low
particular size and design of motor, the steepness and level of insulation (low BIL) compared to an oil-filled
its influence may vary somewhat. transformer.
Dry type equipment, such as rotating machines, have
lower impulse withstand levels compared to a liquid-
Other than the above, the interconnccting cable length type machine, such as a transformer or a switchgear
between the switching device and the machine also plays assembly. A comparison of impulse voltage withstand
an important role in the distribution of the fast-rising levels for the same system voltage for motors (Table
voltage surges as discussed earlier. It is the cable that 11.6) and for switchgear assemblies (Tables 13.2 or 14.1)
will bear the initial severity of the rising surges, before will reinforce this point. A motor is always vulnerable to
the surges reach the motor terminals. Hence, upon the both internal and external voltage surges. Circuit switching
rise time, ti, will depend the safe length of the interconnec- is the most onerous of all and can overstress the windings
ting cables to provide the maximum dampening effect. of a machine if it is not protected adequately, leading to
For very fast-rising waves, in the range of 0.2-0.4 ,us or an eventual breakdown, if not an immediate failure. In
so, cable lengths in the range of 30-300 m are seen to fast-acting devices such as a VCB with Cu-Bi alloy
provide the maximum dampening effect. Actual simulation contacts (Section 19.5.6) the manufacturers provide a
tests or studies of similar installations are, however, surge suppressor.
advisable, for a more accurate assessment. During an interruption, an SF, interrupting device is
For a less steep surge, the situation will be different. found to be normally devoid of a switching surge, as there
Now, the longer the cable, the higher will be the amplitude is no chopping of current. During a closing sequence,
that the surge will attain by the time it reaches the motor however, in both a VCB and an SF, breaker, the switching
terminals. For example, for a surge with a risc time, tl of surges are almost within the same range, say, 1.5 to 2.5 P.u.,
1 ps, the cable must be at least 100 m or more in length as recorded during a simulation test on a 400 kW, 6.6 kV
(considering the speed of propagation as 100 mlps), and motor (see Central Board of Irrigation and Power, 1995).
sometimes it may not be practical to provide this. The
steepness of surge is thus a very vital parameter in deciding Misconception
an ideal cable length to achieve the desired dampening
effect through cables. A slightly shorter length than this 1 It is a misconception that only large high-voltage
may subject the terminal equipment to a near peak motors need be provided with surge protection, in
amplitude of the arriving surge. It is therefore advisable preference to small machines, because they are more
to keep the length of the interconnecting cables as short likely to encounter dangerous surges. Analysis of
as possible, to subject the terminal equipment to only a various motor circuits, as noted earlier, indicates that
moderate amplitude of the arriving surge, much below smaller and higher-speed motors are more subject to
its prospective peak. (For more information on the subject the effects of voltagc surges rather than the larger or
refer to Section 18.6.2 on protective distances of surge lower-speed motors due to one or more of the following
arresters). reasons: