6. Impulse generator size
The factors affecting the length of
The factors affecting the length of the wave tail during factory tests the wave tail during factory tests have been discussed previouslyhave been discussed previously. In . In brief brief these are
these are
a)
a) CaCapapacicitatancnce oe of if impmpululse se gegeneneraratotorr b)
b) LoLoad rad resesisistatancnce of te of the ihe impmpululse cse cirircucuitit c)
c) EffEffectiective ve inducinductance tance and and capaccapacitancitance of e of the the windiwinding bng being eing impulimpulsedsed d)
d) TTerminerminationations on the nonims on the nonimpulsepulsed windind windings whethegs whether they are opr they are open-cien-circuircuited, shorted, short-cit-circuitrcuited, or tered, or ter--minated with resistors, capacitors, or lightning arresters. In general the effective inductance will be minated with resistors, capacitors, or lightning arresters. In general the effective inductance will be largest and the tail longest if the
largest and the tail longest if the nonimpulsed windings are open-circuited, and it will be nonimpulsed windings are open-circuited, and it will be the smallestthe smallest when the nonimpulsed windings
when the nonimpulsed windings are short-circuited.are short-circuited.
The factors that determine the length of the
The factors that determine the length of the wave tail in service are, in brief:wave tail in service are, in brief:
a)
a) ChaCharacracterterististics ics of tof the ihe impumpulse lse at tat the phe poinoint of t of oriorigingin b)
b) SurgSurge ime impedanpedance of ce of the cthe circuiircuit ovt over wher which tich the imhe impulse pulse is pris propagaopagatedted Figure 48ÑGrounding with line current method Figure 48ÑGrounding with line current method
c)
c) CircCircuit paruit parametameters at thers at the terme terminals oinals of the traf the transfornsformer submer subjectjected to the imed to the impulse epulse exclusxclusive oive of the surf the surgege propagating circuit including inductance and capacitance of all connected apparatus including the propagating circuit including inductance and capacitance of all connected apparatus including the transformer
transformer d)
d) SuSurgrge ae arrrresesteter cr chahararactctererisistiticscs e)
e) TTermerminainatiotion of n of othother wer windindingings of s of the the tratransfnsformormerer
It is logical to propose that the transformer be tested as it will be used in the Þeld. However, this is not as It is logical to propose that the transformer be tested as it will be used in the Þeld. However, this is not as simple nor as straightforward as it sounds. The environment of a transformer frequently changes with the simple nor as straightforward as it sounds. The environment of a transformer frequently changes with the years either as it is moved or as equipment or
years either as it is moved or as equipment or lines are added in parallel with it. Also, it is difÞcult in the test-lines are added in parallel with it. Also, it is difÞcult in the test-ing facility to duplicate the effects of lines, cables, generators, lightntest-ing arresters, etc. Even more elusive is ing facility to duplicate the effects of lines, cables, generators, lightning arresters, etc. Even more elusive is the determination of what kind of a lightning stroke to expect and just how it will enter the transmission the determination of what kind of a lightning stroke to expect and just how it will enter the transmission system.
system.
Generally lightning waves vary considerably
Generally lightning waves vary considerably. For . For uniformity in testing, certain wave shapes have been uniformity in testing, certain wave shapes have been stan- stan-dardized. Since the early 1930s, transformers have been impulse tested with a 1.5
dardized. Since the early 1930s, transformers have been impulse tested with a 1.5 ´´40 wave. With very few40 wave. With very few exceptions, this has demonstrated that the transformer, when protected against lightning by the usually exceptions, this has demonstrated that the transformer, when protected against lightning by the usually accepted practices, will give long satisfactory service. Manufacturers have equipped their factories with accepted practices, will give long satisfactory service. Manufacturers have equipped their factories with impulse generators having capacities to apply the 1.5
impulse generators having capacities to apply the 1.5 ´´ 40 wave to the apparatus. This wave has been40 wave to the apparatus. This wave has been changed so that it is now a 1.2
changed so that it is now a 1.2 ´´50 wave. However, with the trend 50 wave. However, with the trend toward larger ratings, it may toward larger ratings, it may sometimessometimes be difÞcult to obtain a 50
be difÞcult to obtain a 50mms tail. In s tail. In these cases, it is imperative that the maximum generator capacitance bethese cases, it is imperative that the maximum generator capacitance be used. To facilitate this, the impulse generator should be arranged so that the banks or stages can be used. To facilitate this, the impulse generator should be arranged so that the banks or stages can be con-nected in various series-parallel arrangements so as to get the maximum generator capacitance for the nected in various series-parallel arrangements so as to get the maximum generator capacitance for the volt-age required. The following minimum capacitance values are suggested for the various BILs when it is age required. The following minimum capacitance values are suggested for the various BILs when it is difÞcult to obtain a tail of 50
difÞcult to obtain a tail of 50 mms.s.
T
Table 2ÑSuggested impulse generator capacitance as a function of able 2ÑSuggested impulse generator capacitance as a function of BILBIL
BIL (kv)
BIL (kv) Suggestion minimumSuggestion minimum capacitance ( capacitance (mmf)f)
110
110 (and (and less) less) 0.60.6
1
15500 00..55
2
20000 00..44
2
25500 00..1166
3
35500 00..1122
450 0.10
450 0.10
5
55500 00..0088
6
65500 00..0055
750 0.05
750 0.05
8
82255 00..001188
9
90000 00..001188
1
1005500 00..001188
1175 0.015
1175 0.015
1
1330000 00..001155
1425 0.012
1425 0.012
1
1555500 00..001122