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IEEE Std C62.55-2020 pdf free download
IEEE Std C62.55-2020 pdf free download.IEEE Guide for Surge Protection of DC Power Feeds to Remote Radio Heads.
9. The issue of multiple strokes and continuing current
Up to this point, only a flash having a single stroke has been considered. These are rare. According to (‘IGRE TB 549 (2013). a typical negative cloud-to-ground flash is composed of three to five strokes (leader/return stroke sequences), with typical interstroke intervals of some tens of milliseconds. The largest number of strokes in a flash reported in clause 2.5 ofCIGRE TB 549(2013) is 26 strokes, although no upper limit has been established. Annex A5.4 of MIL-STD-464 [B 19] describes an extreme multi-surge environment consisting of a 200 kA 6/69 first stroke followed by twenty-three 50 kA 3/35 subsequent strokes spaced 10 ms to 200 ins apart.
Flash strokes do not always occur in the same place. According to 2.7 ofCIGRE TB 549 (2013), one-third to one-hall of all lightning discharges to earth, both single- and multiple-stroke hashes, strike ground at more than one point with the spatial separation between the channel terminations being up to many kilometers. Clause 2.7 of CIGRE TB 549 (2013) talks about strikes to ground, and it is not clear if this observation also applies to strikes to towers. The worst case is to assume that all strokes hit the tower.
Figure 10 illustrates the case of multiple flashes, and also includes continuing current (which can last from a millisecond to hundreds of milliseconds).
Annex A5 .4 of MI L-STD-464- I 997  shows an extreme case where a first stroke can transfer 10 C, a subsequent stroke can transfer 2.5 C. and continuing current can transfer 200 C (400 A for 0.5 s).
Depending on the interval between strokes and the thermal time constant of a device, the thermal energy due to the strokes might accumulate in the device, which could impact its functionality. This view is supported in a paper by Sargent Ct at. [B23] in which halfof a set of 18 mm MOV samples subjected to multi-surge burst 8/20 surges at rated current showed signs of damage; whereas the other halfof the samples tested with a single 8/20 surge at rated current repeated at intervals of 60 s or more showed no damage. Analysis of the failed samples showed cracking and alteration of the material near the surge current conduction channel. Examination of this material suggested that local hot spots formed when a current pulse was passed through the MOV. The material in these hot spots likely resulted from a plasma formed during the current pulse, which rapidly cooled afterwards due to heat conduction to the surrounding ZnO grains. Creation of the altered material was thought to require a local temperature around 1000 °C. which would occur if the pulse energy was concentrated in about 2°/s of the MOV volume. This is an important observation, because a calculation of the energy absorbed in the multi-surge burst test showcd that the temperature rise of the MOV would have been 231 °C if the temperature distribution were uniform, much less than the temperature thought to have caused the damage.
In another multi-surge burst test. Rousseau et al. (1322] subjected an MOV to sixty 20 kA 8/20 surges spaced 60 s apart, with no failure. But when the same type of MOV was subjected to as few as five 20 kA 8/20 surges spaced 50 ms apart, failure occurred.
In addition to forming hot spots, an MOV can fail by surface flashover, as noted in a paper by Darveniza and Merccr [B4]. Surface flashover can occur due to plasma generation. manufacturing defects of the MOV.IEEE Std C62.55-2020 pdf free download.
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