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| Re: PHOTOS: SHOO FLY TWO (1276336) | |||
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Re: PHOTOS: SHOO FLY TWO |
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Posted by Bill West on Thu Feb 20 01:42:46 2014, in response to Re: PHOTOS: SHOO FLY TWO, posted by Jersey Mike on Wed Feb 19 06:59:07 2014. I'm mistaken about lightning arresters, there are none on the transformers which is now unusual. Likewise there are none at the line entry into the station which had been an earlier practice. From the Railway Age feature on PRR electrification, Feb 25, 1933, pg 293:"Primary lightning protection for 132,000-volt transmission lines is insured by the ground or static wire run along the tops of the transmission line poles or columns. No transmission line lighting arresters are used except at the D1 substation (Zoo). The 132,000-volt circuits at the substations are protected from lightning or high-voltage surges in two ways. At the first pole on either side of the substation, one strain insulator string is reduced to 7 units instead of 13, and arcing or grading rings are placed around the two end units of the string. Any high-voltage surge originating on the transmission line will arc over at this point; the rings serve to protect against insulator breakage caused by (the instant heat of the) flashover. In addition bushings (on transformers) are provided with protective spark gaps (top/bottom pointer like rods). Transformers have 154kv class insulation. Lightning poles are used for protecting the substation. They extend above the substation structure to points high enough to protect all apparatus; each foot in height above the structure serves to protect to protect a circle of three-foot radius. Where possible, the lightning rods are mounted on the same structure which supports lines and switches. The poles are run from the ground when the desired location does not conform with the location of the substation structure. Experience thus far has indicated that trouble due to lightning is less frequent on the 132,000-volt line than it is on 44,000-volt lines." Your picture shows that there are more insulator discs in the string now. I'd have to look at Safe Harbor pictures to see if they changed the methodology to modern lightning arresters on the transformers. For background, insulation will withstand substantial short term overvoltages just as conductors withstand short term overcurrents. The difference is that the overvoltage time limits are measured in micro seconds. PRR's 154kv insulation has what is called a Basic Impulse Level of likely 750kv, that's what it will withstand when hit with a transient that rises in 1.5 usec and decays 50% in 40 usec. The insulation strengths are usually graded across the equipment so that the permanently damageable, costly equipment is protected by short circuiting flashovers at simpler equipment. The air around lines is particularly cheap and self heals almost instantly. When the gap in your picture flashes over the voltage drop across it is only 10's of kv thus protecting downstream equipment. A time allowance for the gap to react is all that is needed in setting the downstream strengths. The longer catch is that the regular 25hz power sees the arc as a short too, flows through it and unlike the lightning does not turn off when the lightning stroke finishes discharging. The line circuit breakers will have to open for a few cycles to let the ionized air float away. An automatic or manual reclose then restores service. The sort of protective relays that detect these shorts and reclose the circuit became my career. Next week we'll do Circuit Breakers 101 and Transformers 102, don't forget to send your tuition fees in :). Bill |
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