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Utility Stuff
Posted by Gary S. on 6/30/2005, 1:30 pm About ten years ago I moved to a new area, and my house is only about two miles from what appears to be a pretty major regional power distribution station. A set of lines (six wires, two circuits) comes in from the hinterlands and splits into branches that feed substations farther down the line. Some interesting hardware behind those fences. I've been curious to know just what voltage they're handling on those lines. Can you tell, at least roughly, the voltage carried by the lines from the type of insulators that hold them? I saw a show on the History Channel the other day about linemen who use helicopters to service energized lines, and they mentioned that the 345 KV line they were working on used insulators with 18 "bells." They said two of those were overkill, and that 16 would be the bare minimum to withstand the voltage. Since the feeder lines I see have insulators with 8 bells, and assuming the rule is to have 2 extras, then that would make the proportional voltage for those lines about 130 KV. Does that make sense? What are the common voltages used to distribute power? I see (on wooden poles) lines held with insulators that look like a single knob (these directly feed transformers that feed homes and businesses), others that stand about 6" tall (one line of these seems to feed a substation a few miles from the secondary distribution station near my home). I also see poles with insulators over a foot long (I'd guess a good 18"), set quite high. These also feed substations. I could go on and on with questions here. How about this one: What are the devices that look like transformers, with two high voltage terminals on them, that connect one power wire to another (same phase)? They are inserted in series with the circuit, sometimes manual knife switches are mounted in parallel with them (which always seem to be in the open position). Some of these are BIG, some are the size of smallish coffee cans, but none of them appear to have primary and secondary wires, just the two terminals. No control wires, either. I assume the physical size is related to their KVA ratings. They are almost always installed in sets of three to serve all three lines of the circuit. Could they be transformers? If so, for what? They insulators on each side of them are the same size. That's enough for now! Thanks, Bert, for having this very cool page.
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Re: Utility Stuff
Posted by Bert Hickman on 7/3/2005, 11:32 pm, in reply to "Utility Stuff" Thanks for the kind words - I'm glad you enjoyed the site. : Yes. However, the extra insulator segments are not really overkill, they are there to provide sufficient margin to prevent flashovers, nuisance tripping of circuit breakers, and potential service disruptions. Additional design margin is needed to prevent flashovers from relatively common situations such as overvoltage transients (circuit switching or lightning) or via suspension insulator segments that become leaky or defective (through physical damage, dirt, condensation, corona damage/tracking, or previous flashovers), Yes - your transmission lines are likely either 115 kV or 138 kV. In the US, some commonly used AC power "transmission" voltages are 69 kV, 115 kV, 138 kV, and 240 kV, while long haul bulk power transmission lines operate at 345kV, 500 kV, and 765 kV. "Distribution" voltages are lower - typically 33 kV and below. Older systems may run as low as 2.4 kV, but more commonly used voltages include 4.8 kV, 7.2 kV, 14.4 kV, 24 kV, 33 kV, and 69 kV. Thanks! Small devices like this are typically cutout switches or instrument/control transformers. However, if the devices ONLY have two terminals, they may be inductors. Unfortunately, I haven't seen these in power lines in our area so I can only speculate. Since they may be physically small and are connected in series with the 3-phase circuits, they are likely inductors. Lumped inductance is sometimes added in series to block high frequency signals (such as RF control/monitoring signals that are often used by utilities), but these are most often air core coils. Small inductors are also used to "slow down" high speed transients. These are used along with arresters for lightning and transient overvoltage protection. Also, inductive reactors are sometimes added in series to reduce the available fault current during a short circuit. Another possibility is that these are inductive voltage regulators - these can be bridged across by the parallel shorting switches to remove them from service. It's really hard to tell without seeing what they actually look like...
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Re: Utility Stuff
Posted by Gary S. on 7/4/2005, 9:29 am, in reply to "Re: Utility Stuff" I could send you some pictures if you want, but it does sound like these are inductors. They are in the same type of cylindrical housing as transformers, so a coil of wire would likely be a good fit. There's other hardware on the poles that have me puzzled, too. One was a set of feeds, again in series with all three phases, that went into a fairly large rectangular box (total of six terminals). Plenty of other wires came and went into that box, down the pole to two cabinets mounted three feet from the ground. In addition, a small yagi antenna was bolted to the pole about half way up, its feed also going down to one of the two cabinets. The mystery of what all that was about was solved by a sign on the pole identifying it as "TIE RECLOSER #XXX." OK, that was an easy one! A few poles down from that there's another bunch of poletop hardware, with another six terminal rectangular box. This one has a warning sign to the linemen to "Service capacitors in accordance with directive XXX," and "AVOID INJURIES. Wait five minutes after disconnecting capacitors before servicing." OK, I guess this is a capacitor box of some type. Maybe they use the capacitors to balance their inductors! Thanks so much for the help, Bert.
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Re: Utility Stuff
Posted by Bert Hickman on 7/7/2005, 1:41 pm, in reply to "Re: Utility Stuff" There's also a possibility that these are voltage regulators - automatic transformer-like devices that can either add or subtract an adjustable amount of voltage from the incoming HV mains in order to provide a more regulated source of voltage to downstream loads. Yep! In the event of a phase to ground fault these are designed to open and then automatically reclose after a short time. This allows an arcing fault to clear. If it's a more permanent fault, the recloser will try two more times and then will lock out. Older systems required manual resetting - newer systems (such as the one you saw) can be remotely reset by the dispatcher via an RF link. : You are correct! These are quite common - they're called Power Factor Correction (PFC) capacitors. They are usually rectangular boxes with two or more Frankenstein-like insulators. Many customer loads tend to be inductive in nature - these typically include fluorescent lighting, induction motors, microwave ovens, low voltage transformers, etc. The inductive reactance of these loads tend to cause the current and voltage to get out of phase, which can then lead to excessive heating and voltage drops along the power company's distribution lines. By adding lumped capacitance across the lines, the power company can "correct" for the load's inductive reactance, thereby improving the efficiency of their power distribution system. In some cases the inductive load may not be constant, so the power company may need to switch in the capacitor bank as required. In this case, you'll see can-like instrument/control transformers and high voltage switches in additin to the rectangular PFC capacitors. PFC capacitors also contain bleeder resistors that safely discharge these (over a period of about 5 minutes) so that company personnel can safely work on the system. For more information, see: : My pleasure - best regards, Bert
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