Power 101

[Open Neutral]

When you were a kid, did you play the "Who spotted the VW Bug first" game....? Another EE and I do an adult version.

As we drive various places, we follow the 33 and 69KV PEPCO feeders (and some BG&E) from switchyard to subs, then compare notes trying to reverse-engineer their grid. (Our interest started on the NE Corridor 25Hz system, but I digress.) You can play at home as well, thanks to Google Earth and Bing Birds-Eye imagery...

We've found some ...interesting... designs, such as the 69 KV feeder that crosses Rt 29 underground, emerges, runs exactly one pole & then goes underground again into the sub. Why didn't they just.....?

So I have some dumb questions....

1. Are there any tri-voltage transformers used at subs?
2. What is the typical capacity available on 33 & 69 KV feeders?
3. When do utilities feed customers at anything above the usual? {13.8 here..}
4. Why maintain dead feeders?
5. Why are subs fed underground?

We're guessing that originally the substations were fed with 33, but upgraded to 69 later. On [1] there is a 69->33KV sub by White Oak, and right across the street, a 33->13.8 one. So we wondered if anyone used a triple voltage transformer, saving the 2nd step.

On #3, we have not found anything commercial above 13.8, even going to large shopping malls, but there's a big water treatment plant in Laurel that appears to be fed with 33KV from two directions. In Nowhere, MD there is some ilk of aluminum smelter which he says is fed at 132 KV. {"Guess they have the phone number to PJM taped on the wall" he noted.]

We have followed several feeders with 33KV insulators but one running between subs has 4-5 poles in the middle with ordinary 13KV insulators....and then we noticed neither end went anywhere. They did put it all back up following the derecho... My guess was it was still being included in the rate base but then I have a suspicious mind.

The 69KV sub feeds always seem to be below ground. But in one case, 4 blocks away it descends, and 3 blocks away it emerges again. It would make sense if they bracketed the substation but nope, both are south of it....and the overhead continues north almost past the sub..

Anyone want to enlighten us with their informed speculation?

 

[fmtjfw]

Random stuff

Random stuff

When I'm driving often navigate by high voltage structures -- so many miles to a turn following the 138KV line with the cell phone antennas. My father worked for the power company his entire adult life (when he wasn't in WWII). We used high line maps and outage reports for scrap paper.

Don't let FEMA know you are mapping this critical infrastructure:happyyes:

On any HV line 22KV and above you can tell if it is de-enegized from the ground just by observing the birds. Birds won't land on a 23KV line because the voltage gradient around the conductor is just too great.

We have an aluminum rolling mile in town that has two services 3-phase 138KV and 120/240 single phase. Funny I don't see a placard at either service warning about the other.

Lots of coal mines get their service at 23KV. I can remember line maps that marked possible backfeeds through a large mine from one service point to another. Penn Line (private contractor) maintains miles of mine-owned 23KV and below pole lines.

Here most water treatment plants and large sewer lift pumps are fed from two separate directions. In some cases two poles feet apart with a primary disconnect between the two primary circuits and a transformer and service drop on either side of the disconnect.

In the New England town I lived in for about 30 years we had the terminus of the Hydro Qu?bec DC transmission line.

The line is 3 conductor 900KV DC line to line and 450KV line to neutral. The neutral is insulated at abut 25KV. Capacity 2GW, length 1480KM. The terminals are solid state (con?)inverters. The AC output at Ayer is 345KV

DC was chosen for two reasons:

1) More efficient over long distances (peak voltage = real voltage DC vs. rms for AC)
2) The two networks are not synchronized in terms of 60HZ timing.


Back in the 1960's we started building 500KV lines in WV. WV is an exporter of electricity to other states and low loss transmission was (and is) a good thing. I was in high school and worked summers at the power company. One day the chief accountant, who was handling the accounting for the construction of the lines, came in ask asked if anyone knew what all the directional "SOOKY" signs he saw were for. We told him "500KV".

By the way some of the problems we have seen in widespread outages are caused by synchronization loss. The net load areas are severed from the net generation load areas. When automatic reclosing tries to recover, the load areas have slowed and the generation areas have sped up. DC transmission would help this.

Substation output is often fed underground for a little bit. You can put in more 13KV OCPD in metal unit substation than you've got air space for overhead structures, so you feed it out on cables and up poles to spread it out.

Roughly 1GW coal fired plant. Switch yard on hill 300 feet away. Control circuits copper telephone lines. During switching operations the ground at the plant and at the switch yard had such a difference in potential that the protectors on the phone lines would fire and short out the line. Went to 250V protectors.

Some HV lines carry SCADA and even voice communications as a radio signal. A 138KV line is mush less likely to the destroyed, even in a hurricane than local telephone outside plant.

You can see this carrier system by looking for coils in series with the line (usually just 1 phase). If you look closely you will see a capacitor hooked just before the coil for RF coupling. It will look like a potential transformer.

 

[Hv&Lv]

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We've found some ...interesting... designs, such as the 69 KV feeder that crosses Rt 29 underground, emerges, runs exactly one pole & then goes underground again into the sub. Why didn't they just.....?..}

Could be there is a gas line right of way the PoCo didn't want to cross.

So I have some dumb questions......}

There are no dumb questions.

1. Are there any tri-voltage transformers used at subs?

   You mean XF's with deenergized tap changers? sure. Cooper makes power transformers with 5 or 7 settings.

2. When do utilities feed customers at anything above the usual? {13.8 here..}

   The usual here is 25 kV (14.4)

3. Why maintain dead feeders?

   To hold right-of-way

4. Why are subs fed underground?

Keeps the area neat and safe to work in. It is like working in a spider web in stations that feed all overhead.

 

[mivey]

1. Are there any tri-voltage transformers used at subs?

Yes, but three-winding transformers are usually not preferred because of single-mode failure.

2. What is the typical capacity available on 33 & 69 KV feeders?

Wire size makes a difference, but for 33 kV: Roughly 20-45 MW and for 69 kV 36-96 MW

3. When do utilities feed customers at anything above the usual? {13.8 here..}

Densely populated areas, areas with long feeders, feeders with large loads.

4. Why maintain dead feeders?

To hold right-of-way and for back-up.

5. Why are subs fed underground?

Not all are. UG can be because other stuff is in the way or people don't want to look at poles and wires.

 

[Open Neutral]

You mean XF's with deenergized tap changers? sure. Cooper makes power transformers with 5 or 7 settings.

Nope, I was thinking 66, 33 & 13.8 all in one; to feed both 33 and 13.8 demands.

When do utilities feed customers at anything above the usual? {13.8 here..}

Note I meant customers. The SOP around here is a POCO-owned substation fed with 33 or 69; and outputs 13.8 to various distribution; some may be dedicated. What I sought was exceptions, where a customer gets 33 or above. The water plant is the only one we know of around here, but this is a err non-industrial area. (Hmm, I recall the substation at Fort Fumble; that is surely an exception. I assume most military bases buy at the transmission level.) But for example WMATA's 6-9 MW traction power stations are fed with 13.8.

The unused feeders I see are hung (as are virtually all feeders here) above 13.8 distribution, so there's no question of maintaining RoW that I can see. Part of the mystery is that the one sub at the eastern end appears to be fed with one 69 feeder, period. That did not strike us as kosher, but we thought the 33 was a cross tie...until we grokked neither end connects to anywhere. The Bells were infamous for abandoning cable plant, but still including it in their rate base; so my thoughts wandered to that explanation.

Could be there is a gas line right of way the PoCo didn't want to cross.

Interesting idea; I do know there is an old AT&T Long Lines cable RoW in that immediate area.

Don't let FEMA know you are mapping this critical infrastructure

They are the ones who skimped on the invisibility paint. And the "SECRET - DO NOT SEE THIS" signs all fell down.

 

[fmtjfw]

They are the ones who skimped on the invisibility paint. And the "SECRET - DO NOT SEE THIS" signs all fell down.

Actually they used up all the invisibility paint on the signs. They are now doing a study as to why no one has obeyed the signs.