Reply to post: Re: Can't they upgrade it?

Full shift to electric vans would melt Royal Mail's London hub, MPs told

James O'Shea

Re: Can't they upgrade it?

"If you need more electric capacity, you pay the local electricity distribution network to install bigger wires to your property, and as part of that they'll upgrade substations and interconnect wires as needed - and you pay for all this work."

I used to work for an electricity utility. It's not that simple.

A substation is sized for the local load. You need to have primary transmission lines inbound from the rest of the grid. You need to have a primary busbar and at least one secondary busbar. You need to have transformers (plural) to move the electricity from the transmission side to the distribution side. You need to have primary distribution lines outbound to your local load.

Problem 1: the transmission lines. Transmission lines run at very high voltage, thanks to Ohm's Law, in an effort to reduce the line losses. Typical voltages, depending on the requirements of the local grid, range from 69 kV (69,000 volts) to 750 kV (750,000 volts). The higher voltage levels are usually used in rural transmission lines, because some people get nervous about 750 kV overhead. And others get nervous about induction effects. Typically primary transmission lines might run at currents of 100 to 300 amps. (P=VI. 100 amps at 69 kV is 6.9 MW. 300 amps at 750 kV is 225 MW.) If you have to push more power through a line then your line losses go up. This typically shows up as heat. Enough line losses and your lines start to glow red. A bit more than that and they will, quite literally, melt. Or burn. Or both. If you are going to upgrade the substation for more power, first you must _replace every single transmission line feeding it._ Zambia and other copper-producing countries will love you. And if you think that I'm exaggerating about the care you must take with transmission lines, the reason why I was hired at the power company was that they'd just got a new SCADA system and they needed to get it up and running. One of my tasks was to set up and implement the database containing the line characteristics of every single transmission and primary distribution line and busbar the company owned. (And very boring it was, too) This was necessary so that the system wouldn't allow too much power to run over certain lines, or there'd be problems. The older lines were replaced over several years, because it was bloody expensive to buy a few miles of line capable of running 138 kV at 100 to 250 amps. The company had tried aluminum instead of copper, aluminum was a lot cheaper. It also burns much faster, so they didn't do that after one spectacular accident. (Before my time there, but those who had been there at the time still spoke of it in awe years later.) In the meantime, the guys at System Control had to be careful how they switched the power around.

Problem 2: the primary busbar. This is rated for specific voltages and currents. Run too high a voltage, the busbar has problems, not least being that some stuff may be close enough that the spark gap isn't big enough. The power utility I worked for once had a major blackout caused when a vulture (yes, I blame El Reg's mascot) sunned itself on top of a transformer next to the primary busbar in a very important substation and its wings were enough to bridge the spark gap. Result: 138 kV at 220 amps blew through the vulture, the busbar, and the transformer. It took several hours to clean up the mess and rebuild the fried parts. (We think it was a vulture. Parts of black feathers were recovered, and the only local bird with black feathers big enough to bridge the spark gap was a vulture. The rest of the vulture was vaporized by 138 kV at 220 amps.) You will have to replace the busbar.

Problem 2: the secondary busbar(s). Each secondary busbar feeds the distribution lines. You might have to either up the primary distribution lines to 24 or even 36 kV, or to add more distribution circuits, or both. You'll need a lot of space in your substation to increase the size and loading of the primary busbar, and more to do the same with the secondary busbar(s).

Problem 3: the transformers. They step down the voltage (and up the current) from the transmission side to the distribution side. They are sized for the load on the substation, plus a reserve. Increase the load, and once you're past the reserve (and this project will be blowing past that so fast there'll be a sonic boom) you must replace the transformers. All of the transformers in the substation. Do you know how much transformers of that size cost and how long it takes to get delivery on new ones? Have a look at, for example, and you might have an idea of what's involved. Not to mention that there may not be enough space in the substation to hold the new equipment, so you'll need a new substation. The utility I worked for had to put in a new substation, a substantial distance away from its load, when the station feeding that load proved to be too small to handle the required new equipment, and the cost of acquiring land in the area was prohibitive. I suspect that acquiring land for a new or expanded substation in the City of London might be... expensive.

Problem 4: the distribution lines. Primary distribution tends to be 36 kV or less, with 24 kV and 12 kV being common. Again, you want to have high voltages to cut line losses. Again, if you run the voltages too high you'll have problems with your lines, plus if the voltages get about 50 kV or so you're in transmission line territory. Power company linesmen will hotstick distribution lines, that is will work with distribution lines _without turning the power off_ so as to avoid power cuts. (Yes, they'll hotstick 24 kV lines, and with less enthusiasm, 36 kV lines.) There isn't enough money in the world to pay a linesman who knows what he's doing to hotstick transmission lines, and linesmen who don't know what they're doing and who play with 50 kV or higher lines will be dead or missing various body parts Real Soon Now. One of the linesmen at the power utility I worked for had both arms burned off by an accident involving a 13.8 kV distribution line. Another one was killed by a 24 kV line. You don't fuck with primary distribution lines, or they will fuck you up. Permanently. Transmission lines are worse. In any case, you're replacing all the primary distribution lines as well.

Problem 5: you're outside the substation, but your problems are just beginning. You must step down your primary distribution voltage (and step up the current) from 12/13.8/24/whatever kV to around 400/440 volts, three phase. You do that with local transformers. If you've resized your distribution busbars and distribution lines to have higher voltages, you will have to replace each and every local transformer. All of them. The local transformers are a lot cheaper than the big transformers at the substation, but there are a _lot_ of them.

Problem 6: grid safety equipment. Your current safety systems, including switches, breakers, and reclosers, (no, reclosers are _not_ breakers) are all sized for the voltages and currents on your gird. You've just increased both the voltages and the current. You'll need to strip out each and every breaker, switch, and recloser in the local load area and the substation. All of them. You'll have to strip out the isolators as well. And everything else that comes near the local load.

It's not a matter of they'd pay for the work, it's a matter of they'd have to pay for replacing the local grid. All of it. And that would be very expensive and would take a very long time to do. And, note, I haven't said word one about where and how you're going to get the power to run over your nice new power lines and nice new substation. That's a whole different, and even more expensive, problem.

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