Restarting The Grid When The Grid Is Off The Grid

If you watch YouTube long enough, it seems like going “off the grid” is all the rage these days. But what if the thing that goes off the grid is the grid itself? In the video below the break, [Grady] with Practical Engineering explores the question: How do you restart an entire power grid after it’s gone offline? It’s a brilliantly simple deep dive into what it takes to restore power to large amounts of customers without causing major damage to not just the grid, but the power generators themselves.

What’s A Power Grid Operators Favorite Band?

The hackers among us who’ve dealt with automotive alternators know it must be excited in order to generate electricity. What does that even mean, and how does it affect the grid? Simply put, it takes power to make power. For example, old heavy equipment had what they called pony motors — a small easy to start engine that’s sole purpose was to start a much larger engine. Aircraft have auxiliary power units (APUs) for the same purpose. What do power grids have? You’ll have to watch the video to find out.

Once at least two power generators are online, grid operators can just flip the switch and start feeding power to customers, right? Not quite. [Grady] once again uses a clever test jig and an oscilloscope to show the damage that can occur if things aren’t done just right. It’s a fascinating video well worth watching.

Learn how grid operators use a Power Grid Emulator called LEGOS to help them with keeping the electrons flowing in the right direction.

33 thoughts on “Restarting The Grid When The Grid Is Off The Grid

    1. My goodness, you’re absolutely right. My hearing isn’t so good, I thought I heard Brady. I should know better, having written up one or two of his videos before. Thanks for the correction!

  1. Entire grid black out test have been tried in Finland. Northern part of Finland was isolated from the main grid, shut down and tried to get back online. The result of the test is partly classified secret. Mainly test was successful but time frame for test was not long enough to complete. In finnish (
    Can be done, but needs a lot of practice.

    1. Because there’s “off the grid”, and then there’s “OFF the grid”, which includes not being connected to natural gas, water, sewage, and grid-dependent communications.

  2. One of my colleagues described how, during test-runs, they’d get the apprentice to stand next to the generator to “check for oil leaks” or something like that and then wait until the mains was not-quite-in-sync before throwing the switch and having 3 tons of angry industrial diesel engine attempt to jump off its sled next to the terrified creature.

    Of course these days it’s all automatic (unless something goes badly wrong) so that put paid to their “fun”.

    1. Back in the good old days when you could just stroll into a power plant, I got a nickel tour of a Pelton wheel hydro plant. There was a bare light bulb in a ceramic base mounted on the wall used to sync the generators, somehow connected between the grid and the generators. When the bulb was glowing brightly the generators were out of sync with the grid, and when the bulb went out the operator would throw the switch to connect the plant to the grid. I imagine there’s something higher tech now being as how the plant is automated.

      The lengthy spillway associated with this plant was allowed to fall into disrepair and the plant was offline for several years. Rumor has it that the spillway was rebuilt when the TVA realized that this humble plant was the only one in their grid that could bootstrap itself.

      If you pass by it (Ocoee#2 near Chattanooga) you can see that the middle of the building is laid up with different brick. As the story goes, an engineer and intern inadvertently threw a generator offline allowing it to overspeed and explode. This story is in my engineering ‘at least I didn’t do THAT’ file.

      1. There is a Baby Hydro Plant at the foot of the San Bernardino Mountains on Hyw 38. It Lifts water out of the dry creek bed ( Vertical Turbines ) some 5,000 ft above in the mountains, then Passes it through Thin Wall Steel Pipe till it makes about a 1,000 ft drop into the Peltons.. The Water has now been rescued to provide water to the Cities Below. Which is now it ONLY purpose for it’s existence.

        The Care Taker/Machinist Job was to build the next set of the Buckets and Install them. And do it all over again..

        It needs to run to pump the water out of the ground to feed water to the cities below.. Interesting how it’s function has now changed.. at least it’s still running.. I think it’s a pre 1920 install.

      2. The light bulb trick involves supplying one leg from the grid, and one leg from the generators.

        If the two supplies are in unison, there won’t be a potential difference between the two sides of the light bulb (so no current will flow and it won’t light up).

        If the supplies are out of phase, there will be a difference in potential <= the supply voltage (depending on how out of phase they are, with 180 degrees giving the biggest difference). That'll make the bulb light up.

  3. When I was responsible for the infrastructure supporting a modest data center, I had a 90 KVA Baldor alternator run by a Deere 4039T engine. The unit was capable of being 24/7 prime power if de-rated to 75 KVA. I ran the engine to slowly slip phase lagging utility power at just below 1800 RPM, or about 59.95 Hz. The alternator was connected to the building via an Asco transfer switch. I tested the alternator set on a biweekly schedule even number tests were just running off line to reach engine colorant temperature to open the thermostat to operating spec and to dry about the exhaust silencer. Odd numbered tests, I triggered the switch manually to start the alternator and get the unit speed before pressing the transfer button. By slipping the phase, the switch could transfer within a few seconds. I would let the unit supply power for 20 minutes before transferring back to utility power and allowing a ten minute cool down. The data center had two on-line 20 KVA UPSes supplying separate 208 volt 3-phase busses to servers with dual power supplies, so there was some time to back out if something went wrong with the alternator test.

    I have seen older split-phase 240 volt installations which were completely manual with only two 120 volt light bulbs in series between one leg of alternator power and corresponding leg of utility power. If the lamps were not visibly lit, it was safe to throw a knife switch to go either way between utility or backup power.

  4. So the absolute original way to do this a 250V light bulb. You would run a 120v line from each side to single light bulb and when the light bulb was at its brightest/darkest (depending how it was wired) they would throw the tie breaker. You sometimes still see a random 250v bulb on gear in substations.

    As for damaging your equipment, you would be surprised what it can live through.

  5. The real trick here, particularly if there is a relative island grid (like the UK/Ireland) is matching demand with the generation as it is brought back online. The two need to be matched, but the demand lost is not the demand picked back up. Think of what the fridges, heat pumps etc will do on re energisation

    1. It really doesn’t matter how big or small a grid system is, as in the UK or any other place. All grids are huge systems anymore. Obviously the grid cannot at any time be brought back online all at once, it has to be done in stages for regions and any supply into a grid has to be synchronized in its phasing and load.

  6. I worked at a plant that had one unit capable of a black start. The other three had been upgraded and could no longer black start but all it takes is the one unit to get things going.

    The black start unit still had a mechanical flyball governor and a 24 volt jump starter pack would be used to flash the field on the generator rotor. You got the unit up to speed and then attached the jump starter and waited until the lights came on in the control room.

    Considering the generators were wound in 1898 what is now an emergency start up would have just been an average day.

  7. What about induction generators? I may well be mistaken here, but I thought the majority of the really heavy generators were of the induction type, on account of these don’t try to force phase onto the grid, discrepancies between line frequency and mechanical rotation define the ‘slip’, which in turn determines how much load the generator sees.

    It’d be fun to see this same kind of setup using an induction generator.

  8. While in the Navy (60s), I saw they had to synch up generators occasionally; as I recall they had one (two?) meters they watched and then threw the switch. They did take the process very seriously :) Last thing you want out at sea is to blow the generators (especially in wartime conditions).

  9. wouldn’t the easier way to turn off the grid be to fell a few transmission towers in awkward locations after the generators either before the first step up substations? they are usually spread out across the rural areas and have little protection?

    1. If by “a few” you mean, “most of them”, that might work. The grid is fairly robust when it comes to protecting itself from faults, as long as it isn’t the hottest part of Summer. Taking out one major transmission line may take out a small city, or part of a large city for a little while, but every major generating station has a number of separate transmission lines directly feeding different parts of the cities it supplies, and the grid within the city provides multiple ways for power to get to everywhere in the city from multiple transmission lines. And that’s just from that one generating station. The transmission grid contains many redundant paths, if for no other reason than because it is sometimes necessary to shut down a transmission line for maintenance. When there is a cascading failure blackout, it’s usually because the grid was already overloaded, and then something broke.

      As for awkward locations, yeah, there are places that are hard to get to. And then there are helicopters. Again, every transmission line needs periodic maintenance, so every transmission company knows how to get to every inch of their lines.

      I suggest taking a tour of the transmission lines between you and where your power comes from, using satellite view on a map website, just to see how interconnected the transmission network is. Only, do that from a computer at the public library, because you don’t want to look too interested…

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