In a power plant, what keeps the power at a solid 60hz? It seems like turbines can't always be running at the exact same speeds, given that combustion doesn't always give the exact same temperature (my dad works at a waste to energy plant, which is why I say this).

Also, just for giggles, what would happen if AC power at a different frequency accidently got released into the lines?


2 Answers 2


"The grid" keeps everything locked, more or less.

The AC mains grid is like a very very very big flywheel.
Individual alternators attached to the grid are frequency locked to it and cannot noticeably push it faster or slower.

If you apply more power to your local alternator it will slip in phase relative to grid frequency. Slip too far and you slide off the edge (90 degrees off phase absolute maximum possible, less in practice) and free run relative to mains. At that point it is a race between the impressive magnetic arc blowout breakers and alternator death. The breakers usually win. Usually.

Even a small breaker or fuse "going" - say 100 kVA, can sound like a bomb blast. Ask me how I know ;-).

If enough stations push too much or pull too much overall the whole mains frequency will slowly slip. A central control station observes overall frequency and adjusts power in overall relative to load to keep frequency somewhat stable. Mains frequency is compared to a standard long term and nudged as required. NIST is the supplier of the ultimate US reference. I recall that the need to synchronise the whole grid was removed in the last year of few.

At half time in the US Superb-Owl when 100 million US-Americans get up out of their seats and go to make a cup of coffee the power station controllers who have been waiting this moment of terror put the grid into 'go around power please' mode to take the hit. [[I'm making that up about the terror, BUT having seen the grid power plot for that occasion it is probably true]].

If you have a moderately small grid and input a significant percentage of extra energy from elsewhere via eg a DC link and then artificially make AC from the DC and pump it into the small grid , you can accidentally wave the grid too and fro in the breeze, as it were. Sicily, fed from Italy, has had this happen.

  • 3
    \$\begingroup\$ "US Superb-Owl when 100 million US americans get up out of their seats and go to make a cup of coffee"... That is actually a very big issue for water and sewage utilities. Half time at the super bowl is generally what those utilities size their systems for. It is always nice to remind people that transients can kill a system just as easy as a long term effect can. \$\endgroup\$
    – Kellenjb
    Dec 21, 2011 at 1:40
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    \$\begingroup\$ @Kellenjb - BUT, no +1 for superb Owl, which you managed without a blink ? :-) :-) :-) \$\endgroup\$
    – Russell McMahon
    Dec 21, 2011 at 1:53
  • \$\begingroup\$ Flywheel? What a neat analogy! Now how do you know those breakers sound like bombs? \$\endgroup\$
    – Kyle
    Dec 21, 2011 at 7:04
  • 2
    \$\begingroup\$ I'm giving a +1 for Superb-Owl.... \$\endgroup\$ Dec 21, 2011 at 8:27
  • \$\begingroup\$ But what prevents the whole system frequency drifting? \$\endgroup\$ Sep 3, 2012 at 12:58

I'm not an expert here, but when a generator is connected to the grid it behaves as if it was mechanically connected to every other generator on the grid. This means even if it wanted to turn at a different speed it couldn't (unless it's power contributed a significant percentage of the whole grids power, which it won't be)
To explain this better, if you connected a generator to the grid and then removed mechanical power to it, it would keep turning at the same speed since it is now effectively a motor being driven by all the other generators on the grid. So with many generators on the grid the overall effect is to average any changes out to a very small amount.
Also you have lots of complex equipment monitoring things like frequency, load and making various adjustments as necessary to keep things within defined tolerances.

For the frequency part, other frequencies do get intentionally released into the lines for communications purposes. These are not intended to power anything though.

  • \$\begingroup\$ So what would actually keep the AC frequency "locked?" \$\endgroup\$
    – Kyle
    Dec 21, 2011 at 7:06
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    \$\begingroup\$ @Kyle - energy level is the issue and means of extinguishment to some extent. LARGE breakers cause arcs and provide arc channels for them to rise up with magnetic snuffers sending them to their deaths. There is a street side transformer about 3 lots from my house. Raing unknown but it may serve sevral hundred homes so say 50A x 230V x 200 homes. That's 2 MVA plus but not all at once so maybe an MVA or few. One day Murphy decided to allow one only of its HRC fuses to fuse. I drove the area looking for the LPG explosion or similar. LOUD. \$\endgroup\$
    – Russell McMahon
    Dec 21, 2011 at 11:02
  • \$\begingroup\$ @Kyle - | A man on PICLIST designs HIGH current breakers for in power-plant use. Emergency manual tripping in the case of auomatic control failure requires a long pole, full asbestos and armour suit AND do it wrong and you can die. Really. \$\endgroup\$
    – Russell McMahon
    Dec 21, 2011 at 11:03

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