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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?
Yes, transformers could be lighter. 400 Hz power is used on aircraft for this reason.
The generator wouldn't need to spin faster if it is designed with more poles. Motors would also have to be redesigned to run at normal speeds with 1000 Hz power.
But how often are 60 Hz transformers used in modern electronics? There is a big one on the pole outside of my house, and in my boat-anchor hi-fi amps and tube guitar amps. All the other power supplies in my house use switching supplies that run their transformers at a much higher frequency.
edit: Another thing, transformers tend to vibrate slightly during operation. A 60 Hz transformer will "hum" a little bit. But imagine all the appliances in your house buzzing at 1 kHz. 1000 Hz is two octaves above middle C, and our ears are 30dB more sensitive to this frequency than 60 Hz:
Image source: http://en.wikipedia.org/wiki/Equal-loudness_contour
Why not use an optocoupler? Vishay's SFH6206 has two LEDs in anti-parallel, so it works over the full cycle of the mains voltage. If the input voltage is high enough the output transistor is switched on, and the collector is at a low level. Around the zero crossing, however, the input voltage is too low to activate the output transistor and its collector will be pulled high. So you get a positive pulse at every zero crossing.
The most important parameter for an optocoupler must be its CTR or Current Transfer Ratio, which gives you the ratio between output current and input current. You can compare it with the \$H_{FE}\$ of a common BJT. But while \$H_{FE}\$ for a small signal transistor is often higher than 100, CTR is low. So low that it's expressed in %, like 20%. That's not 20, that's 0.2.
A CTR of 0.2 means that you have to drive the input LED(s) with 1mA to get a mere 200\$\mu\$A out. Usually that's not a problem, since the output is often only used to get a logic level, which will connect to a high impedance input. In that case a high value pull-up resistor can be used, like > 27k\$\Omega\$ in a 5V system. Then the 200\$\mu\$A is sufficient to drive the output low.
Detailed calculations can be found here.
Best Answer
"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.