Tis indeed a fragile system in the US. 
Ron
Not true.
Hi,
To quote the attached document;
FREQUENCY IN THE POWER SYSTEM
Australia’s National Electricity Market (NEM) power system
operates within a set frequency range around 50 Hertz (Hz).
This underpins the safe, secure and reliable transmission of
power through the electricity supply chain from generators
to consumers.
Controlling power system frequency requires the
constant balancing of electricity supply and demand.
If electricity supply exceeds demand at an instant in
time, power system frequency will increase. If electricity
demand exceeds supply at an instant in time, power
system frequency will decrease. If the change is too
great, generation and load can be disconnected. In an
interconnected power system, frequency is usually the
same throughout.
AustralianMainsFreqControl.pdf (322.7 KB)
Tom.... 
PS. Frequency control is not a "She'll be right mate".
If it drops too low it could damage older style line frequency transformers and some motors.
Magnetic devices are designed with a minimum frequency in mind. If the frequency goes too far below that they start to look like a short circuit.
For proof of concept, can you put a variable frequency in at lower voltage (i.e. somewhere further down the circuit).
If so you can use your computers sound card output.
@fluks For testing purposes, just stand your test on it's head. Instead of changing the input frequency, move the standard you're measuring it against. Explanation: If your measurement is of the period of the waveform, and you're comparing it to 20 mS +/- some deviation, change that comparison so the actual 50 Hz waveform is "out of bounds", to test your response.
C
Hi,
Exactly, see post #13.
Tom...
While I figure the thread starter's question(s) have been asked and answered the history of power mains frequency is pretty interesting. My grandparents house was in the Park Slope section of Brooklyn, NY and originally the Edison power delivery was DC. When Consolidated Edison went to AC they gave customers large seilentium rectifiers so the DC compressors in their refrigerators would continue to have DC.
Originally the mains frequency was 25 Hz. The first generators at the Niagara Falls project, built by Westinghouse in 1895, were 25 Hz, because the turbine speed had already been set before alternating current power transmission had been definitively selected. Westinghouse would have selected a low frequency of 30 Hz to drive motor loads, but the turbines for the project had already been specified at 250 RPM. The machines could have been made to deliver 16+2⁄3 Hz power suitable for heavy commutator-type motors, but the Westinghouse company objected that this would be undesirable for lighting and suggested 33+1⁄3 Hz. Eventually a compromise of 25 Hz, with 12-pole 250 RPM generators, was chosen.[3] Because the Niagara project was so influential on electric power systems design, 25 Hz prevailed as the North American standard for low-frequency AC.
This is all covered in the Wickipedia. Interesting reading.
Interesting also the NYC subway system is still powered by DC. Alternating current (AC) operates signals, station and tunnel lighting, ventilation and miscellaneous line equipment. Direct current (DC) is used to operate trains and such auxiliary equipment as water pumps and emergency lighting.
The system's 214 electrical-power substations receive high and low-voltage electrical current from the New York Power Authority. Substations may receive as much as 27,000 volts from the power plants and then convert it for use in the subway. The subway's contact (third) rail requires 625 volts for operating trains.
Before they became obsolete the same was true of all the trollys and busses.
Anyway the history of mains frequency is an interesting one and what drove the changes is interesting.
While I see 50 Hz mentioned the thread starter has not mentioned a tolerance? For example a low limit of maybe 48 Hz and an upper limit of 52 Hz or 50 Hz +/- 2%.
Ron
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Great information! For us on the Left coast, that is all new!.
But we still use DC for power transmission from The Dalles in Oregon to somewhere in the Los Angeles area. 250,000 volts DC. One wire down and one wire back. They tried 500,000 volts using both wires and ground return, but ground currents were shocking livestock and well casing became hot and many other problems.
The 250,000 volts DC begin as 3-phase AC in the dynamos of the Dalles Dam. 6 three phase dynamos connected to transformers, but each dynamo is out of phase a bit from the others. The net result is 18 phase AC going to rectifiers. When we visited in the 1970's they were mercury vapor rectifiers. Now they are all solid state. When we visited, we walked in the rectifier room to see how it all worked. The rectifiers were all inside wire cages for protection.
A DEC mini computer on both ends of the circuit controlled the whole operation.
Power was also returned from LA when they had excess generation available.
So, no Hz also is working today.
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Hi,
Power cables under sea are also DC.
http://www.basslink.com.au/basslink-interconnector/
Tom....
AC power transmission becomes inefficient over longer distances and DC starts to win even given the conversion efficiencies. The UK - EU link has always been DC because the systems though nominally 50Hz are not in sync. Interestingly there has recently been some publications about the effect of DC undersea transmission on various marine species that are sensitive to low frequency magnetic fields.
Though I am sure that GPS is used to sync power systems, not just in the US, I don't think they are actually phase-locked as that would be very difficult given the huge inertia of the system. Certainly frequency-locked. Some interesting data here: 60 Hz AC Mains Frequency Accuracy Measurement The site has a lot of other interesting information and projects. Also this: The Dirty Little Secret about Mains Power Line Frequency
I suspect that is the cause of the glitches experienced when switching from hydro to solar/wind generation.
While we have a few DC transmission lines here in the US our neighbors to the north have several large scale DC transmission lines. While we have drifted well offtopic high voltage transmission lines are a just plain interesting topic. There are several really great videos as to the maintenance and inspection of high voltage transmission lines, this is one. Years ago during a trip to West Virginia, US to visit a close friend I watched a helicopter with a brush hog hanging off of it clearing trees and groth away from high voltage power transmission lines. One mistake and everything is toast.
Ron
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Sorry if I stepped on your post:)
However I wanted to add the use of the sound card. Didn't think without the first paragraph such a suggestion would be clear
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I was under the impression phase "locking" was inherent in the paralleling of the generators. Once "on line" the droop (speed vs load) causes a generator to maintain the grid frequency as increasing in speed will result in a very large increase in electrical load (slowing the generator down). The same for slow speed where the load will become lighter allowing the generator to speed up.
Interestingly I believe the solar panel inverters are designed to always add power the whatever the line frequency is and they don't worry about synchronization. Perhaps could be thought as a current supply.
Sorry, I was unclear. I meant phase-locked to the GPS - the measured results show clearly that the phase and frequency both drift quite significantly over time even if the long-term average is exact. The generators do lock to each other though even then there are transient effects which in extremes can cause alternators to have to be taken off line to avoid catastrophe.
Using the computers sound card or audio out works well enough. I tried it using the Tone Generator in Audacity which also gives waveshape options.
Ron
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