I know I am bringing up a slightly old thread but I have to chime in here. 

With regards to the polarity discussion, there is a good reason why one blade of a plug is larger than the other and its designed to fit one way. Some stuff can be plugged in regardless of polarity but other cannot. Yadda yadda yadda. Best not to force it if there is a reason why its meant to go one way and one way only. 

Brownouts can cause failures of certain electronics. It depends on how sensitive they are. A friend of mine is an avionics mechanic and has told me that certain ground power units hooked up to certain commercial airplanes will not mix as the aircraft rejects the power and trips itself as a form of protection. Some of our equipment in the hospital is highly sensitive and will not accept bad power, and in brownout situations can trip. However, the power situation at the hospital is very regulated to prevent these machines from tripping just willy-nilly and we have a fair amount of equipment to reduce that risk. 

Certain high voltage VFDs I have experience with will also trip if they experience a supply undervolt, under current, or under frequency. Motors are especially prone to brown out issues as someone had mentioned, cooling, is not there like it used to be if its not getting the required jam. 

And Henerik, I think you have it backwards. If the load demand increases on a power plant, the generators speed (which directly relates to grid frequency) slightly (and I mean slightly, like .1 Hz depending on how good the AVR is) decreases and MW output more or less remains the same or slightly (and I mean slightly) decreases. The AVR sees this undervoltage, the turbine speed sensor picks up the slight speed decrease and together the both of them control the boiler to fire more, thus increasing generator output. 

Yes, it is a party when a new plant syncs to the grid for the first time. We had a pizza party at the refinery when we did for the first time. 

Full load rejection on a 150MW turbine is kinda scary. A few fellas I worked with at the refinery worked at power plants in Alberta during the LA earthquake of 1994 and they felt it all the way up in Alberta. Tripped two 300MW units. North America is broken up in to a few different reigons for power that are not interconnected. Western US and Canada is one, Eastern Canada and north eastern US is another. Texas has its own... 

Oh, btw, I used to work at a power plant, and the refinery I worked at we made our own power with a 62MW junk Siemens turbine. 

@chetlaham

The AVR (auto voltage regulator) will pick up in a reduction in voltage output and crank up the DC excitation voltage in the rotor to make up for this slack, adding more excitation volts will only increase real power to a point, but then reactive power production goes up if too much is added. The opposite can be said if there is an over voltage situation. Excitation voltage is also used to control the power factor. The power plant made 13.8kV off the end of all generators we had, and then we stepped it up to 138kV in the switching yard for transmission.

No, speed is not in proportion to the generators MW output. The speed is directly in relation to the frequency made. A two pole generator needs 3600rpm to make 60Hz power, 4 pole needs 1800rpm, 6 pole needs 1200.

Once the turbine was up to speed, 3600rpm (or 1800rpm for that junk Siemens machine) wed switch the boiler control to Boiler Follow (meaning the steam generator would slave to the turbine), then we used a synchroscope to bring it in phase with the grid. This can be done automatically, but best practice as taught to be by a bunch of old fellas was to sync it manually, nail the frequency, then put the syncroscope in auto, then close the breaker to the grid. The idea is if the auto sync system failed and switched to manual, its already set up in manual.

Once the breaker was closed on the turbine and it was on the grid, it was doing 3600rpm with no load, 0MW. On the 150MW turbine the boiler would be about 2% MCR (max continuous rating). We would then release the turbine control to the Electric Control Centre or ECC. At this point the plant was hands off. The turbine followed the grid demand, and the boiler/steam generator followed the turbines load demand. ECC would increase the MW demand at a steady and predictable rate taking care not to over stress or over load any of the equipment. Usually a MW a minute or so.

The entire time the turbine was spinning at 3600rpm, never changing speed. The torque demand is where the power is generated from and that was met by firing the boiler more. And since the boiler was in Boiler Follow mode and slaved to the turbine, if the turbine asked for more the boiler gave it. Adding excitation voltage increases the torque load of the flux lines the rotor has to cut through. Interesting enough the poles on a turbine generator are the rotor, and the stator windings are where the power is actually made. This is because the exciter windings are a lot less mass than the generator windings, and its something like the generator would be 4 times the size if that was the case. The logic for the turbine looked at its speed and the voltage output via the AVR for control and adjusted as necessary.

If you lost a turbine on a generator trip it was pretty scary. The whole place comes to a grinding halt in a split second. The breaker opens, that sends a trip signal to the turbine throttle, which sends a trip signal to the burners. It all cascades back and if youre lucky enough you wont pop a safety valve and wake the neighbors. The junk Siemens turbine had a dump valve on the inlet header so that any full load rejection shouldnt lift a safety valve. When a steam generator is making 1.5 million pounds of steam an hour and all of the sudden it doesnt have a place to put said steam...

Yeah the rotor never really changed speed based off MW load. The controls are so finely tuned that any change was smoothed out almost instantaneously. The speed change was only because more torque was added/subtracted to the rotor from a load demand increase/decrease. Any slip on that turbine would mean were making dirty power, which ECC had the ability to disconnect us from the grid if they started to see weird things. Once the generator was tapped out, it couldnt make anymore and it just sat there pounding out all the MW it could maintaining 3600rpm all day. There are ways to cheat the system by bypassing feedwater heaters and stuff to try and get more MW out of a turbine but the plant becomes inefficient then.

Oh yes, the steam would vent to the outdoors from either the dump valve or the safety valve. Newer plants may have silencers on their safety valves but ours did not, and the plant could be heard from about 15km away. The noise isnt like a scream you would imagine like what you would hear from an old train. It was more just a wad of noise similar to a fighter jet. The refinery had silencers on the dump and safety valves so it was just a low rumble at best. Lifting a safety valve is a very very very rare occurrence. In one province in Canada, lifting one was a reportable event to the regulator.

Some turbines do require an initial purge or venting to atmosphere before steam admission to the blading to ensure high quality dry steam. This is more so low end, low MW dumpy turbines. But most closed loop, regenerative turbines you find in a power plant have lots and lots of drains on the lines so that only screaming hot dry steam is at the turbines inlet valve and is admitted nice and slow, and because the whole thing is under a deep vacuum it just flows through readily requiring no atmospheric dumping. Its a waste of money to vent all that steam, so the drains dump back to the condenser.

Just under 500MW. Three units. The refinery was plated for 80MW but the imagineers at Siemens dropped the ball on the condenser so in the summer you could only get 62MW, and in the winter if you were lucky about 70MW.

Siemens makes crappy steam turbines anyways, and they now service Westinghouse (WH turbines were also 'well made'), AEI, Reyrolle made machines so you can only imagine how well theyre taken care of. The best turbines money can buy are either Toshiba or Hitachi, their service is insane good.