squishy@casbah.acns.nwu.edu (Shishin Yamada) (01/14/91)
In article <10012@as0c.sei.cmu.edu> rsd@sei.cmu.edu (Richard S D'Ippolito) writes: >In article <3122@naucse.cse.nau.edu> Robert Wier writes: > > > I was suprised to learn when the plant was shut down that they didn't > > disconnect from the grid (for example, to clean the pipes and > > turbine valves). They just let the grid power the generators as > > large electric motors. Said it was a whole lot easier than trying > > to bring them up from a dead stop and re-synch them to the grid > > (which had to be done by hand). > > >There's another reason that _very_ large horizontally-mounted generators are >never stopped -- if they sat there for a long time, the weight would cause a >permanent flat spot in the shaft and bearings! > This is probably one of the reasons. The other most likely is that they are using these generators like large synchonous motors. They are then used to store power and correct for minor phase differences, much like a large capacitor (or inductor in other cases as seen by the network). They can then be used to stabilize the power grid. Systems like this were built since the 1920s and are called something like synchronous stabilizers. Nowdays, switching capacitor banks are used as they are cheaper and easier to control. At least this is what we just learned in EECS C61 - Power Transmission and Control class. ---------------------------------------------------------- Shishin Yamada - Northwestern University Electrcial Engineering Class of 1991 ---------------------------------------------------------- PS: It seems the old stabilizers were large motors often without loads attached to their shafts. By their shear shaft inertia, they would _help_ correct the phase back to normal.
rsd@sei.cmu.edu (Richard S D'Ippolito) (01/18/91)
In article <5834@rsiatl.Dixie.Com> John G. DeArmond writes: >Shishin Yamada writes: >> > > I was suprised to learn when the plant was shut down that they didn't > > disconnect from the grid (for example, to clean the pipes and > > turbine valves). They just let the grid power the generators as > > large electric motors. Said it was a whole lot easier than trying > > to bring them up from a dead stop and re-synch them to the grid > > (which had to be done by hand). I wrote: There's another reason that _very_ large horizontally-mounted generators are never stopped -- if they sat there for a long time, the weight would cause a permanent flat spot in the shaft and bearings! John replies: >Both of these stories are silly. I will defend my own: I used to receive the ASEA journal and news magazines, and one of them says exactly that. It dates from the early 70s when I worked on large equipment (3-phase electric arc furnaces). It reported how the generator was turned at 1rpm (as you admit below) to prevent permanent shaft distortion AND bearing flat spots. I guarantee it, John! I'm not one to forget what I read and post what I don't know. In this case, I thought it was an interesting anecdote. Note that the units I read about are probably still in use, (what's 20 years to a power plant?) and that the technology does improve. >The 1200 MWE turbogenerator at the first nuclear plant that I worked at >was assembled fully 5 years before it was turned. The shaft did indeed >sag but was easily reversed by turning it 180 degrees and allowing it to >sit for a couple of weeks. We then spun it up on steam and dynamically >balanced it and away we went. All turbines of any size are equipped with >turning gears and drives that rotate the shaft at perhaps 1 RPM. The >turning gear is designed to equalize sag forces right before startup and >to allow the shaft to cool down equally after shutdown. Cheers. Rich
squishy@casbah.acns.nwu.edu (Shishin Yamada) (01/18/91)
Richard: Sorry, I did not write the initial question. I was the one who stated that the power plant was being run as a large synchronous motor. The reason being that it was being used as storage for reactive power to normalize for disturbances in the transmission network. If I remember correctly, these were called something like synchronous stabilizers. To the Transmission Line it looks like a Thevenin equivalent of a large capacitor (or inductor in other cases). It acts as a generator in this case supplying VARs if needed. These storage devices were first used to correct phase errors around the 1920s. Nowdays, switching capacitor banks are easier and cheaper to use. Phase error causes loss of real power because the current and voltage are out of phase. --------------------------------------------------------------------------- Shishin _Squish_ Yamada Northwestern University Evanston, IL EE Class of 1991 --------------------------------------------------------------------------- Down with Sadam! (At this time, missiles have supposedly hit Tel Aviv: AP news 6:30pm Thurs). God help us.