Power Grid Synchronization and Reactive Power

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- daestrom
  
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posted
17 years ago

Sun, Apr 1, 2007 4:29 PM

Not quite, 4-pole, *single* shaft 'nukees'. One HP and three LP mounted all in-line with the generator. Some of that 40MW (seen it on the oscillograph traces) goes into the exciter I'm sure.

Regarding LP blades, you're right they don't 'like it'. GE warns about running with less than 5% load for any length of time and they recommend not breaking vacumn at all unless it's an emergency. Saw a nice video they developed from one plant where they mounted the camera and strobe light in the exhaust hood. After tripping, the last stages 'wave' back and forth something awesome.

In the units I've worked with, that is what the 'backup unit protection' is for. Some times the 'anti-motoring' is a series of limit switches and the backup is reverse power relay. Sometimes it's the other way around. Usually 'backup' protection uses a different method of detecting the problem from primary, whatever it is.

Also seen a series of valve limit switches in the close permissive. Trying to protect from inadvertant energizing.

Guess it depends on how sensitive the 32 is set at. Can't imagine how long you have to motor to get a high exhaust hood temp. On the 4-pole units I work with, that would be a lot longer than the 32 relay.

Why operators afraid of setting off the 32? I've seen some plants that basically depend on it when shutting down. Basically they say 'reduce the steam flow and verify the turbine trips when MW goes below zero'. Some bug-a-boo about letting the protective relays activate?

One of the things I've seen plants during outages is 'backfeed'. They open the generator disconnects and bring line power backward into the step-up transformer. Then with 24kV on the low side, they feed the 'hotel loads' of the plant from the normal service transformer. I can't stress to them how disasterous it would be if they missed that one step of 'open the generator disconnects'. But your experience brings it into crystal-clear focus.

IIRC, the steam unit near me installed an electric motor. They get them up to about 3000 (2-pole unit) and then pull them into sync from there. Been awhile though, maybe it was higher RPM.

Well....

50MW for 3 minutes is about 15 teraJoules. If it *ALL* went into the water (none into heating steel, or excitation or other losses), that would heat up about 45 metric tonnes of water to boiling. But you mentioned about 10 MW in generator losses, and certainly the steel runner and casing take a lot of heat to warm up as well, and how much water is in one of those casings.

Anyway, seeing is believing (usually:-) and if it doesn't warm up that much, it must be true. Mind you, I wouldn't want to test how long you could go. From what I've seen of hydro-generators, the tolerances are tight on the rotor of the generator (~ 1/8 inch or better). How tight is the fit on the runner?

daestrom

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- Fred Lotte
  
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posted
17 years ago

Mon, Apr 2, 2007 2:04 AM

I got an old tie clip that has that model on it. ;-)

At no load, I'd expect the excitation to be of the order of 2MW not including F+W of the exciter.

The 2 pole machines that I have in mind (both about 800MVA &

600MW) had full load losses of about 2% or less. About half were no load losses. With vacuum maintained the turbine losses were ? but small. My original 4MW is probably about half of what it should be but 40 still seems like a big number. If you saw it, you saw it. I made it a practice when I was working to not doubt what the operating types saw, only try to explain it. Sometime I found that they were right and sometimes their observation or instrument was off a little. But, we found out for each other's mutual benefit and education.

Probably about the same. They were set to about 175dF. They could also be a problem when going on line if the unit wasn't loaded fast enough. I don't recall ever seeing one trip but I'd guess anywhere from 5 to maybe 30 minutes depending on vacuum and any number of other things that take place during turbine run up.

The best inadvertent energization protection is an open disconnect switch and operators that check things out and think before they close a breaker or switch. Every other scheme fails eventually. In this day of remote indications and operation etc. this is a really tough problem. A single failure can turn the machine (including the turbine) into scrap.

I think that's probably it. When the unit protection operates the shutdown of everything is pretty much taken out of the operator's control. They don't like that. I don't blame them. But, I wish they'd calmly reduce the load to slightly motoring then trip the breaker.

During a strike, when I really learned how to operate a power plant, one of my fellow amateur operators in training would actually check the rotor of the unit watthour meter to confirm that he had 0 watts before he tripped a machine. I generally put the watt meter pointer just below 0. These were some 62.5MW 4 pole machines from 1930 or so that were very forgiving trainers. Protection was a differential relay and a smoke detector + an operator with good peripheral vision to catch that sudden meter twitch and a good ear for a disturbance in the hum. Those operating in the turbine room basement or on the firing floor had similar sensitivity for things that go bump in the pump or bang in the boiler. ;-)

I also took a 250MW off line the same way.

I've done this many times when simulating operation to check the unit protection. We didn't have generator disconnects so the phase bus straps were removed for this. I always personally inspected that the generator terminals were disconnected and roped and flagged off limits before we did these tests.

Well, according the the test engineer when we started the plant, about 1% goes into noise. I can believe it. Also, there are some

8 inch recirculation lines that bypass the turbine and there is a little leakage thru the wicket gates and the depression system vents stay open at least til full prime (maybe longer, it's only been about 40 years since I checked that system out). All I know for sure is that it's never been a problem and the unit wattmeter reads 50 MW almost exactly any of the few times I've watched a pump start. The generator/motors are 220MVA/260KHP and regularly pump at 230 to 250MW (which is at the 1.15 service factor limit). The air gap is about an inch.

The turbine/pump (it's a single machine that looks a lot like a pump) has an overall diameter of less than 15 ft. It's hard to estimate because most of the time I've only seen it from the draft tube which is about 6-8 ft in diameter (I'd estimate) or stood on the head cover which is 218 inches +/-. Seal clearance is probably of the order of 1/8 inch or less, probably less. The seals are 'lubed' with really clean water from the high head penstock. It's all encased in a huge monolith of concrete.

The original pump/turbine impellers were changed out about 15 years ago and I haven't attended a pump start with the new ones so the current rule of thumb times and loading may be different. The numbers were very repeatable with the old pumps.