Power Grid Stability

The only trick is getting it close enough in sync so you don't operate the O/C device when you first connect them. Once connected the grid will drag the generator around and keep them in sync.

On Tue, 26 Jun 2007 12:21:58 -0400 snipped-for-privacy@aol.com wrote: | On Mon, 25 Jun 2007 23:10:33 -0700, snipped-for-privacy@yahoo.com wrote: | |>Hi, |>

|>I have heard of cases where engineers were unable to synchronize a |>generator with the grid. The generators and the equipments were |>reported to be in excellent condition and they had the latest safety |>relays etc. The IEEE 1547 standard states for synchronization - |>voltage - ? 5% of base V |>Phase - ? 5 degrees |>Frequency - For a 60 Hz system ? 59.3 Hz to 60.5 Hz (How much for a 50 |>Hz system?) |>

|>If all the above conditions are met, am I correct in assuming that |>synchronization is possible? What is the generator is working |>perfectly and it is the grid is unstable? Is this possible? In such |>instances, how would you check the grid? How can this be corrected, |>assuming the grid is unstable? How can synchronization be possible? |>

|>Thanks, |>LS | | The only trick is getting it close enough in sync so you don't operate | the O/C device when you first connect them. Once connected the grid | will drag the generator around and keep them in sync.

And am I correct in understanding that the voltage can then be tweaked via the excitation current to manage how much load the generator carries?

When you connect a RELATIVELY small generator to "the grid", you don't change anything on the grid. If you vary the excitation current you don't change the power transferred but the "power factor." You can make your local generator "look" like a giant capacitor with high excitation current and like an inductor with low excitation.

The POWER delievered to the grid is just a matter of how much mechanical power you delivered to the generator. When you come up to speed you are adjusting the governor. Once "in sync" further adjustments of the governor just increase the "error" signal and increase power but don't increase speed which is locked to the grid frequency.

There is a special case of an "induction generator." You can think of it as a generator with zero excitation. The generator would be permitted to speed up over sync speed and would provide power to the grid but "look" like a big inductor.

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| There is a special case of an "induction generator." You can think of it | as a generator with zero excitation. The generator would be permitted to | speed up over sync speed and would provide power to the grid but "look" like | a big inductor.

Oh, that's an interesting concept. Of course it would be a _relatively_ small generator and not affect the grid frequency at all.

Mechanical power in = electrical power out (neglecting losses of less than 2% usually)

Change the excitation and you change the var loading or voltage or (usually) both.

Change the mechanical power and you change the electrical power and (if you can measure it) the frequency for a short time until the system controller brings the generation to a level that corresponds to 60Hz (or whatever the desired frequency is). The system controller does this by changing the mechanical power of other generators.

About 35 years ago, the 'morning report' of the utility I worked for crossed my desk on a daily basis. One of the things that was reported was frequency deviations and the probable cause associated. The loss of a 1000MW generator could cause up to a

0.01Hz change in the frequency or (more likely) a 0.005Hz or less change. Usually this would be corrected in 10 to 30 minutes IIRC.

Many years later, I had control of a large unit for thermal sensitivity testing of the rotor and could change the excitation and power independently (to separate turbine induced vibration from vibration induced by shorted turns in the rotor). I noted that for that morning (the test was from about 11PM to 8AM) a change of 30MVAR in the overexcited direction caused a rise of

1kV on the local 345kV system. The system dispatcher and I used this to pace the test. I didn't note any specific change in the voltage caused by power adjustments.

In theory, everything affects everything. In practice, some things move some meters and apparently not others.

Guess that all depends on your idea of 'relatively' :-)

A couple of steam units near hear have disconnected the shafts and use the generators as synchronous motors with nothing connected to the shafts. The whole purpose is to allow them to use this 'trick' of adjusting excitation to supply reactive load to the local substation. These units are/were about

200 MW before the modification.

The bigger fossil units next to them are 900 MW a piece. When they sync to the grid and assume load, they don't change the grid frequency at all (at least, not enough to show up on any instruments). I don't think anyone would classify 900 MW as '_relatively_ small'. :-)

daestrom

I have heard of cases where engineers were unable to synchronize a generator with the grid. The generators and the equipments were reported to be in excellent condition and they had the latest safety relays etc. The IEEE 1547 standard states for synchronization - voltage - ± 5% of base V Phase - ± 5 degrees Frequency - For a 60 Hz system ? 59.3 Hz to 60.5 Hz (How much for a 50 Hz system?)

If all the above conditions are met, am I correct in assuming that synchronization is possible? What is the generator is working perfectly and it is the grid is unstable? Is this possible? In such instances, how would you check the grid? How can this be corrected, assuming the grid is unstable? How can synchronization be possible?

------------------------- The phase angle has to be pretty close to sync the generator to the grid. And to get that, you have to have pretty fine control of the unit's output frequency. Not just keep it within 5%. If the unit frequency stays within even 59.9 and 60.1, but can't be kept stable and wanders back and forth between those values, synchronizing should probably not be attempted.

Determining *which* is the unstable frequency is as simple as placing an accurate meter on each and watching for a couple of minutes. Or if you want to get really fancy, hook up a strip-chart recorder capable of monitoring the frequency of both the unit and the bus (suitable transducers on each side of the sync-metering circuits should work).

I'd bet 'dollars to doughnuts' that its the governor on the unit and not the grid that is unstable. Unless you're talking a tiny, private 'grid'.

One other thing that people sometimes forget (to their dismay), is that polyphase equipment must also have the same phase rotation. This has to be checked in new installations and after disconnecting equipment/wiring. The results of having reversed phase rotation doesn't show up on conventional metering and can be truly disasterous if not corrected before syncing to the grid the first time.

daestrom

In some cases, that ends up being the last time as well. ;-)

It would be nice if the OP had specified why the generator failed to synchronize. Which protection function operated to either prevent breaker closure or to subsequently trip it open again?