Capstone units. First generation. 2

[Skogsgurra]

Hello all! I have been absent for a few weeks. Not dead, not ill. Just a lot of work and no easy internet access. But, just you wait! Back again - with a load of questions.

The first one is this: Do Capstone gas turbine generators aged somewhere between 30 and 40 years have CSI inverters to couple the around 60 000 RPM generator to the grid?

A few facts make this a reasonable assumption:
A. They need an inverter, that's for sure. At 60 000 RPM, it can't be done without an inverter.
B. There are no electrolytic capacitors in there. Life seems to be unlimited. That would not be the case with 'lytics.
C. The turbine is started by letting the inverter run it in motororing fashion for a few seconds and then softly moving to generation. Something that CSI inverters lend them selfselves to easily and without any extra hardware.
D. The output from these units produce ringing in the 11 kV Cable net they are feeding. And that has been observed in other cases where CSI inverters are used. The ringing looks like this:

I have had a similar case around 15 years ago. Siemens Simovert A, which are CSI inverters. So a lot Points to that this is also such a creature. But no documents and no technical description available.

Each commutation in the CSI inverter (usually thyristors at that time) pruduces a steep currentchange which results in Heavy ringing in the grid (between transformer leakage inductance and Cable grid capacitance. The ringing is usually in the kHz range - 7.5 kHz in this case - and produce "unpleasant" effects. Like heating transformer cores, killing PSU's and producing sound from anything from flourescent ballasts to coffee Machines and anything in-between.

Tips? Anyone? Technical details, diagrams, descriptions are needed.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[MikeHalloran]

I had forgotten they'd been around that long.

I worked for a minor Capstone dealer from mid-2007 to early 2010. I could swear I'd snagged a complete set of service manuals and such for 30kW and 50kW models, but I must have left it all on the company computer; sorry.

I got to make a marine exhaust elbow for one, that was subsequently marinized, and they tried to sell it into the yacht market. I doubt there's more than one; it was twice the size of a Deere based genset, and not quite as fuel efficient at any power level.

As you say, there must be an inverter of some kind involved, but I have no idea what a CSI inverter is. I remember the box had a crapload of batteries, and a bigger crapload of power electronics in it.





Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Do you have a model and serial number?
I know a guy who might know a guy...



Mike Halloran
Pembroke Pines, FL, USA

 

[Skogsgurra]

Thanks a lot, Mike!

We were not allowed to take notes or photos. The plant was something of a "Schutzgebiet". I tried to memorize as much as I could and make notes after I left the site. But not any model or any serial numbers. The facts that I have are these:
Power: 250 kW each
Year installed: Probably 1975 - 1995
Turbine speed: Around 60 000 RPM
Grid couplng: Via inverter that is also used to bring system up to speed to start it
Bearings: Air "cushion"
A name, GRIEM, seems to be relevant. But do not know how or why.

There are filters on the inverter output. One of them failed an affected operation so turbine had to be shut down until filter replaced with new unit. It may be that there are other filter failure modes that allow uninterrupted operation.

The main question is: Are the inverters in this type of turbines CSI (with current smooting with large inductor) and not VSI/PWM (with voltage smooting with large capacitors)?





Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[catserveng]

I'm not sure if this will answer your questions, but was a pretty good study done with Capstone and Honeywell micro turbines and their response characteristics.

https://eetd.lbl.gov/sites/all/files/publications/capstone-honeywell-microturbine-report.pdf

Not sure if this will directly answer your questions, but the system description and the test response may help you determine the inverter type.

A couple of years ago I got involved in some power quality monitoring at a site with multiple Capstone units, couldn't get Capstone engaged to help resolve the problems or provide helpful information, but did find this site where I was able to at least get a better understanding of how the units worked, maybe you will find it helpful as well,

http://www.globalmicroturbine.com/search/capstone/index.htm

The engine dealership I worked at several years ago became a Capstone distributor for a short period of time, early 1990's, but we gave it up after a short time due to poor support from Capstone.

MikeL.

 

[MikeHalloran]

ISTR that the turbine idles at ~45,000 rpm, and depending on load may go up to 90,000 rpm.
They are definitely not synchronized to the grid at the alternator, but change rpm with load, using the battery bank to slow down or speed up the turbine to deal with transients.

All the Capstone installation illustrations I ever saw showed a nice neat linear array of turbine units, with the exhausts connected together by a bunch of wyes into what hot rodders call a 'log manifold'.
... which works fine for the cylinders of a single engine.

But if you connect the exhausts of multiple engines together, and have to shut one down, you will soon have a problem, because the exhaust gases from the running engines will corrode the connected exhaust parts of the idle engines, sometimes in a matter of minutes. In SI and DI recips, exhaust valves and cylinder liners are most at risk.

So too, idle turbines.

Capstone tried a series of exhaust check valves to isolate the turbines from each other,
then changed the firmware/software so all turbines in a bank had to be on or off, and they could not be individually shut down for scalability. ... which I think was once a major selling point of their advertising.

Scalability could have been retained without corrosion issues, had they specified individual exhaust pipes all the way to atmosphere, but I guess the 'optics' of multiple big exhaust pipes were just too much for their Marketing people to bear.

On the other hand, I get the impression that the foil air bearings don't give much trouble, and the recuperator is very effective at (almost) compensating for the turbine's otherwise horrible fuel consumption in relation to a Diesel of similar electrical output.




Mike Halloran
Pembroke Pines, FL, USA

 

[Skogsgurra]

Thanks! Both Mikes.

I have been plowing through the Cat-Mike's long list and found some details about the tie inverter. It looks definitely like there could (should, actually) be issues with the synchronization, which seems to have used GTO:s or SCR:s at the time.
There are said to be filters, but those filters have also failed a few times and I am not so sure if they are in working order any more. Or if they were designed to cope with a large capacitive Cable net on the MV side of the transformer.

You got me moving, but would still like to do some actual observations on these old units. The papers Cat-Mike refers to all seem to be for more modern units than the ones I have been looking at.
So, if there are any details on 30-40 yrs old sets - I would love to see them.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Update!

After a thorough search in the documents provided by Cat-Mike, it is clear that this is not a ringing from SCR or GTO switching. It is actually PWM residues that point to defect or inadequate filtering. Ringing would have been damped, like this wave-forms from a CSI inverter with GTO:s

These wave-forms are damped with a Q around 5 while the Capstone wave-forms have no damping at all.

Thanks for good support. We now know that the filters are either defect, inadequate or non-existing. The frequency of the tone is actually 15 kHz, which explains that the tone cannot be heard by all. The reason why it is 15 kHz and not 7.5 kHz is that mechanical forces go with frequency squared (a transformer on 60 Hz, for example, hums along at 120 Hz).


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[davidbeach]

I believe that would be frequency doubled rather than squared.

 

[Skogsgurra]

Sure!
But I hope that no-one notices

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.