Control.com community I am looking for clarification on mechanical vs electrical overspeed on our MKVI controlled Frame 7EA commissioned in 2005. I understand in the days past of Frame machines that there was an overspeed bolt on the accessory gear to trip the unit on mechanical overspeed (we have an older Frame 5 that uses this), and then overspeed logic in the protection core assembly to trip the unit on electrical overspeed.

On our Cimplicity screen there is a choice to select a mechanical overspeed test or an electrical overspeed test. Since we have no overspeed bolt I have been reviewing logic to find out what the mechanical overspeed selection does. In our MKVI application code if either electrical or mechanical overspeed test is selected, a select block sends the control constant (TNKHOS) of 110% to the L12HV1 overspeed detection algorithm in MKVI application code. If the mechanical overspeed test is selected a signal L83HOST_P will also go true which feeds to the protection core as input OfflineOS1test. From what I read in our case this raises the original OS_Setpoint_PR1 in (P) core from 3960 rpm to 4086 rpm via a configured signal space input OS_Tst_Delta with a value of 126, in essence raising the OS trip setpoint for the protection (P core) from 110% to 113.5%. But since we have no overspeed bolt to trip the unit this does nothing but raises the trip setpoint, and because the MKVI application code overspeed trip setpoint is still 110% the application code in the MKVI will still trip the unit at 110%.

So is this just some leftover logic and graphics from GE? Is this a way to test MKVI application code trip logic independent of (P) core trip logic? Or am I over-analyzing this as usual!! Thanks to all for your thoughts, I have been reviewing GEH-6421G volume #1 applications for the VPRO Protection logic-overspeed trip,HP for my studies into this.

 

The display problem is probably a left-over that should have been removed and cleaned up if the unit doesn't have a mechanical overspeed bolt.

There should have been display buttons and logic to select a primary (application code) overspeed test and an emergency (<P> core) overspeed test.

If you want to test the primary overspeed, then set the primary overspeed trip setpoint to something less than the <P> core setpoint, say, 107.5%, or 105%. You can use a frequency generator to test electrical overspeeds if you don't want to perform a running "on-line" overspeed test (on-line does not mean while the generator breaker is closed, just that the turbine is fired/running). Just don't forget to set the primary overspeed setpoint back to the as-found setting when you're done.

 

Thank you CSA for the reply. This confirmed my thoughts that this was indeed a leftover. I hope that someone else can benefit from this info as well, I would think that we are not the only site that has been served some leftovers!!! As usual digging into the manuals more helped me understand more of what is going on inside (P) core.

 

I will suggest before inventing something, maybe you should review the modification reports during the upgrade in 2005. Post upgrading from mechanical OS to EOS, functional test should have been performed and witnessed by your company representative on site.

Normal practice is, post upgrading to EOS, the GT should have been tested for OS. This is in order to check the OS parameters. This upgrade should have also included the additional three speed probes, do you have this?

So, these additional probes should be used for the EOS. Although, altering / deleting the cimplicity screens for the mechanical OS will not harm the OS sequence, you should verify whether the EOS block in the MKVI is also properly configured. Again, if this EOS is already functionally tested, you can ignore my comments above.

Docendo Discumus

 

Ore, thanks for the response. Just to clarify our situation. Our Frame 7 machine has never had a real "mechanical overspeed". In that I mean our turbine never had an overspeed bolt. Our turbine is fitted with 6 speed pickups, 3 for the MKVI cores and 3 for the VPRO protection cores.

My point is there is still logic in the MKVI application code, and signal space entries for the VPRO to raise the VPRO overspeed trip setting to 113.5% speed, and there is a button for "mechanical overspeed" testing which we "technically don't have, unless someone tells me otherwise.

This is confusing to operators and I just want to clarify what we really have for them. When we had GE onsite for a CI inspection I questioned this "mechanical overspeed" selection on the HMI and they just said "its a leftover, just delete the buttons". I have no problem with this, but asked here for clarification, so that if issues come up in the future I can explain what I did and why.

Furthermore I would bet that there are other sites out there that may have the same "leftovers" we have, and may not know why. Mainly I just dislike some of the lack of attention to detail within the MKVI code, and cimplicity screens that gets left when commissioning of a turbine is over. If the selection on the screen says "mechanical overspeed" then that should be what it is!! Otherwise to be clear it should say "primary overspeed test(MKVI application code)" and then "emergency overspeed VPRO" or something like that. And in our case as logic is written the "mechanical overspeed" selection is bogus because it will raise the EOS setting to 113.5% but the primary overspeed setting is still lower than that, so the test does nothing different than selecting an electrical overspeed test.

Furthermore not to harp on GE too much because they do some great things, but many of the people don't take or have the time, or the knowledge for that matter, to do the professional job that some of us demand. I further understand its not always their fault. There is a lack of knowledgeable people out there, and the way they get sent here there and everywhere at a seconds notice would be reason for anyone to burn-out.

We typically have a TA onsite 1-3 times a year. It seems the ones that are knowledgeable and helpful (the old ones) get transferred to a different position or are impossible to get back to site. And the ones sent to commission a site(ours at least) can't wait to leave and will do what it takes to get the machine running and get out. So I digress, sorry for the rant!!!

We perform a true OS test yearly, typically dropping to the overspeed setpoint down to 103% since our insurance only requires a test of overspeed, but does not specify the speed. I simply want to clarify for operations what we have, and be able to explain to operators what is going to happen when they make a selection on the HMI screen. It should just be simple and straightforward, don't you agree?