I would like to have more details on how the load control reference and IPC control reference is calculated in Mark V for ST application. As I understand, IPC mode is selected to enable the turbine to follow the boilers (V1 to control the inlet pressure with bypass valves remained fully closed) while the load control is selected to enable the boilers to follow the turbine (bypass valve modulated by DCS to control the inlet pressure with V1 remains fully opened).

The reason behind this question is that usually my ST will be put into IPC mode after 30MW following a turbine reset. However after a recent startup, the turbine did not go into IPC mode as the TPWR is lower than IPC in the min value gate. The current setting for IPC was 104% with the TPWR being a calculated figure at about 103.94(TN_LD is set at 104% as well).

What I did to resolve the matter is I increased the TN_LD at HMI from 104% to 105% so that the IPC could be within the control range. I got this idea from GEK 104040 where it was stated that the "LOAD SETPOINT must be manually moved to a maximum after enabling IPC so that IPC can respond to the full range of the inlet pressure demand (if the control system does not move it automatically)"

I would like to know what might be the reason behind this scenario as usually this manual shifting of load setpoint is not done and the unit could be loaded to IPC mode without any problems.
Thanks

 

Two years ago our ST was a similar state when the IPC was on. We investigate and discovered that the "hold ramp" selection button (L83HOLD_CMD in 1) was active causing that.

 

Thanks Juan. Will check on that should the same problem come back. Hopefully it doesn't

 

The same problem happened again today. When I checked, the load hold command was not active during the period. From my observation, I noticed that the only changes is on the TPWR which dropped from 107 to 99% and cause the IPC to be out. The rest of the parameters are normal without any changes.

Any comment or advice on what maybe the problem?

 

I'm not very familiar with steam turbine Mark V applications, but it would seem from your posts that something is causing TPWR to increase or decrease when the unit is starting or stopped.

So, you should look at your CSP and find out all the things than can cause TPWR to increase or decrease and then systematically investigate each one to see if somehow, during starting or when stopped, if TPWR is being changed when you don't think it should be.

 

CSA,

The 1st incident occurred during unit startup buy subsequent occurrence happened during unit operation. I went through the BBL from which the TPWR is computed. I did not see anything which could cause the variation as the block is only on speed control. Moreover every incident only last for 5-10 minutes. I believe that it is caused by defective TCEA as I noticed there are quite a number of voter mismatch diagnostics alarms on <S> core caused by TCEA. I will try to replace the module(s) during the next unit shutdown and see whether the problem resolved or not.
Wish me luck

 

Again, I'm not familiar with steam turbine control schemes, and I don't have any steam turbine software to try to look at.

I am guessing (!) but I would imagine that there's some kind of Speed/Load Raise and -Lower function that's involved here, and possibly some external load signal that's somehow also doing a similar thing. You say this happens when you're trying to switch to IPC. In both cases you've had to raise TPWR somehow to get it up out of the way so you could enable IPC. Do you just increase the load setpoint to increase TPWR? Is there some external load setpoint signal that could be driving TPWR high or low during shutdown or start-up?

If you have a lot of Diagnostic Alarms from the TCEA card associated with <S> (that's called <Y>), that shouldn't cause a problem (I wouldn't think) with sequencing in any of the three control processors. The outputs of the TCEA processors are supposed to be independent of the three control processors. There is the cross-trip signal that's communicated from the control processors to the TCEAs, but I'm not aware of any control signals that are communicated from the TCEAs to the control processors. The TCEAs are basically just looking at speed and the rate of change of speed, and if either the rate of change or the magnitude exceed certain levels, then the TCEAs drop out their Emergency Trip Relays (ETRs) on the TCTx card (TCTG for gas turbines; TCTS for small and medium steam turbines; and TCTL for large steam turbines). The control processors drive their Primary Trip Relays (PTRs) on the TCTx cards.

Two out of three of the PTRs *and* two out of three of the ETRs must be picked up for the turbine to run. If you have a lot of Diagnostic Alarms associated with one of the TCEAs, about the only way that happens is if there is a problem with it's speed pick-up, and that should generate Diag. Alarms and usually a Process Alarm to say that there is a voting mismatch between <Y>'s speed sensor feedback and <S>'s speed sensor feedback. It's not the only cause, but it's the most common cause.

So, there's a problem with the TCEA in Loc. 3 (<Y>, the TCEA associated with <S>) but it's not likely causing at least two of the three control processors to alter TPWR. And if it did, I would expect there would be Voting Mismatch Diagnostic Alarms to that effect.

I believe this is the first time we've heard about the limited length of time of the incident. Could that be related to steam flow and inlet pressure increasing? Are you just trying to switch to IPC before it can control the setpoint?

So, there are Diagnostic Alarms you haven't told us about. What alarms (Diagnostic and Process) are present when then incident occurs (all of them)?

Is it possible that you're operating the turbine slightly differently than you have before, using load control to shut it down (lowering TPWR to open the breaker) and you haven't done this before?

When TPWR was above 100% and you had problems, was it because you were using it to raise load and it just wasn't "out of the way? when you tried to select IPC?

I'm sorry; I shouldn't be trying to troubleshoot without looking at the CSP. But I just have a hard time believing that something that has worked has suddenly stopped working.

One thing I do know is that the steam turbine controls engineering group is VERY concerned about good speed signals, because steam turbines can overspeed so quickly. So, it's possible that there's something in the CSP that's holding TPWR because of a problem with <Y>'s speed sensor. But I don't think that's very likely. There are probably six speed sensors on the turbine (one for each of <R>, <S>, <T>, <X>, <Y>, and <Z>) for true redundancy.

Sorry; I'm really reaching here, but this can't be that difficult.

 

Problem solved!

Replaced TCEA module on <P> core and everything is back to normal.. Thx god