We have Mark-VIe system and 6FA machines are there in our plant. In one of our Mark-VIe we are getting "L30NVSSRV_A" "SRV Non-voted S Servo Current Disagree" Alarm. Alarm is appearing intermittently. What are the probable reasons and what actions we should take?
I am not writing this from experience. I am ONLY discussing it theoretically. Hopefully CSA will share his experience.
I am assuming your MarkVIe is TMR. GCV, SRV and IGV have one servo and two LVDTs. Servo is driven by total three outputs one each from R, S and T. There were different types of regulators used for these valves in MarkV. In our case in MarkV we had Regulator type 43 (Position control with Feedback as Maximum of two assigned LVDT) for GCV and IGV. For SRV we had regulator type 77( Pressure control with position feedback while feedback as Pressure_in (V1) and max of 2 assigned LVDT). In MarkV we have other regulator types of 5x (where x can be 1,2 or 3) and 6x (where x can be 4,5 or 6). Regulator type 5x was Flow control and 6x was Flow control with position feedback.
In MarkVIe we have Digital servo regulator types
*Liquid Fuel with position
When we upgraded from MarkV to MarkVIe our regulator for SRV is changed from Pressure control to Position Control with feedback of Max of two LVDTs.
In MarkVIe PCAA (Core Analog Module) is responsible for LVDTs excitation, feedback and servo currents. The signals on PCAA are divided into two groups. We have dedicated (having different instrument for each signal to R,S and T) signals for R,S and T and then we have FANNED (actually one instrument but same is going to R,S and T) signals. PCAA IO pack is mounted on TCAS terminal board which is responsible for handling dedicated IOs. For TMR with dedicated and fanned IOs we have three TCAS terminal boards (and PCAA IO packs) for dedicated IOs and one TCAT terminal board for FANNED IOs. TCAS is connected with TCAT via P1 and P2 connectors.
Excitation for LVDTs is dedicated so it goes through TCAS terminal board, LVDTs feedback is Fanned is received through TCAT terminal board and Servo Outputs are dedicated and are going through TCAS terminal board. Each controller R,S and T receives the LVDT feedback and calculates the servo current based on the algorithm.
Using these three computed servo currents the controller calculates the median value. If any of the three servo currents are greater than or less than a defined difference then you will get this diagnostic alarm. This difference is defined by the parameter TMR_DiffLimit and can be found under the Regulators tab in Core Analog Module. Can you please tell us the servo currents of R, S and T and value of TMR_DiffLimit?
The recommended solution or the possible causes of this alarm as per manual are as follows
1. Adjust TMR_DiffLimit on the regulators tab
2. Also check the mismatch in the coil resistance between R,S and T servo coils. This alarm may be indicative of possible fault of that servo output.
3. Reg#_gain is too high for specified TMR_DIffLimit value.
Can you please apprise us when this problem occurred? After some maintenance job or from the start?
Hope it will help.
Well .... I believe if you look in the application code of the snark VIe you will see the SRV is still a pressure control loop, and the servo regulator is where the LVDT feedback is performed. So, in essence, it's still a pressure control loop with LVDT feedback; just like in the Mark V,. The difference being the pressure control is accomplished in the application code, and the position control is accomplished in the aero-valve output regulator (at the I/O Pack level). In the Mark V, both the pressure control and the position control was done at the I/O card level. I don't know this for certain but it's probably because the application code in the Mark VI and VIe run much faster than the sequencing ran in the Mark V (25 Hz vs. 8 Hz), and it was felt the pressure control could be safely run in the application code.
I don't really understand the "non-voted servo" terminology. My GUESS is the pressure control output of each controller to the I/O Pack is what is being referred to. So, that means the SRV output from <S> is excessively different from the other two controllers. In the Mark V when three P2 pressure transmitters were used, one for each control processor, if one of the drifted excessively a Diagnostic Alarm would be annunciated. I'm wondering if that's what is going on with this Diagnostic Alarm. If something is making <S> try to increase or decrease the P2 pressure excessively I wonder if that's what this means.
However, if I recall correctly in the Mark VI and VIe multiple P2 pressure transmitters are median selected to a single value which is then used by all three controllers. So, the output of the three controllers for P2 pressure should be the same for all three controllers (this is a definite improvement over the Mark V method). So, I may not be correct about this Alarm....
I"m wondering if somehow some Contro Constants related to P2 pressure control were changed in either <R> or <T>, or maybe just <S>???
Or, the I/O Pack for <S> is going bad.?.?.?
I won't have access to a Mark VIe for some time. So I won't be able to investigate this (unusual, for me, anyway) Diagnostic Alarm. Right now (it's been a VERY LONG and difficult day today), I suppose one would have to go on-line from ToolboxST to each controller to check the P2 pressure output to the servo regulator to see what each controller's value is, and if <S>s is excessively different or fluctuating more than the other processor values then one would start working backwards from there to find out why and resolve the problem.
That's if my guess is correct...!
Hope this helps!
Please, someone, write back with the values of P2 pressure control output to the I/O Pack for each controller. It would be very helpful for me; and maybe for the original poster.
>the snark VIe
Now THERE'S a Freudian typographical error if there ever was one. One has to really appreciate spell checkers!
Sometimes, that is.
>So, in essence, it's still a pressure
>control loop with LVDT feedback; just like in the Mark V,.
Spot on. It is pressure control loop. Only regulator used is Position Control. I have only one confusion though. If pressure control can be done in application code then why a separate regulator for SRV (Digital Servo Regulator - Speed Ratio) exists in MarkVIe? Just curious!
>I don't really understand the "non-voted servo" terminology.
In ToolboxST if we go to servo outputs tab and click on any servo, lets say servo1, then at bottom we can see-the non voted values of servo output (of controller R, S and T) as well as the voted value which is the median of the three outputs. If the difference of any non-voted output (i.e. of R, S ant T) is more then 25% then we will get this alarm. Let suppose we click on the Servo1 and we get
R non-voted value: 90%
S non-voted value:90.1%
T non-voted value:90.2%
Voted value: 90.1%
Now for some reason, if R non-voted value changes to 64% then the median (voted value) will remain the same but we should get the diagnostic alarm.
I have one question though. This is the computed value by the controllers. Does MarkVIe has feedback of actual current value? If computed value is 60% and actual value is 68% due to some grounding issue then what will happen? In MarkV I have seen the turbine trip during startup and we found that one wire of the IGV servo was grounded and this particular core was sending more current % (that was noted on Autocal screen) than others and IGV was not able to maintain position within 5 degrees. Can the actual and computed current differ? If wire of the one servo gets grounded or the coil resistance of servo changes (only one) will that affect the computed value of servo output?
>However, if I recall correctly in the Mark VI and VIe
>multiple P2 pressure transmitters are median selected to a
>single value which is then used by all three controllers.
Yes the FPG2 is the median of three interstage (P2) pressure transmitters.
>Please, someone, write back with the values of P2 pressure
>control output to the I/O Pack for each controller. It would
>be very helpful for me; and maybe for the original poster.
Sorry our turbine is shutdown at the moment. Can not note / post P2 pressure actual values.
Once again thanks for your valuable answer.
Thanks for the information; again, I won't be at a Mark VIe site for a month or more, so this is helpful.
>Spot on. It is pressure control loop. Only regulator used is
>Position Control. I have only one confusion though. If
>pressure control can be done in application code then why a
>separate regulator for SRV (Digital Servo Regulator - Speed
>Ratio) exists in MarkVIe? Just curious!
I don't know if I exactly understand the confusion. A bipolar servo can only be driven by a servo-valve output of a Mark VIe. So, any bipolar servo must be connected to a servo-valve output, which is driven by a servo regulator. The servo regulator has to be configured to use the appropriate inputs (high-select of two LVDTs, for example) and the output type has to be appropriate (position control, in this example).
>I have one question though. This is the computed value by
>the controllers. Does MarkVIe has feedback of actual current
>value? If computed value is 60% and actual value is 68% due
>to some grounding issue then what will happen? In MarkV I
>have seen the turbine trip during startup and we found that
>one wire of the IGV servo was grounded and this particular
>core was sending more current % (that was noted on Autocal
>screen) than others and IGV was not able to maintain
>position within 5 degrees. Can the actual and computed
>current differ? If wire of the one servo gets grounded or
>the coil resistance of servo changes (only one) will that
>affect the computed value of servo output?
There's several more than one question above.... Yes, I believe that beginning with Mark V (some later versions of Mark V) the actual servo current output(s) were compared with the calculated value(s) of servo output current, and if the two differed for any output by more than some value a Diagnostic Alarm was identified. I believe this was also done with Mark VI firmware and Mark VIe firmware, also. I have never run across a grounded servo output loop, so I don't know exactly how that would act or what it would look like.
To be honest, I hear lots of "reasons" for why this happened or why the turbine tripped or why that happened. And, some of them are difficult or impossible to understand, or to verify. People, when they are unfamiliar with the equipment and concepts attribute all kinds of things to this or that without really thinking it through. If it makes them happy--and if the unit gets started and returned to service--most don't really re-think their initial impression or attribution. Here on control.com some of the feedback we receive is very brief and sketchy, and when asked for details we don't usually get any more information. So, personally I just leave it at what is said to be. But, I often question some of the explanations and attributions. If the unit is running and producing revenue that's the most important thing.
>A bipolar servo can only be driven by a servo-valve output of
>a Mark VIe. So, any bipolar servo must be connected to a
>servo-valve output, which is driven by a servo regulator.
The servo-valve application I was referring to is for infinite control of high-pressure hydraulic fluid to an actuator for the purpose of very precise control of fuel- or air flow to the gas turbine.
There are MANY older GE-design heavy duty gas turbines which use servo-valves simply as an open/closed valve to permit high-pressure hydraulic fluid to flow to a hydraulic actuator. So, they are used in this application as a "trip valve," or "trip solenoid" (as they are sometimes referred to). They were designated as 20FG-1, or 20FL-1.
So, electro-hydraulic servo-valves can be used for infinite control, or for on-off applications, and do NOT have to be connected to a servo-valve output for the latter (and usually aren't).
My apologies for any confusion.
Meanwhile you can read the post below to get some more insight.