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Servo Current Voting Mismatch for IGV
Will servo current voting mismatch affect IVG
By chiranjeevi on 25 April, 2017 - 3:47 am

In one of my frame 5 machine In MARK VIe IGV servo T (CAGV) voting mismatch coming and getting reset. will it affect the IGV servo spool poiece to change the position and movement of IGV?

CAGV T :-28%,voted:-1.9%. please note GT running at 18MW.

Can anybody explain that this causes the movement of spool in servo or not?

chiranjeevi,

The topic of electro-hydraulic servo-valves as used on GE-design heavy duty gas turbines has been covered extensively over more than twelve years on control.com. And all of the questions and answers--the good ones and the bad ones--are available via the 'Search' feature at the top of every control.com webpage.

I will answer your question with several questions. First, why do you believe that a mismatch in servo currents will cause movement of the spool piece?

Second, what Process-and Diagnostic Alarms are active during the mismatch of servo currents? Please provide ALL of alarms, even if you (or someone else at your site) believe the alarm(s) are not relevant to the issue under discussion. FREQUENTLY, we find there is or has been an alarm (usually Diagnostic!) that is alerting the operator/technician to the cause of the problem. So, help us to help you better understand the system in the future by providing a complete list of Process- & Diagnostic Alarms, whenever you write for help from the kind folks here at control.com. It benefits ALL those who read and follow the GE-design heavy duty gas turbine-related posts here on control.com--and there are a LOT of people following these posts.

Third, what is the indicated IGV angle (from the LVDTs)? (The signal name is usually CSGV.)

Fourth, what is the IGV position reference? (The signal name is usually CSRGV.)

Fifth, was the Mark VIe provided with a new Frame 5, or was the control system upgraded from a previous version of Mark* (Speedtronic) turbine control system, or even some other control system? If the turbine control system was upgraded to a Mark VIe, how long has the unit been operating since it was new?

Sixth, when did this problem start? After a servo was replaced? If so, was the polarity of the individual servo currents verified after the new servo was installed and before the unit was started? Was a "calibration " of the "IGVs " performed as part of the servo replacement? Was an LVDT replaced? We're the IGVs or the IGV actuator worked on during an outage before the servo current mismatch was noted? Was a card or I/O Pack replaced prior to the mismatch being noticed?

Seventh, what are all three servo currents when there is a large mismatch?

Positive servo current serves to close the IGVs; negative serves to open the IGVs. The typical IGV reference is a position reference, but some operating modes can be using exhaust temperature as an outer loop to the position reference loop. If the unit is exhausting into an HRSG to produce steam, then the IGVs may be operating on IGV Exhaust Temperature Control, sometimes called "combined cycle mode," when at less than full load.

Servo current outputs are the result of the comparison of the device's reference (setpoint; command) to the device's actual position or feedback. So, it would seem that <T> believes there to be a large error between the reference and the feedback that are controlling the IGVs when the servo current mismatch is occurring.

However, <R> and/or <S> should be acting to overcome the servo current from <T> to keep the IGVs in or near the desired position--which is one of the benefits of a R control system. And if the IGVs are relatively stable that means the spool piece is also relatively stable and not moving. Any time a servo-operated device is in a stable position the spool piece is not moving. The spool piece moves when the SUM of the three currents is large enough to overcome the null bias spring tension and that allows high-pressure hydraulic oil to flow to the device to change its position. And, when the device moves to the position that makes the feedback equal to the reference then the spool piece returns to the "null" position and stops moving. It's the algebraic sum of the three servo currents acting on the servo valve that make the spool piece move--and even hold it in a steady-state (stable) position to keep the device its operating in a steady-state (stable) position. If the IGVs are relatively stable, then all things being normal with the servo and the polarity of the servo currents being applied to the servo coils, then the sum of the three currents is holding the spool piece in a stable position and it's not moving very much if at all.

So, it's important to know all three servo currents.

The more information you provide in your original post, the better and more concise our responses will be. While there are literally thousands of GE-design Frame 5 heavy duty gas turbines in service around the world there are MANY subtle and not so subtle differences which can affect the response. It's an all-too-common (false) belief that all Frame 5s are alike in most every way, and it's just not true. By providing as much information as possible in your original post we can better understand the unit and the situation. And, also contrary to false and popular belief Alarms are not nuisances, and can be extremely helpful in troubleshooting and resolving issues with GE-design heavy duty gas turbines and their GE-designed turbine control systems. M

LOTS of questions, I know. But the answers to every one of them is important to understanding the problem and what might be the root cause. If you will answer them all to the best of your ability it will save a lot of back-and-forth with single questions.

Since you seem to be somewhat new to the Mark VIe, it's hoped that by gathering the requested information you will also begin to understand some of how the turbine control system works. And, since a lot of people read these threads (questions and answers) it will also be helpful to them in their quest for knowledge and understanding.

Looking forward to hearing back with your replies to the requested information, and providing useful and concise information!