I thought the polarity of faag (servo valve current) is the direction of GCV, and the value accordance with the stroke. In fact, I compared the faag with fsr2x, they are not linearity. So , How can i understand the meaning of servo valve current?

 

The query I think is about GCV of GE gas turbines. Note that there cannot be any kind of linear association between FSR2 and FAG, since the current will be that of "null" position value.

The best way would be to look at FAG when the command (FSR2) and feedback(FSG) are matching or within the deadband. It usually reads -2% to -4% which is the bias applied to the coil against the valve spring. The current will never be beyond this, except while a command is given and there is momentary spike or the valve position is not matching the command. The current at extremes in open and close condition will be + or - 75% which means the valve physically cannot achieve the position asked by the command.

At any given time, when FSR2 and FSG (GCV position) match, the current will be reading + or _ 2% to 4%. The linearity shall be verified between FSR2 & FSG since the GCV is a position controlled valve.

The current is an indicator.

 

Think of the servo current as part of a closed loop. The controller modulates it to open/close the GCV (or any other final control element) according to the required value of the process variable. As far as i can tell you are talking about a combustion turbine: the FAG is set by the controller according to the demand for load/speed. The controller will modulate the output (FAG) to keep load/speed at setpoint but there is no predictable relationship of FAG versus valve opening because there are other variables that cause variations like gas supply pressure, gas btu content, mechanical friction in the valve, the influence of redundant servo coils, etc.

 

The electro-hydraulic servo-valves used in GE-design heavy-duty combustion turbine applications (sometimes referred to as "servo motors") are bipolar devices, meaning they port high-pressure hydraulic oil to a hydraulic actuator (usually a single- or double-acting cylinder) in proportion to the magnitude and polarity of the current signal applied to the servo-valve's torque motor coils.

The servo-valves are rated for -10.0 mA to +10.0 mA. In the GE-design heavy-duty combustion turbine application, positive servo current DECREASES the flow of fuel or air or steam to the unit; negative servo current INCREASES the flow of fuel or steam or air to the unit. "More" positive servo current (i.e., a larger positive magnitude) causes more hydraulic oil to be ported to/from the hydraulic actuator to cause a faster/larger change in position to reduce or shut off the flow of fuel or steam or air to the unit. "More" negative servo current (i.e., a larger negative magnitude) causes more hydraulic oil to be ported to/from the hydraulic actuator to cause a faster/larger change in position to increase the flow of fuel or steam or air to the unit.

When 0.0 mA is applied to the servo-valve coils the flow of high-pressure hydraulic oil to the actuator is shut off--maintaining a stable, steady-state position of the valve or IGVs. HOWEVER, the servo-valves have a small spring which, in the complete absence of all servo current (i.e., a catastrophic failure of the control system with NO servo-valve output currents), would port high-pressure hydraulic oil to/from the actuator to cause the flow of fuel or steam or air to the unit to be quickly shut off.

This spring, called the "fail-safe" spring, applies a constant force at all times to the servo-valve mechanism which must be overcome with a small amount of servo current in order to shut off the flow of high-pressure hydraulic oil to maintain a "neutral," steady-state position of the valve or IGVs or device whose position is being controlled through the servo. The current required to overcome the fail-safe spring tension is called "null bias" current.

In other words, when the feedback is equal to the reference, the output of the servo-valve output summing junction is zero--meaning that no change in position of the valve or IGVs is required. If 0.0 mA were applied to a servo-valve with a fail-safe spring, the device would slowly decrease the flow of fuel or steam or air to the unit to zero because of the force applied by the fail-safe spring.

So, a small amount of current--the null bias current--must be added to the output of the regulator to overcome the fail-safe spring tension in the servo-valve to shut off the flow of high-pressure hydraulic oil to the actuator to keep the valve or IGVs in the steady-state, "neutral" position.

Since the servo-valve requires a negative servo current to increase the flow of fuel or steam or air to the unit, the null bias current must be negative. The servo-valves used in GE-designed heavy-duty combustion turbine TMR applications are designed to have a TOTAL of -0.8 mA, +/- 0.4 mA, of null-bias current.

When the device is operating at steady-state conditions--i.e., when the position feedback (or the liquid fuel flow feedback in the case of the Liquid Fuel Bypass Valve, or the P2 pressure in the case of the Stop-Ratio Valve) is equal to the reference, the amount of current being output to the servo-valve will be slightly negative (because of the null bias current value) and the algebraic sum of all the processor servo-valve outputs to a particular servo-valve should be approximately equal to -0.8 mA, +/- 0.4 mA. (This is a good rule of thumb--not an absolute value.)

If the control system requires an increase in fuel or steam or air to the unit, then slightly more negative current is applied to the servo-valve coils UNTIL the feedback is equal to the reference and then the servo-valve output current "reverts" to the null-bias current value.

Conversely, if the control system requires a decrease in fuel or steam or air, the amount of servo current is increased (by the application of positive servo current or by reducing the negative current) UNTIL the feedback is equal to the reference, and then the servo-valve output current "reverts" to the null-bias current value.

In the case of the Gas Control Valve, the LVDT feedback is compared to the reference (FSR2, or in some cases FSR2X) by the summing junction of the regulator controlling the Gas Control Valve servo-valve output current. If the feedback is equal to the reference, the output of the summing junction is zero, and the null bias current value is added to that to produce the servo-valve output current applied to the servo-valve.

So, on GE-design heavy-duty combustion turbine applications servo-valve output current will always be slightly negative when the device being controlled is in the proper position. Servo-valve output current is NOT proportional to reference and will only increase or decrease from the null bias current value in proportion to amount of error between the reference and the feedback to decrease or increase the flow of fuel or steam or air faster or slower. When the feedback is equal to the reference, the servo-valve output current will "return" to the null bias value.

If the SpeedTronic turbine control panel is a Mk V, the value of null bias current in the I/O Configurator is automatically inverted. In other words, a value of 2.67% input into the I/O Configurator is automatically inverted before being added to the servo-valve output and becomes -2.67% current at the servo-valve coil.... (Are we having fun yet?)

Also in a Mk V turbine control panel, servo current is expressed as +/-100% where 100% is equal to 10 mA. So, 1.0% is equal to 0.1 mA.

 

I watch the FAAGR (SRV servo-valve current) is more than 10% in most times. I think there is something wrong with it compared with your detailed preparation. Actually, the fsagr_err is between 2%~3%. So what should I do to solve this problem? If I do nothing, keep this status, will something worse occur?

 

As long as the error isn't increasing with time, there should be no problem.

The most common reason for such errors is the LVDT calibration process. The description of the process, its variants, and some methods for minimizing the errors is much too lengthy for this forum. Again, as long as the error remains fairly constant and doesn't get much worse than you describe there should be no problem. Most units in the world operate satisfactorily for years with moderately unbalanced servo currents. In fact, unbalanced servo currents are actually introduced by the most common LVDT calibration procedure--they are nearly unavoidable.

What is the value of Null Bias Current for the SRV which has been set in the I/O configurator? (Servo Valve Output #1 is usually configured to be the Stop/Ratio Valve output.) The Null Bias Current Value in the I/O Configurator should be approximately 2.67%. If it's much more or much less than that, that could be a large part of the problem.

But, again, if the SRV is operating properly (i.e., FPG2 is approximately equal to FPRG) and is not hunting or sticking then there's probably no problem.

Units which operate for years at Base Load with relatively stable, constant Gas Fuel Supply pressure can experience actuator wear which can require increased servo currents to overcome the wear (sometimes called "sticktion") but that usually leads to erratic P2 pressures and valve oscillation.