Woodward 505XT, Инструкция по установке и эксплуатации, Страница: 186 / 242

Manual 35018V1 505XT Digital Control System for Steam Turbines
Woodward 184
P = Proportional gain (%)
I = Integral gain (%)
D = Derivative (determined by DR and I)
Tuning P & I Gains
Proportional gain must be tuned to best respond to a system transient or step change. If system response
is not known, a typical starting value is 5%. If proportional gain is set too high the control will appear to be
overly sensitive, and may oscillate with a cycle time of less than 1 second.
Integral gain must be tuned for best control at steady state. If system response is not known a typical
starting value is 5%. If the integral gain is set too high the control may hunt or oscillate at cycle times of
over 1 second.
For best response the proportional gain and integral gain should be as high as possible. To obtain a
faster transient response, slowly increase the proportional gain setting until the actuator or final driver
output begins to oscillate or waver. Then adjust the integral gain as necessary to stabilize the output. If
stability cannot be obtained with the integral gain adjustment, reduce the proportional gain setting.
A well-tuned system, when given a step change, should slightly overshoot the control point then come
into control.
A PID control loop’s gain is a combination of all the gains in the loop, also known as the system gain. The
loop’s total gain includes actuator gain, valve gain, and valve linkage gain, transducer gain, internal
turbine gains, and the 505’s adjustable gains. If the accumulated mechanical gain (actuators, valves,
valve linkage, etc.) is very high, the 505’s gain must be very low to be added to the system gain required
for system stability.
In cases where a small change in the 505's output results in a large speed or load changes (high
mechanical gain) it may not be possible to take the 505's gains low enough to reach stable operation. In
those cases, the mechanical interface (actuator, linkage, servo, valve rack) design and/or calibration
should be reviewed and changed to achieve a gain of one where 0–100% 505 output corresponds to 0–
100% valve travel.
Multiple Dynamics for Speed/Load Control
The Speed PID has two primary sets of dynamics, On-Line and Off-Line; each include Proportional Gain,
Integral Gain, and Derivative Ratio (DR) variables. There are three cases that determine when the
dynamics switch between On-Line and Off- Line:

A “Select On-Line Dynamics” contact input is programmed

Unit is driving a generator

Unit is driving a mechanical drive (not a generator)
If a contact input is programmed to “Select On-Line Dynamics”, it has priority regardless of the driven
device. When the contact is closed, On-Line dynamics are selected; when open, Off-Line dynamics are
selected.
If the unit is driving a generator and no “Select On-Line Dynamics” contact input is programmed, the
Speed Off-Line dynamics are used by the Speed PID when the generator or utility tie breaker contacts
are open. The speed On-Line dynamics are used by the speed PID when the generator and utility tie
breaker contacts are closed. If the speed dynamics select contact is programmed, the generator and
utility tie contacts do not affect the dynamics selection.
If the unit is not driving a generator and no “Select On-Line Dynamics” contact input is programmed, the
Speed Off-Line dynamic settings are used when the turbine speed is below minimum governor speed;
.On-Line dynamics are used if the turbine speed is above minimum governor speed. If the speed
dynamics select contact is programmed, the turbine speed does not affect the dynamics selection.
A relay can be programmed to indicate that the On-Line Dynamics mode is selected.