9440300990 16-1
DECS-250 Stability Tuning
16 • Stability Tuning
Generator stability tuning in the DECS-250 is achieved through the calculation of PID parameters. PID
stands for Proportional, Integral, Derivative. The word proportional indicates that the response of the
DECS-250 output is proportional or relative to the amount of difference observed. Integral means that the
DECS-250 output is proportional to the amount of time that a difference is observed. Integral action
eliminates offset. Derivative means that the DECS-250 output is proportional to the required rate of
excitation change. Derivative action avoids excitation overshoot.
All stability tuning must be performed with no load on the system or
equipment damage may occur.
AVR Mode
BESTCOMSPlus Navigation Path: Settings Explorer, Operating Settings, Gain, AVR
HMI Navigation Path: Settings, Operating Settings, Gains, AVR Gains
Two sets of PID settings are provided to optimize performance under two distinct operating conditions,
such as with the power system stabilizer (PSS) in or out of service. A fast controller provides optimum
transient performance with the PSS in service while a slower controller can provide improved damping of
first swing oscillations when the PSS is offline. BESTCOMSPlus
®
primary and secondary AVR stability
settings are shown in Figure 16-1.
Predefined Stability Settings
Twenty predefined sets of stability settings are available with the DECS-250. Appropriate PID values are
implemented based on the nominal generator frequency selected (see the Configuration chapter of this
manual) and the combination of generator (T’do) and exciter (Texc) time constants selected from the gain
option list. (The default value for the exciter time constant is the generator time constant divided by six.)
Additional settings are available to remove the effects of noise on numerical differentiation (AVR
derivative time constant T
d
) and set the voltage regulator gain level of the PID algorithm (K
a
).
Custom Stability Settings
Stability tuning can be tailored for optimum generator transient performance. Selecting a primary gain
option of “custom” enables entry of custom proportional (K
p
), integral (K
i
), and derivative (K
d
) gains.
When tuning the stability gain settings, consider the following guidelines
• If the transient response has too much overshoot, decrease Kp. If the transient response is too slow,
with little or no overshoot, increase K
p
.
• If the time to reach steady-state is too long, increase K
i
.
• If the transient response has too much ringing, increase K
d
.
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