9440300990 15-1
DECS-250 Power System Stabilizer
15 • Power System Stabilizer
The optional (style xPxxxxx), integrated power system stabilizer (PSS) is an IEEE Std 421.5 type
PSS2A/2B/2C, dual-input, “integral of accelerating power” stabilizer that provides supplementary damping
for low-frequency, local-mode oscillations and power system oscillations.
PSS features include user-selectable, speed-only sensing, two- or three-wattmeter power measurement,
optional frequency-based operation, and generator and motor control modes.
Three-phase current sensing is required for PSS.
PSS settings are configured exclusively through the BESTCOMSPlus
®
interface. These settings are
illustrated in Figure 15-10, Figure 15-11, Figure 15-12, and Figure 15-13.
BESTCOMSPlus Navigation Path: Settings Explorer, PSS
HMI Navigation Path: Settings, PSS
Supervisory Function and Setting Groups
A supervisory function enables PSS operation only when a sufficient load is applied to the generator. Two
separate groups of PSS settings enable stabilizer operation tailored for two distinct load conditions.
Supervisory Function
When PSS control is enabled, a power-on threshold setting determines the level of power (watts) where
PSS operation is automatically enabled. This threshold is a per-unit setting based on the generator
ratings. (The Co nfiguration chapter of this manual provides information about entering the generator and
system ratings.) A hysteresis setting provides a margin below the power-on threshold so that transient
dips in power (watts) will not disable stabilizer operation. This hysteresis is a per-unit setting based on the
generator ratings.
Setting Groups
When setting group selection is enabled, a threshold setting establishes the power level where the PSS
gain settings are switched from the primary group to the secondary group. After a transfer to the
secondary gain settings, a hysteresis setting determines the level of (decreasing) power where a transfer
back to the primary gain settings will occur.
Theory of Operation
The PSS uses an indirect method of power system stabilization that employs two signals: shaft speed
and electrical power. This method eliminates the undesirable components from the speed signal (such as
noise, lateral shaft run-out, or torsional oscillations) while avoiding a reliance on the difficult-to-measure
mechanical power signal.
PSS function is illustrated by the function blocks and software switches shown in Figure 15-1. This
illustration is also available in BESTCOMSPlus by clicking the PSS Model Info button located on the
Control tab.
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