9492600990 13-1
DECS-150 Power System Stabilizer
13 • Power System Stabilizer
This section only applies to generator applications. The optional (style xPxxx), 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 sensing current is required for PSS operation.
A summary of the operational settings appears at the end of this section.
BESTCOMSPlus Navigation Path: Settings Explorer, 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, the 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 Configuration section 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, the 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, the 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 13-1. This
illustration is also available in BESTCOMSPlus by clicking the PSS Model Info button located on the
Control tab.
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