Power System Stabilizer (STAB1)

STAB1 speed-input power system stabilizer. The classic compact PSS: it senses the generator rotor speed ω (wired from the machine's speed output), passes it through a gain-and-washout K·sT/(1+sT) and two lead-lag phase-compensation stages (1+sT1)/(1+sT2) · (1+sT3)/(1+sT4), and outputs a supplementary signal VS (clamped to ±HLIM) for the excitation system's VS summing input — adding damping torque to suppress low-frequency electromechanical oscillations. Transfer function VS = K·sT/(1+sT) · (1+sT1)/(1+sT2) · (1+sT3)/(1+sT4) · ω. The washout K·sT/(1+sT) gives zero output at steady state, so the stabilizer self-initializes to VS = 0 at the load-flow operating point with no startup transient and never biases the voltage regulator in the steady state. Wire ω from the machine speed output and VS to the exciter's VS input (enable the exciter's PSS input). The right-edge VS output lines up with the exciter's left-edge VS input for a direct connection. For the IEEE 421.5 model with a second-order input filter and independent washout time constants use PSS1A.

Category: Control / Excitation

Overview

The STAB1 power system stabilizer is the classic compact speed-input PSS. It adds damping torque to a synchronous machine to suppress the low-frequency electromechanical oscillations (local and inter-area modes, ~0.1–2 Hz) that the voltage regulator alone tends to under-damp.

It senses the machine rotor speed ω, removes the steady component with a gain-and-washout, shapes the remaining swing through two lead-lag phase-compensation stages, and outputs a supplementary signal VS (hard-limited to ±HLIM) that is summed into the exciter's voltage reference.

Because of the washout, STAB1 produces zero output at steady state, so it self-initializes to VS = 0 at the load-flow operating point and a study starts with no excitation transient and no steady bias on the regulator.

For the IEEE 421.5 model with a second-order input filter and independent washout numerator / denominator time constants, use PSS1A.

Block diagram

Transfer function (per-unit, $s=\mathrm{d}/\mathrm{d}t$):

$$V_S=\mathrm{clamp}(K\frac{sT}{1+sT}\cdot \frac{1+s{T}_1}{1+s{T}_2}\cdot \frac{1+s{T}_3}{1+s{T}_4}\omega ,-H_{\text{LIM}},+H_{\text{LIM}}).$$

Block by block:

1. Gain + washout $KsT/(1+sT)$ — the high-pass that gives the stabilizer its zero steady-state output. $K$ is the overall stabilizer gain; the washout time constant $T$ (a few seconds) passes the oscillation band while blocking slow speed drift and the DC operating point. Keep $T>0$ — at $T=0$ the block is identically zero.

2. Lead-lag 1 $(1+s{T}_1)/(1+s{T}_2)$ — phase lead to compensate the exciter/machine phase lag at the oscillation frequency (typically $T_1>T_2$).

3. Lead-lag 2 $(1+s{T}_3)/(1+s{T}_4)$ — a second compensation stage for wider-band or two-mode tuning. Set $T_3=T_4$ (or both 0) to disable it (unity pass-through).

4. Output limiter $[-H_{\text{LIM}},+H_{\text{LIM}}]$ — a symmetric clamp so the PSS can never move the voltage reference by more than $H_{\text{LIM}}$ during a large swing.

Self-initialization (no recipe needed)

STAB1 needs no machine pairing and no load-flow recipe. On the first simulation step every internal state is seeded from the current speed sample so the washout output is exactly zero; since the lead-lag inputs are then zero, VS = 0 at $t=0$ regardless of the operating speed. The start is bumpless by construction.

Wiring

• speed ← the synchronous machine's speed output (ω, pu).
• VS → the excitation system's VS input (enable the exciter's PSS / VS summing input). The right-edge VS output lines up with the exciter's left-edge VS input for a direct connection.

Enable Monitor VS / washout output on the Monitoring tab to record the stabilizer output and the post-washout signal as named signals for plotting.

Tuning notes

• Tune the lead-lag stages first (with a residue / frequency-response method or by trial) so the net phase through the PSS plus the exciter is near zero at the local-mode frequency, then raise K until the damping is adequate without exciting other modes.
• The ±HLIM clamp is a safety limit, not a tuning knob; a PSS that rides the limit in normal swings is over-tuned (reduce $K$).
• A washout T of 1–10 s is typical; shorter T washes out faster but starts to attenuate the low end of the inter-area band.

When to use something else

• PSS1A — the IEEE 421.5 single-input PSS with an optional second-order input filter $1/(1+s{A}_1+s^2{A}_2)$ and independent washout constants $T_5$, $T_6$. Use it when you need the input filter or a non-unity washout high-frequency gain.

Ports

Name	Direction	Value type	Notes
speed	input	double
VS	output	double

Parameters

Gain & Washout

Name	Label	Type	Default	Units	Description
K	K (gain)	double	20	—	Stabilizer gain (pu VS per pu of the washed-out, phase-compensated speed deviation). Sets how much supplementary signal the PSS injects; higher K gives more damping but can destabilize other modes or saturate at ±HLIM if over-tuned.
T	T (washout)	double	10	s (s, ms)	Washout time constant of the K·sT/(1+sT) block. The washout removes the steady (DC) component of the speed input so the PSS contributes zero at steady state; a few seconds passes the electromechanical oscillation band (~0.1–2 Hz) while blocking slow drift. T = 0 makes the block zero (no signal) — keep T > 0.

Lead-Lag

Name	Label	Type	Default	Units	Description
T1	T1	double	0.5	s (s, ms)	Lead (numerator) time constant of the first lead-lag stage (1+sT1)/(1+sT2). Provides phase lead to compensate the exciter / machine phase lag at the oscillation frequency.
T2	T2	double	0.05	s (s, ms)	Lag (denominator) time constant of the first lead-lag stage (1+sT1)/(1+sT2).
T3	T3	double	0.5	s (s, ms)	Lead (numerator) time constant of the second lead-lag stage (1+sT3)/(1+sT4). Set T3 = T4 (or both 0) to disable the second stage (unity pass-through).
T4	T4	double	0.05	s (s, ms)	Lag (denominator) time constant of the second lead-lag stage (1+sT3)/(1+sT4).

Limits

Name	Label	Type	Default	Units	Description
HLIM	HLIM	double	0.05	—	Symmetric output limit on the stabilizer signal VS (pu): the output is clamped to [−HLIM, +HLIM]. Caps how far the PSS can move the voltage reference during a swing.

Monitoring

Name	Label	Type	Default	Units	Description
measure_vs	Monitor VS	enum (Off / On)	0	—	Record the stabilizer output VS (pu, after the ±HLIM limit) as a named observable.
measure_vw	Monitor washout output	enum (Off / On)	0	—	Record the post-washout signal (pu) — the extracted speed deviation before the lead-lag stages — as a named observable.

Signal Names

Name	Label	Type	Default	Units	Description
vs_name	VS name	string	(empty)	—	Observable name for the stabilizer output VS. Blank = skip.
vw_name	Washout name	string	(empty)	—	Observable name for the post-washout signal. Blank = skip.

Observables

Signal	Type	Default name	Enable	Description
VS	signal	from vs_name	measure_vs	Stabilizer output VS (pu) commanded to the exciter's VS summing input, after the ±HLIM limit.
vw	signal	from vw_name	measure_vw	Post-washout signal (pu): the gain-scaled, washed-out speed deviation before the lead-lag phase compensation.