Power System Stabilizer (PSS1A)

IEEE 421.5 Type PSS1A single-input power system stabilizer. Senses the generator rotor speed ω (wired from the machine's speed output), extracts the speed deviation through a washout, shapes it with an optional second-order input filter and two lead-lag phase-compensation stages, scales it by the stabilizer gain KS, and outputs a supplementary signal VS (clamped to [VSTMIN, VSTMAX]) intended for the excitation system's VS summing input — adding damping torque to suppress low-frequency electromechanical oscillations. Transfer function VS = KS · sT5/(1+sT6) · (1+sT1)/(1+sT2) · (1+sT3)/(1+sT4) · 1/(1+sA1+s²A2) · ω. The washout sT5/(1+sT6) 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 Type DC1A exciter's left-edge VS input for a direct connection.

Category: Control / Excitation

Overview

The Type PSS1A power system stabilizer (IEEE Std 421.5, Recommended Practice for Excitation System Models) is a single-input stabilizer that adds damping torque to a generator by modulating the voltage-regulator reference. Left to itself, a high-gain automatic voltage regulator can reduce the damping of the slow electromechanical (0.1–2 Hz) rotor swings; the PSS counteracts that by injecting a small supplementary signal VS into the exciter's summing junction, phased so that it produces a torque component in phase with speed — i.e. positive damping.

PSS1A senses one input — here the generator rotor speed ω, wired from the machine — extracts its deviation through a washout, shapes the phase with lead-lag compensation, scales it by the stabilizer gain, and clamps the result to a small output band. Because the washout removes the steady (DC) component, the stabilizer contributes nothing in steady state and never biases the voltage regulator's set-point.

Block diagram

The transfer function from the input to the (unlimited) output is

$$V_S=K_S\cdot \underset{\text{input filter}}{\underbrace{\frac{1}{1+s{A}_1+s^2{A}_2}}}\cdot \underset{\text{washout}}{\underbrace{\frac{s{T}_5}{1+s{T}_6}}}\cdot \underset{\text{lead-lag 1}}{\underbrace{\frac{1+s{T}_1}{1+s{T}_2}}}\cdot \underset{\text{lead-lag 2}}{\underbrace{\frac{1+s{T}_3}{1+s{T}_4}}}\cdot \omega ,$$

clamped to $[V_{STmin},V_{STmax}]$. Block by block (per-unit, $s=\mathrm{d}/\mathrm{d}t$):

1. Stabilizer gain $K_S$ — sets how much supplementary signal the PSS injects per unit of phase-compensated speed deviation. More gain gives more damping of the target mode, but too much excites other modes or the exciter's own dynamics.

2. Optional second-order input filter$1/(1+s{A}_1+s^2{A}_2)$ — a low-pass / notch ahead of the washout to attenuate torsional (shaft) modes and measurement noise. Leave $A_1=A_2=0$ to bypass it.

3. Washout $s{T}_5/(1+s{T}_6)$ — a high-pass that removes the steady component of the input so the PSS responds only to changes in speed. This is what makes the stabilizer output zero in steady state. With $T_5=T_6$ it is a classic unity-gain washout $s{T}_w/(1+s{T}_w)$; $T_6<T_5$ adds high-frequency gain.

4. Lead-lag stages $(1+s{T}_1)/(1+s{T}_2)$ and $(1+s{T}_3)/(1+s{T}_4)$ — provide the phase lead needed to cancel the lag of the exciter and the generator field at the oscillation frequency, so the injected signal lands in phase with speed. Set the second stage's constants equal (or both 0) to use a single stage.

5. Output limiter $[V_{STmin},V_{STmax}]$ — caps how far the PSS can push the voltage reference during a swing, typically a few percent, so the stabilizer cannot dominate the regulator or trip limiters.

Why a single speed input

PSS1A is the single-input member of the IEEE 421.5 stabilizer family. Classic stabilizer design drives the PSS from shaft speed (or frequency); this component takes the machine's speed output directly and forms the deviation internally through the washout, so you wire a single signal. (Multi-input variants such as PSS2A/PSS2B that blend speed and electrical power to reject mechanical-power transients are a separate model.)

Self-initialization (bumpless start)

The PSS needs no machine pairing to start cleanly. Because the washout removes the DC component, the stabilizer output is identically zero for any constant input. On the first step every internal state — the input filter, the washout lag, and both lead-lag lags — is seeded from the current speed sample, so:

$$V_S(0)=0.$$

The voltage regulator therefore sees no step from the stabilizer at $t=0$, and the PSS only comes alive when the speed actually deviates. There are no initialization parameters to set.

Per-unit convention

The output VS is per-unit on the same voltage base as the exciter's summing junction — it adds directly to the regulator reference the way VREF, VUEL and VOEL do. The limits VSTMAX / VSTMIN are in that same per-unit (typically ±0.05 to ±0.1). The speed input is per-unit of synchronous speed.

Wiring

• speed (ω) ← the synchronous machine's speed output (pu).
• VS → the exciter's VS summing input (enable PSS input on the Type DC1A exciter's Aux Inputs tab).

The PSS's right-edge VS output lines up with the Type DC1A exciter's left-edge VS input, so the two connect with a straight wire. Enable Monitor VS / washout output on the Monitoring tab to record the stabilizer output and the post-washout signal as named channels.

A typical damping loop is therefore: machine speed → PSS → exciter VS, alongside machine Vt → exciter, exciter EFD → machine Efd.

Tuning notes

• Set the lead-lag constants to compensate the phase lag of the exciter + field (GEP(s)) at the local-mode frequency, so the injected torque is in phase with speed.
• Set the washout $T_5=T_6$ in the 1–20 s range — long enough not to attenuate the inter-area / local modes you want to damp, short enough to reject genuine speed offsets.
• Raise the gain $K_S$ until the target mode is well damped, then back off for margin; verify it does not destabilize other modes or the exciter.
• Use the second-order filter ($A_1$, $A_2$) only if torsional modes or noise leak into the output.

Ports

Name	Direction	Value type	Notes
speed	input	double
VS	output	double

Parameters

Gain & Filter

Name	Label	Type	Default	Units	Description
Ks	KS	double	10	—	Stabilizer gain (pu VS per pu of the washed-out, phase-compensated speed deviation). Sets how much supplementary signal the PSS injects; higher KS gives more damping but can destabilize other modes if over-tuned.
A1	A1	double	0	—	First coefficient (s) of the optional second-order input filter 1/(1+sA1+s²A2), used to attenuate torsional / high-frequency noise. Leave A1 = A2 = 0 to bypass the filter (pass-through).
A2	A2	double	0	—	Second coefficient (s²) of the optional second-order input filter 1/(1+sA1+s²A2). Together with A1 it places a low-pass / notch ahead of the washout. Leave A1 = A2 = 0 to bypass the filter.

Washout & Lead-Lag

Name	Label	Type	Default	Units	Description
T5	T5	double	10	s (s, ms)	Washout numerator time constant of the sT5/(1+sT6) block. The washout removes the steady (DC) component of the input so the PSS contributes zero at steady state. The block's high-frequency gain is T5/T6 (set T5 = T6 for a classic unity-gain washout).
T6	T6	double	10	s (s, ms)	Washout denominator (lag) time constant of the sT5/(1+sT6) block. With T6 = T5 the block is a pure washout sTw/(1+sTw); T6 < T5 adds high-frequency gain. T6 = 0 makes the block a pure derivative sT5 (a discrete difference) — usually keep T6 > 0.
T1	T1	double	0.25	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	0.03	s (s, ms)	Lag (denominator) time constant of the first lead-lag stage (1+sT1)/(1+sT2).
T3	T3	double	0.15	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.
T4	T4	double	0.03	s (s, ms)	Lag (denominator) time constant of the second lead-lag stage (1+sT3)/(1+sT4).

Limits

Name	Label	Type	Default	Units	Description
VSTMAX	VSTMAX	double	0.1	—	Upper limit on the stabilizer output VS (pu). Caps how far the PSS can boost the voltage reference during a swing.
VSTMIN	VSTMIN	double	-0.1	—	Lower limit on the stabilizer output VS (pu). Caps how far the PSS can reduce 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 [VSTMIN, VSTMAX] 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 [VSTMIN, VSTMAX] limit.
vw	signal	from vw_name	measure_vw	Post-washout signal (pu): the extracted speed deviation after the second-order filter and washout, before the lead-lag phase compensation.