Custom Exciter Models

This page documents the custom excitation system models available in RAMSES. These models range from a trivial constant-voltage source to full-featured AVR with PSS and OEL. Models without a .txt definition file are implemented directly in Fortran and are described from their source headers.

Exciter models in RAMSES are defined as sub-records within a SYNC_MACH block using the EXC keyword, followed by the model name and its parameters. They are not standalone records. The EXC line appears after the machine parameters and before the TOR (torque/governor) line. The entire block ends with a single ; after the TOR record.

SYNC_MACH name bus FP FQ P Q SNOM Pnom H D IBRATIO
                    XT  Xl  Xd  X'd  X"d  Xq  X'q  X"q  m  n  Ra  T'do  T"do  T'qo  T"qo
              EXC model_name  param1  param2  ...
              TOR model_name  param1  param2  ... ;

exc_constant

Scientific Description

The exc_constant model provides a constant field voltage source. The field voltage $V_f$ is fixed at its load-flow initialisation value and does not respond to any input:

V_f(t) = V_{f0} \quad \forall t

The single algebraic state satisfies:

f(1) = x(1) - V_{f0} = 0

This model is useful for open-loop tests, for machines that are not equipped with a voltage regulator, or as a placeholder during model development.

Implementation details (from Fortran source):

nbxvar = 1, nbzvar = 0, nbdata = 0
One additional parameter: prm(1) = Vf0 (set at initialisation from the load-flow field voltage)
Observables: vf, if

Parameters

Parameter	Description
(none)	No user-supplied data parameters. The field voltage is taken from the initialisation (load-flow) value automatically.

SYNC_MACH GEN1  BUS1  1. 1.  0.  0.   600. 570. 3. 0. 0.95
                    XT   0.15  1.1  0.25  0.2  0.7  *  0.2  0.1  6.0257  0.  5.00  0.05  *  0.1
              EXC CONSTANT
              TOR CONSTANT ;

No parameters need to be supplied; the model self-initialises to the load-flow $V_f$ value.
Useful for simulation of machines operating in manual excitation mode.
The model name in the data file is CONSTANT (not exc_constant).

exc_1storder

Scientific Description

The exc_1storder model is a first-order automatic voltage regulator (AVR). It represents the simplest dynamic exciter: a proportional gain $K_A$ acting on the terminal voltage error, filtered through a single first-order lag $T_A$ , with output limits.

The AVR computes the voltage error and drives the field voltage:

\frac{d V_f}{dt} = \frac{1}{T_A}\left[K_A\left(V_{ref} - V\right) - V_f\right], \quad V_f \in [V_{f,min},\, V_{f,max}]

The reference voltage $V_{ref}$ is initialised from:

V_{ref} = V_0 + \frac{V_{f0}}{K_A}

Implementation details (from Fortran source):

nbxvar = 1, nbzvar = 1, nbdata = 4
Parameters: prm(1) = KA, prm(2) = TA, prm(3) = Vfmin, prm(4) = Vfmax
Additional parameter: prm(5) = V0 (voltage setpoint, computed at initialisation)
The ODE in the continuous mode is:

f(1) = \frac{-V_f + K_A(V_{ref} - V)}{T_A}

With anti-windup: if the limiter is active, f(1) = Vf - Vfmin (or Vfmax).

Parameters

Parameter	Description
`KA`	AVR gain (pu/pu)
`TA`	AVR time constant (s)
`Vfmin`	Minimum field voltage (pu)
`Vfmax`	Maximum field voltage (pu)

Usage Example

RAMSES Data File
Notes

SYNC_MACH GEN1  BUS1  1. 1.  0.  0.   600. 570. 3. 0. 0.95
                    XT   0.15  1.1  0.25  0.2  0.7  *  0.2  0.1  6.0257  0.  5.00  0.05  *  0.1
              EXC 1storder  200.0  0.05  0.0  6.0
                            ! KA    TA    Vfmin Vfmax
              TOR CONSTANT ;

This model is the Fortran-coded equivalent of a simple tf1plim block with input $V_{ref} - V$ .
KA and TA together set the closed-loop bandwidth.
The model name in the data file is 1storder (not exc_1storder).

exc_kundur

Scientific Description

The exc_kundur model implements the excitation system from Kundur’s textbook (Power System Stability and Control, 1994). It includes a first-order voltage transducer, a proportional AVR with transient gain reduction (TGR), and a two-stage PSS acting on rotor speed deviation.

Voltage transducer:

\frac{d V_{fil}}{dt} = \frac{1}{T_R}(V - V_{fil})

AVR with transient gain reduction (lead–lag):

V_f(s) = K_A \cdot \frac{1 + s T_A}{1 + s T_B} \cdot \left(V_{ref} - V_{fil} + V_s\right)

where $V_{ref} = V + V_f / K_A$ is computed at initialisation and $V_s$ is the PSS output.

PSS (two-stage lead–lag with washout):

x_{wash}(s) = \frac{K_{STAB} \cdot s T_W}{1 + s T_W} \cdot \omega(s)

V_s(s) = \frac{1 + s T_1}{1 + s T_2} \cdot \frac{1 + s T_3}{1 + s T_4} \cdot x_{wash}(s)

This is the standard two-stage phase-compensating PSS structure. The CODEGEN blocks in the .txt file are:

Block	CODEGEN type	Function
Voltage filter	`tf1p`	$1/(1+sT_R)$
Main summing junction	`algeq`	$V_{ref} - V_{fil} + V_s - dv$
AVR (TGR)	`tf1p1z`	$K_A(1+sT_A)/(1+sT_B)$
PSS washout	`tfder1p`	$K_{STAB} \cdot sT_W/(1+sT_W)$
PSS lead–lag 1	`tf1p1z`	$(1+sT_1)/(1+sT_2)$
PSS lead–lag 2	`tf1p1z`	$(1+sT_3)/(1+sT_4)$

Parameters

Parameter	Description
`TR`	Voltage measurement filter time constant (s)
`KA`	AVR gain (pu/pu)
`TA`	TGR lead time constant (s)
`TB`	TGR lag time constant (s)
`KSTAB`	PSS gain
`TW`	PSS washout time constant (s)
`T1`	PSS first lead–lag zero time constant (s)
`T2`	PSS first lead–lag pole time constant (s)
`T3`	PSS second lead–lag zero time constant (s)
`T4`	PSS second lead–lag pole time constant (s)

Computed at initialisation: Vref = V + Vf / KA

Usage Example

RAMSES Data File
Notes

SYNC_MACH GEN1  BUS1  1. 1.  0.  0.   600. 570. 3. 0. 0.95
                    XT   0.15  1.1  0.25  0.2  0.7  *  0.2  0.1  6.0257  0.  5.00  0.05  *  0.1
              EXC kundur  0.02  200.0  0.02  10.0  9.5  1.41  0.154  0.033  0.154  0.033
                          ! TR   KA     TA    TB    KSTAB TW    T1     T2     T3     T4
              TOR CONSTANT ;

Parameters match the Example 12.6 from Kundur (1994), applicable to a medium-speed generator.
The PSS stabilises local modes in the 0.5–2 Hz range.
No output limiter is defined in the base model; add clipping externally if required.
The model name in the data file is kundur (not exc_kundur).

exc_generic1

Scientific Description

The exc_generic1 model is a generic AVR with OEL (Over-Excitation Limiter) and PSS, implemented in Fortran. It includes a proportional–integral AVR, a PSS acting on rotor speed or active power, and an inverse-time OEL.

AVR:

The excitation system uses a first-order integrating controller (washout or PI-type, set by the gain $G$ and time constants $T_A$ , $T_B$ ) with output limits $[L_3,\, L_4]$ :

V_f(s) = G \cdot \frac{1 + s T_A}{1 + s T_B} \cdot \left(V_{ref} - V\right) \cdot \frac{1}{1 + s T_E}

PSS (speed or power input, selectable):

Input signal is selected by SPEEDIN: 1 = rotor speed $\omega$ , 0 = electrical power $P$ .

V_{PSS}(s) = K_{PSS} \cdot \frac{s T_W}{1 + s T_W} \cdot \frac{1 + s T_1}{1 + s T_2} \cdot \frac{1 + s T_3}{1 + s T_4} \cdot u(s)

Output is clamped to $[DV_{MIN},\, DV_{MAX}]$ .

OEL (inverse-time limiter):

The OEL monitors field current $I_f$ . When $I_f > IFLIM$ , a timer begins accumulating via:

Dead zone $d$ and phase $f$ parameters control OEL activation thresholds.
The OEL integrates the overcurrent error and reduces $V_{ref}$ once the timer reaches threshold $L_1$ , with output range $[L_1, L_2]$ .

Parameters from Fortran source (prm array):

Index	Name	Description
1	`IFLIM`	Field current OEL threshold (pu)
2	`d`	OEL dead zone (pu)
3	`f`	OEL phase characteristic
4	`S`	AVR setpoint input (usually 0 = $V$ )
5	`K1`	AVR lead numerator gain
6	`K2`	AVR lag denominator gain
7	`L1`	OEL integrator lower limit (pu)
8	`L2`	OEL integrator upper limit (pu)
9	`G`	AVR overall gain
10	`TA`	AVR lead time constant (s)
11	`TB`	AVR lag time constant (s)
12	`TE`	Exciter time constant (s)
13	`L3`	AVR output lower limit (pu)
14	`L4`	AVR output upper limit (pu)
15	`SPEEDIN`	PSS input signal: 1 = speed, 0 = power
16	`KPSS`	PSS gain
17	`Tw`	PSS washout time constant (s)
18	`T1`	PSS lead–lag 1 zero (s)
19	`T2`	PSS lead–lag 1 pole (s)
20	`T3`	PSS lead–lag 2 zero (s)
21	`T4`	PSS lead–lag 2 pole (s)
22	`DVMIN`	PSS output lower limit (pu)
23	`DVMAX`	PSS output upper limit (pu)

Usage Example

RAMSES Data File

SYNC_MACH GEN1  BUS1  1. 1.  0.  0.   600. 570. 3. 0. 0.95
                    XT   0.15  1.1  0.25  0.2  0.7  *  0.2  0.1  6.0257  0.  5.00  0.05  *  0.1
              EXC GENERIC1  1.05  -0.1  0.5  0.0  1.0  1.0  0.0  0.5  200.0  0.02  0.10  0.05  0.0  6.0
                            ! IFLIM  d    f    S   K1   K2   L1   L2   G      TA    TB    TE    L3   L4
                            1     9.5   1.40  0.15  0.03  0.15  0.03  -0.10  0.10
                            ! SPEEDIN KPSS  Tw    T1    T2    T3    T4    DVMIN  DVMAX
              TOR CONSTANT ;

exc_generic2

Scientific Description

The exc_generic2 model is a more detailed generic AVR (formerly known as exc_generic3 internally), developed and validated with Hydro-Québec data. It includes a voltage compensator, a combined PI–proportional regulator, an AC exciter with saturation, a PSS, and an OEL.

Voltage measurement with line-drop compensation:

V_c = V + X_c \cdot I_q - R_c \cdot I_d \quad (\text{or simplified})

PI–proportional AVR with transient gain reduction:

V_{AVR}(s) = \left(K_P + \frac{K_I}{s} + C \cdot \frac{1 + s T_C}{1 + s T_B}\right) \cdot K_A \cdot \frac{1}{1 + s T_A}(V_{ref} - V_c)

AC exciter with saturation:

\frac{d V_E}{dt} = \frac{1}{T_E}\left[V_R - K_E \cdot V_E - S_E(V_E)\right]

where $S_E(V_E)$ is the piecewise-linear saturation function defined by $(E_1, S_{E1})$ and $(E_2, S_{E2})$ .

PSS (type selectable by typss):

V_s(s) = K_{PSS} \cdot \frac{s T_W}{1 + s T_W} \cdot \frac{1 + s T_1}{1 + s T_2} \cdot \frac{1 + s T_3}{1 + s T_4} \cdot u(s)

OEL (inverse-time, three-segment):

The OEL uses a three-level piecewise-linear characteristic $(s_1, T_{oel1})$ , $(s_2, T_{oel2})$ , $(s_3, T_{oel3})$ to compute the allowable overcurrent time.

Parameters from Fortran associate block (39 data parameters + 4 additional):

Index	Name	Description
1	`Xc`	Line-drop compensation reactance (pu)
2	`Tr`	Voltage measurement time constant (s)
3	`C`	TGR numerator gain
4	`Tc`	TGR lead time constant (s)
5	`Tb`	TGR lag time constant (s)
6	`Ka`	AVR proportional gain
7	`Ta`	AVR time constant (s)
8	`Ki`	AVR integral gain
9	`Kp`	AVR proportional path gain
10	`VrmaxD`	Regulator max (dynamic, pu)
11	`VrminD`	Regulator min (dynamic, pu)
12	`G`	Exciter overall gain
13	`Ke`	Exciter self-excitation coefficient
14	`Te`	Exciter time constant (s)
15	`E1`	Saturation breakpoint 1 (pu)
16	`SeE1`	Saturation factor at $E_1$
17	`E2`	Saturation breakpoint 2 (pu)
18	`SeE2`	Saturation factor at $E_2$
19	`Vrmax`	Field voltage upper limit (pu)
20	`Vrmin`	Field voltage lower limit (pu)
21	`typss`	PSS input type (1 = speed, 2 = power)
22	`W`	PSS input gain
23	`Tf`	PSS derivative filter time constant (s)
24	`Kpss`	PSS gain
25	`Tw`	PSS washout time constant (s)
26	`T1`	PSS lead–lag 1 zero (s)
27	`T2`	PSS lead–lag 1 pole (s)
28	`T3`	PSS lead–lag 2 zero (s)
29	`T4`	PSS lead–lag 2 pole (s)
30	`Lim`	PSS output symmetric limit (pu)
31	`LimSup`	PSS upper asymmetric limit (pu)
32	`s1`	OEL segment 1 (pu $I_f$ )
33	`toel1`	OEL time at segment 1 (s)
34	`s2`	OEL segment 2 (pu $I_f$ )
35	`toel2`	OEL time at segment 2 (s)
36	`s3`	OEL segment 3 (pu $I_f$ )
37	`toel3`	OEL time at segment 3 (s)
38	`perm`	OEL permanent current limit (pu)
39	`LimInf`	PSS lower asymmetric limit (pu)

Usage Example

RAMSES Data File

SYNC_MACH GEN1  BUS1  1. 1.  0.  0.   600. 570. 3. 0. 0.95
                    XT   0.15  1.1  0.25  0.2  0.7  *  0.2  0.1  6.0257  0.  5.00  0.05  *  0.1
              EXC generic2  0.0  0.02  1.0  0.10  1.00  200.0  0.05  5.0  1.0  6.0  -6.0  1.0  0.0  0.50
                            ! Xc   Tr   C    Tc    Tb    Ka     Ta    Ki   Kp  VrmaxD VrminD G   Ke  Te
                            3.10  0.066  4.18  0.91  6.0  -6.0  1.0  1.0  0.05  9.5  1.40  0.154  0.033  0.154  0.033
                            ! E1   SeE1   E2   SeE2  Vrmax Vrmin typss W   Tf   Kpss  Tw    T1     T2     T3     T4
                            0.10  0.10  1.1  200.  1.2  60.  1.5  10.  1.05  -0.10
                            ! Lim  LimSup s1  toel1 s2  toel2 s3  toel3 perm  LimInf
              TOR CONSTANT ;

exc_GENERIC3

Scientific Description

The exc_GENERIC3 model (note capital letters) is a generic AVR with integrated PSS and OEL, defined via the CODEGEN .txt description. It provides a clean, modular three-block structure: AVR, PSS, and OEL.

AVR (transient gain reduction + exciter):

\Delta V = V_0 - V + V_{PSS} - V_{OEL}

V_1(s) = G \cdot \frac{1 + s T_a}{1 + s T_b} \cdot \Delta V(s)

V_f(s) = \frac{1}{1 + s T_e} \cdot V_1(s), \quad V_f \in [V_{f,min},\, V_{f,max}]

where $V_0 = V + V_f/G$ is computed at initialisation.

PSS (washout + two lead–lag stages):

V_{PSS}(s) = \frac{K_{PSS} \cdot s T_W}{1 + s T_W} \cdot \frac{1 + s T_1}{1 + s T_2} \cdot \frac{1 + s T_3}{1 + s T_4} \cdot \omega(s)

Output is limited to $[-C,\, +C]$ .

OEL (inverse-time integrator):

x_{oel1} = I_f - if_{1lim}

\frac{d x_{oel2}}{dt} = \frac{1}{T_{oel}} \cdot \min(x_{oel1},\, if_{2lim}), \quad x_{oel2} \in [L_1,\, L_2]

V_{OEL} = \text{clip}(K_{oel} \cdot x_{oel2},\, 0,\, L_3)

The CODEGEN block structure from the .txt file:

Block	CODEGEN type	Signal
AVR summing junction	`algeq`	$V_0 - V + dv_{PSS} - dv_{OEL} - \Delta V$
Transient gain reduction	`tf1p1z`	$G(1+sT_a)/(1+sT_b)$
Exciter	`tf1plim`	$1/(1+sT_e)$ with $[V_{f,min}, V_{f,max}]$
PSS washout	`tfder1p`	$K_{PSS} \cdot sT_W / (1+sT_W)$
PSS lead–lag 1	`tf1p1z`	$(1+sT_1)/(1+sT_2)$
PSS lead–lag 2	`tf1p1z`	$(1+sT_3)/(1+sT_4)$
PSS limiter	`lim`	$[-C, +C]$
OEL error	`algeq`	$I_f - if_{1lim}$
OEL upper bound	`min1v1c`	$\min(\cdot,\, if_{2lim})$
OEL integrator	`inlim`	$1/T_{oel}$ with $[L_1, L_2]$
OEL multiplier	`algeq`	$K_{oel} \cdot x_{oel2}$
OEL output limiter	`lim`	$[0, L_3]$

Parameters

Parameter	Description
`G`	AVR gain (pu/pu)
`Ta`	AVR lead time constant / TGR zero (s)
`Tb`	AVR lag time constant / TGR pole (s)
`Te`	Exciter time constant (s)
`vfmin`	Minimum field voltage (pu)
`vfmax`	Maximum field voltage (pu)
`Kpss`	PSS gain
`Tw`	PSS washout time constant (s)
`T1`	PSS first lead–lag zero (s)
`T2`	PSS first lead–lag pole (s)
`T3`	PSS second lead–lag zero (s)
`T4`	PSS second lead–lag pole (s)
`C`	PSS output symmetric limit (pu)
`if1lim`	OEL field current threshold (pu)
`if2lim`	OEL maximum overcurrent error (pu)
`Toel`	OEL integrator time constant (s)
`Koel`	OEL output gain
`L1`	OEL integrator lower limit
`L2`	OEL integrator upper limit
`L3`	OEL output upper limit (pu)

Computed at initialisation: Vo = V + Vf / G

Usage Example

RAMSES Data File

SYNC_MACH GEN1  BUS1  1. 1.  0.  0.   600. 570. 3. 0. 0.95
                    XT   0.15  1.1  0.25  0.2  0.7  *  0.2  0.1  6.0257  0.  5.00  0.05  *  0.1
              EXC GENERIC3  200.0  0.05  10.0  0.04  0.0  5.0
                            ! G     Ta    Tb    Te    vfmin vfmax
                            9.5  1.40  0.154  0.033  0.154  0.033  0.10
                            ! Kpss Tw   T1     T2     T3     T4     C
                            1.05  0.30  30.0  1.0  0.0  1.0  2.0
                            ! if1lim if2lim Toel Koel L1  L2  L3
              TOR CONSTANT ;

exc_GENERIC4

Scientific Description

The exc_GENERIC4 model is a compiled version of exc_GENERIC3, implemented directly in Fortran using the new-style RAMSES API (single subroutine with mode selector). The mathematical structure is identical to exc_GENERIC3:

V_f(s) = \frac{1}{1 + s T_e} \cdot G \cdot \frac{1 + s T_a}{1 + s T_b} \cdot \left(V_0 - V + V_{PSS} - V_{OEL}\right)

V_{PSS}(s) = K_{PSS} \cdot \frac{s T_W}{1 + s T_W} \cdot \frac{1 + s T_1}{1 + s T_2} \cdot \frac{1 + s T_3}{1 + s T_4} \cdot \omega(s)

The OEL integrates the overcurrent error $(I_f - if_{1lim})$ , capped at $if_{2lim}$ , then multiplies by $K_{oel}$ to produce $V_{OEL}$ .

Implementation details (from Fortran source):

nbdata = 20, nbaddpar = 1 (Vo computed at init), nbxvar = 16, nbzvar = 5
The advantage over exc_GENERIC3 is performance: all equations are compiled, not interpreted.

Parameters

Parameter	Description
`G`	AVR gain
`Ta`	Lead time constant (s)
`Tb`	Lag time constant (s)
`Te`	Exciter time constant (s)
`vfmin`	Field voltage lower limit (pu)
`vfmax`	Field voltage upper limit (pu)
`Kpss`	PSS gain
`Tw`	PSS washout time constant (s)
`T1`	PSS lead–lag 1 zero (s)
`T2`	PSS lead–lag 1 pole (s)
`T3`	PSS lead–lag 2 zero (s)
`T4`	PSS lead–lag 2 pole (s)
`C`	PSS output symmetric limit (pu)
`if1lim`	OEL threshold (pu)
`if2lim`	OEL max overcurrent (pu)
`Toel`	OEL time constant (s)
`Koel`	OEL gain
`L1`	OEL lower limit
`L2`	OEL upper limit
`L3`	OEL output limit (pu)

Usage Example

RAMSES Data File
Notes

SYNC_MACH GEN1  BUS1  1. 1.  0.  0.   600. 570. 3. 0. 0.95
                    XT   0.15  1.1  0.25  0.2  0.7  *  0.2  0.1  6.0257  0.  5.00  0.05  *  0.1
              EXC GENERIC4  200.0  0.05  10.0  0.04  0.0  5.0
                            ! G     Ta    Tb    Te    vfmin vfmax
                            9.5  1.40  0.154  0.033  0.154  0.033  0.10
                            ! Kpss Tw   T1     T2     T3     T4     C
                            1.05  0.30  30.0  1.0  0.0  1.0  2.0
                            ! if1lim if2lim Toel Koel L1  L2  L3
              TOR CONSTANT ;

exc_GENERIC4 is the compiled-code equivalent of exc_GENERIC3. Use when simulation speed is a priority, as the Fortran subroutine is faster than the interpreted CODEGEN version.
Parameter order and meaning are identical to exc_GENERIC3.
The model name in the data file is GENERIC4 (not exc_GENERIC4).