# EXAC1 _Source: `GridKit/Model/PhasorDynamics/Exciter/EXAC1/README.md`_ ## **IEEE Type AC1 Excitation System Model (EXAC1)** EXAC1 is an IEEE Type AC excitation system with a terminal-voltage transducer, lead-lag compensated voltage regulator, alternator field-voltage state, stabilizing feedback, exciter saturation, rectifier loading, and optional speed multiplier. Notes: - Internal voltage and current signals are on model base unless otherwise stated. - The rectifier loading block $F_{\mathrm{ex}}=f(I_N)$ is the source AC-exciter loading curve from Fig. 1; it is not a CommonMath helper. - The source diagram labels the optional multiplier input as `Speed`; GridKit uses machine speed deviation, so the enabled multiplier is $1+\omega$. ### Block Diagram Standard model of the EXAC1 Exciter. ```{image} ../../../../../Figures/PhasorDynamics/EXAC1_diagram.png :align: center ``` Figure 1: Exciter EXAC1 model. Figure courtesy of [PowerWorld](https://www.powerworld.com/WebHelp/) ### Model Parameters Symbol | Units | JSON | Description | Typical Value | Note ------------------------------------|----------|-----------|---------------------------------------------------------|---------------|------ $T_R$ | [sec] | `Tr` | Transducer time constant | 0.0 | Block name: `Tr`; if zero, $V_C$ is algebraic $T_B$ | [sec] | `Tb` | Lag time constant for voltage-regulator input lead-lag | 0.0 | Block name: `Tb`; if $T_B=T_C=0$, the lead-lag block is bypassed $T_C$ | [sec] | `Tc` | Lead time constant for voltage-regulator input lead-lag | 0.0 | Block name: `Tc` $K_A$ | [p.u.] | `Ka` | Voltage-regulator gain | 40.0 | Block name: `Ka` $T_A$ | [sec] | `Ta` | Voltage-regulator time constant | 0.1 | Block name: `Ta` $V_R^{\max}$ | [p.u.] | `Vrmax` | Maximum voltage-regulator output | 1.0 | Block name: `Vrmax` $V_R^{\min}$ | [p.u.] | `Vrmin` | Minimum voltage-regulator output | -1.0 | Block name: `Vrmin` $T_E$ | [sec] | `Te` | Exciter alternator time constant | 0.5 | Block name: `Te` $K_F$ | [p.u.] | `Kf` | Stabilizing feedback gain | 0.05 | Block name: `Kf` $T_F$ | [sec] | `Tf` | Stabilizing feedback time constant | 0.7 | Block name: `Tf`; if zero, $V_F$ is algebraic $K_C$ | [p.u.] | `Kc` | Rectifier loading current coefficient | 0.0 | Block name: `Kc`; forms $I_N$ $K_D$ | [p.u.] | `Kd` | Demagnetizing factor feedback gain | 0.0 | Block name: `Kd` $K_E$ | [p.u.] | `Ke` | Exciter field-resistance line-slope margin | 0.1 | Block name: `Ke` $E_1$ | [p.u.] | `E1` | First saturation voltage point | 2.8 | Block name: `E1` $S_E(E_1)$ | [p.u.] | `SE1` | Saturation value at $E_1$ | 0.08 | Block name: `Se1` $E_2$ | [p.u.] | `E2` | Second saturation voltage point | 3.7 | Block name: `E2` $S_E(E_2)$ | [p.u.] | `SE2` | Saturation value at $E_2$ | 0.33 | Block name: `Se2` $s_{\mathrm{spd}}$ | [binary] | `Spdmlt` | Speed multiplier flag | 0 | Block name: `Spdmlt`; 1 enables the speed multiplier #### Parameter Validation Invalid EXAC1 parameter sets are rejected by the following checks. ```{math} \begin{aligned} &K_A > 0 \\ &T_R \ge 0,\quad T_A > 0,\quad T_B \ge 0,\quad T_C \ge 0,\quad T_E > 0,\quad T_F \ge 0 \\ &T_B > 0\quad\text{or}\quad(T_B = 0\ \text{and}\ T_C = 0) \\ &V_R^{\min} \le V_R^{\max} \\ &s_{\mathrm{spd}} \in \{0,1\} \end{aligned} ``` The saturation points are either disabled together, ```{math} \begin{aligned} S_E(E_1) = 0,\quad S_E(E_2) = 0 \end{aligned} ``` or define a valid two-point quadratic saturation fit: ```{math} \begin{aligned} &E_1 > 0,\quad E_2 > 0,\quad E_1 \ne E_2 \\ &S_E(E_1) > 0,\quad S_E(E_2) > 0,\quad S_E(E_1) \ne S_E(E_2) \end{aligned} ``` #### Model Derived Parameters The saturation curve is fitted from the two supplied saturation points. If both saturation factors are zero, use $S_A=0$ and $S_B=0$. Otherwise: ```{math} \begin{aligned} C &= \sqrt{\dfrac{S_E(E_2)}{S_E(E_1)}} \\ S_A &= \dfrac{C E_1 - E_2}{C - 1} \\ S_B &= \dfrac{S_E(E_1)}{(E_1 - S_A)^2} \end{aligned} ``` ### Model Variables #### Internal Variables ##### Differential Symbol | Units | Description | Note ------------------------------------|--------|---------------------------------------------------------|------ $V_E$ | [p.u.] | Exciter alternator voltage state before output multipliers | State 1 in Fig. 1; source label: `VE` $V_C$ | [p.u.] | Sensed compensated voltage | State 2 in Fig. 1; source label: `Sensed Vt`; algebraic when $T_R=0$ $V_R$ | [p.u.] | Voltage-regulator output | State 3 in Fig. 1; source label: `VR` $x_{\mathrm{ll}}$ | [p.u.] | Lead-lag block state | State 4 in Fig. 1; source label: `VLL` $V_F$ | [p.u.] | Stabilizing feedback washout output | State 5 in Fig. 1; source label: `VF`; algebraic when $T_F=0$ ##### Algebraic Symbol | Units | Description | Note ------------------------------------|--------|---------------------------------------------------------|------ $e_V$ | [p.u.] | Voltage-regulator input error before lead-lag block | Summing junction after sensed voltage $V_{\mathrm{ll}}$ | [p.u.] | Lead-lag output | Input to voltage regulator $S_E$ | [p.u.] | Saturation coefficient evaluated at $V_E$ | Uses derived saturation curve $I_N$ | [p.u.] | Normalized exciter loading current | Source label: `IN`; satisfies $V_E I_N=K_C I_{\mathrm{fd}}$ $F_{\mathrm{ex}}$ | [p.u.] | Rectifier loading factor | Source label: `FEX`; source curve $F_{\mathrm{ex}}=f(I_N)$ $V_{\mathrm{fe}}$ | [p.u.] | Exciter feedback signal | Sum of saturation/resistance and $K_D I_{\mathrm{fd}}$ paths $E_{\mathrm{fd}}$ | [p.u.] | Field-voltage output | Output after rectifier loading and optional speed multiplier #### External Variables ##### Differential None. ##### Algebraic Symbol | Units | Description | Note ------------------------------------|--------|---------------------------------------------------------|------ $E_C$ | [p.u.] | Compensated terminal voltage magnitude | Source label: `EC` $V_{\mathrm{ref}}$ | [p.u.] | Voltage-control reference | Source label: `VREF` $V_S$ | [p.u.] | Stabilizer input signal | Source label: `VS`; optional, defaults to zero $V_{\mathrm{uel}}$ | [p.u.] | Under-excitation limiter input | Source label: `VUEL`; optional, defaults to zero $V_{\mathrm{oel}}$ | [p.u.] | Over-excitation limiter input | Source label: `VOEL`; optional, defaults to zero $I_{\mathrm{fd}}$ | [p.u.] | Machine field current | Source label: `IFD` $\omega$ | [p.u.] | Machine speed deviation | Source label: `Speed`; optional when $s_{\mathrm{spd}}=0$ ### Model Equations #### Differential Equations ```{math} \begin{aligned} 0 &= -T_R\dot V_C - V_C + E_C \\ 0 &= -T_B\dot x_{\mathrm{ll}} - x_{\mathrm{ll}} + e_V \\ 0 &= -T_A\dot V_R + \text{antiwindup}\!\left( V_R, -V_R + K_A V_{\mathrm{ll}}, V_R^{\min}, V_R^{\max} \right) \\ 0 &= -T_E\dot V_E + V_R - V_{\mathrm{fe}} \\ 0 &= -T_F\dot V_F - V_F + K_F\dot V_E \end{aligned} ``` CommonMath defines the [Anti-Windup](../../../common-math.md#derived-functions) target and smooth approximation. #### Algebraic Equations ```{math} \begin{aligned} 0 &= -e_V + V_{\mathrm{ref}} + V_S + V_{\mathrm{uel}} + V_{\mathrm{oel}} - V_C - V_F \\ 0 &= -T_B(V_{\mathrm{ll}} - x_{\mathrm{ll}}) + T_C(e_V - x_{\mathrm{ll}}) \\ 0 &= -S_E + S_B\,q(V_E - S_A) \\ 0 &= -V_E I_N + K_C I_{\mathrm{fd}} \\ 0 &= -F_{\mathrm{ex}} + f(I_N) \\ 0 &= -V_{\mathrm{fe}} + (K_E + S_E)V_E + K_D I_{\mathrm{fd}} \\ 0 &= -E_{\mathrm{fd}} + \left(1+s_{\mathrm{spd}}\omega\right)F_{\mathrm{ex}}V_E \end{aligned} ``` CommonMath defines the primitive [quadratic ramp](../../../common-math.md#primitives) $q$. The rectifier loading function $f(I_N)$ is the source curve shown in Fig. 1. When $T_B=T_C=0$, the lead-lag block is bypassed so $V_{\mathrm{ll}}=e_V$. ### Initialization The machine initializes $E_{\mathrm{fd}}$ and $I_{\mathrm{fd}}$ first. For a standard unsaturated start, EXAC1 reads those values along with attached $\omega$, $E_C$, $V_S$, $V_{\mathrm{uel}}$, and $V_{\mathrm{oel}}$, and sets all internal derivatives to zero. First solve the coupled rectifier-loading equations: ```{math} \begin{aligned} 0 &= -V_{E,0}I_{N,0} + K_C I_{\mathrm{fd},0} \\ 0 &= -F_{\mathrm{ex},0} + f(I_{N,0}) \\ 0 &= -E_{\mathrm{fd},0} + \left(1+s_{\mathrm{spd}}\omega_0\right)F_{\mathrm{ex},0}V_{E,0} \end{aligned} ``` Then evaluate: ```{math} \begin{aligned} S_{E,0} &= S_B\,q(V_{E,0} - S_A) \\ V_{\mathrm{fe},0} &= (K_E + S_{E,0})V_{E,0} + K_D I_{\mathrm{fd},0} \\ V_{R,0} &= V_{\mathrm{fe},0} \\ V_{\mathrm{ll},0} &= \dfrac{V_{R,0}}{K_A} \\ V_{C,0} &= E_{C,0} \\ V_{F,0} &= 0 \\ x_{\mathrm{ll},0} &= e_{V,0} = V_{\mathrm{ll},0} \\ V_{\mathrm{ref},0} &= e_{V,0} + V_{C,0} + V_{F,0} - V_{S,0} - V_{\mathrm{uel},0} - V_{\mathrm{oel},0} \end{aligned} ``` This standard start requires $1+s_{\mathrm{spd}}\omega_0\ne 0$, $V_{E,0}\ne 0$, and $V_R^{\min}\le V_{R,0}\le V_R^{\max}$. Saturated regulator starts are outside these closed-form equations. ### Model Outputs Output | Units | Description | Note ----------------|--------|-------------------------------------|------ `efd` | [p.u.] | Field-voltage output | $E_{\mathrm{fd}}$ `ve` | [p.u.] | Exciter alternator voltage state | $V_E$ `vc` | [p.u.] | Sensed compensated voltage | $V_C$ `vr` | [p.u.] | Voltage-regulator output | $V_R$ `vll` | [p.u.] | Lead-lag output | $V_{\mathrm{ll}}$ `vf` | [p.u.] | Stabilizing feedback state | $V_F$ `in` | [p.u.] | Normalized exciter loading current | $I_N$ `fex` | [p.u.] | Rectifier loading factor | $F_{\mathrm{ex}}$ `se` | [p.u.] | Saturation coefficient | $S_E$