"""
According to P. Sauer "Power System Dynamics and Stability" p. 67 the
relationship between v_f_d and i_in1 is nonlinear due to saturation
of the exciter iron.

The saturation is then often described through two characteristic points on the
saturation curve: S_E_max (saturation at maximal excitation voltage)
and S_E0.75(saturation at 0.75* maximal excitation voltage).

These can be used to fit the following function:
S_E(E_f_d) = A_x * exp(B_x * E_f_d)               P.Sauer, P.70, Eq(4.22)

In a steady-state it is also given that:
0 = - (K_e + S_E_max) * E_f_d_max + V_R_max       P.Sauer, P.69, Eq(4.21)
Inputs:
    - K_e: Exciter constant related to self-excited field [pu]
    - S_E_max: Saturation at maximal excitation voltage [pu]
    - S_E_tq: Saturation at 0.75 * maximal excitation voltage [pu]
    - V_R_max: Maximal reference voltage for the AVR [pu]
Outputs:
    - E_f_d_max: maximal excitation voltage
    - A_x: Fitting Parameter
    - B_x: Fitting Parameter
"""


function ExciterSaturationEq(K_e, S_E_max, S_E_tq, V_R_max)
    E_f_d_max = V_R_max / (K_e + S_E_max)
    A_x = S_E_tq^4 / S_E_max^3
    B_x = 4/3 * log(S_E_tq / A_x) / E_f_d_max
    return E_f_d_max, A_x, B_x
end
