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This module is based on the IEEE Std 412.5 type AC8B excitation system model [1]. The block diagram of this excitation system model includes an AC alternator with non-controlled rectifiers. The model presents alternate options for the under excitation limiter input.

Mask and Parameters

AVR Parameters


Expanding the "AC8B diagram" displays the block diagram in the parameters window.

AC8B AVR block diagram



Regulator input time constant. This input filter is not part of the standard and there are no typical values recommended. It was added for flexibility.

NB This parameter can be set to zero if the filter is not used.

TdrDerivative time constant control PID
KprProportional gain PID control

KdrDerivative gain PID control

KirIntegral gain PID control

KaAVR filter gain
TaAVR filter time constant
VRmaxMaximum regulator internal voltage
VRminMinimum regulator internal voltage

Exciter Parameters

Expanding the "Exciter diagram" displays the block diagram in the parameters window.

KeExciter constant related to self-excited field
TeExciter time constant
KcRectifier loading factor
KdDemagnetizing factor
Se1, Se2Exciter saturation factor at the corresponding Vex
Ve1, Ve2Exciter voltage for the exciter saturation function
VEminMinimum exciter output voltage
VfemaxMaximum exciter field current

Initial Values Mask Parameters

AC8B-Initial Values mask parameters

Ifd0Synchronous machine field current initial value


Efd0Exciter output voltage initial value

The parameters Ifd0 and Efd0 can be set manually by entering a numerical value. It can also be set automatically, based on load flow calculations, by entering a referenced synchronous machine variable.  For instance, if the name of the synchronous machine on which the excitation system is connected is “SM1”:

  • If a thermal machine or a hydraulic machine is used, Ifd0 shall be set as “=SM1.IfdInit” multiplied by the synchronous machine parameter Xad = Xd – Xl, and Efd0 shall be set as “=SM1.EfdInit”;
  • If a pu standard or pu fundamental machine is used, Ifd0 shall be set as “=SM1.IF_Init” and Efd0 shall be set as "=SM1.EFD_Init".

The HYPERSIM® simulation option Set Initial Conditions must be checked for the automatic initialization to work properly.

Inputs and Outputs and Signals Available for Monitoring



Signal proportional to compensated terminal voltage

If a load compensator block is not used upstream from the exciter block, then Vc1 is equal to Vt (main alternator terminal voltage)

VrefVoltrage regulator reference voltagepu
VsIs defined as the output voltage of a power system stabilizer (PSS)pu
IfdSynchronous machine field current*pu

*If a Synch. Machine (Hydraulic or Thermal) from the Network Machines and Generators library is used, the machine observable Ifd must be multiplied by the synchronous machine parameter Xad = Xd – Xl prior to its input to the exciter. This multiplication is not needed if a Synchronous Machine (pu Standard) or (pu Fundamental) is used.


 Efd Exciter output voltagepu


1. “IEEE Recommended Practice for Excitation System Models for Power System Models for Power System Stability Studies,” IEEE Standard 421.5-2005.

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