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ST1A Excitation System

Implements IEEE type ST1A excitation system model


Simscape / Electrical / Specialized Power Systems / Electrical Machines / Synchronous Machine Control

  • ST1A Excitation System block


This block models an excitation system where the supply of power to the controlled bridge rectifier is provided by the generator terminals through a transformer to lower the voltage to an appropriate level. The voltage regulator gain and excitation system time constants are represented by Ka and Ta, respectively.

This block is an adaptation of the ST1A excitation system of the IEEE® 421 standard, copyright IEEE 2005, all rights reserved.


Controllers Tab

Low-pass filter time constant

The time constant Tr of the first-order system representing the stator terminal voltage transducer. Default is 20e-3.

Voltage regulator gain and time constant

The gain Ka and time constant Ta of the first-order system representing the main regulator. Default is [210 0.001].

Voltage regulator input limits

The voltage regulator input limits VImin and VImax, in p.u. Default is [-999 +999].

Voltage regulator internal limits

The voltage regulator internal limits VAmin and VAmax, in p.u. Default is [-999 999].

Voltage regulator output limits

The voltage regulator output limits VRmin and VRmax, in p.u. Default is [-6.0 6.43].

Damping filter gain and time constant

The gain Kf and time constant Tf of the first-order system representing the derivative feedback. Default is [0.001 1].

Transient gain reduction lead and lag time constants

The time constants Tb, Tc, Tb1, and Tc1 of the second-order system representing the lead-lag compensator. Default is [0 0 0 0].

Exciter and Rectifier Tab

Exciter output current limiter gain KLR

The exciter output current limiter gain. Default is 1.

Exciter output current limit reference ILR

The exciter output current limit reference. Default is 2.

Rectifier loading factor Kc

The rectifier loading factor proportional to commutating reactance. Default is 0.038.

Initial Values Tab

Initial values of terminal voltage and field voltage

The initial values of terminal voltage Vt0 and field voltage Efd0, both in p.u. Initial terminal voltage is normally set to 1 pu. The Vt0 and Efd0 values can be determined using the Powergui Load Flow tool. Default is [1 1].

Sample time

Specify a value greater than zero to discretize the block at the given sample time. Set to -1 to inherit the simulation type and sample time parameters of the Powergui block. Default is 0.



The reference value of the stator terminal voltage, in p.u.


The measured value in p.u. of the stator terminal voltage of the controlled Synchronous Machine block.


The measured value in p.u. of the stator field current of the controlled Synchronous Machine block.


Connect this input to a power system stabilizer to provide additional stabilization of power system oscillations. When you do not use this option, connect to a Simulink® ground block. The input is in p.u.


The field voltage to apply to the Vf input of the controlled Synchronous Machine block. The output is in p.u.


[1] “IEEE Recommended Practice for Excitation System Models for Power System Stability Studies.” IEEE Standard, Vol. 421, No. 5, 2005 (Revision of IEEE 521.5-1992).

Version History

Introduced in R2012b