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AC4A Excitation System

Implements IEEE type AC4A excitation system model


Fundamental Blocks/Machines/Excitation Systems (sps_avr)


This block models an ac alternator connected to a thyristor rectifier to produce the field voltage Vf required by the Synchronous Machine block. A voltage regulator controls the thyristors and the excitation system stabilization is accomplished by a series lead-lag compensator.

This block is an adaptation of the AC4A 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 [200 0.015].

Voltage regulator input limits

The voltage regulator input limits VImin and VImax, in p.u. Default is [-10 10].

Voltage regulator output limits

The voltage regulator output limits VRmin and VRmax, in p.u. Default is [-4.53 5.64].

Transient gain reduction lead and lag time constants

The time constants Tb and Tc of the first-order system representing the lead-lag compensator. Default is [10 1.0].

Exciter and Rectifier Tab

Rectifier loading factor Kc

The gain Kc represents the rectifier loading factor proportional to the commutating reactance. Default is 0.

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.


The power_machines example contains a Configurable Subsystem block that allows you to select between seven types of excitation systems to control the terminal voltage of the Synchronous Machine block. This configurable block refers to the power_machines_lib example library that contains seven pretuned excitation system blocks that fit simulation requirements for this example.

Right-click the EXCITATION configurable block, then select AC4A from the Block Choice menu to control the Synchronous Machine block using the AC4A Excitation System block.


[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).

Introduced in R2012b

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