# AC1A Excitation System

Implements IEEE type AC1A excitation system model

## Library

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

## Description

This block models an ac alternator driving a diode rectifier to produce the field voltage Vf required by the Synchronous Machine block. A non-controlled voltage regulator provides a voltage in p.u. with a lower limit of zero imposed by the diode rectifier.

This block is an adaptation of the AC1A excitation system of
the IEEE^{®} 421 standard, copyright IEEE 2005, all rights reserved.

## Parameters

### 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

`[400 0.02]`

.**Voltage regulator internal limits**The voltage regulator internal limits VAmin and VAmax, in p.u. Default is

`[-14.5 14.5]`

.**Voltage regulator output limits**The voltage regulator output limits VRmin and VRmax, in p.u. Default is

`[-5.43 6.03]`

.**Damping filter gain and time constant**The gain Kf and time constant Tf of the first-order system representing the derivative feedback. Default is

`[0.03 1.0]`

.**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

`[0 0]`

.

### Exciter and Rectifier Tab

**Exciter gain and time constant**The gain Ke and time constant Te of the first-order system representing the exciter. Default is

`[1.0 0.80]`

.**Exciter alternator voltage values**The exciter saturation function is defined as a multiplier of exciter alternator output voltage to represent the increase in exciter excitation requirements due to saturation [1]. The saturation function is determined by specifying two voltage points,Ve1 and Ve2 in p.u., on the air-gap line and no-load saturation curve and providing the corresponding two saturation multipliers SeVe1 and SeVe2. Default is

`[4.18 3.14]`

.Typically, the voltage Ve1 is a value near the expected exciter maximum output voltage, Ve2 value is about 75% of Ve1 [1].

**Exciter saturation function values**The exciter saturation function is defined as a multiplier of exciter alternator output voltage to represent the increase in exciter excitation requirements due to saturation [1]. The saturation function is determined by specifying two voltage points, Ve1 and Ve2 in p.u., on the air-gap line and no-load saturation curve and providing the corresponding two saturation multipliers SeVe1 and SeVe2. Default is

`[0.10 0.03]`

.SeVe1 and SeVe2 multipliers are equal to C-B / B, C is the value of exciter field current on the no-load saturation curve corresponding to the specified Ve voltage, and B is the value of exciter field current on the air-gap line corresponding to the selected Ve voltage [1].

If you do not want to model the saturation effect, set SeVe1 and SeVe2 values to zero.

**Demagnetizing factor Kd**The gain Kd represents the demagnetizing factor, a function of exciter alternator reactances. Default is

`0.38`

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

`0.20`

.

### 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`

.

## Ports

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

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

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

- Vstab
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.- Efd
The field voltage to apply to the

`Vf`

input of the controlled Synchronous Machine block. The output is in p.u.

## Examples

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 **AC1A** from
the **Block Choice** menu to control the Synchronous
Machine block using the AC1A Excitation System block.

## References

[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**