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# AC1A Excitation System

Implements IEEE type AC1A excitation system model

sps_avr

## 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.

## Dialog Box and Parameters

### Controllers Tab

Low-pass filter time constant

The time constant Tr of the first-order system representing the stator terminal voltage transducer.

Voltage regulator gain and time constant

The gain Ka and time constant Ta of the first-order system representing the main regulator.

Voltage regulator internal limits

The voltage regulator internal limits VAmin and VAmax, in p.u.

Voltage regulator output limits

The voltage regulator output limits VRmin and VRmax, in p.u.

Damping filter gain and time constant

The gain Kf and time constant Tf of the first-order system representing the derivative feedback.

Transient gain reduction lead and lag time constants

The time constants Tb and Tc of the first-order system representing the lead-lag compensator.

### Exciter and Rectifier Tab

Exciter gain and time constant

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

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.

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.

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.

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

### 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.

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.

## 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.

## Example

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).