Documentation

SM AC1C

Synchronous machine AC1C excitation system including an Automatic Voltage Regulator (AVR) and an exciter

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Description

The SM AC1C block implements a synchronous machine type AC1C excitation system model in conformance with IEEE 421.5-2016[1].

Use this block to model the control and regulation of the field voltage of a synchronous machine operating as a generator using an AC rotating exciter.

You can switch between continuous and discrete implementations of the block by using the Sample time parameter. To configure the integrator for continuous time, set the Sample time property to 0. To configure the integrator for discrete time, set the Sample time property to a positive, nonzero value, or to -1 to inherit the sample time from an upstream block.

The SM AC1C block is made up of four major components:

  • The Current Compensator modifies the measured terminal voltage as a function of terminal current.

  • The Voltage Measurement Transducer simulates the dynamics of a terminal voltage transducer using a low-pass filter.

  • The Excitation Control Elements component compares the voltage transducer output with a terminal voltage reference to produce a voltage error. This voltage error is then passed through a voltage regulator to produce the exciter field voltage.

  • The AC Rotating Exciter models the AC rotating exciter, producing a field voltage to be applied to the controlled synchronous machine. The block also feeds the exciter field current (given the standard symbol VFE) back to the excitation system.

This diagram shows the overall structure of the AC1C excitation system model:

In the diagram:

  • VT and IT are the measured terminal voltage and current of the synchronous machine.

  • VC1 is the current-compensated terminal voltage.

  • VC is the filtered, current-compensated terminal voltage.

  • VREF is the reference terminal voltage.

  • VS is the power system stabilizer voltage.

  • EFE and VFE are the exciter field voltage and current, respectively.

  • EFD and IFD are the field voltage and current, respectively.

The following sections describe each of the major parts of the block in detail.

Current Compensator and Voltage Measurement Transducer

The current compensator is modeled as:

VC1=VT+ITRC2+XC2,

where:

  • RC is the load compensation resistance.

  • XC is the load compensation reactance.

The voltage measurement transducer is implemented as a Low-Pass Filter block with time constant TR. Refer to the documentation for this block for the exact discrete and continuous implementations.

Excitation Control Elements

This diagram illustrates the overall structure of the excitation control elements:

In the diagram:

  • SP is the summation point input location for the overexcitation limiter (OEL), underexcitation limiter (UEL), and stator current limiter (SCL) voltages. For more information about using limiters with this block, see Field Current Limiters.

  • The Lead-Lag block models additional dynamics associated with the voltage regulator. Here, TC is the lead time constant and TB is the lag time constant. Refer to the documentation for this block for the exact discrete and continuous implementations.

  • The Low-Pass Filter block models the major dynamics of the voltage regulator. Here, KA is the regulator gain and TA is the major time constant of the regulator. The minimum and maximum anti-windup saturation limits for the block are VAmin and VAmax, respectively.

  • TO is the take-over point input location for the OEL, UEL, and SCL voltages. For more information about using limiters with this block, see Field Current Limiters.

  • The Filtered Derivative block models the rate feedback path for stabilization of the excitation system. Here, KF and TF are the gain and time constant of this system, respectively. Refer to the documentation for the Filtered Derivative block for the exact discrete and continuous implementations.

  • EFEmin and EFEmax are the minimum and maximum saturation limits for the output exciter field voltage EFE.

Field Current Limiters

You can use various field current limiters to modify the output of the voltage regulator under unsafe operating conditions:

  • Use an overexcitation limiter to prevent overheating of the field winding due to excessive field current demand.

  • Use an underexcitation limiter to boost field excitation when it is too low, risking desynchronization.

  • Use a stator current limiter to prevent overheating of the stator windings due to excessive current.

Attach the output of any of these limiters at one of these points:

  • The summation point as part of the AVR feedback loop

  • The take-over point to override the usual behaviour of the AVR

If you are using the stator current limiter at the summation point, use the single input VSCLsum. If you are using the stator current limiter at the take-over point, use both an overexcitation input VSCLoel and an underexcitation input VSCLuel.

AC Rotating Exciter

This diagram illustrates the overall structure of the AC rotating exciter:

In the diagram:

  • The exciter field current VFE is modeled as the summation of three signals:

    • The nonlinear function SE(VE) models the saturation of the exciter output voltage.

    • The proportional term KE models the linear relationship between exciter output voltage and the exciter field current.

    • The demagnetizing effect of the load current on the exciter output voltage is modelled using the demagnetization constant KD in the feedback loop.

  • The Integrator block integrates the difference between EFE and VFE to generate the exciter alternator output voltage VE. TE is the time constant for this process.

  • The nonlinear function FEX models the exciter output voltage drop from rectifier regulation. This function depends on the constant KC which itself is a function of commutating reactance.

Ports

Input

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Voltage regulator reference set point, in per-unit representation.

Data Types: single | double

Input from the power system stabilizer, in per-unit representation.

Data Types: single | double

Terminal voltage magnitude in per-unit representation.

Data Types: single | double

Terminal current magnitude in per-unit representation.

Data Types: single | double

Input from the overexcitation limiter, in per-unit representation.

Dependencies

  • To ignore the input from the overexcitation limiter, set Alternate OEL input locations to Unused.

  • To use the input from the overexcitation limiter at the summation point, set Alternate OEL input locations to Summation point.

  • To use the input from the overexcitation limiter at the take-over point, set Alternate OEL input locations to Take-over.

Data Types: single | double

Input from the underexcitation limiter, in per-unit representation.

Dependencies

  • To ignore the input from the underexcitation limiter, set Alternate UEL input locations to Unused.

  • To use the input from the underexcitation limiter at the summation point, set Alternate UEL input locations to Summation point.

  • To use the input from the underexcitation limiter at the take-over point, set Alternate UEL input locations to Take-over.

Data Types: single | double

Input from the stator current limiter when using the summation point, in per-unit representation.

Dependencies

  • To ignore the input from the stator current limiter, set Alternate SCL input locations to Unused.

  • To use the input from the stator current limiter at the summation point, set Alternate SCL input locations to Summation point.

Data Types: single | double

Input from the stator current limiter to prevent field overexcitation when using the take-over point, in per-unit representation.

Dependencies

  • To ignore the input from the stator current limiter, set Alternate SCL input locations to Unused.

  • To use the input from the stator current limiter at the take-over point, set Alternate SCL input locations to Take-over.

Data Types: single | double

Input from the stator current limiter to prevent field underexcitation when using the take-over point, in per-unit representation.

Dependencies

  • To ignore the input from the stator current limiter, set Alternate SCL input locations to Unused.

  • To use the input from the stator current limiter at the take-over point, set Alternate SCL input locations to Take-over.

Data Types: single | double

Measured per-unit field current of the synchronous machine.

Data Types: single | double

Output

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Per-unit field voltage to be applied to the field circuit of the synchronous machine.

Data Types: single | double

Parameters

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General

Initial per unit voltage to be applied to the field circuit of the synchronous machine.

Block sample time. Set this to 0 to implement a continuous AC1C system. Set this to -1 or a positive number to implement a discrete system.

Pre-control

Resistance used in the current compensation system. Set this and XC to 0 to disable current compensation.

Reactance used in the current compensation system. Set this and RC to 0 to disable current compensation.

Equivalent time constant for the voltage transducer filtering.

Control

Gain associated with the voltage regulator.

Major time constant of the voltage regulator.

Equivalent lag time constant in the voltage regulator. Set this to 0 when the additional lag dynamics are negligible.

Equivalent lead time constant in the voltage regulator. Set this to 0 when the additional lead dynamics are negligible.

Rate feedback block gain for stabilization of excitation system.

Rate feedback block time constant for stabilization of excitation system.

Maximum per-unit output voltage of the regulator.

Minimum per-unit output voltage of the regulator.

Maximum per unit field voltage to be applied to exciter.

Minimum per unit field voltage to be applied to exciter.

Select overexcitation limiter input location.

Select underexcitation limiter input location.

Select stator current limiter input location. To specify the SCL input:

  • If you select Summation point, use the V_SCLsum inport port.

  • If you select Take-over, use the V_SCLoel and V_SCLuel inport ports.

Exciter

Proportional constant for exciter field.

Time constant for exciter field.

Rectifier loading factor proportional to commutating reactance.

Demagnetization factor related to exciter alternator reactances.

Exciter output voltage for first saturation factor.

First exciter saturation factor.

Exciter output voltage for second saturation factor.

Second exciter saturation factor.

References

[1] IEEE Recommended Practice for Excitation System Models for Power System Stability Studies. IEEE Std 421.5-2016. Piscataway, NJ: IEEE-SA, 2016.

Introduced in R2017b

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