SM AC11C

Discrete-time or continuous-time synchronous machine AC11C excitation system including an automatic voltage regulator and an exciter

Since R2023a

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Libraries:
Simscape / Electrical / Control / SM Control

Description

The SM AC11C block implements a synchronous machine type AC11C excitation system model in conformance with IEEE Std 421.5-2016 [1].

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

Switch between continuous and discrete implementations of the block by using the Sample time (-1 for inherited) parameter. To configure the integrator for continuous time, set the Sample time (-1 for inherited) parameter to 0. To configure the integrator for discrete time, set the Sample time (-1 for inherited) parameter to a positive scalar. To inherit the sample time from an upstream block, set the Sample time (-1 for inherited) parameter to -1.

The SM AC11C block comprises five major components:

The Current Compensator component modifies the measured terminal voltage as a function of the terminal current.
The Voltage Measurement Transducer component 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 value. The component then passes this value through a voltage regulator to produce the exciter field voltage.
The AC Rotating Exciter component models the AC rotating exciter, which produces a field voltage that is applied to the controlled synchronous machine. The block also feeds the exciter field current (V_FE) back to the excitation system.
The Power Source component models the dependency of the power source for the controlled rectifier from the terminal voltage. The Power Source comprises multiple compound circuits. You can select this additive circuit as a boost circuit, that is applied when the faulted conditions in the system cause a drop in the terminal voltage of the generator. The SM AC11C block then releases the circuit when the generator voltage recovers.

This diagram shows the structure of the AC11C excitation system model:

In the diagram:

V_T and I_T are the measured terminal voltage and current of the synchronous machine, respectively.
V_C1 is the current-compensated terminal voltage.
V_C is the filtered, current-compensated terminal voltage.
V_REF is the reference terminal voltage.
V_S is the power system stabilizer voltage.
SW₁ is the power source switch that you specify for the controlled rectifier.
V_B is the exciter field voltage.
E_FE and V_FE are the exciter field voltage and current, respectively.
E_FD and I_FD are the field voltage and current, respectively.

Current Compensator and Voltage Measurement Transducer

The block models the current compensator by using this equation:

$V_{C 1} = V_{T} + I_{T} \sqrt{R_{C}^{2} + X_{C}^{2}},$

where:

R_C is the load compensation resistance.
X_C is the load compensation reactance.

The block implements the voltage measurement transducer as a Low-Pass Filter block with the time constant T_R. Refer to the documentation for the Low-Pass Filter block for information about the exact discrete and continuous implementations.

Excitation Control Elements

This diagram shows the structure of the excitation control elements:

In the diagram:

The Summation Point Logic subsystem models the summation point input location for the overexcitation limiter (OEL), underexcitation limiter (UEL), and stator current limiter (SCL). For more information about using limiters with this block, see Field Current Limiters.
There are two Take-over Logic subsystems. They model 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 SM AC11C block models the controller of the automatic voltage regulator by using three different PID controls. The activation of a specific limiter (UEL or OEL) immediately changes the transfer function of the automatic voltage regulator to the controller path which corresponds to the activated limiter. The SW_UEL and SW_OEL Switch blocks activate the appropriate control path when the V_UEL and/or V_OEL signals are connected to their respective alternate positions. The SW_UEL and SW_OEL Switch blocks are on position B when you set the Alternate UEL input locations (V_UEL) and Alternate OEL input locations (V_OEL) parameters to Take-over at voltage error.
The Power source selector SW1 parameter controls the origin of the power source for the controlled rectifier. The power source comprises a boost circuit that you can apply when the faulted conditions in the system cause a drop in the terminal voltage of the generator. The voltage regulator command signal V_R is then summed to and/or multiplied by the exciter field voltages, V_B1 and V_B2. For more information about the logical switch for the power source of the controlled rectifier, see Power Source and Boost Circuit.

Field Current Limiters

You can use different types of field current limiter 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, which risks 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:

Summation point — Use the limiter as part of the automatic voltage regulator (AVR) feedback loop.
Take-over points — Override the usual behavior of the AVR.

If you are using the stator current limiter at the summation point, use the input V_SCLsum. If you are using the stator current limiter at the take-over point, use the overexcitation input V_OELscl, and the underexcitation input V_UELscl.

AC Rotating Exciter

This diagram shows the structure of the AC rotating exciter:

In the diagram:

The exciter field current V_FE is the sum of three signals:
- The nonlinear function V_x models the saturation of the exciter output voltage.
- The proportional term K_E models the linear relationship between the exciter output voltage and the exciter field current.
- The subsystem models the demagnetizing effect of the load current on the exciter output voltage using the demagnetization constant K_D in the feedback loop.
The Integrator with variable limits subsystem integrates the difference between E_FE and V_FE to generate the exciter alternator output voltage V_E. T_E is the time constant for this process.
The nonlinear function F_EX models the exciter output voltage drop from the rectifier regulation. This function depends on the constant K_C, which itself is a function of commutating reactance.
The parameters V_Emin and V_FEmax model the lower and upper limits of the rotating exciter.

Power Source and Boost Circuit

You can use different power source representations for the controlled rectifier by setting the Power source selector SW1 parameter value. To derive the power source for the controlled rectifier from the terminal voltage, set the Power source selector SW1 parameter to Position A: power source derived from terminal voltage. To specify that the power source is independent of the terminal voltage, set the Power source selector SW1 parameter to Position B: power source independent from the terminal conditions.

The power source section also contains an additive circuit that you can use as a boost circuit. This additive circuit is applied when the faulted conditions in the system cause a drop in the terminal voltage of the generator. The SW_Boost Switch block controls the logic associated with the boost circuit. If the terminal voltage, V_T, is greater than the value of the Reference value for applying additive (boost) circuit, V_BOOST (pu) parameter, the switch is on position “A,” otherwise the switch is on position “B.”

If you set the Reference value for applying additive (boost) circuit, V_BOOST (pu) parameter to a high value, the additive component is always active.

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