SM ST6C

Discrete-time or continuous-time synchronous machine ST6C static excitation system with automatic voltage regulator

Since R2023a

Libraries:
Simscape / Electrical / Control / SM Control

Description

The SM ST6C block implements a synchronous-machine-type ST6C static 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.

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 ST6C block comprises four 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 field voltage.
The Power Source component models the power source for the controlled rectifier when it is independent from the terminal voltage.

This diagram shows the structure of the ST6C 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.
V_B is the exciter field voltage.
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:

There are two Summation Point Logic subsystems. The subsystems model the summation point input locations for the overexcitation limiter (OEL), underexcitation limiter (UEL), stator current limiter (SCL), and power switch selector (V_S) voltages. For more information about using limiters with this block, see Field Current Limiters.
There are two Take-over Logic subsystems. The subsystems model the take-over point input location for the OEL, UEL, SCL and PSS voltages. For more information about using limiters with this block, see Field Current Limiters.
The PI subsystem models a PI controller that functions as a control structure for the automatic voltage regulator and allows the representation of an equipment retrofit with a modern digital controller. The minimum and maximum anti windup saturation limits for the block are V_Amin and V_Amax, respectively.
An inner field voltage control loop is utilized to increase the dynamic response and it is composed of the gains K_M and K_FF. The minimum and maximum anti windup saturation limits for the block are V_Mmin and V_Mmax, respectively. If the field voltage regulator is not implemented, the corresponding parameters K_G and K_FF are set to 0.
The Power source selector parameter controls the origin of the power source for the controlled rectifier. The subsystem then multiples the output of the rectifier bridge V_M by the exciter field voltage V_B. For more information about the logical switch for the power source of the controlled rectifier, see Power Source.

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_SCLoel, and the underexcitation input V_SCLuel.

Power Source

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

This diagram shows a model of the exciter power source utilizing a phasor combination of the terminal voltage V_T and terminal current I_T:

Ports