SM AC8C

Discrete-time or continuous-time synchronous machine AC8C excitation system including an automatic voltage regulator and an exciter

Since R2020a

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Libraries:
Simscape / Electrical / Control / SM Control

Description

The SM AC8C block implements a synchronous machine type AC8C 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 that operates as a generator using an AC rotating exciter.

You can 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) property to 0. To configure the integrator for discrete time, set the Sample time (-1 for inherited) property to a positive, nonzero value, or to -1 to inherit the sample time from an upstream block.

The SM AC8C block is made up of five major components:

The Current Compensator modifies the measured terminal voltage as a function of the 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, which produces a field voltage that is applied to the controlled synchronous machine. The block also feeds the exciter field current (which is given the standard symbol V_FE) back to the excitation system.
The Power Source models the dependency of the power source for the controlled rectifier from the terminal voltage.

This diagram shows the overall structure of the AC8C excitation system model:

In the diagram:

V_T and I_T are the measured terminal voltage and current of the synchronous machine.
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 user-selected power source switch 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.

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:

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

where:

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

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

Excitation Control Elements

This diagram illustrates the overall 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), stator current limiter (SCL), and the 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. They 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 PID_R subsystem models a PID controller that functions as a control structure for the automatic voltage regulator. The minimum and maximum anti-windup saturation limits for the block are V_PIDmin and V_PIDmax, respectively.
The Low-Pass Filter block models the major dynamics of the voltage regulator. Here, K_A is the regulator gain and T_A is the major time constant of the regulator. The minimum and maximum anti-windup saturation limits for the block are V_Rmin and V_Rmax, respectively.
The Logical switch 1 parameter controls the origin of the power source for the controlled rectifier. The voltage regulator command signal V_R is multiplied by the exciter field voltage, V_B. For more information about the user-selected logical switch for the power source of the controlled rectifier, see Power Source.

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, 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:

The summation point as part of the automatic voltage regulator (AVR) feedback loop
The take-over point to override the usual behavior of the AVR

If you are using the stator current limiter at the summation point, use the single input V_SCLsum. If you are using the stator current limiter at the take-over point, use both 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

It is possible to use different power source representations for the controlled rectifier by selecting the relevant option in the Logical switch 1 parameter. The power source for the controlled rectifier can be either derived from the terminal voltage (Position A: power source derived from terminal voltage) or it can be independent of the terminal voltage (Position B: power source independent from the terminal conditions).

Ports