SM ST3C
Discrete-time or continuous-time synchronous machine ST3C static excitation system
with an automatic voltage regulator
Description
The SM ST3C block implements a synchronous-machine-type ST3C static
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.
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 ST3C block comprises four 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
field voltage.
The Power Source models the power source for the controlled rectifier when
it is independent from the terminal voltage.
This diagram shows the overall structure of the ST3C 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.
VB is the exciter field
voltage.
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:
where:
The voltage measurement transducer is implemented as a Low-Pass
Filter block with the time constant
TR. Refer to the documentation for the
Low-Pass Filter 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.
The Take-over Logic subsystem models 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 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 VPImin and
VPImax,
respectively.
The Lead-Lag block models additional dynamics associated with
the voltage regulator and represents the system stabilizer. 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.
An inner field voltage control loop is utilized to linearize the
exciter control characteristic and it is composed of the gains
KM and
KG and the time
constant TM. The minimum and
maximum anti-windup saturation limits for the block are
VMmin and
VMmax,
respectively.
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:
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 the overexcitation
input, VSCLoel, and the underexcitation
input, VSCLuel.
Power Source
It is possible to adopt a different representation of the power source for the controlled
rectifier by selecting the relevant option in the Power source
selector parameter. The power source for the controlled rectifier can
be either derived from the terminal voltage (Position A: power source
derived from generator terminal voltage) or it can be independent
of the terminal voltage (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, VT,
and terminal current, IT:
Ports
Input
expand all
V_REF — Voltage reference
scalar
Voltage regulator reference set point, in per-unit representation, specified as a
scalar.
Data Types: single | double
V_S — Input from stabilizer
scalar
Input from the power system stabilizer, in per-unit representation, specified as a
scalar.
Data Types: single | double
V_T — Terminal voltage
scalar
Terminal voltage magnitude in per-unit representation, specified as a scalar.
Data Types: single | double
I_T — Terminal current
scalar
Terminal current magnitude in per-unit representation, specified as a scalar.
Data Types: single | double
V_OEL — Overexcitation limit signal
scalar
Input from the overexcitation limiter, in per-unit representation, specified as a
scalar.
Dependencies
To ignore the input from the overexcitation limiter,
set Alternate OEL input locations
(V_OEL) to
Unused.
To use the input from the overexcitation limiter at
the summation point, set Alternate OEL input
locations (V_OEL) to
Summation point.
To use the input from the overexcitation limiter at
the take-over point, set Alternate OEL input
locations (V_OEL) to
Take-over.
Data Types: single | double
V_UEL — Underexcitation limit signal
scalar
Input from the underexcitation limiter, in per-unit representation, specified as a
scalar.
Dependencies
To ignore the input from the underexcitation limiter,
set Alternate UEL input locations
(V_UEL) to
Unused.
To use the input from the underexcitation limiter at
the summation point, set Alternate UEL input
locations (V_UEL) to
Summation point.
To use the input from the underexcitation limiter at
the take-over point, set Alternate UEL input
locations (V_UEL) to
Take-over.
Data Types: single | double
V_SCLsum — Summation point stator current limit signal
scalar
Input from the stator current limiter when using the summation point, in per-unit
representation, specified as a scalar.
Dependencies
To ignore the input from the stator current limiter,
set Alternate SCL input locations
(V_SCL) to
Unused.
To use the input from the stator current limiter at
the summation point, set Alternate SCL input
locations (V_SCL) to
Summation point.
Data Types: single | double
V_SCLoel — Take-over stator current limit (OEL)
scalar
Input from the stator current limiter that prevents field overexcitation when using the
take-over point, in per-unit representation, specified as a
scalar.
Dependencies
To ignore the input from the stator current limiter,
set Alternate SCL input locations
(V_SCL) to
Unused.
To use the input from the stator current limiter at
the take-over point, set Alternate SCL input
locations (V_SCL) to
Take-over.
Data Types: single | double
V_SCLuel — Take-over stator current limit (UEL)
scalar
Input from the stator current limiter that prevents field underexcitation when using the
take-over point, in per-unit representation, specified as a
scalar.
Dependencies
To ignore the input from the stator current limiter,
set Alternate SCL input locations
(V_SCL) to
Unused.
To use the input from the stator current limiter at
the take-over point, set Alternate SCL input
locations (V_SCL) to
Take-over.
Data Types: single | double
Ifd_pu — Measured field current
scalar
Measured per-unit field current of the synchronous machine, specified as a
scalar.
Data Types: single | double
Output
expand all
Efd_pu — Field voltage
scalar
Per-unit field voltage to to apply to the field circuit of the
synchronous machine, returned as a scalar.
Data Types: single | double
Parameters
expand all
General
Initial field voltage, Efd0 (pu) — Initial output voltage
1 (default) | real number
Initial per-unit voltage to apply to the field circuit of the
synchronous machine.
Initial terminal voltage, Vt0 (pu) — Initial terminal voltage
1 (default) | real number
Initial per-unit terminal voltage.
Initial terminal current, It0 (pu) — Initial terminal current
1 (default) | real number
Initial per-unit terminal current.
Sample time (-1 for inherited) — Block sample time
-1 (default) | 0 | positive scalar
Time between consecutive block executions. During
execution, the block produces outputs and, if appropriate, updates its internal state.
For more information, see What Is Sample Time? and Specify Sample Time.
For inherited discrete-time operation, specify -1. For
discrete-time operation, specify a positive integer. For continuous-time operation,
specify 0.
If this block is in a masked subsystem, or other variant subsystem that allows you to
switch between continuous operation and discrete operation, promote the sample time
parameter. Promoting the sample time parameter ensures correct switching between the
continuous and discrete implementations of the block. For more information, see Promote Parameter to Mask.
Pre-Control
Resistive component of load compensation, R_C (pu) — Compensation resistance
0 (default) | positive number
Resistance used in the current compensation system. Set this parameter
and Reactance component of load compensation, X_C
(pu) to 0 to disable current
compensation.
Reactance component of load compensation, X_C (pu) — Compensation reactance
0 (default) | positive number
Reactance used in the current compensation system. Set this parameter
and Resistive component of load compensation, R_C
(pu) to 0 to disable current
compensation.
Regulator input filter time constant, T_R (s) — Regulator time constant
0 (default) | positive number
Equivalent time constant for the voltage transducer filtering.
Control
Voltage regulator gain, K_A (pu) — Regulator gain
200 (default) | positive number
Gain associated with the voltage regulator.
Regulator denominator (lag) time constant, T_B (s) — Regulator lag time constant for first lead-lag block
10 (default) | positive number
Equivalent lag time constant in the voltage regulator. Set this
parameter to 0 when the additional lag dynamics are
negligible.
Regulator numerator (lead) time constant, T_C (s) — Regulator lead time constant for first lead-lag block
1 (default) | positive number
Equivalent lead time constant in the voltage regulator. Set this parameter to
0 when the additional lead dynamics are
negligible.
Voltage regulator proportional gain, K_PR (pu) — Regulator proportional gain
1 (default) | positive number
Proportional gain associated with the voltage regulator PI control
block.
Voltage regulator integral gain, K_IR (pu/s) — Regulator integral gain
0 (default) | positive number
Integral gain associated with the voltage regulator PI control
block.
Maximum voltage regulator output, V_PImax (pu) — Upper limit of regulator output for PI control
99 (default) | real number
Maximum per-unit output voltage of the regulator PI control
block.
Minimum voltage regulator output, V_PImin (pu) — Lower limit of regulator output for PI control
-99 (default) | real number
Minimum per-unit output voltage of the regulator PI control
block.
Maximum regulator output, V_Rmax (pu) — Upper limit of regulator output
10 (default) | real number
Maximum per-unit output voltage of the regulator.
Minimum regulator output, V_Rmin (pu) — Lower limit of regulator output
-10 (default) | real number
Minimum per-unit output voltage of the regulator.
Maximum voltage error (regulator input), V_Imax (pu) — Voltage upper limit
0.2 (default) | real number
Maximum per-unit voltage given as input to the regulator.
Minimum voltage error (regulator input), V_Imax (pu) — Voltage lower limit
-0.2 (default) | real number
Minimum per-unit voltage given as input to the regulator.
Forward gain of inner loop field regulator, K_M (pu) — Inner loop forward gain
7.93 (default) | positive number
Per-unit forward gain in the inner loop field regulator.
Forward time constant of inner loop field regulator, T_M (s) — Inner loop forward time constant
0.4 (default) | positive number
Forward time constant in the inner loop field regulator, in
seconds.
Maximum output of field current regulator, V_Mmax (pu) — Upper limit of field current regulator output
1 (default) | real number
Maximum per-unit output voltage of the field current regulator.
Minimum regulator output, V_Mmin (pu) — Lower limit of field current regulator output
0 (default) | real number
Minimum per-unit output voltage of the field current regulator.
Feedback gain of field current regulator, K_G (pu) — Feedback gain of field current regulator
1 (default) | positive number
Per-unit feedback gain in the field current regulator.
Maximum feedback voltage for field current regulator, V_Gmax (pu) — Maximum feedback voltage for field current regulator
5.8 (default) | positive number
Maximum per-unit feedback voltage for the field current regulator.
Alternate OEL input locations (V_OEL) — OEL input location
Unused (default) | Summation point | Take-over
Overexcitation limiter input location.
Alternate UEL input locations (V_UEL) — UEL input location
Unused (default) | Summation point | Take-over
Underexcitation limiter input location.
Alternate SCL input locations (V_SCL) — SCL input location
Unused (default) | Summation point | Take-over
Stator current limiter input location:
If you select Summation point,
use the V_SCLsum input port.
If you select Take-over, use
the V_SCLoel and
V_SCLuel input ports.
Exciter
Rectifier loading factor proportional to commutating reactance, K_C
(pu) — Rectifier loading factor
0.2 (default) | positive number
Rectifier loading factor proportional to the commutating reactance.
Potential circuit (voltage) gain coefficient, K_P (pu) — Potential circuit voltage gain coefficient
6.15 (default) | positive number
Per-unit voltage gain coefficient in the power source circuit.
Dependencies
To enable this parameter, set Power source selector to
Position A: power source derived from generator
terminal voltage.
Compound circuit (current) gain coefficient, K_I (pu) — Potential circuit current gain coefficient
0 (default) | positive number
Per-unit current gain coefficient in the power source circuit.
Dependencies
To enable this parameter, set Power source selector to
Position A: power source derived from generator
terminal voltage.
Reactance associated with potential source, X_L (pu) — Potential source reactance
0.081 (default) | positive number
Per-unit reactance in the power source circuit.
Dependencies
To enable this parameter, set Power source selector to
Position A: power source derived from generator
terminal voltage.
Potential circuit phase angle (degrees) — Potential circuit phase angle
0 (default) | positive number
Phase angle of the power source circuit, in degrees.
Dependencies
To enable this parameter, set Power source selector to
Position A: power source derived from generator
terminal voltage.
Maximum available exciter voltage, V_Bmax (pu) — Upper limit of exciter voltage
6.9 (default) | positive number
Maximum per-unit available exciter voltage value.
Power source selector — Power source selector
Position A: power source derived from
generator terminal voltage (default) | Position B: power source independent of generator
terminal conditions
Position of the power source selector, specified as Position A: power
source derived from generator terminal voltage or
Position B: power source independent of generator
terminal conditions.
References
[1] IEEE Recommended
Practice for Excitation System Models for Power System Stability
Studies. IEEE Std 421.5-2016. Piscataway, NJ: IEEE-SA,
2016.
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