Description

Transformer models with tap changer are designed to be used when the voltage needs to be set. This is often the case with electronic power converters. The 2-winding saturable transformer adds modelling of saturation and hysteresis to the linear transformer at the expense of some computation time. This model also integrates a decoupling element helping to separate the network into two distinct tasks that can be computed on separate cores to optimize performance.

Table of Contents

Mask and Parameters

General Parameters

Description	Use this field to add all kinds of information about the component
Flux-Current model	Model saturation only or saturation with hysteresis
Iteration in saturation model	Enable or disable iteration to achieve more accurate results at the expense of computation time when the saturation segment changes
Base primary/secondary/tertiary winding voltage (rmsLL)	Base value for PU conversion (kV). Voltage expressed in kV rms LL This base voltage and nominal voltage will change, if the corresponding winding connection switches between delta and Y.
Base power (total)	Base value for PU conversion (MVA)
Base frequency	Base value for PU conversion (Hz)

Magnetization Impedance Parameters

Rm: Equivalent resistance of iron losses of the magnetic circuit (Ω)

Winding Parameters

Primary connection	Forced to Y ground
Secondary connection	Forced to Y floating
Tertiary connection	Forced to Delta; choice of: Delta lead: Delta connection with lead of 30° Delta lag: Delta connection with lag of 30°
Voltage (rmsLL)	Rated voltage of the winding (kV) Y connections: Voltage expressed in kV rms LL Delta connections: Voltage expressed in kV rms LL
R1, R2, R3	Leakage resistance of the winding (Ω)
L1, L2, L3	Leakage inductance of the winding (H)

Neutral Impedance Parameters

R1	Neutral resistance of the primary winding; only applies to Y ground (Ω)
L1	Neutral inductance of the primary winding; only applies to Y ground (H)
C1	Neutral capacitance of the primary winding; only applies to Y ground (F)

Saturation Parameters

The saturation is characteristic of the core, thus of the winding and not the type of 3-phase connection (Y, Delta or Zigzag). It is represented only for the magnetization branch (schematically using line segments).

Number of data points	Number of segments of the current-flux saturation curve; only the positive part of the curve must be specified, the negative part being completed by symmetry
Saturation current	Current for each segment of the saturation curve; the origin (0,0) is implied (A)
Saturation flux	Flux for each segment of the saturation curve; the origin (0.0,0.0) is implied (V.s)

Hysteresis Parameters

The main hysteresis cycle is characterized by four parameters. It is measured in DC so as not to include the Foucault losses, which are considered by the parallel resistance (Rm). The initial trajectory is characterized by only one parameter, the initial flux. Two other special parameters serve to minimize the generation of internal nested loops, and their corresponding trajectories, saved in memory (e.g. the loops that are too small will be ignored and their trajectories modelized by a straight line segment).

This is useful since their number must be limited (100 which suffices in most simulated cases). Above this cycle (limited to ±Is), the saturation zone is entered. The saturation is then characterized either by a series of points on the curve or by an inductance that the curve approaches asymptotically. In this last case, the model generates automatically segments (in the positive and negative saturation zone) of equal length (Is).

Saturation data type	Determines if the saturation curve is calculated by the model or defined by a series of segments (Equation, Curve)
Air core inductance	Value of the saturation inductance that the curve approaches asymptotically (H)
Slope at Ic	Flux slope at coercive current (H)
Coercitive current - Ic	Positive coercive current at null flux (A)
Saturation current - Is	Current value of the first point in the saturation zone (A)
Current tolerance	Special parameter limiting the generation of minor nested loops. When the magnetizing current values at the last inversion point and the preceding inversion point are closer than the specified tolerance (in % of Ic), it is assumed that there is a displacement on a trajectory represented by a straight line segment.
Remnant flux - Φr	Positive remnant flux at null current (V.s)
Saturation flux - Φs	Flux value of the first point in the saturation zone (V.s)
Flux tolerance	Special parameter limiting the generation of minor nested loops. When the flux values at the last inversion point and the preceding inversion point are closer than the specified tolerance (in % of Φs), it is assumed that there is a displacement on the current loop.
Initial flux (peak)	Initial flux determining initial trajectory which is calculated by supposing that it has an inversion point on the main cycle (V.s)
Number of points	Number of segments of the current-flux saturation curve; only the positive part of the curve must be specified, the negative part being completed by symmetry
Saturation current	Current for each segment of the saturation curve; the first value must be equal to Is (A)
Saturation flux	Flux for each segment of the saturation curve; the first value must be equal to Φs (V.s)

Tap Changer Parameters

The tap changer effect is simulated by changing the transformer ratio.

Control type	Source of the command Internal (combined with LCC): The parameter "Manual tap position" defines the set point for the tap. It is reached after the temporization and operation times. Internal (manual tap position): The parameter "Manual tap position" defines the set point for the tap. It is reached after the temporization and operation times. External (input sensors): The parameter "Manual tap position" defines the initial value of the tap and is not used thereafter. The temporization and operation times apply to the external control.
Manual position	Manual position required; either a set point or initial value depending on the control type
Number of taps	Number of tap(s); maximum 50
Nominal voltage (rms LG)	Primary voltage to generate the rated voltage on the secondary winding at the nominal tap (kV)
Minimum voltage (rms LG)	Primary voltage linked with position 1 to generate the rated voltage on the secondary winding (kV)
Maximum voltage (rms LG)	Primary voltage linked with the last position of the tap changer to generate the rated voltage on the secondary winding (kV)
Temporization time	Minimum duration required for a signal order to start changing tap (s)
Operation time	Time between tap changes (s)

Ports, Inputs, Outputs and Signals Available for Monitoring

Ports

Net_1	Primary winding connection (supports only 3-phase connections)
Net_2	Secondary winding connection (supports only 3-phase connections)
Net_3	Tertiary winding connection (supports only 3-phase connections)

Inputs

None

Outputs

None

Sensors

Down	"Down one position" order from internal command
DownBuf	Signal specifying the start of a change issued by a “Down” order
DownEXT	"Down one position" order from an external command
FLUX(a,b,c)	Magnetization flux for each phase (V.s)
IMAG(a,b,c)	Magnetization current for each phase (A)
IPRIM(a,b,c)	Primary current for each phase (A)
PosA.B%	Specifies by how much the tap is above or below the rated position
PosSign	Specifies that the tap is above or below the rated position
RatioXfo	Winding ratio
SEG(a,b,c)	Segment number of the saturation curve In the saturation model, the numbering is always positive starting at 1 for the last segment in the negative saturation zone. In the hysteresis model, the numbering is positive/negative starting at 1/-1 in the positive/negative saturation zone. In the hysteresis zone, it takes a null value.
Tap	Tap position
Up	"Up one position" order from internal command
UpBuf	Signal specifying the start of a change issued by an “Up” order
UpEXT	"Up one position" order from an external command

References

«Hysteresis Modeling in the Matlab/Power System Blockset», Silvano Casoria, Patrice Brunelle, Gilbert Sybille, Mathematics and Computers in Simulation, Volume 63, Issues 3-5, Pages 237-248

3-Winding 3-Phase Saturable Transformer with Tap Changer and Decoupling Element