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# Multi-Winding Transformer

Implement multi-winding transformer with taps

Elements

## Description

The Multi-Winding Transformer block implements a transformer where the number of windings can be specified for both the primary (left side windings) and the secondary (right side windings).

The equivalent circuit of the Multi-Winding Transformer block is similar to the one of the Linear Transformer block and the saturation characteristic of the core can be specified or not. See the Saturable Transformer block reference pages for more details on how the saturation and the hysteresis characteristic are implemented.

The equivalent circuit of a Multi-Windings Transformer block with two primary windings and three secondary windings is shown in the next figure.

You can add equally spaced taps to the first primary winding (the upper-left winding) or to the first secondary winding (the upper-right winding). The equivalent circuit of a Multi-Winding Transformer block with one primary winding and eight taps on the first of the two secondary windings is shown in the next figure.

The winding terminals are identified by the corresponding winding number. The first winding is the first one on the primary side (upper-left side) and the last winding is the last one on the secondary side (bottom-right side). The polarity of the windings are defined by a plus sign.

The tap terminals are identified by their winding number followed by a dot character and the tap number. Taps are equally spaced so that voltage appearing at no load between two consecutive taps is equal to the total voltage of the winding divided by (number of taps +1). The total winding resistance and leakage inductance of a tapped winding is equally distributed along the taps.

## Dialog Box and Parameters

### Configuration Tab

Number of windings on left side

Specifies the number of windings on the primary side (left side) of the transformer.

Number of windings on right side

Specifies the number of windings on the secondary side (right side) of the transformer.

Tapped winding

Select no taps if you don't want to add taps to the transformer. Select taps on upper left winding to add taps to the first winding on the primary side of the transformer. Select taps on upper right winding to add taps to the secondary winding on the right side of the transformer. The number of taps is specified by the Number of taps (equally spaced) parameter.

Number of taps (equally spaced)

This parameter is not visible if the Tapped winding parameter is set to no taps.

If the Tapped winding parameter is set to taps on upper left winding, you specify the number of taps to add to the first winding on the left side.

If the Tapped winding parameter is set to taps on upper right windings, you specify the number of taps to add to the first winding on the right side.

Saturable core

If selected, implements a saturable transformer. See also the Saturation characteristic parameter on the Parameters tab.

Simulate hysteresis

Select to model hysteresis saturation characteristic instead of a single-valued saturation curve. This parameter is visible only if the Saturable core parameter is selected.

Hysteresis Mat file

The Hysteresis Mat file parameter is visible only if the Simulate hysteresis parameter is selected.

Specify a .mat file containing the data to be used for the hysteresis model. When you open the Hysteresis Design Tool of the Powergui, the default hysteresis loop and parameters saved in the hysteresis.mat file are displayed. Use the Load button of the Hysteresis Design tool to load another .mat file. Use the Save button of the Hysteresis Design tool to save your model in a new .mat file.

Measurements

Select Winding voltages to measure the voltage across the winding terminals of the Saturable Transformer block.

Select Winding currents to measure the current flowing through the windings of the Saturable Transformer block.

Select Flux and excitation current (Im + IRm) to measure the flux linkage, in volt seconds (V.s), and the total excitation current including iron losses modeled by Rm.

Select Flux and magnetization current (Im) to measure the flux linkage, in volt seconds (V.s), and the magnetization current, in amperes (A), not including iron losses modeled by Rm.

Select All measurement (V, I, Flux) to measure the winding voltages, currents, magnetization currents, and the flux linkage.

Place a Multimeter block in your model to display the selected measurements during the simulation.

In the Available Measurements list box of the Multimeter block, the measurements are identified by a label followed by the block name.

Measurement

Label

Winding voltages

U_LeftWinding_1:,
U_TapWinding_2.1:,
U_RightWinding_1:, etc.

Winding currents

I_LeftWinding_1:,
I_TapWinding_2.1:,
I_RightWinding_1:, etc.

Excitation current

Iexc:

Magnetization current

Imag:

Flux:

### Parameters Tab

Units

Specify the units used to enter the parameters of the Multi-Winding Transformer block. Select pu to use per unit. Select SI to use SI units. Changing the Units parameter from pu to SI, or from SI to pu, will automatically convert the parameters displayed in the mask of the block. The per unit conversion is based on the transformer rated power Pn in VA, nominal frequency fn in Hz, and nominal voltage Vn, in Vrms, of the windings.

Nominal power and frequency

The nominal power rating, in volt-amperes (VA), and nominal frequency, in hertz (Hz), of the transformer. Note that the nominal parameters have no impact on the transformer model when the Units parameter is set to SI.

Winding nominal voltages

Specify a vector containing the nominal RMS voltages, in Vrms, of the windings on the left side, followed by the nominal RMS voltages of the windings on the right side. You don't have to specify the individual tap nominal voltages.

Winding resistances

Specify a vector containing the resistance values of the windings on the left side, followed by the resistance values of the windings on the right side. You don't have to specify the individual tap resistances.

Winding leakage inductances

Specify a vector containing the leakage inductance values of the windings on the left side, followed by the leakage inductance values of windings on the right side. You don't have to specify the individual tap leakage inductances.

Magnetization resistance Rm

The magnetization resistance Rm, in ohms or in pu.

Magnetization inductance Lm

The Magnetization inductance Lm parameter is not accessible if the Saturable core parameter on the Configuration tab is selected.

The magnetization inductance Lm, in Henry or in pu, for a nonsaturable core.

Saturation characteristic

This parameter is accessible only if the Saturable core parameter on the Configuration tab is selected.

The saturation characteristic for the saturable core. Specify a series of current/ flux pairs (in pu) starting with the pair (0,0).

Break Algebraic loop in discrete saturation model

When you use the block in a discrete system, you will get an algebraic loop. This algebraic loop, which is required in most cases to get an accurate solution, tends to slow down the simulation. However, to speed up the simulation, in some circumstances, you can disable the algebraic loop by selecting Break Algebraic loop in discrete saturation model. You should be aware that disabling the algebraic loop introduces a one-simulation-step time delay in the model. This can cause numerical oscillations if the sample time is too large.

## Example

The power_OLTCregtransformerpower_OLTCregtransformer example uses three Multi-Winding Transformer blocks to implement a three-phase On Load Tap Changer (OLTC) transformer. Because of long simulation times required for illustrating operation of the tap changer (simulation time is set to 120 s), the example uses the Phasor simulation method.

Phasor Simulation of On Load Tap Changer (OLTC) Regulating Transformers

A 25 kV distribution network consisting of three 30-km distribution feeders connected in parallel supplies power to a 36 MW /10 Mvar load (0.964 PF lagging) from a 120 kV, 1000 MVA system and a 120kV/25 kV OLTC regulating transformer. The OLTC transformer is used to regulate system voltage at the 25 kV bus.

The same circuit is duplicated to compare the performance of two different models of OLTC transformers:

• Model 1 is a detailed model where all OLTC switches and transformer windings are represented. This model uses three Multi-Winding Transformer blocks to implement a three-phase regulating transformer, with the OLTC connected on the high voltage side (120 kV). This model can be used with either continuous or discrete solvers to get detailed waveshapes or with the phasor simulation method (as in the present example) to observe variations of phasor voltages and currents.

• Model 2 is a simplified phasor model where the transformer and OLTC are simulated by current sources. This model can be used only with the phasor solution method. It is faster to execute and it should be the preferred model for transient stability studies, when several such devices are used in the same system.

Both OLTC transformer models implement a three-phase regulating transformer rated 47 MVA, 120 kV/25 kV, Y/ Delta, with the OLTC connected on the high voltage side (120 kV).

Select the Model 1 transformer block and use the Edit/Look_under mask menu to see how the regulating transformer is implemented. The OLTC transformer model is built from three Multi-Winding Transformer blocks. Each phase consists of two main windings (winding 2 = 120 kV and winding 3 = 25 kV) and one regulation winding (tapped winding 1). The seven taps of the regulation winding allow eight steps of voltage variation either in positive or negative direction. Three OLTC subsystems contain switches performing tap selection and reversal of the regulation winding.

Run the example. Double-click the Double click for info block to get details.