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Special transformers are objects that require a different modelling than normal transformers. There is a large variety of control transformers, both in terms of voltage levels and the structure.The following are examples of transformers that can be modelled as a special transformer:

•Auto transformer

•Auto booster transformer

•Auto transformer with built in zig-zag coil

•Low Voltage control transformers with continues regulation

•Low Voltage control transformers with rotating regulation

 

A number of these special transformers are already implemented in Vision. They have in common that the short circuit voltage and the short circuit losses both have a strong relation to the tap position.

 

 

PARAMETERS

 

General

In this form the switches on both sides of the transformer can be opened or closed.

 

 

The special transformer runs from the From-node towards the To-node. The From-node and the To-node of the special transformer are determined automatically. To switch the From-node and the To-node use the <> button.

 

Transformer

 

 

1) The default value of the nominal voltage is chosen equal to the nominal voltage of the node to which the winding is connected

 

Type

The type list contains all transformers which have an Unom,w1 and Unom,w2 between 0.8*Unom and 1.2*Unom for both nodes.

See also: Type

 

Sort

A number of special transformers are modelled. The possible types which can be chosen are: Yd11 Auto transformer, Y0 Auto transformer, Yn0 Auto transformer, Yn0 Auto transformer asymmetric, Booster transformer, Shunt transformer, Quadrature boost R-S, Quadrature boost R-T, AXA Low Voltage regulator and RELO Low Voltage regulator.

 

Unom

With a new transformer, the nominal voltage of the nodes concerned is adopted for Unom,w1 and Unom,w2.

 

Z0 and R0

Due to the construction the zero sequence impedance resembles that of a normal connection, so it is not dependent on the transformers' tap position. In the model the values for Z0 and R0 are interpreted as the zero sequence longitudinal impedances.

An exception is made for the Low Voltage AXA control transformer, where Z0 and R0 are used for the zig-zag coil impedance. The AXA zero sequence longitudinal impedance equals the normal longitudinal impedance.

 

Tap min, nom and max

The indication of the tap position can be defined by the user by indicating the minimum, nominal and maximum tap position. Note that, for example, the minimum tap position can be defined as the tap position at the smallest number of windings and thus (depending on the tap side) can give the largest voltage ratio!

 

Control transformer

Based on an MS control transformer, with which the asymmetry can be reduced in some (especially above-ground) MS networks, a sort of "YN0, asymmetric auto transformer" has been added. With this transformer, the voltage control is designed such that the tap switches of the three phases can each be controlled independently, so that the voltages of the three individual phases will fall within the desired voltage band. Also, on the "General" tab of the special transformer, each tap can be set manually.

 

 

Connection

 

The earthing of the neutral point of the transformer is defined in the transformer model.

 

 

Voltage control

 

 

1) The default value is chosen equal to the nominal voltage of the transformer winding, which is on the measurement side.

 

Current compensation

 

In load flow calculations, Vision can use the voltage control to determine a correct tap position, taking into account the secondary current (Ij) and a compounding impedance Zc. The tap position is determined in such a way that the voltage on the measured side (w1 or w2) will be within the specified voltage limits Uset ± ½ * Uband, corrected with the product of Ij and Zc. The figure below shows an example of transformer with voltage control with tap side w1 (i), measurement side w2 (j) and a fictitious measurement point on the w2 side (note the direction of Ij).

 

 

The voltage Umeas, on the basis of which the voltage control chooses a different tap, is:

 

 Umeas = Uj + Ij * Zc

 

where:

 Zc = Rc + jXc

 

The compounding in Vision takes into account the direction of the current due to the complex multiplication. Note: in practice it can occur that the absolute current value is assumed. In those cases, the model of the voltage regulation will respond differently when it is delivered back than in practice.

 

If Umeas> Umax or Umeas <Umin on the w2 side, a different tap mode is selected on the w1 side (until the minimum or maximum tap position is reached).

If the voltage regulation has to compensate the voltage loss over a certain connection, this can be done by indicating a compounding impedance Zc. The way in which Zc can be determined from a graph U = f (I) is indicated by the following figure.

 

If Rc/Zc = cos(load), the following applies:

 U / I = Zc

 

which can be used to determine Rc and Xc:

 Rc = Zc * cos(φ)load

 Xc = Zc * sin(φ)load

 

If the values found for Rc and Xc are given in the form, the transformer voltage will be dependent on the load current.

 

The dead band is located around the to be controlled voltage. The transformer will not change tap if the measured voltage is between Uregel - ½*Uband and Uregel + ½*Uband.

 

 

Power control

 

The types "quadrature boost" and "shunt" can be equipped with power control. The active power is controlled by the load flow within a band, by adjusting the tap position.

 

 

Reliability

 

 

 

 

MODELLING

 

For all calculations, the transformer is modelled in accordance with the figure shown below, in which R is principally determined by Pk, and X principally by uk. The tap changer is usually located on the primary side (w1).

 

 

The voltage ratio is determined in accordance with the following, depending on the tap side:

 Tapside w1: ( Unom,w1 + tapstandardised * tapsize ) / Unom,w2

 Tapside w2: Unom,w1 / (Unom,w2 + tapstandardised * tapsize )

 

Introduction of Inom' and Snom' for branches

For all branches the variables Inom' and Snom' have been introduced for a uniform overload indication in the load flow. For a special transformer the value is obtained as follows:

•Snom' is set to the Snom input or to the special input (formerly Smax)

 

 

IEC 60909 and fault analysis

In asymmetrical short-circuit calculations and fault analyses, the inverse impedance is equal to the normal impedance (Z1 = Z2).

 

IEC 60909

A short-circuit calculation in accordance with IEC 60909 can be determined using a nominal tap (transfer ratio: Unom,w1 /Unom,w2) or using the transfer ratio which follows from the set tap.

 

The voltage control has no influence on IEC 60909 calculations. Transformers with voltage control are modelled in the same way as transformers without  voltage control.

 

For the calculation of short circuits with earth contact, it is checked whether: Z0> 0. If not, a warning is generated.

 

If For motor start is enabled, the secondary rated voltage of the transformer (eventually corrected with the tap position in case of set tap calculation setting) is used as the c*Unom voltage of the voltage source in case of short circuits at nodes connected to the secondary side of the transformer. The adjustment takes place up to a maximum of three coupled nodes. If more nodes are coupled on the secondary side, an error message is generated. This additional functionality (not explicitly reported in the IEC60909 standard) is useful if the current voltage during motor start differs significantly from the rated voltage of the motor node. Then the short-circuit current also differs significantly from the short-circuit current in the rated situation.

 

Fault analysis

Special transformers with voltage control are modelled for sequential fault analysis in the same way as special transformers without voltage control. A load flow calculation is carried out to determine the "pre-fault" situation (sequence 0). In this load flow calculation, the tap can be determined via the voltage control.

 

Harmonics

The distributed parameter model is used for calculating harmonics. See: Harmonics: Model