Adjustable Transformer

The 2-winding tap-changing transformers and angle-changing phase shifters are connected between a pair of AC buses, individually or as a section of a sectional branch. They are represented by a two-inductor symbol with an arrow in the Bus Tree and diagrams.

The model of an adjustable transformer consists of an ideal transformer with variable tap ratio or phase shift on the From side, a fixed-tap ideal transformer on the To side, a series impedance between them, as well as magnetizing admittances at the From and To buses and shunt (charging) admittance.

A tap-changer can control the voltage of a bus or its own MVAr flow by adjusting the From-side turns ratio within limits. A phase-shifter can control its own MW flow by adjusting its phase shift angle within limits.

The attributes of each adjustable transformer, shown under Main, Impedances, Controls and MVA Ratings tabs in Adjustable Transformer Properties dialog, are the following:

To-Bus: Terminal bus at the “To” end. This bus must exist in the powerflow data.

To connect the transformer to a different From or To bus, click on the browse button beside the bus number/name to open the Set AC Bus dialog, select the new bus in that dialog and click the OK button.

ID: 8 character identifier to distinguish among parallel adjustable

transformers.

Section: Section number of the transformer if it belongs to a sectional branch. It will be 0 (blank in the Transformer table) if the transformer is not a section of a sectional branch. Sections of a sectional branch must have the same From-bus, To-bus (in the same order) and ID, and consecutive Section numbers (1, 2, . . .) starting from 1 for the section at the From-bus. Note that a sectional branch cannot have more than one fixed or adjustable transformer or static tap changer/phase regulator.

Status: In-service or Out-of-service status.

Base MVA: Rated (base) MVA. Note that pu impedance and admittances are in the system MVA base, not the transformer base MVA.

Meter end: Metered end, which can be set to one of the following:

 From

 To

From-Side Tap Ratio: Off-nominal turns ratio of the From side in pu.

To-Side Tap Ratio: Off-nominal turns ratio of the To side in pu.

Maximum Ratio: Maximum of From side tap ratio in pu.

Minimum Ratio: Minimum of From side tap ratio in pu.

Ratio Step: Step size, for From side tap ratio adjustment, in pu.

Phase Shift Angle: Phase shift angle (phase angle of From side minus To side at no load) in degrees.

Maximum Ph. Sh.: Maximum of phase shift angle in degrees.

Minimum Ph. Sh.: Minimum of phase shift angle in degrees.

Resistance: Series resistance in pu.

Reactance: Series reactance in pu.

For transformers with Impedance Correction Table, resistance and reactance values specify the reference impedance. The actual impedance will depend on the tap ratio or the phase shift angle.

Correction Table: Number of the Impedance Correction Table. To choose one of the existing correction tables, click on the pull-down list and select a table number. To specify no correction table (remove the specified number), choose the blank (first) item in the pull-down list. To view or modify the correction tables or add a new one, click on the Add/View Correction

Tables button to open the Impedance Correction Tables Dialog.

From-Magnetizing: Magnetization conductance and susceptance in pu at the From-bus.

To-Magnetizing: Magnetization conductance and susceptance in pu at the To-bus.

Charging: Charging (shunt) conductance and susceptance in pu.

Control Flag: Control flag, which can be set to one of the following:

 Enabled

 Disabled

Tap ratio or phase angle of Enabled transformers will be adjusted in powerflow solution if the corresponding solution parameter is set. Tap ratio and phase angle of Disabled transformers will remain fixed.

Control Variable: Control variable, which can be set to one of the following:

 Voltage

 Real Power (MW flow)

 Reactive Power (MVAr flow)

Control Side: Control side, which can be set to one of the following:

 From

 To

For voltage control of a remote bus, this indicates whether that bus is on the From side or To side. For MW or MVAr flow control, this indicates whether the flow is measured at the From side or To side, but always in the From-to-To direction.

Controlled Bus: Controlled bus (for voltage control). It can be the From-bus, To-bus or a remote bus. To change the controlled bus, click on the browse button (. . .) beside it to open the Set Controlled Bus dialog, select the desired bus in that dialog and click the OK button. To remove the existing controlled bus, click on the browse button (. . .) beside it to open the Set Controlled Bus dialog and click the Remove Bus button in that dialog.

Upper Voltage Limit: Upper limit for controlled bus voltage in pu.

Lower Voltage Limit: Lower limit for controlled bus voltage in pu.

Upper MVAr Limit: Upper limit for controlled reactive power in MVAr.

Lower MVAr Limit: Lower limit for controlled reactive power in MVAr.

MVA Ratings 1 to 6: Thermal ratings of the transformer in MVA at nominal (1 pu) voltage.

Rating Group: An integer number representing the rating group.

Name: 16 characters long characteristic name. This name is not required to be unique.

Owner: 16 characters long transformer ownership name.

The Adjustable Transformer Properties dialog also shows the voltage of the controlled bus and MW/MVAr flows (for comparison with the upper/lower limits), as well as the MW, MVAr and current (A) flows at the From and To buses (flows entering the transformer from each bus) under the Flows tab.