Branch Options: Transformer Control

See Also

 

Transformers are used to transfer power between different voltage levels or to regulate real or reactive flow through a particular transmission corridor. Most transformers come equipped with taps on the windings to adjust either the voltage transformation or the reactive flow through the transformer. Such transformers are called either load-tap-changing (LTC) transformers or tap-changing-under-load (TCUL) transformers.

Another type of transformer is known as a phase-shifting transformer (phase shifter or phase angle regulator). Phase-shifting transformers, which are less common than LTC transformers, vary the angle of the phase shift across the transformer in order to control the MW power flow through the transformer.

Options for specifying transformer control are found on the Transformer Control tab of the Branch Options dialog when in Edit mode and the Transformer tab of the Branch Information Dialog when in Run mode.

 

Off-nominal Turns Ratio and Phase Shift Degrees

The Off-nominal Turns Ratio field indicates the voltage transformation, while the Phase Shift Degrees field show the phase shift angle. If the transformer is not on automatic control, these values can be changed manually. The off-nominal tap ratio determines the additional transformation relative to the nominal transformation. This value normally ranges from 0.9 to 1.1 (1.0 corresponds to no additional transformation). For phase-shifting transformers the phase shift value normally ranges from about -40° to 40°. The phase angle field can be non-zero for LTC and fixed transformers, most notably +/- 30° if the transformer configuration is a delta-wye or wye-delta configuration. The transformer configuration is very important when performing a fault analysis study.

 

Automatic Control Type provides the type of transformer. Valid types are 1) No Automatic Control (in which the taps are assumed fixed), 2) voltage regulation (AVR), 3) Reactive Power Control, and 4) Phase Shift Control.

Simulator provides you with a great deal of flexibility in being able to specify which transformers will actually be used for automatic control in the Power Flow Solution. For a transformer to be used for voltage or flow control, three criteria must be met:

• For an LTC transformer, the transformer's Automatic Control Enabled field must be checked. This field can also be modified on the Transformer Records display. When the transformer is a phase shifter, the Automatic Control Enabled field has three options. Select Not Enabled to completely disable the phase shifter for automatic control. Select Enabled for Power Flow to allow automatic control actions during the power flow solution. Select Enabled for OPF Only to allow the phase shifter to be a control as part of the OPF solution but not on automatic control as part of the power flow solution.
• The transformer's area must have automatic transformer control enabled. This is specified on the Options Tabof the Area Records display.
• Transformer control must not be disabled for the entire case. This is specified on the Power Flow Solution Tab of the Simulator Options Dialog. Automatic control for LTC transformers and phase shifters can be disabled separately at the case level.

The type of transformer can be changed on the Transformer Control Info dialog opened by clicking the Change Automatic Control Options... button. This dialog contains different information depending on the type of transformer control that is selected. The Common Options tab of this dialog allows changing the control type while in Edit mode and displays the type of control while in Run mode. The type CANNOT be modified while in Run mode. The Time Step Options tab contains control options specific for the Time Step Simulation tool.

The type-specific information contained on the Common Options tab is described below:

No Automatic Control

On this control setting the transformer will operate at the given off-nominal turns ratio and phase shift, and will remain fixed at those values during the entire solution process unless manually changed by the user.

AVR (Automatic Voltage Regulation)

When on automatic voltage control, the transformer taps automatically change to keep the voltage at the regulated bus (usually one of the terminal buses of the transformer) within a voltage range between the minimum voltage and maximum voltage values (given in per unit). The Transformer Control Info dialog is configured as the Transformer AVR Dialog when this control type is selected. Note that automatic control is possible only if a regulated bus has been specified.

The tap position for an LTC transformer is indicated on the oneline by the number of tap step positions from the nominal position (i.e., the position when the off-nominal tap ratio is equal to 1.0). When the off-nominal ratio is greater than 1.0, the transformer's tap is said to be in the "raise" position, and an 'R' appears after the number. Likewise, when the off-nominal ratio is less than 1.0, the transformer's tap is said to be in the "lower" position, and an 'L' appears after the number. For example, with a step size of 0.00625 and an off-nominal ratio of 1.05, the tap would be in position 8R. The tap position can be changed manually only when the transformer has been set off automatic voltage control.

Simulator will also detect instances when controlling transformers are in parallel, and will employ checks during the solution routine to prevent the controllers from fighting each other and potentially going to opposite tap solutions, which could result in unwanted loop flow through the transformer objects. This option is enabled by default, but can be turned off in the Power Flow Solution Advanced Options.

Transformer Reactive Power Control

When on automatic reactive power control, the transformer taps automatically change to keep the reactive power flow through the transformer (measured at the from bus) within a user-specified range. When this control type is selected, the Transformer Control Info dialog is configured as the Transformer Mvar Control Dialog.

Phase Shift Control

When a transformer is on phase shift control, the transformer phase shift angle automatically changes to keep the MW flow through the transformer (measured at the from bus) between the minimum and maximum flow values (given in MW, with flow into the transformer assumed positive). The limits on the phase shifting angles are specified in the minimum and maximum phase fields (in degrees). When this control type is selected, the Transformer Control Info dialog is configured as the Transformer Phase Shifting Dialog. The phase shift angle changes in discrete steps, with the step size specified in the Step Size field (in degrees). The MW Per Phase AngleStep Size provides an estimate of the change in the controlled MW flow value if the phase angle is increased by the step size value.

 

Area and Case Control Options

This set of options is available when in Run mode and provides a summary of the area and case level options for allowing automatic transformer control. These settings are provided for informational purposes only and cannot be set here.

Area Transformer Control Enabled has to be checked for any transformer in that area to be controlled (phase shifter control must be enabled for power flow); Case LTC Transformer Control Enabled has to be checked for any LTC type (Voltage Regulation (AVR) or Reactive Power Control) transformer in the entire case to be controlled, and Case Phase Shifter Control Enabled has to be checked for any phase shifter in the entire case to be controlled.

 

Specify Transformer Bases and Impedances

Shows the Transformer Bases and Impedances Dialog. This dialog allows the user to specify the transformer parameters in per unit on the transformer base (taken as its rating). Click OK to convert all the transformer parameters values to the system base specified in the Power Flow Solution General Options.