Transformer AVR Dialog
The Common Options tab of this dialog has the following fields:
Automatic Control Type
This section is only visible on the dialog in edit mode. The control type of the transformer can be changed between no control, voltage regulation, reactive power control, or phase shifter control. Changing the control type will update the dialog to reflect the type of control that is selected.
Regulated Bus Number
The number of the bus whose voltage is regulated by the control.
Present Regulated Bus Voltage
The present voltage of the regulated bus.
Voltage Error
If the regulated bus' voltage falls outside the regulating range of the transformer (as defined by the Regulation Minimum Voltage and Regulation Maximum Voltage fields), the Voltage Error field indicates by how much the voltage deviates from the control range.
Regulation Minimum Voltage
The minimum acceptable voltage at the regulated bus.
Regulation Maximum Voltage
The maximum acceptable voltage at the regulated bus.
Regulation Target Type
As long as the regulated voltage is inside the regulation minimum and maximum, the transformer will not change its tap ratio. When the regulated voltage moves outside of this regulation range, Simulator will calculate a new tap ratio in an attempt to bring the regulated voltage back inside of its range. The Regulation Target Type determines what value is used as a target when calculating this change in tap ratio. Middle is the default and means that the target is the average of the regulation minimum and maximum regardless of whether the voltage is high or low. Max/Min means that the regulation maximum is used as the target when the regulated voltage is above the maximum, and regulation minimum is used as the target value when the regulated voltage is below the minimum.
Present Tap Ratio
The tap ratio of the transformer for the current system state.
Minimum Tap Ratio, Maximum Tap Ratio
Minimum and maximum allowable off-nominal tap ratios for the LTC transformer. Typical values are 0.9 and 1.1.
Tap Step Size
Transformer off-nominal turns ratio increment. The off-nominal turns ratio is either incremented or decremented from 1.0 in integer multiples of this value. Default value is 0.00625.
Voltage to Tap Sensitivity
Shows the sensitivity of the voltage magnitude at the regulated bus to a change in the transformer's tap ratio. You can use this field to assess whether or not the transformer can effectively control the regulated bus voltage. In an ideal case, such as when the LTC transformer is being used to control the voltage at a radial load bus, the sensitivity is close to 1.0 (or -1.0 depending upon whether the tapped side of the transformer is on the load side or opposite side of the transformer). However, sometimes the transformer is very ineffective in controlling the voltage. This is indicated by the absolute value of the sensitivity approaching 0. A common example is a generator step-up transformer trying to control its high-side voltage when the generator is off-line. Simulator automatically disables transformer control if the transformer sensitivity is below the value specified on Power Flow Solution Tab of the Simulator Options dialog.
Line Drop/Reactive Current Compensation
Typical specification of the bus at which the voltage should be controlled by a transformer is an actual bus in the power system model. However, some control schemes require that the voltage at some fictitious bus a specified impedance away from the transformer be controlled instead. This can be accomplished using the line drop and reactive current compensation control method.
This method requires that the regulated bus for a transformer be specified as one of the transformer's terminal buses. This provides a reference point for specifying the location of the fictitious bus that is the actual controlled point. An impedance must also be specified that will determine the location of the fictitious bus in reference to the selected regulated bus.
Check the Use Line Drop/Reactive Current Compensation checkbox to use this control method. Specify the impedance for the location of the fictitious controlled bus using the Line Drop/Reactive Current Compensation Resistance (R) and Line Drop/Reactive Current Compensation Reactance (X) fields. The impedance should be specified in per-unit.
Impedance Correction Table
This field specifies the number of the transformer's corresponding transformer impedance correction table. Transformer impedance correction tables are used to specify how the impedance of the transformer should change with the off-nominal turns ratio. If this number is 0, then no impedance correction table is associated with the transformer, and the impedance of the transformer will thus remain fixed as the tap ratio changes. To assign an existing impedance correction table to the transformer, enter the existing table's number. To view the existing impedance correction tables, click the Insert/View Impedance Correction Table button, which brings up the Transformer Impedance Correction Dialog. To define a brand new impedance correction table for the transformer, enter an unused table number and then click Insert/View Impedance Correction Table to prescribe the correction table. Note that the association between a transformer and an impedance correction table is not finalized until you select either OK or Save on the Branch Information Dialog or Branch Options dialog.
View Transformer Correction Table or Insert Transformer Correction Table
Click on this button either to view or to insert transformer correction tables. Clicking on this button displays the Transformer Impedance Correction Dialog. Note that a table must contain at least two points in order to be defined.
The Time Step Options tab contains control options specific for the Time Step Simulation tool.