Three Winding Transformer - LTC (Load Tap Changer) Tab

Figure 1: LTC tab of Three-Winding Transformer Data Dialog Box

Option Descriptions
Option Description

Step Size

The default is 0.625 but you can type in your own percentage step size both here and in the power flow three-winding transformer temporary dialog box.

Min Tap kV

Minimum tap kV is used to determine the bottom limit to which the tap can be adjusted during a power flow analysis. This value should be the lowest tap on the transformer to get meaningful results. The default of 0.12 kV is not realistic and should be changed to the actual values if the LTC is used.

Max Tap kV

Maximum tap kV, which is used to determine the upper limit that the tap can be adjusted to during a power flow analysis. This value should be the highest tap on the transformer to get meaningful results. The default of 1500 kV is not realistic and should be changed to the actual values if the LTC is used.

Tap

Load tap changers (LTC) can be placed on one or two windings of a three-winding transformer. Any combination of primary, secondary or tertiary is allowed, provided that they do not specify the same winding. Also, if only one LTC is used, it must be LTC #1. If the transformer does not have an LTC, select None for standard fixed taps. Fixed off-nominal taps can be entered in the main dialog's Tap kV field.

Control Value (Ctrl Value)

Control value, which is determined by the Control Type field.

  • Voltage: The control value should be entered in per-unit voltage. A typical range would be .975-1.01 per-unit, with 1.0 per-unit as a starting point.
  • MVAR: The value should be entered in actual MVAR rather than adjusted per-unit. To determine what MVAR range is appropriate for a given transformer size, determine the MVAR flow without using an LTC then multiply by .975-1.01 for an approximate starting place. LTC control is much more limited in strength and range than large generator control.

Control Type

Determines how the LTC model is used.

  • Voltage: The LTC attempts to control the voltage of the bus on the same side of the LTC. For example, if LTCs are selected on both the secondary and tertiary sides, voltage is controlled at those buses. This works well if the secondary and tertiary sides are load buses as is common for split winding transformers. However, if the primary side is selected for the LTC and the secondary and tertiary sides are load buses, the primary side (source side) cannot control the load bus voltage. For this type of system standard, fixed taps work well.
  • MVAR: The LTC attempts to control the MVAR flow through the transformer to a given value.

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