Conclusiones

The transformer is another type of branch that connects between two existing nodes. PSS/ADEPT allows you to adjust the transformer’s physical and electrical characteristics.

To change the properties for a transformer:

1. Double-click the transformer to select it and display the Transformer Property sheet, or, to select a transformer or a group of transformers, right-click, and choose Properties to display the Transformer Property sheet (Figure 3-31).

Notice that there are five tabs for this property sheet: Main, Tap Control, Regula-tion, Harmonics and DRA.

Figure 3-31. Transformer Property Sheet: Main Tab

2. In the Main tab, enter/select the properties for your transformer:

Name: Each item in the PSS/ADEPT network must have a unique name identifier. The transformer name has no relation to the FROM and TO nodes identified in the upper right of the Transformer Property sheet. The FROM and TO nodes may not be modified on the property sheet.

Phasing: Phasing is specified on the FROM side of the transformer (e.g., for a wye-delta transformer, the wye side is the FROM side). When specifying phasing, the

designation A, B, and C mean the first winding, second winding, and third winding.

Thus, if a wye-delta (+30º) transformer with A phasing is specified, the winding on the wye side of the transformer from phase A to ground exists, and the winding on the delta side from C to A is installed. Conversely, if a wye-delta (-30º) transformer with A phas-ing is specified, the windphas-ing on the wye side of the transformer is still phase A to ground, but the winding on the delta side from A to B is installed. Refer to Appendix A, Section A.1.2 for more information about phasing.

Type: Select the transformer connection type:

• Wye-Wye

• Center Tapped Delta-Wye (±30º)

• Wye-Wye with Phase Shift

• Wye Auto

• Z Wye (±30º)

• Z Wye (±150º)

• Wye-Wye +180º

In addition, three-winding transformers, three-legged core transformers, and grounding transformers may be modeled by referring to Appendix A, Sections A.2.3 through A.2.7.

Tapped node: Specify which of the two connecting nodes has the load tap-changing capability. In PSS/ADEPT most transformers have the taps on the TO side, so chang-ing the tapped node may also cause the transformer type to flip, e.g., a wye-delta trans-former will become a delta-wye.

Nameplate rating (kVA/phase): The transformer size is specified in kVA per phase.

For single-phase transformers, the value is usually the nameplate rating; for three-phase transformers, the entered value will usually be one-third of the transformers nameplate rating. See Appendix A, Section A.2.1 for rules and hints when specifying transformer size.

Construction type: Construction type is a 1- to 10-character alphanumeric identifier that refers to a construction type defined in the Construction Dictionary. You may select one of the construction types available in the Construction Dictionary, or you may enter your own (user-defined) unique construction type.

Phase shift (deg): Specify in degrees the angle of phase shift. This field will be enabled only when a wye-wye with phase shift transformer type is selected.

Voltage: By default, the transformer voltages will be set to the FROM and TO node base voltages. In some cases, the transformer voltages will not match the node

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voltages, e.g., a 13.8 kV transformer being used on a 13.2 kV system. To set the trans-former voltage independently:

a. Place a check mark in the box labeled User defined. The FROM and TO voltage fields will become enabled.

b. Change the FROM side and/or TO side voltage of the transformer.

Impedance: All transformers require a leakage impedance; some units also require a second impedance to be specified. A few of the transformers also require a second impedance; the field for entering this second impedance will be enabled when it is needed. Some transformers can have grounding impedances on the FROM, TO or on both sides. Again, the required fields will be enabled when needed. The impedance val-ues needed for each type of transformer, is shown below.

Leakage impedance, Full-winding leakage impedance, Half-winding leakage impedance, Zero-sequence impedance: Enter the value in per-unit (pu) based on the Nameplate rating that was specified.

Grounding impedance: Enter resistance (R) and reactance (X) in Ohms. Setting R = X = 0 results in the winding solidly grounded.

Ratings: The four values are pu of the specified transformer nameplate rating. They are used to check transformer overload after a loadflow. The four values can be man-ually entered or come from a construction dictionary.

3. To display the transformer on the diagram, click once in the Visible check box (on the Main tab) to place a check mark there.

4. To indicate that the selected transformer is in service, click the In service check box, which is the default setting. If In service is not checked, the transformer is out of service and is disconnected at both ends.

Type Required Values

Wye-Wye, Wye-Wye with phase shift, Wye-Wye +180° Delta-Delta, Delta Auto Regulator Leakage Impedance

Wye Auto, Wye Auto Regulator Leakage Impedance Grounding Impedance

Center Tapped Delta-Delta Full-winding Leakage Impedance Half-winding Leakage Impedance Center Tapped Delta-Wye (±30°) Full-winding Leakage Impedance

Half-winding Leakage Impedance TO Side Grounding Impedance Z-Wye (±30°, ±180°) Leakage Impedance

Zero Sequence Impedance FROM Side Grounding Impedance

5. Place a check mark in the box labeled Results to display results for this transformer on the diagram.

6. Click the Tap Control tab (Figure 3-32) and enter/select the Tap Control properties for your transformer.

Figure 3-32. Transformer Property Sheet: Tap Control Tab

Tap Adjustment: Select the load tap changing operation. Your choices include:

• Taps in the phases are adjusted independently of each other. For Z-Wye trans-formers independent tap adjustment cannot be selected

• Taps in all phases are ganged together and have the same setting. The first set of taps control the operation; the others follow. For example, with a phase ABC Wye-Wye transformer, the phase A taps adjust to control the phase A voltage of the regulated node.

• Taps locked in present position in which the transformer taps will remain

"locked" in their current tap position for all subsequent loadflows.

Tap Settings: Specify the present position of the three taps, the Max and Min possible tap settings, and the tap change increment (step). Manually, you can set the taps to

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they correspond to one of the increments. After setting the taps manually, if you do not want them moved during a subsequent loadflow, the "Taps locked in present position"

option should be selected.

For auto wye transformers, the load changing tap side can be designated as either the TO or FROM side. The taps on the other side can then be used as no load taps.

Time delay: There are situations where, two or more transformers, two or more capac-itor banks, or a combination of transformers and capaccapac-itor banks may be regulated by voltage at some location in a network. In such situations, the transformer tap (capacitor bank) controllers may fight one another trying to control voltage. The time delay is used to prevent these controllers from interacting by defining the order in which they attempt control. Controllers with a short time delay will operate before controllers with a long time delay. A short time delay (i.e., zero) is generally assigned to transformer tap con-troller closest to the source; increasingly longer time delays are assigned to down-stream controllers. In this way, updown-stream transformers are first used to correct voltage problems. Only if unsuccessful will controllers downstream of the first be used. Time delay is a floating-point number (e.g., 1.5). No specific units are assumed.

7. Click the Regulation tab (Figure 3-33) and enter/select additional Tap Control proper-ties for your transformer:

Figure 3-33. Transformer Property Sheet: Regulation Tab

Regulated node: Specify the node at which the voltage is to be monitored/regulated to determine if tap changing is necessary. If the specified regulated node is not the transformer TO or FROM node, specify whether the regulated node is on the tapped or untapped side of the transformer so that the tap controller will know which way to move the taps during voltage regulation. If the specified regulated node is not the transformer FROM or TO node, then the compensating impedance, discussed further below, is ignored. When the specified regulated node is not the FROM or TO node, the voltage at the specified node is regulated directly. It is easy for a computer program to look at the voltage of a distance node and do the direct regulation. However, this can be done in the field only if there is some form of communication to send the information back to the tap controllers.

Controlled Voltage: Specify the range in which the transformer will attempt to control the voltage at the regulated node. If the regulated node is on the side of the transformer that has a wye winding, the phase-to-ground voltage will be monitored to determine if tap changing is necessary. If a regulated node is on the side with a delta winding, the phase-to-phase voltage will be monitored. Normally, you set the range of the controlled voltage to a value greater than the minimum tap step.

Compensating Impedance: The compensating impedance is designed to be used with the auto regulator transformers, either wye or delta; it is enable only for these two transformer types. The value of resistance and reactance entered should be the dial setting of the actual regulator; for this reason the units are volts (V). The PT ratio and CT rating entered should also be that of the actual regulator.

Compensating Impedance Calculation: This is a separate activity that calculates auto regulator dial settings for you; it is enabled only for the wye auto or delta auto reg-ulator transformers. The regreg-ulator CT and PT values should be filled in before doing the calculation. The downstream node (load center node) is selected and the position of the load center node in relation to the transformer (on the TO side or FROM side) is designated. Clicking the Calculate Impedance button results in the R and X values being filled in. Rotating impedances by 30° for the delta auto regulators is done auto-matically; the values returned by this activity are meant to be the actual dial settings.

At the present time, the single-phase and three-phase delta auto regulators in PSS/ADEPT are connected lagging; leading regulators will be added in a later version.

Thus the compensating impedance and its calculation can be used only for lagging connected regulators at the present time. If an open delta auto regulator is specified (delta auto regulator with AB, BC, or CA phasing), then in PSS/ADEPT the first phase is connected lagging and the second phase is leading. For example, with a delta auto regulator with AB phasing specified, the first (A) phase is connected lagging (from A to B) and the second (B) phase is leading (from C to B). In Figure 3-34, you can see pic-tures of the configuration. If you have an open delta regulator and perform the imped-ance calculation, you will see that the calculated compensating impedimped-ances are different for the two phases; the difference is because the 30° impedance rotation is opposite for leading and lagging regulators.

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Figure 3-34. Delta Autoregulator with AB Phasing

At the present time, PSS/ADEPT calculates the compensating impedance only for the wye and delta autoregulators. This compensating impedance is then used, in a loadflow, to calculate a synthetic transformer output voltage. This synthetic voltage, an estimate of the voltage at the remote load center, is used in the transformer tap changing.

Although the automatic calculation is available only for the two transformers mentioned above, it can also be used for any transformer with a wye or Z winding on the load cen-ter side. If you are using a wye or Z transformer (not a regulator) and want PSS/ADEPT to perform the compensating impedance calculation for you, momentarily insert a wye autoregulator transformer on the downstream side of the transformer and have PSS/ADEPT make the calculation. Copy this value into the Transformer Property sheet, delete the autoregulator and restore the original connection.

8. Click the OK button to accept your changes to the transformer properties.

To change the harmonic properties for a transformer (visible if you are licensed for the harmonics module):

1. Click the Harmonics tab to modify the harmonic properties of a transformer (Figure 3-35).

Figure 3-35. Transformer Property Sheet: Harmonics Tab

Name: The name of the line. This property is for information only and is not editable.

Click the Main tab to modify this item.

FROM node: The name of the FROM node to which this transformer is connected. This property is for information only and is not editable.

To node: The name of the TO node to which this transformer is connected. This prop-erty is for information only and is not editable.

Type: Select the type of line representation from the drop-down list.

• IEEE Model – Represents an IEEE line. The modeling of an IEEE Line is described in Chapter 8, Section 8.9.6.

• Custom – Represents a user-defined line model for harmonics. Selection of this

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For custom types, enter the requested values for the following:

• Positive sequence resistance

• Positive sequence reactance

• Grounding sequence resistance

• Grounding sequence reactance

To change the reliability properties for a transformer (visible only if you are licensed for the DRA module):

1. Click the DRA tab to modify the reliability properties of a transformer (Figure 3-36).

2. Refer to Chapter 9, Section 9.4.2 for further instructions.

Figure 3-36. Transformer Property Sheet: DRA Tab