Adding Transformer, Generator, and Load Records to the IEEE 9-Bus Case

Welcome back. Your topology-only case should now contain nine buses and the six 230 kV transmission lines, with buses 1, 2, and 3 intentionally isolated. This lesson closes those three gaps: add the generator step-up transformers, establish the three generator buses as voltage-controlling sources, and place the three constant-power loads.

The result will be a complete steady-state network model, although it will not yet be a validated solved case. In the next lesson, you will run the AC power flow and treat the mismatch, reactive limits, voltages, and flows as engineering evidence rather than simply accepting a “solved” message.

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The records behind the remaining one-line equipment

The full WSCC/P.M. Anderson one-line contains three distinct equipment classes that must remain distinct in PSS®E:

1. Two-winding transformers connect each generator terminal bus to the 230 kV transmission system.
2. Plant and machine records establish scheduled voltage, active-power dispatch, reactive limits, and the reference angle.
3. Load records represent specified real and reactive demand at buses 5, 6, and 8.

A useful distinction is that a bus is an electrical node, while a record type describes the equipment attached to or between nodes. The connection from bus 1 to bus 4 is visible as a single stroke on the one-line, but it is not a line record: it transforms from 16.5 kV to 230 kV and must therefore be a transformer record.

Likewise, do not hide transformer impedance inside the machine record when you have explicitly modeled the transformer as a separate branch. In practical interconnection models, this distinction becomes essential at a renewable plant: inverter terminal, collector, GSU high side, substation bus, and POI are different electrical locations with different data records and control implications.

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Fix the reference dataset before entering values

Published “IEEE 9-bus” cases differ in details, particularly load placement, generator limits, ratings, and dynamic-machine data. Do not merge individual rows from several files because they happen to share the same topology.

For this course, use the following consistent 100 MVA-base steady-state teaching dataset. It is the conventional three-machine WSCC/P.M. Anderson arrangement used in the preceding topology lesson.

Three explicit generator step-up transformers

Enter these as two-winding transformer records, all in service, with circuit ID 1.

Transformer	Winding 1 bus	Winding 2 bus	Winding 1 nominal kV	Winding 2 nominal kV	$R$ p.u.	$X$ p.u.
GSU 1	1	4	16.5	230.0	0.0000	0.0576
GSU 2	2	7	18.0	230.0	0.0000	0.0625
GSU 3	3	9	13.8	230.0	0.0000	0.0586

The stated impedances are already on the system 100 MVA base. Therefore, select the transformer impedance-entry option corresponding to per unit on system base, typically the PSS®E CZ = 1 convention.

For winding voltages, use the option that accepts kV values, typically CW = 2, and enter nominal winding voltages exactly as shown in the table. This approach makes the intended voltage transformation visible in the record itself.

For this benchmark:

• set magnetizing conductance and susceptance to zero;
• use nominal taps;
• specify no automatic tap control;
• use zero phase shift for the positive-sequence power-flow model; and
• retain provisional blank or zero thermal ratings if your source does not provide them.

Do not label a zero or blank rating as “unlimited.” It means the rating has not been established for this small teaching case and therefore cannot support a credible thermal conclusion. A production planning model would require normal and emergency ratings for every line and transformer.

Why the winding order is intentional

Use the terminal-side, lower-voltage bus as winding 1 and the 230 kV side as winding 2:

• bus 1 and bus 4 for GSU 1;
• bus 2 and bus 7 for GSU 2;
• bus 3 and bus 9 for GSU 3.

For an untapped transformer this order will not change ordinary steady-state power-flow physics. But establishing a consistent convention early is valuable because winding order matters when a transformer has tap control, phase shift, metering conventions, or sequence-data implications. In larger SPP cases, an incorrectly ordered regulating transformer can cause control to act on the wrong bus or in the wrong direction.

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Add the transformer records first

Begin from your saved topology milestone, not from an empty case. Save a new revision before editing, for example:

IEEE9_SSBUILD_R00.sav

Open the Transformer Data or Two-Winding Transformer Data interface. Menu labels vary by release, but the following field logic is stable.

For each GSU, set:

PSS®E field concept	Recommended entry
Winding buses	Use the bus pair in the transformer table
Circuit ID	1
Status	In service
Winding-voltage code	kV-based entry, typically CW = 2
Impedance code	System-base per unit, typically CZ = 1
Magnetizing branch	Zero
$R$, $X$	Values in the transformer table
Winding 1 voltage	16.5, 18.0, or 13.8 kV, respectively
Winding 2 voltage	230.0 kV
Tap position	Nominal
Transformer control mode	No control
Phase shift	Zero

The absence of automatic tap control is deliberate. These generator step-up transformers should not become hidden voltage-control devices merely because a generic transformer dialog presents control fields. The generators, through their reactive-power response within limits, will regulate terminal-bus voltage in this simple case.

After entering all three transformers, the electrical connectivity should change substantially:

• bus 1 becomes connected to bus 4;
• bus 2 becomes connected to bus 7;
• bus 3 becomes connected to bus 9;
• no buses remain intentionally isolated.

At this point, the network is connected but still has neither generation nor demand. Do not solve it yet.

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Establish plant control and generator records

A generator bus has two related but separate representations:

• The plant record defines the voltage-control target and regulated location.
• The machine record defines dispatch, capability limits, machine base, source impedance, status, and unit ID.

For this simple system, each generator regulates its own terminal bus. Set the regulated bus to local control, commonly represented by an IREG value of zero in PSS®E.

Plant records and bus types

Create plant records for buses 1, 2, and 3. Use these scheduled voltages:

Bus	Bus role	Scheduled voltage, p.u.	Regulated bus
1	Swing/reference generator	1.040	Local bus
2	PV generator	1.025	Local bus
3	PV generator	1.025	Local bus

Then revise the bus types you assigned provisionally in the topology lesson:

Bus	Final power-flow bus type	Meaning
1	Swing	Holds voltage magnitude and reference angle; supplies the active-power balance required by the solved case
2	PV	Holds specified active power and voltage magnitude unless a reactive limit binds
3	PV	Holds specified active power and voltage magnitude unless a reactive limit binds
4–9	PQ	Network or load buses without a local voltage-controlling generator

A PV bus is not a promise that voltage will always equal the scheduled value. If the required reactive output exceeds $Q_{\max }$ or drops below $Q_{\min }$, PSS®E releases voltage control and treats the bus as a PQ bus at the applicable reactive limit. That behavior is physically important: a voltage setpoint without reactive capability is not a guaranteed voltage outcome.

Machine records

Add one in-service machine at each of buses 1, 2, and 3. Use machine ID 1 at all three buses; the combination of bus number and ID makes each machine unique.

Bus	$P_{\text{GEN}}$, MW	Initial $Q_{\text{GEN}}$, Mvar	$Q_{\max }$, Mvar	$Q_{\min }$, Mvar	$P_{\max }$, MW	$P_{\min }$, MW	MBASE, MVA
1	71.64	27.05	300	-300	250	10	100
2	163.00	6.70	300	-300	300	10	100
3	85.00	-10.90	300	-300	270	10	100

These values establish a consistent initial operating point for this exercise. The reactive values are initial values, not the final answers. Once you solve, PSS®E will adjust generator reactive outputs as necessary to hold scheduled voltage, subject to the specified limits.

The machine at bus 1 is the swing machine. Its entered active-power value is a starting condition, but the solved output at bus 1 will become whatever is necessary to balance total demand, transmission losses, and the fixed dispatch at buses 2 and 3.

The scheduled non-swing generation is:

$$P_{\text{bus 2}}+P_{\text{bus 3}}=163+85=248\text{ MW}$$

The three loads you will enter total 315 MW, so the swing generator must supply the remaining demand plus real-power losses. Its pre-solution value of 71.64 MW anticipates approximately 4.64 MW of losses.

Source impedance and the explicit-GSU rule

If your PSS®E machine-data form requires source impedance, use the course reference value:

$$Z_{\text{source}}=0+j0.20\text{ p.u. on the 100 MVA machine base}$$

This is machine source impedance, not transformer impedance. It normally does not affect an ordinary AC power-flow solution, but it matters later for fault and dynamic-study representation.

Because you have explicitly entered GSU 1, GSU 2, and GSU 3 as transformer records, do not also enter $j0.0576$, $j0.0625$, or $j0.0586$ as an implicit transformer impedance inside the machine data. Doing both would place two transformer impedances in series and distort voltage drop, fault strength, and dynamics.

This is a recurring error in renewable interconnection models. A project data package may provide inverter source impedance, pad-mount transformer impedance, collector equivalent impedance, and GSU impedance. Each must be placed at the electrical location and in the model record where it belongs; none should be silently duplicated.

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Add the three load records

Add an in-service constant-MVA load record at buses 5, 6, and 8, each with load ID 1.

Bus	Load label	$P_{\text{LOAD}}$, MW	$Q_{\text{LOAD}}$, Mvar	Constant-current part	Constant-admittance part
5	Load A	125	50	0	0
6	Load B	90	30	0	0
8	Load C	100	35	0	0

In PSS®E load records, positive $P_{\text{LOAD}}$ and positive $Q_{\text{LOAD}}$ represent demand. Thus all three loads consume real power and inductive reactive power.

The total specified demand is:

$$P_{\text{load,total}}=125+90+100=315\text{ MW}$$ $$Q_{\text{load,total}}=50+30+35=115\text{ Mvar}$$

Enter these as constant-MVA components only:

• $P_{\text{LOAD}}$ and $Q_{\text{LOAD}}$ contain the values above.
• Constant-current load fields are zero.
• Constant-admittance load fields are zero.

A ZIP representation can be appropriate when the study specifies voltage-dependent demand. For this compact reference case, use pure constant-power load to avoid adding behavior that is not part of the selected benchmark definition.

In a production large-load or data-center assessment, the load model must be more explicit than this. You would distinguish firm versus interruptible portions, UPS and emergency-generation behavior, ramp rate, power factor, load-shedding scheme, and whether the demand remains constant, current-like, or voltage-dependent through a disturbance. The 9-bus model is deliberately simpler: it develops correct PSS®E record discipline before those operational refinements are introduced.

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A disciplined record-entry sequence

Use the following sequence rather than entering devices in the order you notice them on the one-line:

1. Save a new revision of the topology case as IEEE9_SSBUILD_R00.sav.
2. Enter the three explicit GSU transformers and verify buses 1, 2, and 3 are connected to the transmission system.
3. Create plant records at buses 1, 2, and 3 with local voltage regulation.
4. Change bus types to swing at bus 1 and PV at buses 2 and 3.
5. Enter the three machine records, including unit IDs, voltage targets, active dispatch, reactive limits, active limits, MBASE, and source impedance.
6. Enter the three load records at buses 5, 6, and 8.
7. Save again before attempting a power-flow solution.

This ordering avoids an easy mistake: creating a voltage-controlling generator before the underlying plant control data and bus classification are coherent.

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Verify the case without solving it yet

The case now has all equipment records required for an AC power-flow model. Before solving, inspect PSS®E reports or the network-data tables and confirm the following.

Check	Expected result
Bus records	9
Generator step-up transformers	3: 1–4, 2–7, 3–9
Transmission-line records	6
In-service machine records	3: buses 1, 2, 3
Load records	3: buses 5, 6, 8
Total specified load	315 MW and 115 Mvar
Swing bus	Bus 1
PV buses	Buses 2 and 3
Isolated buses	None
Explicit GSU impedance duplicated in machine records	No
Transformer automatic control	Disabled
Load current and admittance components	Zero

Your manifest should now identify this revision as structurally complete but not yet validated:

Manifest field	Example entry
Case ID	IEEE9_SSBUILD_R00
Parent case	IEEE9_TOPO_R00
System base	100 MVA
Added transformers	1–4, 2–7, 3–9
Added machines	1:1, 2:1, 3:1
Added loads	5:1, 6:1, 8:1
Intended status	Complete steady-state data entry; unsolved and unvalidated
Known limitations	No source-verified equipment ratings or sequence-data validation

Do not quietly describe this as a “validated base case” merely because every device is present. A valid planning base case must also solve satisfactorily, have plausible voltage and reactive-power behavior, and be reconciled against an appropriate reference result. That is the purpose of the next lesson.

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Key takeaways

You have converted the remaining equipment on the IEEE 9-bus one-line into appropriate PSS®E records:

• three explicit two-winding GSU transformers between generator terminals and the 230 kV network;
• three plant and machine records that define the swing and PV buses, voltage schedules, dispatch, and capability limits; and
• three constant-MVA load records totaling 315 MW and 115 Mvar.

The most important modeling safeguard is to keep explicit transformer impedance separate from generator source impedance. This same discipline will later prevent duplicated collector, pad-mount, GSU, and inverter impedances in solar and BESS interconnection models.

Next, you will solve the IEEE 9-bus AC power flow, inspect mismatches and convergence, and diagnose voltage, reactive-limit, topology, and interchange issues before treating the case as a usable reference.