ETAP Project 2: Load Flow and Fault Analysis for HV Transmission Network

The diagram below shows a HV transmission system supplied by two generating stations (GS1 & GS2). Both generators are operating at rated voltage. At each station the voltage is stepped up to a transmission level voltage. The HV buses are connected by a double circuit line. A load bus is located part way along one of the lines. Equipment characteristics are listed in the ETAP Project 2 data file. This project is to be completed using the Project 2 data which has been assigned to you.

Report Requirements 

1. Prepare a model of the system in ETAP and attach a printout of the one-line diagram (showing impedance values and transformer Δ-Y configuration) and a copy of the Transformer Schedule and the Line Data Schedule (available from the Schedule Report Manager in Edit mode). (5 marks)

Load Flow Analysis

1. Configure the system with all devices closed, and run a Load Flow study. Attach the Load Flow Report and discuss the results. (10 marks)

1. Based on the Load Flow Report, and using the voltage magnitudes and angles at the load bus and at each of the two buses immediately upstream, calculate the Power Transfer from each of the two buses and the total Power Transfer. Compare to the values obtained from the Load Flow Report. (20 marks)

1. Open the Line 2 breaker, repeat the Load Flow study, and attach the Report. Discuss the effect on the voltage at the load bus and the reason for the change. (10 marks)

Fault Analysis

1. Configure the system with all devices closed. Using the Short Circuit Analysis tool, configure the system for a fault at the load bus.

1. Perform a Maximum SC study and attach the 3 Phase, Line to Line, and Line to Ground fault reports. (15 marks)

1. Using a value of S_BASE = 100MVA and the system device impedances listed in the project data file, calculate the expected fault currents for each type of fault using the Per Unit method and symmetrical components. Assume that all generators are operating at their rated voltage.Compare to the values obtained from ETAP. (30 marks)

1. Draw a symmetrical component sequence network diagram for the Line to Ground fault. (10 marks)

Brief summary of assessment requirements

You are asked to model and analyse a two-generator high-voltage (HV) transmission system in ETAP (Project 2 data) and produce a report that covers:

1. ETAP model & schedules (5 marks)

   • Build the one-line model (show transformer Δ–Y configuration and impedance values).

   • Attach one-line printout, Transformer Schedule and Line Data Schedule.

2. Load flow analysis (total 40 marks)

   • Run load flow with all devices closed and attach the Load Flow Report (10).

   • Using voltage magnitudes & angles at the load bus and the two upstream buses, calculate power transfer from each upstream bus and total and compare to ETAP (20).

   • Open the Line-2 breaker, re-run load flow, attach report and discuss the voltage effect at the load bus and why it changed (10).

3. Fault analysis

   • Configure system for a fault at the load bus, run Maximum Short-Circuit (SC) study and attach 3-phase, line-to-line and line-to-ground fault reports (15).

   • Using SBASE = 100 MVA and the device impedances from the data file, compute expected fault currents (3-phase, LL, LG) by Per-Unit and symmetrical-component methods assuming generators at rated voltage; compare with ETAP results (30).

   • Draw the symmetrical component sequence-network diagram for the Line-to-Ground fault (10).

How the Academic Mentor approached the assessment 

Below is the practical, reproducible workflow the Academic Mentor used to guide the student through the assignment. Each step describes the purpose, required actions, and the brief rationale.

Step 1 : Understand the data & set up the project file

• Actions: Review the Project-2 data file (equipment impedances, transformer ratings, line parameters, generator data, bus/load data). Confirm base quantities to be used in calculations (SBASE = 100 MVA specified).

• Why: Accurate modelling depends on correct nameplate data and consistent per-unit base. Mentor ensured the student had all numeric values and units clarified before modelling.

Step 2 : Build the ETAP one-line model

• Actions:

  • Create buses for GS1, GS2 (generator step-up transformers), intermediate transmission buses, and the load bus.

  • Add transformers (set Δ/Y config where required), lines (double circuit represented appropriately), generator internal impedances, and the load.

  • Enter impedances exactly as in the data file (ohms or p.u. consistent with chosen bases).

  • Save a one-line printout and generate Transformer Schedule and Line Data Schedule from the Schedule Report Manager (Edit mode).

• Why: This produces the documentation required by the brief and guarantees ETAP is modelling the same network used in hand calculations.

Step 3 : Run the initial Load Flow (all devices closed)

• Actions: Execute a load flow (e.g., Newton-Raphson) with all breakers/lines closed. Export the Load Flow Report (voltages, angles, branch flows, losses).

• Rationale: This gives the reference operating point and the ETAP values the student must compare against.

Step 4 : Manual power transfer calculations (using voltages & angles)

• Goal: Compute power transferred from each upstream bus to the load location using the voltages reported by ETAP.

• Method (guided formula approach taught by mentor):

  1. Treat the branch between bus i and bus j with series impedance ZijZ_{ij}Zij.

  2. Compute branch current phasor:

     Iij=Vi∠θi−Vj∠θjZijI_{ij} = \frac{V_i \angle\theta_i - V_j \angle\theta_j}{Z_{ij}}Iij=ZijVi∠θi−Vj∠θj

  3. Compute complex power injected from bus i into the branch:

     Sij=Vi∠θi⋅Iij∗S_{ij} = V_i \angle\theta_i \cdot I_{ij}^*Sij=Vi∠θi⋅Iij∗

     Real power transferred Pij=ℜ{Sij}P_{ij} = \Re\{S_{ij}\}Pij=ℜ{Sij}.

  4. For multi-branch feeding the load (two upstream buses), calculate each upstream contribution separately using the relevant series impedances or by forming nodal phasors and summing branch powers.

• Why: These formulas are general and match ETAP’s internal branch-flow computations; mentor had the student compute by hand (or in a spreadsheet) and compare to ETAP branch flows and generator outputs.

Step 5 : Contingency: Open Line-2 breaker and re-run load flow

• Actions: Open the Line-2 breaker in ETAP, run load flow, export report.

• What to examine: Voltage magnitude & angle at the load bus, line loading, generator outputs, and losses.

• Conceptual explanation (mentor explanation): Removing Line-2 reduces network stiffness and reroutes current over the remaining circuit(s). Expect a change in voltage at the load bus : typically a voltage drop (or increased angle shift) if the remaining path has higher impedance or greater loading. Mentor guided the student to explain the change in terms of increased series impedance and altered power sharing.

Step 6 : Short-Circuit (Fault) analysis in ETAP

• Actions:

  • Configure a fault at the load bus and run Maximum SC studies for 3-phase, LL, and LG faults. Export the ETAP reports listing fault currents and contributing source impedance.

• Why: These ETAP results are the benchmark for comparison with analytical (per-unit / symmetrical components) calculations.

Step 7 : Theoretical fault current calculations (Per-Unit & Symmetrical Components)

• Preparation: Convert all device impedances to per-unit on SBASE = 100 MVA (mentor ensured consistent base voltages across transformer taps and converted generator Xd'' etc. as required).

• Guided calculation steps taught to the student:

  Three-phase fault:

  • Use the positive-sequence Thevenin equivalent seen at the fault:

    I3ϕ=EthZth1I_{3\phi} = \frac{E_{th}}{Z_{th1}}I3ϕ=Zth1Eth

    where Zth1Z_{th1}Zth1 is the equivalent positive-sequence impedance to the fault.

  Line-to-line fault:

  • Symmetrical components yield:

    ILL=EZ1+Z2I_{LL} = \frac{E}{Z_1 + Z_2}ILL=Z1+Z2E

    (current magnitude in the two faulted phases, derivation via sequence networks).

  Line-to-ground fault:

  • For single line-to-ground at bus k:

    ILG=3EZ1+Z2+Z0I_{LG} = \frac{3E}{Z_1 + Z_2 + Z_0}ILG=Z1+Z2+Z03E

    where Z0Z_0Z0 is zero-sequence impedance and EEE is the positive-sequence internal voltage (per-unit).

• Comparison: The mentor had the student compute numeric values after per-unit conversion, then compare percent differences with ETAP reports, and investigate causes of any discrepancies (e.g., rounding, transformer grounding connections, generator sub-transient reactances used in ETAP vs assumed values in hand calc).

Step 8 : Draw the symmetrical component sequence network for LG fault

• Action: Sketch the three sequence networks (positive, negative, zero) and show how they are connected in series at the fault point for a single line-to-ground fault: positive sequence source → series Z1 → fault node → series Z2 → series Z0 → return to neutral. Include grounded transformer neutrals and generator sequence impedances as per data.

• Mentor tip: Label sequence impedances, neutral grounding impedances, and the fault connection point.

Step 9 : Validate, document and submit

• Action: Collect printouts: one-line diagram, Transformer Schedule, Line Data Schedule, Load Flow Reports (both configurations), Short-Circuit reports, hand-calculations (or spreadsheet), sequence network diagram, and a concise discussion/comparison.

• Why: This completes the deliverables and demonstrates both practical ETAP competence and theoretical understanding.

How the outcome was achieved 

Outcome (what the student delivered)

• ETAP model: Fully built one-line with transformer Δ–Y configurations and correct impedance entries; Transformer and Line Data Schedules exported.

• Load Flow: Base case load flow completed and report attached. Manual power transfer calculations from upstream buses matched ETAP branch-flow values to within acceptable numerical tolerance after using the correct branch impedances and phasor voltages.

• Contingency: On opening Line-2 breaker, ETAP showed reduced voltage at the load bus and shifted power sharing : explained by increased path impedance and reallocation of current through the remaining circuit.

• Fault studies: Max SC reports for 3-phase, LL and LG obtained from ETAP. Hand calculations using per-unit (SBASE = 100 MVA) and symmetrical components produced fault currents that closely matched ETAP values once correct generator sub-transient reactances, transformer grounding and line zero-sequence data were included.

• Sequence diagram: Complete L-G sequence network drawn, with annotations for Z1, Z2, Z0 and the series connection at the fault.

Learning objectives met

• Build and parameterise a transmission network model in ETAP using given equipment data.

• Interpret one-line diagrams, transformer vector groups (Δ–Y), and line schedules.

• Run and interpret load flow results; perform hand calculations of power flows from phasor voltages and line impedances.

• Analyse contingency effects (line outage) and explain impacts on bus voltages and power sharing.

• Perform short-circuit analysis in ETAP and compute fault currents using per-unit and symmetrical components.

• Draw and interpret sequence networks for single line-to-ground faults.

• Reconcile simulation outputs with theoretical calculations and identify sources of discrepancies (modelling detail, grounding, Xd'' vs Xd', rounding).

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