Generator Step-Up Transformer Fault Contribution Analysis in Ontario Wind Farm Interconnection

Scenario

Project Type: Renewable interconnection study Location Context: 200 MW wind farm in southwestern Ontario connecting via 230 kV transmission; required short-circuit contribution assessment per Hydro One Grid Code Annex G. Constraints: Manufacturer provided generator step-up (GSU) transformer impedance as 12.5% on its own 105 MVA / 34.5–230 kV nameplate; but system studies required actual impedance referred to 230 kV side using network-wide 100 MVA / 230 kV base.

Given Data

• Per-Unit Impedance (z_per_unit): 0.125 pu (12.5% on nameplate base)
• Base Voltage (voltage_base): 230.0 kV (system reference, line-to-line)
• Base Apparent Power (power_base): 100.0 MVA (Hydro One standard base)

Calculation

First, convert nameplate impedance to system base using base change formula — but the tool assumes input z_per_unit is already referenced to the given voltage_base and power_base. Since the engineer performed base conversion offline (per IEEE Std 141, Eq. 3-12), the adjusted zpu on 100 MVA / 230 kV base was calculated as:

$$ Z_{\text{pu,new}} = Z_{\text{pu,nameplate}} \times \frac{S_{\text{base,new}}}{S_{\text{base,nameplate}}} \times \left(\frac{V_{\text{base,nameplate}}}{V_{\text{base,new}}}\right)^2 = 0.125 \times \frac{100}{105} \times \left(\frac{230}{230}\right)^2 = 0.11905~\text{pu} $$

Then applied tool formula:

$$ Z_{\text{actual}} = 0.11905 \times \frac{(230{,}000)^2}{100{,}000{,}000} = 0.11905 \times \frac{52{,}900{,}000{,}000}{100{,}000{,}000} = 0.11905 \times 529.0 = 62.9775~\Omega $$

Rounded to 4 decimal places → 62.9775 Ω

Result and Decision

The actual impedance (62.9775 Ω) was entered into ETAP v22.1 to model GSU contribution to asymmetrical 3-phase fault current at the 230 kV bus. Results showed peak fault current remained below 45 kA—within breaker interrupting rating (50 kA). No series reactor or neutral grounding modification was required.

Lesson

Per-unit impedance must be referred to the correct system base before using conversion tools; blindly entering nameplate %Z without base adjustment would have yielded 66.3225 Ω (12.5% × 530.58 Ω), overestimating impedance by 5.3% and underestimating fault current—potentially masking a thermal stress risk.