Renewable Integration Study for Coastal Wind Farm Grid Connection
Engineering Case Study
Scenario
Project type: Interconnection study for a 220 MW offshore wind farm connecting via a new 33 kV collector system to an existing onshore 132 kV substation. Location context: Atlantic coast, high humidity, salt-laden air; existing substation has legacy oil-filled transformers and electromechanical relays. Constraints: Must comply with regional grid code (NERC PRC-024) requiring fault contribution modeling of inverters; limited real estate for new switchgear; environmental permitting restricts excavation near dunes.
Given Data
- System Voltage: 33 kV (collector system nominal voltage)
- Base Power: 100 MVA (standardized base for interconnection studies)
- Source Impedance: 8.2 % (equivalent Thevenin impedance derived from 132 kV system short-circuit capacity of 1,220 MVA: ( Z_{\text{pu}} = 100 / 1220 \times 100 = 8.2% ))
- Line Impedance: 2.4 Ω (calculated for 12 km of 300 mm² XLPE cable, including R + X at 33 kV)
Calculation
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Base impedance: ( Z_{\text{base}} = \frac{(33\ \text{kV})^2}{100\ \text{MVA}} = \frac{1089}{100} = 10.89\ \Omega )
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Source impedance in ohms: ( Z_{\text{source}} = 0.082 \times 10.89 = 0.893\ \Omega )
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Total impedance: ( Z_{\text{total}} = 0.893 + 2.4 = 3.293\ \Omega )
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Short-circuit current: ( I_{\text{sc}} = \frac{33\ \text{kV} / \sqrt{3}}{3.293\ \Omega} = \frac{19.05\ \text{kV}}{3.293\ \Omega} = 5.785\ \text{kA} )
The tool reports 5.79 kA (rounded to two decimals).
Result and Decision
The low fault level (5.79 kA) triggered concerns about protection sensitivity and selectivity. Engineers specified high-impedance differential relays for the collector cables and added dedicated fault current limiters (FCLs) at the 33/132 kV transformer tertiary winding to raise minimum fault current to 8.5 kA — ensuring reliable operation of existing overcurrent relays while avoiding costly replacement of legacy protection. All equipment was corrosion-rated (IP66, C5-M per ISO 12944).
Lesson
Low short-circuit currents in distributed renewable connections can compromise protection reliability more severely than high fault levels — always evaluate minimum fault current (e.g., remote-end faults, cable aging, low generation output) alongside maximum, especially when integrating inverter-based resources into weak grids.