500 kW Offshore Wind Farm: Offshore Substation + GIS + Special Isolation Transformer Deep-Read
1×500 kW WTG · 800 kVA box (0.69/35 kV) · offshore substation GIS · special isolation transformer · 110 kV submarine cable
1 · TL;DR Conclusion
Conclusion: this 500 kW offshore wind farm (1 × 500 kW WTG) uses an 800 kVA one-turbine-one-transformer box transformer (0.69/35 kV) → an offshore substation with 35 kV GIS → a special isolation transformer → 110 kV submarine-cable export + ±1 Mvar SVG, with C5-M marine anti-corrosion (GIS/isolation transformer priced separately) — configurator total ≈ $491,625 (FOB), EPC ≈ $640,850, 9 yr payback, 10.6% IRR (feasibility-level).
- •1×500 kW WTG → 800 kVA box transformer (0.69/35 kV) one-turbine-one-transformer; offshore-platform integrated
- •Offshore substation with 35 kV GIS (anti-corrosion, compact, low-maintenance) + special isolation transformer (submarine-cable capacitance / insulation isolation)
- •110 kV submarine-cable export + ±1 Mvar SVG (PF ±0.95) + C5-M marine anti-corrosion (flagged in config)
- •Generation 1,040,250 kWh/yr (P50), P90 986,906 kWh, revenue $83,220/yr @ $0.08/kWh PPA
- •Economics: $640,850 EPC, 9 yr payback, 10.6% IRR, 181% 25-yr ROI
2 · Solution Overview
An offshore wind farm differs from onshore in environment: salt-spray corrosion (C5-M), constrained platform space and weight, and submarine-cable export. On top of the onshore box-transformer + collector + step-up skeleton, the 35 kV collector is upgraded to GIS, a special isolation transformer handles submarine-cable capacitive charging, and the marine anti-corrosion class is flagged.
3 · Design Process & Rationale
1. Requirement identification
500 kW offshore wind, 100% export; the offshore-platform environment and submarine-cable export are the key constraints beyond onshore.
2. Source sizing
One 500 kW WTG (turbMw 0.5); installed 500 kW, effective supply 400 kW, 100% renewable share.
3. Machine step-up
0.69 kV terminal steps up to 35 kV on-site (800 kVA box) to cut offshore collection losses.
4. Collector upgraded to GIS
On the offshore platform the 35 kV collector uses GIS (SF6 gas-insulated) — salt-spray resistant, maintenance-free, compact — the inevitable choice for platform space and O&M cost.
5. Special isolation transformer
The 110 kV submarine cable draws large capacitive charging current; a special isolation transformer provides insulation/capacitance isolation for safe export.
6. Export & POI
110 kV submarine cable to shore; step-up main transformer 35/110 kV (engine floor 10 MVA).
7. Anti-corrosion & power quality
C5-M marine anti-corrosion class (flagged in config, priced separately); ±1 Mvar SVG for PF ±0.95 grid compliance.
Pain Points → Solution → Evidence
Marine salt-spray (C5-M) corrodes primary equipment and enclosures — onshore-standard equipment life collapses.
Config flags C5-M marine anti-corrosion; the 35 kV collector is upgraded to GIS (fully gas-insulated SF6).
special.corrosion=C5-M flagged; C4+ triggers anti-corrosion treatment.
Offshore platforms are space- and weight-constrained — conventional AIS switchgear is bulky and needs frequent maintenance.
GIS is compact and maintenance-free; the special isolation transformer reduces cable charging-current stress on equipment.
35 kV GIS + special isolation transformer form the offshore substation core.
Submarine cables draw large capacitive charging current with high insulation demands — direct connection risks overvoltage/resonance.
A special isolation transformer provides insulation/capacitance isolation; 110 kV submarine cable runs to shore.
The 110 kV submarine-cable export path is explicit in the scheme.
4 · Key Parameter Deep-Read
| Parameter | Value | Basis | Impact & Boundary |
|---|---|---|---|
| Box transformer | 800 kVA (0.69/35 kV) | 500 kW ÷ 1.0 PF × 1.05 = 525 kVA → 800 kVA | One-turbine-one-transformer, offshore-platform integrated >3 MW turbines switch to oil-immersed combined box |
| Collector type | 35 kV GIS | Offshore platform selects GIS (fully gas-insulated SF6) | Salt-spray resistant, maintenance-free, compact Onshore may use AIS; GIS costs more |
| Special isolation transformer | 110 kV submarine-cable isolation | Submarine-cable capacitive charging + insulation isolation | Suppresses overvoltage/resonance, protects export Rating/impedance need design-institute confirmation |
| Export method | 110 kV submarine cable | hvKv=110 | To-shore grid connection Near-shore may drop to 35 kV submarine cable |
| Corrosion class | C5-M (marine) | special.corrosion=C5-M | Triggers anti-corrosion treatment (C4+) Priced separately; engine FOB does not include it |
| SVG rating | ±1 Mvar | 25% of 500 kW = 125 kvar → 1 Mvar | PF ±0.95, LVRT ≥1 Mvar engine floor |
5 · Drawing Deep-Read
The following drawings are shown for reference (full set in the configurator “View design document”):
6 · Operation Demo (Deep-Read)
Energy flow
WTG 500 kW → 0.69 kV → 800 kVA box up to 35 kV → offshore GIS → special isolation transformer → 110 kV submarine cable to shore.
Reactive & power quality
±1 Mvar SVG continuous dynamic reactive, PF ±0.95; submarine-cable capacitance isolated by the isolation transformer.
Fault & protection
LVRT on dips, staged collector feeder protection, isolation/step-up transformer differential — safeguarding offshore export.
Economics
1,040,250 kWh/yr, $83,220/yr; $640,850 EPC, 9 yr payback, 10.6% IRR.
Other views (shown): LCC (life-cycle cost) · Power quality & harmonics · Voltage profile · Scheme features · Environmental derating
7 · Economics & Payback
Equipment FOB ≈ $491,625 (WTG $352,500 + box $37,000 + step-up $29,500 + SVG $50,000 + HV switchgear $16,750 + RMU $5,875); EPC ≈ $640,850. Offshore GIS, special isolation transformer, C5-M anti-corrosion and submarine-cable laying are extra POA items not in the FOB. Annual 1,040,250 kWh → $83,220/yr; 9 yr payback, 10.6% IRR, 181% 25-yr ROI.
8 · FAQ
Why does an offshore wind farm need GIS?
What does the special isolation transformer do?
What box transformer for a 500 kW offshore turbine?
What does C5-M corrosion class mean?
What voltage is the submarine cable?
What is the payback?
Is offshore priced the same as onshore?
Why is an SVG needed?
How much energy per year?
Special Considerations
- C5-M marine anti-corrosion class is flagged in the config, but the engine FOB price does not include the anti-corrosion treatment cost — price it separately.
- Offshore GIS, the special isolation transformer and submarine-cable laying are extra POA items; this quote is the equipment-FOB basis of the onshore-equivalent topology.
- The step-up main transformer is engine-floored at 10 MVA (35/110 kV); a real offshore substation rating must be sized by the design institute to the farm scale.
- LVRT / grid code (GB/T 19963) is not explicitly modelled; offshore farms face stricter fault-ride-through — confirm with the grid operator.
- Receiving-side calcs (10/0.4 kV short-circuit, load flow, arc flash, N-1) are stale in generation-export mode — ignore them; plant-side data is in windCalc/genPlant.
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