Turbines per feeder
6
Wind Farm Box Transformer Sizing Calculator
Size the box transformer for each wind turbine from its rated power and power factor, then lay out the collector feeders and the step-up main transformer.
For wind-farm developers and electrical engineers selecting box-type substations, collector voltages and grid-connection transformers for onshore and offshore projects.
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Worked example
A pre-computed reference example (crawlable — no JavaScript required). Enter your own parameters above for a live result.
Example 1 — 4 MW turbines, 20 units, 35 kV collector
Box transformer per turbine
5000 kVA
4 MW turbine · 20 units · 35/0.69 kV · Oil-immersed combined box transformer (35 kV)
Collector feeders
4
Main transformer
100 MVA
Total collector
100.0 MVA
Sizing calculation
| Turbine apparent power | 4 MW ÷ 1 × 1.05 = 4200 kVA (with 5% margin) |
| Standard box transformer | → 5000 kVA |
| Feeder capacity | 35 kV feeder ≈ 30 MVA |
| Turbines per feeder | 30 MVA ÷ 5.00 MVA = 6 (capped at 10) |
| Number of feeders | 20 ÷ 6 = 4 |
| Step-up main transformer | 100.0 MVA → 100 MVA · 35/110 kV |
Collector scheme
35 kV collector is suitable for feeder capacity up to ~30 MVA (6 turbines × 4 MW).
Bill of materials
| Equipment | Spec | Qty | Subtotal |
|---|---|---|---|
| Oil-immersed combined box transformer (35 kV) | 5000 kVA · 4 MW · 35/0.69 kV | 20 | POA |
| Collector network | 4 feeder(s) · 35 kV · 6 turbines per feeder | 1 | POA |
| Collector cable | YJV 3-core 35 kV XLPE · radial feeder (per feeder) | 4 | POA |
| Collector switchgear | 35 kV feeder breaker panels · 4 feeders + PT/SPD | 1 | POA |
| Step-up main transformer | 100 MVA · 35/110 kV | 1 | POA |
| Grounding & protection | collector system earthing + relay protection 50/51/51N + metering | 1 | POA |
| Total equipment (box transformers) | POA |
📐 Single-line diagram
Example 2 — 2 MW turbines, 50 units, 10 kV collector
Box transformer per turbine
2500 kVA
2 MW turbine · 50 units · 10/0.69 kV · Oil-immersed box transformer (10 kV)
Turbines per feeder
7
Collector feeders
8
Main transformer
150 MVA
Total collector
125.0 MVA
Sizing calculation
| Turbine apparent power | 2 MW ÷ 0.95 × 1.1 = 2316 kVA (with 10% margin) |
| Standard box transformer | → 2500 kVA |
| Feeder capacity | 10 kV feeder ≈ 18 MVA |
| Turbines per feeder | 18 MVA ÷ 2.50 MVA = 7 (capped at 10) |
| Number of feeders | 50 ÷ 7 = 8 |
| Step-up main transformer | 125.0 MVA → 150 MVA · 10/35 kV |
Collector scheme
10 kV collector limits each feeder to ~18 MVA; consider 35 kV for larger clusters.
Bill of materials
| Equipment | Spec | Qty | Subtotal |
|---|---|---|---|
| Oil-immersed box transformer (10 kV) | 2500 kVA · 2 MW · 10/0.69 kV | 50 | POA |
| Collector network | 8 feeder(s) · 10 kV · 7 turbines per feeder | 1 | POA |
| Collector cable | YJV 3-core 10 kV XLPE · radial feeder (per feeder) | 8 | POA |
| Collector switchgear | 10 kV feeder breaker panels · 8 feeders + PT/SPD | 1 | POA |
| Step-up main transformer | 150 MVA · 10/35 kV | 1 | POA |
| Grounding & protection | collector system earthing + relay protection 50/51/51N + metering | 1 | POA |
| Total equipment (box transformers) | POA |
📐 Single-line diagram
How it was calculated
- · Box transformer capacity: S = (P_turb × 1000) ÷ PF × margin, rounded up to a standard rating.
- · Feeder loading: each collector feeder carries a design MVA (≈30 MVA at 35 kV, ≈18 MVA at 10 kV).
- · Turbines per feeder = floor(feeder MVA ÷ box-transformer MVA), capped at ~10 for voltage reasons.
- · Number of feeders = ceil(turbines ÷ turbines per feeder).
- · Step-up main transformer rated ≥ total collector MVA at collector/grid voltage.
Referenced standards
| Standard | Scope |
|---|---|
| IEC 60076 | Power transformers |
| IEC 61400-21 | Wind energy generation systems — power quality characteristics of grid-connected turbines |
| GB/T 19963 | Technical rule for connecting a wind farm to the power system |
| GB/T 17468 | Guide for the selection of power transformers |
Frequently asked questions
What is a wind farm box transformer?
A wind farm box Transformer (or pad-mounted transformer) steps each turbine's output (typically 0.69 kV) up to the collector voltage (35 kV). It is sized to the turbine rating, usually 1.1-1.3x the generator capacity. A 3 MW turbine uses a ~3.5 MVA box transformer.
How do I size a box transformer for a wind turbine?
Box transformer MVA = turbine generator MVA plus auxiliary and margin, typically 10-30% above generator rating. A 2.5 MW generator at 0.9 pf needs about 3.2 MVA, so select 3.5 MVA. The calculator sizes the transformer and the collector feeder scheme.
What is the typical collector voltage in a wind farm?
Wind farm collector systems run at 33 kV or 35 kV (sometimes 10 kV for small farms), gathering each turbine through a box transformer then ring or radial feeders to the Substation. 35 kV is standard for farms above ~20 MW. The calculator models the collector feeder scheme.
What standards govern wind farm transformers?
Wind farm box transformers follow IEC 60076 (transformers) and GB/T 17467 for pad-mounted designs, plus IEC 61400 for the wind turbine electrical interface. They must withstand frequent load cycling and grid fault ride-through. The calculator cites these standards in its design basis.
How do box transformers handle turbine load cycling?
Wind turbines cycle between partial and full load continuously, causing thermal stress and tap-changer wear. Box transformers use vacuum tap-changers or fixed taps, low-loss cores, and are designed for frequent energization and harmonic content from converters. The calculator selects ratings with this duty in mind.
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Results are engineering estimates for preliminary design and reference only. Always verify with the applicable standards, the equipment nameplate and a licensed engineer before procurement or installation.
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