IEC 60038: Standard Voltages — The First Decision in Grid Design

Why Voltage Levels Are Not Arbitrary

If you have ever designed a substation, you know: pick the wrong voltage level, and your project cost can double. IEC 60038:2009 defines global standard voltage levels — not as a rigid list of numbers, but as the first critical decision in every power system design.

Why does China use 10 kV for MV distribution rather than 11 kV or 9 kV? Because the IEC 60038 standard sequence — 3.3/6.6/10/11/20/33/35 kV — was derived through extensive optimization of equipment manufacturing cost, line losses, and insulation coordination. Standards are not invented in a vacuum.

Voltage Hierarchy at a Glance

Classification	Nominal Un	Highest Um	Typical Application
Low Voltage	230/400 V	—	Residential and commercial
Medium Voltage	10 kV	12 kV	Urban distribution backbone
Medium Voltage	20 kV	24 kV	Industrial parks (common in Europe)
Medium Voltage	35 kV	40.5 kV	Large factories, wind farm collector lines
High Voltage	110 kV	123 kV	Urban perimeter transmission ring
High Voltage	220 kV	245 kV	Regional transmission grid
Extra-High Voltage	500 kV	550 kV	Inter-provincial trunk transmission
Ultra-High Voltage	1000 kV	1100 kV	Cross-regional ultra-long-distance transmission

U_n vs. U_m: Why the 5–15% Gap Matters

This is the single most misunderstood concept among junior engineers:

• U_n (Nominal Voltage): The system’s “name” — “this is a 220 kV substation”
• U_m (Highest Voltage for Equipment): The maximum continuous operating voltage equipment insulation must withstand

The gap exists because real grid voltages fluctuate. On an unloaded 220 kV line, the Ferranti effect can push voltage to 235–240 kV. A transformer’s on-load tap changer can further elevate the secondary side. U_m = 245 kV is not an arbitrary buffer — it is the insulation coordination baseline that surge arrester selection and BIL ratings depend on.

Quick design rule: Insulation levels are based on Um, not Un.
Example: For a 220 kV system, arrester rated voltage Ur ≥ 0.8 × Um = 0.8 × 245 = 196 kV.
But accounting for TOV during single-phase faults, typically use Ur = 200–216 kV.

Three Engineering Trade-offs in Voltage Selection

1. Power transfer capacity: A single-circuit 110 kV line has a Surge Impedance Loading (SIL) of ~30 MW. 220 kV: ~120 MW. 500 kV: ~1000 MW. Exceeding SIL requires series compensation or stepping up voltage.
2. Transmission distance: Rule of thumb — Voltage (kV) ≈ 0.5–0.6 × distance (km). To move power 200 km, you need at least 110–132 kV.
3. Short-circuit constraint: Higher voltage means higher short-circuit capacity. Connecting to a 500 kV system means circuit breakers rated for 50–63 kA breaking current — a massive cost driver.

China’s Optimized Voltage Sequence

China uses a simplified voltage sequence: 1000/500/220/110/35(或 20)/10/0.4 kV. Each step has a transformation ratio of approximately 4.5–5× — the result of extensive techno-economic optimization.

Compared to European sequences (400/220/132/33/20/0.4 kV or 380/110/20/0.4 kV variants), China’s approach eliminates one intermediate transformation stage. Every eliminated stage saves roughly 20% in substation investment and reduces losses by about 2%. The cost savings at national scale are enormous.

DC Voltages: When AC Standards Are Not Enough

IEC 60038 also covers DC voltages. Traditional LCC-HVDC (thyristor-based) uses standardized voltages like ±500 kV, ±660 kV, and ±800 kV — optimized for point-to-point bulk power transfer. VSC-HVDC (IGBT-based), however, is more flexible: commercial projects range from ±80 kV to ±525 kV.

Notably, VSC-HVDC voltages are not strictly constrained by IEC 60038 — the modular multilevel converter (MMC) architecture allows manufacturers to customize voltage levels per project requirements. This is a case where new technology is challenging old standards.

Three Costly Voltage Mistakes

1. Specifying equipment by U_n alone: GIS insulation must be rated to U_m. A “220 kV” GIS actually needs 245 kV insulation.
2. Ignoring short-circuit escalation: Upgrading a distribution network from 10 kV to 20 kV doubles capacity — and doubles fault current. Existing 16 kA switchgear becomes obsolete overnight.
3. Copy-pasting voltage levels across borders: Middle Eastern grids commonly use 13.8 kV (60 Hz). South America has 34.5 kV MV networks. Both are IEC 60038 compliant — but neither behaves like your familiar 10 kV system.

TN Lab — Standard voltages look simple. Pick wrong, and the entire project pays the price.