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IEC 60044-2: Inductive Voltage Transformers — Specification and Testing
Metering VT vs. Protection VT: Same Form, Different Performance
IEC 60044-2:2003 specifies inductive voltage transformers (VTs). A common engineering mistake: using a metering-class VT for protection, or vice versa. They look identical on the outside but are optimized for completely different voltage ranges.
| Type | Accuracy Class | Key Requirement |
|---|---|---|
| Metering VT | 0.1 / 0.2 / 0.5 | Precision near rated voltage (80–120% Un) |
| Protection VT | 3P / 6P | Accuracy under fault voltage sag (5–150% Un) |
The critical distinction: Metering VTs guarantee accuracy only near rated voltage. Protection VTs must maintain accuracy from 5% to 150% Un — because distance protection relays need correct voltage phasors during severe sags to determine fault direction. A metering VT at 10% voltage may produce severe phase-angle errors, causing relay misoperation.
Standard secondary voltages: phase-earth 100/√3 ≈ 57.7 V; phase-phase 100 V. A common mistake is specifying 110/√3 V — a legacy holdover from older standards, incompatible with modern digital relays.
TN Lab — VT accuracy class is just the beginning. The voltage range over which accuracy is guaranteed is what really matters.
