Transformer Impedance Selection: Complete Technical Guide
What is Transformer Impedance?
Impedance voltage (Uk%) expresses the transformer's internal impedance as a percentage of rated voltage. It directly affects short-circuit current levels, voltage regulation, and system coordination. Standard values range from 4% to 10% depending on capacity and application.
Standard Impedance Values
| Capacity (kVA) | Typical Uk% (Distribution) | Typical Uk% (Power) |
|---|---|---|
| ≤ 630 | 4% | — |
| 800–1600 | 4.5–5% | — |
| 2000–3150 | 5–6% | 6–7% |
| ≥ 4000 | 6–7% | 7–10% |
How Impedance Affects Your System
Lower Impedance (4%)
- Pros: Better voltage regulation, lower voltage drop under load, higher efficiency
- Cons: Higher short-circuit current — requires higher-rated switchgear
Higher Impedance (6–8%)
- Pros: Limits short-circuit current, lower stress on downstream equipment
- Cons: Higher voltage drop (poorer regulation), slightly lower efficiency
Special Applications
- Generator Step-Up: 8–10% impedance recommended to limit fault current contribution from generators
- Parallel Operation: All paralleled transformers must have matching Uk% (within ±10%) for proper load sharing
- Motor Starting: Lower impedance (4%) preferred to minimize voltage dip during large motor starts
Short-Circuit Calculation
Formula: Isc = (I_rated / Uk%) × 100
Example: 1000kVA, 400V secondary, Uk = 5% → I_rated = 1443A → Isc = 1443 / 0.05 = 28,860A
Your downstream switchgear must be rated above this short-circuit current. Contact our team for impedance recommendations based on your single-line diagram.
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