Transformer Impedance & Short Circuit Calculator
Understanding Transformer Impedance Calculation
Transformer impedance, typically expressed as a percentage (Z%), represents the ratio of voltage drop across the transformer windings at full load compared to the rated voltage. This parameter is critical for electrical engineers to determine short-circuit current ratings, protective device coordination, and voltage regulation.
How the Calculation Works
To convert the nameplate percentage impedance to actual Ohms (Ω), we use the following steps:
- Calculate Rated Current: For a 3-phase system, I = kVA / (√3 × kV).
- Calculate Base Impedance: Zbase = Vphase / Iphase or kV² / MVA.
- Calculate Actual Ohms: Zohms = (Z% / 100) × Zbase.
Why Is Transformer Impedance Important?
The impedance value determines how much current will flow during a fault. A lower impedance results in higher short-circuit currents, which may require more expensive circuit breakers with higher interrupting capacities. A higher impedance reduces short-circuit current but causes a larger voltage drop when the transformer is under heavy load.
Example Calculation
Consider a 1000 kVA, 3-phase transformer with a 480V secondary and 5.75% impedance:
- Rated Current: 1000 / (1.732 * 0.480) = 1,202.8 Amps.
- Short Circuit Current: 1,202.8 / (5.75 / 100) = 20,918 Amps.
- Ohmic Impedance: (0.0575 * 0.480²) / 1.0 MVA = 0.0132 Ω.