Sizes interconnected star (zigzag) earthing transformers used to establish an artificial neutral for ungrounded delta systems, calculating zero-sequence impedance from factory single-phase injection test measurements and verifying thermal withstand.
Governing Formulas & Standards
Standards Basis: IEEE Std C57.32 / IEC 60076-1 / IS 3043
Z_0 = 3 \times \frac{V_{test}}{I_{test}} \quad ; \quad I_N = \frac{3 \cdot V_{LN}}{Z_0 + 3 R_N}
Because zero-sequence current splits equally into 3 parallel phase windings, the per-phase zero-sequence impedance equals 3 times the measured test impedance.
Worked Engineering Example: 33 kV Delta System Zigzag Earthing Transformer Test Evaluation
- Zero-Sequence Impedance per Phase: Z_0 = 3 × (415 / 52.0) = 3 × 7.98 Ω → 23.94 Ω
- Prospective Ground Fault Current: I_f = 3 × (33000 / √3) / 23.94 = 57157 / 23.94 → 2,387 A
Final Solution: Zero-Sequence Impedance: 23.94 Ω/phase | Earth Fault Current: 2,387 A
Frequently Asked Questions
- How does a zigzag transformer provide a neutral without drawing high continuous current?
- Each phase of a zigzag transformer has two halves wound on different core limbs in opposite directions. For normal balanced positive-sequence voltages, the flux vectors cancel out, resulting in high magnetizing impedance and negligible idle current.
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