Calculates unsymmetrical fault currents by interconnecting symmetrical sequence networks: SLG (networks in series), LL (positive and negative in parallel), and LLG (positive in series with parallel combination of negative and zero). Evaluates healthy phase voltage rise under ground fault conditions.
Governing Formulas & Standards
Standards Basis: IEC 60909-0 / IEEE Std 141 / Anderson Analysis of Faulted Power Systems
I_{a1}^{\text{SLG}} = \frac{E_a}{Z_1 + Z_2 + Z_0 + 3Z_f}, \quad I_{a1}^{\text{LL}} = \frac{E_a}{Z_1 + Z_2 + Z_f}, \quad I_{a1}^{\text{LLG}} = \frac{E_a}{Z_1 + \frac{Z_2(Z_0 + 3Z_f)}{Z_2 + Z_0 + 3Z_f}}
Applies classical sequence network interconnection boundary equations for each unsymmetrical fault type.
Worked Engineering Example: Single Line-to-Ground (SLG) Fault on 11 kV Feeder
- Sum Sequence Loop Impedances: Z_sum = Z1 + Z2 + Z0 = (0.12+0.12+0.35) + j(0.85+0.85+2.40) → 0.59 + j4.10 Ω (|Z| = 4.14 Ω)
- Compute Sequence Current (I1 = I2 = I0): I1 = 6351 V / 4.14 Ω → 1.534 kA
- Compute Fault Current (Ia = 3·I1): Ia = 3 × 1.534 kA → 4.60 kA RMS
Final Solution: Fault Current: 4.60 kA RMS | Fault MVA: 87.7 MVA | Healthy Phase Voltage: 1.02 pu
Frequently Asked Questions
- Why do healthy phases experience overvoltage during an SLG fault?
- In an ungrounded or high-impedance grounded system, the neutral shifts to the faulted phase potential, causing the healthy phases to experience full line-to-line voltage (1.732 times normal line-to-neutral voltage) with respect to ground.
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