Fault Level Calculation
Understanding fault level calculations is critical for electrical system design and protection. This guide explains how to calculate fault currents considering transformer impedance and cable impedance in electrical distribution systems.
Basic Parameters
- Base kVA:
- Reference power for per-unit calculations
- Base Voltage:
- System nominal voltage (V)
- Base Current:
Ibase = (Base kVA × 1000) / Base Voltage- Base Impedance:
Zbase = Base Voltage / Base Current
Calculation Steps
- 1.Cable Impedance (Zc):
Zc = Distance × Cable Volt Drop Rating / 1000 - 2.Total Impedance (Ztotal):
For transformers in parallel, use worst case (2 in parallel):
Ztotal = ZT / (ZT² + Zc) - 3.Fault Current:
Ifault = Ibase / Ztotal
Key Formulas:
Power of TX: P = Base kVA × Number of Transformers
Transformer Impedance (p.u.): ZT = %Z / 100
Cable Impedance (p.u.): Zc(p.u.) = Zc(Ω) / Zbase
Worked Example
This example demonstrates a fault level calculation for a system with two 1600 kVA transformers operating in parallel at 400V, with 6.0% impedance and a cable run of 147m.
Given Parameters:
1) Cable Impedance, Zc = Distance × Cable Volt Drop Rating
= 147 × (0.15 / 4 / 1.732) / 1000
= 0.00318 Ohms
= 0.0318 p.u.
2) Total Impedance, Ztotal = ZT1 / (ZT2 + Zc)
The equivalent impedance for the transformers shall be taken as the worst case of 2 in parallel operation. (ZT1 / (ZT2+Zc))
= 0.06 / (0.06 + 0.0318)
= 0.06 / 0.0918
= 0.036 p.u.
3) Fault Current = Ibase / Ztotal
= 2309.401 / 0.036
= 64.15 kA
Result:
Ifault = 64.15 kA
Important Notes
- For parallel transformers, use the worst-case scenario (2 in parallel operation) as shown in the formula: ZT / (ZT² + Zc)
- Cable impedance must be converted to per-unit values using the base impedance
- The fault current calculation assumes a bolted three-phase fault at the point of interest
- This calculation is essential for selecting appropriate protective devices and ensuring system safety