In my previous article, I have provided the different formula for calculating fault currents in different fault scenarios. We will put that into the application at this time.

A utility has given these values to our design team:

Voltage: 11kV
3-Phase Fault Current: 2424 A
Phase-ground Fault Current: 294 A

System Impedances:
Z_{1} = {1.06} + j{2.4}~Omega
Z_{0} = {2.55} + j{16.30}~Omega

From the above values, whichever values your use, current or impedance, your calculation will provide the same result. To verify this,

Three Phase Fault:
I_{3-phase-fault} = {V_{an}} / {Z_{1}}

I_{3-phase-fault} = {lbrace 11000/sqrt{3} rbrace} / {1.06 + j{2.4}}

I_{3-phase-fault} = {6350.85} / {2.62< 66.17^o}

I_{3-phase-fault} = 2423.988~A~ or~ 2424~ A~ (as~ provided~ by~ the~ utility)

Phase-to-Ground Fault:
I_{phase-to-ground-fault} = {V_{an}} / {lbrace{Z_{1}} + { Z_{2} + Z_{0} }rbrace}

Note: Since we are distant from the generator, Z_{1} = Z_{2}.

I_{phase-to-ground-fault} = { lbrace 11000 / sqrt{3} rbrace } / { lbrace (1.06 + j{2.4}) + (1.06 + j{2.4}) + (2.55 + j{16.30}) rbrace }

I_{phase-to-ground-fault} = { 6350.85 } / { (4.67 + j{21.1}) } = 6350.85 / {(21.610 < 77.52^o)}

I_{phase-to-ground-fault} = 293.884~A~ or~ 294~ A~ (as~ provided~ by~ the~ utility)

Author

  • A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia, and Professional Electrical Engineer (PEE 2574 - 1st Place April 1991) - Philippines with extensive experience in concept selection, front-end engineering, HV & LV detail design, construction, and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and wastewater pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).

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