Replacing a transformer with the same kVA and voltage ratings but a lower %Z has which impact on available fault current downstream?

Replacing a transformer with the same kVA and voltage ratings but a lower %Z has which impact on available fault current downstream?

• A. Fault currents are decreased
• B. No change
• C. Fault currents are increased
• D. It depends

Answer: (C)

The amount of fault current flowing downstream is set by the impedance in the fault path. Replacing the transformer with the same kVA and voltage but a lower percent impedance means the transformer presents a smaller impedance to a fault. Since fault current is roughly the system voltage divided by the impedance in the fault loop, lowering that impedance causes the available fault current to rise. So, the downstream fault currents increase. The other possibilities don’t fit because increasing impedance would reduce fault current, unchanged impedance would keep it the same, and “depends” isn’t accurate here since the change is directly determined by the percent impedance.

The amount of fault current flowing downstream is set by the impedance in the fault path. Replacing the transformer with the same kVA and voltage but a lower percent impedance means the transformer presents a smaller impedance to a fault. Since fault current is roughly the system voltage divided by the impedance in the fault loop, lowering that impedance causes the available fault current to rise. So, the downstream fault currents increase. The other possibilities don’t fit because increasing impedance would reduce fault current, unchanged impedance would keep it the same, and “depends” isn’t accurate here since the change is directly determined by the percent impedance.