Apparatus and Procedure for Calculating Short-Circuit Impedance to Determine Bus Fault Currents and Smart Inverter Settings

This paper describes a new procedure and test apparatus that accurately calculates the short-circuit resistance and inductance to the energized bus where measurements are taken. The calculated impedance can be used to predict the available bus fault current to properly size protection devices and inform of arc flash hazards. The calculated impedance can also be used to determine power factor settings to reduce adverse voltage impacts from distributed energy resources such as photovoltaics. The novelty of this apparatus is that it uses a diode-bridge rectifier load that draws less than 0.5% of the available bus fault current to make the required measurements. The apparatus is tested and validated on 120Vac and 277Vac circuits at the Electric Power Research Institute (EPRI) in Knoxville, TN.