PV system/available fault current

[Dsg319]

I assume available fault current calculation is rarely done in the residential setting. But just out of curiosity I began to wonder about you guys who install PV systems(never have). How do you guys add the PV contribution into the calculation?

 

[Carultch]

I assume available fault current calculation is rarely done in the residential setting. But just out of curiosity I began to wonder about you guys who install PV systems(never have). How do you guys add the PV contribution into the calculation?

The short answer is that it is usually insignificant.

On the DC side, this is the short circuit current (Isc). The limiting case on the I-V curve, where V=0. You would add the 125% enhancement factor to account for irradiance exceeding standard test conditions.

On the AC side, look for the term "maximum fault contribution current" if this information is available in the inverter product documentation. It is common for utilities to ask for this information on the applications, because a source with rotating machinery has significantly higher fault currents than its operating current. For other kinds of sources like generators, you could expect this to be tens or even hundreds of times the full load amps of the source. This is not the case with inverters, and the maximum fault contribution current might even be as low as the inverter's own full load amps.

 

[WasGSOHM]

Or you can load the system until you get a 1% drop in voltage and use a formula to calculate source resistance.
No guarantees as to how close this value will be to any other posted value.

 

[Carultch]

Or you can load the system until you get a 1% drop in voltage and use a formula to calculate source resistance.
No guarantees as to how close this value will be to any other posted value.

Inverter output circuits don't really work that way. The concept of source resistance won't work the same as it would for other sources. This is a device that continuously monitors the voltage of the existing grid, and uses a feedback loop to sense the resistance of the output circuit. This feedback loop solves the algebra problem of Ohm's law to determine the necessary voltage the inverter needs to produce. It generates a waveform slightly higher in voltage from the existing grid, and synchronized with the grid voltage waveform, in order to be large enough that it can push its power from the output terminals to the point of interconnection.

 

[WasGSOHM]

So you'd measure zero source impedance or some meaningless value.

 

[Carultch]

So you'd measure zero source impedance or some meaningless value.

I don't know what the inverter manufacturers do behind the scenes to determine the "maximum fault contribution current". I've seen some inverters where this value directly copies the maximum output current. I've seen other inverters where it is about 10% higher than the max output current. Perhaps they have a way of disabling the internal relay and current-limiting circuitry, so it can be tested on a controlled fault in a circuit to measure the extreme case.

In any case, if you tried to operate an grid-tied inverter as it is sold to the installer, you would be unable to do so without first connecting it to a working grid that is within the programmed tolerances of the nominal voltage and frequency (usually the IEEE standard values). If you short circuit the inverter's output, it would not turn on in the first place. The source impedance you'd measure as you compare its voltage drop to its current, would primarily be the ohmic resistance of the AC circuit. Voltage drop on an inverter output becomes voltage rise, as the unit compensates for the ohmic resistance of the output circuit.

 

[Hv&Lv]

The short answer is that it is usually insignificant.

On the DC side, this is the short circuit current (Isc). The limiting case on the I-V curve, where V=0. You would add the 125% enhancement factor to account for irradiance exceeding standard test conditions.

On the AC side, look for the term "maximum fault contribution current" if this information is available in the inverter product documentation. It is common for utilities to ask for this information on the applications, because a source with rotating machinery has significantly higher fault currents than its operating current. For other kinds of sources like generators, you could expect this to be tens or even hundreds of times the full load amps of the source. This is not the case with inverters, and the maximum fault contribution current might even be as low as the inverter's own full load amps.

We dont consider max fault current from a PV system. It’s generally considered to be what the invertor max output is. And thats with full sun.. If it’s at capacity there isn’t rotational inertia to produce fault current.

 

[WasGSOHM]

Today I have learned something.

"Evaluating rotational inertia as a component of grid reliability with high penetrations of variable renewable energy"

 

[Dsg319]

Thanks everyone. Was just a random curiosity.