Short circuit ratio

[klapton]

Hello

I understand that strength of a power grid can be defined by the respective short circuit ratio or SCR. A lower SCR (i.e. 'weak grid') implies a high impedance grid which means fluctuation in current will cause voltage to change more so than a higher SCR (i.e. 'strong grid'). A weak grid also means protection may not operate correctly as fault levels may be too low.

I have been looking at large utility scale solar inverters and often they specify what the minimum SCR is for their inverters to operate.

What does this minimum inverter SCR mean and what is implied by it?

 

[cranky108]

Inverters are current source devices, and under a fault will self limit current output, thus if all you have on a grid is limited current sources, a fault will cause the voltage to collapse, and the inverter will no longer be able to follow the grid frequency (a heartbeat), and will shut off.

The requirements for power generators, including grid sized inverters, is to "ride through" a fault, but that is impossible if the voltage drops too low during a fault, and the inverter can not find the grid heartbeat.

A rotating generator acts differently, in that it will not self limit beyond the R and X values, and will not shift a great deal during such a fault, and if it does shift, as long as it is within some limits it will re-establish it's connection to the grid. Said another way, it will establish its own heartbeat, until it can re-sync with the grid.

I don't know that I can say it any better.

 

[xnuke]

There's a pretty good NERC document on inverter-based source integration here that discusses problems associated with low SCR.

xnuke
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