Short-circuit ratio

The short-circuit ratio (SCR) is a figure for how strong the grid is where an inverter plant connects. It is usually stated at the point of interconnection or at the inverter AC terminals. At very low SCR values, the impedances of the inverters, the transformer and the grid can combine to cause resonance on the AC voltage waveform. If the resonance gets strong enough, an inverter such as the XGI 1500 sees the grid voltage as too high and shuts down safely. To prevent this, the manufacturer limits how many inverters can share one point of interconnection on a weak grid. Its configuration tables cover SCR scenarios from 2.0 to 6.0. Some central inverter datasheets give a minimum short-circuit ratio at the AC terminals. For the SMA Sunny Central UP-US it is greater than 2. SCR is closely tied to grid impedance: the short-circuit power available from the grid at the medium-voltage connection is used to work out the grid impedance on the low-voltage side of the transformer.

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• sentence 1: support check: partial - The passages show SCR stated at the POI and at AC terminals, but never define SCR as a measure of grid strength.
• sentence 5: support check: partial - The >2 minimum SCR at the AC terminals is stated, but the passages don't name the model as SMA Sunny Central UP-US or say it is a feature of some central inverters.
• sentence 6: support check: partial - u32 shows grid short-circuit power at the MV connection used to compute LV-side grid impedance, but it doesn't mention SCR or say they are closely tied.
• sentence 1: review: partial - The passages show SCR stated at the POI and at the AC terminals, but they never describe SCR as a measure of grid strength. -->