Blackouts occur when the grid stops sending any voltage your...

Blackouts occur when the grid stops sending any voltage your way. "Brownouts" occur when the grid sends reduced voltage (usually deliberately). You still receive power, but might be 5-15% below the rated level (e.g. 220v rather than 240v).

Brownouts produce duller lighting (lower lumens), but even a small voltage reduction can cause greater problems with electrical equipment; some shut down completely below a specific voltage.

I assume that brownout power from the grid is fine for some plant requirements (e.g. lighting), but the back-up generators might need to completely replace the grid power for much equipment during brownouts.

But for RED, I think the greater problem is that the 16Kv line doesn't deliver enough power even in normal power situations, so the back-up generators continuously or often fill in the shortfall. And when there is a brownout or blackout, the back-up generators are even more in demand.