# Effects of low voltage on power grids?

I understand that if load is suddenly ramped up on a power grid:

1. A sudden increase in current is caused by adding these parallel loads to the circuit.
2. The current increase thus increases the torque required from spinning generation in order to maintain the speed required for its designed voltage and frequency output. For example a generator designed to output 32 kV at 3600 RPM will see a voltage drop if the speed (and thus Hz) drop.
3. Since the generator cannot immediately increase its torque there is a brief slow down, which in turn causes a frequency drop and thus a voltage drop.

My question is what some of the dangers of this drop are for the grid? How can this lead to a cascading problem (a blackout)?

• Capacitor banks used for voltage support drop off per $$\Q=\frac{V^2}{X_C}\$$ so a 10% drop in voltage means the cap bank will put out 19% less reactive var support. Here is good overview by Carson Taylor on how this affects the "nose curve" used in voltage stability analysis.