Effects of low voltage on power grids?

Question

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)?

Answer 1

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)?

• Under voltage will cause induction motor load to start drawing excessive reactive (var) from the system. Further exacerbating the under voltage condition and possibly leading to voltage collapse.
• 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.
• Tripping of elements during a stressed condition (e.g. generators tripping or lines tripping) overloads remaining lines and can lead to further tripping. NERC PRC-23 (for transmission line protection) and NERC PRC-025 (for generators) now require relays to be much more resistant to tripping on load as compared to pre-2003 (in North America). Here is a good read on the specific topic of distance relays (introduction is good short read).

Answer 2

If you read into major blackouts in history (which have been attributed to cascading effects) - e.g. the 2003 Italy blackout or the 2003 US Northeast blackout, you'd see that the common pattern is that the changes to the grid load were too quick for reactive action. It's not that the undervoltage or underfrequency itself was the problem, because you can always shed load as to restore the balance between generation and consumption, and provision enough spare capacity to cope with yet more failures should they happen to arise. However if you don't act quickly enough, the load that has been shifted to other power lines or generating stations can cause further problems, exacerbating the issue.

Also see this: If a large AC generator is overloaded, will it lose frequency or voltage?

I think most power plants are constructed in a way that the electrical generator is undersized with respect to the prime mover (likely because you don't want to be able to stall it at full power!). So you're more likely to lose voltage than speed unless the overloading of the grid is particularly severe.

Yet another aspect is that not all power input to the grid is via synchronous generators. Solar power uses inverters. You may want to dismiss them as insignificant, but they are a significant % of generation in some countries. In yet another countries you may have HVDC links to neighbouring grids, and if you view these as a "power producer", their generation can also be quickly ramped up without losing frequency (whether the other grid will like it is a different story though, you only see the DC so you don't care).