Comment by cesarb
5 years ago
The key word is "suddenly".
In the electricity grid, demand and generation must always be precisely matched (otherwise, things burn up). This is done by generators automatically ramping up or down whenever the load changes. But most generators cannot change their output instantly; depending on the type of generator, it can take several minutes or even hours to respond to a large change in the demand.
Now consider that, on modern servers, most of the power consumption is from the CPU, and also there's a significant difference on the amount of power consumed between 100% CPU and idle. Imagine for instance 1000 servers (a single rack can hold 40 servers or more), each consuming 2kW of power at full load, and suppose they need only half that at idle (it's probably even less than half). Suddenly switching from idle to full load would mean 1MW of extra power has to be generated; while the generators are catching up to that, the voltage drops, which means the current increases to compensate (unlike incandescent lamps, switching power supplies try to maintain the same output no matter the input voltage), and breakers (which usually are configured to trip on excess current) can trip (without breakers, the wiring would overheat and burn up or start a fire).
If the load changes slowly, on the other hand, there's enough time for the governor on the generators to adjust their power source (opening valves to admit more water or steam or fuel), and overcome the inertia of their large spinning mass, before the voltage drops too much.
>generated; while the generators are catching up to that, the voltage drops, which means the current increases to compensate...
Close- you won't see an increase in load of a synchronous machine operating at constant throttle manifest as a voltage sag, you'll see it manifest as a decrease in frequency (this generators literally slow down, like a guy on a bike going uphill). Voltage sags are more related to transmission line phenomenon.
I get that lots of servers can add up to lots of power, but what is a "lot"? Is 1MW really enough demand to destabilize a regional power grid?
No. All balancing authorities are required to keep a certain amount of "spinning reserve" available for fast adjustments like this. But if I do it and the next guy does it and a transmission like is down and...etc
A lot of horror stories start that way.
If it's all at once at the end of one leg and unplanned? Yes.
The question is somewhat similar to a thought experiment. If a ship is docked and loading cargo, is it a good idea to use all the cranes to suddenly fill up one outer side of the ship?