# In the USA, the grid transmits power at/around 60Hz. How big of a frequency deviation can the grid handle before relays trip and cause a blackout?

I understand that small frequency deviations will result in automatic load shedding or other control actions to restore system frequency, but when does a blackout occur? When the deviation exceeds + or - .05Hz? + or - 0.1Hz?

Power IT Labs has posted numerous simulations that are cool, but I've struggled to fully understand them.

• Sorry, my lack of understanding is obvious. So technically, the entire grid could operate at 30Hz, it's just the end-use electronics are designed for 60Hz? And if you try to reconnect a generator operating at anything but 60Hz, it will trip the relays? Jun 22, 2017 at 21:00
• Have you tried wikipedia. Don't limit yourself to articles about the US or 60 Hz. Jun 22, 2017 at 21:03
• Transformers and motors are designed to be most efficient at some particular frequency, but probably could tolerate a lot of variation. Many motors run at a speed that is determined by the power-line frequency. How much variation they could tolerate would be application dependent. Modern (non-transformer) electronic power supplies usually are designed to operate over a range of input frequencies anyway. Jun 22, 2017 at 21:08
• If you try to connect a generator that is not spinning at the same speed, and also in phase you will trip relays. Or, if you try to re-connect two disconnected parts of the grid and they have gotten more than a tiny bit of phase with each other, that will trip something too. Jun 22, 2017 at 21:10

Short answer: The range is usually held within ±0.5%, so its from 59.7Hz to 60.3Hz for a 60Hz grid.

Long answer: Frequency is regulated tightly because it's how the overall load in the grid is controlled. If there's a runaway to lower frequencies, that usually means there is a short-circuit near a major power station or hub. So that will drop out soon. Then there usually is a runaway to higher frequencies because of the dropped load. So things escalate very quickly if the grid has many gigawatts power stations. And you don't want that.

In weak grids on the contrary, the frequency may swing much more. Eastern European countries typically allowed 47Hz to 53Hz. That's acceptable if there's only a few power stations and a few big consumers. The same for emergency generators and isolated grids on islands.

What limits the frequency downwards is power conversion in transformers and AC motors. The lower the frequency is, the lower the primary voltage needs to be not to overexcite the iron parts by the magnetic fields. So a substantial frequency drop has to be accompanied by a voltage drop too, and that's what is done in weak grids to allow lower frequencies.

I may be corrected by someone with better understanding of grid control but it may help to consider this from another angle: what causes the frequency to drop?

The answer, generally, is that power demand is exceeding the combined generator capability. This means that excessive current is being drawn. Since generation is distributed around the grid it is likely that one generator somewhere will have a high local load which exceeds its circuit breaker trip setting - particularly if it is a small generator. Once that trips the others try to take up the load and a cascade of trips may occur.

If load shedding is effected before trips occur then obviously the demand will reduce and frequency can be maintained with the acceptable frequency limits.

Frequency is generally allowed to drop a little during peak demand and then run a little higher than nominal during off peak to keep the overall cycles per day correct so that synchronous clocks are kept within a minute or so of correct time over many, many years.

• Interesting. This is not a topic I know much about but I am interested in learning why the frequency drops with an increased load Jun 23, 2017 at 4:46
• It's for the same reason your push-bike drops speed when it reaches a hill - the engine (you) can't provide the power for the increased load. Try it in a car: turn the engine on and let it idle until revs are steady. Then turn on all the lights and you will find that the increased load on the alternator causes the engine revs to drop a little. You can fix this by increasing the throttle a little. If you keep increasing the load on the grid then at some point you literally "run out of steam" and can increase the power no further. Jun 23, 2017 at 5:53