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EU residents might have noticed that some of the clocks in their house have been running out of sync with other clocks. Apparently, the cause for this is that power suppliers had to reduce the frequency at which the AC network alternates the current flow. I understand that the speed at which these clocks tick is based on the frequency of the AC network, so thus far it makes sense. The thing I don't understand is that according to reports, the reason the power suppliers had to reduce the frequency was because a regional provider in the Balkan did not provide their fair share of power to the grid. I read the article of the Swiss power grid website at https://www.swissgrid.ch/swissgrid/en/home/experts/topics/frequency.html which states:

If the consumption of electrical power is lower than production, the frequency is higher; if consumption is higher than production, the frequency is lower. The reason for this is as follows: the electrical generators of an electricity grid rotate more readily and faster when consumption is low. Consequently, they rotate with a higher frequency. Conversely, the electrical generators rotate more laboriously and with a lower frequency when consumption is greater.

So the generators have to rotate at a slower pace if demand is greater than supply. Why? I mean, I don't even understand how the grid remains functional if demand is greater than supply. usually if demand is greater than supply, part of the demand doesn't get fulfilled. I might misunderstand how the power grid as a whole deals with a situation like that. I especially don't understand what the link is between the undersupply on the grid and the mechanical speed of the generators. I know basically nothing about the power grid, so that doesn't help either.

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    \$\begingroup\$ Because the extra load on them slows them down, just like riding a bicycle uphill. \$\endgroup\$ – Brian Drummond Mar 7 '18 at 12:55
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    \$\begingroup\$ Someone's up to date with their hackernews \$\endgroup\$ – Makoto Mar 7 '18 at 12:56
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So the generators have to rotate at a slower pace if demand is greater than supply. Why?

That is not exactly true. Power torque X speed. To increase power it is not necessary to increase speed; torque can be increased. Torque is increased by increasing the throttle setting. All generation systems have some kind of throttle mechanism. However, the operation of the throttle is not instantaneous. That leads to a brief reduction in speed when the load (demand) increases. As the various generators connected to the grid slow down, their throttles are automatically adjusted to correct the speed decrease. Electrical grids generally have enough reserve capacity to handle the increased demand. If there is not enough reserve, the voltage and frequency drop may persist for an longer time.

When the demand drops, the generators will run faster than normal for a brief period. That would tend to correct the clocks so that there would be little or no net error over the course of 24 hours. However, if the grid capacity is not sufficient, extended periods of reduced voltage and frequency will result.

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So the generators have to rotate at a slower pace if demand is greater than supply. Why?

Mechanical power into a generator is torque x rotational speed so, if the output electrical power is too high you can reduce that output power by slowing down the rotational speed.

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  • \$\begingroup\$ I mean, isn't the output in this case too low because the demand outpaces supply? \$\endgroup\$ – Nzall Mar 7 '18 at 13:08
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    \$\begingroup\$ Demand cannot outpace supply. If the loading gets too great then something has to give and to avoid serious mechanical or electrical failure, the problem of supplying too much mechanical torque is solved by reducing speed (which also reduces torque). \$\endgroup\$ – Andy aka Mar 7 '18 at 13:10
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    \$\begingroup\$ @Andyaka "Demand" can be larger than supply. "actual consumption" has to be equal to the supply. If demand is larger than supply, then everybody gets a bit less than what they demand. \$\endgroup\$ – asdfex Mar 7 '18 at 13:34
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    \$\begingroup\$ But electrical power from a generator is voltage x current. While it's true that voltage is in turn proportional to rotation speed, there are other ways to influence voltage (generator field winding current, transformer taps). Why doesn't the grid only reduce the voltage (independently of the frequency), so that the frequency deviation is minimized? \$\endgroup\$ – jms Mar 8 '18 at 12:10
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    \$\begingroup\$ @jms Yes that is of course true but if you reduce frequency and the majority of your loads (big factores and plants) are 3 phase induction motors then reducing F reduces the speed of those armatures and mechanical output power is reduced. It's a balancing act - there will be plenty of loads that have control loops that will correct a lowering of frequency or voltage to maintain output power but most (greater than 50%?) won't, and the small reduction in speed will be unimportant. \$\endgroup\$ – Andy aka Mar 8 '18 at 12:23
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If demand outpaces supply, the gernerators slow down and eventually come to a halt within a couple of minutes. This has not happend yet because there are emergency procedures in place to prevent it.

Normally, each power plant operator produces exacly the amount of power their customers ordered. In this case, demand equals supply and the grid operates at a nominal 50Hz.

If someone draws more power than they ordered, a cycle commences:

  1. Demand outpaces supply, so the generators start to slow down.
  2. Once the generators slowed down to 49.95Hz or less, emergency procedures kick in. Sirens go off in all power plants, people shout commands and more coals are shoveled into the ovens to protect the grid.
  3. The increasing supply of emergency power matches demand. The generators accelerate back to 50Hz over the course of some minutes.
  4. Once frequency is back up, emergency procedures cease and power plants return to normal operation.
  5. Rinse and repeat.

During a cycle, the grid oscillates between 50Hz and normal power and 49.95Hz and emergency power. Average power always equals demand and average frequency will be slightly below 50Hz. The actual averaging is done by the momentum of the generators.

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Grid frequency fluctuates slightly according to demand So at any moment, the frequency will be within a tolerance set.

The cummulative difference is compared with standard clock and corrected by adjusting power generated to compensate for the electric clock (synchronous).

So, there should not be any noticebale time gain/loss on the clock face, except total power black out.

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A question that needs to be addressed is: "why are the electrical providers allowing their customer's consumption to out-pace their production capability?"

Usually electrical utilities have arraignments with other (nearby) utilities to buy their excess power when some of their own generating capacity is off line. (due to failure or maintenance)

I recommend buying a battery powered clock or two for your living space or use a cell phone's time as your standard. (which are set by cell tower from GPS satellites)

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