<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>pjc50</author><text>Synchronisation is purely local. They are, from the point of view of a sky observer, out of phase by the amount you say, but that doesn&amp;#x27;t matter. Each generator need only sync up at the time of connection.&lt;p&gt;Imagine you are on half a tandem bicycle. You are in a room with the chain of the tandem vanishing beyond the wall. You can&amp;#x27;t see, but clearly someone else is pedalling the thing as the pedals are rotating. Getting on it is tricky, as you have to rotate your feet at the right speed, but once you&amp;#x27;re on you can sit there and let it carry them round - or you can start applying pressure through your feet to do work and accelerate the chain system.&lt;p&gt;At this point you discover the thing on the other end of the chain is not a person but a 60Hz synchronous motor-generator. Congratulations, you are in sync with the grid and (when pushing on the pedals) contributing power.&lt;p&gt;(A corollary of this is that if your grid connection is down or the grid is split into two pieces, you can&amp;#x27;t start up again until you get a grid input to know you&amp;#x27;re in phase. See &amp;quot;black start&amp;quot; for details on this.)</text><parent_chain><item><author>CompelTechnic</author><text>I have a question about the power grid I&amp;#x27;ve never gotten a satisfactory answer to.  I hope someone here can help.&lt;p&gt;If the USA&amp;#x27;s power grid is about 4,000,000 meters wide, and the electric signal propagates at roughly the speed of light (3 x 10^8 m&amp;#x2F;s) (&lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;physics.stackexchange.com&amp;#x2F;questions&amp;#x2F;358894&amp;#x2F;speed-of-light-vs-speed-of-electricity&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;physics.stackexchange.com&amp;#x2F;questions&amp;#x2F;358894&amp;#x2F;speed-of-...&lt;/a&gt;), then it will take roughly .013 seconds for the electrical signal to reach from one station to another. This is a lot out of phase! (the period of 60 Hz is .016 sec)  How is this managed?&lt;p&gt;If two generating stations on opposite ends of the country are both contributing to the electrical signal, how is this time lag accounted for?  I have two guesses:&lt;p&gt;1.  The power network is &amp;quot;mapped&amp;quot; such that there is one central generating station and all other generating stations are time lagged based on their graph distance from this generating station&lt;p&gt;2.  All stations try to generate simultaneously, with their output signals interfering with eachother to a certain degree.  This might not matter much because their local loads dampen the strength of their signal.&lt;p&gt;Either way seems to introduce inefficiencies.</text></item><item><author>Slartie</author><text>&amp;gt; Apparently to match the demand at given instance electricity plants rotate their turbines a bit faster or a bit slower instead of switching a complete plant on and off, which results in slight deviations from the 50Hz standard but it is fine as long as it stays between the limits.&lt;p&gt;This is slightly imprecise. It actually works the other way round: the demand is &amp;quot;communicated&amp;quot; through the network by slight changes in the frequency.&lt;p&gt;This can be described best for the example of steam-based generators, as they are typical for the majority of coal&amp;#x2F;gas&amp;#x2F;oil&amp;#x2F;nuclear power plants. When demand for electricity grows, for example because a new factory is powered up, the turbines in the power plants are experiencing larger forces attempting to slow them down, starting with the power plant closest to the new factory.&lt;p&gt;The speed (and thus the frequency) is constantly measured, and if it drops, the steam valve is opened just a tiny bit more to counteract the drop. If this is not possible because the power plant is already at maximum output, either other power plants have to take the load (and the electricity is then routed through the network to the factory) or additional power plants may have to be added to the network.&lt;p&gt;The important thing is: the frequency change is a means of communication, it originates from demand changes, and because the entire network is kept in sync, it works to communicate demand changes across the entire network. Power plants providing energy to a sub-part of the entire network always attempt to run their turbines with the exact same speed of 50 rotations per second, which should ideally always conform to the frequency in all other sub-networks that a specific sub-network is attached to.&lt;p&gt;If just one of these sub-networks does not perform its duty of re-establishing 50 Hertz by powering up their own plant output, the rest of the network must either channel enough power into this sub-network to allow its local generators to re-establish 50 rotations per second by taking some of the load off of them or - if that is not possible or decided against - must also deliberately drop their frequency (and thus the rotational speed of their generators) to match the sub-network that is deviating.&lt;p&gt;The only alternative to this would be to drop off the deviating sub-network entirely, but that is usually only done in extreme cases of deviation.</text></item><item><author>mrtksn</author><text>The short version of the story is that grid electricity in Europe is produced by rotating the turbines 50 times a second and that&amp;#x27;s where the 50Hz AC electricity comes from.&lt;p&gt;Apparently to match the demand at given instance electricity plants rotate their turbines a bit faster or a bit slower instead of switching a complete plant on and off[edit: this is not exactly right, see the reply below], which results in slight deviations from the 50Hz standard but it is fine as long as it stays between the limits.&lt;p&gt;At the end of the day, as the demand increases and decreases, the average frequency would be 50Hz and engineers took advantage of that fact to create clocks that may not be accurate to the second but accurate on average. How do they do that? They count the change in the electricity and assume that 50 changes are exactly 1 second.&lt;p&gt;Unfortunately, due to political issues in the Balkans, the grid was undersupplied or oversupplied for a prolonged period and this created a deviation from the average of 50Hz and the clocks that depend on this average to be 50Hz also lost accuracy that currently amounts to 6 minutes.</text></item></parent_chain></comment><story><title>Continuing frequency deviation in the Continental European Power System</title><url>https://www.entsoe.eu/news-events/announcements/announcements-archive/Pages/News/2018-03-06-press-release-continuing-frequency-deviation-in-the-continental-european-power-system.aspx</url></story>
<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>radiowave</author><text>Stations within the same grid are in sync, plus or minus propagation delays. When a newly started up station wants to connect to the grid, it has to get its generators in sync with the rest of the grid before being switched in. (See wikipedia entry for Synchrocope.)&lt;p&gt;Once the generators are switched in, they&amp;#x27;ll inherently maintain sync, as even if there&amp;#x27;s too little motive power being supplied by the station&amp;#x27;s turbines, the difference will be made up by the power grid itself, which will drive the generators round like an electric motor. (Typically this would happen for a brief period after coming online, after which the station would throttle up and start helping to push the generators round.)</text><parent_chain><item><author>CompelTechnic</author><text>I have a question about the power grid I&amp;#x27;ve never gotten a satisfactory answer to.  I hope someone here can help.&lt;p&gt;If the USA&amp;#x27;s power grid is about 4,000,000 meters wide, and the electric signal propagates at roughly the speed of light (3 x 10^8 m&amp;#x2F;s) (&lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;physics.stackexchange.com&amp;#x2F;questions&amp;#x2F;358894&amp;#x2F;speed-of-light-vs-speed-of-electricity&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;physics.stackexchange.com&amp;#x2F;questions&amp;#x2F;358894&amp;#x2F;speed-of-...&lt;/a&gt;), then it will take roughly .013 seconds for the electrical signal to reach from one station to another. This is a lot out of phase! (the period of 60 Hz is .016 sec)  How is this managed?&lt;p&gt;If two generating stations on opposite ends of the country are both contributing to the electrical signal, how is this time lag accounted for?  I have two guesses:&lt;p&gt;1.  The power network is &amp;quot;mapped&amp;quot; such that there is one central generating station and all other generating stations are time lagged based on their graph distance from this generating station&lt;p&gt;2.  All stations try to generate simultaneously, with their output signals interfering with eachother to a certain degree.  This might not matter much because their local loads dampen the strength of their signal.&lt;p&gt;Either way seems to introduce inefficiencies.</text></item><item><author>Slartie</author><text>&amp;gt; Apparently to match the demand at given instance electricity plants rotate their turbines a bit faster or a bit slower instead of switching a complete plant on and off, which results in slight deviations from the 50Hz standard but it is fine as long as it stays between the limits.&lt;p&gt;This is slightly imprecise. It actually works the other way round: the demand is &amp;quot;communicated&amp;quot; through the network by slight changes in the frequency.&lt;p&gt;This can be described best for the example of steam-based generators, as they are typical for the majority of coal&amp;#x2F;gas&amp;#x2F;oil&amp;#x2F;nuclear power plants. When demand for electricity grows, for example because a new factory is powered up, the turbines in the power plants are experiencing larger forces attempting to slow them down, starting with the power plant closest to the new factory.&lt;p&gt;The speed (and thus the frequency) is constantly measured, and if it drops, the steam valve is opened just a tiny bit more to counteract the drop. If this is not possible because the power plant is already at maximum output, either other power plants have to take the load (and the electricity is then routed through the network to the factory) or additional power plants may have to be added to the network.&lt;p&gt;The important thing is: the frequency change is a means of communication, it originates from demand changes, and because the entire network is kept in sync, it works to communicate demand changes across the entire network. Power plants providing energy to a sub-part of the entire network always attempt to run their turbines with the exact same speed of 50 rotations per second, which should ideally always conform to the frequency in all other sub-networks that a specific sub-network is attached to.&lt;p&gt;If just one of these sub-networks does not perform its duty of re-establishing 50 Hertz by powering up their own plant output, the rest of the network must either channel enough power into this sub-network to allow its local generators to re-establish 50 rotations per second by taking some of the load off of them or - if that is not possible or decided against - must also deliberately drop their frequency (and thus the rotational speed of their generators) to match the sub-network that is deviating.&lt;p&gt;The only alternative to this would be to drop off the deviating sub-network entirely, but that is usually only done in extreme cases of deviation.</text></item><item><author>mrtksn</author><text>The short version of the story is that grid electricity in Europe is produced by rotating the turbines 50 times a second and that&amp;#x27;s where the 50Hz AC electricity comes from.&lt;p&gt;Apparently to match the demand at given instance electricity plants rotate their turbines a bit faster or a bit slower instead of switching a complete plant on and off[edit: this is not exactly right, see the reply below], which results in slight deviations from the 50Hz standard but it is fine as long as it stays between the limits.&lt;p&gt;At the end of the day, as the demand increases and decreases, the average frequency would be 50Hz and engineers took advantage of that fact to create clocks that may not be accurate to the second but accurate on average. How do they do that? They count the change in the electricity and assume that 50 changes are exactly 1 second.&lt;p&gt;Unfortunately, due to political issues in the Balkans, the grid was undersupplied or oversupplied for a prolonged period and this created a deviation from the average of 50Hz and the clocks that depend on this average to be 50Hz also lost accuracy that currently amounts to 6 minutes.</text></item></parent_chain></comment><story><title>Continuing frequency deviation in the Continental European Power System</title><url>https://www.entsoe.eu/news-events/announcements/announcements-archive/Pages/News/2018-03-06-press-release-continuing-frequency-deviation-in-the-continental-european-power-system.aspx</url></story>