Wave Interference - Page 1
Forum section: electronics / general

author: c7aig
author_rank: Newbie
timestamp: May 28, 2019, 07:33:31 am
content: Hi,I understand how 2 waves of equal frequency meet in the air and can produce constructive or destructive interference.But does mains AC 50hz waveform experience this (hopefully constructive!) effect when multiple power stations or residential solar grid tie inverters are added to the grid?Craig

author: hamster_nz
author_rank: Super Contributor
timestamp: May 28, 2019, 07:46:00 am
content: Quite possibly, but a 50Hz wave is pretty long, so seeing this effect would be hard...It is quite visible in RF systems.

author: Berni
author_rank: Super Contributor
timestamp: May 28, 2019, 08:00:17 am
content: Here is an excellent explanation how reflected waves work in cablesBut seeing this happen with 50Hz is difficult because electricity is much much faster than sound. The wavelength of a 50Hz signal is 6000km for radio. A typical coax cable has about 2/3 velocity factor so a signal travels down the cable at 2/3 the speed of light. So you need a 4000km long cable to see it happen.Faster RF signals like 100MHz brings wavelengths down to a few meters so the phenomenon becomes easily visible on a spool of cable. Going above 1GHz brings the wavelengths to cm levels where you can see waves traveling over traces on a circuit board.EDIT: By the way the actual challenge is keeping the grid at exactly 50Hz. All of the big generators are of the synchronous type so they are forced to rotate at exactly the 50Hz mains frequency. If there is more load on the power grid this sucks more energy out of them forcing them to slow down and causing the 50Hz frequency of the entire grid to dip down to say 49.9Hz. Only way to bring it back up is to put more power into the grid by running the generators harder or bringing extra generators online. This creates an excess of power on the network so the excess power goes into spinning up the generators faster and things get back to 50Hz. This is why power plants need to be regulated all the time produce exactly as much power as is being used, nothing less nothing more.

author: c7aig
author_rank: Newbie
timestamp: May 28, 2019, 08:53:51 am
content: Thank you for the replies.So does the amplitude of the wave have to be monitored because of this effect? Or is it just not an issue?

author: hamster_nz
author_rank: Super Contributor
timestamp: May 28, 2019, 09:31:14 am
content: On a power grid he bulk of the generators are mechanical, and any phase difference is 'pulled back into line'. if the grid is lightly loaded the cycles per second increases somewhat, if t is heavily loaded the grid frequency drops.These differences prohibit large power grids from being tied together, as power will surge through the connection point. Seehttps://en.wikipedia.org/wiki/Continental_U.S._power_transmission_gridWhen you have lots of small generators (e.g Grid tied solar) things could become unstable. Plenty of research papers if you go looking, e.g:https://web.stanford.edu/~gorin/papers/POWERCON2012FreqStab.pdf

author: Berni
author_rank: Super Contributor
timestamp: May 28, 2019, 09:56:20 am
content: Oh i had no idea the US had so many independent power grids.Since its all one country i assumed it was all one grid. After all most of Europe apart from island nations is part of the same synchronized grid. Tho there are oddities in Europe such as Norway,Sweden,Finland and Denmark being on a separate Nordic 50Hz grid.

author: GlennSprigg
author_rank: Super Contributor
timestamp: May 28, 2019, 10:42:37 am
content: Maybe I'm missing the point here, but I interpret the question/answer this way...Anything that canre-connectto the Mains Supply, like a U.P.S. system, or a HomeSolar Power system, especially after a power-failure & re-commencement of supply,MUSTSyncitself first, with the supply, before connecting to it. This is done automatically withinthe Inverter system. Otherwise they will not be IN PHASE when connected, and be catastrophic!!While the Mains is present, the Inverters constantly ensure Phase Synchronization.

author: c7aig
author_rank: Newbie
timestamp: May 28, 2019, 10:56:46 am
content: Actually I understand the phasing, what I'm still not sure about is whether adding more power sources to the grid (inverters, generators etc) causes the amplitude of the wave to increase.If a large solar/wind/generator was suddenly turned on that was feeding into the grid (and it was phased the same) would this combine with the grid and increase the amplitude?Craig

author: c7aig
author_rank: Newbie
timestamp: May 28, 2019, 10:58:21 am
content: Also thanks for the links and video.

author: GlennSprigg
author_rank: Super Contributor
timestamp: May 28, 2019, 11:39:58 am
content: Quote from: c7aig on May 28, 2019, 10:56:46 amFor the 'Amplitude' to Increase, the VOLTAGE would then be slightly higher. The 'Home' solar systemsdetect this imbalance of current flowfrom/tothe 'Grid', and your 'Bill' will be adjusted accordingly.That's at a 'local' level. But remember that loads produce an overall voltage drop, so someone downthe street should not see a 'jump' in voltage. I don't know how to explain it in a simple way, sorry...

author: Berni
author_rank: Super Contributor
timestamp: May 28, 2019, 12:34:42 pm
content: Ah yes the what controls the voltage thing in AC power systems can be quite confusing indeed.This works different depending on how the AC system is built. Its mostly split into "grid connected operation" or "island operation". Things like generators on ships are a example of "island operation", here there is a single synchronous AC generator supplying the whole grid. Because of this you have complete control over the voltage and frequency in the grid. If you spin the generator faster than the frequency will go up, if you increase the magnetic field strength inside the generator then the voltage goes up. For this reason large generators always have field coils instead of permanent magnets. This lets them use a DC current to control the field strength and with that control the generators output voltage. There is typically a box of electronics that monitors the voltage and constantly adjusts the field strength to keep it constant. The more load is put on the grid the more current flows trough your generators output and this causes a countering field in the generator so it will take more torque on the generators shaft to keep turning it at the same speed.When a synchronous AC generator is grid connected things work differently. This could be imagined as connecting yourself in parallel with a massive generator that is thousands of times bigger than yours. Because of this you can no longer control the voltage or frequency because that generator is much stronger than you and forces its own voltage upon you. Because the frequency and rpm of a generator are a fixed constant means that your generator will always spin at whatever speed the grid wants it to spin due to its frequency. Even if you put zero torque into the generator it will continue to spin, it just becomes an electric motor at that point and will be drawing power from the grid. But if you try to spin the generator faster than it already is then it will fight back with torque and start pushing power out into the grid. The harder you try to spin it faster the more power it will output into the grid but the voltage and frequency will not change. However because you are sending power back out means you are helping the giant generator supply power to the grid so the giant generator will have to act accordingly and reduce the torque input so it doesn't speed up (It wants to hold the same frequency) and perhaps adjust the field strength inside to keep voltage constant (due to resistive losses and such)This "giant generator" would be unpractically large to supply the entire Europe, so it takes form of thousands of smaller generators placed in power plants all over the place. Each generator having its own control system that keeps it running at the right speed and voltage.The field strength adjustments can happen very quickly as it just needs a slight change in the field winding current. The regulation loop can keep the voltage output of these generators very stable. But an instantaneous increase in the load current (That's typically associated when you raise voltage) also instantaneously makes the generator harder to turn. So the turbine that is spinning it needs to quickly "step on the throttle" to provide the extra turning effort. If they can't provide the extra needed turning ommph then the generator simply wont output any more power, this means other power plants on the grid have to take up the load your plant is failing to provide and put that missing torque into there own generators. If the other power plants are also unable to turn the generators hard enough to keep up, then there is nobody else to take the load off them, this forces the generators to start taking the needed power out of there own rotational inertia so they start to slow down and with that the frequency drops. If nothing is done about this and it drops too far safety systems will start to kick in and disconnect parts of the grid to reduce the load, otherwise the frequency would continue to drop to 0Hz and all the generators stop turning rendering the entire grid dead. So this is why overloading the grid causes the frequency to drop but not voltage.As far as your solar inverter raising the mains voltage, it does happen to some extent. But that is purely because of resistive voltage drop from you to the transformer station. By outputing power you are taking up some of the neighborhoods electrical load on to yourself so the transformer doesn't need to supply as much current and so there is less voltage drop on the wires from the transformer station.

author: soldar
author_rank: Super Contributor
timestamp: May 28, 2019, 12:38:32 pm
content: Quote from: c7aig on May 28, 2019, 10:56:46 amAmplitude means voltage and the answer is "yes" but with some additional considerations. Does dumping a glass of water into lake Michigan increase water level? Yes it does.But even if you put on line a very large generator, the entire distribution network is continually monitored and adjusted to demand. A generator at the other end of the network does not know or care if voltage at its end is rising because demand is decreasing or because other suppliers went online. All they see is less demand and they will decrease their output accordingly.In other words, the reason the voltage ("amplitude") is stable is because it is being monitored continually and supply is continually adjusted to meet demand.

author: c7aig
author_rank: Newbie
timestamp: May 28, 2019, 01:05:36 pm
content: Berni/Soldar.Thank you for the detailed explanations, it perfectly answered my questions.Craig