Naked Science Forum
Non Life Sciences => Technology => Topic started by: peppercorn on 01/07/2010 10:41:15
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I was considering how electrical conditioning units for connecting micro-generation to the national grid can represent a costly addition to a small-scale generating system.
The key aspects of tying into the grid are matching the frequency, voltage and phase shift of the mains. However, I believe that the mains is guaranteed to (in general) stay within fairly tight limits.
So, for instance in the UK, the generator's voltage could be set at 243V with an average 3V dumped by a crude regulator.
I did wonder whether frequency variation could be matched by changing the genny's revs, but I doubt this would offer a quick enough response.
I think the most common way of matching is to convert AC-gen to DC to AC-mains, but this seems overkill if you can start with a machine that gives more-or-less 240V at 50Hz.
Ideas welcome! ..... [?]
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I believe a synchronous generator will stay in sync with the grid. The trick is to get it running very close to the same frequency and phase before you connect it. If you don't do that, very bad things can happen.
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I believe a synchronous generator will stay in sync with the grid. The trick is to get it running very close to the same frequency and phase before you connect it. If you don't do that, very bad things can happen.
Thanks. Yes, I thought there was some sort of synchronicity trick one could use with alternators, etc. I think so long as you put in a (relatively simple) phase sensitive cct breaker all should be okay ... hopefully [:o]
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BTW, this works with polyphase machines - three or more phases. It could be a bit trickier with a single phase connection.
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Synchronising is easy, you just connect a light bulb ( incandescent NOT CFL) rated at at least 2x the generator output voltage across the breaker for each phase. Then you get the generator running at the correct frequency and when the light is at it's dimmest ( it will brighten and dim as it approaches the correct phase) you close the breaker. If the generator is small compared to the line ( in most cases it is) it will be phase locked and stay that way while providing power. You are basically only connecting when the generator and line are almost in phase, the difference, if small, will not have any effect. If you connect at the wrong time the generator will blow fuses ( at best) or blow up. Big bang is generally proportional to size, but if you are adding a big set onto a line then you better buy a proper synchroniser, most likely a big set intended for on line operation will have this.Really big gensets almost always leave via the roof if you get this wrong.
If you are using solar panels to augment the electricity grid then the inverter that it uses will have the ability to do so built in, do not use any inverter other than this type for this application, they are not meant for this type of use, smoke always comes out.
If you are using a genset for standby power when the mains fails you just need a suitable changeover switch, which can either be manual or automatic. In each case the switch is physically designed that the load can be only connected to one source at a time, and the 2 sources cannot apply power to each other no matter what happens during switching.
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Synchronising is easy, you just connect a light bulb ( incandescent NOT CFL) rated at at least 2x the generator output voltage across the breaker for each phase. Then you get the generator running at the correct frequency and when the light is at it's dimmest ( it will brighten and dim as it approaches the correct phase) you close the breaker. If the generator is small compared to the line ( in most cases it is) it will be phase locked and stay that way while providing power. You are basically only connecting when the generator and line are almost in phase, the difference, if small, will not have any effect.
Thanks for the reminder SeanB. We ran that experiment in Elec. Apps. class at college and yes, it's a remarkably simple idea that should work for single or three-phase, I think.
Replacing the bulb with a resistor and measuring the current through it would allow a circuit controlling the throttle to match the genset speed to the mains frequency.
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A variant of that is used at every power station to connect each turbogenerator to the interior infinite bus. Still possible to do manually but often controlled by a PLC to reduce the chance of an error happening.