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Need to regulate AC mains frequency

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Jul 4, 2009
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Aberdyfi, West Wales, UK
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I'm only at the initial stages of gathering information to find the optimum solution for this but ideas from others would be welcomed. This is the rather unusual situation:

I've got about 1.5KW of PV generation here, currently feeding a grid-tied inverter to offset electricity company bills. I also have an unusual load situation where one or two pumps may be intermittently operating with a load of about 1.5KW each. This is as well as 'normal' domestic use which sits at around 350W most of the day but reaches peaks of about 6KW. It works fine as it is and the PV helps to keep the bills down. I cannot re-sell electricity to the grid so making an income from excess production isn't possible, however this isn't an issue.

The problem is the incoming AC supply (230V) is liable to frequent interruptions, possibly lasting for days. With the PV being grid-tied, when the incoming power fails, the anti-islanding kicks in and also stops the PV feed. It's a commercialy made inverter and I don't want to reverse engineer it to fake incoming power when there isn't any, and it would be illegal to do so anyway.

I have a 5KW Diesel backup generator which gets used when the grid is off but it has a mechanical speed governor which is slow to react to load changes, hence the grid-tied inverter drops out every time it senses a frequency change on the line. So although I have power from the generator, I can't simultaneously use the PV to augment it.

Faster regulation of the generator speed isn't really a feasible solution, even with near instantaneous fuel control, the mass of the motor and generator wouldn't keep it stable enough. So I'm looking at electrical solutions. My first thoughts are to build a hefty PWM inverter, maybe rated at 10KW but rather than feed it from low voltage DC, to simply hang a bridge rectifier on the output of the generator so I would have around 340V DC ahead of the PWM switches. A crystal controlled PWM inverter would be stable enough for the PV to continue working. I can forsee some problems, in particular the likely chances of the PV back-driving the PWM output.

Has anyone experience of doing this kind of thing?


I have come across this sort of problem, 250 KW of consumption, pull the mains, the Chloride battery power inverter takes the load (max time support is 10 minutes), diesel fire up and runs. This is where it went pear shaped, the diesels output was set by a mechanical governor to +-1 HZ, Chlorides frequency window was +- .5 Hz, so it never saw the "new" mains, so never changed over to it. Hasty restoration of the true mains!!!. I am sure that the solution was a relaxation of the Chlorides frequency window. I would go down this route with your inverter. or even build a second PV inverter which can lock onto your diesel feed (only 1.5 KW).

Thanks Frank.

The trouble is the PV inverter is already installed and I don't really want to either replace it or modify it. Listening to the Diesel when a pump kicks in makes me think it would need maybe +/- 10Hz anyway which would be pushing my luck even with relaxed timing. It does have a frequency meter on it's 'dashboard' but I don't relish sitting there and watching the numbers all day waiting for a load to kick in. Sitting next to a 400cc single cyclinder Diesel engine for a long time is bad for your ears and your sanity!

It did occur to me that I could use two bridge rectifiers, one across the generator output and the other across the incoming power lines. The DC from both being joined and feeding a big PWM "re-generator". It would save having to switch the power sources during a black out but the electricity company would probably go ballistic with me!


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