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I am trying to design an electrical generator that can supply power to my house but without cutting off the utility power supply. The idea is that I could put the generator in parallel with the utility power supply so that if the utility power goes out I still have the generator and vice versa. However I want to make sure that the output of my generator is in phase with the utility power, otherwise I'll get surges or weird wave interference patterns

I live in the US so utility power is about 117 VAC at 60 Hz.

I was thinking that it needs two specific kinds of circuits: one that changes the phase of the generator and then some kind of controller that detects the phase difference between the two signals.

I thought that perhaps this could be done by combining the two out-of-phase AC signals and rectifying them into a DC signal. At the same time the signals from the generator and utility power supply could be rectified and added to represent what the in-phase sum of the power signals would look like. These who DC signals could then be compared by an op-amp controlling the phase shifting circuit, but I have no idea how the details would work out. I'm afraid I have a lot of theoretical knowledge but little practical experience with electronics.

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  • \$\begingroup\$ These boxes can be bought. You must speak with an electrician to sort out the practical and legal issues - it's not enough just to cover the theory. \$\endgroup\$ – user253751 Sep 21 '17 at 1:41
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The power feed to you home will almost certainly be 120V/240V single phase.

You will normally require a transfer switch to totally isolate the commercial power feed from your generator, to prevent your "home-made" power from feeding out to the commercial power grid, potentially endandgering the power company's technicians. This means you can use either commercial power, or your generator to power your home, but not both at once (then you don't need to worry about phase synchronization).

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This is not an easy task. Keep in mind that, if you are to connect a power device such as yours to the utility grid that does not follow the correct US standards, you are endangering other peoples lives/equipment as well. Remember, everything fails sooner or later. You have to take precautions.

With that said, here are some things you need to consider:

Your device,

  • Should have appropriate IP level (Your device should be enclosed within a metal chasis, but it will be heating. So, how much enclosure?)
  • HV and grounding protection (Important! - Bad UPS will get you tingly when you touch them!)
  • Should have appropriate fuse boxes, cabling, emergency shut down, etc.(Or you'll start a fire in case stuff go wrong)
  • Should not create islanding(When grid power is off, if you give power to the grid this is called islanding) on the utility grid. (Important! - If you do this someone might die)
  • Should have fire safety, or a plan on what to do if it catches on fire (Happens)
  • Follow US standards, this will lower the chance of failure for a great deal. (They are sold for a price)

There are a number of solutions for your problem. I think the best solution for your case is to create a line interactive UPS. Here are some UPS types.

After you decide on a method, you'll probably be creating a PCB for inverter/converter stuff, writing code for your DSP/ARM chip (microcontrollers are slow for this stuff, get something that works 75 MHz or higher), debuging your code/ debugging PCB errors. (This will take some time). Hopefully, though, after some time you'll get it working.

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  • \$\begingroup\$ Thanks, Maximus. I didn't think about the power from my generator hurting someone. I will modify my designs to take your safety advice into account \$\endgroup\$ – Gabe Feb 14 '17 at 20:14
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An easy way to accomplish this, is with a motor and flywheel driving a generator. This will, however, only run for a few seconds without some kind of OTHER motor than an AC-driven one from your power grid, after the power drops. Uninterruptible power supplies (battery powered) and generator sets (with complex control circuitry and interlocks) are commercially available.

In short, the generator is the EASY part; handling the switchover, safely, while your diesel engine is warmed up to handle the load, is more difficult.

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It is not a good idea to run the generator during a power outage without cutting off the utility power supply.

At first it is dangerous for the technicians of the power company. They assume there is no power on the grid but they don't know of your generator backfeeding the grid. If somebody gets injured or even killed by the electricity from you generator you will need a good lawyer.

If your generator is still connected to the grid, you will feed all your neighbours. Even if you have a very strong generator for all your loads in your house, it won't be enough for all the extra load from the neighbourhood. The primary circuit breakers for your house will trip finally or the generator will stop under the overload. If the generator stops, you can not restart it under all the extra load.

You can not run the generator on every hour of every day, the fuel will be too expensive and you will need frequent service of the motor. The motor has a very limited lifetime and will run for some few years only if operated continously.

If the voltage of the grid is a little lower than that of your generator, your generator will partially feed the grid but you don't get money for that.

The one and only way to supply your house by your own generators is to disconnect your house from the grid when the generator is running.

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Let me add a bit.

When you convert from ac to dc you loose a lot of power in the process depending on the efficiency of the converter. So best if you buy professionally designed converter box (if available in market) for that purpose which has maximised efficiency by professional factory designers.

Many house gadgets wont work under dc supply.

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  • 1
    \$\begingroup\$ Welcome to EE.SE. The original question is not talking about converting power to DC but rather obtaining a small DC control signal somehow representing the phase of the mains supply and one for the generator supply and using these to synchronise the two. The question of efficiency does not arise. \$\endgroup\$ – Transistor Feb 4 '18 at 22:16

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