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Connecting a generator to the main at the same time

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New Member
Hi guys,

Im new to this forum, so have not read much about this before. so if this has been covered, please bear with me. Perhaps direct me to the relevant topic.

What I would like to do is connect a hydraulic turbine from a nearby stream to my house. The turbine will at times be able to supply more power than I can use, and at other times it will only supply much less than is required to power everything in the house.

The problem here in South Africa at the moment is that our electrical service provider(Eskom) cannot accept grid tie systems.

So if the turbine produces more than we can use we need to dump the power through a resistive load, this is fine.

my problem is how to wire the house so that the equipment in the house uses the green power first and then draws the surplus from the national grid.

The problem is how will the turbine/generator know that its supplying to much and dump the energy instead of forcing it into the grid?

Does the grid and green power have to be completely seperated? What equipment is sits in between the 2 power supplies for it to be managed properly?

please help, have been struggling on this for a while, thanks


New Member
Right now what your proposing dosn't really exist that I know of. What you can do is either be grid tied or off grid. Your best bet if the power company dosn't allow grid tie like you said is to wire separately for your green power and use it when you have it. Like lighting. You could have two lights in each room. One on the grid with a switch, one off grid. When the one off grid isn 't bright enough you switch on the grid one, for extra light. Or charge batteries, if that's an option for you. But anyways how big is your hydro generator? And what kind of power are you expecting out of it? I dont really know if there's a controller that will let you use your power, and when it's not enough use the grid, and when you have excess to dump it somewhere.


Most Helpful Member
there is a way to do it but your going to spend a fair amount of money while the overall systems will loose some efficiency in the process.

Basically you need a big enough inverter system that can handle your entire house that preferably runs off of a near line voltage battery bank.
What you do is use the inverter and battery bank as the primary power.
The hydro system and utility line provide constant charging power to the battery bank.
The hydro system is set up to provide the primary battery charging power when it can. when it cant the utility line provides the rest.

It rather complex and less efficient but its the only way I have heard of with two or more independent power sources feeding a common load without a direct interconnection between them.


Well-Known Member
If you know the amount of power your house is drawing from the grid (ie have a specialised power meter), you can control the power that your turbine feeds into the house wiring, and provided you feed in less than the power your house is consuming at all times there will never be a situation where power is fed back to the grid.

And now it starts... :D


New Member
At Mr RB,

So you saying install a normal grid tie system and hope the house always draws more than the hydro generator produces, that makes sense but... :)

For some more info, the generator I plan will be 2 stage, one turbine producing 300W in summer and an addition 1.5kW when the sensing equipment knows the river is really flowing, then it will open both and produce 1.8kW

The house is equipped with solar geysers so in summer during the day it is likely that after heavy rain the hydro genny will produce more than the house can draw.

Apparently the hydro genny comes with a load controller that works in a stand alone mode. If the genny produces more than is drawn, it dumps load to resistive element.

What if the load controller could sense the total drawn by the house, and the total power it is producing and then do a computation saying, if generator produces more than house, dump load? Would this not prevent reverse current back into the grid?


Well-Known Member
What I said was largely "tongue in cheek" which is why i used the big smiley face.

If your local power company doesn't want you feeding power back into their grid then you should choose a system where you WON'T be feeding power back into their grid.

You last sentence seems sensible enough, and should be workable from an engineering point provided you can sense the power the house consumes from the grid and your controller is fast enough to make sure nothing goes back to the grid.

So I would say "yeah that might work" but then I'm NOT a South African lawyer so I can't say whether you SHOULD do it, that would be between you and the energy company etc.


New Member
I would think tcmtech's idea would be the best. I have seen the inverters that also charge your batteries for you when they get low, If you can find one that accepts two inputs that is. This would be about your only legal option, unless you went off grid completely. You definately don't want some huge fine, that kinda negates the whole point of saving any money.


Well-Known Member
Is you hydro generator an A.C. or D.C. generator?

Is there a governor control on the hydro gen?

If D.C. do you have a grid tie hybrid inverter?

An A.C. generator would have to syncronized to grid which if hard to do in a small system.

D.C. generator feeding an inverter grid-tie hybrid would be like having PV panels or wind generator.


Well-Known Member
Most Helpful Member
You could look into this:-

Obtain a generator large enough to run the house.

Turn that with water turbine.

Assist it as needed with a motor driven from the mains. Add a ratchet so that the turbine can't drive the motor as a generator.

You would also need some overspeed protection.


Well-Known Member
How wonderfully industrial! :)

When I was an elec tech apprentice in the early '80s in industry they had big "substations" everywhere with massive MG (motor/generator) sets spinning 24 hours per day converting AC to DC, higher voltages to lower voltages etc. This was the system left over from the 1950's and even earlier. These things had motors and generators the size of cars, on reinforced concrete slabs 2 foot thick. At 1500RPM you couldn't talk near them. On hot days we would walk through the substation and when going past the vents on the big MGs the windage would almost blow your shirt off, you would be totally dry, free from sweat and a few degrees cooler by the time you exited the substation.

And they would run for 10+ years at a time, never stopping. Ahh the good old days. Now I'm sure the substations are full of panels packed with buzzy little mosfets and heatsinks.


Well-Known Member
I my 28 years working industrial electric power supply I have never seen what your talking about.
I have seen big transformers that use big fans to keep cool that could all most nock you off your feet.
Substations still use transformers


Well-Known Member
These weren't for electricity distribution, it was inside a steelworking mill. The substations were mainly generating LOTS of DC for DC cranes, and massive DC motor driven steel rolling mills.

The real big MG sets were just for powering the 5-stand cold rolling mill, these MG's were the size of buses but that area was locked off, not the sort of area us lowly apprentices could just choose to walk through on hot days. But the "little" MG sets in the other substations were good enough...

Much more fun (more manly?) than your cute little substation above... ;)


you may face problem in synchronizing with grid,
during summer you will consume a part of power from grid,
during winter it’s not required to dump it, you can make your system like islanding and let the governor of the turbine control the load.

since your capacity of generation is very small, inverters may not be a good option(efficiency) so better you do a separate circuit for the green power if you don’t have the option to sync with grid.

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