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How to limit A/C current draw from gird and draw power from secondary grid tied solar inverter ? Answered

Micro grid tie inverters are an excellent power source and can be linked together to generate energy, however they do not function when grid is down, this is for the safety of line workers. In a real world scenario I want to ensure that all power is drawn from the energy produced by the micro solar inverters as the primary source while it can produce power and not from the grid which will act as a secondary power source, this will require some sort of current limiting solution from the grid without reducing the voltage to keep the inverter working.  

For the safety of line workers I would like to have an automatic transfer switch which will have both the primary power source from grid and a tertiary inverter backed battery backup so that when the grid is off, the power is drawn from the Inverter and there is no power fed back into the line keeping lineworkers safe and since pure sine wave produced by the inverter can activate the grid-tie inverters and it should theoretically increase the VA of the UPS to the size of the installed Grid-tie micro inverter capacity, the automatic transfer switch should take care of switching between grid when power is on and to the backup inverter when power is off. 

TIA for any advice in this regard !

Discussions

Here is one solution I think may work, however its bit more expensive than I first imagined, I wish someone could simplify it, I will use 400v capacitor and zener diode circuit to convert 240v to 12v DC, for the second part I would need to add 12v dc to ac pure sine wave to generator. I think that will limit current flow to a great extent as there are more ways to limit power draw in DC electronics than AC, or is it too much thinking :) ?

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iceng

2 years ago

OK the safety of power line workers. If you are supplying power to the main grid and a car accident takes out a pole the employee assumes the disconnected portion is Safe... That's why you must have a disconnect circuit box that will immediately stop your generated power feeding the mains... It could kill a utility service guy trying to reconnect power to a live line.

GE General Electric makes a FOUR_QUADRANT_POWER_SUPPLY which can handle overhauling loads that turn from motor to generator. GE is expensive but there are relatively simple box devices that do this... I can't remember what it is called right now

Thanks for the response, however, four or two quadrant power supply does not provide what I need which is the AC limiting function, I don't know if that can be done, to simply reduce the amperage drawn from grid and keep the same voltage. The only function I need to complete this equation is something to reduce the load on the grid. Kindly advise.

A tiny phase angle variation will decide which way current will try to flow.

I checked the logic, but if I understand correctly the power flows through one way cutting off the other, but what I need is where both flows are active, but power is drawn mainly through one source which is solar, and a limited amount from grid source to keep the whole system active, so I need both power sources active, but I need to limit one, not close it off completely. I cannot find the right solution yet. Someone told me once that the flow of current is like a pipeline, and by increasing and limiting the diameter of the pipe you are essentially increasing and reducing the flow that is what I want to translate into my model, but since I'm not an electrical engineer, it takes me more time to read and grasp concepts.

Consider the action of a cloud and the atmosphere including day night... Eventually you will adjust your solar settings to match your solar / power environment to allow the switching of power between your energy sources to minimize dependence on the grid... Smart users keep unconnected power reserves to account for unexpected scenarios that usually do happen...

Keep in mind people have been successfully doing this sharing between energy sources for a long time before you were born using windmills.

Consider that as one source feeds more current its internal resistance lowers the voltage shifting the balance back to the other source...

Yes, even the grid has a resistance (reactance) that lowers its AC voltage as you and your neighbors change your power needs.

Consider separating functions... Unlike where you live, my geographic location has cold winters... Each home has a heating plant that costs oil, gas or electrical heat pump energy heating... All these work on a thermal sensor that controls power to maintain a certain temperature as desired by the home owner's schedule...

Now, what my friend did was use a standard AC motor as a self excited induction generator that simply drives current through resistors under his floor... The conversion efficiency of resistors is 100%... The variable heat radiates through the floor and reduces the amount of oil needed to heat the home...

All this action occurs without any fancy regulation, batteries or load sharing electronics and saves over half the normal winter heating cost...

Click the picture to view the whole image.

WindGenLoop.bmp

Thanks my friend, however we don't have winters ;)

You have hot weather, fans, and thermometric coolers.

Grid usage is very expensive, so I am trying to limit usage, that is the primary reason in limiting the consumption.

I think the problem is with the concept.
These inverters are meant to run together with the main grid.
One of the reasons for this to avoid batteries.
If I understand correctly you want to basically use whatever comes from your solar panels and only if that is not enough you want to tap into the mains grid.
Without proper buffering by means of batteries you won't be able to do this, even if the inverter would allow such a configuration.
If a cloud passed in front of the sun you might loose too much to sustain power.
A normal battery operated system would now draw power from the batteries.
I might be wrong but you need an inverter that that is capable of using mains power as an input source instead of only a being on the receiving end.

One way I could think of would be to eliminate the mains connection and only attach it to a self regulating DC power supply.
By self regulating I mean:
I should have at least the same power as all solar panels combined and be able to sense the load.
This way you create a dual power supply for your inverter.
As long as there is enough sun the solar will basically work alone, without it the power supply kicks in and provides what the solar system can't.

Thanks for your comments, you are right, it was meant to work with the grid, but the problem is that in third world countries, the grid goes offline from 1-8 hours per day, so when you have a lot of sunshine, it will be silly if we cannot utilize our solar system it when the grid is down during the day, so I have the automatic transfer switch to fix this problem where an inverter kicks in and pure sine wave is fed to the micro inverters to activate them. The minimum power bills for any average shop is around 2000USD, and one tenth for homes, which is daylight robbery, so the primary purpose is to not utilize the grid, if the grid is down I want to tap into the backup inverter to just activate the micro grid tie inverters, but what I am not able to fix is that the load should not draw power from the grid and solar, but I need to figure out something that can increase resistance to the grid thereby lower consumption from grid. Primarily this solution is to work only during the day, and not for night usage for powering air conditioners and other machinery. The focus is to exponentially expand the number of panels required to support any industry as a one time investment and allow scaling if required with an affordable and modular approach..

I am not sure if you or me are thinking in different directions here but I will try anyway...
Again, just to confirm:
For as long as there is sufficient power from the solar panels you want to be completely off the grid?
If so I think some circuit digging might help.

Your inverter needs mains power to supply any output at all, so the big question would be wether or not the inverter is actually ablte to provide all the power you need.
I know you don't want to hear this but IMHO the best option would be to use a stand alone inverter and a battery system.
To keep losses and problem low for your current system it could make sense to actually investigate on the inverter you have.
It has to sense mains power somehow but I highly doubt it actually checks how much power is available as it is safe to assume mains power on means there is enough juice in the lines.
So somewhere in the electronic guts of your inverter is some little logic telling it that is mains power available and that it should activate.
Finding this part of the circuit and understanding it would mean you can totally bypass the mains check with a tiny switch or by feeding some control voltage to the logic.
Eliminates the need and losses for an inverter to feed your inverter ;)
In an ideal case you would abviously modify the inverter with some external logic so it will use mains power when no sun is available, otherwise and if just using a simple switch you would need to include some sort of undervoltage protection that shuts the inverter down if there are too many clouds.

Thanks again, but I did try to read up how they function, and circuit digging is not an option, since there is too much involved, they first detect power source and sync up with the grid A/C waveform to push power back to the grid and by no means a simple feat. Moreover it voids warranty on the microinverters. For a cloudy day, I could compensate by increasing the number of panels installed, so each panel will produce a minimum threshold. But the using the old system is a no go, due to the inherent disadvantages, and it is very costly when compared to micro inverters, and does not scale and acts as a single point of failure and even partial shading in one solar panel reduces the output of all panels, best output = weakest link. So we must make this new technology work, and an A/C limiting device on the grid source should suffice, unless such a thing does not exist or cannot be made.