AC in to AC out grid Tie..

[mrpackethead]

I'd rather not discuss the why.. however...

If i had a 'cheap' supply of 230VAC 50Hz Power ( or possibly 415V 3phase),  and i wanted to push that back onto the grid , how woudl you go about doing it.    You can assume that the two systems will be in phase.

[capt bullshot]

Assuming systems are in phase (at no phase shift), you'd have to insure the cheap supplying system voltage is "somewhat" higher than the grid voltage and connect it. That would ensure energy flow from the cheap source to the grid and would keep the systems in phase (grid always wins or fuse blows).
How to do that, depends on your source. Some primitive systems like a water mill just use an induction motor (as generator) to achieve the goal.
For some kind of unknown source of unknown properties, but with a stiff voltage and grid-locked phase, one could imagine a "weak" step-up transformer a a coupling element.

[mrpackethead]

Assuming systems are in phase (at no phase shift), you'd have to insure the cheap supplying system voltage is "somewhat" higher than the grid voltage and connect it. That would ensure energy flow from the cheap source to the grid and would keep the systems in phase (grid always wins or fuse blows).
How to do that, depends on your source. Some primitive systems like a water mill just use an induction motor (as generator) to achieve the goal.
For some kind of unknown source of unknown properties, but with a stiff voltage and grid-locked phase, one could imagine a "weak" step-up transformer a a coupling element.

The strange situation is that the 'input' energy source would in fact be the same grid as the grid that power is being supplied to.    Theres a crazy pricing scheme which makes this a viable proposition.     

Using a transformer, how would you limit the current..

[capt bullshot]

Using a transformer, how would you limit the current..

By impedance, either inherent to the transformer or something external, preferably reactive, not resistive.
The transformer ideally would have a range of fine steps to adjust the voltage difference which in turn controls the current through the impedance.

[sibeen]

This is done by thousands of inverters already in New Zealand. They're called solar inverters

OK, firstly the safety shit. Any grid connect inverter in Australia or New Zealand needs to comply with AS/NZ 4777. This is for safety of the user but even more importantly the safety of electrical workers who may be performing work on lines that are connected to your inverter. If you decide to connect something to the grid without taking into account the strict guidelines and rules around anti-islanding then you could be a gaol term waiting to happen. This is not to be stuffed around with.

[mrpackethead]

This is done by thousands of inverters already in New Zealand. They're called solar inverters

OK, firstly the safety shit. Any grid connect inverter in Australia or New Zealand needs to comply with AS/NZ 4777. This is for safety of the user but even more importantly the safety of electrical workers who may be performing work on lines that are connected to your inverter. If you decide to connect something to the grid without taking into account the strict guidelines and rules around anti-islanding then you could be a gaol term waiting to happen. This is not to be stuffed around with.

Please go back and read the last few posts.   I dont' ahve a DC power source.

[sibeen]

OK, I've given a warning.

To adjust the power fed into the grid you adjust the phase angle. To adjust the VARs you adjust the voltage.

[mrpackethead]

OK, firstly the safety shit. Any grid connect inverter

in this therotical system, there is no invertor.  The 'input'  is the 'grid'.  The 'output' is also the same grid.   The cost of power on the 'input' side is cheap.  The revenue on the 'output' side is more than the cost of the power on the input.

Islanding cant' happen for the simple reason that shoudl teh 'grid' go down, there will be no energy. 

[capt bullshot]

in this therotical system, there is no invertor.  The 'input'  is the 'grid'.  The 'output' is also the same grid.   The cost of power on the 'input' side is cheap.  The revenue on the 'output' side is more than the cost of the power on the input.

Money for nothing

[IDEngineer]

The strange situation is that the 'input' energy source would in fact be the same grid as the grid that power is being supplied to.    Theres a crazy pricing scheme which makes this a viable proposition.

My first impression is that one customer's power is going to be sold back into the same grid by a second customer. The "crazy pricing scheme" might simply be that a next-door-neighbor is a high volume consumer with a better per-kilowatt negotiated price. The coupling between the two might be as simple as a heavy extension cord dug under a fence. Did I guess correctly? What do I win?

If this scheme is some variation of the above, you can be pretty confident the power company attorneys anticipated this and included penalties in their customer contracts for trying it. Please keep this thread updated as events unfold!

As a purely academic question, this raises a topic I've long wondered about: How do multiple grid power sources (dams, nukes, coal, NatGas, whatever) "push" power to the shared grid. At first glance it would seem that any individual source would need to increase their voltage to insure they are a source and not a sink, but that would quickly lead to a game of oneupsmanship with little increments in voltage causing each to further bump their voltage, ad infinitum. A bit of research revealed that in the interesting world of multiphase AC, frequency is used to measure and control this behavior. If your frequency is lagging, you are a net sink. So the goal of power plant control loops is to precisely match the frequency (and, of course, phase) of the grid. Doing so insures you are not sinking current.

[dmills]

The big three phase generator sets have a magnetic field coupling the rotor and stator and this results in a torque on the shaft proportional to the angle between the rotor and stator fields (Actually there is some trig in there, but go with it, at small angles it is close enough). If the rotor lags on the grid then it acts as a motor, if it leads on the grid then it generates.

Because of this torque all the machines on a grid will run at the same speed and in phase with whatever the local grid connection point is doing, to supply power to the grid you open the throttles and the resulting torque will (once the transient damps down) move the rotor angle to be such that the rotor field leads on the stator field by an mount that causes the torques to again match. In effect your torque contribution is trying to speed up the whole grid, and the loads are trying to slow it down.

If you have a nuke hydro or something similar (cheap per GJ but with large fixed costs) you probably wind up setting the speed regulator at the top of the range so that as long as there is any significant load on the grid your plant will be going at full production. Conversely if you have a open cycle gas turbine (Cheap plant but really expensive power per GJ) you set the governor such that the thing only produces torque once the grid frequency drops well below nominal, that way you only make power when it is critical to do so.

Reactive power (and thus network voltage) is controlled by the terminal voltage.

An interesting funny is a synchronous motor connected to a flywheel, not only does it provide inertia making grid frequency changes slower, but by varying the exciter current you can make it appear as a capacitor or an inductor, quite cool.

Regards, Dan.

[dmills]

I knew a guy who had an early subsidised solar system that got a really quite nice feed in tariff (But really shitty panels), the guy was running the inverter off a honking great transformer/rectifier combo from the grid, it apparently paid quite well!

Regards, Dan.

[ejeffrey]

The strange situation is that the 'input' energy source would in fact be the same grid as the grid that power is being supplied to.    Theres a crazy pricing scheme which makes this a viable proposition.

My first impression is that one customer's power is going to be sold back into the same grid by a second customer. The "crazy pricing scheme" might simply be that a next-door-neighbor is a high volume consumer with a better per-kilowatt negotiated price. The coupling between the two might be as simple as a heavy extension cord dug under a fence. Did I guess correctly? What do I win?

If this scheme is some variation of the above, you can be pretty confident the power company attorneys anticipated this and included penalties in their customer contracts for trying it. Please keep this thread updated as events unfold!

Almost certainly.  In most places, users are not allowed to feed power back into the grid unless specifically approved.  When you get that approval, it generally specifies how you can do it.  For instance, I can get approval to feed in solar or wind only.

The other way this can happen is if a single customer has multiple meters with different rates.  For instance, here EV users can get a second meter installed for EV charging that gets a lower rate.  But you are only allowed to use that for EV charging -- powering general loads or feeding back into the grid through your primary meter is not allowed.

Islanding cant' happen for the simple reason that shoudl teh 'grid' go down, there will be no energy. 

Your belief that islanding can't happen does not eliminate the need to comply with those rules.  Also, are you really sure islanding can't happen?  You don't only have to protect against islanding when the "entire grid" goes down, but also when the service disconnect is used.  Unless your two feeds share a single service disconnect and don't have individually dedicated disconnects, then you can create an island situation.

[mrpackethead]

Did I guess correctly? What do I win?

Just the knowledge of knowing you were able to work it out.   :-)   Yes, it is an academic question, but as you have gone on to say below, theres some interesting thoughts.

[jmelson]

OK, firstly the safety shit. Any grid connect inverter

in this therotical system, there is no invertor.  The 'input'  is the 'grid'.  The 'output' is also the same grid.   The cost of power on the 'input' side is cheap.  The revenue on the 'output' side is more than the cost of the power on the input.

Islanding cant' happen for the simple reason that shoudl teh 'grid' go down, there will be no energy.

I really think you cannot do this the "simple" way.  Maybe, a variable transformer could be tied between input and output, and some mechanism adjusts it to get the desired energy transfer.  Assuming that the two meters (I'm guessing the scheme is to buy power from one meter and sell to another meter) are off the same branch of the grid, then there's not going to be any phase difference, and the voltages ought to track.

But, really, the right way to get this to work is to get a big rectifier/filter on the source meter, and drive a grid-tie inverter to the load meter.  Then, the grid-tie inverter can be set up to deliver the desired power to the load.

Jon

[NiHaoMike]

The other way this can happen is if a single customer has multiple meters with different rates.  For instance, here EV users can get a second meter installed for EV charging that gets a lower rate.  But you are only allowed to use that for EV charging -- powering general loads or feeding back into the grid through your primary meter is not allowed.

What if then you put your Bitcoin miners or whatever in the EV, running from the same supply? Or what if you tap off the EV's battery in order to use it as a UPS?

Some years ago, Big Clive explained how a transformer can be used to reverse an electric meter using an illegal tap. I can see it working here, minus the illegal tap.