Putting energy into the National Grid

[Geo1000]

I would like to know how it is possible to connect a device to the grid that enables energy to be put into the grid? The grid runs at huge voltages which are then stepped down to the 240 volts of the average mains supply. To my uneducated mind, anything connected to the mains supply is going to 'see' a powerful source of energy. As soon as anything is connected to the mains - energy will come out. In simplistic terms - how can a secondary device 'overcome' the mains voltage and actually shove current into the grid in the 'reverse' direction? Would it not require a greater EMF to push energy in? And how is this metered? Surely to affect the grid it has to get all the way back to the network that runs at hundreds of thousands of volts?

From the web I have found the following facts... some may be inaccurate.

A special transformer is required. The connection is made directly to the mains. Care has to be taken with the phasing.The domestic meter will run backwards when energy is going into the grid

The Web has thousands of pages saying that it can be done, but I have not yet found one that describes the actual process.

Can anyone help?

[Zevon]

Waht are you connecting or hypothetically connecting to the grid. The water analogy doesn't work here so try to ignore the idea of pushing electrcity into an overflowing pipe thats gushing electricity out.

To put electricity into the grid would require you to put the power in the same way as it is already flowing, then the emf of your supply doesn't matter it is just added to the existing voltage.

To make any difference the voltage would have to be huge and the current pretty large.

[Zevon]

The actaul process may be something like:

1)Find a mains cable and find which way the current is flowing.

2)Attach your power source to the cable in the same direction, ideally in series but cutting the cable would probably injure you and the voltage flowing woyuld probably damage the component you introduce so introduce the power in parallel to split the voltage.

To achive this get say a car battery and with jumper cables attch it to thre existing cable in the same direction. You'll put power in but no-one will notice.

The transformer is not strictly necessary, but to add power that will be noticed thena transformer is used.

[rojash]

Zevon's reply seems to ignore the fact that mains current is AC. But he is quite correct in saying that to add to the current available you don't need to push in the opposite direction!

Imagine a cart being pushed down the road. If you want to help push it, you don't stand in front and push in the opposite direction! In the same way, if you want to add to the energy available in the grid, you don't try to feed it in against the flow.

Just go with the flow, man...

[Geo1000]

I am not really suggesting that putting electricity into the grid would require current going in the reverse direction. I am sure a Physicist would get to basics and talk about 'work done'. What I want to find out is the practicalities. A circuit diagram of how the connection is made would be invaluable. I imagine that the grid would not really 'know' that a connection had even been made, and that the meter would sit between the generator and the grid and register the power going into or out of the grid.

If the potential (EMF) of the generator exactly matched that of the grid then no current would flow. If it was slightly greater then current would flow into the grid - if slightly less then current would flow out. Because we are dealing with AC it is meaningless to talk about current directions. If work is done by the generator then energy is going into the grid. If work is done on the generator then energy is drawn from the grid.

[Zevon]

Soz about the Ac, replace car battery with source like the rest of the answrer and it makes sense.

A circuit diagram woul dbe exactly as I have described, it would be the same as the grid is without the new source except for the source attached in parallel or series. The power supplied does not depend on emf of the existing grid as a generator always produces a positive source of power, otherwise it isn't a generator.

EMF is defined as the energy produced per unit charge, the emf of the existing grid does not matter, the new source simply adds its value of emf to the existing emf. the energy supplied per unit charge is added up, otherwise the old grid would have positive energy per coulomb and your new source would have negative energy per coulomb to cancel out as you imply.

[rojash]

You still seem to have a hangup with the idea that you need to overcome some sort of opposition from the energy in the grid.

If, as you say, "If the potential (EMF) of the generator exactly matched that of the grid then no current would flow", then how do you explain connecting a pair of batteries in parallel? According to your theory, the second battery would have no effect whereas, of course many devices which require higher current work in exactly this way!

[Geo1000]

Zevon - The emfs are only added if the generators are in series - which would be a bit tricky in this instance - if you connected in series you would have to interrupt the mains circuit and then all the current in the mains would flow through your 'generator'.

So Series is definitely out - it has to be parallel.

Rojash - you cannot just connect the generator in parallel and hope for the best. If the emfs are not right you could get current from the mains going in the 'wrong' direction. If you add batteries of similar emf and internal resistance in parallel to other batteries then I agree - you will not have to worry about currents - the added batteries will boost the total current available without changing the emf. But if the battery you add in parallel has a lower emf (perhaps it is going 'flat') then the other batteries will push current through the duff battery in the 'wrong' direction. After all, this is how rechargeable batteries are charged. What I am trying to find out is how this is prevented when a generator is connected to the grid. If it happens to be a windmill type device, then the energy output is going to fluctuate wildly according to windspeed. From what I have read they usually generate DC and inverters are used to make AC to connect to the grid. I am guessing that some monitoring circuitry will disconnect the device if it is not generating sufficient 'oomph'.

[rojash]

Sorry Zevon, I wasn't trying to imply that it's as straightforward as just bunging it in in parallel, I was trying to give a simple example of how you can add to power output. But I see now, that you did in fact already have a correct understanding of the problem - something which wasn't clear (at least to me) from from you previous posts :-)