How to grid-locked/tied synchronous inverters work in altern

[Peter]

I am curious of the design of so-called synchronous inverters or
grid-locked inverters as used in alternative energy, wind and solar
power configurations.

I am quite familiar with inverters such that a windmill, at any
windspeed/shaftspeed could generate 230v AC at any phase, but of
course at hugely variable current. However I am curious of the
technique to 'blend' this power with the household supply such that
according to local usage requirement all the generated energy will
either be used locally or redirected back into the grid.

Does anyone have any technical references / comments?

 

[Ian Stirling]

Peter <usenet_AT_tecno.demon.co.uk@junkblock.com> wrote:

I am curious of the design of so-called synchronous inverters or
grid-locked inverters as used in alternative energy, wind and solar
power configurations.

I am quite familiar with inverters such that a windmill, at any
windspeed/shaftspeed could generate 230v AC at any phase, but of
course at hugely variable current. However I am curious of the
technique to 'blend' this power with the household supply such that
according to local usage requirement all the generated energy will
either be used locally or redirected back into the grid.

Does anyone have any technical references / comments?

If you connect up a synchronus motor (or generator) to the network,
then the torque is proportional to the lead/lag from the mains to its
rotational phase.
It's moderately trivial to arrange for some means of synchronisation,
so there is no net lead/lag at all from your inverter, you're just
pushing amps in in phase with the mains.

 

[Peter]

It's moderately trivial to arrange for some means of synchronisation,
so there is no net lead/lag at all from your inverter, you're just
pushing amps in in phase with the mains.

Sure, but my question was not about how to synchronise but the 'blend'
for want of a better word such that the surplus power goes to either
the mains supply of the local load, I.e. in neither case is the
locally generated power thrown away.

 

[Tony Williams]

In article <0k7ga152f8s6bg0c8mqf0sb6aeiokcl2i4@4ax.com>,
Peter <usenet_AT_tecno.demon.co.uk@JUNKBLOCK.COM> wrote:

Sure, but my question was not about how to synchronise but the
'blend' for want of a better word such that the surplus power
goes to either the mains supply of the local load, I.e. in
neither case is the locally generated power thrown away.

Search on the keywords 'asychronous alternator' or
'induction generator'.

It is an ordinary induction motor, driven by any source
of mechanical power, usually intermittant in nature.

Any time the mechanical power drives the shaft of the
induction motor at greater than synchronous speed, the
motor is connected to the mains. Because it is running
faster than sync speed it generates AC power, and always
at the frequency of the mains.

--
Tony Williams.

 

[Peter]

I am looking designs and explanatory notes for entirely electronic
methods. Nothing using rotary mechanical parts.

 

[Ian Stirling]

Peter <usenet_AT_tecno.demon.co.uk@junkblock.com> wrote:

It's moderately trivial to arrange for some means of synchronisation,
so there is no net lead/lag at all from your inverter, you're just
pushing amps in in phase with the mains.

Sure, but my question was not about how to synchronise but the 'blend'
for want of a better word such that the surplus power goes to either
the mains supply of the local load, I.e. in neither case is the
locally generated power thrown away.

It's irrelevant.

You feed all the power to the grid, and connect your appliances to the
same point.
You need some scheme to check that you've not been spectacularly unlucky,
and the power you are using is exactly the power you are generating and the
line has gone down just outside your house.

For example, don't feed the grid at 0-20V, and don't turn on your inverter
unless the 0-20V slope looks right.

 

[Terry Given]

Peter wrote:

I am looking designs and explanatory notes for entirely electronic
methods. Nothing using rotary mechanical parts.

its pretty simple really.

Firstly, the "blend":

the energy source connects to the grid (as does the house and its
appliances), and attempts to pump in power (assuming V,f etc are at
appropriate levels to do so). The impedance of the grid is low, and the
impedance of the house is governed solely by the load it draws (IOW
whats turned on).

The energy source pumps current into the connection. some of that
current flows into the house, the rest into the grid. The division is
governed by the three impedances - grid, house and inverter. Should the
house load decrease (whilst power generated remains constant) then the
portion of current flowing to the house decreases, and the portion
flowing to the grid increases. A "T" network of three series R-L
circuits is a pretty good model.


secondly, the box of tricks:

normally one has two inverters - one to connect to the grid (fixed
voltage & frequency), one to the AC generator (variable voltage &
frequency). In between the two is a DC bus, containing a sufficient
amount of capacitance to smooth the input (and output) power
[P(t) = Ppksin^2(wt)] to its average value.

the grid-tie interface is often done by increasing the voltage and/or
frequency to push the desired power (IOW that being sucked out of the
generator) into the grid (there are limits on the percentage change in V,f).

HTH

Cheers
Terry