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Thread: Solar PV panels
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face picon face BY : RussellMc email (remove spam text)


My friend Ken has useful experience in the interface aspects of grid tie
inverters.
I sent him a portion of the discussions here and he provided the following
comments:


Russell,

None of this is likely to be "news" to you, but some of your audience may
find it helpful.

The principle at the core of any grid-tied inverter is that it acts as an
AC current source with a voltage compliance that exceeds the peak voltage
of the AC mains.

Once you have that, and a means to synchronise the current injection
waveform from the inverter with the AC mains voltage, you can happily
deliver current (in any phase relationship to the voltage) into
​​
the grid supply.

The fact that the grid has a very low impedance makes the job easier
because the process of injecting current does not significantly perturb the
voltage waveform  - thus making continued synchronisation reliable.  Detection
of any such voltage perturbation should it occur is one means by
which grid-tied inverters detect islanding.

Well-designed grid-tied inverters can also be set up to work with a
relatively "soft" AC supply.  For instance you can use an SMA Sunny Island
inverter to establish a "local" grid and then add Sunny Boy (or Windy Boy)
grid-tied inverters to inject further power in order to supplement that
grid supply.

The basic principle has been around more or less since the beginning of AC
power distribution  - but in early times took the form of synchronous
rotary machines that delivered reactive power (current out of phase with
the voltage) to the supply in order to effect power factor correction.

Note that for the purposes of many grid-tied inverters an AC current source
is just a DC current source (usually implemented as a high-frequency
switch-mode power converter  - often in boost topology) plus a means of
commutation (often a full bridge operating at the supply frequency of 50 or
60 Hz) to deal with the voltage reversals of the AC grid supply.  In effect
the commutation process takes a half-wave "rectified" output generated by
the current source and converts it to a full-wave AC output compatible with
the (sinusoidal) grid supply.

The issue of drawing current from the grid while simultaneously injecting
current into the grid is easiest to understand if you just regard the grid
as an AC bus capable of absorbing or delivering any amount of power you
like.  Ignoring the added complexity of reactive power (when the current
and voltage are not in phase), if you draw more current than you inject net
power is delivered to you, and if you draw less current than you inject net
power is delivered to the grid.

In the real world things get a bit more hazy because (in this part of the
world at least) utility companies pay a lot less for energy (kWhrs) you
inject into the grid than they charge for energy that you take from the
grid.  They will often use separate energy meters  - one operating
conventionally to record the energy consumed by your electrical load and
the other connected "backwards" between your own source of electrical
energy (PV panels, wind turbine, micro-hydro, etc.) and the incoming grid
supply.  The AC bus still exists (on the grid side of the two meters),
but your electrical load and your own electrical energy source are not
directly connected.  Many modern digital energy meters can combine the
separate measurement functions into one device  - in which case the grid
connects to one port of the meter, and both your load and your own energy
source (connected in parallel) connect to the other.  Then, by monitoring
the phase relationship between the voltage and the current flowing through
the meter, energy consumed and energy delivered can be separately metered.

As an interesting experiment that will almost certainly lead to an
epiphany, take two suitable identical iron-cored transformers.  Supply one
primary directly from the mains and the other primary via a Variac.  Set
the Variac output to match the mains voltage.  Connect the two secondaries
together via a low-resistance current shunt  - taking care to get the
phasing the same.  Use an oscilloscope to display the secondary voltage on
the transformer fed from the Variac, and the voltage across the current
shunt.  Adjust the Variac voltage up and down slowly while watching the
waveforms.  You can alternatively conduct the experiment in simulation  -
using LTSPICE or similar.

There is a widespread misapprehension that loss of synchronisation between
a grid tied inverter and the grid supply will necessarily result in
destruction of the inverter.  While that may be the case for a
poorly-executed design, the fact that the inverter is a current source
means that it is potentially capable of driving a controlled current into
the grid supply regardless of the instantaneous voltage of that supply
(within the normal limits of the peak AC voltage).  So for instance if the
inverter has an internal DC bus (often called a DC link) of say 600V, it
can deliver current into a 230VAC (rms) supply when the supply voltage is
at its positive peak (+325V) and the voltage difference is 275V, or when
the supply voltage is at its negative peak (-325V) and the voltage
difference is 925V.  Not all inverters are designed to deliver current over
the full range of voltage difference (because that's not typically
necessary for most applications)  - but it is certainly technically
possible.

In the past, most grid-tied inverters have been unable to accept
significant power from the grid (i.e. anything greater that what they need
for their own internal "housekeeping"), but there is a class of grid-tied
inverters that can have an associated battery bank  - and these (often
referred to as inverter-chargers) can take significant energy from the grid
and deliver it in order to charge the storage batteries.
Inverter-chargers will become increasingly common as battery technology
improves and becomes more cost-effective and systems like Tesla's
"PowerWall" which support temporal "load-shifting" become popular.


Regards,

Ken
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