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'[EE} Solar PV panels'
2016\06\22@193213 by David C Brown

picon face
I am looking into installing solar PV panels on the estate but I am
struggling to understand how they integrate with the existing power system./

I have a utility supply rated at about 25 kW and would install a solar
system rated at about 5 kW.  If I now impose a 6kW load how will it be
supplied?   Simple engineering suggests that 5kw willbe supplied by the
utility and 1kW by the solar panels.

But the instillation company assure me that all load up to the capacity of
the  panels will be met by them and that I only need to use utility power
as a top up.   And that excess solar power will be fed back to the utility

How is this implemented?
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak                           Phone: 01663 733236
Derbyshire                eMail: spam_OUTdcb.homeTakeThisOuTspamgmail.com
SK23 7ND          web: http://www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>



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2016\06\22@201546 by Brent Brown

picon face
On 23 Jun 2016 at 0:32, David C Brown wrote:

> I am looking into installing solar PV panels on the estate but I am
> struggling to understand how they integrate with the existing power system./
>
> I have a utility supply rated at about 25 kW and would install a solar
> system rated at about 5 kW.  If I now impose a 6kW load how will it be
> supplied?   Simple engineering suggests that 5kw willbe supplied by the
> utility and 1kW by the solar panels.
>
> But the instillation company assure me that all load up to the capacity of
> the  panels will be met by them and that I only need to use utility power
> as a top up.   And that excess solar power will be fed back to the utility
>
> How is this implemented?

Hi David,

I've recently installed a 5kW system on my own house, will see if I can help explain. My system consists of 20 x 250W panels, 10 x dual input grid-connect micro inverters. The inverters connect directly to the (in my case 230V 50Hz) AC mains at the main switchboard... this is the switchboard where every electrical load in my house is conencted. It's is important to note this is downstream, on the consumers side, of the utility power meter.

Let's say the inverters are generating 5kW of power (FYI, you should only expect to see such a peak this at midday on a very sunny summers day with your panels set to the best possible angle). First let's say there is zero load presented by your house, then all 5kW will be "exported" back to the grid (ideally you have an import/export meter fitted and you get credit for this. Now let's say the house load is 5kW... supply and demand are balanced, zero power is imported or exported.... your power meter indicates zero. Now say the house load is 6kW, 5kW will be supplied by the inverters, the grid supplies the remaining 1kW and your meter should indicate 1kW of power is being imported.

Hope this helps. It does take a while to get a handle on power flows etc. Extra for experts: think about what the voltage/current waveforms look like during export vs import.


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2016\06\22@212541 by John Gardner

picon face
If I've got it right (& I probably don't),  the PV system,  after converting

the DC output of the panels to AC grid specs,  adjusts the phase angle

between grid power & PV power to suit the needs of the moment?
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2016\06\23@035656 by RussellMc

face picon face

                  BCC Ken - comments?

On 23 June 2016 at 11:32, David C Brown <.....dcb.homeKILLspamspam@spam@gmail.com> wrote:

{Quote hidden}

​_________________

The installation company is ~= correct as long as the inverter is designed
correctly.
The inverter will produce maximum power from the DC input (if set to do so).
The inverter can configure its voltage and phase angle such that it will
transfer excess power to the grid if excess power is available and allow
power drawer from the grid i Pinverter < Pload. . ​
​By ...

​________

E&OE.
No guarantee that this is fully correct.
As it in part MAY disagree in some degree with Brent it's probably ​in some
degree wrong (probably for both  correlated and non correlated versions of
some)
* I changed disagrees' to 'may disagree'  as I'm talking about V and he may
have been talking about I.
We appear to still not be in full agreement.  eg I say max power at V +/-
90 lead/lag of Vgrid. .

A simplistic but useful look at power transfer is given by:

V1 = invertervoltage
V2 = grid voltage
X12 or just X is coupling impedance between inverter and grid. This usually
includes a physical inductor or sustem inductance.
Delta = d is phase angle between inverter and grid.
sind = sin(d) , cod = cos(d)

Real power transfer = V^2/X = (V1 x V2)/X x sind
So may transfer occurs at d = 90 degrees.
V1 leading V2 gives max real powerexport.
V1 lagging V2 gives max real power import.
Power is also controllable by varying V1.
[Murphy suggests that getting V1 leading V2 may require V1 > V2 but that's
a technicality].

Imaginary power transfer = V1^2/X - (V1 x V2)/X x cosd

As cosd = max = 1 when sind = 0 and d=0, pur  reactive power max occurs
when V1 and V2 are in phase BUT if V1 = V2 you then get zero reactive
power. By then altering V1 relative to V2 you generate negative or positive
reactive current in X and thus overall reactive power.

______________

I dislike "slideshare" pages but this slide set gives the simplest
treatment I found.
Slide 5 says much as above but other slides are variably useful

         http://www.slideshare.net/niteshjha3705/grid-tie-inverter

Pages 52-55 here say similar
MODELING AND ANALYSIS OF A PV GRID-TIED SMART INVERTER’S SUPPORT FUNCTIONS
Thesis 2013m 112 pages

 http://www.slideshare.net/niteshjha3705/grid-tie-inverter

Nicish TI grid tied microinverter reference design with software available
as a module in the free development suitye
Grid-tied Solar Micro Inverter with MPPT
          http://www.ti.com/tool/TIDM-SOLARUINV

ST 3 kW equivalent - 65 pages but maybe no software

             <
http://www.st.com/content/ccc/resource/technical/document/application_note/0b/16/e1/a7/0e/db/49/09/CD00253868.pdf/files/CD00253868.pdf/jcr:content/translations/en.CD00253868.pdf
>




*Relevant*


Mainly for schematic.
One topology of many

        http://solar.smps.us/grid-tie-inverter-schematic.html


IEEE 2012
6 pages
Reactive Power Control of Single Phase Grid Tied Voltage Sourced Inverters
for Residential PV Application


http://www.ele.utoronto.ca/wp/wp-content/uploads/2014/11/C/X_Zong-C39.pdf


Google: Simplified Reactive Power Control for Grid-connected Photovoltaic
Inverters

*Less relevant:*


https://yhipower.co.nz/downloadhandler.axd?type=2&id=100246&ins=1

http://www.cleanenergyministerial.org/Portals/2/pdfs/A_Guidebook_for_Minigrids-SERC_LBNL_March_2013.pdf


Am I starting to sound like Brent?
:-)


       Russell
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2016\06\23@035834 by RussellMc

face picon face

On 23 June 2016 at 13:25, John Gardner <goflo3spamKILLspamgmail.com> wrote:

> If I've got it right (& I probably don't),  the PV system,  after
> converting
> the DC output of the panels to AC grid specs,  adjusts the phase angle
> between grid power & PV power to suit the needs of the moment?
>
> ​Yes, + voltage.

R​
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2016\06\23@234938 by RussellMc

face picon face

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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