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Photovoltaic Line Interactive Inverter

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Mr.Cool

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inverter synchronization

hello everyone. i am currently designing a Photovoltaic inverter for residential use. i was wondering if there are any people out there that can provide some advice?

currently, i am using a full H-bridge, single phase inverter. it connects to the grid via a single inductor (not an active filter). i am using analog method to generate the gate functions (compare a triangle wave to the sine wave from the grid). between the solar panal and the inverter lies a Maximum Power Point Tracker (MPPT) to force the solar panel to produce its maximum power. (more on this later).

my questions are, how can i ensure that power flows from my inverter to the Grid at ALL TIMES ?
what i have come up with so far is to force the phase of the current to be at unity power factor by adjusting the voltage phase angle.
the forumula looks like:

I*(phase_angle) = Vinverter*(inv.phase_angle) - Vgrid*(0)
--------------------------------------------
Z(90degrees)

note that the grid phase angle is taken as the reference and is hence equal to 0. The Z (impedance) of the inductor is 90 degrees (or is that -90?). Thus, the phase angle of the inverter voltage can be adusted to control unity power factor.

is it true taht power will ALWAYS flow from inverter to Grid so long as the magnitue of the inverter output voltage is ALWAYS greater than the grid voltage? or is there some other method?

my second question, since my solar panals will produce a varying DC supply to the inverter how do i make my inverter "immune" to a varying DC supply? a varying DC supply will cause lower order harmonics which must be avoided. i do not want to use a regulator at the output of the solar panal as i believe this is a waste of power dissipation. any suggestions?

thanks for any help
Mr.Cool
 

flatulent

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grid force inverter

I vaguely remember from school (35 years ago) that in the utility system, generators are varied in phase relative to the line to control the power flow. Can you get a power book from the library to consult?
 

Mr.Cool

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synchronizing inverter

hi. yes i have already gotten a book. i know much on the subject already, as i worked on big rotating generators synchronizing to the grid. but i have never done a simple (??) inverter synchronization.

i agree with your phase suggestion. i believe that controlling the phase angle is important.
 

Mr.Cool

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line interactive inverter

since Magnitude of the inverter voltage is :

Vmag,inverter = sqrt[(Re^2 + Im^2)]

and the phase angle is:

Vangle,inverter = tan^-1(Im/Re)

thus i believe it is possible to vary the phase angle to achieve Unity power factor (current Io in phase with Vgrid). at the same time, because of the mathematical relationship shared between the Magnitude and Phase, i believe it is possible to maintain a Vmag,inverter > Vmag,grid. thus the power flow will be in the inverter --> grid direction.

will this work?

Mr.Cool
 

Mr.Cool

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pdf + solar + grid synchronized inverter

by the way, i have not found a power electronics book that describes (at all!) how to *actually* connect an inverter to the grid. the books always talk about frequency, phase and magnitude for synchronization. but how do you garauntee power flow in one direction? yes i can match the frequency, magnitude and phase, and yes i can close the breaker. but i need to understand further how the grid affects the inverter. the grid is not like a dummy load. it controls your inverter and dictates all aspects after you close the breaker.

can anyone recommend a good book? preferably an eBook.

Mr.Cool
 

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photovoltaic line

I have discovered today that many utility companies have information packets for people doing what you are doing.

If all else fails, you can do some emperical measurements. Back in my army days we would put a light bulb between a generator and the system to keep from damaging the generator while making adjustments. When the phase and voltage were correct, we would jumper around the bulb.

you could start with a low current 220 V bulb to prevent damage while you make measurements and adjustments. Continue in steps of raising the current rating of the bulb until you have enough emperical data to formulate a theory and feel confident about jumpering around the bulb.
 

Mr.Cool

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interactive invertor

unfortunately this does not answer my question

but thanks anyways :)

Mr.Cool
 

TekUT

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photovoltaic, inverter, thesis, pdf

I know that this topic was rather old but I've some interest in it. Do you've found the needed information? Could you please give some infos about the subject (how to be sure that the generated power came from the inverter to the grid)?

Thanks in advance
Pow
 

FvM

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how we connect an inverter to line

In a short, the arrangement of control loops achieves a correct current waveform and phase (PF = 1) and an energy flow according to your demand.
 

Mr.Cool

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how do grid interactive inverters work

agreed, been a long time!!!

i did finish this project & connect to the grid without any problems. the power factor which is code for phase angle is the key. it is certianly possible to deliver ACTIVE power to the grid while simumltaneously consume REACTIVE power from the grid, or vice-versa.

the best information i found was to read up on electric generators connected to the grid - power systems book. think of your inverter as producing the same waveforms of voltage (assuming you have sinusoidal filter at output of your inverter) and current as your generator - except you have more and better control.

my suggestion is to use fuses everywhere and initially connect your inverter's output to a Variac which is plugged into the wall. then you can connect to the voltage of your choice, rather than grid voltage (120V for north american residential).

lots of information also in the renewable energy field and books on this subject.

this is easy to simulate in Matlab/Simulink because it is primarily a systems architecture + controls problem.

then, once you have it working you are left with the 3 biggest problems:
1) how will you make it comply with regulations, IEEE-519 for example
2) how will you make it safe and fault proof
3) how will you make it idiot proof (i.e. installer connects your inverter's DC supply backwards)

good luck,
Mr.Cool
 

TekUT

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thesis simulink model of mppt

Mr.Cool said:
agreed, been a long time!!!
...cut...
Nice to know that all working fine, may be a chanche to see somethings about your work on this board (schematics, some firmware hint and so on)?

Best regards
Pow
 

Mr.Cool

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photo voltaic inverter

i don't have any of my files handy anymore, it was a long time ago. and i only made a single prototype, i didn't go on to do the more difficult stuff i listed above so i have only the basic functionality, nothing worth while.

but nowadays there is lots of info out there. just type in "thesis" with your search string in google. for example, i found this:

http://www.energy-based.nrct.go.th/...usoidal hysteresis current control.thesis.pdf

and

http://www.kuleuven.be/ei/Public/publications/EIWP03-02.pdf

Mr.Cool
 

TekUT

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modele mppt simulink

Mr.Cool said:
i don't have any of my files handy anymore, it was a long time ago. and i only made a single prototype, i didn't go on to do the more difficult stuff i listed above so i have only the basic functionality, nothing worth while.

but nowadays there is lots of info out there. just type in "thesis" with your search string in google. for example, i found this:

http://www.energy-based.nrct.go.th/...usoidal hysteresis current control.thesis.pdf

and

http://www.kuleuven.be/ei/Public/publications/EIWP03-02.pdf

Mr.Cool
Thank you very much for these link, if you can retrieve your old work and make it as a sort of tutorial I think should be of great interest for all, of course I can understand that this may require some time, but if you can do it I think will be great.

Bye
Pow
 

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