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PostPosted: Tue Sep 19, 2017 1:03 pm 

Joined: Wed Sep 13, 2017 5:06 pm
Posts: 4
Dear André and PVsyst users!

We are working on some large projects (bigger than 1MW), where we need to make simulations on our solar plants. We just started using PVsyst and I think we are getting better and better at it, but there are some questions which we cannot answer after trying many scenarios.
Our biggest problem now is that we do not really know how does the software calculate the AC ohmic losses for the whole system. We need to give the total length of the wires between the transformer and the inverters, and also the total cross-section of the several AC cables. The problem is that the AC ohmic losses that are shown in the report don't match with our estimations (much bigger than the estimations). Usually between 6 and 13%.
Please let us know, if only we are overcomplicating this issue?
On the other hand, can you please tell us what is the accurate way to give the correct parameters of the AC wiring?
(For example: if we have 42 inverters each with 50mm2 cables to the transformer, then we need ~2100mm2 cross-section totally, but we need to calculate with 42*100m length?)

I am looking forward to your response.


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PostPosted: Fri Sep 22, 2017 12:25 pm 

Joined: Wed Sep 13, 2017 5:06 pm
Posts: 4
I am curious that anybody who makes a design for a solar power plant, how does he setup these parts of the project? We got way too much losses if we give the datas mentioned above.

Thanks.


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PostPosted: Fri Sep 22, 2017 8:38 pm 

Joined: Thu Aug 02, 2012 3:38 am
Posts: 105
PVSyst uses an equivalent single resistance and source to calculate losses. So you have to convert the mesh of conductors into a single equivalent resistance that would carry the current from a single equivalent source to the POCC. This is a basic series parallel reduction of resistances. Best to set it up in a spreadsheet that calculates the equivalent resistance as you vary the size and length of the actual conductors.

Keep in mind that the actual losses are going to be computed dynamically based on the current through the equivalent resistance on an hourly basis. It’s difficult to compare that to a hand calculation that is based on some assumed average current. You should also calculate a worse case voltage drop for each actual run from an inverter to the POCC to make sure you do not have problems with overvoltage shutdown of the inverters. That’s going to set an upper limit on the resistance to ensure system operation.


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PostPosted: Tue Nov 07, 2017 4:20 pm 

Joined: Wed Sep 13, 2017 5:06 pm
Posts: 4
I really appreciate your answer!

As we want to truly understand, how PVsyst's calculation works, we need some further advices about how we should calculate the AC Ohmic losses. We think that our calculations are wrong if we are giving the equivalent length of the conductor, because we have some real time measurements from older projects that we brought to life --> and the losses ended up close to our previous hand calculations.

If we have 14 inverters, each with 50mm2 cross-section and a length of 'L' cables to the transformer, then what is the method to get the equivalent conductor?
According to our previous calculations (sum the cross-sections and lengths), the simulation end up giving way too much losses.

We are looking forward to your answer!


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PostPosted: Tue Nov 07, 2017 7:56 pm 
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Joined: Tue Nov 07, 2017 6:54 pm
Posts: 1
Location: Genk, Belgium
Dear Andrew, Hammon, PVsyst users,

I'm also trying to understand the calculation behind the "AC Ohmic losses" in the "detailed losses" section. I tried several calculation but can't calculate the same values as calculated by in the section "AC circuit: Inverter to injection point".

Further I have the following questions that adds on:
- From where do the electrical values (Pac and I) behind the "STC:" writing come? These values aren't the same as the given values of the inverter? Please check the values of the inverter and values given in the section "AC circuit: Inverter to injection point".

- If we have one tri-phase inverter and we connect it to the grid with a 3x10 mm² (fase) + 10 mm² neutral cable, we have to choose 30mm² (=3x10mm²) as cross section or 10 mm²? If right what Hammon says, the answer on this question should be 30mm².
=> If the length of cables (3x10mm²) is 10 meter, than the length to be filled in should be 10 meter, OK? Now if we have two inverters that are connected the same, should we fill in as cross section 60 mm² and the lengst stays 10 meter?

Thanks for your help and reply!

_________________
MSc. Alessio Oliva - www.oliva-technics.be


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PostPosted: Fri Nov 17, 2017 9:33 pm 

Joined: Fri Feb 05, 2016 4:20 pm
Posts: 4
To expand on the previous, I am also confused as to how the Pac is calculated in the "AC Circuit:" inverter to injection point". The value that is shown does not match the nameplating rating of the inverter or the STC value of the solar array. Changing the DC value changes this Pac, and changing the inverter quantity changes it slightly. There doesn't seem to be a solid correlation, just that the Pac is roughly 2-3% of DC kW at STC


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PostPosted: Fri Jul 05, 2019 4:40 pm 

Joined: Wed Jul 03, 2019 10:29 pm
Posts: 3
Location: Houston
I want to also add into it as well, I ma designing a 200 MW AC with bi-facial with tracking modules system project but my transformer losses are too high. External transformer losses which I am talking are (Iron loss .10%, resistive losses .90%). Are these number high? Furthermore, what else I can do to decrease transformer losses?
Your help is appreciated.


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