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Kevin, Licensed Electrical Contractor
Category: Electrical
Satisfied Customers: 3667
Experience:  30 years Licensed Electrical Contractor in Illinois, Adjunct College Electrical Instructor, Former Electrical Inspector, Diploma: Digital Electronics, FCC Technician License
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Why the current in a string( a branch of a Photovoltaic

Customer Question

Why the current in a string( a branch of a Photovoltaic array that consists of many modules connected in series and strings connected in parallel ) is dropped down when one cell out of 36 cells gets shaded? Based on Ohm's law that the current should be
fixed in series connection but it is not the case in here. If you can help me in this issues I can show numerical example? I need mathematical explanations ?
Submitted: 2 years ago.
Category: Electrical
Expert:  Kevin replied 2 years ago.

Hello and welcome to Just Answer. My name is ***** ***** I will be happy to assist you with your electrical question.

1) What is the DC voltage rating of the PV panels? 12, 24, 36 or 48 VDC?

2) Do you have a "clamp-on" combination amp meter and a digital volt meter available?

Customer: replied 2 years ago.
I want a mathematical interpretation for the drop in the current I1(A) in column#1 in table IV(due to the shadow effect) in the attached paper, I want you to prove it using formula, I did build the same model in Simulink so, when I shaded one cell out of 36 cells in one module, then the entire current of the string drops down, so please prove that using the math formula?
Expert:  Kevin replied 2 years ago.

Thanks for the replies.

1) Mathematical formulas nor Ohm's Law are irrelevant in your application.

You didn't reply to my questions:) What is the individual PV panel voltage and do you have a combo clamp-on amp meter or a volt meter available? This is a troubleshooting issue and not an electrical circuit analysis issue:). You need to prove what you have in place by isolating the component and using a meter:)

2) Since you have a PV string wired in series which are then parallel to other series strings, you either have a faulty PV panel or a loose wire connection or the questionable PV panel resides in the shade and is acting as an "open circuit".

3) In a series PV application, the voltages of each individual panel are incremented and totaled to a final DC voltage output at the end of the string. In a series circuit, their is only one current flow and if one panel is faulty or if one loose connection or one panel resides in shade, the entire string is not providing an output voltage due to an "open circuit".

4) I suggest de-installing the questionable PV panel and measure the open voltage on it under full sun light conditions. Compare the measured open voltage to the panel electrical spec's and see if you're close or accurate.

Customer: replied 2 years ago.
What I am doing here is just a simulation using MATLAB/SIMULINK software. To develop a fault detection method. So, my question is why I am getting the current in the entire string almost zero while each string has 6 modules and each module has 36 cells that means each module generates 36*0.6=21.6 V so in case one of these cell shaded the entire current of the string drops. How it becomes open circuit? Could u explain it using formula?
Expert:  Kevin replied 2 years ago.

There is nothing to explain via a formula nor any mathematical calculation. You have an open circuit within a series circuit.

Real simple..........An open circuit in a series circuit results in (0) zero amps current flow. --------> Therefore, no current flow in the PV string. No current flow in a circuit means that it is not functional and working.

Doesn't matter on the PV panel voltage. If an open conductor or a faulty PV panel or a shaded PV panel, the PV string will always act like an "open circuit"

You either have a faulty PV panel or a loose wire connection or the questionable PV panel resides in the shade and is acting as an "open circuit". Nothing else to explain as the problem is one of these 3.

Disconnect either the positive or the negative lead on your car battery and try starting it. You won't get very far since the battery contains an "open circuit" and no current flow. The car battery example is no different than the solar panel example.

Customer: replied 2 years ago.
Thanks for your reply but the issue is not open circuit as you can see in the paper the results of open circuit fault are shown in tableII and they are clear and I know what open circuit means but what it is not clear for me is the hot spot(shadow) how it becomes open circuit? This is the whole misunderstanding I have. There is no disconnect at all, I did create the shadow by setting the irradiance to zero instead of 1KW/m2 for one cell of the 36 cells connected in series in one module that results in the I1=0.11A and that does not make sense for me.?
Expert:  Kevin replied 2 years ago.

Thanks for the replies but I can only comment on actual field conditions and not those representative of algorithm's or mathematical calculations or software applications. I can only comment from practical field experience in the many solar panel installations that I've installed over the past 8 years.

I will opt out of the question and perhaps another electrical expert here is more intuitive to the mathematical anomalies that apply to your specific question.

No need to reply back to me as you will be notified once another expert picks up your question.

Take care and have a great evening.............Thanks.............Kevin!