Which inspection method do you prefer? (1/3)

There are various inspection methods that can be done to a solar asset. Depending on the dimension and the purpose, you can choose the type of inspection that suits you the most.

Indeed, it is necessary and important to know the solar photovoltaic system condition, both during the construction as the O&M phase.

So, we’ll talk about 3 different inspection methods for solar parks, their benefits, and limitations, namely: I-V Curves, Thermography and Electroluminescence.

In this blogpost we will talk about the I-V Curves.

What is the I-V curve and how it works?

The I-V Curve is the curve that represents the power as a function of current and voltage, produced by the photovoltaic module or string (depending on what you are testing – from now on we will always refer as string). The measurement equipment records the relation between the current and the voltage, varying the load, since the open circuit voltage until the short-circuit current. The graphic below shows the typical I-V curve and the respective power curve.

Figure 1 – Standard I-V curve.

In the field, it is necessary to record the irradiance level, ensuring that the irradiance is measured at the same inclination and orientation than the string to be tested, and the cell temperature. It is also needed to define the string (number and type of solar panels) and the length and gauge of wire between the PV modules and the PV analyzer. Depending on the test equipment, this information could be made before or after the measurements. I’ll drop a list of some manufactures of I-V curve equipment:

What about the results?

When the I-V curve have a deviation from the standard curve, it means one of two things: or the string have a problem, or the test went bad because of misconnections or the low irradiance levels. Beside the fact that you must connect the equipment accordingly (may vary from test equipment), the standard IEC 61829:2015 Photovoltaic (PV) array —On-site measurement of current-voltage characteristics says that the minimum level of irradiance should be, at least, 700 W/m2.

Each measurement gives you information about the PR and the FF, where:

PR – Performance Factor – is the most important information that the equipment could give you about the string performance. The values vary between 0% and 100% and a good and wealthy string have PR values above 90%.

Performance Factor (PF, %) = 100 * (Measured Pmax/Predicted Pmax)

FF – Fill Factor – It is the square defined by the quotient of the three important points of the I-V curve. The Isc, the Voc and the Pmáx (Imp,Vmp).

Figure 2 – Fill Factor = Imp * Vmp / (Isc * Voc). [Source: PVA User Guide].

Basically, is the way to classify the efficiency of each module and PV technology. The range of FF is from 0 to 1 and it is dimensionless.

How to “read” the curve?

The curve is influenced by 3 essential factors: solar module temperature, irradiance, and conservation state.

When you have higher values of temperature, the values of the YY axis (current) are slightly higher. On the other hand, with the increase of temperature, the voltage output reduces significatively so does the power output.

Figure 3 – Effects of temperature on solar modules [SOURCE: Seaward FAQs].

In its turn, the increase of irradiance levels increases the current output significatively and does not have an impacting effect on the voltage. Consequently, the power output increase with the rise of irradiance levels.

Figure 4 – Effects of irradiance on solar modules [SOURCE: Seaward FAQs].

These 2 first factors only change the position of the curve in the axes, with the variations of Isc and Voc. However, the curve could have different shapes, and that is an indicator of the existence of defects.

There are several defects that can be identified by I-V curve analysis, such as module Isc mismatch, tapered shade or dirt dams, shunt paths exist in PV cells, effect of shading or damage cell, effect of series resistance, effect of module degradation or soiling, etc.

The graphic below shows 3 examples of possible defects:

Figure 5 – Possible curve variations accordingly each type of defects.

Conclusions

It is a good inspection method to know if the performance of the string or module is in accordance with the expected. It could be quickly analyzed by the PF (should be between 90% and 100%).

However, it isn’t an expeditious method, and it could take some time to evaluate all photovoltaic system strings. Additionally, with the increasing use of string inverters that are able to provide the strings output in real-time through an APP, this inspection method could be used only to discover specific modules of a string previously identified by the inverter’s software string analyses.

All the best,

Solarud Team

SOURCES:

Solmetric

Seaward

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