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I-V Curve Tracing In Solar PV
Michael Middlemast
Business Development Manager for Renewables
IEC 62446:2016 Category 2 Tests
From the IEC: “It is relatively common, particularly for large systems, that come Category 2 tests are performed on a selected sample of the system (a fixed percentage of the strings/modules). Such a selective approach and percentage of the system to be tested will be agreed with the client prior to commissioning. ”
Category 2 tests include:
I-V Curve Tracing IR Camera Inspection
Voltage to ground
Blocking diode test
Shading analysis
What is I-V Curve Tracing in Solar PV?
The electrical power produced by a solar PV cell or module is a function of the current (I) and voltage (V) characteristics. Measuring the relationship between current and voltage whilst varying the electrical load connected to the PV cell or module from open circuit to short circuit produces a characteristic current vs voltage (I-V) curve as shown below. The points where the curve meets the current and voltage axis are the short circuit current Is/c and open circuit voltage Vo/c respectively.
Power is the product of voltage and current and so the power vs voltage curve shown can be generated from the measured voltage and current data. The power vs voltage curve shows the point at which the power is a maximum (Pmax). The corresponding maximum power point Mpp is shown on the I-V curve. Loading the PV module such that the current is Impp and voltage is Vmpp will operate the PV module at the maximum power point ( Mpp) and result in the maximum power generation.
What is the Fill Factor?
Fill factor (FF) is the ratio of the actual maximum obtainable power, represented by the dark blue box, to the product of short circuit current Is/c and open circuit voltage Vo/c, represented by the light blue box.
𝐹𝐹 =𝐼𝑚𝑝𝑝 ∗ 𝑉𝑚𝑝𝑝
(𝐼𝑠/𝑐 ∗ 𝑉𝑜/𝑐) =
𝐴𝑟𝑒𝑎 𝐴
𝐴𝑟𝑒𝑎 𝐵
The Fill Factor is essentially a measure of the efficiency of a PV module, the theoretical maximum value depending on factors such as the type of silicon used to construct the module. However, deviation from the expected value or changes in Fill Factor can provide an indication that a fault is present. • Great for comparing curve shapes • Gives valuable hints on series or shunt resistance
Understanding Datasheet Curves
Solar module datasheets show I-V curves that give an indication of the maximum power output of the module.
An example of a typical solar module datasheet
They often show several curves that indicate performance at varying irradiance and temperature levels.
By comparing these curves to our measured curve transposed to STC (standard test conditions) we get a good indication of predicted performance vs actual performance.
When to use I-V Curve Tracing
Pre-installation Stage
Testing the modules prior to installation saves time in the long run
It also gives you an indication of the power output prior to installation so you can monitor the LID effect (light induced degradation)
This gives piece of mind to your clients and saves you money and time on trouble shooting at a later date.
When to use I-V Curve Tracing
Commissioning Stage
The PV200 from Seaward can perform all required solar PV commissioning tests included in IEC 61829 and IEC62446
Taking an I-V curve of an array at the commissioning stage gives us a bench mark for future comparisons.
An example of a linear performance warranty from Trina Solar
Helps us to diagnose issues before going off site
When to use I-V Curve Tracing
Operations and Maintenance Stage
I-V curve tracing is the most powerful tool in the PV operations and maintenance engineers arsenal.
Quickly trouble shoot loss in generation by interpreting the I-V curve of an array.
Test at a string level to identify issues on a modular level.
Send reports from the field back to the office in an instant with the PV200 from Seaward
Advantages over displays
The shape of the I-V curve provides a visual indication of the performance of a module or string...
But real time monitoring systems, such as an inverter screen, a personal display unit or a generation meter only give us the total output. While a good first indication that something is wrong, they do not give you detailed information about the problem.
But how do we interpret the curve?
Interpreting the I-V Curve
Reduced Current Operator Error Wrong modules selected Wrong information entered Irradiance Measurement Issues Irradiance meter not mounted correctly Uniform Soiling When the modules are soiled equally
Interpreting the I-V Curve
Reduced Voltage Possible shorted bypass diodes When combined with other deviations this could point to potential induced degradation (PID) The could be especially true if we also have a reduced fill factor.
Interpreting the I-V Curve
Stepped I-V Curve This is when we see notches or steps in the curve. Could be spotted shading on the modules. Potential shorted bypass diodes.
Microcracks in the cells are the most common cause of this curve.
Selective Shading Technique
• Trace the curve of the entire array, this gives you the problem curve, but does not identify which module is faulty
• Partially shade one module at a time. This will reduce the power of the curve but the faulty shape will remain.
• When the curve returns to a natural shape, you have isolated the faulty module in the array.
A unique irradiance meter with added features such as a digital compass, inclinometer and thermometer. A great tool for site surveying, commissioning and an integral part of the I-V curve tracing process.
• Irradiance (in W/m2 or BTU/hr-ft2)
• Roof orientation and pitch
• Ambient and panel temperature (in °C or °F)
• Complete with a on board datalogging mode for 5000 records
Highly accurate measurement of irradiance using a monocrystalline PV cell – more accurate than using a photodiode
Suitable for both photovoltaic and solar thermal installations
Compare system performance against long-term irradiance variation
Solar Survey 200R
Solar Cert Elements V2.0
Easily create site reports and analyse curve data. Organise by plant hierarchy and compare multiple strings at once
World Leading Electrical Test Equipment from Seaward Seaward has over 70 years experience in the design manufacture of market-leading electrical test equipment. Seaward’s unrivalled range of electrical test equipment serves a wide variety of safety testing and precision measurement applications. For 25 years Seaward has been developing market-defining portable appliance testers, software and accessories for the PAT testing market. The Cropico and Clare ranges of precision instruments and industrial electrical test equipment bolster Seaward’s product offering and demonstrate the result of 70 years of technical expertise across a number of industries including manufacturing, utilities and aerospace. The market-defining range of solar PV test equipment makes meeting IEC 62446 and other PV test and documentation requirements faster, safer and easier. From solar site-survey to commissioning tests, PV system maintenance and PV system certification, Seaward Solar has it covered.
