What is I-V Curve
Power P is given by P=V*I and Solar Modules in any given operational conditions have a single operating point where the values of the current (I) and voltage (V) of the cell results in maximum power output. An I-V curve is a graphical representation of the relationship between the voltage applied across an electrical device and the current flowing through it.
I-V curve is one of the simplest and common method to determine the functioning of an electronic device. The shape and details of the curve provides greater insights into the functioning of the electronic device.
What is I-V Curve Testing
I-V curve testing is performed to the PV Modules and Strings to determine the performance of the array in a PV system. I-V curve testing reveals about the PV Module performance and condition better than other conventional testing methods.
I-V testing is performed by applying a series of voltages to the device. The current flowing through the device is measured at each point of the voltage. The current is measured by an ammeter connected in series and the supplied voltage is measured by a voltmeter connected in parallel to the device. A source measuring unit a device capable of simultaneously supplying the voltage and measuring the current with high accuracy can also be used for measuring the I-V curve.
The I-V curve (current versus voltage) represents all possible operating points of a PV module, string or array at existing environmental conditions. The curve starts at the short circuit current and ends at the open circuit voltage as shown in Figure-2. The maximum power point, located at the knee of the I-V curve, is the operating point that delivers the highest output power. It is the job of the inverter’s maximum power point tracking circuit (MPPT) to find that point as irradiance and temperature change. The P-V curve (power versus voltage) is zero at either end or a maximum at the knee of the I-V curve.
Any impairment that affects the shape of the I-V curve as shown in Figure-3 will reduce the peak power value and diminish the value of the array. The impact of a mismatch impairment on output power is shown in Figure 4 below
What is the Requirement of I-V Curve Testing
I-V curve testing helps in identifying the anomalies and pinpoint the root cause of underperforming PV Modules or Strings.
Requirement or the benefits of the I-V curve measurement are as tabulated below.
|Reduced Test Time||I-V curve tracing measures array performance with a single electrical connection at each combiner box, and a single measurement per string.|
|No need to bring the inverter on-line to test PV string performance||Traditional test methods required the inverter to be brought on-line in order to measure the operating current of each string under load|
|Reduced start-up and commissioning risk||Testing the array before the inverter is brought on-line means less risk of array-side problems showing up during start-up or commissioning.|
|More detailed measurement results||I-V curve tracing is the most comprehensive test possible for PV arrays. In addition to measuring the short circuit current and open circuit voltage, it uniquely measures the maximum power point of each string.
The PV Analyzer measures 100 points along the I-V curve. With this resolution, the measurement can detect more physical effects that may be degrading the performance of the system.
|Efficient data management||I-V curve measurement data is saved electronically, eliminating data recording errors.|
|Detailed performance baseline||PV arrays are extremely robust and reliable, but performance does gradually degrade. Occasionally a module will fail. I-V curves captured and stored at start-up or commissioning time give you a detailed baseline against which to compare future measurements over the life cycle of the PV system.|
|More efficient troubleshooting||Curve tracing is an invaluable tool when troubleshooting PV strings, detecting excess series resistance, low shunt resistance, module mismatch, or shorted bypass diodes directly from the shape of the I-V curve.|
Which Stage I V Curve Can Carryout
-To Check the Quality Before Dispatch
|-Time of Pre-commissioning
-To Check the Performance Before Commissioning
-To Check the Performance Periodically
Expected Results of an I-V Curve Testing
The expected results of an I-V curve are as tabulated below
|Type of Losses||Probable Cause|
|Series Losses (Losses due to excess series resistance show up in the I-V curve as a decreased slope, or inward tilt, of the curve near Voc.)||Corroded or poorly connected array wiring|
|Shunt Losses (Losses due to shunt resistance show up in the I-V curve as an increased slope, or downward tilt, of the curve near Isc.)||Cracked cell or damage Cell at the time of Manufacturing, Transportation, installation|
|Mismatch Losses (Substantial Mismatch effect shows up as Notch or Kinks in the I-V curve.)||Shading, Uneven soiling, cracked PV cells, Shorted bypass diodes and mismatch module/Cell.|
|Reduced Current (Reduction in the Height of the I-V Curve can be caused by Uniform Soiling, Edge Soiling or Weather condition that reduce the Irradiance.)||Uniform soiling, edge soiling, PV module degradation or weather condition|
|Reduced Voltage (The Width of the I-V Curve is affected by Module Temperature. Poor Air-Circulation.)||Module temperature, Poor air circulation and issue with the heat dissipation|
|Series Losses (Losses due to excess series resistance show up in the I-V curve as a decreased slope, or inward tilt, of the curve near Voc.)||Corroded or poorly connected array wiring.|
|Shunt Losses (Losses due to shunt resistance show up in the I-V curve as an increased slope, or downward tilt, of the curve near Isc.)||Cracked cell or damage Cell at the time of Manufacturing, Transportation, installation.|
Standards of I-V Curve Testing
Applicable standards of I-V curve testing are tabulated below
|IEC 61829:2015||Photovoltaic (PV) array – On-site measurement of current-voltage characteristics|
|IEC 62446-1:2016||Photovoltaic (PV) systems – Requirements for testing, documentation, and maintenance – Part 1: Grid connected systems – Documentation, commissioning tests and inspection|
|IEC 60891:2009||Photovoltaic devices – procedures for temperature and irradiance corrections to measured I-V characteristics.|
Limitations of I-V Curve Testing
General limitations of I-V Curve tracing are as tabulated below
|Cannot be used as real-time monitoring||I-V curve testing reports data from a single point in time and is heavily dependent on environmental conditions. This cannot be used for real-time monitoring of the Module performance.|
|Diagnostic Information is confined to that reference time stamp||Defects can go undetected for longer periods of time until the next I-V curve tracing activity has been conducted. Hence periodic I-V curve tracing activity must be planned every 6 months to 1 year for better diagnostics.|
|Module Level Analysis||Performing I-V curve testing to strings would not bring out anomalies or defects of neighbouring Modules which are underperforming. However, if the neighbouring Modules exhibit considerable degradation it can be found during String I-V measurement activity.|
|Requires costly labour & equipment||Searching for the problem by the technicians in the site is a time instinctive process and without the support of the real time and continuous diagnostics this exercise becomes more challenging.
However, this problem can be avoided by interacting with the O&M team and identifying the low performing blocks/areas for the measurement and performing the measurement in those blocks/areas.
|Cannot be used for raising Warranty Claims||The I-V curve testing only provides the information about probable degradation and cannot confirm the same. The Modules have to be sent to the Third-Party labs for ascertain the degradation.
However, I-V curve can help in identifying the defective Modules and minimize the financial risk of losing the warranty claim which may arise if in case the Modules are selected randomly without performing I-V curve testing.
I-V curve testing is an affordable, compact, fast, and easy to use measurement technique. The shape of I-V curve provides critical information about potential causes of the performance problems in the Modules and provide us with preliminary data to infer the probable cause of underperformance in the PV Modules. However, only I-V testing would not be enough to determine the root cause of the performance degradation. The combination of Thermography, I-V curve and EL testing would be required for effective and reliable results.