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101 renewable - PV module electrical output profile

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Last Updated
4th of October, 2018

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A Guide for PV System Technicians and Engineers

pv panel graph

I-V curve illustrates the electrical output profile of a PV module, string or array and certain points on an I-V curve are used to rate the module or string performance. That is achieved in conjunction with additional reporting software. An I–V curve characteristics are changing with the temperature or with the level of the irradiance. In order to trace an I-V curve on all four channels: current, voltage, temperature and irradiance we have to sample both the Idv and Vdc respective temp and irradiance with a frequency of hundreds of measurements per minute  and the I–V curve analyzer has to provide a capacitive load at its own end to resonate the outgoing and incoming waves signals against a calibrated probe. 

The I–V curve analyzers promoted by Clean Energy Brands are used to commission, troubleshoot or to retrofit PV power plant installations. Modules are rated based on electrical performance. Every PV module is delivered with a “Name Plate” rating at Standard Test Conditions (STC) corresponding to NEC requirements. The most significant rated values are I-V standard values and the "Pick Power" value. Those are listed on the nameplate attached to the back of the solar module. The I-V values measured are short circuit ampacity and open circuit voltage at standard test conditions (STC). Manufacturers have to guaranty the pick power value for a +/- 10% from the power (Pmax) listed on the nameplate.

Some states respectively some of the utility companies require different another type of rating certificates provided by independent laboratories. This type of ratings will try to match the real life cycle of a PV module.  In  this way, we will get a flattening of the I –V curve with a reduction in "Pick Power" value listed for that specific module.

Solar Panels  commissioning and testing will assure the end user - the installation beneficiary - that the PV power plant is working with high efficiency. Checking the modules before install and during maintenance allows to spot problems and isolate the defects. During the tests of the solar panels we will map solar panel intrinsic values as I-V curves and electric parameter: Nominal power (PMAX, measured in W), open circuit voltage (VOC), short circuit current (ISC, measured in amperes), maximum power voltage (VMPP), maximum power current (IMPP), peak power, kWp, and module efficiency (%). We will compare those to the product data sheet containing the OEM values.

In general, solar PV modules are electrically tested for voltage and current values? Here are the voltage and current pair values we must sample:

Open-circuit voltage (Voc): Open circuit voltage or VOC is the maximum voltage that the panel can produce when not connected to an electrical circuit or PV system. VOC can be measured with a meter directly on an illuminated panel's terminals or on its disconnected cable

Maximum power voltage (Vmp): Voltage at maximum power point (Vmp) is the voltage at which the module puts out the most power. All voltage measurements are made at the solar panels electrical terminals mounted on the module’s back. These measurements are usually made with a calibrated voltmeter and MC3 or MC4 test leads.

Short-circuit current (ISC): Short Circuit Current (Isc) is the amount of current that the solar panel outputs into a dead short.

Maximum power current (IMPP): Current at maximum power point (Imp) is the number of Amperes delivered by the module at its maximum power point.

Maximum Power / Maximum power point (Pmax): The Maximum Power Point is measured in Watts (W) and is equal to Amperes times Volts. Every solar panel has a specific point on its power curve where the product of Amps times Volts yields the greatest Wattage. This is the Maximum Power Point, and the solar panels wattage. In order to find the module’s maximum power point, the flash test takes data over the entire range of voltage and current. This way the wattage for each Current and Voltage data point can be calculated (IV curve).

After we collect the information in a set of tests carried over different intervals we can determine two of the main parameter values of the solar panels:

Panel efficiency (%) or Fill Factor (FF):  The Fill Factor is defined as the maximum power produced (at MPP) divided by the product of Isc and Voc. One can see that the Fill Factor will always be less than 1. Due to the difficulties in measuring conversion efficiency quickly, it is common to measure the fill factor instead.

Peak Power Rating (kWp): The peak power rating, kWp, is the maximum output according to standard test conditions (not the maximum possible output).

The values measured are logged into a data acquisition log and compared to the label values. The match must come in a 3% tolerance. In some situations after more as 10 years in service, a derating factor is applied. In general, the industry warrants 80% efficiency after 20 years from the time they the panels have been commissioned.

 

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