Guidelines for selecting PV Modules
PV modules in case of large scale power plants are selected based on the integration of modules, voltage and current rating with the inverter voltage and current rating. Typically for a grid connected plant the inverters operate in the voltage range of 450-750 volts and accordingly the modules are selected to match the desired operating voltage level through parallel and series combinations.
While selecting PV modules, it is important to consider the power tolerance limits under the standard temperature conditions. Typically power tolerance in PV modules range between -5 % to 5%. For example- a 280 watt module with -5% to 5% tolerance will give output of the order of (280* (1-.05) = 266 and similarly (280* (1+.05)) =294.
While selecting PV modules, it is important to negotiate for a minimum tolerance level or at least modules with positive tolerance should be selected. If the modules of different wattage are connected in a single string, this leads to module mismatch losses and output of the string is governed by the weakest module. For example- in our case, the string has module ranging from 266 watt to 294 watt are connected in series, it will lead to higher module mismatch losses. In order to avoid the module mismatch losses, proper sorting of modules should be carried out while installing the modules in plant, it advisable that same wattage modules are connected in a single string through proper module sorting. For example, in our case a single string should have 266 W module only, and the other string should have 294 W module only in order to avoid module mismatch losses.
The power output of PV module is governed by operating voltage and current, hence in above example if the voltage is 36 volt the current of each module will be 7.8 amps. While 294 W module with same voltage level may produce 8.2 amps. And module of 266 W module, will produce 7.38
If the modules with positive and negative tolerance are connected in single string, the output of the string will be governed by minimum performance module (266 W, 7.38 amps). The module characteristic is typically represented by I-V characteristic curve and the maximum power is produced by at a voltage and current rating known as maximum power point (MPP). If different modules are connected in series, they will have various MPP’s and output of string is not decided as average MPP. It is decided based on minimum MPP condition of module which is connected in the string.
While the module ratings are not always identical and it is decided after the flash test analysis at the manufacturing unit and accordingly the rating of module is specified on each module, it is important that during planning phase the overall plant capacity is estimated based on the minimum power capacity of the modules based on their tolerance limit. Module mismatch losses are critical and avoidable by proper sorting of modules while installation in the plant. Also, the project developers can negotiate for minimum power tolerances for the specified modules during their purchase negotiations.
Module mismatch factor = 1-(.25 * warranted power tolerance range)
Module mismatch for 280 W module = .975