varying voltages and currents. Additionally, a partially shaded string will have no effect on the unshaded string since they are electrically isolated and tracked by their own MPPT.
SHADE-ACCOMMODATING MPPT
One of the biggest leaps in inverter technology is shade-accommodating MPPT algorithms. These algorithms perform an I-V sweep of the PV array, much like an I-V curve tracer, and then shift the MPP from a lower array power local peak up to the maximum power global peak.
For those who prefer MLPE, module-level strategies can also be deployed to address partial shading, but they come at a higher cost.
MODULE-LEVEL POWER ELECTRONICS IN RESIDENTIAL AND COMMERCIAL APPLICATIONS
There are compelling reasons to use MLPE, even in applications where other topologies would make better sense with respect to ROI. Micro inverters and optimizers share these favorable attributes for some business models:
For installers in states that do not offer attractive rebate structures, micro inverters allow them to add on to a residential PV system every year or so, as the homeowner is able to afford the expansion.
Other installers, such as Nathan Charles of Pennsylvania-based Paradise Energy Solutions, cite high employee turnover rates and costly labor training for string inverters as reasons why they choose micro inverters. According to Charles, he can have a fully trained crew ready to deploy and install micro inverter systems within a matter of weeks. “As a whole, micro inverters require less training and personal protection equipment,” he said.
For Charles, “The failure rate of micro inverters is an acceptable risk.” He informs his customers, who might be monitoring their module-level data on a daily basis, that failed units will be replaced annually and not when each one fails. “A failed inverter costs the homeowner about $30 per year,” he added, “which isn’t enough to justify rolling a truck.”
Module-level monitoring does come in handy for quickly finding defective inverters and for locating ground faults caused by rodents. Ground fault localization is a time-consuming practice that is virtually eliminated when using MLPE. Knowing how many inverters will be replaced and which tools will be required for servicing are advantages of using MLPE.
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Optimzing PV Systems eFeature | July 2014
OPTIMIZING PV SYSTEMS - PART 1: INVERTERS