String Current Mismatch After a Replacement Module
A 5 kW residential array has two strings of ten 360 W modules. After a tree limb damaged one module, a contractor replaced it with a spare 300 W module because it was the only matching frame size in stock. The original modules have Voc 40.0 V, Vmp 33.5 V, and Imp 9.8 A. The replacement has Voc 39.8 V, Vmp 33.0 V, and Imp 8.2 A. On a clear day at 950 W/m², string A produces 9.4 A and string B produces 8.0 A; both strings have normal open-circuit voltage. The system uses a central string inverter with two MPPT inputs. The homeowner reports that total production dropped by about 8% and asks whether a bypass diode has failed or whether the replacement module is defective. The contractor has a clamp meter and an irradiance meter but no module-level IV curve tracer on the truck.
1. Diagnose the current difference. Why does one lower-current module reduce the whole string’s current? What measurements support your conclusion?
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In a series string, the current is essentially the same through every module. The replacement module has Imp = 8.2 A at STC, while the original modules have Imp = 9.8 A. At 950 W/m², the original modules would be expected to produce about 9.8 × 0.95 = 9.3 A, and string A’s 9.4 A is consistent with that. The replacement would be expected to produce about 8.2 × 0.95 = 7.8 A; string B’s 8.0 A is close.
Normal open-circuit voltage makes an open string or a failed bypass diode less likely. The measured current tracks the replacement module’s lower Imp, so the string is current-limited by the lowest-current module. The roughly 8% total loss is consistent with one string losing about 15% of its current.
2. Evaluate the contractor’s proposed fixes: (a) replace the 300 W module with a matched 360 W module, (b) add a bypass diode across the 300 W module, (c) leave it because the loss is only 8%.
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(a) Replacing the 300 W module with a matched 360 W module is the correct fix. It restores current compatibility and the expected string output.
(b) Adding an external bypass diode across the 300 W module is not a valid fix. A bypass diode conducts when a module is reverse-biased, such as under severe shading or failure; it does not make a low-current module produce more current. An improperly installed external diode could short the module, overheat, or create a safety hazard.
(c) Leaving it is a customer-relations and performance problem. The array is permanently limited by the mismatched module, and the as-built documentation would no longer match the design. The module should be replaced or the replacement should be paired with module-level electronics that can mitigate mismatch.
3. What safety and documentation steps should accompany the repair?
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Shut down the inverter and use the rapid-shutdown or DC disconnect means. Treat the DC conductors as energized until verified otherwise with a meter rated for the system voltage. Isolate the affected string, verify zero voltage, and use insulated tools and appropriate PPE. Replace the module with the matched unit, torque the connectors to the manufacturer’s specification, and confirm polarity. Then measure Voc and Isc for the repaired string and compare it with the good string at similar irradiance and temperature. Update the string label, as-built drawing, and module serial-number record, and note the warranty replacement.