String Imbalance Three vs One String Decision Tree

Why this matters

A four-string commercial PV array where three strings are producing as modeled and one is producing 60 to 80 percent of model is showing a classic isolated-string fault. The fault could be a single failed module, a partial shade window, a junction box connection burn, a wire-loss problem, an MPPT-channel hardware fault, or a polarity reversal at the recombiner. Each has a different remedy. Driving across town to swap a module that wasn't the cause is the most expensive misdiagnosis in commercial O&M. The decision tree below isolates to the failure mode in a defined order so the truck-roll resolves the fault.

Symptom presentation

Monitoring portal shows three strings on the same inverter at expected production. One string is at 60 to 80 percent and tracks the others in shape but not in magnitude. The inverter may not report a fault if MPPT tracking is still finding a maximum. Module-level monitoring (where present) may pinpoint a specific module that has dropped.

Quick checks

1. Read string DC voltage and DC current at the inverter MPPT input during peak irradiance. A healthy string should match the parallel strings in voltage; current should match if module count and orientation match.
2. Confirm module count per string. A 16-module string and a 17-module string at the same inverter MPPT input mismatch voltage and one or the other operates off-peak.
3. Look at the irradiance plot for the day. A string showing depressed production during a specific hour pattern that matches a known shade source (chimney, vent stack, neighboring building) is shading, not fault.
4. Read module-level data if present (SolarEdge optimizer, Tigo, Enphase). A single module reporting below the others isolates the location.
5. Open the recombiner box and read string current at the fuse holder. Compare each string at the same irradiance.

Isolation tree

Step 1: With irradiance steady, measure each string's open-circuit voltage (Voc) at the inverter input or recombiner with the string isolated. All four strings should read within 2 percent of each other and within 2 percent of the count-times-module-Voc spec. A low Voc on one string indicates a missing module-voltage contribution: failed module, bypass-diode fully conducting on a hot module, or open-circuit somewhere in the string.

Step 2: With irradiance steady, measure short-circuit current (Isc) per string. A low Isc indicates a current limitation: connector burn, conductor undersize, or a partial-shade condition.

Step 3: If Voc is low: walk the string with module-level data or with an IR camera. Look for a module with bypass diodes fully on (hot string under load, hot diodes). Burn marks on the back-side junction box confirm internal failure. Replace the module.

Step 4: If Isc is low and Voc is on spec: inspect MC4 connector pairs along the string. A connector pair that pulled apart, was bird-pecked, or rodent-chewed creates a series resistance that drops current under load. Thermal-image at midday; the bad connector glows.

Step 5: If both Voc and Isc are on spec at the string entry but the inverter still reports low production: suspect the MPPT input channel. Swap the underperforming string to a known-good MPPT channel (where the inverter supports independent MPPTs) and observe. If the string now produces normally, the original MPPT channel has a hardware fault; the inverter needs board service or replacement.

Step 6: At the recombiner or string-combiner: confirm fuse condition and polarity. A reversed polarity at a recombiner string entry causes the string to act as a load on the others; the symptom looks like an MPPT problem.

Confirming diagnosis

Failed module confirms by isolated Voc deficit matching a single module Voc, plus IR or visible inspection. Connector fault confirms by IR camera showing a hot spot at a connector pair on a steady-irradiance day. MPPT channel fault confirms by swapping strings between channels. Polarity reversal at recombiner confirms by physical inspection at the recombiner terminals.

Remediation

For failed module: replace with a like-for-like module. Cross-brand swaps may not match Voc and Isc closely enough; the new module pulls the string off the model.

For connector fault: replace the MC4 connector pair. Always replace as a pair; mixing brands of MC4 (Stäubli original vs Amphenol vs Multi-Contact) violates UL 6703 listing and creates exactly this kind of fault.

For MPPT channel hardware fault: service inverter through OEM (Fronius, SMA, SolarEdge). Some MPPTs are field-serviceable, most are board-level replacements.

For polarity reversal: stop production, correct the polarity at the recombiner, retest each string.

After repair, re-baseline production at the next sunny day and confirm all four strings match within tolerance.

References

1. NEC 2023 Article 690.7 Maximum Voltage and 690.8 Circuit Sizing and Current.
2. NEC 2023 Article 690.13 Disconnecting Means and 690.31 Wiring Methods.
3. UL 6703 Standard for Connectors for Use in Photovoltaic Systems.
4. IEEE 1547-2018 Standard for Interconnection and Interoperability of Distributed Energy Resources.
5. SolarEdge Multiple MPPT Inverter Installation Guide and Fronius Symo Service Manual on MPPT channel diagnostics.