Reverse Current Flow in Solar PV Systems: Detection and Prevention

What is reverse current?

In a PV array with parallel strings, a faulted or heavily shaded string can be driven backwards by the healthy strings. Instead of delivering power to the bus, the weak string absorbs power—turning into a heater. Reverse current can also occur:

• From a battery back into the PV array (nighttime backfeed) if blocking/control fails.

• From an external DC source (e.g., DC-coupled wind, rectifiers) sharing the same bus.

• Due to wiring mistakes (polarity reversal) during commissioning.

Key risks:

• Thermal runaway in cells or junction boxes (bypass diode overstress).

• Conductor overheating (I²R losses).

• Nuisance trips or miscoordination of OCPDs.

• Permanent module damage and accelerated degradation.

When does it happen? (Root causes to look for)

1. String mismatch or shading: One string’s current capability collapses while others keep pushing current.

2. String short/ground fault: A low-impedance path invites current from healthy strings.

3. Open bypass diode or shorted sub-strings inside a module.

4. Polarity error at combiner or during maintenance.

5. Battery/inverter path: Faulty charge controller, failed MOSFET/diode, or incorrect settings let energy flow back at night.

6. Too many parallel strings without proper string fusing.

Engineering the limits (the math you need)

Let:

• $n$ = number of parallel strings

• $I_{SC}$ = short-circuit current (per string) at worst-case conditions (use datasheet STC and consider irradiance/temperature multipliers for local worst-case if required by your practice)

• $I_{\text{rev,max}}$ = maximum reverse current the module/string can safely withstand (often tied to the module’s maximum series fuse rating; check datasheet)

Worst-case reverse current into a faulted string

Safe number of parallel strings without string fusing

If your array exceeds this, string fusing is mandatory (and is standard practice for most commercial/utility systems).

Reverse heating power in the weak string (order-of-magnitude)

where $V_{\text{op}}$ is the string operating voltage at the time of the event.

Worked Example 1 — Parallel string backfeed and fuse selection

Given

• 8 parallel strings (n = 8)

• Module datasheet at STC: $I_{SC} = 11.0 \text{ A}$ per string

• Module max series fuse rating (often the proxy for $I_{\text{rev,max}}$) = 20 A

• String operating voltage near MPP/operating point: $V_{\text{op}} \approx 480 \text{ VDC}$

Step 1: Compute worst-case reverse current

Step 2: Can you run without string fuses?

You have 8 strings—well above 2. Therefore, string fusing is required.

Step 3: Select the string fuse rating

• Use a fuse not exceeding the module’s max series fuse (20 A here).

• Size per local code and conditions (commonly 1.25× to ~1.56× $I_{SC}$ for continuous currents and temperature considerations).

• With $I_{SC}=11.0 \text{ A}$, a 20 A string fuse is typical and within the module limit.

Step 4: Fault energy & clearing
In a worst-case reverse event, the faulted-string fuse will see ~77 A. That’s >3.8× a 20 A fuse’s rating—ample to drive fast clearing (verify with manufacturer’s time–current curve and ensure DC interrupt rating at the array’s max voltage).

Step 5: Thermal reality check
Approx reverse power into the weak string:

This shows why fast clearing is critical—substrings, diodes, and junctions can overheat very quickly.

Worked Example 2 — Nighttime battery backfeed

Scenario: DC-coupled system, the charge controller’s ideal-diode MOSFET fails short, allowing the battery to feed the PV bus at night.

Given

• Battery bus: 500 VDC

• Nighttime leakage path across array ≈ $R_{\text{leak}} = 2 \text{ k}\Omega$ (cables, electronics, contaminants)

• Potential reverse current:

Power dissipated:

Across an 8-hour night:

That’s 1 kWh every night lost as heat—plus localized hot-spots. Proper reverse-blocking (healthy MOSFET ideal-diode or a backup series diode/relay) prevents this.

Blocking diode loss check (daytime):
If you add a Schottky diode per string: $V_f \approx 0.5 \text{ V}$, string current $I \approx 9 \text{ A}$ →

For 100 strings and 5 sun-hours:

This is why modern grid-tied plants avoid blocking diodes and rely on string fuses + controller/inverter reverse-blocking instead.

How to detect reverse current (fast and reliable)

Live/online

• String-level current monitoring (SCADA/MLPE): Look for negative or near-zero current on one string while others are producing normally. Sudden divergence is a flag.

• Inverter logs: Nighttime “consumption” from the array, abnormal night tare, or DC reverse-current alarms.

• Thermal imaging (IR): Hotspots at module corners or junction boxes (bypass diode stress) during production.

• Clamp meter snapshot (Hall-effect DC): Clip around individual string conductors at the combiner and check polarity—negative readings on a string indicate backfeed.

Offline/commissioning

• Polarity checks with a multimeter at every combiner input before landing wires.

• Insulation resistance (megger) to screen for leakage paths and ground faults.

• I-V curve tracing: Abnormal curves (fold-back, kinks) identify failing diodes or substrings likely to invite reverse current.

Design measures to prevent reverse current

1. String fusing at the combiner

   • One series fuse per string sized within the module’s max series fuse rating.

   • DC voltage interrupt rating ≥ array max DC voltage.

   • Use gPV (PV-specific) fuses with appropriate I²t and temperature capability.

2. Miscoordination control

   • Ensure string fuses clear before upstream feeder/combiner fuses. Use manufacturer time–current curves to verify coordination.

3. Limit parallel strings per input

   • Respect the no-fuse limit; beyond that, fuses are mandatory. For very large block combiner inputs, consider re-combining at multiple levels.

4. Inverter/charger reverse-blocking

   • For DC-coupled systems, use ideal-diode MOSFET stages (or redundant series devices) in the charge controller.

   • For AC-coupled, ensure the inverter has anti-backfeed/anti-islanding and nighttime reverse current protection enabled.

5. Wiring quality and polarity control

   • Keyed connectors, color codes, ferrules, torque specs, and test sheets that include a polarity sign-off at each term point.

6. Bypass diode health

   • Choose modules with robust diode specs and maintain good thermal paths in junction boxes. Replace modules that show chronic diode heating.

7. Environmental hardening

   • Keep junction boxes and connectors dry/clean; moisture increases leakage. Use correct IP-rated enclosures and UV-stable components.

8. Monitoring and alarms

   • Deploy string-level or block-level current measurement with alarms on negative or out-of-family values.

Commissioning & O&M checklist 

• Verify polarity at every string with labeled test sheets.

• Record open-circuit voltage and I-V curves per string (baseline).

• Confirm string fuses: rating, DC voltage class, and coordination.

• Enable inverter/controller reverse-current protections and alarms.

• Perform night audit: confirm zero/near-zero array current at night.

• Thermal scan under load; investigate any localized hot areas.

• Schedule annual I-V / thermal scans; compare to baseline.

FAQs

Q1: Do I always need string fuses?
No—only if the number of parallel strings can push reverse current above the module’s limit. In practice, most commercial systems do use string fuses because arrays often exceed the no-fuse limit.

Q2: Are blocking diodes recommended?
Not for modern grid-tied plants. They add continuous losses and heat. Use string fuses and ensure the inverter/charge controller provides reverse blocking.

Q3: How do I know my fuse will clear safely?
Check the time–current curve for the selected gPV fuse at the actual DC voltage and expected fault current. Also check the interrupt rating and coordination with upstream devices.

Q4: Can reverse current damage bypass diodes?
Yes. Sustained reverse current drives diodes into high dissipation, causing junction box overheating and eventual diode failure.

Q5: What alerts should I configure?
Set alarms for negative string current, nighttime array current > 0.1–0.2 A per string (adjust for noise), and out-of-family performance versus peer strings.

Key takeaways 

• Reverse current is (n−1)×Isc into the weak string—easy to exceed module limits.

• Use string fuses sized within module limits and verified for DC interrupt and coordination.

• Avoid blocking diodes in grid-tied systems; rely on proper protection and controller/inverter reverse-blocking.

• Detect issues early with string monitoring, IR scans, and night audits.