Hot Module, Open Bypass Diode and Output Loss in an 8 kWp Array
An 8 kWp array has two strings of ten 400 Wp modules. Each module has Voc 46 V, Vmp 37 V, Isc 9.5 A and Imp 8.6 A. After a water tank was installed on the roof, the inverter reports string 1 at 8.4 A and 370 V, while string 2 is at 7.2 A and 333 V. An infrared scan shows one module in string 2 about 12 °C hotter than its neighbours, with a concentrated hot cell area. A diode-mode test across that module’s bypass diode shows an open circuit. The string open-circuit voltage is normal, and the short-circuit current is slightly low. The site has an average 5.5 peak sun hours and a tariff of ₹7/kWh. The customer reports that output was normal before the tank was installed and asks whether the module can simply be left in service until the next service visit.
1. Calculate the power loss in string 2 and estimate the daily and monthly energy and revenue loss.
Read the worked answer
String powers at the reported operating points
String 1: 8.4 A × 370 V = 3108 W.
String 2: 7.2 A × 333 V = 2398 W.
Loss = 3108 - 2398 = 710 W.
As a fraction of string 1: 710/3108 = 22.8%. As a fraction of the whole array, the loss is about 11.4%, because only one string is affected.
Energy and revenue
Daily loss = 0.710 kW × 5.5 h = 3.9 kWh/day.
Monthly loss = 3.9 × 30 = 117 kWh.
At ₹7/kWh, monthly loss = 117 × 7 = ₹819.
Annual loss ≈ 3.9 × 365 = 1424 kWh, worth about ₹9,970/year.
The loss is large enough to justify immediate investigation. The calculation assumes the reported currents and voltages are representative; use the inverter’s logged data and a calibrated clamp meter to confirm.
2. Give a logical diagnostic sequence to distinguish a failed bypass diode, a cracked cell, and shading from the water tank. State what the evidence already indicates.
Read the worked answer
Sequence
- Measure irradiance and module temperature at the array so the expected current can be estimated.
- Measure string open-circuit voltage and short-circuit current; compare with string 1.
- Measure each module’s Voc and Isc, and clamp the operating current in the string.
- Perform an I-V curve trace on both strings; look for a step or a reduced current plateau.
- Test each bypass diode in forward and reverse bias with the module isolated and covered.
- Use the thermal image to locate the hot cell/substring and inspect for cracked glass, burnt backsheet, or a damaged junction box.
- Check the water tank’s shadow path at different times; shading moves, while a cracked cell hotspot remains.
What the evidence indicates
String 2 has normal Voc but reduced current and a Vmp about 37 V lower, which is approximately one module’s Vmp. That suggests one module is being bypassed or is limiting the string. The hot module plus an open bypass diode points to a faulty substring: the diode should conduct and bypass the substring, but because it is open, the substring is reverse-biased and dissipates power. Shading from the tank may have initiated the mismatch, but an open diode is a confirmed defect. A slightly low Isc with normal Voc is consistent with a series/current-limiting fault rather than an open string.
3. Advise the customer on whether the module can remain in service. Give the repair and verification steps.
Read the worked answer
The module should not be left in service. An open bypass diode means the affected cell substring has no protection. The hotspot can delaminate the module, crack glass, burn the backsheet, or develop into a DC arc or fire. The risk is higher because the fault is already producing a 12 °C temperature rise.
Repair steps:
- Isolate the string, cover the modules, and follow DC safe-work procedures.
- Replace the faulty module with a matched model and rating. If the junction box is serviceable, a qualified technician may replace the bypass diode, but a cracked cell cannot be repaired.
- Remove or relocate the water tank, or move the array, so the shadow no longer falls on the modules. If relocation is impossible, use module-level power electronics only as a mitigation, not as a repair for a faulty module.
- Check string connectors, torque, fuses, and cable insulation.
- Re-test string Voc, Isc, and I-V curve; compare with string 1. Repeat the thermal scan under load.
Record the pre- and post-repair values. The customer should understand that the lost revenue of roughly ₹800/month is secondary to the fire risk. A temporary reduced-output operation is acceptable only if the faulty module is fully bypassed or removed, not left with an open diode.