NABCEP System Maintenance & Troubleshooting 4 — Questions and Answers
Question 1: A soiling loss study on a utility-scale PV plant shows a 3% monthly energy loss. The O&M team wants to determine the optimal cleaning frequency. This decision should primarily be based on:
- The inverter manufacturer's warranty requirements
- A cost-benefit analysis comparing cleaning costs against the value of recovered energy (Correct answer)
- The local fire department's water use restrictions
- The maximum tilt angle of the array
Correct answer: A cost-benefit analysis comparing cleaning costs against the value of recovered energy
The economically optimal cleaning interval is determined by comparing the marginal cost of cleaning against the marginal revenue from recovered energy production.
Question 2: While measuring insulation resistance of a PV array with a megohmmeter (megger), a technician should test at a DC voltage that is:
- Equal to the array's Voc at STC
- At least equal to the maximum system voltage rating per NEC and the instrument manufacturer's guidance (Correct answer)
- Limited to 500V regardless of system voltage
- Equal to twice the array's Vmpp
Correct answer: At least equal to the maximum system voltage rating per NEC and the instrument manufacturer's guidance
Insulation resistance testing must be conducted at a voltage level appropriate for the system's maximum rated voltage to stress-test the insulation adequately per NEC and IEC standards.
Question 3: An MPPT charge controller in a PV-battery system continuously operates in bulk charge mode and never transitions to absorption. The MOST likely cause is:
- The battery bank capacity is too small for the array
- PV array is undersized relative to battery bank capacity, and charge current is insufficient to raise battery voltage to the absorption set point (Correct answer)
- The battery temperature compensation sensor has failed
- The DC disconnect between array and controller is undersized
Correct answer: PV array is undersized relative to battery bank capacity, and charge current is insufficient to raise battery voltage to the absorption set point
If the array's available current is too small relative to the battery's capacity, the battery voltage may never rise to the absorption set point, keeping the controller in bulk mode.
Question 4: A PV module with a failed bypass diode that is NOT conducting will cause which condition during normal operation?
- The module will produce no output at all
- The affected cell group will develop excessive heat as a hot spot, potentially damaging the module (Correct answer)
- The entire string will disconnect automatically
- The module's Voc will increase above nameplate value
Correct answer: The affected cell group will develop excessive heat as a hot spot, potentially damaging the module
A non-conducting (open) bypass diode means shaded or underperforming cells must pass full string current, dissipating power as heat and creating damaging hot spots.
Question 5: When inspecting conduit runs during a maintenance visit, a technician finds conduit fill has increased due to an added circuit by a previous contractor. The primary concern regarding NEC compliance is:
- Voltage drop in the added circuit
- Whether total conductor cross-sectional area exceeds the NEC Chapter 9 conduit fill allowance (Correct answer)
- Whether the conduit material is rated for outdoor use
- The added circuit's wire color coding
Correct answer: Whether total conductor cross-sectional area exceeds the NEC Chapter 9 conduit fill allowance
NEC Chapter 9 limits conduit fill to protect conductors from overheating and facilitate pulling; exceeding the fill limit is a code violation requiring larger conduit or separate raceway.
Question 6: A battery energy storage system (BESS) management system logs show that one cell module in a lithium-ion battery rack repeatedly hits its high voltage limit during charging while adjacent modules do not. This indicates:
- The rack's DC bus voltage is set too high
- Cell module imbalance requiring BMS balancing or possible cell module replacement (Correct answer)
- The inverter is producing excessive AC voltage
- Normal variation within manufacturer tolerances
Correct answer: Cell module imbalance requiring BMS balancing or possible cell module replacement
A single cell module consistently hitting its voltage ceiling while others do not indicates capacity or impedance imbalance that the BMS passive balancing may not fully correct, potentially requiring module replacement.
Question 7: During annual maintenance of a tracking PV system, a technician should verify that tracker stow position functionality works correctly primarily because:
- Stow position maximizes energy production during low-sun hours
- Stow positions reduce structural wind loads during high-wind events, protecting the racking and modules (Correct answer)
- Stow position is required for NEC compliance
- Stowing the array improves inverter efficiency
Correct answer: Stow positions reduce structural wind loads during high-wind events, protecting the racking and modules
Tracker stow mode positions modules to minimize wind profile and structural loading, and failure to stow during high-wind events can result in catastrophic mechanical damage.
NABCEP System Maintenance & Troubleshooting 4
A soiling loss study on a utility-scale PV plant shows a 3% monthly energy loss.
The O&M team wants to determine the optimal cleaning frequency.
This decision should primarily be based on: