NABCEP Commissioning and Troubleshooting 4 — Questions and Answers
Question 1: A transformer-less string inverter produces an audible high-pitched noise during operation but otherwise functions normally. What is the most likely source?
- The DC disconnect switch is vibrating
- Switching frequency components from IGBT or MOSFET operation (Correct answer)
- Ground fault in the PV array
- AC grid frequency mismatch
Correct answer: Switching frequency components from IGBT or MOSFET operation
Transformerless inverters use high-frequency switching transistors that produce audible tones; this is normal but should be within manufacturer-specified noise limits.
Question 2: During commissioning of a commercial rooftop system, string voltages measured at the combiner box are 10% lower than calculated. All modules show normal Isc. What is the most likely cause?
- Module temperature is above STC
- One module per string has been wired in reverse polarity, reducing net string voltage (Correct answer)
- The combiner box has failed
- Module soiling is reducing voltage
Correct answer: One module per string has been wired in reverse polarity, reducing net string voltage
A single reverse-polarity module subtracts its voltage from the string total, causing a predictable voltage reduction without affecting Isc.
Question 3: What is the recommended sequence for powering up a newly commissioned grid-tied PV system for the first time?
- Close AC breaker first, then DC disconnect, then verify inverter startup
- Verify all connections, close DC disconnect first, then AC breaker, observe inverter startup sequence (Correct answer)
- Open all disconnects, power the inverter from AC, then connect DC
- Connect DC with all breakers open, then close AC, then DC disconnect
Correct answer: Verify all connections, close DC disconnect first, then AC breaker, observe inverter startup sequence
Proper startup requires verifying connections, then energizing DC first so the inverter can detect PV voltage before connecting to the AC grid.
Question 4: A technician is troubleshooting a PV system where the inverter shows MPPT voltage significantly below the expected Vmpp range. Which fault is most consistent with this symptom?
- Inverter is undersized for the array
- A string has developed an open circuit, reducing the number of series modules (Correct answer)
- Grid voltage is too high
- The AC output fuse has blown
Correct answer: A string has developed an open circuit, reducing the number of series modules
An open circuit in a string reduces the effective number of series modules, lowering the total MPPT voltage presented to the inverter.
Question 5: When verifying proper anti-islanding operation during commissioning, what standard test method is employed?
- Measure inverter output voltage with a voltmeter while the grid is connected
- Disconnect the utility feed and confirm the inverter shuts down within the required time per IEEE 1547 (Correct answer)
- Measure inverter harmonic distortion with a power quality analyzer
- Verify DC open-circuit voltage is within operating range
Correct answer: Disconnect the utility feed and confirm the inverter shuts down within the required time per IEEE 1547
Anti-islanding verification requires disconnecting the utility and confirming the inverter de-energizes within the time limits specified in IEEE 1547.
Question 6: A technician finds that the PV array's maximum system voltage, calculated at the site's record low temperature, exceeds the inverter's rated maximum input voltage. What is the proper corrective action?
- Derate the system by 10% and proceed with commissioning
- Remove modules from strings to reduce voltage below the inverter's maximum rating (Correct answer)
- Increase the inverter's voltage limit through configuration software
- Add a voltage-limiting diode to each string
Correct answer: Remove modules from strings to reduce voltage below the inverter's maximum rating
The only safe correction is to reduce the number of series modules so that Voc at minimum temperature does not exceed the inverter's maximum DC input voltage.
Question 7: During final commissioning walkthrough, the inspector notices PV conduit runs without visible expansion fittings on a 120-foot rooftop run. Why is this a concern?
- It increases voltage drop in the conductors
- Thermal expansion of conduit without expansion fittings can stress connections and damage conduit (Correct answer)
- It violates the minimum bending radius requirement
- It reduces the system's ground fault protection
Correct answer: Thermal expansion of conduit without expansion fittings can stress connections and damage conduit
Long conduit runs exposed to daily and seasonal temperature swings require expansion fittings to accommodate thermal expansion and prevent mechanical damage.
NABCEP Commissioning and Troubleshooting 4
A transformer-less string inverter produces an audible high-pitched noise during operation but otherwise functions normally.
What is the most likely source?