NABCEP Commissioning and Troubleshooting 2

Question 1

During PV system commissioning, you measure an open-circuit voltage significantly higher than the STC-rated Voc. What is the most likely cause?

Accepted Answer

Ambient temperature is much lower than 25°C STC

Answer

Shading on one or more modules

Answer

A bypass diode has failed short

Answer

The inverter is in sleep mode

Question 2

A string of 10 modules shows an Isc that is 15% lower than expected while all other strings test normally. What is the first diagnostic step?

Accepted Answer

Inspect each module in the string for shading or soiling

Answer

Replace the string combiner fuse

Answer

Reprogram the inverter MPPT settings

Answer

Check the AC output voltage at the main panel

Question 3

When commissioning a battery-based off-grid system, what parameter must be verified to ensure the charge controller protects the battery bank?

Accepted Answer

Maximum PV array open-circuit voltage vs. charge controller input rating

Answer

AC output frequency set to 60 Hz

Answer

Nominal battery voltage matches the inverter output voltage

Answer

Ground fault detection threshold on the inverter

Question 4

An inverter repeatedly faults with an 'AC voltage high' error during afternoon peak. What is the most probable cause?

Accepted Answer

Utility grid voltage rises above the inverter's upper trip threshold

Answer

The DC input voltage is too low

Answer

The inverter needs a firmware update

Answer

PV modules are overheating

Question 5

During commissioning, an IR thermal scan of a module shows one cell running 20°C hotter than surrounding cells. What does this indicate?

Accepted Answer

A localized cell defect such as a crack or hot spot

Answer

The module has the highest irradiance exposure

Answer

The bypass diode is operating correctly

Answer

Soiling on the module surface

Question 6

What is the correct procedure when an electrolytic capacitor is suspected of failing inside an inverter?

Accepted Answer

De-energize the inverter, wait the manufacturer's specified discharge time, then inspect

Answer

Power cycle the inverter three times to reset it

Answer

Measure capacitor voltage while the inverter is operating

Answer

Replace the inverter's DC disconnect and retest

Question 7

A grid-tied PV system produces less energy than modeled but all electrical measurements appear normal. What non-electrical factor should be investigated?

Accepted Answer

Actual site shading vs. design assumptions

Answer

String fusing size

Answer

Inverter transformer tap setting

Answer

Ground electrode resistance