NABCEP PV System Design Principles 3 — Questions and Answers
Question 1: The purpose of a module-level power electronics (MLPE) device such as a microinverter or DC optimizer is primarily to:
- Mitigate the impact of shading and mismatch on overall system output (Correct answer)
- Increase the module's rated wattage
- Reduce the system's DC operating voltage
- Eliminate the need for overcurrent protection
Correct answer: Mitigate the impact of shading and mismatch on overall system output
MLPEs allow each module to operate at its own maximum power point, reducing losses caused by shading, soiling, or module mismatch.
Question 2: For a roof-mounted residential PV array, which factor is most important when evaluating roof pitch for energy production?
- The angle's effect on tilt, which influences annual irradiance capture (Correct answer)
- The roof's load-bearing capacity
- The distance from the roof edge for fall protection
- The type of roofing material used
Correct answer: The angle's effect on tilt, which influences annual irradiance capture
Roof pitch determines array tilt angle, which directly affects how much solar irradiance the modules intercept throughout the year.
Question 3: A combiner box in a large PV array is used to:
- Combine multiple source circuits into a single output circuit with overcurrent protection per string (Correct answer)
- Convert DC to AC before sending power to the inverter
- Monitor each module's individual output
- Provide grounding for the array structure
Correct answer: Combine multiple source circuits into a single output circuit with overcurrent protection per string
A combiner box parallels multiple strings, providing fuse or breaker protection for each source circuit before connecting to the inverter.
Question 4: According to NEC 690.12, rapid shutdown of a PV system is required primarily to:
- Reduce voltage in roof-mounted conductors to protect first responders (Correct answer)
- Prevent battery overcharge during emergencies
- Disconnect the utility grid automatically during outages
- Limit inrush current when the system restarts
Correct answer: Reduce voltage in roof-mounted conductors to protect first responders
Rapid shutdown requirements protect firefighters and other first responders from energized conductors on and near the roof.
Question 5: The term 'specific yield' in PV system performance refers to:
- Annual energy production per kilowatt of installed DC capacity (kWh/kWp) (Correct answer)
- The ratio of AC output to DC nameplate rating at STC
- Energy produced per square meter of module area
- The percentage of irradiance converted to usable energy
Correct answer: Annual energy production per kilowatt of installed DC capacity (kWh/kWp)
Specific yield, expressed in kWh/kWp, normalizes annual energy output to system size and enables comparison across different system sizes and locations.
Question 6: When calculating the maximum number of modules per string, the designer must consider the module's Voc at:
- The lowest expected ambient temperature at the site (Correct answer)
- Standard test conditions (25°C)
- The highest expected cell operating temperature
- The inverter's maximum power point voltage
Correct answer: The lowest expected ambient temperature at the site
Module Voc increases as temperature decreases, so the coldest expected temperature produces the highest string voltage, which must remain within inverter limits.
Question 7: What is the primary advantage of a central inverter compared to multiple string inverters in a large commercial PV system?
- Lower cost per watt and simpler maintenance at a single location (Correct answer)
- Better shade tolerance for each module
- Higher efficiency at partial load
- Easier rapid shutdown compliance
Correct answer: Lower cost per watt and simpler maintenance at a single location
Central inverters typically have a lower cost per watt and consolidate maintenance at one location, making them economical for large uniform arrays.
NABCEP PV System Design Principles 3
The purpose of a module-level power electronics (MLPE) device such as a microinverter or DC optimizer is primarily to: