NABCEP PV System Design Principles 5 — Questions and Answers
Question 1: The purpose of an anti-islanding protection feature in a grid-tied inverter is to:
- Automatically disconnect the PV system when the utility grid goes down (Correct answer)
- Prevent the battery from discharging back into the grid
- Limit the PV system's output during peak demand periods
- Protect modules from lightning-induced surges
Correct answer: Automatically disconnect the PV system when the utility grid goes down
Anti-islanding protection detects grid loss and shuts down the inverter to prevent energizing downed utility lines, protecting utility workers.
Question 2: Which irradiance condition is used as the standard for rating PV module power output (STC)?
- 1000 W/m² at 25°C cell temperature with AM1.5 spectrum (Correct answer)
- 800 W/m² at 20°C ambient temperature
- 1000 W/m² at 45°C NOCT cell temperature
- 1200 W/m² at 25°C with direct-beam irradiance only
Correct answer: 1000 W/m² at 25°C cell temperature with AM1.5 spectrum
Standard Test Conditions (STC) are defined as 1000 W/m² irradiance, 25°C cell temperature, and AM1.5 solar spectrum.
Question 3: Nominal Operating Cell Temperature (NOCT) is used to estimate module temperature under what conditions?
- 800 W/m² irradiance, 20°C ambient, 1 m/s wind speed, open-circuit module (Correct answer)
- 1000 W/m² irradiance, 25°C ambient, no wind
- 800 W/m² irradiance, 25°C ambient, open-rack mounting
- 1000 W/m² irradiance, 20°C ambient, maximum power point operation
Correct answer: 800 W/m² irradiance, 20°C ambient, 1 m/s wind speed, open-circuit module
NOCT is measured at 800 W/m², 20°C ambient temperature, 1 m/s wind, and open-circuit electrical conditions to represent typical outdoor operating conditions.
Question 4: A PV system designer wants to estimate annual energy production. Which data source provides the best site-specific solar resource information for the US?
- NREL's National Solar Radiation Database (NSRDB) or PVWatts (Correct answer)
- Local weather station average cloud cover records
- Manufacturer's energy yield estimates based on module wattage
- The nearest airport's historical sunshine hours data
Correct answer: NREL's National Solar Radiation Database (NSRDB) or PVWatts
NREL's NSRDB provides high-resolution historical solar irradiance data integrated into tools like PVWatts for accurate US site energy estimates.
Question 5: In a transformer-less (transformerless) PV inverter, which additional safety concern must the designer address?
- The potential for DC ground faults and leakage current due to lack of galvanic isolation (Correct answer)
- Higher risk of islanding compared to transformer-based inverters
- The inability to meet NEC rapid shutdown requirements
- Greater vulnerability to voltage surges from the utility grid
Correct answer: The potential for DC ground faults and leakage current due to lack of galvanic isolation
Transformerless inverters lack galvanic isolation between the DC and AC sides, making DC ground fault detection and leakage current management critical safety concerns.
Question 6: What is the primary purpose of a PV system's grounding electrode conductor (GEC)?
- To bond the system to earth ground, limiting voltage rise during lightning or fault events (Correct answer)
- To carry the normal operating current from the array to the inverter
- To provide a return path for AC current from the inverter to the panel
- To protect modules from electrostatic discharge during installation
Correct answer: To bond the system to earth ground, limiting voltage rise during lightning or fault events
The GEC connects the electrical system to a grounding electrode, dissipating fault and lightning-induced currents safely into the earth.
Question 7: When evaluating a site for a ground-mounted PV array, inter-row spacing is determined primarily by:
- The sun's lowest elevation angle during the winter solstice to avoid self-shading (Correct answer)
- The size of the maintenance vehicles needed for access
- Local zoning setback requirements from property lines
- The length of the module string to keep wiring runs short
Correct answer: The sun's lowest elevation angle during the winter solstice to avoid self-shading
Inter-row spacing must be sufficient so that the shadow cast by the front row does not shade the rear row during the worst-case low-sun-angle condition (winter solstice).
NABCEP PV System Design Principles 5
The purpose of an anti-islanding protection feature in a grid-tied inverter is to: