NABCEP Inverter Systems 5 — Questions and Answers
Question 1: What happens to a grid-tied inverter's output when the utility grid voltage rises above the inverter's high-voltage trip threshold?
- The inverter increases its output voltage to match the grid
- The inverter disconnects from the grid and waits for voltage to return to normal range (Correct answer)
- The inverter switches to battery backup mode automatically
- The inverter reduces DC input power by deactivating MPPT
Correct answer: The inverter disconnects from the grid and waits for voltage to return to normal range
When grid voltage exceeds the high-voltage trip threshold, the inverter disconnects (trips offline) and waits a specified reconnect delay before resynchronizing, protecting both the inverter and connected equipment.
Question 2: In an AC-coupled battery system, the battery inverter synchronizes to the frequency signal from the grid-tied solar inverter. How does the battery inverter signal the solar inverter to reduce output when the battery is full?
- It sends a digital command over a Modbus RS-485 communication link
- It slightly raises the AC frequency above 60 Hz, causing the solar inverter to reduce output via frequency-watt droop (Correct answer)
- It opens the AC disconnect between the two inverters momentarily
- It increases the DC bus voltage to force the solar MPPT to a lower power point
Correct answer: It slightly raises the AC frequency above 60 Hz, causing the solar inverter to reduce output via frequency-watt droop
In AC-coupled systems (e.g., Victron + SolarEdge or Enphase), the battery inverter raises AC frequency slightly above 60 Hz, causing the solar inverter's frequency-watt (Freq-Watt) function to curtail output.
Question 3: What does the 'power factor' of a grid-tied inverter output represent?
- The ratio of DC input power to AC output power
- The ratio of real (active) power to apparent power, indicating how much current contributes to useful work (Correct answer)
- The percentage of time the inverter operates above 50% of rated capacity
- The ratio of fundamental frequency power to total harmonic power
Correct answer: The ratio of real (active) power to apparent power, indicating how much current contributes to useful work
Power factor is the ratio of real power (watts) to apparent power (volt-amperes); a unity power factor (1.0) means all current contributes to useful work with no reactive component.
Question 4: A homeowner reports their grid-tied solar system stops producing around noon on sunny days despite the panels appearing clean and unshaded. Which inverter issue is most likely?
- Anti-islanding protection falsely tripping due to load imbalance
- High-temperature thermal derating causing the inverter to reduce output (Correct answer)
- Ground fault protection activating due to moisture in junction boxes
- Low-voltage DC trip occurring as battery bank is depleted
Correct answer: High-temperature thermal derating causing the inverter to reduce output
Inverters have thermal management systems and will derate (reduce output) or shut down when internal temperatures exceed safe limits, which often occurs at peak solar production on hot days.
Question 5: What is the purpose of the DC disconnect switch (also called PV disconnect or array disconnect) required between the solar array and the inverter?
- To prevent the inverter from back-feeding the array during nighttime
- To provide a means for safely de-energizing the DC conductors for maintenance and emergency disconnection (Correct answer)
- To automatically switch the array to a battery bank when grid fails
- To limit DC current from the array during cloud-edge events
Correct answer: To provide a means for safely de-energizing the DC conductors for maintenance and emergency disconnection
The DC disconnect provides a manual means to safely isolate the PV array DC conductors from the inverter, required by NEC 690.15 for maintenance, inspection, and emergency de-energization.
Question 6: Which of the following best describes 'reactive power support' (also called VAR support) that utility companies may require from large PV inverter systems?
- The inverter must store excess energy in batteries to supply reactive loads later
- The inverter absorbs or injects reactive power to help maintain stable grid voltage at the point of interconnection (Correct answer)
- The inverter must produce power at a fixed power factor of 0.85 lagging at all times
- The inverter provides backup reactive power only during grid voltage sag events
Correct answer: The inverter absorbs or injects reactive power to help maintain stable grid voltage at the point of interconnection
Reactive power support allows inverters to inject or absorb VARs (reactive power) on demand, helping utilities regulate voltage on distribution circuits especially in areas with high PV penetration.
Question 7: A NABCEP-certified installer is sizing a string for a transformerless inverter with an MPPT range of 200-480V DC. The string has 10 modules, each with Vmp of 38.5V at STC. What is the string Vmp at STC, and does it fall within the MPPT range?
- 385V; yes, it is within the 200-480V MPPT range (Correct answer)
- 385V; no, it falls below the minimum MPPT voltage
- 420V; yes, after temperature correction it enters the range
- 350V; no, it exceeds the maximum MPPT voltage
Correct answer: 385V; yes, it is within the 200-480V MPPT range
String Vmp = 10 × 38.5V = 385V, which falls within the 200-480V MPPT range; however, temperature-corrected Vmp at high temperatures must also be checked to ensure it stays above 200V.
NABCEP Inverter Systems 5
What happens to a grid-tied inverter's output when the utility grid voltage rises above the inverter's high-voltage trip threshold?