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Inverter Systems Flashcards

7 cards from real NABCEP practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. What happens to a grid-tied inverter's output when the utility grid voltage rises above the inverter's high-voltage trip threshold?

    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.

  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?

    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.

  3. What does the 'power factor' of a grid-tied inverter output represent?

    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.

  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?

    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.

  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?

    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.

  6. Which of the following best describes 'reactive power support' (also called VAR support) that utility companies may require from large PV inverter systems?

    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.

  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?

    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.