NABCEP Inverter Systems 2 — Questions and Answers
Question 1: What is the primary function of an anti-islanding protection feature in a grid-tied inverter?
- Prevent backfeed into the utility grid during normal operation
- Disconnect the inverter from the grid when utility power is lost (Correct answer)
- Regulate DC input voltage from the solar array
- Limit maximum output current to the load
Correct answer: Disconnect the inverter from the grid when utility power is lost
Anti-islanding protection automatically disconnects the inverter from the utility grid when grid power is lost, preventing the PV system from energizing downed lines and endangering utility workers.
Question 2: A string inverter is rated for a maximum DC input voltage of 600V. On a cold winter morning, the open-circuit voltage of the array measures 620V. What is the most likely result?
- The inverter will operate normally with a slight efficiency reduction
- The inverter may be damaged or its warranty voided (Correct answer)
- The MPPT algorithm will reduce voltage to the safe limit
- The inverter will trip a ground fault protection circuit
Correct answer: The inverter may be damaged or its warranty voided
Exceeding the inverter's maximum DC input voltage can permanently damage internal components and typically voids the manufacturer's warranty.
Question 3: Which inverter topology uses a high-frequency transformer to provide galvanic isolation between the DC array and the AC output?
- Transformerless inverter
- High-frequency transformer-based inverter (Correct answer)
- Line-frequency transformer inverter
- Micro-inverter without isolation
Correct answer: High-frequency transformer-based inverter
High-frequency transformer-based inverters use a transformer operating at high frequency (kHz range) to provide galvanic isolation while remaining more compact than line-frequency transformer designs.
Question 4: When multiple string inverters are installed in a commercial system, each inverter must have:
- A shared AC disconnect switch for all inverters
- Its own AC disconnect accessible to utility personnel (Correct answer)
- A DC combiner box mounted within 3 feet
- A dedicated utility meter for production monitoring
Correct answer: Its own AC disconnect accessible to utility personnel
NEC 690.15 requires each inverter to have its own AC disconnect means that is accessible to utility workers for safe de-energization.
Question 5: What does the term 'MPPT range' refer to in inverter specifications?
- The AC output voltage regulation range of the inverter
- The range of DC input voltages over which the inverter can perform maximum power point tracking (Correct answer)
- The efficiency range across different load conditions
- The operating temperature range for optimal inverter performance
Correct answer: The range of DC input voltages over which the inverter can perform maximum power point tracking
The MPPT range specifies the DC voltage window (minimum to maximum) within which the inverter's maximum power point tracking algorithm can operate to extract maximum power from the array.
Question 6: A single-phase string inverter with CEC efficiency of 96% is producing 5,000W of AC output. Approximately how much power is being dissipated as heat?
- 50W
- 104W
- 200W
- 208W (Correct answer)
Correct answer: 208W
At 96% efficiency, 4% of DC input power is lost as heat; DC input ≈ 5000/0.96 = 5,208W, so heat dissipated ≈ 208W.
Question 7: Which of the following is a key advantage of a transformerless inverter compared to a transformer-based inverter?
- Built-in galvanic isolation between DC and AC sides
- Higher weight due to robust magnetic components
- Higher efficiency and lower cost due to elimination of transformer losses (Correct answer)
- Better performance at very low irradiance levels
Correct answer: Higher efficiency and lower cost due to elimination of transformer losses
Transformerless inverters eliminate transformer copper and core losses, resulting in higher peak efficiencies (often >97%) and reduced weight and cost compared to transformer-based designs.
NABCEP Inverter Systems 2
What is the primary function of an anti-islanding protection feature in a grid-tied inverter?