NABCEP Inverter Systems 4 — Questions and Answers
Question 1: A grid-tied inverter displays a 'Ground Fault' error and shuts down. What is the most likely cause?
- Utility grid frequency has drifted outside acceptable limits
- An unintended current path exists between a current-carrying conductor and ground (Correct answer)
- The battery state of charge has dropped below minimum threshold
- AC output voltage is too high due to utility overvoltage
Correct answer: An unintended current path exists between a current-carrying conductor and ground
A ground fault error indicates that current is flowing through an unintended path to ground, typically caused by insulation failure or wiring errors in the DC array.
Question 2: What is 'frequency-watt' (Freq-Watt) control mode in a grid-tied inverter?
- A mode that adjusts inverter output frequency based on available DC power
- A grid support function that reduces inverter output power when grid frequency rises above threshold (Correct answer)
- A feature that increases inverter output when grid voltage drops
- An efficiency optimization mode that operates the inverter only at 60 Hz
Correct answer: A grid support function that reduces inverter output power when grid frequency rises above threshold
Frequency-Watt (or Freq-Watt) control reduces inverter active power output as grid frequency increases above a set threshold, helping stabilize grids with excess generation.
Question 3: For a transformerless grid-tied inverter, NEC requires which of the following to reduce leakage current risk?
- A DC-rated fuse on each string
- An equipment grounding conductor sized at 250% of the array output
- Ground fault protection device (GFPD) integrated into or paired with the inverter (Correct answer)
- A ferrite bead filter on each PV module connector
Correct answer: Ground fault protection device (GFPD) integrated into or paired with the inverter
NEC 690.5 requires ground fault protection for ungrounded PV systems (common with transformerless inverters) to detect and interrupt ground fault currents that could cause fires or shock hazards.
Question 4: A string inverter has two independent MPPT inputs. How does this benefit the system designer?
- It allows the inverter to produce twice the power output
- It permits strings with different orientations or shading conditions to be optimized independently (Correct answer)
- It provides redundancy so the inverter continues operating if one MPPT fails
- It enables AC coupling with a battery system on the second MPPT input
Correct answer: It permits strings with different orientations or shading conditions to be optimized independently
Dual independent MPPT inputs allow strings facing different directions (e.g., east and west) or with different shading profiles to each operate at their individual maximum power points rather than compromising to a single point.
Question 5: What is 'volt-VAR' (Volt-VAR) control in advanced grid-interactive inverters?
- A mode that varies DC input voltage to maximize VAR production
- Reactive power (VAR) output adjusted in response to local grid voltage to help regulate voltage (Correct answer)
- A variable frequency drive mode for motor loads connected to the inverter output
- An automatic voltage regulator that limits AC output to 240V maximum
Correct answer: Reactive power (VAR) output adjusted in response to local grid voltage to help regulate voltage
Volt-VAR control allows inverters to absorb or inject reactive power (VARs) in response to grid voltage deviations, helping utilities maintain voltage within acceptable limits on distribution feeders.
Question 6: An off-grid inverter/charger is rated for 4,000W continuous AC output. A customer wants to run a 3-ton central air conditioner with a 5,400W LRA (locked rotor amperage) starting current. What is the key specification to check?
- The inverter's CEC weighted efficiency at 50% load
- The inverter's surge capacity rating to handle motor starting current (Correct answer)
- The inverter's maximum DC input voltage from the battery bank
- The inverter's output power factor at full rated load
Correct answer: The inverter's surge capacity rating to handle motor starting current
Motor starting (LRA) can require 3-6× the running current for 1-3 seconds; the inverter's surge/peak capacity rating must exceed the motor's LRA to successfully start the load without shutdown.
Question 7: According to UL 1741-SA (Supplement A), what capability must inverters have to qualify for use in Hawaiian Electric and California Rule 21 interconnection?
- The ability to operate in island mode without grid connection
- Advanced grid support functions including voltage and frequency ride-through and reactive power control (Correct answer)
- Built-in battery storage with minimum 4-hour backup capacity
- DC-coupled battery backup with bidirectional charging at minimum 90% efficiency
Correct answer: Advanced grid support functions including voltage and frequency ride-through and reactive power control
UL 1741-SA certifies inverters for advanced grid support functions required by Hawaiian Electric and California Rule 21, including voltage/frequency ride-through, volt-VAR, and volt-watt controls.
NABCEP Inverter Systems 4
A grid-tied inverter displays a 'Ground Fault' error and shuts down.
What is the most likely cause?