A solar technician is troubleshooting a newly installed string inverter that is frequently reporting 'Grid Undervoltage' errors and shutting down. All AC and DC wiring has been verified as correct. What is the most likely cause of this issue?
- AIncorrect programming of the inverter's maximum power point tracking (MPPT) settings.
- BOver-sizing the PV array relative to the inverter's DC input rating.
- CA faulty bypass diode within one of the PV modules.
- DInsufficient conductor size for the AC output circuit, leading to excessive voltage drop.
Show answer & explanationAnswer & explanation
Correct answer: D. Insufficient conductor size for the AC output circuit, leading to excessive voltage drop.
When an inverter pushes power onto the grid, it causes a slight increase in voltage (voltage rise). If the AC output conductors are undersized, their resistance is too high, leading to an excessive voltage drop along the circuit. This makes the inverter 'see' a lower voltage at its terminals than the actual grid voltage, potentially triggering a 'Grid Undervoltage' error and causing it to disconnect, even if the grid itself is fine.
Why the other options are wrong
- A. MPPT settings primarily affect DC power harvest, not the AC grid voltage sensing that causes undervoltage errors.
- B. Over-sizing the DC array typically leads to 'Clipping' errors (power limiting) or 'DC Overvoltage', not directly 'Grid Undervoltage' on the AC side.
- C. A faulty bypass diode would primarily affect the DC power output of a string, not cause a 'Grid Undervoltage' error on the AC side of the inverter.
Inverter Grid Undervoltage (Conductor Sizing)
A 'Grid Undervoltage' error on a grid-tied inverter can be caused by undersized AC output conductors, which result in excessive voltage drop (or voltage rise) between the inverter and the point of interconnection, making the inverter sense a lower-than-actual grid voltage.
- Inverters monitor grid voltage for safe operation.
- Voltage drop in AC conductors reduces the voltage available at the inverter terminals.
- Excessive voltage drop can make the inverter falsely detect a grid undervoltage.
- Proper conductor sizing (NEC 210.19(A)(1) & 690.8) is crucial to prevent this.
Memory trick: Tiny Wires Make Tiny Volts, Triggering Trip-Outs.