UN-BUS: DC bus undervoltage

The "UN-BUS" error means the DC voltage level within the inverter is below the required threshold for proper operation. The DC bus is responsible for converting the DC power from solar panels into a stable DC voltage before transforming it into AC power for the grid.

Why it's problematic

Insufficient DC voltage can prevent the inverter from starting or cause it to shut down, interrupting power generation.
Prolonged under-voltage conditions can potentially damage inverter components.

Possible causes

Insufficient solar irradiance: Low light conditions or shading on the solar panels can result in low DC voltage output.
Faulty or disconnected wiring: Issues in the wiring between the panels and the inverter, such as loose connections, damaged cables, or incorrect polarity, can cause voltage drops.
Malfunctioning solar panels: Damaged or underperforming solar panels can produce lower voltage than expected.
Internal inverter issues: Problems within the inverter itself, like a faulty capacitor or sensor, can affect voltage regulation.

Before you begin

Safety First: Working with electrical equipment always involves risks. Ensure you are qualified and follow appropriate safety precautions. Disconnect the inverter from both AC and DC sources before performing any physical inspection or adjustments.

Gather Information: Before starting troubleshooting, have the inverter manual on hand. You should also gather information about the inverter model and serial number, the installation date, and the PV array configuration (number of panels, capacity, and number of strings).

Troubleshooting steps

Check for sufficient sunlight: Ensure the solar panels are receiving adequate sunlight and are not shaded.
Inspect DC wiring and connections: Thoroughly check all wiring and connections between the solar panels and the inverter. Look for any signs of damage, loose connections, corrosion, or incorrect polarity.
Isolate and test individual strings (if applicable): If your inverter has multiple MPPT inputs, disconnect each string one at a time and see if the error clears. This can help pinpoint a problematic string.
Measure string voltage: Use a multimeter to measure the voltage of each string. Compare the readings to the expected output based on the panel specifications and the amount of sunlight.

Verifying the solution

Monitor the inverter’s display after troubleshooting. If the "DC Bus Under Voltage" error disappears and the inverter starts generating power, the issue is resolved.

If the problem is not resolved

If the error persists after following all the above steps, there might be an internal fault within the inverter.

As a system owner: In this case, contact the inverter installer or distributor for further assistance. You can find the details of your installer in the MyAutarco app.
As a certified installer: Contact Autarco Support, you can submit a support case via Helios.