What Does This Error Mean?

Fronius State 307 means the DC voltage from your solar panels exceeded the inverter's maximum input voltage (typically 800V or 1000V depending on the model). This is most common on cold clear mornings when panel output voltage is at its peak — check that the panel string was designed within the inverter's maximum input spec.

Affected Models

Fronius Primo (max 800V DC)
Fronius Symo (max 800V DC)
Fronius Galvo (max 500V DC)
Fronius IG Plus
All Fronius string inverters

Common Causes

Too many panels connected in series — total string voltage in cold conditions exceeds the inverter's DC maximum
Very cold morning temperatures boosting panel Voc above the design calculation
Design error by the installer — string voltage was calculated at STC (25°C) rather than minimum temperature Voc
Defective MPPT input allowing voltage above the rated maximum
Surge or transient on the DC wiring from a nearby lightning strike

How to Fix It

Record the frequency and conditions of State 307.

State 307 most often occurs on very cold, clear mornings (below 10°C) when panel Voc is at its highest. Log in to solarweb.com and note the exact time — if it always appears at sunrise in winter, the string design is marginal for your climate. A single event is less concerning than regular occurrences.
Have the installer review the string design.

Solar string design must account for the maximum panel Voc at the minimum expected temperature for your location. If your installer calculated string voltage at 25°C (STC), the actual cold-morning voltage may exceed the inverter's rated maximum. The installer should recalculate string voltage using the lowest historical winter temperature for your area and may need to remove one panel from the string.
Check the DC surge protection device.

If State 307 appeared suddenly after a storm, a lightning-induced surge may have delivered a voltage spike above the inverter's limit. Locate the DC surge protection device (SPD) on the DC input wiring — a tripped or failed SPD should be replaced. An SPD that has absorbed a surge will often have a visible indicator or show a fault reading on its indicator window.
Check the MPPT input range for your Fronius model.

Different Fronius models have different maximum DC input voltages: - Fronius Galvo: 420V max - Fronius Primo: 800V max - Fronius Symo: 800V max Verify that the total string voltage under worst-case cold conditions is below the listed maximum for your model.
Do not operate the system if State 307 is frequent.

Repeated DC overvoltage events can permanently damage the inverter's MPPT input circuitry. If State 307 appears regularly (more than once a month), shut the DC isolator off until your installer redesigns the string. Continuing to operate a system with a marginal string design risks expensive inverter failure.

When to Call a Professional

State 307 can permanently damage the inverter's DC input stage over time. Contact your solar installer to review the string design. If overvoltage occurred from a lightning transient, have an electrician inspect the DC surge protection device (SPD).