Problem diagnosis · Fronius

Fronius Grid Overvoltage — Frequency Fault & State 102

Your Fronius inverter is disconnecting because the grid voltage at your property exceeds 253 volts — the upper limit set by UK grid code G98. This is not an inverter fault. It is a network issue, and it is increasingly common on UK streets with multiple solar installations.

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If state 102 trips daily during peak sun, you are losing significant energy production. We quantify the loss from your Solar.web data and provide the evidence your DNO needs to fix the network.

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Written by Ronald · Independent Solar Engineer Last updated 10 November 2026

UK grid voltage limits (G98 / G99)

Nominal — 230V AC

Permitted range — 216.2V to 253V

Trip thresholds — 10-minute average above 253V, or above 262.2V for 1 s, or above 273.7V for 0.5 s

The inverter disconnects automatically when voltage passes one of these trips. It reconnects after approximately 60 seconds once voltage returns to the safe range.

Diagnostics

6-step grid overvoltage diagnosis

Grid overvoltage is a network problem, not an inverter fault. These steps help you identify whether the issue is one-off or persistent, and what to do about it.

Identify which grid fault code you are seeing

Fronius uses several state codes for grid conditions. State 102 (AC voltage too high) is the most common — the inverter has detected voltage above the 253V G98 limit. State 103 means voltage too low. State 105 means frequency too high and state 106 means frequency too low. State 107 means no grid detected, and state 108 means the anti-islanding protection has triggered.

Check the inverter display or the Solar.web event log for the exact code, timestamp, and how often it has occurred. A single state 102 event during unusual grid conditions is not a concern — a daily pattern is.

Understand that this is a grid issue, not an inverter fault

All 1xx state codes are protective responses to abnormal grid conditions. The inverter is working exactly as it should — it has detected that the electricity network at your property is outside safe limits and has disconnected as required by UK grid code G98.

The inverter reconnects automatically after approximately 60 seconds once grid conditions return to normal. You do not need to reset it manually. If voltage remains high, the inverter will trip again — creating a cycle of short disconnections that costs you generation during the best producing hours of the day.

Check for a recurring overvoltage pattern

Open the Solar.web event log and review the last 30 days. State 102 appearing every sunny afternoon between midday and 3pm is the classic pattern. Multiple solar systems on your street are exporting simultaneously, pushing the local voltage above 253V.

This is increasingly common across the UK as solar adoption grows — particularly on rural networks with long distribution lines, weak infrastructure, and transformers that were set to high baseline voltages before solar was widespread. The combined export from several homes creates a cumulative voltage rise that the local network was not designed to handle.

Monitor your grid voltage

If your system has a Fronius Smart Meter, you can see real-time grid voltage in Solar.web. Even without one, the inverter logs the voltage at the time of each state 102 event.

Look at the peak readings. Grid voltage consistently above 248V during daylight hours means there is very little headroom before the 253V trip threshold. Anything regularly above 250V with solar export active is a strong indicator that the network transformer tap is set too high for the level of solar generation now connected in your area.

Report persistent overvoltage to your DNO

If state 102 is recurring and costing you generation, contact your Distribution Network Operator (UK Power Networks, National Grid, Northern Powergrid, etc.). Provide your Solar.web event log showing dates, times, and frequency of trips. Request that they fit a voltage monitor at your property.

The most common DNO fix is adjusting the transformer tap setting — lowering the baseline voltage by 5 to 10 volts to create headroom for solar export. This is a straightforward change that resolves overvoltage for all properties on that transformer.

If the DNO is unresponsive, escalate to Ofgem (the energy regulator), then the Energy Ombudsman if still unresolved.

Consider export limitation as a temporary measure

While waiting for the DNO to act, export limitation can reduce the frequency of overvoltage trips. A Fronius Smart Meter measures grid voltage in real-time and automatically reduces inverter output when voltage approaches the trip threshold. The system still generates for self-consumption but reduces export during high-voltage periods.

Export limitation does not fix the underlying grid problem — it is a mitigation that prevents generation loss while the DNO resolves the network issue. STS can assess whether this is appropriate for your situation and configure it remotely.

Why rural UK networks are hit hardest by grid overvoltage

UK distribution transformers have adjustable output taps — typically five to nine positions, each representing a 2.5% voltage step. Rural transformers were historically set to the highest tap to compensate for voltage drop over long distribution lines to remote properties. With no solar generation and low daytime load, this worked well for decades.

The problem appeared when rooftop solar became widespread. To export power, an inverter must push at a voltage slightly higher than the grid. On a rural network where the transformer is already delivering 245 to 250 volts at the nearest properties, even a small amount of solar export pushes the voltage above the 253-volt trip threshold. On a street where four or five homes have solar, the cumulative effect makes overvoltage a daily occurrence during peak sun.

Urban networks are less affected because they have shorter distribution lines, lower impedance, more customer loads consuming power during the day, and transformers set to lower baseline voltages. The long-term solution is for DNOs to adjust transformer taps downward and, in some areas, install active voltage management equipment. Until that happens, affected homeowners lose generation during the most productive hours.

Reference

102

AC voltage too high

Grid voltage above 253V. The most common grid fault in the UK. Auto-reconnects after ~60 seconds when voltage drops. Report to DNO if recurring.

103

AC voltage too low

Grid voltage has fallen below the inverter's lower limit. Less common than overvoltage. May indicate a weak supply connection, a high-draw appliance on the same circuit, or a wider network brownout.

105 / 106

Frequency outside limits

Grid frequency has drifted outside the permitted 50 Hz range. Caused by system-wide grid events, not local conditions. Usually brief and self-correcting. Rarely recurring.

107

No grid detected

The inverter cannot detect the grid at all. Check the AC isolator at the consumer unit, the main breaker, and whether there is a wider power cut in your area.

108

Island operation detected

Anti-islanding protection has triggered. The inverter detected it may be powering a section of the grid independently — a safety hazard for line workers. Reconnects automatically once the grid is confirmed stable.

Grid overvoltage — common questions

What causes Fronius state 102 grid overvoltage?

State 102 triggers when grid voltage exceeds 253V — the UK G98 upper limit. The most common cause is high solar penetration on your local network. When multiple homes export simultaneously, the combined feed-in pushes voltage above the safe threshold. This is worse on rural networks with weak infrastructure and transformers set to high baseline voltages. The inverter is working correctly — it disconnects as required by regulation.

Why does my Fronius inverter keep tripping on overvoltage?

Recurring trips during sunny afternoons mean grid voltage in your area is consistently near or above 253V when solar systems export. This is a network infrastructure problem, not an inverter fault. The transformer serving your area may have its tap set too high. Contact your Distribution Network Operator — they can adjust the tap, lowering baseline voltage by 5–10V to create headroom for solar export.

Can Fronius voltage trip settings be adjusted?

Fronius inverters have configurable voltage protection parameters, but they are locked to the G98/G99 country setup for UK installations. A qualified installer can make limited adjustments within the range permitted by the grid code, but the trip threshold cannot be raised above the regulatory limit. Persistent overvoltage above 253V requires a network-level fix from the DNO rather than an inverter settings change.

What is the difference between a voltage fault and a frequency fault?

Voltage faults (102, 103) are caused by local network conditions — the voltage at your property is too high or too low. These can be persistent, recurring daily. Frequency faults (105, 106) are caused by system-wide grid events — a sudden imbalance between generation and demand nationally. Frequency faults are typically brief, self-correcting, and rare. If you see state 105 or 106 regularly, it may indicate an inverter measurement issue rather than a genuine grid frequency problem.

How much generation am I losing to grid overvoltage trips?

It depends on how frequently the inverter trips. Each event triggers a minimum 60-second disconnection. If voltage stays high, the inverter trips again immediately after reconnecting — creating repeated cycles during peak sun. In severe cases, homeowners lose 20–40% of potential midday generation. STS can analyse your Solar.web data to calculate the actual loss and provide evidence for your DNO complaint.

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Overvoltage costing you generation? We quantify the loss.

We analyse your Solar.web data to calculate exactly how much energy you are losing to grid trips — and provide the documented evidence your DNO needs to take action.

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No fix, no fee
DNO complaint evidence included

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