Why does my inverter shut down on hot afternoons?

Three different faults produce the same notch in an afternoon generation curve. How to tell temperature derating from a grid over-voltage trip, and what the design should have allowed for at the inverter position.

The generation curve is clean until about half past one, then it steps down, sits flat for two hours and recovers by four. From March to May it happens on every clear day. In December it never happens at all. Before anybody replaces anything, pull the inverter's event log, because three different faults draw almost the same shape on a graph and they have different fixes.

Three faults, one shape

• Temperature derating. The inverter is not faulting, it is reducing output to protect its power stage. The curve rounds off rather than dropping vertically, and the log shows a derating or power limit flag rather than a fault code.
• Over temperature shutdown. Output goes to zero, then returns after a cool-down delay, often repeatedly. The log names an internal temperature fault and usually records the temperature reached.
• Grid over-voltage trip. Output drops to zero at the same time of day, but the log says AC over-voltage. This is not a heat problem. It is the local network voltage sitting high at midday, plus the drop across your own AC cable, forcing the inverter's terminal voltage past its permitted limit. Replacing the inverter changes nothing.

If it is the third one, the fixes are a larger AC cable, a shorter run, or a transformer tap adjustment agreed with the utility. Everything below applies to the first two.

Where the heat comes from

Every inverter datasheet carries a derating curve: full output up to a stated ambient temperature, reduced output above it. The temperature that governs is the air entering the unit, not the shade temperature outdoors, and the gap between the two is where most of these problems live.

• Direct sun on the enclosure. A unit on a west-facing wall is being solar heated all afternoon. The rating assumed it was not.
• Mounted under the array on a metal roof, sitting in a pocket of trapped hot air with no cross flow.
• A small sealed room. The room warms to whatever the inverters put into it. Even at high conversion efficiency, a few hundred kilowatts of inverters reject a continuous few kilowatts of heat, and in a closed room with no path out, the air temperature simply climbs until something derates.
• Units packed too closely. Manufacturers state minimum clearances above, below and to the sides. Line up more units than the room's air movement supports and the exhaust of one becomes the intake of the next.
• Blocked filters and heat sinks. On a jute, textile, rice or cement site the filters clog in weeks rather than months, and fins pack solid with fibre. Bird nests behind a wall-mounted unit are common enough to check for by default.
• Low DC input voltage. For the same power the inverter carries more current, and more current means more heat. A string design that lets Vmp sag on a hot roof makes the thermal problem worse at exactly the wrong hour.

What the design should have allowed for

None of this is difficult to get right at design stage, and all of it is expensive to correct afterwards.

1. Pick the design ambient from the site and add the expected rise inside the room or enclosure, rather than working from a national average temperature.
2. Site the inverters out of direct sun, on a shaded or north-facing wall, or in a room with genuine cross ventilation.
3. Size the ventilation as a calculation, not a guess: the air flow needed follows from the total heat rejected by the inverters divided by the temperature rise you are willing to accept in the room. If the room has to be air conditioned instead, that cooling becomes a critical auxiliary, and it must not sit on a circuit that gets shed.
4. Respect the manufacturer's clearances, and let the room's air movement decide how many units go in a row.
5. Specify filters that can be changed without shutting the plant down, and put a cleaning interval set by your site's dust into the maintenance contract.
6. Compare derating curves at your design ambient when selecting the inverter. Two units with the same nameplate can behave very differently at high ambient, and the cheaper one often starts derating earlier. That is a legitimate trade-off, but make it knowingly rather than by accident.

Read the log before you argue with your contractor. It will tell you in a few minutes whether you are looking at a ventilation problem, a cable problem or a grid problem, and those three conversations go to three different people.

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