Hi maybe you could help me. I am trying to repair my Growatt inverter that is showing error 08 when changing the batteries. Something on the main board is pushing high voltage back to the solar output via the sense resistors. I have removed the diode and igbt on the inverter and I still see 70v on the solar output with nothing connected and only running on batteries.

I also removed the protection caps and it’s still there.

The inverter does give 230V AC and works. Just can’t get it to charge the batteries.

Please help Coulomb.

I don’t know the Growatts, though they seem similar in design and firmware to the Axperts.

All these "transformerless" inveters with high maxmum PV voltage have a 50Hz negative square wave of bus voltage (320-500V) amplitude between PV- and earth/chassis, any time that the DC-AC converter (the "inverter proper") is running. This can cause confusing measurements. It's also the cause of strange faults if the PV panels have leakage to earth. On Axpert inverters, fault code 08 (bus voltage too high, same as with Growatt) can be directly caused by PV leakage to earth.

So my guess is that PV leakage to earth is causing the fault code 08, and the voltage at the PV inputs is an expected result of the voltage between PV- (connected to bus-) and earth. I don't know why it only happens when battery charging for you. Axpert owners report that this fault mainly occurs at night, so some disconnect PV at night with a relay to work around it.

The voltage sense resistors typically connect to a circuit that is referenced to the neutral output, which should be at near-earth potential. Earth has a 50 Hz positive square wave with respect to bus- and PV- when the DC-AC converter is running so these will leak some voltsge, at very low current, into the PV+ input. It won't be perfectly half the bus voltage, since your multimeter will load the weak leakage. So this is not the cause of the problem.

PV leakage to earth can be hard to find and fix. I'm lucky not to have this issue, since I have 145V max PV voltage inverters, which don't connect PV- with bus-. They connect with battery- instead.

I think I've finally figured out how this happens:

Consider the time (~49% of a generated sine wave) when the neutral upper IGBT is on. Now BUS+ is connected to neutral and hence to earth, so BUS- and PV- are at about -400V with respect to earth. If there is leakage from PV- to earth (say one of the panels has water ingress), then a current will flow as shown from BUS+ through earth, the leakage, the inductance of the wiring, and back to BUS- via the PV- input. Half a wave later, the neutral upper IGBT turns off. There is a dead time before the neutral IGBT turns on, so that the bus isn't shorted by both IGBTs conducting (they take a microsecond or so to turn off). Typical PV wiring has a lot of inductance; the inductance wants to keep the current flowing (inductors oppose a sudden change of current). This causes the PV- terminal to fly positive with respect to earth to allow the current to continue. It continues via the bypass diodes in the PV panels, through the PV boost diode, into the DC bus capacitors. This increases the voltage on the bus capacitors, and ramps down the inductor current. As soon as the neutral lower IGBT turns on, any remaining inductor current can return via the neutral lower IGBT.

During the day, the bypass diodes will be reverse biased by the PV voltage, but the current can continue through the panels instead. The "inductor" voltage will simply adjust to whatever it takes to keep the leakage current going through the panels.

So the bus capacitors will get a small, brief increase to their voltage. If the load on the DC bus is small, then this increase may be enough to keep the bus voltage increasing. Eventually, when the bus voltage reaches its limit, the inverter will stop with fault code 08, bus voltage too high. It might take a long time, but even a milliamp will increase 2000μF bus capacitors by 100 V in 200 seconds:

dV/dt = I/C = 10⁻³/2000·10⁻⁶ = 0.5V/s so 100V in 200 seconds or 3.3 minutes.

One milliamp from 400 V is R = V/I = 400 / 10⁻³ = 400kΩ. 400k is a pretty high resistance. Of course, if there is 10 mA of bus current (4W), then the leakage will have to be 11 mA for an excess of 1mA, requiring 36kΩ of resistance, a more substantial leakage.

The reason for this being more common at night may be that the inverter's load is typically lower at night.

However, when the inverter is battery charging, the bus current would be much higher, and I don't see this theory working.

Edit: I've just realised that the leakage current comes from the DC bus, so unless there is perpetual motion (of electrons?) here, this can't be quite right. The firmware regulates the DC bus to maintain the DC bus voltage within certain limits. It does this by adjusting how hard the DC-AC converter is operating, and/or how much current is fed back to the battery via the buck converter (if charging). During the day, power is fed from the PV panels to the DC bus. All I can think of is that the algorithm (which is quite complex) gets confused by the leakage current somehow. But during the day, it has to cope with the PV voltage and current fluctuating literally with the breeze at times, so it seems to me that it should be able to cope with a little extra current effectively at the PV input.

If anyone can see what I'm missing, I'd be quite interested.

Edited May 20May 20 by Coulomb

Thanks for your time Coulomb.

Hi does anyone know what can cause dc voltage to be present on the Growatt solar voltage sense line. The voltage can be measured at the solar input running on battery. This is with only the sense wire connected to the mppt board. I think this voltage is causing my error 08. With nothing but the ribbon cable connected to the mppt board the voltage on the solar input rises to about 74vdc measured with a multi meter. After about 70v it also shows up on the inverter lcd. I just can find where this voltage is coming from. I know it’s a fault on the main board, as I have swapped control cards and removed the diode and igbt from the mppt board and the voltage is still there. The voltage is still there with the dc bus disconnected from the mppt board.