Why does the Sun2000/1000 keep having AC shorts when using it at rated specs?
Update 3: At about 2.5 months, I acquired a broken Sun2000 with an AC short. This had only about 14 kWh logged in as the total kWh which means it probably died within a week. I fixed it with the new faster IGBTs and it is now stacked with my original unit under test. They are both set with a limit of 1800 W connected to the same 48V battery bank running 13 hours/day. This 2nd unit went into service on Jan 27, 2025.
Update 2: Reaching two months of continual operation at the extended time and limit. It's already been working longer at this setting than ever before so it looks good to becoming a real fix. I will run another month and then run it with no limit.
Update 1: It's been just over a month since we installed the faster IGBTs fix and it's still working great. We've been running it 13 hours/day (an increase of 5 hours/day from previous testing) with a power output limit of 1800W (also increase from 1500W) the entire time. Previous tests (with IGBTs with higher turn off delays) only lasted less than a month with the reduced hours and power so we are already doing much better than before. More details are added below.
At first glance the Sun2000/1000 grid tied inverter appears to be a decent value inverter. However, it seems the only way to make it work is running it at a derated spec with a lot of external protection and many best practice 'rules' making it too fragile to use for most users. This inverter has been around for a long time and latest is a 2nd generation version. One would think that over the many years of production and with a 2nd generation, improvements would have been made to increase its robustness and reliability. But overall, the reliability doesn't seemed to have improved significantly especially with AC shorts. If you do the research, there are plenty of support from user communities such as Facebook and YouTube that have documented their issues with using the Sun2000/1000 in various configurations.
We've been playing around with one since April 2024 from Y&H and had our share of failures. However, all our failures have been when the AC side is shorted which not only causes the inverter not to work, but also the LCD screen won't power up so it appears completely dead with no error messages or any ways to troubleshoot except to open up the unit and use the following guide.
When the AC side is shorted, the AC fuse will eventually blow, but usually not after other components are damaged. This blog will attempt to document our journey and hopefully offer solutions to completely eliminate the AC shorts when used within the manufacturer's specifications along with the usual surge protection devices without having to "baby" the inverter. We will update this blog over time since we are in the process of testing our solutions. We are mindful of people's time so this blog includes a quick summary section and a more details section.
A quick summary:
From a real simplistic explanation, the AC lines (L1 and L2 or N) are each connected to an IGBT (fast transistor switch). Each IGBT feeds each AC line the current from inverting the DC input. The AC short problem happens when there is a direct path between L1 and L2 or N. Under normal operation, each IGBT is alternating turning on and off at high speed to ensure both are never on simultaneously to create a short between L1 and L2 or N.
When our failures occurred, we were using our Sun2000 in accordance with the operation manual. We're using it with batteries and have installed the basic standard surge protection devices (one for the main panel to protect the whole house and one for the dedicated circuit of the inverter). We believe there are at least two possible reasons why the AC shorts are occurring in our scenario, either they were inadvertently commanded to both turn on or one of the IGBT fails as a short and then the other one is commanded to turn on as part of its normal cycle. The damage could be widespread and extensive. It can take more than 1.5 secs (at the maximum rated current for the IGBTs) before the stock 15 A time-delay fuse blows. During this second or so, the gate can short internally causing the driver to blow, the SCRs can blow along with some current sensing resistors and even the PCB traces can melt open. Below are our theories as to the causes for these two possibilities.
Cause 1: Both IGBTs inadvertantly turn on when the inverter's microcontroller is in an unknown (brown out) state.
Symptoms: AC short happens when unit is not generating or when there is momentary power loss on the AC side creating a brown out condition for the microcontroller.
Our Fix*: Add a battery powered UPS on the DC portion of the microcontroller circuit. This can be done by buying a mini UPS board from AliExpress (select 9V1A) and 3.7 V lithium polymer battery (at least 1100 mAh) and wiring it up yourself (wiring diagram) or buying a complete kit called SunScreen.
Cause 2: During normal operation, the IGBT(s) get(s) too hot and fail as shorts. We believe the IGBT's junction temperature is getting too hot because the IGBTs are not switching off fast enough and the heat isn't being dissipated fast enough.
Symptoms: AC short happens while the unit is generating at or near rated power.
Our Fix**: Change out the IGBTs to faster ones like these. We are calling this fix SunSpeed.
*Note, it looks like a later version of the motherboard (2023-08-09) might invadvertainly fix this by adding 2 relays that engage the AC lines when the 12 V DC is powered on or after the microprocessor powers up.
**Note, it looks like a later version of the motherboard (2023-08-09) is attempting to fix this by using higher rated full body TO-247 IGBTs (1200 V, 25 A vs 600 V, 20 A), but it is slower by 2.5 times for the turn off delay.
Detail explanation:
For Cause 1
The signal to turn on the IGBTs are not being controlled properly. We believe under normal operating conditions, this cannot happen because the inverter's firmware controls the driver chip that is used to control the IGBTs. We are assuming that the inverter's firmware has been optimized over the many years to not have any defects that can accidently turn on both IGBTs simultaneously. The only way is if the firmware is not controlling the driver chip as in the case of a brown out condition. This is when the power supply voltage is below the microcontroller's recommended operating voltage and the microprocessor's outputs are usually in an undetermined state. Since the outputs are connected to the driver and then the IGBTs, the undetermined state could be inadvertently turning on both IGBTs causing an AC short.
Our fix is to never allow the microcontroller to power down during a drop in AC voltage (potential for brown out) by having an uninterruptible power source (UPS). If the firmware is always running during this time, there is no chance for the outputs to go into an undetermined state.
Our first failure was an AC short after less than a month when the unit wasn't even generating back in April 2024. We installed our brown out prevention fix in June 2024 and we have been running our Sun2000 (45-90 V) with 48 V lithium ion batteries without having any failures during drops in AC voltage regardless if it is generating or not.
However, since then we have had AC shorts only when the unit was generating near or at its rated power when the AC voltage was fine. We believe this is the second cause for AC shorts.
For Cause 2
In attempt to finding the cause for this type of AC short, we installed a 5 A fast acting fuse (ordered 8 A but found out we received a different fuse rated at 5A) instead of the original 15 A fuse to see if we could isolate the failure to a single component since previously the AC short would destroy multiple components so it was hard to determine which actually failed first. The 8 A fuse that we ordered was chosen due to the reaction time was about 0.07 secs when the max rated current of the IGBT (40 A @ 25 deg C) is reached. The original 15 A time-delay fuse would probably take at least 1.5 seconds to blow at 40 A. It turns out the 5A fuse that we received actually has a reaction time of 0.05 secs at the max rated current and would allow 10 A without blowing. When we got the AC short failure after installing the 5 A fuse, only the IGBTs and fuse were blown. This tells us that the IGBTs were the first to fail and the other component failures were the result of the IGBT failures. So we can now focus on what is causing the IGBTs to fail.
At first, we thought of trying IGBTs with higher voltage and current ratings, but looking at the voltage and current waveforms on the AC side, there appears to be enough headroom with 600 V and 40 A for continuous operation. We know that ultimately too much heat (specifically the junction temperature) is what kills them, but didn’t know exactly how. The next thought was to look at the heat generated during switching. This would be looking at the turn on and off delays. The turn on delay is much less than the turn off delay and the turn off delay increases as temperature rises, but the turn on delay does not. So the more current it was switching off at high speed, the more the temperature would rise which would then increase the off delay which would then increase the temperature at a faster rate potentially causing a “thermal runaway” condition if not sufficiently cooled. We think that this would still slowly damage/degrade the IGBT every time it gets close to this runaway condition even though there wasn't a catastrophic failure. It seems to fit with the other symptoms that we have experienced and read about that others were having, more so than having under-rated components.
So at first, we put IGBTs with a longer turn off delay of 199 ns vs 125 ns (original IGBT) and got the AC short failure in about a month. We noticed that prior to a complete failure, the unit would intermitently turn off generating and then back on generating. It did this for a few days before the complete failure. Our theory is the longer delay in turning off allowed the IGBT to heat up quicker and higher during the off transition thus increasing the chance for thermal runaway and chance to cause damage.
For our current test, we put in IGBTs with a 78 ns turn off delay vs the 125 ns and are waiting for the results. It is also a full body type TO-220 which has better heat dissipation. It is a 650 V vs 600 V rated part with the same current rating as the original one. However, the power dissipation is only 45 W compared to the original IGBT with 140 W. This seems like a significant difference! So far, we are already seeing overall cooler internal temperatures and it seemed to be more stable in tracking the power output with our limiter CTs. We are able to use SunBeam (our WiFi remote control product) and Home Assistant to analyze real time data for power output, internal temperature, and load power. We started this test on 15th of Nov 2024 and we want to run this for at least 6 months in order to declare success. We don't see how this new IGBT can make things worse, so we want to go ahead and tell people what we are doing so that if you happen to need to replace your IGBTs, you might want to try the ones we are using to help confirm our theory that these IGBTs are better and may be the solution to fixing this cause.
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