Information to clarify the query, and replies
Information to clarify the query, and replies
Unfortunately there was no error message. This was found out only via monitoring system. We now have spread out the strings from this inverter to other inverter evenly. We don't see any issue until now after restringing. Lets see what happens!
Based on the original query by TheElectrician, and replies to that query from others, I'd like to clarify several points that have been posted, most of which are excerpted for brevity:
electrofelon: I think the Imp can exceed the Inverter's max without damage.
DL-PE: This is correct. Inverters won't be damaged if the maximum power point current from the PV array exceeds the inverter's maximum rated DC input current.
The query by TheElectrician implies much more PV DC wattage is connected to the inverter input than is required to generate maximum AC output. By UL 1741 requirement, grid-interactive inverter AC output current may not exceed the maximum current rating in the inverter's specification, regardless of the total DC power available. This in turn limits the DC input current. If an oversized array of (say) 175% is connected to an inverter, the output power waveform through the course of the day has a steeper slope up early in the day and will last later into the afternoon than an inverter with an array that's 100% of the inverter's DC current rating. It may mean the system isn't going to operate
The only parameter of importance in this aspect of a system is to ensure the PV array output voltage does not exceed the inverter's maximum DC input voltage rating under all conditions of operation. (PV voltage goes up with colder temperature.)
TheElectrician
... and in this case the energy loss is acceptable.
DL-PE:
No energy is lost. It's simply not converted from solar DC to grid-connected AC. However, in this context, it's easily understood you're referring to this unconverted energy. It simply means the system efficiency is less than ideal. Typical DC-to-AC sizing is 1.25 to 1. (125 kW of PV for 100 kW of inverter-rated AC output.) This may vary depending on circumstances. 1.75 to 1 is much higher than good design practice would suggest.
TheElectrician
But we are experiencing random shutdown and restart of the inverter during peak hours. The inverter manual says even in worst conditions 1.75 DC/AC ratio is acceptable as long as I meet the inverters Isc and Voc, which are fine in my case. I will have to check the thermal limitation to see if that causes this problem.
DL-PE:
There's an important clue here. "Random shutdown and restart of the inverter during peak hours."
Shutdown could be due to any of several possibilities:
1. Inverter over-temperature due to continuous operation at 100% of its ratings for extended periods of time. Ambient temperature would also be a factor.
2. Abnormal grid conditions. In this case, most likely the grid voltage
at the inverter exceeds the allowed maximum. This can be caused by poor connections in the inverter AC output circuit, insufficient ampacity in the inverter conductors .. or simply a high grid voltage. When forcing current into conductors, and back into the grid, there's a certain amount of voltage rise in the circuit. Removing load from the grid also can cause a voltage rise. 10% is the maximum allowed variance. Thus presuming a 277/480 volt three-phase topology, if any phase should exceed the 10% voltage allowance, it's required that output from all three phases cease. The inverter must sense "good" grid within acceptable parameters for a period of five minutes before it can again begin exporting power.
3. Out-of-tolerance line frequency is also a possibility, albeit somewhat distant. Grid-interactive inverters are required to cease exporting power if the frequency falls below 59.7 Hz, or goes above 60.5 Hz. Depending on the location of the installation, the inverter may be required to operate outside of these limits.
* jaggedben
I very much doubt that the inverter will shut down or trip. There is almost no way I can imagine that an inverter could allow a 1.75 DC/AC ratio and not end up in this situation. It will almost certainly just limit the current to something less than the IMP of the array and therefore clip energy production. And since you have a DC/AC ratio of 1.5 this should be something you are expecting. It's pretty common these days as module prices have dropped and interconnection costs haven't.
Consult the inverter manual and manufacturer to double check that you are not voiding the warranty.
DL-PE:
I included the entire context of Ben's reply, because it's all relevant.
The inverter will not cease to generate power simply because of a high DC-to-AC ratio. There may be more DC available than the inverter can convert to AC, but all that happens is the inverter output attains its specific maximum output power, and does not go above it. Thus, when the daily energy is plotted, the chart has a flattened area during the middle of the day. Due to the resemblance of this chart to an audio waveform that has been "clipped", the same term is often mistakenly applied to a flattened energy plot in AC systems. "Clipping" implies distortion. There is no distortion taking place here. Instead, the correct term would be "limited". However, most people take them to mean the same thing.
* electrofelon
There is a potential issue with DC/ac ratios being too high, where the point on the I/V curve needed to clip that power is outside of the inverter's capabilities. It seems that with some basic measurements and discussion with the manufacturer would identify of that is what is going on. I also seems to me that specifying a DC/ac ratio that is acceptable is too crude, and one would need to look at where the array parameters sit within the inverter's capabilities.
DL-PE:
No. This is incorrect. There is no point at which the DC-to-AC ratio can be "too high". Inverters will use as much DC power as is needed to attain their maximum rated AC output power. The rest goes unused. Yes, this may result in the inverter operating at a point on the array's voltage-current curve that isn't exactly at the "maximum power point", but this is harmless. The only "damage" is to the bank account of the system owner, when they pay much more for the system than was necessary.
* pv_n00b
* What's the shut down error message? Most inverters these days are pretty good about telling you why they are shutting down. Random shutdowns during peak production are also a sign of possible AC overvoltage due to a high AC voltage drop and high AC voltage from the utility.
DL-PE:
The reference to AC over-voltage shutdown is absolutely correct. If this is what's occurring, you can watch for the inverter to resume generating power after a required five-minute wait time, during which the grid voltage and frequency must remain within acceptable limits. If during that five-minute period another anomaly should occur, the five-minute time starts back at zero.
* TheElectrician
* Unfortunately there was no error message. This was found out only via monitoring system. We now have spread out the strings from this inverter to other inverter evenly. We don't see any issue until now after restringing. Lets see what happens!
DL-PE:
If in the rewiring the total power to any one inverter is such that it never attains 100% of its rated output power, this may be sufficient reduction to keep the grid voltage within acceptable limits.
For those who may not be familiar with solar panels, their "rated" output and what's actually achieved tends to be around 80% of the ratings. Thus, a solar panel rated 300 watts DC under "Standard Test Conditions" will deliver around 240 watts in real life. Sunlight intensity and ambient temperature directly affect their total output power. Sun angle and other variables also factor into this equation.
Summary:
Based on information provided by "TheElectrician", if the random issues continue .. my first recommendation is to monitor the voltage of all three phases. If the voltage on any phase exceeds 110% of nominal, the entire inverter will cease to export power (within two seconds), and will stay in that mode until the grid has been within acceptable parameters for five minutes. If you're present when the inverter goes offline, and it resumes exporting five minutes later, this is clear evidence of an abnormal grid condition. If the voltage is within a volt or two of 110%, this is an indication of pending problems, and it takes only a small increase in the voltage to cause the inverter to cease. It was indicated some of the PV were moved to another inverter. Does it also suffer the same intermittent behavior? If so, it strengthens the "high line voltage" assertion by PV Noob. If the issue affects just one inverter, it indicates bad connections in the output between the inverter and the point of connection to the utility.
An over-temperature shutdown is not clear a possibility, as it would result in the generation of an error code, and would leave the inverter offline for a period of time to cool. Many inverters also include a pre-shutdown warning, and may even reduce the AC output to prevent additional heating.
As a brief aside .. please don't be distracted by the "Junior Member" status I seem to have acquired here. This is my 48th year involved in the solar industry.
My apologies for the lengthy reply.
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