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Grid-connected Inverters -Cloudy Day Intermittency

Grid-connected Inverters -Cloudy Day Intermittency

(OP)
Hi again folks.

With PV systems connected to distribution utility (DU)grids, most inverter manufacturers for such applications would be designed and tested per UL 1741 and functionally in compliance with IEEE 1457.

There is a concern of possible voltage fluctuation or sudden load drawn from DU when clouds are passing by and the PV panels input power will fluctuate affecting inverter output and power quality. My question is, how do PV grid connected inverters mitigate this potential concern. The same was indicated in IEEE 1547 but no details on mitigations.

Does anyone have good opinions on this?

Thank you

RE: Grid-connected Inverters -Cloudy Day Intermittency

There is no way to mitigate that with a direct from solar system. That is essentially 'The Whole Problem.'

Keith Cress
kcress - http://www.flaminsystems.com

RE: Grid-connected Inverters -Cloudy Day Intermittency

(OP)
So does this mean that it is inevitable even with inverter controls?

If the DU grid is stiff, say the PV plant is only 25% of source DU substation transformer and feeder rating...is the probability of causing power quality issues going to be incidental.

I was wondering how much factor is DU going to br considered “stiff” to have embedded PV generation not cause any issue?

What other folks did to be able to connect with no issues.

RE: Grid-connected Inverters -Cloudy Day Intermittency

Interconnected grids are generally very large, clouds are comparatively small. California rode out the eclipse without much difficulty.

RE: Grid-connected Inverters -Cloudy Day Intermittency

I can point to Vermont's Public Service Board as an example... when the DER is 15% or less of the annual peak load of the zone of protection in which the interconnection is made, among other things, a fast track screening process can be enacted rather than a full blown system impact study.

I think it will depend quite a bit on the line tying to the DER as well. A PV inverter will drop 80% of its peak rating within seconds during intermittent cloud cover (according to my own observations and another study). If you're feeding it with #2 ACSR, there could be far more significant flicker than with 1/0.

You might look to similar Public Service Board allowances on fast track screening criteria to get a feel for when a PQ problem is generally considered to be more probable. The answer is to evaluate your specific instance to see if flicker or other PQ problems could be a worry, I'm afraid.

RE: Grid-connected Inverters -Cloudy Day Intermittency

Older PV inverters do NOTHING to mitigate the voltage drop. Instead utilities adjust voltage regulation equipment such as Substation LTCs, line voltage regulators and switched capacitor banks. Utility devices typically take 30 seconds to 90 seconds to operate, so customers will experience short voltage excursions for large changes in solar output.

There are several different efforts to change the requirements for inverters including revising IEEE 1547, the California Smart Inverter Working Group and the German inverter retrofit. New inverters will likely be sucking in reactive power while at full production to control high voltage at the end of distribution lines, then providing reactive power during cloudy periods to keep the voltage up when all real power is supplied from the substation.

While California did just fine during the eclipse, it was a highly predictable and slow moving event spread across a large region. I think the OP was asking a more localized question. Voltage issues on feeders are more of an issue in areas less than 10 miles where a cloud bank can cover the entire feeder very quickly.

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