DC String Inverters, High voltage DC when disconnected?

[TallTimber]

I am pretty sure i am not understanding this correctly. For a DC String system(SolarEdge SE10000 for example). when the system is disconnected from the grid do the DC strings stay energized? If they do disconnect i think the only thing that can do that is the optimizers. Would a system without optimizers just stay at series DC voltage? Seems like a huge negative over a microinverter setup. Thanks in advance!

 

[texie]

I am pretty sure i am not understanding this correctly. For a DC String system(SolarEdge SE10000 for example). when the system is disconnected from the grid do the DC strings stay energized? If they do disconnect i think the only thing that can do that is the optimizers. Would a system without optimizers just stay at series DC voltage? Seems like a huge negative over a microinverter setup. Thanks in advance!

You need to study the rules for rapid shutdown requirements. And yes, this is a major advantage of micro invertor systems as there is no high DC voltage outside the array boundary.

 

[TallTimber]

You need to study the rules for rapid shutdown requirements. And yes, this is a major advantage of micro invertor systems as there is no high DC voltage outside the array boundary.

I understand the requirements for rapid shutdown. But I am more wondering about the voltages beyond the 30 second requirement. Do systems tend to go to zero volts(1,5,10 minutes) after shutdown. Or does a system typically hover at a DC voltage? Thanks for the quick reply!

 

[texie]

I understand the requirements for rapid shutdown. But I am more wondering about the voltages beyond the 30 second requirement. Do systems tend to go to zero volts(1,5,10 minutes) after shutdown. Or does a system typically hover at a DC voltage? Thanks for the quick reply!

If you have a string inverter and you lose the AC and there are no MLEs then the string voltage will be there all the way to the inverter. Hence rapid shut down requirements and the complexity that comes with it.

 

[Carultch]

I am pretty sure i am not understanding this correctly. For a DC String system(SolarEdge SE10000 for example). when the system is disconnected from the grid do the DC strings stay energized? If they do disconnect i think the only thing that can do that is the optimizers. Would a system without optimizers just stay at series DC voltage? Seems like a huge negative over a microinverter setup. Thanks in advance!

The optimizers stop the module voltage at the optimizers, and the optimizers only output a shutdown standby voltage of 1 Volt, when the circuit is off. This allows for diagnostics and polarity confirmation, but it also keeps the string voltages within the rapid shutdown limit.

The uncontrolled open circuit voltage of the module is still present on the input leads to the optimizer, but the optimizers use an internal switch (or solid-state transistor), to prevent the full voltage from accumulating with the next optimizer.

Uncontrolled PV would produce the full string open-circuit voltage when disconnected, so this will happen for a ground-mount or a pre-2017 array, prior to module-level rapid shutdown was a requirement on buildings. Optimizers and micro-inverters are not the only option for rapid shutdown, but module-level power electronics (MLPE) in general will be required in some form or another, if this rule applies. Module-level rapid shutdown isn't written in the NEC in those exact words, but it is a logical consequence of the voltage constraints from 690.12.

The simplest version of MLPE that can do this, are devices that use a power line carrier signal to maintain the device's ON-state. When the signal ceases to be produced, the device shuts off the module's input circuit to the device, and prevents its voltage from accumulating with the rest of the string. The transmitter can be integrated in the inverter, or it can be a separately-installed transmitter. This kind of MLPE only turns the circuit on or off, and doesn't have any other functionality like monitoring or power processing for optimization (which optimizers and microinverters provide).

 

[tortuga]

Disconnecting from the grid does not necessarily trigger a rapid shutdown event though right?

 

[Carultch]

Disconnecting from the grid does not necessarily trigger a rapid shutdown event though right?

It's very common that it does, but it doesn't necessarily need to. The answer is specific to the details of the technology used.

Unless you are designing a system with battery backup, where there's a specific reason to do otherwise, you might as well design a system so that disconnecting the AC to the inverters also initiates rapid shutdown.

 

[jaggedben]

I am pretty sure i am not understanding this correctly. For a DC String system(SolarEdge SE10000 for example). when the system is disconnected from the grid do the DC strings stay energized?

Personally I wouldn't refer to a SolarEdge as a string inverter, or any other category, precisely because it won't work without optimizers and has very particular behavior that's unique to the technology design. Questions about SolarEdge do not necessarily have the same answers as any other brand of solar technology.

... Would a system without optimizers just stay at series DC voltage? ...

Yes.

 

[TallTimber]

Personally I wouldn't refer to a SolarEdge as a string inverter, or any other category, precisely because it won't work without optimizers and has very particular behavior that's unique to the technology design. Questions about SolarEdge do not necessarily have the same answers as any other brand of solar technology.

I used SolarEdge as i see a lot of them. However i am not familiar with all brands of inverters. But based on what i have read, anything newly installed will abide by the rapid shutdown requirements. However there is a possibility that in the event of an improper shutdown (grid outage, manual disconnect from grid, fire, etc.) There exists the possibility that the system could still be at full series voltage(~1000V in some cases)?

 

[jaggedben]

I used SolarEdge as i see a lot of them. However i am not familiar with all brands of inverters. But based on what i have read, anything newly installed will abide by the rapid shutdown requirements.

Er, no. It's not that simple. Some installations (ground mounts) are not required to have rapid shutdown and so not all inverters are designed for it. Some designs also rely on installing third party optimizers. Then there is also an option for installing a listed racking system to be compliant, which requires following the instructions to the most minute detail.

However there is a possibility that in the event of an improper shutdown (grid outage, manual disconnect from grid, fire, etc.) There exists the possibility that the system could still be at full series voltage(~1000V in some cases)?

Possibly.

 

[Carultch]

However there is a possibility that in the event of an improper shutdown (grid outage, manual disconnect from grid, fire, etc.) There exists the possibility that the system could still be at full series voltage(~1000V in some cases)?

Depends on exactly how the rapid shutdown is meant to be initiated for a proper shutdown.

A lot of rapid shutdown technology is based on a "keep alive" signal that needs to be actively transmitted to the MLPE devices. If the inverter, or separately-installed transmitter loses power, the devices switch to their OFF mode. This would mean an "improper shutdown" as you've called it, is just as proper of a shutdown as any other method.

If instead, you have a shutdown method that only works by a manual push of a separate button, then you would be correct. An example of an NEC2014-compliant system that did this, was Midnite solar's birdhouse system, which maintained a charged capacitor, and then discharged the capacitor when the user initiates rapid shutdown, to shunt trip the output breaker in the combiner. This would require a separate action from the user, in addition to throwing the AC disconnect.