Open Circuit Voltage with DC to DC Converters

[Canton]

Looking for some help guys.

How is the Open Circuit Voltage calculated when DC to DC Converters/Economizers are used in the source circuit strings?

Would 690.7 still be in play, and use the Voc of the series modules?
OR
Would the max voltage output of each series connected DC converter be used?

The example would be 10 PV modules each with its own DC converter, and connected in series.

 

[Carultch]

By strict definition, the open circuit voltage of the output of DC-to-DC converters (i.e. optimizers) would be the shutdown standby voltage. Such as units that default to outputting 1 Volt per device, when the inverter is turned off, or when the inverter isn't connected yet. I would recommend calling this the shutdown standby voltage to be more specific than open circuit voltage. Because this is primarily a voltage to allow you to test polarity and string size, and to allow the optimizers to begin communicating when the inverter does get turned on. Even though this is technically an open-circuit voltage, it is by no means the maximum voltage that people usually expect Voc to be.

The max voltage by contrast, is not set by the optimizers or by the modules. Instead, the inverter governs this. The inverter datasheet will specify a maximum system voltage (perhaps 500V), and a typical operating voltage (perhaps 400V). These numbers are inverter-specific, so these are not universal numbers for this technology. The maximum system voltage (e.g. 500V) takes the place of how you used to use Voc with ordinary strings. It governs the selection of the voltage rating for every material between the optimizers and the inverter, and gets labeled on the inverter in place of the open circuit voltage. This is the maximum voltage that the optimizer string will produce, even if the optimizers have a maximum output voltage that seemingly can add up to greater than 500V. The optimizers won't all be operating at their full output voltage capacity simultaneously. The typical operating voltage (e.g. 400V) takes the place of how you use Vmp for ordinary strings, which is where you typically will find the inverter operating. The inverter aims to run at 400V when it can. If necessary to increase this voltage, then it has the 400V to 500V range available as headroom above its "preferred" operating voltage.

The way optimizer strings work, is that the inverter sets the operating voltage, and the optimizers will communicate with one another to "solve an algebra problem". The current must be identical in each optimizer, due to being in series. The optimizers then produce a voltage that is proportional to the power available to each one. This voltage of each optimizer adds up to the voltage set by the inverter. Optimizers are analogous to transformers, in the sense that they satisfy the equation V_in*I_in = V_out*I_out*efficiency. Optimizers change V_in to V_out by a ratio of A, and they correspondingly change current by a ratio equal to 1/A*efficiency. The optimizer will dynamically adjust the ratio A, as needed to "cooperate" with the other optimizers in the string.

 

[Canton]

Carultch,

Thanks for the information. The question arose out of the need for the Permit Submission for the Voc of the strings. Is there no calculation to be done then? Meaning that the Inverter (communicating with economizers) will not let the DC String Voltage exceed a set limit? The only limitation being the Max Module/Economizers that can be stringed together set by the manufacturer.

I am looking at this from a traditional string inverter set up where the Voc of the series strings cannot exceed a certain voltage in extreme cold weather conditions. Apparently with economizers this not an issue.

For Current calculations is the Max Current (Isc) x 1.56 still applicable? If I read the artilcle correctly it would appear that with DC to DC Converters (Economizers) it is only (converter output current x 1.25). Is this correct?

 

[jaggedben]

I think the short answer is probably that 690.7 is not in play anymore for the series string. That is certainly the case for Solaredge. Other brands I'm not as familiar with, but you would look to the optimizer datasheet or instructions for info on how they would behave.

 

[jaggedben]

And note that with optimizers the string is a DC-to-DC Converter source circuit, not a PV source circuit. So 690.7A applies only to the conductors between the modules and optimizers.

For permit submission I would probably put either the max system voltage as described in the optimizer literature, or I would put 'NA'.

 

[Canton]

Jaggedben,

I was starting to think the same that 690.7 would not apply.

For Current calculations is the Max Current (Isc) x 1.56 still applicable? If I read the article correctly it would appear that with DC to DC Converters (Economizers) it is only the (converter output current x 1.25). Is this correct?

 

[jaggedben]

Yes usually the optimizer output circuit current is 1.25 of the datasheet max output. You don't need the additional 1.25 multiplier that you need for source circuits. As far as the optimizer input, usually it's just module leads going to the optimizer so it's just a matter of making sure that module and optimizer specs are in accord. Optimizers usually have a max Isc input which cannot be less than module nameplate Isc.

 

[Canton]

Guys, Greatly appreciated!

 

[Carultch]

For Current calculations is the Max Current (Isc) x 1.56 still applicable? If I read the article correctly it would appear that with DC to DC Converters (Economizers) it is only the (converter output current x 1.25). Is this correct?

Optimizers are current-limited, unlike ordinary modules. If modules experience more than 1 kW/m^2 of sunlight, a "sun" at standard test conditions, the current will exceed the values on the datasheet, which is why the first 1.25 factor applies to modules in general. The first 1.25 factor for a "super sun" of irradiance doesn't apply, except on the optimizer input where you don't really need to size anything anyway. The 1.25 factor for continuous loads is the only one of the 1.25 factors that applies for sizing anything between the optimizer and inverter. Termination amps and OCPD's.

1.25*Imax takes the place of 1.56*Isc
Imax itself takes the place of 1.25*Isc
Imax comes off the optimzier datasheet. Solaredge values are 15A and 18A, depending on which family of optimizers you use.

 

[photovoldanic83]

These gents are all correct about the optimizer (what you call an economizer), that the optimizer DC-DC converter output circuit (Figure 690.1(a)) will most likely be controlled at all times. In the case of solaredge, depending on the optimizer, operating output voltage will be 60-85V. During RSD times, the optimizer puts out 1.01V.

The problem that I see (somewhat unrelated and regarding rapid shutdown) is that, with some soalredge dual optimizers (P730, P801, P850, P950) the modules are wired in series (presumably within the optimizer) which causes an over 80V condition at TC VOC. If the firefighters have to for some reason saw through the module, because someone installed a "space invaders" array, they could hit that optimizer and get over 80V.

 

[Carultch]

The problem that I see (somewhat unrelated and regarding rapid shutdown) is that, with some soalredge dual optimizers (P730, P801, P850, P950) the modules are wired in series (presumably within the optimizer) which causes an over 80V condition at TC VOC. If the firefighters have to for some reason saw through the module, because someone installed a "space invaders" array, they could hit that optimizer and get over 80V.

As of 2019 in any location where 2017 or later is in effect, you can no longer use the dual-module optimizers that connect input modules in series, in any application where rapid shutdown applies. Newer optimizers (e.g. P860's) are built with the 80V limit in mind, and therefore are built to allow parallel connected modules, instead of series-connected.

So any dual-module optimizer that uses series connections of the two modules is either restricted to a location where NEC2014 is in effect, or restricted to non-building applications where rapid shutdown doesn't apply.