AC Voltage Drop vs. DC Voltage Drop

[andrew.tkelly]

Hi,

Consider this hypothetical situation: When designing a photovoltaic system, let's say that the main electrical panel is far away from the array ( ~200 ft away). Would it make more sense to place a single inverter near the far away main electrical panel, or closer to the array itself? I'm guessing it comes down to whether we want a DC voltage drop or an AC voltage drop. Are there advantages/disadvantages to each configuration?

Regards,
Andy

 

[Besoeker]

Hi,

Consider this hypothetical situation: When designing a photovoltaic system, let's say that the main electrical panel is far away from the array ( ~200 ft away). Would it make more sense to place a single inverter near the far away main electrical panel, or closer to the array itself? I'm guessing it comes down to whether we want a DC voltage drop or an AC voltage drop. Are there advantages/disadvantages to each configuration?

Regards,
Andy

PV isn't my field but DC will give you a lower voltage drop. That might be a consideration at 200ft.

 

[SolarPro]

The voltage drop associated with 600 Vdc will be lower than the voltage drop associated with 240 Vac. Also the dc voltage drop won't hurt you, but ac voltage rise (a byproduct of voltage drop in the ac lines) might. So generally speaking you want to run the long distance with the PV circuit conductors in a scenario like this.

 

[ggunn]

The voltage drop associated with 600 Vdc will be lower than the voltage drop associated with 240 Vac. Also the dc voltage drop won't hurt you, but ac voltage rise (a byproduct of voltage drop in the ac lines) might. So generally speaking you want to run the long distance with the PV circuit conductors in a scenario like this.

Of course, there is also rapid shutdown to consider if the AHJ has adopted the 2014 NEC. For that reason it is sometimes advisable to set the inverter within 10' of the array and bite the bullet with larger conductor sizes on the AC side.

 

[SolarPro]

Very true. If the array is mounted on a building and subject to NEC 2014, complying with 690.12 likely takes precedent over voltage drop considerations.

 

[Carultch]

Hi,

Consider this hypothetical situation: When designing a photovoltaic system, let's say that the main electrical panel is far away from the array ( ~200 ft away). Would it make more sense to place a single inverter near the far away main electrical panel, or closer to the array itself? I'm guessing it comes down to whether we want a DC voltage drop or an AC voltage drop. Are there advantages/disadvantages to each configuration?

Regards,
Andy

The consequence of too much DC voltage drop is that the inverter MPPT system will find a lower voltage for the same current. Simply put...power performance loss, proportional to your percentage.

The consequence of too much AC voltage drop is that the inverter will experience voltage rise above grid voltage at its output terminals to compensate. If the grid voltage starts particularly high, this could nuisance trip its internal relays. If grid voltage starts particularly low, not as much of an issue.

Typical default specs often call for not more than 2% on either the DC or AC sides, and not more than 3% overall. Not an NEC requirement, but more of an interpretation of the NEC's 3% feeder voltage drop recommendation.

Given a choice, rank order your voltage situations by best kcmil utilization/ohmic performance. Then travel with DC as much as possible.

Voltage drop is a more fluid question. Not a rigid rule. What do you want it to be? What is your customer willing to pay for?

 

[electrofelon]

I worked on a 1MW ground mount recently and since have been contemplating the question of which to send further in a situation like this, the AC or DC. I didnt do any design on this but quick stats were 480V service, 1000V string voltages, 30 24KW inverters scattered throughout the array. 10 Combiner panel boards were located throughout the array with direct bury aluminum USE back to the service equipment area (some pretty long runs). This was the only system of this scale I have worked on so I guess I cant make accurate judgements on doing it a different way, however in hindsight i think I would go for sending the strings further, back to a central point near the service where there are maybe 4- 6 inverters. Granted 1kv (actually this job used 2 kv) copper is more expensive then 600v aluminum, but with 1000-v strings, the voltage is obviously a substantial gain, and less connections, less digging, and less equipment scattered all over.

 

[Carultch]

The voltage drop associated with 600 Vdc will be lower than the voltage drop associated with 240 Vac. Also the dc voltage drop won't hurt you, but ac voltage rise (a byproduct of voltage drop in the ac lines) might. So generally speaking you want to run the long distance with the PV circuit conductors in a scenario like this.

I find this particular chart quite useful for rank ordering your options of a "travelling voltage". Given that you're name is SolarPro, I'll assume you are familiar with Blake's article as well.

 

[SolarPro]

Given that you're name is SolarPro, I'll assume you are familiar with Blake's article as well.

Yep, that's a very helpful table. Pretty much the crux of Blake's article.

To the OP: In my previous message, I linked to both Blake Gleason's voltage drop article for SolarPro, as well as a subsequent article about voltage rise. These are related topics. "Voltage rise," as seen by the inverter, is a function of voltage drop between the inverter and the grid.

 

[Zee]

If Blake writes it, it is well though out.
---------------------
And remember: every time you up the Volts, you ALSO drop the Amps! Double benefit for V drop.

On a recent ground mount I had about 260' distance. (I know because my 250' fish tape didn't make it)

I cancelled the micro-inverters at 240 VAC to go to a SolarEdge system.
This means a constant V of 350 VDC.

What always surprises me on V drop calcs is:
The funny math end result of higher V is really that of TWO significant factors: for a given Wattage, if we up the V, then the A also drops lower. Both affect V drop positively. So even though 350Vdc is just 50% higher than 240Vac....the Amps were also 2/3 as low...

I ended up pulling a pair of #6, for 28 modules. Don't ask what the alternative was...i erased the unpleasant memory.

Made a huge difference.
The solaredge optimizers and their "Safe DC feature" also solved rapid shutdown req.s!

 

[Zee]

FYI:
on DC systems use
Vmp
and
Imp for Voltage Drop calcs!
(not Max Sys. V, Voc or Isc)

 

[Cheese_boy]

What always surprises me on V drop calcs is:
The funny math end result of higher V is really that of TWO significant factors: for a given Wattage, if we up the V, then the A also drops lower. Both affect V drop positively. So even though 350Vdc is just 50% higher than 240Vac....the Amps were also 2/3 as low...

And it is nice that they multiply together to give even lower % of power lost in the transmission. (as well as the more obvious benefit of being able to use smaller wire/less costly for the longer runs)