New PV grounding config (pv ground mounted attached to a civil building)

[josealjim]

THANKS IN ADVANCE

Id like to give a brief explanation about my situation in order to make u understand my ignorance on these issues. I have 2 years of working experience, 1 year as internship working on solar thermal plants simulations and troubleshooting on site, on my second year (already with an engineer contract) ive been developing the automation of biogas generation and biogas upgrading plants, the executives told me that they wanted to introduce photovoltaic systems in our portfolio, so i started to study theory and simulation making simulations. There is no technical supervisor for pv projects here and we dont have any consultant company for advices, and the electrical engineers here (Thailand) dont care about how installers do. Thanks to this forum and information given by suppliers Im having a better idea about how a good installation should be made, so I really appreciate every suggestion given.

The engineers where I work told me that the lighting ground system and and main service ground system were not but after digging a bit I saw that we have one earth ring system for the building where LPS and MDB busbar are connected. There is a lot of controversy about the best PV grounding system when you dont rely on any code and I would like to have the PV parking we are building grounded with the best configuration possible.

After knowing that we have a common earth ring for LPS and building services and with all the information provided by this forum i made two configurations:



On this configuration, all the modules and structure are bonded and connected to a busbar at the array. From the busbar there is an PVC 16sqmm cable going to a 18 m far handhole, from there to the building ring and connected to the SPDs, the same PVC 16sqmm continues to the inverters for ground fault protection.
Although i dont need to rely on NEC code, it says that we can not use EGC as GEC. I dont know if this config I accomplish that

Is this config correct? Would it compromise the groundfault protection of the inverter?

What about this other one?



Here there is a GEC to the handhole and from there to the building ring. SPSs are attached to the GEC at the handhole. An EGC goes from the array busbar directly to the inverter for the groundfault protection. Is this config better?
There are no junction box for parallel connection outside the inverters, each string to its inverter dc input but the thai legislation makes you include a circuit breaker right before the inverter. If I size the EGC according to NEC 250.122. it would be an AWG14 wire but the installers already ran a pvc 16sqmm (they havent connected yet, we are waiting the inverters).

On this second config, the connection from the handhole to the earth ring of the building, should it be by bare copper with the same size than the ring?
Is correct to use both GEC and EGC with the same size 16sqmm although with a much smaller one for EGC would be ok?


In brief,
Which of both would you choose?
What would you change to make it better?

IM A BIT DESPERATE WITH THIS ISSUE SO I REALLY APPRECIATE YOUR HELP

 

[jaggedben]

I don't see a difference electrically between the diagrams. In both cases your array is bonded to the EGC terminal of the inverter with a green wire so your groundfault protection should be functional. What is not clear in the diagrams is whether you intend to run the green wire in the same raceway as the DC conductors going back to the inverter. The NEC would require the EGC to be run with the circuit conductors. It seems to me that in either diagram you could do that, though.

Besides that, long as all your bonding is done with a proper method, it shouldn't matter if you do it at the array or the SPD. The SPD seems like less wire.

...
... Although i dont need to rely on NEC code, it says that we can not use EGC as GEC....

I think the prohibition was the other way around, but either way it's a slightly silly rule, and partially contradicted by 690.47(C)(3). Personally I see no general problem with combining an EGC and GEC into one wire in any section of raceway where you would otherwise be running two separate wires. That's as long as the combined section follows the various requirements for both purposes.

 

[shortcircuit2]

NEC 250.134(B) ex#2 allows EGC to be separate for DC circuits...

 

[josealjim]

Thanks a lot for your support

 

[Smart $]

Using a combined EGC/GEC under the provisions of 690.47(C)(3), you could wire it like this:

 

[josealjim]

Using a combined EGC/GEC under the provisions of 690.47(C)(3), you could wire it like this:

Thanks but shouldn't the SPD protections box be attached to the nearest point to earth (there is a building ground ring test box near the spds where i was thinking to connect it) in order to keep the building out of high induced voltages by lightning?
There also SPD at the AC side of the inverter.
Thanks

 

[Smart $]

Thanks but shouldn't the SPD protections box be attached to the nearest point to earth (there is a building ground ring test box near the spds where i was thinking to connect it) in order to keep the building out of high induced voltages by lightning?
There also SPD at the AC side of the inverter.
Thanks

The diagram as revised shows [one means of] minimum compliance with the NEC. As long as no requirement or its intent is subverted, anything beyond that is a design decision. I believe what you suggest is compliant with the NEC... at least I can't think of anything offhand that would not be compliant.

 

[josealjim]

Hi, guys i just received a suggestion from the moderator of another forum. He suggests not to connect the EGC to the inverters to avoid multipoints connections loops due lightning strikes.

http://forum.solar-electric.com/showthread.php?24543-PV-grounding-configuration&p=200121#post200121

His suggestion:

" If I understand your questions (and I am certainly not a lightning ground expert)--My choice is #1, and to not make the connection from the "local" PV grounding rod to the GEC on the inverter. The reasons:

1) this brings the potential for lightning energy directly into the building/GT inverter.
2) creates AC power ground loops through your equipment--I would try to avoid that. "

"The second drawing simply makes it even easier for lighting to be brought into the building/GT inverter/Electrical system (directly attached to PV array safety/lightning ground).

In both drawings, you have a separate green wire from the building single point AC ground rod brought directly into the GT Inverter GEC bar. Plus you are bringing the local Green Wire ground to the inverter chassis (which is probably connected to the GEC ground bar too).

Again--multi-point grounding. While not usually a bad thing for lighting--It does give you more chance of a ground loop, and bringing in energy (somehow) from different points in the building). Not neccesarly bad to multi-point ground, but, from my point of view, voltages are "local" to the GT inverter. You have the local green wire going with the AC leads together--They both will have the same surge voltages present (if any)--And that is "OK". Voltage differences are what cause the problems. With two different ground paths (and potentially two different ground reference voltages/locations), you now are bringing in two "ground" wires that can have large voltage differentials--Because of different ground bond locations (i.e., voltage gradient in ground from lighting or AC voltage injection) or because of the "loop" nature of the two ground paths (wires that follow the same physical path/bundled together do not develop differential voltages. Wires that follow different paths/have different reference points will develop differential voltages).

I have seen 60 VAC differences between grounds in a large salt water tank pumping/filtering system 75' apart (enough to give me a shock). With lightning, the voltage differences can be much more.

In reality--Lighting energy wants to only follow wire for relatively short distances 10-20 feet maximum or so, before impedance becomes high enough that it may want to find a different path. Taking the shortest/straightest path to ground with lighting bonds (large diameter, flat/woven cables, large bend radius, etc.), vertical ground rounds at corners/opposite sides of buildings (parallel current flow creates same polaritiy magnetic fields, lightning current flow wants to flow at outside edges of buildings on opposite parallel wires, not parallel wires next to each other)--So you don't want to bring lightning grounding (i.e., solar panel frames) down the middle/inside of the building--As the lighting may jump towards the the outside walls via any "better" conductive path (lightning is something that NEC has not always be good at).
"
My understanding is that lighting energy will go to earth though to the easiest path so , why that energy would prefer to cross the building through the inverter and main service ground bussbar in order to reach earth if there is a much shorter easier way right before entering into the building were the SPDs are attached (connection to ground ring showed on my picture)?

Too much divergence on this topic.... it is turning me crazy

THANKS FOR YOUR SUPPORT

 

[GoldDigger]

As Mike's video on the subject points out, not bonding the auxiliary electrode to your other electrodes to make it a single GES invites lightning induced earth gradients to send current through the EGCs and the equipment.
But not bonding the panel and racking metal to an EGC is a code violation (!!), so the proper solution is to bond the PV ground electrode to the rest of the GES through wire which is outside the building if possible.
Oh, and actual lightning current will go wherever it darned well pleases, unless it is completely diverted from the house by a properly engineered and installed lightning protection system, which has little to do with NEC grounding systems.

 

[josealjim]

As Mike's video on the subject points out, not bonding the auxiliary electrode to your other electrodes to make it a single GES invites lightning induced earth gradients to send current through the EGCs and the equipment.
But not bonding the panel and racking metal to an EGC is a code violation (!!), so the proper solution is to bond the PV ground electrode to the rest of the GES through wire which is outside the building if possible.
Oh, and actual lightning current will go wherever it darned well pleases, unless it is completely diverted from the house by a properly engineered and installed lightning protection system, which has little to do with NEC grounding systems.

Thanks GoldDigger,

The panels frames and structure are grounded. Each string has 4 sqmm wire that ends up in a busbar at the local array, from that buscar the PVC 16sqmm buried cable goes to the nearest connection to the bulding's ground ring and from there the same PVC 16sqmm continues to the dc isolators and inverter for the correct function the GFDI protection inside the inverter. I guess this configuration accomplish Mike's and NEC (even though i dont have to follow NEC in thailand, just to make it right).

" unless it is completely diverted from the house by a properly engineered and installed lightning protection system "

In your opinion, is the configuration a good lighting protection systems?

The LPS of the building ( 22m high and 17 m detached from the solar parking ) covers part of the solar installation according to our electrical engineers.
We tried to install separated ground rods right near the array but it was impossible to get bellow 25 ohms even using 3 rods (thai standards require 5 ohms). The resistance to earth of the building ring is pretty good, about 1 ohm.



Thanks
Jose

 

[Smart $]

Not sure about lightning protection, but that looks okay to me. The main thing is that only one grounding electrode system is used for the entire system. Otherwise, as pointed out by GD, you end up with a voltage gradient between electrode systems during an electrical storm.

 

[ggunn]

Threading weirdness...

...and that fixed it.

 

[jaggedben]

...Oh, and actual lightning current will go wherever it darned well pleases, unless it is completely diverted from the house by a properly engineered and installed lightning protection system, which has little to do with NEC grounding systems.

Yes. This.

... The LPS of the building ( 22m high and 17 m detached from the solar parking ) covers part of the solar installation according to our electrical engineers. ...

Wait, we've gone through two threads and now you tell us the building has an LPS? Did I miss that before? :slaphead:

My advice is to stop worrying about lightning so much, or get a LPS engineer to weigh in. If you/we keep trying to conflate equipment grounding, AC system grounding, and lightning protection, you're going to end up with something not right.

Oh, and I think you misinterpreted BB's words on the other forum. To repeat : you need an EGC from the array to the inverter for the GFDI to work. There's no point in risking fire damage to avoid lightning damage.

 

[josealjim]

Yes. This.

Wait, we've gone through two threads and now you tell us the building has an LPS? Did I miss that before? :slaphead:

My advice is to stop worrying about lightning so much, or get a LPS engineer to weigh in. If you/we keep trying to conflate equipment grounding, AC system grounding, and lightning protection, you're going to end up with something not right.

I think i mentioned about the building LPS but i dont know it was on this forum or another, sorry for that . Anyway the LPS of the building covers less than 10% of the Array, and id like to know how to arrange any future configs (not just our single case).

Oh, and I think you misinterpreted BB's words on the other forum. To repeat : you need an EGC from the array to the inverter for the GFCI to work. There's no point in risking fire damage to avoid lightning damage.

As I understood from BBs, there is a ground loop in my configuration because the first egc connection from the building ring and its continuity through the inverter to another connection to the ring thought the MDB busbar,

The ways i see to solve this are: (please correct me if im wrong)

- removing any bond inside the inverter between inverter dc busbar and inverters AC ground.



- Eliminating the first connection to the building ring. What i think would bring any possible high induced voltage inside the building


- Not bringing the EGC to the inverter (it will contradict what u say about GFCI) and connect it to the MDB ground busbar.
My point here is, if the array is connected to the ring, the impedance from dc wires to ground can be measured either from the EGC or from the MDB busbar that is connected to the ring aswell. Assuming this hypothesis is correct, the GFCI would work without bringing the EGC into the house (avoiding the ground loop) because if there is fault on the array it will also be detected with the MDB green wire. Is this right?



I know ive been posting for a long time about this issue, but forums and publications are the only resource i have to understand and be able to make future good configurations. Ill gldly pay u a diner if you guys come to thailand

In order to close this issue, which configurations would u choose in order to divert possible induced voltages inside the building, ground loops and at the same time allowing the GFCI to work correctly?

THANKS AGAIN

 

[Smart $]

In what way are ground loops undesirable on a power distribution system, with regard to lightning-induced voltages and currents??? You make it sound like we are talking about low-voltage communications circuits.

 

[josealjim]

In what way are ground loops undesirable on a power distribution system, with regard to lightning-induced voltages and currents??? You make it sound like we are talking about low-voltage communications circuits.

So should I avoid ground loops or not?. Im getting crazy with this :?

Thanks Smart for you reply

 

[Smart $]

So should I avoid ground loops or not?. Im getting crazy with this :?

Thanks Smart for you reply

IMO, no... at least not of the wire/metal type.

What you want to avoid are parallel pathways for normal current on grounding conductors.

 

[jaggedben]

The ways i see to solve this are: (please correct me if im wrong)

- removing any bond inside the inverter between inverter dc busbar and inverters AC ground.
View attachment 10937

I just need to say that you should not try to do this for any reason whatsoever, ground loop or otherwise. It would violate the manufacturer's warranty, and would probably be futile in any case since both are typically bonded to the inverter's metal chassis.

I agree with Smart$ that 'ground loops' on the grounding electrode system shouldn't be a concern.

 

[ggunn]

...

The ways i see to solve this are: (please correct me if im wrong)

- removing any bond inside the inverter between inverter dc busbar and inverters AC ground.
View attachment 10937 .

I will add my "don't do that" to the other responses. This will, in addition to what else has been said, defeat the GFI capability of the inverter and thereby create a significant fire hazard.

 

[josealjim]

Then, ill definitely go for the previous one:



Thanks everybody for your patience and support. I hope this thread helps someone else