AC current when grounding AC and DC conductors in PV system? 3

[Blutooth]

Hi all,

I am reading a book called Photovoltac Systems (by James P. Dunlop, I recommend it to everyone trying to get a better understanding on the subject).

There is a part on grounding that I can't get my head around, I'v included a picture below that shows two possible lay-outs. The equipment grounding, DC grounding and AC grounding are all connected in the two lay-outs. I can't help but think this will mess up the AC and DC current flows as you basically connect your AC with your DC side. Will this not make an AC current flow between the two grounded current carrying conductors (AC and DC) as there will be a AC voltage difference between them?

Thanks in advance!

Blutooth

http://s16.postimg.org/qqznxftyt/IMG_20150728_124521.jpg

edit: I hope I'm not violating the Posting guidelines by saying it's a good book or by posting an image from it. Let me know if it's not OK.

 

[VEBill]

Having a common ground doesn't necessarily cause a current to flow from one side to the other. Such currents may flow during certain fault conditions, hopefully popping a circuit breaker.

The primary purpose of grounding is to ensure that the systems stay grounded (e.g. chassis) for human safety, as opposed to becoming a shock hazard relative to ground.

On the diagrams: One might point out that the 'AC Power Distribution Panel' chassis should itself be grounded, and the external grounding conductors should be connected inside to the grounding bus bar.

Also, having two grounding points can contribute to lightning damage.

Standby for others to chime in in case I have any details wrong.

 

[No1BEE]

Blutooh,

The depicted diagrams are layout of a grounded inverter system (vs. an ungrounded system). In such a system, the AC and DC sides are isolated from each other via a built-in isolation transformer. Therefore, you have a separately derived (inverter) system where the DC grounded conductor and equipment ground are bonded and get their own single connection to earth through the depicted grounding electrode as the AC side has its own grounded system. The two electrode systems are bonded together to avoid any potential rise between two systems due to any possible electrical failure modes and render both system safe by creating the same reference ground potential to anyone who may come in contact with the system. Remember that electrical failure modes can be due to lightning, downing of a transmission line on the system, or short circuit failures that exist in DC or AC mode. Furthermore, you should be aware that when there is DC side ground fault, the PV array would not stop generating the DC current, despite of the DC ground fault within the inverter opening.

 

[irmalcol]

I agree with VE1BLL. Under normal operating conditions, there should be no current in the grounding/bonding conductors. I would like to add that it is important to note that bonding and grounding are different and serve different purposes.

Bonding is the connection of all metallic (i.e. electrically conductive) non-current-carrying parts of a system. The intent of bonding is to provide a low impedance path from any point in the system back to the power source. This is very important for protection, since we want any fault to result in a large current to trip a breaker/fuse quickly.

Grounding is the electrical connection of bonded components to ground. The intent of grounding is to dissipate any charge on the bonded parts of a system to eliminate or reduce the shock hazard for anyone who may touch the metallic case of a piece of equipment (the solar inverter, for example) while standing on the ground.

I would also like to hear if anyone has anything else to add (or if I got anything wrong).

 

[Blutooth]

Thank you all for your replies, but I still don't get it. Are you saying it's Okay to electrically connect one of the wires of an AC system to one of the wires of a DC system? Because that sounds very counter intuitive and possibly dangerous to me, maybe it's just me...

 

[No1BEE]

Blutooh,

Only the ground electrodes of the two systems are connected together. The ground electrodes are in direct contact to earth, thus creating earth connection for both systems. There are normally no other current flowing through them into the ground, except for the systems leakage current, and the magnitude of these currents are normally very low to be of any concerns in grounded systems. The only time there is a substantial current flow (AC or DC) in the ground would be under a failure mode. And even then, the ground potential rise at the point of the fault in reference to ground in the AC side of the system is usually rapidly removed by the AC overcurrent device and on the DC side the bond between the grounded conductor and the earth is removed within the inverter via the DC ground fault circuit detector/interrupter converting the grounded DC system to an ungrounded DC system; however, the ground fault will remain in the DC system, until someone locates it and removes it.

 

[Blutooth]

Thanks for yoru help No1BEE,

I can see how the ground electrodes are connected and that this doesn't form a problem. Yet there is an electrical path (through the bonding conductor) between the grounded conductor on the DC side and the grounded conductor on the AC side (especially visible on the lower picture). It is this connection that I don't understand.

Am I not seeing something?

 

[IRstuff]

Again, grounds are NOT intended to carry operational current. They typically serve two purposes:

> as discussed above, to prevent shock hazards by having a high conductance path to ground that is more preferable for current to go through than a human body. These grounds are tied together to ensure that most combinations of shock faults will still be blocked by the grounds

> to provide a voltage measurement reference point, i.e., 10V DC is referenced to the ground, as is 0V AC. There are often circuits on the different systems that communicate with each other. If the circuit grounds are not tied together, power-on transients can rapidly damage transceivers like RS-422.

TTFN
faq731-376

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[No1BEE]

Blutooth,

To see the answer to your question, see if you can answer this simple question without looking at mine:

Is there a potential difference between the grounded DC and AC side of the system under normal operating condition?

And the answer is.... NO, sorry I couldn't wait.

So, why is that? If you look back at the previous posts it will come to you real fast .