Does this choice have any impact on high energy fault damage?

[Electric-Light]

Does anyone have any idea if the choice of connection via A(uninsulated) or B(10kV basic insul level) would make a significant difference in the likelihood or the extent of damage that would be expected to happen to those inside the blue circle from surge traveling back through the feeder if something like this were to happen?

 

[mbrooke]

Can you give more detail/info on the scenario? Where is the 7.2kv conductor landing exactly?

 

[mivey]

An isolator for what? The path is fried through the ground connection at least. I'm not seeing much difference unless you give more detail.

 

[Electric-Light]

Car hits a pole and smashes a live 7.2kV conductor on a solar panel, which will seek to find a return path to the substation.
Ordinarily, hot side conductor is not so readily exposed to contact by another electrical source, but solar panels with large surface area has much greater chance of aerial wire eventually landing on it.

Common mode current cannot flow through line conductors if there is an isolation transformer blocking common mode current path from the solar panels. If there is no galvanic isolation between the solar system and the point of common coupling as in the case of unisolated inverters, it could easily share relatively low impedance path through the PCC, through neighboring homes' stuff then to ground unless the 7.2kV had first struck a solidly grounded item first.

 

[mivey]

Unless you maintain the insulation all the way through, the 7.2 is going to find a path. It could even be on the roof when it arcs from the panel wiring to roof flashing or from one panel to another to a bracket, wiring, etc.

You have to have the HV insulation all the way through including panels, brackets, conduit, screws, or anything that would make a path.

Supposing it only lands on your panel, if it can find a low insulation path, it may still run through some of the PV equipement in its path to the LV stuff even if it does not go through the isolation transformer.

The way the HV stuff works is: insulate and use equipement rated up to a certain surge voltage and time and shunt to ground (arrestor) to operate protective equipement before the high voltage causes damage.

So you need to insulate to HV level and provide a great ground on the insulated stuff that will draw enough current to operate the protective equipement. If the insulated stuff can't handle the shunting voltage level or live long enough for the protection to operate then it is damaged.

PS: The ground is a path back to the source. On HV, that may even include Earth. The idea is for the connections on either side of your shunt to surround the protected equipment and to keep the shunt voltage, including lead voltage drop, below what your equipement can handle (with a safety factor thrown in, say at least 1.5-2X).

 

[Electric-Light]

Yeah I would expect the solar asset and their property to get screwed either way. I was curious about the effectiveness of isolation transformer at the solar asset in reducing others on same PCC from getting sacrificed. I see the low voltage isolation transformer has an insulation rating of 10kV BIL between windings.
So basically I wouldn't want my place being at greater risk from feeder feedback damage because of a neighbor's yard and roof full of solar panels.

One guess is that isolation transformer shuts out this path and current will seek other than the feeder and raise the impedance thus reduce fault energy somewhat and let the house hit by the pole sustain damage

My guess with the use of non-isolated inverter is that fault would punch through inverter transitors like nothing and a substantial portion of such fault might go through PCC, then seek ground through neighbor's feeders to their gas pipes, through appliances etc.

 

[jaggedben]

It seems to me you're making way too much of the inverter type. What you're positing is a high voltage fault to equipment with grounded frames. The fault should find it's way to the grounded AC service conductor regardless of inverter type. It's possible it won't even flow in meaningful amounts on the circuit conductors from the roof.

 

[mbrooke]

Car hits a pole and smashes a live 7.2kV conductor on a solar panel, which will seek to find a return path to the substation.
Ordinarily, hot side conductor is not so readily exposed to contact by another electrical source, but solar panels with large surface area has much greater chance of aerial wire eventually landing on it.

Common mode current cannot flow through line conductors if there is an isolation transformer blocking common mode current path from the solar panels. If there is no galvanic isolation between the solar system and the point of common coupling as in the case of unisolated inverters, it could easily share relatively low impedance path through the PCC, through neighboring homes' stuff then to ground unless the 7.2kV had first struck a solidly grounded item first.

Those solar panel would be grounded, which in turn would be connected to the building's grounding system and in turn the neutral in the service drop and ultimately the multi grounded neutral which would complete the path back to the substation.

 

[mbrooke]

Yeah I would expect the solar asset and their property to get screwed either way. I was curious about the effectiveness of isolation transformer at the solar asset in reducing others on same PCC from getting sacrificed. I see the low voltage isolation transformer has an insulation rating of 10kV BIL between windings.
So basically I wouldn't want my place being at greater risk from feeder feedback damage because of a neighbor's yard and roof full of solar panels.

One guess is that isolation transformer shuts out this path and current will seek other than the feeder and raise the impedance thus reduce fault energy somewhat and let the house hit by the pole sustain damage

My guess with the use of non-isolated inverter is that fault would punch through inverter transitors like nothing and a substantial portion of such fault might go through PCC, then seek ground through neighbor's feeders to their gas pipes, through appliances etc.

The POCO's MGN plays a major role in that which I think you are leaving out.

 

[mivey]

The weak path is the ground to POCO neutral. The POCO transformer provides isolation from the 7.2 kV traveling on the service phases.

 

[mivey]

It seems to me you're making way too much of the inverter type. What you're positing is a high voltage fault to equipment with grounded frames. The fault should find it's way to the grounded AC service conductor regardless of inverter type. It's possible it won't even flow in meaningful amounts on the circuit conductors from the roof.

Agreed. A good ground path back to POCO neutral would be a good thing.

Add: A good shunt to ground for the ungrounded stuff, or at least a means to establish a good fault path.

 

[Electric-Light]

Agreed. A good ground path back to POCO neutral would be a good thing.

Add: A good shunt to ground for the ungrounded stuff, or at least a means to establish a good fault path.

Well, I don't know what type of ground they (PoCo) use at what part. (HRG, LRG, or solid)
So basically the only answer I am curious about is if the isolation affects errant current magnitude/presence within the blue boundaries

 

[GoldDigger]

The frames and racking of the panels will be solidly grounded and connected to EGC in either case, so the direct impact of distribution level voltage to the panel circuit will be minimal.
I would also expect the front glass of the panel to provide very effective insulation.
If we still assume high current connection from 7.2kV to DC + and -, I would expect that the semiconductor parts on the panel side of the transformer will break down to ground, removing any significant difference between the two cases.
The only difference I see is that for the transformerless the fault current will be distributed over the Ls and the neutral and the EGC instead of being confined to the EGC only.

mobile

 

[Ingenieur]

The iso xfmr would minimize damage to the convertors but increase likelyhood to the solar panels

 

[Electric-Light]

The iso xfmr would minimize damage to the convertors but increase likelyhood to the solar panels

is there any chance of over-voltage appearing on low voltage side? (i.e. other people sharing the point of common coupling) due to HV incident at the solar property?

 

[mbrooke]

is there any chance of over-voltage appearing on low voltage side? (i.e. other people sharing the point of common coupling) due to HV incident at the solar property?

There will be over voltage to anyone who is sharing the pole pig assuming 7.2kv to any hot conductor. Though due to the relatively low impedance of the low voltage 120/240 winding this should trigger an OCPD on the 7.2kv side.

 

[Electric-Light]

There will be over voltage to anyone who is sharing the pole pig assuming 7.2kv to any hot conductor. Though due to the relatively low impedance of the low voltage 120/240 winding this should trigger an OCPD on the 7.2kv side.

That's the main concern. The risk of that happening is more than negligible when the panels occupy a large surface area in vicinity of aerial in the trajectory of downed lines. Rural areas with even bigger panel area and farm equipment related aerial line incident is possible, its an additional risk. PoCo's circuit breakers are quite often auto resetting so it might power back on a few times.

Can it make sense for them to require insurance endorsement to cover losses to other people's stuff and utility equipment in order to hook up solar without isolation for damage coming from solar asset that would have been avoided by a 10kV BIL isolation transformer?

Someone much smarter and experienced and I in risk science can decide if it's a worthy risk or if its along the line of same risk as getting abducted by flying monster from the outer space.

 

[mbrooke]

That's the main concern. The risk of that happening is more than negligible when the panels occupy a large surface area in vicinity of aerial in the trajectory of downed lines. Rural areas with even bigger panel area and farm equipment related aerial line incident is possible, its an additional risk. PoCo's circuit breakers are quite often auto resetting so it might power back on a few times.

Can it make sense for them to require insurance endorsement to cover losses to other people's stuff and utility equipment in order to hook up solar without isolation for damage coming from solar asset that would have been avoided by a 10kV BIL isolation transformer?

Someone much smarter and experienced and I in risk science can decide if it's a worthy risk or if its along the line of same risk as getting abducted by flying monster from the outer space.

I agree with you- and now that I think about it you might be on to something. However, keep in mind that the 7.2kv is AC, so it will technically pass through the transformer if 7.2kv goes in one side and out the other into ground.

 

[Electric-Light]

I agree with you- and now that I think about it you might be on to something. However, keep in mind that the 7.2kv is AC, so it will technically pass through the transformer if 7.2kv goes in one side and out the other into ground.

It would have to transfer magnetically across the transformer and my guess is that fault energy magnetically coupled through a one digit kVA home solar transformer would be magnitudes less than a direct contact with a primary. A knocked over pole can rip away from the low side and leave the solar house and neighbors connected together like a powerstrip with the source plug pulled.

It is possible for it to hit the low side cabling directly but that's the difference between making it in the hoop vs landing the ball anywhere on the court.

 

[mivey]

is there any chance of over-voltage appearing on low voltage side? (i.e. other people sharing the point of common coupling) due to HV incident at the solar property?

Yes. Your diagram is incomplete.

The diagram illustrates infinite impedance from the contact point to L1L2 except through the isolation xfmr. In reality, due to the high voltage, there are other non-zero impedance paths such as I described in my earlier post.