Ungrounded Vs Grounded Inverters

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Electric-Light

Senior Member
You keep talking about this theoretical possibility that the AC source can short to ground through the DC conductors, while ignoring that the inverter controls effectively make that impossible.

Power semiconductors usually fail shorted out. When one of the wires are dropped to solidly grounded conductor, it causes a head-on collision as the thyristor or transistor switches on. High speed current limiting fuse is the only defense at this point. The fuses will prevent catastrophic fireworks show, but the static converter can still get destroyed.

He explained it right.
http://forums.mikeholt.com/showthread.php?t=171411&p=1671456#post1671456
 

Electric-Light

Senior Member
This should help you understand it a bit better.

Actual load is not a heating element. Picture you're pedaling a cement roller bicycle with the rear wheel coupled by gears. When you're pedaling, you're exporting power. But if your foot gets caught in the gears "a fault condition", then the roller will extrude you through the gears, because it is directly coupled. In other words, grid is "backflowing" into DG system or feeding the fault energy.

In a non-isolated inverter, a fault would cause energy to flow from the grid back into the point of fault.Since the grid is AC, it would initially start like a half wave diode put between hot and neutral. Fast acting current limiting will prevent the static converter from erupting into a giant ball of flame, but there's a fair chance that power components will fry.

I pulled the PV out of circuit just for shows. You can mentally put it back in, but you can see how it makes no difference in fault current. I put a star at the point of fault and the black arrow is the direction of fault energy flow. I jumpered one of the transistors to show how they normally fail. (shorted).

In an isolated system, the fault energy is limited by the size of isolation transformer. In a non-isolated system, the limit is the fault current of the utility's transformer.

l70KUTN.jpg
 

Electric-Light

Senior Member
Non-isolating and isolating each have their functional advantages and disadvantages.
Weight is not a significant advantage in most permanently installed stationary applications.

Solar marketing companies have significant interest in promoting non-isolated design to reduce hardware costs since it would increase the filler gap between retail price and manufacturing cost. As for the effect on final installed cost, it could be rather insignificant.

The largest expenses are soft cost such as parasitic losses of money such as markups in each layer of multi-leveled marketing structure.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The largest expenses are soft cost such as parasitic losses of money such as markups in each layer of multi-leveled marketing structure.
Yeah, yeah, we know; it's all a huge conspiracy.

The vast majority of PV systems that are installed in the territories of AHJ's I work in are installed by companies who buy directly from the equipment manufacturers or at worst from distributers who do. There is nothing "multi-lveveled" about it.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
This should help you understand it a bit better.

I don't believe I've had any trouble understanding what you're saying from the start. I just disagree that it practically applies to PV inverters.

Actual load is not a heating element. Picture you're pedaling a cement roller bicycle with the rear wheel coupled by gears. When you're pedaling, you're exporting power. But if your foot gets caught in the gears "a fault condition", then the roller will extrude you through the gears, because it is directly coupled. In other words, grid is "backflowing" into DG system or feeding the fault energy.

This, I think, is a horrible analogy, for many, many reasons. For example, gears have pretty much nothing in common with electrical circuits. Perhaps the most salient reason, though, is that if I were a PV inverter I would be pedaling in the opposite direction of the grid.

In a non-isolated inverter, a fault would cause energy to flow from the grid back into the point of fault.

Only if it overcame several physics and engineering constraints that would keep that from happening.

Since the grid is AC, it would initially start like a half wave diode put between hot and neutral. Fast acting current limiting will prevent the static converter from erupting into a giant ball of flame, but there's a fair chance that power components will fry.

Fast acting current control is how a PV inverter operates. So I think it has the ability to quickly limit fault current. Also, you keep referring to 'static converters', and a PV inverter, I think, is not one.

I pulled the PV out of circuit just for shows. You can mentally put it back in, but you can see how it makes no difference in fault current.

It makes all sorts of difference. The key difference is that the DC current is flowing the opposite way. Think about that. Draw it out if you need to. (I did). Physics and UL 1741 dictate that if the inverter can inject energy into the grid in the opposite polarity of the grid voltage, then a fault on one of the DC conductors must result in one of two things:
a) the inverter causes DC current to flow from the faulted conductor (i.e. on the EGC)
b) the inverter must stop operating, if it can no longer be correctly pushing against the grid voltage

If (a) then the fault current is DC on the EGC and flows the opposite direction (on the AC grounded conductor) of how the utility fault current would flow
If (b) then the inverter has shut down, meaning the DC conductors are now isolated, meaning AC fault current can't flow. (See response to previous quote in this post.)

I actually have no idea which of these will prevail; it could be either one, I suppose, depending on a lot of other factors. What I can see is that neither scenario involves AC fault current through the DC conductor. And if it's (a), then the GFDI is supposed to detect it and shut down the inverter.

Solar marketing companies have significant interest in promoting non-isolated design to reduce hardware costs since it would increase the filler gap between retail price and manufacturing cost. As for the effect on final installed cost, it could be rather insignificant.

The largest expenses are soft cost such as parasitic losses of money such as markups in each layer of multi-leveled marketing structure.

Actually non-isolated inverters have brought along significant end-user cost reductions. And 'soft costs' in the PV industry is everything that isn't hardware. That post really shows you are not familiar with solar industry economics.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
In an isolated system, the fault energy is limited by the size of isolation transformer. In a non-isolated system, the limit is the fault current of the utility's transformer.

And it's also limited by the AC OCPD, of course. And since the inverter can continuously pass about that much current while operating, it seems reasonable it could withstand the fault current if that OCPD is sized correctly, like a lot of other equipment. Nobody (before you) has ever told me I need to worry about PV inverters being damaged by DC faults.
 

Electric-Light

Senior Member
This, I think, is a horrible analogy, for many, many reasons. For example, gears have pretty much nothing in common with electrical circuits. Perhaps the most salient reason, though, is that if I were a PV inverter I would be pedaling in the opposite direction of the grid. Only if it overcame several physics and engineering constraints that would keep that from happening.
Pedal forward and you're delivering power to the roller. The huge momentum of the roller will however shred you if you get your foot caught in the gear.

Fast acting current control is how a PV inverter operates. So I think it has the ability to quickly limit fault current. Also, you keep referring to 'static converters', and a PV inverter, I think, is not one.
Are you going to tell me that the PV drives a motor that drives an alternator? :roll:


It makes all sorts of difference. The key difference is that the DC current is flowing the opposite way. Think about that. Draw it out if you need to. (I did). Physics and UL 1741 dictate that if the inverter can inject energy into the grid in the opposite polarity of the grid voltage, then a fault on one of the DC conductors must result in one of two things:
a) the inverter causes DC current to flow from the faulted conductor (i.e. on the EGC)
b) the inverter must stop operating, if it can no longer be correctly pushing against the grid voltage

If (a) then the fault current is DC on the EGC and flows the opposite direction (on the AC grounded conductor) of how the utility fault current would flow
If (b) then the inverter has shut down, meaning the DC conductors are now isolated, meaning AC fault current can't flow. (See response to previous quote in this post.)

I actually have no idea which of these will prevail; it could be either one, I suppose, depending on a lot of other factors. What I can see is that neither scenario involves AC fault current through the DC conductor. And if it's (a), then the GFDI is supposed to detect it and shut down the inverter.
You don't understand or see the diagram we're talking about. Maybe you're viewing from mobile. Do you see the diagram?
 
But I think it's absolutely correct for the NEC to refer to systems which have no grounded DC conductors as 'ungrounded'. They are no more grounded than the ungrounded conductors of an AC system.

I think it is confusion over CONDUCTORS vs SYSTEMS. In the context of system grounding, we use the term grounded/ungrounded to refer to whether the SYSTEM is grounded and it makes no difference if the grounded conductor is used. Yes a TL inverter has no grounded DC conductors, but that is not what the grounding is referring to. So I think the terminology should be consistent with the rest of the NEC and TL inverters should not be called "ungrounded"


The key difference is that the DC current is flowing the opposite way. Think about that. Draw it out if you need to. (I did). Physics and UL 1741 dictate that if the inverter can inject energy into the grid in the opposite polarity of the grid voltage, then a fault on one of the DC conductors must result in one of two things:
a) the inverter causes DC current to flow from the faulted conductor (i.e. on the EGC)
b) the inverter must stop operating, if it can no longer be correctly pushing against the grid voltage

I do agree with EL that in theory there could be a fault through the inverter. It makes no difference which way energy is flowing at the moment the fault occurs. Think about jumping two cars. IT makes no difference if the dead car is cranking and drawing 100 amps from the other - you would still have a fault if (while jumping) you connected the positive of the dead car to the frame of the good car, its a path back to the source. That being said, I think: A) the inverter would sense that very quickly and shut down B) it wouldnt be a "hard" fault as I suspect there would be a fair amount of impedance in the inverter depending on the state of charge of the inductors and the "position" of the transistors, etc C) I suspect inverters are built "tough" enough so that a AC/DC fault through the inverter wont damage the inverter.


The largest expenses are soft cost such as parasitic losses of money such as markups in each layer of multi-leveled marketing structure.

No, modules are the single largest cost. In my experience, equipment is drop shipped from the manufacturer and has not gone through a bunch of middle men
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I do agree with EL that in theory there could be a fault through the inverter. It makes no difference which way energy is flowing at the moment the fault occurs. Think about jumping two cars. It makes no difference if the dead car is cranking and drawing 100 amps from the other - you would still have a fault if (while jumping) you connected the positive of the dead car to the frame of the good car, its a path back to the source.
A significant difference is that in your example both cars' DC systems are grounded to the frames of the cars. The negative conductors are grounded conductors; you can jump a car with one wire (or two jack handles - I have done it) if they have metal bumpers and you put them in contact with each other.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Pedal forward and you're delivering power to the roller. The huge momentum of the roller will however shred you if you get your foot caught in the gear.

Except that I'm already counteracting the huge momentum of the roller, making it spin slower in the direction it normally spins. The momentum is precisely what I'm engineered to overcome. Also, gears are a hopelessly inaccurate analogy here.

Are you going to tell me that the PV drives a motor that drives an alternator? :roll:

That's a horrendous strawman.

If you look up 'static converter' you find references to things like phase converters that operate off AC power. A PV inverter is not one of those. It does not convert AC power. You're diagrams are all accurate for devices that convert AC power, and not accurate for PV inverters.

You don't understand or see the diagram we're talking about. Maybe you're viewing from mobile. Do you see the diagram?

I already told you that I did, and I do. It's a horribly messy diagram, but I still understand it. And no, I'm not doing this from a mobile.

Do you understand physics? Like, for example, current can't travel in two directions on the same conductor at the same time?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I think it is confusion over CONDUCTORS vs SYSTEMS. In the context of system grounding, we use the term grounded/ungrounded to refer to whether the SYSTEM is grounded and it makes no difference if the grounded conductor is used. Yes a TL inverter has no grounded DC conductors, but that is not what the grounding is referring to. So I think the terminology should be consistent with the rest of the NEC and TL inverters should not be called "ungrounded"

Right now, DC conductors is 'what the grounding is referring to'. The DC system is regarded as a separate system for grounding

I understand what you're getting at, and maybe the code should be clarified. But again, but from a code-making point of view I still think what matters is that the rules are technically justified. If what actually matters is whether there's an intentionally grounded DC conductor, then we should just keep the terminology, and define it somewhere such that it applies to DC conductors regardless of the nature of the AC side. If what actually matters is whether the inverter is isolated, then you're right, the language should be changed.

FWIW, I don't think it will change because this stuff comes (I believe) from UL standards, which are based whether the DC side is grounded, not whether the inverter is isolated.


I do agree with EL that in theory there could be a fault through the inverter. It makes no difference which way energy is flowing at the moment the fault occurs.

Again, it makes all sorts of difference. If you understand how it's possible for the inverter to backfeed the grid at all, then you have to ask yourself why that should get overpowered and stopped. In order for AC power to backfeed through to the DC side and fault to ground, the DC current has to stop flowing. Because when you draw it out, they are going opposite directions, and current can't flow in two directions at the same time.

I'm pretty sure that in the real world the inverter detects something is wrong and shuts down faster than the AC breaker can trip.
 

Electric-Light

Senior Member
Right now, DC conductors is 'what the grounding is referring to'. The DC system is regarded as a separate system for grounding

I understand what you're getting at, and maybe the code should be clarified. But again, but from a code-making point of view I still think what matters is that the rules are technically justified. If what actually matters is whether there's an intentionally grounded DC conductor, then we should just keep the terminology, and define it somewhere such that it applies to DC conductors regardless of the nature of the AC side. If what actually matters is whether the inverter is isolated, then you're right, the language should be changed.

FWIW, I don't think it will change because this stuff comes (I believe) from UL standards, which are based whether the DC side is grounded, not whether the inverter is isolated.




Again, it makes all sorts of difference. If you understand how it's possible for the inverter to backfeed the grid at all, then you have to ask yourself why that should get overpowered and stopped. In order for AC power to backfeed through to the DC side and fault to ground, the DC current has to stop flowing. Because when you draw it out, they are going opposite directions, and current can't flow in two directions at the same time.

I'm pretty sure that in the real world the inverter detects something is wrong and shuts down faster than the AC breaker can trip.

This is what I mean by unisolated, grounded that solar marketing and sales industry calls "ungrounded". It's simplified, but the energy can transfer from left to right or if you apply forward torque to the shaft, it will from from right to left and the "DC system" located on the ungrounded line supplies the light bulb from the battery independent of what the AC system is doing, but if any part in the red cirlcle comes in contact with grounded conductor, it goes kapoof with whatever fault current available in the 480v system, not what the D cell can provide.
 

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
This is what I mean by unisolated, grounded that solar marketing and sales industry calls "ungrounded". It's simplified, but the energy can transfer from left to right or if you apply forward torque to the shaft, it will from from right to left and the "DC system" located on the ungrounded line supplies the light bulb from the battery independent of what the AC system is doing, but if any part in the red cirlcle comes in contact with grounded conductor, it goes kapoof with whatever fault current available in the 480v system, not what the D cell can provide.
This is such BS.

In the first place, Article 690.35 in the NEC explicitly refers to "Ungrounded Photovoltaic Power Systems", but I guess the authors of the NEC are in on the deception being foisted upon the world by the Evil Solar Multilevel Marketing Conspiracy (ESMMC).

In the second place your little drawing bears no resemblance whatsoever to a PV system of any type.

In the third place, Underwriters Laboratories have put their reputation on the line by listing inverters with ungrounded DC circuits; wouldn't a rational person think if the faults you claim inverters of this type are so vulnerable to are a real risk that the UL might balk at listing them? But I guess the UL are in the pocket of the ESMMC as well.

In the fourth place, the NEC stipulates that in the event of a ground fault on the DC side the inverter must interrupt the fault current (690.5(A)). Even in the event of a fault nothing "goes kapoof".

In the fifth place, there are thousands (millions?) of installed PV systems with ungrounded DC circuits; can you document a single case of a fire or an injury attributable to the kind of failure you claim is possible? But I guess they are all being covered up by the ESMMC.

That darned ESMMC has got its tendrils in everything, doesn't it?

Seriously, we get it. You hate PV.
 
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A significant difference is that in your example both cars' DC systems are grounded to the frames of the cars. The negative conductors are grounded conductors; you can jump a car with one wire (or two jack handles - I have done it) if they have metal bumpers and you put them in contact with each other.

I dont see that that makes any difference in the point I was trying to make. Current already flowing nor energy direction does not make current suddenly not follow a path back to its source. Yes the current on the common paths of the flow and fault will add or subtract but the fault will still be fed.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I dont see that that makes any difference in the point I was trying to make. Current already flowing nor energy direction does not make current suddenly not follow a path back to its source. Yes the current on the common paths of the flow and fault will add or subtract but the fault will still be fed.
Only that if the DC systems of the cars were not referenced to the frames then contact of either wire to the body of either car would not provide a path for fault current.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I dont see that that makes any difference in the point I was trying to make. Current already flowing nor energy direction does not make current suddenly not follow a path back to its source. Yes the current on the common paths of the flow and fault will add or subtract but the fault will still be fed.

With respect to a PV inverter, you've still got to consider the fact that the device that is closing the circuit is a device that only closes the circuit if all the voltage and frequency parameters are checking out every handful of cycles. So yes, the current will subtract, but that physically entails the voltage going out of whack, and pretty much as soon as that happens the circuit will be opened and the fault cleared. And the GFDI and anti-islanding shutdown requirements for the devices are really fast, faster than max trip times for thermal magnetic breakers, as far as I can tell.

If you have a fault that subtracts less than the inverter output current, it will get caught by the GFDI; if the fault occured while the inverter wasn't operating (at night), then the inverter won't even turn on and there will never be an AC fault. If you have an AC fault that is larger than the inverter is rated for, it would pretty much have to be from some freak occurrence causing sudden major damage to the wiring while it was operating. (Say, a tree falling on the array). A blown inverter is likely to be the least concern.

Finally: Even if Electric-Light is completely right about the possiblity of a non-isolated inverter getting destroyed by utility AC flowing through it, that is a safer, less costly outcome than an isolated PV system setting a building on fire because the first fault in the grounded conductor was never detected. The latter has been documented publicly. The former, to my knowledge, has not.
 

Electric-Light

Senior Member
This is such BS

Seriously, we get it. You hate PV.

What I said here is common to all types of non-isolated static converters including grid tie solar inverters. Solar photovoltaic systems may defy gravity, but they defy what I said here.

Transformerless, isolationless , galvanically conducting inverters rely on fast, current limiting fuses to kill direct faults and GFCI to kill high resistance faults.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What I said here is common to all types of non-isolated static converters including grid tie solar inverters. Solar photovoltaic systems may defy gravity, but they defy what I said here.

Transformerless, isolationless , galvanically conducting inverters rely on fast, current limiting fuses to kill direct faults and GFCI to kill high resistance faults.

Defy gravity? Hyperbole.
 
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