Ungrounded Vs Grounded Inverters

[SolarPro]

Namely, do you have V from either pos. or neg. to ground, in an ungrounded PV system?

Don't both legs tend to have roughly the same potential (but opposite polarity) relative to ground?

Google isn't helping me find a source, but that's the way I've always heard it described by people who would know. The legs just naturally want to "float" on either side of the ground reference.

 

[GoldDigger]

Don't both legs tend to have roughly the same potential (but opposite polarity) relative to ground?

Google isn't helping me find a source, but that's the way I've always heard it described by people who would know. The legs just naturally want to "float" on either side of the ground reference.

Any leakage paths (only resistive, not also capacitive or inductive as with AC), however small, will tend to form a voltage divider. If the leakage paths in both legs are symmetrical, the leg voltages will tend to be balanced with respect to ground.
But this will be a phantom voltage only seen with a high impedance meter.
A low impedance meter will form its own leakage path and will tend to read close to zero on both sides when measured individually.
If you put in two meters, one from each leg to ground at the same time, the sum of the readings must equal the DC source voltage.

 

[Carultch]

Don't both legs tend to have roughly the same potential (but opposite polarity) relative to ground?

Google isn't helping me find a source, but that's the way I've always heard it described by people who would know. The legs just naturally want to "float" on either side of the ground reference.

In general, yes. But there is no guarantee that they do, especially given fault conditions.

 

[Zee]

Any leakage paths (only resistive, not also capacitive or inductive as with AC), however small, will tend to form a voltage divider. If the leakage paths in both legs are symmetrical, the leg voltages will tend to be balanced with respect to ground.
But this will be a phantom voltage only seen with a high impedance meter.

Can that phantom voltage hurt you?

 

[Zee]

Don't both legs tend to have roughly the same potential (but opposite polarity) relative to ground?

But why would pos+ or neg- have potential to gr.? I am genuinely curious, i am not sure.
After all, in a typical 120/240 VAC system we only have potential to gr. because gr. is bonded to one hot leg ( which becomes the neutral).
Whereas in an an ungr. PV system neither pos+ or neg- are bonded to gr.

Literally no electrons are taken from ground......they are taken from one polarity and added to the other.

Admittedly, my grasp of electrical theory is weaker than my practical knowledge.
It is these "leakage paths, the phantom V" i have always heard about and do not understand.....so I have apprehension over.....:ashamed1:

CAN I GRAB THE POS OR NEG AND TOUCH A RAIL?

 

[GoldDigger]

Can that phantom voltage hurt you?

You would probably feel it, but whether it could injure you depends on the magnitude of the leakage resistance.
The leakage resistances to ground are real, and could be measured with a Megger. The voltage in this case I'd called a phantom voltage just because the current is so limited.
Electrons are going from one leg to ground and from ground to the other leg. That is real current.

 

[Electric-Light]

I think each type have advantages of their own, but there are additional issues with ungrounded systems.

Don't both legs tend to have roughly the same potential (but opposite polarity) relative to ground?

Right, which means either side can short to ground.(or more precisely damaged wire can short to something solidly bonded such as building frame) Just like either leg will short to ground on 120-N-120.

Google isn't helping me find a source, but that's the way I've always heard it described by people who would know. The legs just naturally want to "float" on either side of the ground reference.

That's why you have to use an insulated transformer to make a proper 120/240 service from 208.

Any leakage paths (only resistive, not also capacitive or inductive as with AC), however small, will tend to form a voltage divider. If the leakage paths in both legs are symmetrical, the leg voltages will tend to be balanced with respect to ground.
But this will be a phantom voltage only seen with a high impedance meter.
A low impedance meter will form its own leakage path and will tend to read close to zero on both sides when measured individually.
If you put in two meters, one from each leg to ground at the same time, the sum of the readings must equal the DC source voltage.

Insulated ungrounded
insulated bonded and grounded
To bond the +, or - to grounding point or to leave them completely floating are choices available.

non-insulated AC side grounded.
If you bond either conductor on DC side to ground, it will short out the utility power through static converter and EGC

 

[ggunn]

Right, which means either side can short to ground.(or more precisely damaged wire can short to something solidly bonded such as building frame) Just like either leg will short to ground on 120-N-120.

Except that in a 120-N-120 system N is tied to ground so shorting either leg to ground is a hard short and lots of current flows. In ungrounded DC a short to ground just references the system to ground at that point, which is why a different mode of ground fault detection is necessary than in grounded systems.

 

[Zee]

You would probably feel it, but whether it could injure you depends on the magnitude of the leakage resistance.
The leakage resistances to ground are real, and could be measured with a Megger. The voltage in this case I'd called a phantom voltage just because the current is so limited.
Electrons are going from one leg to ground and from ground to the other leg. That is real current.

Thanks!
I would like to know numbers: Exactly how many amps?
Also you say "current is ....limited" then add "[t]hat is real current." Those statements seem contradictory.
I don't know the answer myself, wondering who does?

What is needed for heart muscle fibrillation? .oo5 amps or something??? Is this more or less ?

 

[Zee]

I think each type have advantages of their own, but there are additional issues with ungrounded systems.

OK, what are they?

 

[Electric-Light]

Except that in a 120-N-120 system N is tied to ground so shorting either leg to ground is a hard short and lots of current flows. In ungrounded DC a short to ground just references the system to ground at that point, which is why a different mode of ground fault detection is necessary than in grounded systems.

The distribution transformer is tied to the ground at neutral in most cases, so there's no galvanic isolation between the input and output sides of the inverter and any part of DC circuit can hard short to ground. It's misleading to call non-isolated inverters by anything else.

Current can flow between input and output without an isolation. Again, they don't always make it clear that transformerless means lack of electrical isolation.

Isolated means the two sides are electrically separated just like a motor and generator connected back to back.

Older homes with outlets without 3rd pin maybe called "ungrounded" but it's a grounded system unless the neutral is not attached to ground at the transformer.

OK, what are they?

Possibility of DC offset getting added beyond point of service entry.
Hard short if either conductor touches grounded object.
Some panels appear to require back side bonded to negative.

High resistance fault from any part of DC side will cause current to flow to ground, which triggers the GFCI.
Basically, transformerless non-isolated inverters skips isolation, then fall back on GFCI for high resistance fault, and fuses for hard fault.

 

[Engineer#4827502]

Seems to me there's a return path through the grounded AC conductor and the loads or transformer, as well as the other side of the inverter. But I don't know what really happens through the power electronics of the inverter.

What I am fairly certain of is that utility current does not fault to ground against the polarity of the PV array. The current has to flow through the inverter or PV panels or both. That's not a short circuit fault to ground.

Looking a bit further into it, I believe you are right, at least while the inverter is operating. Looking at a circuit diagram, a ground fault in the PV array could pass current through the AC neutral. If the inverter stops, it will isolate the array from the AC system which would cut the return path. The inverter will also not be able to operate if one of the PV poles becomes grounded as one of the poles would be at the same voltage as ground.

As far as current backfeeding from the AC side to the DC side, I've never heard of that happening and inverter manufacturers have told me as much. That diagram does not really support that as it could work as a rectifier through the flyback diodes. I imagine there is another diode on the DC side to prevent backfeed in a typical inverter.

 

[GoldDigger]

Thanks!
I would like to know numbers: Exactly how many amps?
Also you say "current is ....limited" then add "[t]hat is real current." Those statements seem contradictory.
I don't know the answer myself, wondering who does?

What is needed for heart muscle fibrillation? .oo5 amps or something??? Is this more or less ?

If the leakage is a typical insulation resistance of 5 megohms and the voltage is 500VDC, then leakage current on one side could be as much as .1ma.
Not very large. But real rather than theoretical current. It could only flow if there were a matching leakage on the other leg of ungrounded 1000VDC or there is a fault in the system.
The threshold for heart interference from DC is much higher than for 60Hz AC.
So something an order of magnitude or two higher than 6ma. I did not look it up, so that is just a SWAG.

 

[jaggedben]

Any leakage paths (only resistive, not also capacitive or inductive as with AC), however small, will tend to form a voltage divider. If the leakage paths in both legs are symmetrical, the leg voltages will tend to be balanced with respect to ground.
But this will be a phantom voltage only seen with a high impedance meter.
A low impedance meter will form its own leakage path and will tend to read close to zero on both sides when measured individually.
If you put in two meters, one from each leg to ground at the same time, the sum of the readings must equal the DC source voltage.

Not sure exactly what you're saying here, but that red word at least seems misleading to me. Surely you're not saying that the sum of both legs to ground is the same as from one lead to the other. Or else you're saying this is only with low impedance meters.

Or are you talking about when the inverter is operating, rather than when DC source is disconnected?

FWIW, the actual results I typically see when I test ungrounded (floated) arrays to ground with (I believe) a high impedance meter is a small voltage (<10 volts) that dissipates down to zero the longer I hold my meter to it. I've never really understood what's going on there, maybe you can explain. Sunpower modules show higher phantom voltages of this nature.

 

[jaggedben]

The distribution transformer is tied to the ground at neutral in most cases, so there's no galvanic isolation between the input and output sides of the inverter and any part of DC circuit can hard short to ground.

Except that it can't, and doesn't.

 

[jaggedben]

Right, which means either side can short to ground.(or more precisely damaged wire can short to something solidly bonded such as building frame)

That's not what SolarPro meant, and it's not the case with ungrounded DC. It would require a ground fault on both conductors to short circuit the DC source.

Just like either leg will short to ground on 120-N-120.

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.

 

[GoldDigger]

Not sure exactly what you're saying here, but that red word at least seems misleading to me. Surely you're not saying that the sum of both legs to ground is the same as from one lead to the other. Or else you're saying this is only with low impedance meters.

Or are you talking about when the inverter is operating, rather than when DC source is disconnected?

FWIW, the actual results I typically see when I test ungrounded (floated) arrays to ground with (I believe) a high impedance meter is a small voltage (<10 volts) that dissipates down to zero the longer I hold my meter to it. I've never really understood what's going on there, maybe you can explain. Sunpower modules show higher phantom voltages of this nature.

I am saying that if there are no leakage paths and the only possible current path from the DC wiring to ground is through the two meters, then the sum of the two DC readings to ground will be exactly the same as the line to line reading. Simple basic voltage divider.
If you add in additional leakage resistances, particularly unbalanced ones, then for a high impedance meter the sum of the two readings will still equal the line to line reading, even though the two line to ground readings may not be identical. If the meter impedance is low enough, swamping any leakage, then the two meter readings will be the same in magnitude.
Note carefully that I am NOT talking about two separate meter readings with the same meter moved from one side to the other. I am talking about using two identical (or different) meters connected simultaneously.

If you add in some hypothetical reference bias circuit that pumps the whole DC source up relative to ground by more than the source voltage itself, then you will have to be careful of the signs of the two voltages when adding them, but the result will still be the same.
(Depending on how you connect the meters, you might actually want to use the difference between the two signed readings rather than the sum, but the principle is still the same.)

The dissipating down to zero behavior you see with one meter implies that either there is a capacitance involved or a charge storage between the cell substrate and ground, of the kind that causes performance losses with some panels if the correct polarity of the string is not grounded.
I am not sure that type of panel can be used successfully in a floating DC system.

 

[jaggedben]

I am saying that if there are no leakage paths and the only possible current path from the DC wiring to ground is through the two meters, then the sum of the two DC readings to ground will be exactly the same as the line to line reading. Simple basic voltage divider.
If you add in additional leakage resistances, particularly unbalanced ones, then for a high impedance meter the sum of the two readings will still equal the line to line reading, even though the two line to ground readings may not be identical. If the meter impedance is low enough, swamping any leakage, then the two meter readings will be the same in magnitude.
Note carefully that I am NOT talking about two separate meter readings with the same meter moved from one side to the other. I am talking about using two identical (or different) meters connected simultaneously.

I get it now. I was being dense. :slaphead:

I'd be curious to try this with low-end (high impedance?) Fluke meters and see if the voltages are balanced or not. (Not sure when I'll have the chance though, since I usually have loan my meter to my crew, not the other way around. :roll

The dissipating down to zero behavior you see with one meter implies that either there is a capacitance involved or a charge storage between the cell substrate and ground, of the kind that causes performance losses with some panels if the correct polarity of the string is not grounded.

I believe I've read elsewhere that there's capacitance between the cells and module glass/frames.

I am not sure that type of panel can be used successfully in a floating DC system.

That's correct with regard to Sunpower modules, which is stressed in their installation manuals. They use a different technology with no solder lines on the front of the cell.

Potential Induced Degradation does happen in other modules but is not serious for most them.

 

[electrofelon]

The terms non-isolated vs isolated are better description than grounded vs not.

I agree. The NEC and industry really has to stop calling non isolated systems "ungrounded". Perhaps they should be called "already grounded"

Are/were there any transformer inverters that are ungrounded?

 

[jaggedben]

I agree. The NEC and industry really has to stop calling non isolated systems "ungrounded".

First, they describe different things. (see below)

I won't debate the merits of which one the NEC should refer to for having certain rules. That depends at least in part on the technical justification for those rules. 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.

Are/were there any transformer inverters that are ungrounded?

All of the currently produced models from Enphase fit that bill.