electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
This is,a small rural county public safety building. The supply is 13. 2 kv. I am,suprised they take,service at MV
zig zag transformer
- Thread starter electrofelon
- Start date
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This is,a small rural county public safety building. The supply is 13. 2 kv. I am,suprised they take,service at MV
wbdvt
Senior Member
- Location
- Rutland, VT, USA
- Occupation
- Electrical Engineer, PE
ok, I am surprised also but can't see the rest of the dwg that shows more of the NG side of things. That may shed some light but all this is off topic from OP's original question. More of curiosity now.
- Location
- Connecticut
- Occupation
- Engineer
The zig-zag transformer is used to reduce current flowing in the neutral due to harmonics.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
David,
Are harmonics an issue with grid interactive inverters? I have never had a zig zag spec'ed on a PV system before.
pcanning87
Member
- Location
- New York
Effective Grounding
Effective Grounding
National Grid generally requires any PV plant >500kW, and sometimes smaller ones, to have what they call "effective grounding". This can take the form of either a grounding transformer/Zig-Zag transformer or a Neutral Ground Reactor. I believe both would fall under the general category of impedance grounding. I generally see Zig-Zag transformers used with Wye-G:Wye-G and NGRs used with Wye-G
elta, although I'm not sure of the exact reasons behind that.
The best way I understand it is the utility is concerned with the impact of the PV system during a nearby ground fault on the transmission system. With a solidly grounded Wye-Wye system the fault current has a return path through the PV system secondary, which could desensitize the utility ground fault detection and cause high fault currents to flow. It makes sense to me that adding impedance between the PV system neutral and ground could reduce these fault currents.
I believe there are also concerns with temporary overvoltage under some of these fault scenarios, which NGRs/Zig-Zags can mitigate. I'm less clear on how this works.
I admit I don't truly understand some of the fundamentals here, this is just what I've picked up in dealing with NGRID. It also often appears that the utilities themselves don't understand what they mean by "effective grounding". We just suck it up and buy a zig-zag to make them happy.
Solectria has published some good documentation on this issue that may be helpful.
Effective Grounding
National Grid generally requires any PV plant >500kW, and sometimes smaller ones, to have what they call "effective grounding". This can take the form of either a grounding transformer/Zig-Zag transformer or a Neutral Ground Reactor. I believe both would fall under the general category of impedance grounding. I generally see Zig-Zag transformers used with Wye-G:Wye-G and NGRs used with Wye-G
elta, although I'm not sure of the exact reasons behind that.The best way I understand it is the utility is concerned with the impact of the PV system during a nearby ground fault on the transmission system. With a solidly grounded Wye-Wye system the fault current has a return path through the PV system secondary, which could desensitize the utility ground fault detection and cause high fault currents to flow. It makes sense to me that adding impedance between the PV system neutral and ground could reduce these fault currents.
I believe there are also concerns with temporary overvoltage under some of these fault scenarios, which NGRs/Zig-Zags can mitigate. I'm less clear on how this works.
I admit I don't truly understand some of the fundamentals here, this is just what I've picked up in dealing with NGRID. It also often appears that the utilities themselves don't understand what they mean by "effective grounding". We just suck it up and buy a zig-zag to make them happy.
Solectria has published some good documentation on this issue that may be helpful.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
National Grid generally requires any PV plant >500kW, and sometimes smaller ones, to have what they call "effective grounding". This can take the form of either a grounding transformer/Zig-Zag transformer or a Neutral Ground Reactor. I believe both would fall under the general category of impedance grounding. I generally see Zig-Zag transformers used with Wye-G:Wye-G and NGRs used with Wye-G
The best way I understand it is the utility is concerned with the impact of the PV system during a nearby ground fault on the transmission system. With a solidly grounded Wye-Wye system the fault current has a return path through the PV system secondary, which could desensitize the utility ground fault detection and cause high fault currents to flow. It makes sense to me that adding impedance between the PV system neutral and ground could reduce these fault currents.
I believe there are also concerns with temporary overvoltage under some of these fault scenarios, which NGRs/Zig-Zags can mitigate. I'm less clear on how this works.
I admit I don't truly understand some of the fundamentals here, this is just what I've picked up in dealing with NGRID. It also often appears that the utilities themselves don't understand what they mean by "effective grounding". We just suck it up and buy a zig-zag to make them happy.
Solectria has published some good documentation on this issue that may be helpful.
Thank you. That sheds some light on where the requirement came from. I was more leaning toward that it was the designer's decision not a utility requirement because I have worked on a dozen other large systems in NGRID territory, but they were all just under 500KW, so these requirements likely didnt kick in.
So there is a chance this is a silly requirement accomplishing little/nothing, however I am also willing to accept that perhaps this is just above my knowledge level
FWIW, I have worked on a number of ther 1-2 meg systems in other utilities' territory, and the interconnect was solely throught a Wye:Wye transformer.Thanks for the solectria reference, I will look into it.
epimetheus
Member
1200A is the smallest practical size switchgear breaker available. Not much market for a lower ampacity breaker. Most of the time with MV switchgear you're going to have a programmable relay anyway. Just set it as needed and take advantage of mass produced 1200A breakers. Actually, I'm not aware of a switchgear manufacturer that offers a MV breaker less than 1200A.
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National Grid generally requires any PV plant >500kW, and sometimes smaller ones, to have what they call "effective grounding". This can take the form of either a grounding transformer/Zig-Zag transformer or a Neutral Ground Reactor. I believe both would fall under the general category of impedance grounding. I generally see Zig-Zag transformers used with Wye-G:Wye-G and NGRs used with Wye-G
The best way I understand it is the utility is concerned with the impact of the PV system during a nearby ground fault on the transmission system. With a solidly grounded Wye-Wye system the fault current has a return path through the PV system secondary, which could desensitize the utility ground fault detection and cause high fault currents to flow. It makes sense to me that adding impedance between the PV system neutral and ground could reduce these fault currents.
I believe there are also concerns with temporary overvoltage under some of these fault scenarios, which NGRs/Zig-Zags can mitigate. I'm less clear on how this works.
I admit I don't truly understand some of the fundamentals here, this is just what I've picked up in dealing with NGRID. It also often appears that the utilities themselves don't understand what they mean by "effective grounding". We just suck it up and buy a zig-zag to make them happy.
Solectria has published some good documentation on this issue that may be helpful.
Exactly correct here. For even more interesting information on the subject you can read IEEE 142 the Green Book on grounding and IEEE 1547 interconnection requirements for PV systems. Effective grounding is a method used for decades on rotating generators, it's questionable that it does anything for static inverters used in PV systems but utilities are used to requiring them on generators so they thought, "why not just keep going with that."
Effective grounding usually calls for the neutral to be grounded through an impedance and not solidly, so that's a little unusual here. I would not be surprised if this effective grounding system was not improperly designed for the application.
Effective grounding is supposed to limit ground fault overvoltage and load rejection overvoltage after the local substation isolates a distribution line with PV attached and the PV is slow to disconnect. The alternative is direct transfer trip where the local substation directly commands the PV systems to disconnect at the same time it isolates the line. That is usually much more expensive so people were happy to have the effective grounding option.
The remote trip on the zig-zag breaker is to make sure the PV system is disconnected from the distribution system if it has a fault or if it's intentionally disconnected, but usually that only trips the PV system interconnection and not the whole site.
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electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Ok this is starting to make a littler more sense. It sounds like the transformer was supposed to be delta on the PV side and the system grounding comes from the zig zag. The designer misunderstood the intent of the utility requirement. Do others agree with this hypothesis?
If the inverter is star connected with a neutral, the phase voltages could become unbalanced, should connection to the ground of the neutral is lost. To avoid it happening, the zigzag transformer is provided as an additional neutral grounding measure.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I dont see that being likely. You have the inverter's anti island and voltage parameters. It will shut down very quickly if things run too far amuck.
Remember ungrounded power systems. While line voltages remain balanced, phase to ground voltage would vary. So that can happen between phase and neutral when the latter is not grounded.
kwired
Electron manager
- Location
- NE Nebraska
Shouldn't phase to neutral voltage remain fairly steady as well? I can understand phase to ground and even neutral to ground voltages varying if there is no ground reference in the system.
That phase voltages become unbalanced when connection of neutral to ground is lost is the experience of public distribution utility engineers.
kwired
Electron manager
- Location
- NE Nebraska
Open neutral will create voltage imbalances that vary depending on connected load, but loss of the ground connection simply leaves the system ungrounded and variances in voltage to ground from phase conductors as well as neutral conductor, but phase to neutral voltages will remain unchanged - you haven't changed the source, just what point is referenced to ground. You could lift the ground from the neutral and move it to a phase and the system voltages still remain the same just the ground reference changes.That phase voltages become unbalanced when connection of neutral to ground is lost is the experience of public distribution utility engineers.
You are leaving out the capacitive currents flowing in each phase with ground acting as other conductor which may become unequal and cause different voltage drops in each phase unless there is a common grounding point in which case the phase to ground capacitance is shorted out and phase voltages become balanced
kwired
Electron manager
- Location
- NE Nebraska
What I was likely leaving out is how that may be different with a solid state supply vs a core and coil supply, which I guess is sort of the issue with OP.
Yes. The neutral may be solidly grounded. There appears to be no need for a zigzag transformer.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Yes, but as electrofelon stated, in this case the source will simply shut down if the phase voltages are too out of balance. It is probably the case that for this reason the zig-zag is not necessary; the requirement is probably based on a need that arises only with a different type of generator. I say 'probably' because "I am also willing to accept that perhaps this is just above my knowledge level."
If it is one time phase unbalance, the system may shut down to pass the unbalance condition. What if it persists which is usually the case? In such cases, neutral grounding or a zigzag transformer is required.
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