Wye to wye questions

[synchro]

If I'm understanding Option A correctly, the return path for a 600V phase to EGC fault would be through the EGC to 15KV switchboard, then through the concentric of MV cable to X0. If that's true, will the concentric and even the 15KV switchboard adequately handle the fault current available from the 600V secondary? I'm thinking probably no, but I don't know the details of the installation you're proposing.

Also if the secondary fault current is enough to completely open the concentric conductor path over to X0, then H0/X0 will be left floating. In this case unless all three MV fuses have blown there could be high voltages on X0.

 

[electrofelon]

If I'm understanding Option A correctly, the return path for a 600V phase to EGC fault would be through the EGC to 15KV switchboard, then through the concentric of MV cable to X0. If that's true, will the concentric and even the 15KV switchboard adequately handle the fault current available from the 600V secondary? I'm thinking probably no, but I don't know the details of the installation you're proposing.

Also if the secondary fault current is enough to completely open the concentric conductor path over to X0, then H0/X0 will be left floating. In this case unless all three MV fuses have blown there could be high voltages on X0.

Yeah good point I did previously say I didn't like A because of the longer and convoluted fault clearing path but I didn't think about sizing.
​​​​​ That concentric conductor is the fault clearing path so should be sized for such. That conductor is obviously usually sized for the relatively low current of a MV system so it could be way undersized to handle a fault on the 600v side.

 

[hhsting]

Attached sketch. I am not sure wiring is correct concentric neutral goes to gnd pad or not but something like that would be realistic. Not sure if gnd pad is tank ground or not. Sketch has important terminals as shown on datasheet of the transformer unless i miss something.

Their is EGC also HV side going to gnd pad HV compartment.

**sketch attached for reference only and discussion not sure if it is correct way of doing this or not***

 

[tortuga]

Yeah good point I did previously say I didn't like A because of the longer and convoluted fault clearing path but I didn't think about sizing.
​​​​​ That concentric conductor is the fault clearing path so should be sized for such. That conductor is obviously usually sized for the relatively low current of a MV system so it could be way undersized to handle a fault on the 600v side.

I think we have concluded that option A can't safely clear a fault 250.4(A)(5) and 250.122, and could be problematic in terms of harmonics.
Option B is NEC compliant.
.

 

[hhsting]

I think we have concluded that option A can't safely clear a fault 250.4(A)(5) and 250.122, and could be problematic in terms of harmonics.
Option B is NEC compliant.
.

Umm... manufacturer of this xfmr has provided neutral to ground strap on LV compartment which can be disconnected ofcourse but what does that tell you to provide N to G at MV side?

 

[hhsting]

Yeah good point I did previously say I didn't like A because of the longer and convoluted fault clearing path but I didn't think about sizing.
​​​​​ That concentric conductor is the fault clearing path so should be sized for such. That conductor is obviously usually sized for the relatively low current of a MV system so it could be way undersized to handle a fault on the 600v side.

Good point but that was mentioned sometime post #129 option A question #2 and someone post #130 said yes and now mind has been changed. I dont think anyone here knows. Is anyone actually reading what I am saying?

 

[ggunn]

Good point but that was mentioned sometime post #129 option A question #2 and someone post #130 said yes and now mind has been changed. I dont think anyone here knows. Is anyone actually reading what I am saying?

Reading, yes, but understanding, sometimes not so much. My view only, of course.

 

[hhsting]

HV and LV neutral have to be tied together otherwise there would be problem of harmonics no? post #141

 

[electrofelon]

HV and LV neutral have to be tied together otherwise there would be problem of harmonics no? post #141

I don't know where this harmonics thing came from. I don't see what tying the neutrals together or not has to do with harmonics, nor does a PV system create significant harmonics as far as I know. If someone has experience on the matter feel free to correct and educate me, but otherwise let's not throw more gobledegook into the mix.

 

[electrofelon]

Umm... manufacturer of this xfmr has provided neutral to ground strap on LV compartment which can be disconnected ofcourse but what does that tell you to provide N to G at MV side?

I don't know what else to say, I feel like I already stated my position on this. You can't rebond the neutral under 1000 volts in a non SDS, so you bond it on the MV side and pull your 600v EGC off of that point and keep the XO bushing floating. Perhaps it's a nitpicky distinction but that is my interpretation of what the code says.

 

[ggunn]

I don't know what else to say, I feel like I already stated my position on this. You can't rebond the neutral under 1000 volts in a non SDS, so you bond it on the MV side and pull your 600v EGC off of that point and keep the XO bushing floating. Perhaps it's a nitpicky distinction but that is my interpretation of what the code says.

Speaking of nitpicky, X0 isn't floating if it's bonded to a grounded H0. Carry on.

 

[hhsting]

I don't know what else to say, I feel like I already stated my position on this. You can't rebond the neutral under 1000 volts in a non SDS, so you bond it on the MV side and pull your 600v EGC off of that point and keep the XO bushing floating. Perhaps it's a nitpicky distinction but that is my interpretation of what the code says.

X0/h0 floating are you saying still 600V neutral from x0/h0 terminal to 600V swbd be only connection and their should not be any other connection?

 

[electrofelon]

Speaking of nitpicky, X0 isn't floating if it's bonded to a grounded H0. Carry on.

Right, bad word choice. How about "appear to float"?

 

[electrofelon]

X0/h0 floating are you saying still 600V neutral from x0/h0 terminal to 600V swbd be only connection and their should not be any other connection?

Also an EGC from 600v SW board to the MV neutral on the "MV side".

 

[hhsting]

Complaint I suppose per code no n to g bond 600V however three items:

1. floating neutral is dangerous as stated in why of wye wye transformer article some post ago.

2. Where does it say muliple N to G bond okay over 1000V it just says multiple grounding of neutral only ok not n to g bond multiple.

3. If I have mbj at 13.2kV swbd I am going to have egc going to the transformer. At 1000V xfmr side will have egc and neutral bond again creating loop.

 

[hhsting]

Speaking of nitpicky, X0 isn't floating if it's bonded to a grounded H0. Carry on.

In this xfmr x0 is same terminal as h0 on LV compartment so if you ground h0 then you ground x0. Same.

 

[hhsting]

I don't know where this harmonics thing came from. I don't see what tying the neutrals together or not has to do with harmonics, nor does a PV system create significant harmonics as far as I know. If someone has experience on the matter feel free to correct and educate me, but otherwise let's not throw more gobledegook into the mix.

I dont know wye wye transformer neutral floating. Following questions for solar panel experts:

1. Does solar inverter produce harmonics?

2. The wye to wye xfmr feeds utility grid and the utility grid feeds who knows what maybe plants, houses, warehouses etc etc. If some them use power from solar panel can harmonics come to the wye to wye transformer?

 

[electrofelon]

I dont know wye wye transformer neutral floating. Following questions for solar panel experts:

1. Does solar inverter produce harmonics?

2. The wye to wye xfmr feeds utility grid and the utility grid feeds who knows what maybe plants, houses, warehouses etc etc. If some them use power from solar panel can harmonics come to the wye to wye transformer?

1. No
2. No

Note NONE of the options we are discussing have a floating neutral. Study those diagrams a bit. Neutral is always grounded.

 

[ggunn]

Complaint I suppose per code no n to g bond 600V however three items:

1. floating neutral is dangerous as stated in why of wye wye transformer article some post ago.

Per my post #151 and electrofelon's reply, X0 is not floating, just not bonded to ground on the LV side.

 

[hhsting]

1. No
2. No

Note NONE of the options we are discussing have a floating neutral. Study those diagrams a bit. Neutral is always grounded.

Ok lets back up. Ruled out option A. Option B post #154 make N to G bond at MV side. So revised option B would be as attached on this post sketch.

NOTE: Their is no one X0 termnial and no another separate H0 terminal. Their is One H0/X0 terminal on LV side only. There are two gnd pads one each compartment.


You do realize that LV side neutral is now connected to HV phase conductors. So now where will zero sequence go to HV phase conductors?

You also realize LV EGC is now connected to HV concentric neutral and HV EGC so following: 1. Can this produce objectionable current LV side? 2. Fault from LV EGC fault path still go to LV EGC to concentric neutral and HV EGC to MBJ in 13.2kV swbd to HV phase conductors to 600V winding. Is not concentric neutral undersize, 13.2kV SWBD handle this and how is this different then option A?


Sketch You see the x0/h0 terminal on LV side its floating and its not grounded. If you ground it then its N to G bond. If not correct then what is the revised sketch option B looks like?