wye to wye transformer grounding 1

[hhsting]

I have customer owned 2000kva transformer wye to wye primary 13.2kv to secondary 600V used for solar farm. The transformer primary is fed from customer owned 13.2kV SWBD. The service point as defined in NEC 2014 is at 13.2kV Swbd which has fused disconnect to the transformer. I have 3 concentric neutral cables coming to primary transformer which has H1, H2, H3, ground pad which maybe for tank ground not sure. The transformer secondary has X1, X2, X3, H0/X0, ground neutral strap, ground pad which maybe tank ground not sure. H0 and X0 is bonded internally. The transformer secondary feeds 600V SWBD which has neutral to ground bond, grounding electrode gnd ring and grounding electrode conductor. 13.2kV Swbd, xfmr and 600v swbd are all in one concrete pad outside in fenced compound. AHJ that project located is in NEC 2014.

I am having hard time figuring out how to properly ground and bond the transformer. Where does concentric neutral land HV compartment their is no neutral bushing or goes to X0/H0 LV compartment where their is also LV neutral? Is the transformer separately derived? Can their be neutral to ground bond and 600V SWBD have grounding electrode conductor to ground ring?

Anyone experience with wye to wye grounding and bonding can you help? Will post transformer name plate and compartment datasheet.

 

[RRaghunath]

13.2kV distribution is three phase only without neutral conductor, hence 13.2kV switchboard will not have neutral busbar and the incomer to the switch board as well as outgoing feeders will be without neutral cable. 13.2kV neutral of 13.2/0.600kV transformer will be connected to earth grid at the transformer location itself.
On 600V side, the transformer neutral will be taken to the switchboard. Since the neutral bar in the switchboard is already connected to earth, the neutral will not be earthed at transformer end.
Speaking of body and neutral earth connections for Transformer in general, neutral is supposed to be connected to earth grid (only 13.2kV neutral in this case) with two earth conductors for redundancy.
With regard to transformer body earthing, there will usually be two earthing points provided for transformer body which need to be connected to the earth grid.

 

[waross]

This water is too deep for you and you are in way over your head.
You need more help than it is safe to get over the internet.
Hire an electrician with high voltage experience.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hhsting]

If the 13.2kV does not come with neutral then what is MV-90 with concentric neutral cable? Also how can you make neutral to ground bond at 13.2kV switchboard. NEC 2014 Article 250.186?

 

[jghrist]

The concentric neutrals should be connected to:

1. The tank ground in the primary compartment
2. The tank ground in the secondary compartment
3. The H0/X0 bushing in the secondary compartment
4. The grounding electrode.

 

[bacon4life]

hhsting- Understanding separately derived systems is pretty fundamental to safely grounding and bonding a transformer, thus I agree with Bill's advice. This kind of transformer cannot produce a separately derived system.
RRaghunath-If the 13.2 kV system is 3 wire only without a neutral, connecting a grounded wye primary transformer would be unsafe.

 

[hhsting]

Attached sketch shows wiring from 13.2kV SWBD to 13.2kv to 600V wye to wye xfmr. Would this be something that is normal wiring for transformer such as this?

 

[stevenal]

I'll go with waross on this, get some real help. My colleagues here have not spoken of available fault current, clearing time, which conductors carry fault current, which carry load unbalance, touch potential, etc.

 

[waross]

Do you have experience terminating high voltage cables?
Have you installed terminating kits?
If kits are not available have you experience building stress cones?
If so, The grounding depends on the local codes and inspectors.
The number and the point of connection to the grounding electrodes is a matter of codes.
I have had a young inspector fail a simple grounding scheme. I was able to get a re-inspection by an old, mature inspector who said the grounding was perfect.
The code is the code, but:-
The AHJ interprets the code.
Check with your AHJ. (Authority Having Jurisdiction)


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hhsting]

I hate to say this but I am contracted reviewer for an AHJ and the previous attachment is what I came across at my desk with plans. It has not been build. I am having hard time figuring this out as well NEC code wise and how wye to wye is typically wired but any help or opinions is appreciated from anyone who has done anything like this.

 

[waross]

My thoughts.
If this were my project I would submit the following connections to the AHJ.
1.

The transformer primary is fed from customer owned 13.2kV SWBD.

The 13.2 kV system should be properly grounded at the SWBD.
The 13.2 kV transformer is a load. The neutrals should not be grounded at the load. Grounding at the load end may interfere with protection schemes at the SWBD.
Neutral current may split between the neutral conductor and the ground.
Ground fault current may split between the neutral conductor and the ground.
2. The jumper between H0 and X0 should be removed. Same reasons as above.
3. The 600 Volt system is a separately derived system. X0 should be directly connected to the ground pad in the 600 Volt compartment, and from that pad directly to the grounding electrodes.
4.The sketch shows a pad in the high voltage side that seems to be used as the main ground pad. That is the neutrals, H0, EGCs nd the ground electrode are all connected to that pad.
Only the 13.2 kV neutrals and a jumper to H0 should be on that pad. If the pad is grounded it may not be used to collect the neutrals.
5. The pad in the low voltage side: Connect the EGCs, a system grounding quality jumper to X0, and a system grounding quality conductor to the grounding electrodes to this pad.
Short version:
Ground the primary neutrals at the SWBD and not at the transformer.
Ground the 600 Volt neutrals at the transformer, together with the equipment grounds.
Do not interconnect H0 and X0.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hhsting]

1.Would not some of the neutral current from xfmr still split 600V neutral is tied at bar which is grounded solidly and so is 13.2kV swbd same gnd electrode?

2. Are neutral floating? How can zero sequence current pass thru?

3. On the attachment do you see H0 terminal 13.2kV side? Note their is no separate H0 terminal and separate X0 terminal. Their is only one X0/H0 terminal on 600V side. Can you see two separate one x0 and other h0 Attach nameplate xfmr.

4. Heard of multigrounding neutral scheme 13.2kV?

 

[waross]

That is a distribution transformer.
Distribution circuits do have multiple grounded neutrals.
Distribution circuits are not subject to the same codes.
Suggest a delta/wye transformer.
The 13.2 kV side will not need a neutral with a delta/wye transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hhsting]

Not sure what you mean by not subject to same code? I can only recommend not enforce. I am not the designer or electrical engineer of this project. AHJ reviewer contractor only can enforce NEC 2014 and which xfmr to use is not in NEC. The designer will not change to delta wye.

 

[bwen08]

I don't adhere to NEC in my field so can't provide definite answers in regards to that.

waross: I'm betting H0 and X0 are tied internally inside the tank possibly with a removable link (common unit). We've isolated H0 and X0 on some existing wye-wye units for an industrial customer, however this required H0 to be bonded internally to the tank to keep the neutral point tied to the 12kv system neutral. I would strongly suggest not removing the H0-X0 link unless considerations are made to re-establish the H0 neutral.

Typical utility practice (not NEC!!!) is to have a ground loop within the vault/transformer cabinet. A bare ground is not typically pulled in with the 12kV underground cables. At the transformer, the concentric neutrals are tied together and then to the ground loop, ground loop is connected to both tank grounding pads, ground rods and also to the H0-X0 bushing which typically also has a strap to the tank. From there, 4 wires are provided for the customer connection (A,B,C,N) where they follow NEC and have the singular neutral-ground bond.

EDIT: On further thought after reading through all this again, I'll bite...
hhsting, this transformer is a common utility purchase, while I've never dabbled in wind farm collectors, as we don't have them, I'm betting this setup is all over in the US. This is a cheaper unit to buy, the H0 and X0 are connected internally and then brought to a single H0X0 bushing in the low-voltage compartment, the H0 and X0 would be tied together anyways so this reduces one more external connection. The ground pads in the HV and LV compartment are just to ground the tank, they have no physical connection internally to either the HV or LV winding.

 

[hhsting]

@bwen08 you are correct it is factory installed internally bonded and from submitals xfmr when I look at HV compartment their is H1, H2, H3 terminals, gnd pad. When I look at LV compartment their is X1, X2, X3, H0/X0 terminals, gnd pad. LV compart has strap from H0/X0 to gnd pad. I have the nameplate post #12 from top.

However in my case xfmr falls downstream of service point where everything is governed by NEC.

In terms of wiring it what I posted post #7 from top is what you are describing?

 

[RRaghunath]

The transformer nameplate indicates that the transformer is for step-up application.
Does it mean, the 600V switchboard has power generating source connected??

 

[hhsting]

Post #1 says its for solar panel farm. There are inverters feeding fdr bkrs in 600V switchboard. 600V is solar side.

 

[RRaghunath]

Thanks for pointing out. I missed obviously.
So, the transformer 13.2kV and 600V star points are together and will be earthed at the transformer itself. This will help earth fault protection (SLG) in 13.2kV feeder to detect a fault in the feeder cable or transformer itself or fault on 600V side.
Inverter associated with the PV source will not feed current in case of a SLG fault in 600V system.

 

[stevenal]

Generating plants are subject to NEC? The scope of NESC specifically includes solar generation as well as other forms.

Perhaps the discussion should move to the ethics forum, regarding the acceptance of work outside an engineer's competence.