Neutral Grounding Resistors and Wye Wye Transformers

[OdessaEE]

I think I already know the answer to this, but I will ask anyway.

A substation with a 10MVA 138KV/4160V Delta Wye transformer feeds our facility. The power company is planning to install a neutral grounding resistor during our next shutdown.

Here is where my question comes in. The transformer feeds our 4160V switchgear in the plant. A feeder for a 300KVA 4160/480V Wye Wye transformer comes off this switchgear. The transformer has an internal bonding jumper that is connected to both XO connections and is grounded to the plant grounding system. Will the neutral being grounded on the primary side of the transformer effectively make our neutral grounding system useless if we experienced a line to ground fault on the 4160V system?

 

[jghrist]

It will make the resistance part of it useless by shorting out the resistor. If you want a resistance grounded 4160 system and a solidly grounded 480 system, you should use a delta-wye 4160-480/277 transformer.

 

[ePOWEReng]

the ngr is limiting the SLG contribution from the large transformer, this will not change from what you are describing. the 4160/480 transformer will make an additional SLG contribution if you ground the HS wye. the additional contribution of the smaller transformer is going to be relatively small. you need to consider this contribution in protection considerations. also its possible for the small transformer to be damaged if the fault is present long enough and the transformer is small enough.

 

[jghrist]

I'll have to disagree with ePOWEReng. The 4160-480 wye-wye transformer is not a ground source. The NGR is shorted out by the neutral and plant grounding system.
[tt]
138000-4160 4160-480
Xfmr Xfmr
X0------------------------------------X0
| neutral |
N |
G |
R |
| |
+-------------------------------------+
plant grounding system
[/tt]

 

[ePOWEReng]

jghrist,

I agree with you.

thanks.

 

[wareagle]

I agree with jgrist. You have 3 phases an a neutral to the 4.16 kv switch. Take 3 phases to the 4.16kv/480 volt transformer and connect primary delta. Remove the neutral connection. Normally your would install an EGC from the 4kv switch to the transformer tank. The secondary is grounded wye and bonded to the grounding system. The question I am having is how do you ground the transformer without messing up the NGR system. Does the bonding of the secondary neutral do this? I don't think so.

 

[rbulsara]

I am curious, are X0(10MVA) to x0 (300kva)indeed interconnected (hard wired). What if not?

Rafiq Bulsara

http://www.srengineersct.com

 

[racobb]

As was I wondering if the utility supply transformer might have a 4 bushing secondary? Otherwise why even consider the NGR? Would think a delta/wye downstream would be much better as was indicated before.

I have never considered the installation of a res. grd. wye to grd. wye. Seems like a connection I would want to avoid.

Alan

 

[powersupply1]

Hi OdessaEE,
From the electric point of view, the NGR on the 480 WYE side will further limit the SLG fault; however I am not sure why you want to add a NGR on the 480 WYE as I believe the power company has already determined the maximum SLG fault to be limited.
The disadvantage here is that your plant will see the SLG fault flowing from the power company 4160V system because of the 300kVA transformer winding configuration WYE (HV/LV sides). Ideally, you should specify a 300kVA Tfrm, Delta/Star (WYE)so your plant(480V system)will be protected from SLG flowing from the power company transformer (10MVA, 138/4160V, Delta-WYE), providing there is not other WYE/WYE transformer in your plant.
This network configuration SLG fault can be easily viewed if you apply the positive (+), negative (-) and zero (0) sequence diagram on both transformers.
I hope this help.

 

[odlanor]

Will the neutral being grounded on the primary side of the transformer effectively make our neutral grounding system useless if we experienced a line to ground fault on the 4160V system?
YES, OF course!
In the zero sequence circuit of YY transformer ZL is connected with ZH.

 

[odlanor]

lets clarify:
1- a resistor will be installed at neutral of 138KV/4160V Delta Wye transformer1 where is the source.
2- 300KVA 4160/480V Wye Wye transformer2 has both neutral solid grounded.
3- trafo1 is interconnected with trafo2
4- You experienced a line to ground fault on the 4160V system between trafo1 and trafo2

Well, there will be zero sequence circulating in the 4160 V between neutrals of trafo1 and trafo2(currents up) and ground-fault point(current down).
If you include a resistance in the neutral of trafo1, you will reduce ground fault current level in the 4160 system.