Generator 400v step up to 11kv vector group confirmation 2

[AHSEN_MEP]

We have 1250kva two generators of CAT which is giving output of 400voltage . These genset will be connected to synchronizing panel. After Sync Panel we will connect it with 2500KVA step up transformer .4/11kv .
This transformer will convert voltage into 11kv . vector group of this transformer is YNd 11 so primary side of transformer has three busing and secondary side has four bushings . neutral bushing will be grounded and generator neutral will also be grounded .
i need to know that this vector group is ok and will work or we need to change vector group .
secondly please confirm that as there is no neutral at primary/LV side so there will be no neutral in synchronizing panel and ACB breakers will be three pole

 

[che12345]

Dear Mr. AHSEN_MEP (Electrical)
1. I try to understand your present set-up and raise some points for your consideration.
(a) 2 x LV Gen each [1.25 MVA 400V with Neutral]. Both N1 and N2 are solidly Grounded, without any switching.
(b) one 2.5 MVA step-up trafo LV is 0.4kV Star with Neutral solidly grounded. The HV is 11kV Delta.
2. FYI: The usual convention the HV side is denoted by [upper-case lettering] and the LV is by [lower-case], irrespective of whether it is [step-up or step-down]. For your case it is Dyn11.
3. You solidly Grounded both N1 and N2 could be problematic for GF protection. Consider separate Neutral and add switching with only One ground when in sym. Caution: this needs some attention.
4. Each Gen shall be with [3-phase + separate Neutral] respectively. Therefore, both LV ACBs shall be 4-pole .
5. Each Gen or symed shall be with [3ph + Neutral] connected to the Trafo phases and Neutral. Trafo Neutral shall be solidly grounded.
6. The trafo is Dyn11 with 11kV on HV side. It does NOT matter what is the vector group output at the 11kV.
Che Kuan Yau (Singapore)

 

[xnuke]

che12345, I don't think you read the OP closely enough. You seem to think the step-up transformer is wye on the LV side and delta on the HV side, but that is the opposite of what the OP says. I believe he described things correctly, with the transformer as YNd11, as this reduces the SLG fault current on the LV side for faults between the generators (esp. if they are 2/3 pitch) and the transformer to less than the bolted fault current for solidly-earthed generators. If the transformer is truly YNd11, only three-pole CBs are needed for the generators, there will be no need for a neutral in the synchronizing switchgear.

AHSEN_MEP, a single-line diagram would be helpful. Also, I've been discouraging the use of the terms primary and secondary as related to transformers to avoid confusion (especially since power flow can be bidirectional in some transformers), even though I believe you used them correctly. Please use HV and LV throughout. I'm not sure why you are concerned about the vector group unless your step-up transformer output would need to be in parallel with that of another transformer's output that has a different vector group, resulting in circulating currents are a requirement to derate the parallel transformer power ratings. Care to state the cause of your concern?

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[che12345]

Dear Mr AHSEN_MEP (Electrical)
@ Mr. xnuke (Electrical)
I read the description again and admit my mistake. My sincere apology. Thank you for your advice.
1. So, the trafo is YNd11, where YN HV at 11kV, d at 0.4kV.
2. The LV Gen N1 and N2 are NOT distributed but solidly grounded. The LV 0.4kV system would be a 3-ph 3-W (without Neutral). The ACBs shall be 3-pole.
3. The Gen1 and Gen2 Neutrals are solidly earthed. Caution: I am afraid it may need further study?
4. The HV 11kV YN where Neutral is solidly grounded is fine.
5. Agreed with your learned advice ".... unless your step-up transformer output would need to be in parallel with that of another transformer's output that has a different vector group, resulting in ......."; the vector group is immaterial.
Che Kuan Yau (Singapore)

 

[AHSEN_MEP]

Dear Che12345 & xnuke
Thank you for your response .

From your relies followings are clarified :
1. Step up transformer .4/11kv yNd11 that LV side is Delta and MV side is star and solidly grounded
2.Gensets neutral will be soldily grounded
3.There is no requirement of Neutral bus bar in Synchronizing Panel
4.Breakers will be 3P ACBs in Synchronizing Panel
5. Vector group is yND11 (LV is Delta and MV / secondary side is Star.

Please confirm the following also
Neutral of Genset will be grounded . 20mm copper 3 mtrs rod is enough for Grounding ? Conductor size between neutral of Genset and Grounding rod ?
Can we use stepdown Dyn11 transformer 11/.4kv by connecting in reverse to turn it in step up .4/11kv

 

[RRaghunath]

AHSEN MEP,
When two generators are paralleled, it is recommended to avoid earthing both the neutrals in order to prevent circulation of third harmonics with associated heating of windings and neutral circuit.
As Che Kuan Yau Sir mentioned in his post, please do study this configuration further.
Further, when you mention YNd11, Y in capitals refers to HV winding connection and d n small letters refers to LV winding connection of the transformer as per the international standard convention (just in case you are not aware).

 

[che12345]

Dear Mr. AHSEN_MEP (Electrical)(OP)16 Jan 23 12:53

From your relies followings are clarified :
1. Step up transformer .4/11kv yNd11 that LV side is Delta and MV side is star and solidly grounded.
You mean YNd11; where yNd11 is a typo error ?

2.Gensets neutral will be soldily grounded
FYI: Caution, need further study of the consequences.

3.There is no requirement of Neutral bus bar in Synchronizing Panel
Agree, the LV system is a 3-phase 3-Wire (no Neutral) system, fine.

4.Breakers will be 3P ACBs in Synchronizing Panel.
Both LV ACBs are 3-pole, fine

5. Vector group is yND11 (LV is Delta and MV / secondary side is Star.
If LV is in Delta and HV is in Star with Neutral; It is YNd11. See above 1.

Please confirm the following also
1. Neutral of Genset will be grounded . 20mm copper 3 mtrs rod is enough for Grounding ? Conductor size between neutral of Genset and Grounding rod ?
FYI: Caution, check the local Earthing/grounding regulation (i.e. IEC, NEC ....).
Attention: single 20mm copper 3 m rod may? NOT be able to comply with the local regulation.

2. Can we use stepdown Dyn11 transformer 11/.4kv by connecting in reverse to turn it in step up .4/11kv
Any transformer irrespective of whether in the LV or HV is Star or Delta, the input can be on the LV or the HV. That is, it can be a "step-up" or "step-down" transformer.
Che Kuan Yau (Singapore)

 

[AHSEN_MEP]

Thank you for response

@Che12345 Refer to my below message and your response , Please advise the number of ground rod and size , thank you
1. Neutral of Genset will be grounded . 20mm copper 3 mtrs rod is enough for Grounding ? Conductor size between neutral of Genset and Grounding rod ?
FYI: Caution, check the local Earthing/grounding regulation (i.e. IEC, NEC ....).
Attention: single 20mm copper 3 m rod may? NOT be able to comply with the local regulation.

 

[che12345]

Dear Mr. AHSEN_MEP (Electrical)(OP)19 Jan 23 15:22
"....@Che12345 Refer to my below message and your response ,... #1Please advise the number of ground rod and size , ..... #2. .... Neutral of Genset will be grounded . ....with 20mm copper 3 mtrs rod is enough for Grounding ? Conductor size between neutral of Genset and Grounding rod ?..."[/i

We are here acting on pro bono basis, the opinion is the personal. We cannot act as a free consultant. Caveat emptor. You have to do your part

1. FYI: Caution, check the local Earthing/grounding regulation (i.e. IEC, NEC ....).
Attention: single 20mm copper 3 m rod may? NOT be able to comply with the local regulation.
2. Neutrals of both Gensets will be solidly grounded needs further study. This could? be problematic even the 400V system is changed to 3-ph 4-W, with both ABCs 4-pole.
Che Kuan Yau (Singapore)

 

[FreddyNurk]

Can we use stepdown Dyn11 transformer 11/.4kv by connecting in reverse to turn it in step up .4/11kv

The transformer itself will work happily supplied on the low side and feeding the high side. What that means for your earthing and protection system is an entirely different question.

The circulating currents issue for sets paralleled in LV is really only an issue when there are different alternator winding pitches (e.g. Caterpillar versus Stamford, although Caterpillar may well have gone to 2/3 pitch as well). I've done plenty of LV paralleled generator sites with 3P breakers and a common neutral bus in the main switchboard, but I get the impression that's not the concept here, particularly with a synchronising panel and YNd transformers. What is likely to be just as much of a problem is how the HV Earth Fault protection works with multiple star points on the 11kV side.


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