Selection of Step-up Transformer

.it is yet an unproven theory but practically it desirable to have your neutral grounded...you mentioned that harmonics effect is limited that is because your neutral serves as a path for your 3rd and other zero sequence harmonics....lastly, what is the configuration of your transformer connection?..wye-wye? wye-delta? or delta-wye?

i think the connection Ynd is the best for generator step-up transformers with using delta connection for the low voltage side for the following reasons:

• The delta connected winding keep the zero sequence impedance of the transformers reasonably low.
• The line current in delta connection equal to the line current divided by √3 which makes the winding smaller by using smaller bundle of wiring conductors

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Generator step-up transformers

Thanks for your comments... transformer configuration we have is dyn where delta is on low voltage side.

So far in your comments you said that this configuration is better for generator step-up transformer. Does this configuration has no effect on alternator?

Selecting the required transformer may be dependent on a power utility's electricity network vector and the utility's regulatory standard practices in a country. I addition to "rockraiden" post, the dyn11 step-up Tx earthing is done in two; the L.V earthing and the H.V earthing. >The L.V side earthing is effective in reducing the short-circuit faults, surges, and harmonics. The neutral is connected to the earth wire at a link, from there earth wire of insulated hard drawn copper conductor. This earth wire is run through protection conduit to ground. It is run and connected to earth stakes 0.5-1.0m underground. Earth stakes are placed in a loop diameter of 1.5-2m, by which the cable is run. Any fault on the neutral will be grounded. >The H.V side earthing is effective in protecting the H.V load side, transformer and equipments supporting the transformer. It runs from arrestors, connected to transformer chassis, connected to the supporting structure, and then is run to ground, connected to earth stakes at 0.5-1m underground in a loop of 1.5-2m diameter, and buried. The earthing protection of the dyn11 and the ydn11 is a great advantage is protecting the load, supply source and equipment, and the transformer itself. Either the dyn11/ydn11 transformer is areal pole mounted or is a kiosk ground mount transformer; the earthing standard practice is similar, despite the pole top construction and underground cable termination requirements for the kiosk type transformer.

Also consult transformer manufactures, they may be of great assistance....they designed it, built it and tested it....

I doubt your understanding of the transformers vector notation. In the vector notation, the capital letter denotes how the HV winding is connected (In your case, it is 'D', which means that the HV winding or the 11kV winding is Delta connected, which is quite normal, as it is a three-phase three wire system); the small letter denotes how the LV winding is connected (in your case, this is 'y', which means that the LV side or the 400V side is star connected, which is also quite normal as the 400 V side is also towards the generator, which would be star connected too with the neutral solidly earthed). 11 means that the LV voltage leads the HV voltage by 30 degrees. In your case, as LV is the primary, the HV voltage or 11kV would "lag" the LV voltage or 400V by 30 degrees. It is quite normal to have such a connection for a generator transformer. Don't worry!

Your confusion is due to the fact that you associate the capital letter in the vector symbol to be that of the "primary winding" and the small letter to be that of the "secondary winding", which is not correct. It is always that the capital letter is HV and the small letter is LV.

Hope it is clear now!

Hi mate,

your reply was probably the easiest and logical answer I have come across on this subject.

I am trying to work out the protection requirements for a similar installation. I have a 2mva 415v - 11KV Tx Dyn11 which is going to be fed by a generator bus system on a 2500amp feeder. The 415v feeder is earth leakage and overload protected. The HV 11KV side is also earth leakage and overload protected through a ringmaster rn2c unit. Is there any requirements on earthing arrangements that I should be aware of in your opinion.

If you get a chance be good to hear your comments

ok. let i clear for you. i have step-up transformer where generated 400V is stepped-up to 11KV. Transformer 400V side is delta & 11kv side is star connected. Now i want to ask that what are the effects on alternator of this configuration(dYn) & what will be effects on alternator if i reverse this configuration(ynD).?