Autotransformer Vector Group

This link explains it better than I could.

https://electrical-engineering-portal.com/understanding-vector-group-transformer-1

Dear sirs,

It appears to me that YNa0d11 means star connected HV winding with neutral brought out, autotransformer with 0 degree displacement. LV winding delta connected leading HV by 30 degrees.

Please refer to my original question. The mentioned vector group does not give any information on secondary and tertiary winding. Is this to be understood that primary and secondary winding both are star connected?

Regards,

You cannot have an auto transformer with a phase shift to LV [except due to reactance], because the LV terminal is just a tap on a "single" grounded winding. It would be the tertiary winding which was delta connected and the theoretical vector of a point of the Δ to ground would have the 30 degree shift.

In other words, the vector group gives information about the connection of the tertiary winding. It is likely that the voltage in the tertiary winding is "LV", hence d11 not D11.

Primary and secondary must be star, with zero vector group phase shift, because there is no other way to use auto transformers on 3 phase.

The percentage reactance is unlikely to be same for tertiary winding as from line to LV winding.

I would think the rating would be 50MVA for the winding A from HV terminal to tap & 50VA for LV winding B from tap terminal to ground i.e. 50VA delivered to external LV load. Since the voltage on winding A is less than HV incoming voltage its current rating will be higher than if it were at the HV voltage, for a specified MVA.

I may be wrong, but I cannot see another way of getting the correct terminal current limits from MVA rating.

This is obviously necessary because winding A must carry current for the tertiary winding as well as the current exiting LV part of the winding at LV voltage. With the tertiary winding voltage, you would know the current rating of tertiary winding. The HT terminal current x HV voltage would be greater than LV terminal current x LV voltage, with difference due to tertiary.

Apologies for error in post#4, I wrote 50VA for LV winding, when it should be 50MVA

Is primary mva = secondary mva + tertiary mva ?

I think the MVA of the winding A from HT terminal to LV hot terminal must be given to get a correct current limit for the HV terminal - it is feeding both LV & tertiary windings.

The MVA of the external LV load must be supplied as well as the LV winding B contributing to tertiary winding C & losses as part of primary circuit.

There are only 2 voltages possible for HV MVA, HV line to LV tap or HV line to Ground. Using the second will only allow 50 MVA input, with 50 MVA LV output that leaves no power for 10 MVA tertiary or losses.

The relevant information for use is HV voltage, HT terminal current, LT terminal voltage & current and tertiary winding voltage & current which you can meter for operation and relay for protection.

You have not given HV - ground voltage or LV - ground voltage. Nor am I clear if this is 3 single phase transfo or one 3 phase transfo. All I know is that HV auto transfos have low ratios around 4 :1.

If you look at the actual voltages & work out currents as above you may find that HV input = LV output + tertiary MVA + [number for losses/magnetising current] which is believable.

I would consult the manufacturer on how to interpret the data to get the currents & impedances required. Also how they actually test for impedances - you could test from HV with only LV shorted, only tertiary shorted or both shorted. The usual 3 phase transfo with 3 limbs is not symmetrical - not all phases will give same impedance.

Do you have the factory test report? - it should give voltages & currents measured.

While I can’t figure how the EE who came up with how to design a tertiary winding even imagined it, I did run across an interesting article on how to set up your protective relaying, may be of help designing protection.

https://www.eiseverywhere.com/file_uploads/a99e2aa556135afc2ea43c68303bd462_zha_pap.pdf

Tertiary delta winding on star auto transformer, that is

This is 3 single phase atotransformer forming a three phase bank. The voltage rating is 220/132/33 kV.

Regards,

Yes, still looks like a recipe for disaster for an auto transformer, especially analyzing zero sequence current circulation. Someone had to say, "Sure why not", and figured out how you could interconnect the middle of one auto transformer winding to the middle of another, and make a cheap transformer.

Not a good answer by me [except to consult test results]. Wrong! Wrong! Wrong! that is a 3 phase wrong!

As analysed in paper referenced by R Williams, the tertiary can be closed delta so it does not draw power in normal operation when phase-ground voltages are equal. The 10MVA/33kV indicates the current rating & ratio for the tertiary winding, while 50MVA/50MVA indicates normally all HV power goes to LV.

67model

To answer the Original Post question, correctly, primary input 50MVA/LV 50MVA/Tertiary 10 MVA suggests that if auxiliary power is taken from tertiary [at maximum 10 MVA] then LV MVA must be reduced to 40 MVA.

The primary power could not be increased if the primary windings are designed for the losses/current at 50MVA, 60 MVA would increase primary winding losses by about 50%.

This is a simplification, because reduced losses in LV may allow some increase of losses in HV.

The OP is in a position to know for what actual continuous simultaneous current loadings, LV & tertiary, the transfo was ordered & tested. The factory test ought to be based on the the ordered continuous loadings. Similarly, OP should be able to find if tertiary is connected to a 33kV load or not.

If the transfo were designed for a tertiary winding functional only for harmonic and fault currents, then 50/50/10 MVA makes sense. For a continuous tertiary auxiliary load of 10MVA, then 60/50/10 MVA seems correct for continuous 50MVA from LV.

If, on load test at 50MVA input, 50MVA LV load, zero tertiary load; the HT winding [the one connected to the HV terminal], or LV temperature rise by resistance was already at maximum permitted, then there is no margin for tertiary load.

You have just reminded me why I studied Mechanical Engineering.

This can be a very complicated subject,as there are many possible configurations of secondary.

The tertiary windings are normally connected in Delta.

It is too deep to go into here,and to repeat the info that is freely available.

If you truly understand transformers and how they work,things will become easier to comprehend.

I suggest further study in the area.

Much info is available on Google.

As someone once said,"I can teach it,but I cannot learn it for you."

I wish you good luck in your pursuit of knowledge.

Here is a link that explains tertiary windings:

https://www.electrical4u.com/tertiary-winding-of-transformer-three-winding-transformer/