Unusual 3 Winding Transformer Calculations 1

[Nik10]

Hello there!

Please forgive me but this is my first post to any forum. I am working on my own to suggest the team (management) here that renewable energy sources can used melt scrap!

I am a manufacturing engineer at steel plant in Port Talbot (so mostly hands-on guy), Wales. I am designing (well trying!) a 3 winding step up transformer 33kV (P) /132kV (S) , 90MVA. One the 132kV(S) side, I want to create a two windings. S1 will be 99kV rated and S2 will be 33kV rated. During normal operation, the two S1& S2 will work in series. However, during the start-up, I want only 33kVv(S2) winding to work. This way i can get more current into the electric rods to melt the scrap in an electric arc furnace.

Please can anyone suggest how I can rate the windings resistance and reactance values. I have attached this excel sheet, and some calculated values. It would be kind (and great) if anyone can provide some feedback on my calculations and correct them for the three different scenarios with some comments about pros and cons.I really need to understand if the values make any sense as I have no sounding board here.

many thanks in advance!

 

[7anoter4]

In my opinion, it will be better to contact a manufacturer-for instance GE:

https://www.gegridsolutions.com/pro...formers-brochure-en-2018-09-grid-ptr-0199.pdf

 

[argotier]

Unusual transformer indeed!

Are you sure that this tranformer will be used for an electric arc furnace?

Usually you need several kA for this application and so its not rare secondary windings of very low voltage (around or less than 1 kV).

Secondary voltages of 132 or 33 kV and currents of only 1000 A seems odd. Please check again this.

Regarding your question, 7anoter4 is right, you are overthinking it. You need to define just what you need and let the transformer manufacturer deal with the specifics. Imagine the transformer as a black box and only specify what you need in each stage "from the bushings out".

Do you really need a specific set of values for R and X for each winding?

This 132-33 kV change, it's supposed to be made on-load? If so beware that this is not easily achievable and it will be very costly.

Arc furnace transformers are very special machines and it's best if you seek advice from an experienced manufacturer in this field.

BTW you can´t allocate R and X values for each winding in the way you tried in the spreadsheet, on one hand pu impedances are defined for a pair of windings and on the other hand the X of a pair of windings depends entirely on the geometry of the windings (diameter, height...). So you can come up with different values on each winding that will comply with your needed impedance on transformer terminals. The manufacturer can and will select the best solution for this.

 

[prc]

As argotier mentioned, please give your requirements and transformer engineers will give Optimum solution. From what you mentioned, it will require a regulating transformer and furnace transformer in series. Such an arrangement is widely used in aluminium smelters- a regulating transformer to vary output Voltage from 10% -110 % of 132 kV by means of a 104 position tap tap-changer feeding Rectifier transformer. Similar solutions can be used in your case also.

 

[Nik10]

Many thanks 7anoter4, argotier & prc!

I am trying to contact the manufacturers but I had not much from them due to lock-down. Moreover, because its an investigating inquiry, many of them do are not interested in this.

Anyways, many thanks for your support.

Regards,